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Classifications

Harrison's Part

IUIS Category

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Inheritance

1

Autosomal recessive inheritance HP:0000007

HKLLS1 is inherited in an autosomal recessive manner; affected individuals carry biallelic (homozygous or compound heterozygous) CCBE1 mutations, frequently in the setting of parental consanguinity.

Autosomal recessive inheritance

Show evidence (1 reference)

PMID:19935664 SUPPORT Human Clinical

"Homozygous and compound heterozygous mutations in seven subjects paired with functional analysis in a zebrafish model identify CCBE1 as one of few genes causing primary generalized lymph-vessel dysplasia in humans."

Homozygous and compound heterozygous CCBE1 genotypes establish autosomal recessive inheritance for HKLLS1.

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Discussions and Knowledge Gaps

6

How do CCBE1 variants produce the variable neurodevelopmental and craniofacial findings beyond their established effect on lymphatic development?

HUMAN MODEL MISMATCH OPEN mismatch_ccbe1_models_nonlymphatic_phenotype

Attached to

pathophysiology#Generalized lymphatic dysplasia phenotypes#Intellectual disability phenotypes#Midface retrusion phenotypes#Depressed nasal bridge phenotypes#Hypertelorism phenotypes#Epicanthus phenotypes#Delayed speech and language development phenotypes#Microcephaly

The models summarized here establish a role for CCBE1 in lymphatic development, while human CCBE1-positive cases establish variable cognitive, speech, head-size and facial findings. The clinical sources do not demonstrate a causal route to each finding. Intrauterine facial edema is a proposed explanation for facial morphology, but a direct developmental contribution has not been excluded. These observations therefore remain without asserted causal edges.

Recorded as HUMAN_MODEL_MISMATCH rather than KNOWLEDGE_GAP because model-system evidence for the lymphatic mechanism is strong; what is uncertain is how far that evidence transfers to the non-lymphatic arms of the human phenotype.

Show evidence (2 references)

PMID:21778431 SUPPORT Model Organism

"Mutant mice die prenatally. Proximity ligation assays indicate that vascular endothelial growth factor receptor 3 activation appears unaltered in mutants."

Prenatal lethality is the concrete reason the mouse model cannot report on the postnatal neurodevelopmental phenotype; the assays reported in the same sentence pair are lymphatic-signalling readouts, which is the only arm of the disease this model addresses.

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"We conclude thatCCBE1mutations cause a widespread and severe form of lymphatic dysplasia, associated with variable intellectual disabilities, unusual facial morphology and infrequently malformations."

This conclusion concerns CCBE1-positive patients and establishes the human nonlymphatic findings whose mechanisms remain unresolved.

Does the domain in which a CCBE1 variant falls predict clinical severity in HKLLS1?

KNOWLEDGE GAP OPEN gap_ccbe1_domain_genotype_phenotype

Attached to

genetic#CCBE1 pathophysiology#CCBE1 loss of function

The domain-deletion knock-in series shows a clear functional hierarchy - collagen-repeat loss phenocopies the null while EGF-domain loss still permits rudimentary lymphatics - and the same ordering held for two Hennekam patient mutations, one per domain. Two alleles is a demonstration, not a predictor. With roughly a dozen reported CCBE1 variants across about seventeen probands and no systematic functional assay applied across them, it is not established whether domain position explains the wide clinical variability, which ranges from recurrent prenatal hydrops to a child with imaging-detected lymphangiectasia and no measurable protein loss.

Show evidence (2 references)

PMID:25814692 SUPPORT Model Organism

"Furthermore, a Hennekam syndrome mutation within the collagen domain has a stronger effect than a Hennekam syndrome mutation within the EGF domain."

Establishes the domain-dependent severity signal that this gap asks whether to generalize; it rests on two patient alleles.

PMID:30564329 SUPPORT Human Clinical

"at least 12 different CCBE1 variants have been reported in at least seventeen probands"

Bounds the size of the reported allelic series, which is what makes a genotype-phenotype rule unestablished rather than merely uncurated.

What fraction of clinically diagnosed Hennekam syndrome remains molecularly unexplained after CCBE1, FAT4 and ADAMTS3 testing, and how many further genes account for it?

KNOWLEDGE GAP OPEN gap_hennekam_unexplained_molecular_fraction

Attached to

genetic#CCBE1 prevalence#Worldwide

CCBE1 explains only about a quarter of clinically diagnosed Hennekam syndrome. FAT4 and ADAMTS3 account for further subsets, but a substantial residue has no molecular diagnosis, and additional candidate genes have been proposed on single families. This directly limits the epidemiology of HKLLS1: published case counts are for the clinical syndrome rather than for molecularly confirmed CCBE1 disease, so no prevalence figure specific to HKLLS1 can be derived from them.

Show evidence (2 references)

PMID:24913602 SUPPORT Human Clinical

"It can be caused by mutations in CCBE1 which are found in approximately 25 % of cases."

Quantifies the unexplained fraction that motivates this gap.

PMID:28985353 SUPPORT Human Clinical

"They account for about one third of all primary lymphedema cases, underscoring the existence of additional genetic factors."

Documents that the known primary-lymphedema gene set leaves most cases unexplained, the wider context in which the Hennekam residue sits.

Which historical Hennekam findings are established specifically in CCBE1 disease?

KNOWLEDGE GAP OPEN gap_ccbe1_subtype_phenotype_attribution

Attached to

genetic#CCBE1

The original 1989 family was later shown to have FAT4 disease. Its narrow mouth, dental and ear findings cannot alone establish those phenotypes in HKLLS1, and a general statement that ADAMTS3 cases have similar faces does not supply CCBE1-specific observations. The Lee rehabilitation patient with pulmonary lymphangiectasia had no genetic testing. These findings are omitted pending direct subtype support. Total lymphopenia is a recognized consequence of intestinal lymph loss generally, but the reviewed CCBE1 reports document normal total lymphocyte counts or selected lymphocyte subsets rather than a confirmed low total count. This remains a possible complication rather than an observed subtype phenotype. Seizures were absent in the 13 CCBE1-positive cohort; this does not establish absolute exclusion.

Show evidence (1 reference)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"Seizures did not occur in the group of molecularly confirmed HS patients, but were found in 6 of the HS cases without molecular confirmation."

The genotype-stratified comparison shows why historical Hennekam seizure estimates should not be imported into HKLLS1.

What mechanisms explain the associated thyroid, hernia and fetal structural findings?

KNOWLEDGE GAP OPEN gap_ccbe1_additional_findings_mechanisms

Attached to

phenotypes#Hypothyroidism phenotypes#Inguinal hernia phenotypes#Umbilical hernia phenotypes#Cystic hygroma phenotypes#Pulmonary hypoplasia phenotypes#Metopic synostosis phenotypes#Congenital diaphragmatic hernia

The clinical reports document these findings without establishing their causal routes. The C174Y siblings had hypothyroidism and hernias; whether these reflect lymphatic disease or additional developmental effects is unresolved. Cystic hygroma and pulmonary hypoplasia were observed in an ungenotyped fetus from the CCBE1 family, so both individual genetic attribution and a detailed mechanism remain uncertain. Causal edges are withheld. Metopic synostosis and diaphragmatic hernia were isolated CCBE1-positive cohort findings without a demonstrated causal route. Growth hormone deficiency was also named in one cohort case, but the narrative does not supply the testing needed to choose a specific HPO binding; this is retained as a curation lead. Isolated neoplasms and uncertain polydactyly are not generalized into established CCBE1 disease associations.

Show evidence (2 references)

PMID:26686525 SUPPORT DIRECT Human Clinical

"In adulthood, both P1 and P2 developed hypothyroidism treated with levothyroxine."

Documents the thyroid phenotype without establishing its mechanism.

PMID:30564329 SUPPORT INDIRECT Human Clinical

"An autopsy described a female fetus with a cystic hygroma, serous (nonchylous) pleural and pericardial effusions, congested liver, hypoplastic lungs, and a “markedly enlarged, hyperplastic thyroid gland”."

The fetal structural findings were observed in the second pregnancy, which could not be genotyped.

Which drainage, leakage and oncotic mechanisms produce pleural effusions and ascites in CCBE1 disease?

KNOWLEDGE GAP OPEN gap_ccbe1_compartment_specific_fluid_mechanisms

Attached to

pathophysiology#Generalized lymphatic dysplasia phenotypes#Pleural effusion phenotypes#Ascites phenotypes#Chylous ascites

CCBE1-positive patients have pleural effusions and fetal ascites, and other confirmed patients have recurrent chylous ascites. These observations establish the clinical associations but do not resolve the compartment-specific routes of fluid accumulation. Impaired drainage, lymphatic leakage and reduced oncotic pressure may contribute differently. The ultrasound of the confirmed fetus did not characterize the fluid, and the sibling report did not locate a peritoneal leak. These passages do not themselves attribute each effusion to dysplasia as a causal claim, so those arrows are withheld. Unknown intermediates alone would not preclude an indirectly supported causal edge; the missing evidence here is for the compartment-specific causal attribution.

Show evidence (2 references)

PMID:30564329 SUPPORT Human Clinical

"Ultrasound images of hydropic fetus from G4 pregnancy. A, Fetal chest demonstrating pleural effusions; B, fetal head demonstrating scalp edema; C, fetal abdomen demonstrating ascites"

The confirmed fetus had pleural effusions and ascites; ultrasound establishes their presence rather than the mechanism or composition of the fluid.

PMID:26686525 SUPPORT Human Clinical

"We report two HS siblings, born to consanguineous parents of Turkish ancestry, whose clinical phenotype also includes protein losing enteropathy, painful relapsing chylous ascites, and hypogammaglobulinemia."

The molecularly confirmed siblings establish chylous ascites, but this co-occurrence does not identify the anatomical route of lymph leakage or exclude other contributors.

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Pathophysiology

6

CCBE1 loss of function

Biallelic loss-of-function mutations in CCBE1 reduce the activity of a secreted extracellular-matrix protein required for lymphangiogenesis.

CCBE1 hgnc:29426

Animal models

CCBE1 domain-deletion knock-in mice Mus musculus recapitulates Maps loss of CCBE1 function onto specific protein domains, and shows that a Hennekam syndrome mutation in the collagen repeats is more damaging than one in an EGF domain - a structural account of why HKLLS1 alleles differ in severity.

Downstream

Impaired pro-VEGF-C proteolytic activation CCBE1 is the cofactor that lets ADAMTS3 cleave pro-VEGF-C into its mature active form; without it, that cleavage step fails.

Show evidence (5 references)

PMID:19935664 SUPPORT Human Clinical

"Homozygosity mapping identified a critical chromosomal region containing CCBE1, the human ortholog of a gene essential for lymphangiogenesis in zebrafish."

Identifies CCBE1 as the causal gene and situates it as an essential lymphangiogenesis factor.

PMID:25814692 SUPPORT Model Organism

"deleting the collagen domains of CCBE1 has a much stronger effect on CCBE1 activity than deleting the EGF domains"

Domain-dissection knock-in mouse and zebrafish data show the collagen repeats of CCBE1 are crucial for its VEGF-C-activating, lymphangiogenic function that is lost in HKLLS1.

PMID:26686525 SUPPORT In Vitro

"Our findings show that the mutant CCBE1 C174Y protein is not loss-of-function by loss-of-expression."

Qualifies what "loss of function" means at this node for one recurrent patient allele: C174Y is not loss-of-function by failure to make the protein, since transfected cells show increased expression and secretion. That is an in vitro result and does not describe patient tissue, where the same paper finds CCBE1 scarce in affected intestinal lymphatics. The paper offers two unresolved explanations, quoted in the items below, so the node should be read as loss of CCBE1 activity without a settled account of whether the protein is absent, misfolded, or degraded in vivo.

+ 2 more references

Impaired pro-VEGF-C proteolytic activation

CCBE1 and the protease ADAMTS3 form a complex that cleaves the poorly active 29/31-kDa form of pro-VEGF-C into the mature 21/23-kDa ligand. Loss of CCBE1 removes the cofactor, so this proteolytic maturation step fails and little active VEGF-C is produced. CCBE1's two domains contribute differently: the C-terminal collagen-like domain drives the cleavage activity, while the N-terminal EGF-like domain colocalizes pro-VEGF-C with its protease at the lymphatic endothelial cell surface.

metalloendopeptidase activity of the ADAMTS3-CCBE1 complex GO:0004222 ↓ DECREASED

Animal models

CCBE1 domain-deletion knock-in mice Mus musculus measures The same allelic series was read out directly on VEGF-C processing, so it assays the proteolytic activation step itself rather than only its downstream consequence. Collagen-domain deletion abolishes processing while EGF-domain deletion does not, which assigns the cleavage-supporting function of CCBE1 to the collagen repeats.

Downstream

Failed lymphangiogenesis Without mature VEGF-C, VEGFR-3 signaling in lymphatic endothelial cells is not driven and lymphatic vessels fail to form.

Show evidence (3 references)

PMID:24552833 SUPPORT In Vitro

"it promotes proteolytic cleavage of the otherwise poorly active 29/31-kDa form of VEGF-C by the A disintegrin and metalloprotease with thrombospondin motifs-3 protease, resulting in the mature 21/23-kDa form of VEGF-C, which induces increased VEGF-C receptor signaling."

Defines the cleavage this node names and the cofactor role CCBE1 plays in it.

PMID:27159393 SUPPORT In Vitro

"an ADAMTS3-CCBE1 complex can form independently of VEGFR3 and is required to convert VEGFC, but not VEGFD, into an active ligand"

Establishes that CCBE1 and ADAMTS3 act as a complex and that the complex is required for VEGF-C activation specifically, which is why loss of CCBE1 is not compensated by VEGF-D.

PMID:28687807 SUPPORT In Vitro

"The N-terminal EGF-like domain of CCBE1 increased VEGFR-3 signaling by colocalizing pro-VEGF-C with its activating protease to the lymphatic endothelial cell surface."

Resolves the two separable contributions of CCBE1 to this step, which is what makes domain-specific patient alleles differ in severity.

Failed lymphangiogenesis

Without VEGF-C/VEGFR-3 drive, lymphatic endothelial cells are still specified but fail to bud, migrate and assemble into lymphatic vessels.

lymphatic endothelial cell CL:0002138

lymphangiogenesis GO:0001946 ↓ DECREASED

Animal models

Ccbe1-null mouse Mus musculus recapitulates Loss of Ccbe1 in the mouse blocks budding and migration of Prox1-positive lymphatic endothelial cells from the cardinal vein, which is exactly the cellular step this node names.

ccbe1 mutant zebrafish Danio rerio recapitulates ccbe1 mutants fail lymphangioblast budding and venous sprouting, and ccbe1 acts at the same developmental stage as vegfc, which is the cellular lymphangiogenic step this node names.

Downstream

Generalized lymphatic dysplasia Defective lymphangiogenesis produces malformed, dysfunctional lymphatic vessels throughout the body.

Show evidence (2 references)

PMID:19287381 SUPPORT Model Organism

"Ccbe1 acts at the same stage of development as Vegfc and is required for lymphangioblast budding and angiogenic sprouting from venous endothelium."

Identifies the cellular step that fails - lymphangioblast budding and venous sprouting - and places it at the same developmental stage as VEGF-C.

PMID:26686525 SUPPORT DIRECT Human Clinical

"we found a strong decrease of lymphatic vessels, with a corresponding diminished expression of CCBE1, by immunohistochemistry of the patients' intestinal biopsies"

Human intestinal tissue from CCBE1-mutant patients shows the lymphatic vessel loss this node asserts. The rest of the support for this node is from zebrafish and mouse, so this is the observation that anchors it in human disease rather than in model systems.

Generalized lymphatic dysplasia

Widespread lymphatic vessel dysplasia impairs lymph drainage and underlies the intestinal lymphangiectasia, lymphedema, and serous effusions of the syndrome.

lymphatic endothelial cell CL:0002138

Animal models

Ccbe1-null mouse Mus musculus partially recapitulates Complete absence of definitive lymphatics models the severe end of the human generalized lymphatic dysplasia, and is consistent with the recurrent hydrops fetalis seen in some HKLLS1 pregnancies.

Downstream

Intestinal lymphangiectasia Dysplastic intestinal lymphatics dilate and leak lymph into the gut lumen.

Lymphedema Impaired lymphatic drainage in the limbs, genitalia, and face causes lymphedema.

Nonimmune hydrops fetalis Severe congenital lymphatic dysplasia can manifest as lymphedema sufficiently extensive to produce hydrops; the detailed fluid pathway was not measured in the cohort.

Pericardial effusion Impaired lymphatic fluid drainage can contribute to recurrent pericardial fluid accumulation.

Show evidence (2 references)

PMID:19935664 SUPPORT Human Clinical

"Lymphedema, lymphangiectasias, mental retardation and unusual facial characteristics define the autosomal recessive Hennekam syndrome."

Establishes the generalized lymphatic phenotype (lymphedema and lymphangiectasias) as defining features of the disorder.

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"All patients carryingCCBE1mutations had lymphedema of the limbs, unusual face and lymphangiectasia in the gut, representing the most distinctive features of the syndrome."

The CCBE1-positive cohort establishes widespread lymphatic involvement.

Intestinal lymphangiectasia

Dilated intestinal lymphatics leak protein-rich lymph into the gut, producing protein-losing enteropathy with hypoalbuminemia and hypogammaglobulinemia. Lymphocyte loss is a recognized complication of intestinal lymphangiectasia generally but is not established as total lymphopenia in the reviewed CCBE1 cases.

Downstream

Protein-losing enteropathy Enteric loss of lymph depletes serum albumin and immunoglobulins.

Show evidence (1 reference)

PMID:25925991 SUPPORT DIRECT Human Clinical

"She was diagnosed with a protein losing enteropathy secondary to segmental primary intestinal lymphangiectasia."

Direct CCBE1 evidence links intestinal lymphangiectasia to enteric protein loss.

Protein-losing enteropathy

Enteric loss of protein-rich lymph depletes serum albumin and immunoglobulins and can compromise nutrition and growth.

Downstream

Hypoalbuminemia Enteric protein loss lowers serum albumin.

Decreased circulating immunoglobulin concentration Enteric loss of lymph depletes serum immunoglobulins.

Growth delay Enteric protein loss and associated malabsorption can impair nutrition and growth; this is a possible route in affected children, not an explanation for every growth abnormality.

Failure to thrive Enteric protein loss and associated malabsorption can impair nutrition and growth; this is a possible route in affected children, not an explanation for every growth abnormality.

Show evidence (1 reference)

PMID:25925991 SUPPORT DIRECT Human Clinical

"She was diagnosed with a protein losing enteropathy secondary to segmental primary intestinal lymphangiectasia."

Direct observation of enteropathy secondary to lymphangiectasia in a homozygous CCBE1 child.

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Pathograph

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Phenotypes

26

Blood 1

Decreased circulating immunoglobulin concentration HP:0004313

Show evidence (2 references)

PMID:26686525 SUPPORT DIRECT Human Clinical

"We report two HS siblings, born to consanguineous parents of Turkish ancestry, whose clinical phenotype also includes protein losing enteropathy, painful relapsing chylous ascites, and hypogammaglobulinemia."

Hypogammaglobulinemia is directly reported in the homozygous C174Y siblings.

PMID:32629717 SUPPORT DIRECT Human Clinical

"Once admitted, ultrasonography showed a large amount of ascites, while blood laboratory investigations revealed severe hypoproteinemia, hypoalbuminemia and hypogammaglobulinemia."

The homozygous p.Arg158Cys infant also had hypogammaglobulinemia, with concomitant CMV infection.

Cardiovascular 2

Intestinal lymphangiectasia HP:0002593

Show evidence (2 references)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"All patients carryingCCBE1mutations had lymphedema of the limbs, unusual face and lymphangiectasia in the gut, representing the most distinctive features of the syndrome."

All 13 CCBE1-positive patients in this genotype-stratified cohort had intestinal lymphangiectasia.

PMID:25925991 SUPPORT DIRECT Human Clinical

"She was diagnosed with a protein losing enteropathy secondary to segmental primary intestinal lymphangiectasia."

The homozygous CCBE1 p.C98W child had segmental intestinal lymphangiectasia causing enteropathy.

Pericardial effusion HP:0001698

Show evidence (2 references)

PMID:30564329 SUPPORT INDIRECT Human Clinical

"An autopsy described a female fetus with a cystic hygroma, serous (nonchylous) pleural and pericardial effusions, congested liver, hypoplastic lungs, and a “markedly enlarged, hyperplastic thyroid gland”."

Observed at autopsy of the second affected pregnancy in the CCBE1 family. This fetus could not be genotyped; molecular confirmation was obtained only in the fourth pregnancy.

PMID:40394495 SUPPORT DIRECT Human Clinical

"This study presents a middle-aged male with long-term refractory PE without apparent cause and non-specific symptoms, and eventually diagnosed as Hennekam lymphangiectasia with lymphedema syndrome (HKLLS)."

The adult had recurrent pericardial effusion and two reported CCBE1 variants. The report assigns HKLLS1, but parental phasing was not demonstrated.

Digestive 5

Protein-losing enteropathy HP:0002243

Show evidence (2 references)

PMID:25925991 SUPPORT DIRECT Human Clinical

"She was diagnosed with a protein losing enteropathy secondary to segmental primary intestinal lymphangiectasia."

Documents enteropathy secondary to intestinal lymphangiectasia in the homozygous CCBE1 p.C98W child.

PMID:26686525 SUPPORT DIRECT Human Clinical

"We report two HS siblings, born to consanguineous parents of Turkish ancestry, whose clinical phenotype also includes protein losing enteropathy, painful relapsing chylous ascites, and hypogammaglobulinemia."

Two homozygous C174Y siblings provide additional direct CCBE1 evidence.

Ascites HP:0001541

Show evidence (1 reference)

PMID:30564329 SUPPORT DIRECT Human Clinical

"Ultrasound images of hydropic fetus from G4 pregnancy. A, Fetal chest demonstrating pleural effusions; B, fetal head demonstrating scalp edema; C, fetal abdomen demonstrating ascites"

Ultrasound of the fourth-pregnancy fetus, confirmed to carry both CCBE1 variants, documents this finding. The fluid was not characterized as chylous.

Chylous ascites HP:0012281

Show evidence (2 references)

PMID:26686525 SUPPORT DIRECT Human Clinical

"We report two HS siblings, born to consanguineous parents of Turkish ancestry, whose clinical phenotype also includes protein losing enteropathy, painful relapsing chylous ascites, and hypogammaglobulinemia."

The homozygous C174Y siblings had painful relapsing chylous ascites.

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"Recurrent chylous ascites was present in 3 molecularly confirmed patients, in none of the unconfirmed patients and in patient 37 (Noonan syndrome)."

The cohort separately identifies three CCBE1-positive cases with recurrent chylous ascites; the additional Noonan case is not counted as HKLLS1 evidence.

Inguinal hernia HP:0000023

Show evidence (1 reference)

PMID:26686525 SUPPORT DIRECT Human Clinical

"Both siblings since infancy have developed a distinctive clinical phenotype marked by limb lymphedema, relapsing chylous ascites (Fig. 2), inguinal hernias, enteric protein loss, hypoproteinemia, and hypogammaglobulinemia."

Direct observations in two molecularly confirmed siblings document inguinal hernias.

Umbilical hernia HP:0001537

Show evidence (1 reference)

PMID:26686525 SUPPORT DIRECT Human Clinical

"In addition, he had inguinal and umbilical hernia requiring surgical correction by 4 months of age."

This sentence describes the genotyped male sibling.

Endocrine 1

Hypothyroidism HP:0000821

Show evidence (1 reference)

PMID:26686525 SUPPORT DIRECT Human Clinical

"In adulthood, both P1 and P2 developed hypothyroidism treated with levothyroxine."

Both molecularly confirmed siblings developed hypothyroidism; the cause of thyroid dysfunction was not established.

Eye 1

Hypertelorism HP:0000316

Show evidence (1 reference)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"aPhenotype of patient 10 with aCCBE1mutation at age 2 years showing hair whorl in anterior hairline, hypertelorism, thick lateral eyebrows, epicanthi, depressed nasal bridge, and (pitting) lymphedema of distal limbs."

The figure caption identifies this finding in patient 10 with a CCBE1 mutation, rather than in an ungenotyped Hennekam case.

Head and Neck 6

Midface retrusion HP:0011800

Show evidence (1 reference)

PMID:32629717 SUPPORT DIRECT Human Clinical

"At first examination, she presented an abdominal circumference of 60 cm, edema of the lower extremities and vulva, and facial dysmorphisms (hypertelorism, flat nasal bridge, flat mid-face)."

Flat mid-face is directly observed in the molecularly confirmed CCBE1 infant, supporting the more specific midface term.

Depressed nasal bridge HP:0005280

Show evidence (1 reference)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"aPhenotype of patient 10 with aCCBE1mutation at age 2 years showing hair whorl in anterior hairline, hypertelorism, thick lateral eyebrows, epicanthi, depressed nasal bridge, and (pitting) lymphedema of distal limbs."

The figure caption identifies this finding in patient 10 with a CCBE1 mutation, rather than in an ungenotyped Hennekam case.

Cystic hygroma HP:0000476

Show evidence (1 reference)

PMID:30564329 SUPPORT INDIRECT Human Clinical

"An autopsy described a female fetus with a cystic hygroma, serous (nonchylous) pleural and pericardial effusions, congested liver, hypoplastic lungs, and a “markedly enlarged, hyperplastic thyroid gland”."

Observed at autopsy of the second affected pregnancy in the CCBE1 family. This fetus could not be genotyped; molecular confirmation was obtained only in the fourth pregnancy.

Epicanthus HP:0000286

Show evidence (1 reference)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"aPhenotype of patient 10 with aCCBE1mutation at age 2 years showing hair whorl in anterior hairline, hypertelorism, thick lateral eyebrows, epicanthi, depressed nasal bridge, and (pitting) lymphedema of distal limbs."

The figure caption identifies this finding in patient 10 with a CCBE1 mutation, rather than in an ungenotyped Hennekam case.

Microcephaly HP:0000252

Show evidence (1 reference)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"Growth parameters were disturbed in a similar way in those with and without aCCBE1mutation, but microcephaly was more common in those with aCCBE1mutation."

The cohort explicitly records microcephaly in mutation-positive patients; no population frequency is inferred.

Metopic synostosis HP:0011330

Show evidence (1 reference)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"Unusual clinical manifestations inCCBE1positive patients included metopic craniosynostosis, diaphragmatic hernia, growth hormone deficiency (each occurring in one case), and hypothyroidism (n = 3)."

This is an observation in the CCBE1-positive group. An isolated cohort finding does not establish penetrance or its causal mechanism.

Metabolism 4

Lymphedema HP:0001004

Temporal: CHRONIC

Edema is often described as progressive in Hennekam syndrome, but the course is not uniformly progressive in CCBE1 disease. Peripheral lymphedema regressed after dietary optimization in a molecularly confirmed adult, so no uniform PROGRESSIVE qualifier is assigned.

Show evidence (3 references)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"All patients carryingCCBE1mutations had lymphedema of the limbs, unusual face and lymphangiectasia in the gut, representing the most distinctive features of the syndrome."

The genotype-stratified cohort documents limb lymphedema in the CCBE1-positive group.

PMID:32629717 SUPPORT DIRECT Human Clinical

"At first examination, she presented an abdominal circumference of 60 cm, edema of the lower extremities and vulva, and facial dysmorphisms (hypertelorism, flat nasal bridge, flat mid-face)."

The homozygous CCBE1 p.Arg158Cys infant had lower-limb and vulvar edema.

PMID:26686525 SUPPORT DIRECT Human Clinical

"On this optimized diet, peripheral lymphedema regressed with normalization of serum protein (7.8 gm/dL) and immunoglobulin (1400 mg/dL) levels, without necessitating albumin or immunoglobulin replacement for 15 months."

The CCBE1 C174Y female had regression of peripheral edema, supporting the variable-course caveat.

Hypoalbuminemia HP:0003073

Show evidence (2 references)

PMID:32629717 SUPPORT DIRECT Human Clinical

"Once admitted, ultrasonography showed a large amount of ascites, while blood laboratory investigations revealed severe hypoproteinemia, hypoalbuminemia and hypogammaglobulinemia."

Documents hypoalbuminemia in the homozygous CCBE1 p.Arg158Cys infant, who also had CMV infection; the concurrent infection may have aggravated the intestinal disease.

PMID:18294365 SUPPORT INDIRECT Human Clinical

"dilated intestinal lacteals resulting in lymph leakage into the small bowel lumen and responsible for protein-losing enteropathy leading to lymphopenia, hypoalbuminemia and hypogammaglobulinemia"

Names hypoalbuminemia as a consequence of the protein-losing enteropathy caused by intestinal lymphangiectasia. Cited for the shared mechanism only: this review is about primary intestinal lymphangiectasia (Waldmann disease) and lists Hennekam syndrome as a separate entity, so it is not evidence about HKLLS1 patients and is secondary to the on-disease citation above.

Nonimmune hydrops fetalis HP:0001790

Show evidence (2 references)

PMID:30564329 SUPPORT DIRECT Human Clinical

"In conclusion, WES was able to identify compound heterozygous pathogenic variants in the CCBE1 gene as a cause of HKLLS1 and recurrent fetal hydrops."

The fourth affected pregnancy carried both CCBE1 variants; the maternal antibody screen was negative, supporting nonimmune hydrops.

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"The lymphedema was sufficiently marked at birth to diagnose hydrops foetalis in 3 of theCCBE1positive cases, in 1 of theCCBE1negative HS cases and in 1 of the cases thought to have another entity (patient 37)."

Three CCBE1-positive cohort members had hydrops, independently confirming the severe prenatal presentation.

Pleural effusion HP:0002202

Show evidence (1 reference)

PMID:30564329 SUPPORT DIRECT Human Clinical

"Ultrasound images of hydropic fetus from G4 pregnancy. A, Fetal chest demonstrating pleural effusions; B, fetal head demonstrating scalp edema; C, fetal abdomen demonstrating ascites"

Ultrasound of the fourth-pregnancy fetus, confirmed to carry both CCBE1 variants, documents this finding. The fluid was not characterized as chylous.

Musculoskeletal 1

Congenital diaphragmatic hernia HP:0000776

Show evidence (1 reference)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"Unusual clinical manifestations inCCBE1positive patients included metopic craniosynostosis, diaphragmatic hernia, growth hormone deficiency (each occurring in one case), and hypothyroidism (n = 3)."

This is an observation in the CCBE1-positive group. An isolated cohort finding does not establish penetrance or its causal mechanism.

Nervous System 2

Intellectual disability HP:0001249

Show evidence (2 references)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"We conclude thatCCBE1mutations cause a widespread and severe form of lymphatic dysplasia, associated with variable intellectual disabilities, unusual facial morphology and infrequently malformations."

The conclusion specifically attributes variable intellectual disability to the CCBE1-positive phenotype.

PMID:25925991 SUPPORT DIRECT Human Clinical

"The patient we describe here has a lymphatic dysplasia without intellectual disability or dysmorphism caused by mutation in CCBE1, highlighting the phenotypic variability that can be seen with abnormalities in this gene."

The p.C98W case establishes variability; this observation does not refute intellectual disability in other CCBE1 patients.

Delayed speech and language development HP:0000750

Show evidence (1 reference)

PMID:26686525 SUPPORT DIRECT Human Clinical

"Furthermore, both siblings had speech delays up until age 4 years with gain of full age-appropriate speech capabilities by age 7 years."

Direct sibling observations support speech delay without asserting global developmental delay.

Respiratory 1

Pulmonary hypoplasia HP:0002089

Show evidence (1 reference)

PMID:30564329 SUPPORT INDIRECT Human Clinical

"An autopsy described a female fetus with a cystic hygroma, serous (nonchylous) pleural and pericardial effusions, congested liver, hypoplastic lungs, and a “markedly enlarged, hyperplastic thyroid gland”."

Observed at autopsy of the second affected pregnancy in the CCBE1 family. This fetus could not be genotyped; molecular confirmation was obtained only in the fourth pregnancy.

Growth 2

Growth delay HP:0001510

Show evidence (1 reference)

PMID:26686525 SUPPORT DIRECT Human Clinical

"However, he continued to have growth delay until puberty, after which both height and weight normalized for age."

The homozygous CCBE1 C174Y male had persistent childhood growth delay with later catch-up.

Failure to thrive HP:0001508

Show evidence (1 reference)

PMID:26686525 SUPPORT DIRECT Human Clinical

"At 3 weeks of age, she developed failure to thrive, protein losing enteropathy, and massive generalized edema."

The homozygous C174Y female had failure to thrive from early infancy.

🧬

Genetic Associations

1

CCBE1 (Causal biallelic loss-of-function variant)

Gene: CCBE1 hgnc:29426

Show evidence (4 references)

PMID:19935664 SUPPORT Human Clinical

"Homozygous and compound heterozygous mutations in seven subjects paired with functional analysis in a zebrafish model identify CCBE1 as one of few genes causing primary generalized lymph-vessel dysplasia in humans."

Establishes CCBE1 as the causal gene for HKLLS1 with biallelic variants.

PMID:24913602 SUPPORT Human Clinical

"It can be caused by mutations in CCBE1 which are found in approximately 25 % of cases."

Quantifies the contribution of CCBE1 to the overall Hennekam syndrome cohort (~25% of cases).

PMID:30564329 SUPPORT Human Clinical

"at least 12 different CCBE1 variants have been reported in at least seventeen probands"

Summarizes the reported CCBE1 variant spectrum in HKLLS1.

+ 1 more reference

Variants (7)

p.Leu229fs (c.683_684insT)

Maternally inherited pathogenic loss-of-function frameshift CCBE1 variant; listed in gnomAD in 14 of 277,032 chromosomes, consistent with a rare recessive disease allele.

Show evidence (2 references)

PMID:30564329 SUPPORT Human Clinical

"A maternally inherited variant, p.Leu229fs (c.683_684insT), was detected and confirmed by Sanger sequencing"

A specific pathogenic loss-of-function frameshift CCBE1 allele.

PMID:30564329 SUPPORT Human Clinical

"listed in the Genome Aggregation Database (gnomAD) Browser in 14 out of 277 032 chromosomes"

Population allele frequency of the frameshift variant, consistent with a rare recessive disease allele.

p.Arg158Cys (c.472C>T)

Recurrent missense CCBE1 allele, reported homozygous in an infant with intestinal lymphangiectasia and ascites and separately as one half of a compound heterozygous genotype (with c.521G>A) in a second child.

Show evidence (2 references)

PMID:32629717 SUPPORT Human Clinical

"Genetic investigations identified the genomic variant c.472C>T of the CCBE1 gene, coding for a protein variant (p.Arg158Cys), in homozygosity."

Establishes the allele and its protein consequence in a homozygous HKLLS1 patient.

PMID:32472549 SUPPORT Human Clinical

"The patient was found to carry compound heterozygote variants (c.521G>A and c.472C>T) of the CCBE1 gene, which were respectively inherited from his parents."

Independent recurrence of c.472C>T, here in trans with c.521G>A, which is what distinguishes a recurrent disease allele from a private one.

p.Cys174Tyr (C174Y)

Recurrent homozygous missense CCBE1 allele, reported in two Turkish siblings of consanguineous parents and previously in a Dutch kindred. It is one of a cluster of alleles hitting cysteine residues in the EGF domain, whose disulfide linkages make them likely to disrupt the domain's tertiary structure. Functionally it is notable for a split result: in transfected cells the mutant protein shows increased intracellular expression and increased secretion, yet CCBE1 is scarce in the patients' own intestinal tissue by immunohistochemistry. The authors leave the reconciliation open between a secreted-but-misfolded protein and one that is rapidly degraded in vivo, so this allele is not loss-of-expression at the synthesis level without being a protein that is present where it is needed.

Show evidence (2 references)

PMID:26686525 SUPPORT Human Clinical

"We identified by whole exome and Sanger sequencing the homozygous CCBE1 C174Y mutation in both siblings. This mutation had been previously reported in another HS kindred from the Netherlands."

Establishes the allele, its homozygous state in affected siblings, and its independent recurrence in a second kindred.

PMID:25925991 SUPPORT Human Clinical

"All of these mutations affect cysteine residues within the EGF domain and given the disulfide linkages within EGF domains, it is reasonable to assume all of them will disrupt the tertiary structure of the domain to a certain degree."

Places C174Y in the EGF-domain cysteine cluster and gives the structural reason those alleles are damaging, which complements the domain-level result from the knock-in mouse series.

p.Cys98Trp (p.C98W)

Homozygous missense CCBE1 allele identified in a consanguineous family. Notable because the child had lymphatic dysplasia with protein-losing enteropathy but no intellectual disability and no dysmorphism, marking the mild end of the HKLLS1 spectrum.

Show evidence (2 references)

PMID:25925991 SUPPORT Human Clinical

"We identified several large stretches of homozygosity in the patient that allowed us to sort the variants found in the patient's exome to identify p.C98W in CCBE1 as the likely pathogenic variant."

Establishes the allele and how it was ascertained.

PMID:25925991 SUPPORT Human Clinical

"The patient we describe here has a lymphatic dysplasia without intellectual disability or dysmorphism caused by mutation in CCBE1, highlighting the phenotypic variability that can be seen with abnormalities in this gene."

Documents the attenuated phenotype attached to this allele, and is the source for the wide expressivity recorded in clinical_burden.

p.Asp104Asn (c.310G>A)

Paternally inherited missense CCBE1 allele in two affected brothers with primary lymphedema, in trans with p.Leu27Pro. It sits adjacent to other known pathogenic CCBE1 mutations, and zebrafish assay showed loss of function.

Show evidence (2 references)

PMID:27345729 SUPPORT Human Clinical

"The presumed paternally inherited NM_133459.3:c.310G>A; p.(Asp104Asn), lies adjacent to other known pathogenic CCBE1 mutations"

Establishes the allele, its inheritance, and its position relative to known pathogenic variants.

PMID:27345729 SUPPORT Model Organism

"Functional analysis in a zebrafish model of lymphatic disease showed that both mutations lead to CCBE1 loss of function, confirming the pathogenicity of these variants and expanding the genotypic spectrum of lymphatic disorders."

Functional confirmation that this allele is loss-of-function rather than an incidental rare variant.

p.Leu27Pro (c.80T>C)

Maternally inherited missense CCBE1 allele in the CCBE1 signal peptide, in trans with p.Asp104Asn. The signal peptide had not previously been implicated in disease, so this allele extends the mutable region of the protein beyond the collagen and EGF domains.

Show evidence (1 reference)

PMID:27345729 SUPPORT Human Clinical

"the maternally inherited NM_133459.3:c.80T>C; p.(Leu27Pro) lies in the CCBE1 signal peptide, which has not previously been associated with disease"

Establishes the allele and the novelty of its location within CCBE1.

p.Thr112Ile (c.335C>T)

Paternally inherited likely-pathogenic missense CCBE1 variant, in trans with the frameshift allele (compound heterozygosity).

Show evidence (1 reference)

PMID:30564329 SUPPORT Human Clinical

"A paternally inherited variant, p.Thr112Ile (c.335C>T) variant was detected and confirmed by Sanger sequencing"

A specific likely-pathogenic missense CCBE1 allele in trans with the frameshift variant.

💊

Medical Actions

6

Dietary management of protein-losing enteropathy

Action: dietary intervention Ontology label: Dietary Intervention NCIT:C15447

Platform: Behavioral / lifestyle

A low-fat, high-protein diet enriched in medium-chain triglycerides reduces intestinal lymphatic load and enteric protein loss from intestinal lymphangiectasia; supplemental nutrition supports growth.

Mechanism Target:

Protein-losing enteropathy — Reducing long-chain-triglyceride load lowers intestinal lymphatic pressure, so less protein-rich lymph is lost into the gut. This acts on the enteropathy itself rather than replacing what has been lost.

Show evidence (1 reference)

PMID:19449286 SUPPORT INDIRECT Human Clinical

"17 of 27 cases (63%) treated with MCT had complete resolution of symptoms while only 10 of 28 (35.7%) patients in group B showed complete resolution."

Quantifies the effect of the MCT diet on the enteropathy across 55 collected cases. The series is primary intestinal lymphangiectasia rather than HKLLS1, so this is the general efficacy of the intervention against the mechanism HKLLS1 produces.

Show evidence (4 references)

PMID:29560340 SUPPORT Human Clinical

"patients may require a high-protein, low-fat diet with medium-chain triglyceride supplementation in the case"

Directly supports a high-protein, low-fat, medium-chain-triglyceride diet for the protein-losing enteropathy of Hennekam syndrome.

PMID:32629717 SUPPORT Human Clinical

"serum albumin and IgG rose to normal values after 3 months (4.3 g/dL and 501 mg/dL, respectively)"

In a homozygous-CCBE1 infant managed with parenteral then enteral nutrition, correcting the enteric protein loss restored both albumin and IgG. This is the on-disease evidence that the nutritional arm addresses the cause; no immunoglobulin was given in that case.

PMID:19449286 SUPPORT INDIRECT Human Clinical

"Mortality for Group A was 1 out of 27 (3.7%), while mortality in group B was 5 of 28 (17.85%) patients."

Reports the mortality difference between MCT-treated and untreated intestinal lymphangiectasia, which is why this diet is the cornerstone rather than one option among several. The series is primary intestinal lymphangiectasia, not HKLLS1.

+ 1 more reference

Lymphedema supportive care

Action: physical therapy Ontology label: Physical Therapy NCIT:C15302

Platform: Behavioral / lifestyle

Complete decongestive therapy (compression, manual lymphatic drainage, exercise, skin care) manages the lymphedema; a Hennekam case showed measurable limb-circumference reduction with this approach.

Show evidence (2 references)

PMID:29560340 SUPPORT Human Clinical

"rehabilitative treatments including complete decongestive therapy for edema control appeared to be beneficial"

The case report concludes that complete decongestive therapy was beneficial for edema control in a patient with Hennekam syndrome.

PMID:29560340 SUPPORT Human Clinical

"upper arm, from 22 cm to 19 cm; forearm, from 22.5 cm to 21 cm, respectively."

Complete decongestive therapy produced a measurable reduction in upper-arm and forearm circumference in a Hennekam syndrome patient.

Supportive and multidisciplinary care

Action: supportive care Ontology label: Supportive Care NCIT:C15747

Management is symptomatic and multidisciplinary, including nutritional support, monitoring for hypoalbuminemia and immunodeficiency, and developmental and educational support according to the individual's findings.

Show evidence (1 reference)

PMID:26686525 SUPPORT INDIRECT Human Clinical

"Both siblings had a history of mild development delay requiring extra scholarly assistance to pass primary school, compared to their healthy unaffected siblings."

The CCBE1 siblings required educational support alongside treatment for enteropathy; this supports individualized multidisciplinary care rather than a subtype-specific protocol.

Genetic counseling

Action: Genetic Counseling NCIT:C15240

Counsel families about autosomal recessive inheritance, 25% sibling recurrence risk, carrier testing, and reproductive options.

Show evidence (1 reference)

PMID:19935664 SUPPORT Human Clinical

"Homozygous and compound heterozygous mutations in seven subjects paired with functional analysis in a zebrafish model identify CCBE1 as one of few genes causing primary generalized lymph-vessel dysplasia in humans."

Autosomal recessive biallelic inheritance is the basis for recurrence-risk counseling.

Immunoglobulin replacement therapy

Action: immunoglobulin replacement therapy Ontology label: Immunoglobulin Therapy NCIT:C62710

Agent: human immunoglobulin G NCIT:C80829

Platform: Protein replacement

The hypogammaglobulinemia of HKLLS1 is a secondary immunodeficiency driven by enteric loss of immunoglobulin, not a primary defect of B-cell development, so the dietary control of lymph loss is the treatment that addresses the cause. Immunoglobulin replacement is used for symptomatic hypogammaglobulinemia alongside it; intravenous administration does not always achieve satisfactory plasma levels, and subcutaneous administration has been reported to do better in intestinal lymphangiectasia. Two genotype-confirmed CCBE1 patients in one kindred document its use in HKLLS1 itself, one of whom no longer needed replacement once the medium-chain-triglyceride diet was optimized.

Mechanism Target:

Decreased circulating immunoglobulin concentration — Replaces the immunoglobulin lost into the gut rather than correcting the enteric loss itself.

Show evidence (1 reference)

PMID:26934740 SUPPORT INDIRECT Human Clinical

"We describe here the case of a patient with PIL and severe hypogammaglobulinemia treated with immunoglobulins."

Establishes that this treatment is directed at the hypogammaglobulinemia of intestinal lymphangiectasia, which is the node this link names. The case is primary intestinal lymphangiectasia rather than HKLLS1, so the link rests on the shared enteric-loss mechanism.

Show evidence (3 references)

PMID:26934740 SUPPORT INDIRECT Human Clinical

"The administration of intravenous immunoglobulins does not always lead to satisfactory plasma levels and therefore the replacement therapy with immunoglobulins is controversial."

Documents both the use of immunoglobulin replacement for the secondary hypogammaglobulinemia of intestinal lymphangiectasia and the caveat about intravenous dosing. The report is of a primary intestinal lymphangiectasia patient rather than a molecularly confirmed HKLLS1 patient, so the transfer rests on the shared enteric-loss mechanism.

PMID:26686525 SUPPORT DIRECT Human Clinical

"She was treated with albumin and immunoglobulin replacements (750 mg/kg every 2 weeks), aggressive diuresis (furosemide and aldactone), and modified diet (20 % protein, 50 % carbohydrate, 30% fat of which 50% are composed of medium-chain triglycerides) with significant improvement in edema,..."

Immunoglobulin replacement with a stated dose in a genotype-confirmed CCBE1 C174Y patient, with improvement in the hypogammaglobulinemia it targets. This is the on-disease support the link previously lacked; note it was given alongside a medium-chain-triglyceride diet, so the two arms were not separated in this patient.

PMID:26686525 SUPPORT DIRECT Human Clinical

"hypogammaglobulinemia (615 mg/dL) requiring intravenous immunoglobulin substitution (30 gm every 3 to 4 weeks)"

A second genotype-confirmed patient in the same kindred on ongoing intravenous immunoglobulin, which establishes the treatment as a continuing requirement rather than a one-off in this disorder.

Octreotide for refractory protein-losing enteropathy

Action: Pharmacotherapy NCIT:C15986

Agent: octreotide CHEBI:7726

Platform: Peptide

Second-line pharmacotherapy for intestinal lymphangiectasia when the medium-chain-triglyceride diet fails to control enteric protein loss. It is costly and requires parenteral administration. No HKLLS1-specific trial exists; the evidence is from intestinal lymphangiectasia generally.

Mechanism Target:

Protein-losing enteropathy — Used to reduce enteric protein loss when dietary management alone is insufficient.

Show evidence (1 reference)

PMID:14723832 SUPPORT INDIRECT Human Clinical

"Octreotide has been helpful in cases in which treatment with MCT has failed, but it is costly and requires parenteral administration."

Places octreotide's action on the enteropathy itself, after dietary management, which is the node and the position this link asserts.

Show evidence (1 reference)

PMID:14723832 SUPPORT INDIRECT Human Clinical

"Octreotide has been helpful in cases in which treatment with MCT has failed, but it is costly and requires parenteral administration."

Positions octreotide as second-line after MCT for intestinal lymphangiectasia, with its cost and route as the stated drawbacks. The source addresses primary intestinal lymphangiectasia rather than HKLLS1, so this is the general management of the enteropathy this disease causes.

🔬

Diagnosis

3

Molecular genetic testing of CCBE1

Sequencing and deletion/duplication analysis of CCBE1 confirms the diagnosis in individuals with syndromic generalized lymphatic dysplasia; a panel or exome approach also covers FAT4 and ADAMTS3.

genetic testing NCIT:C15709

Results: Biallelic pathogenic CCBE1 variants confirm HKLLS1.

Show evidence (1 reference)

PMID:19935664 SUPPORT Human Clinical

"Homozygosity mapping identified a critical chromosomal region containing CCBE1, the human ortholog of a gene essential for lymphangiogenesis in zebrafish."

Supports molecular confirmation by identification of causal CCBE1 variants.

Lymphoscintigraphy

Radionuclide lymphatic imaging demonstrates the site of lymph leakage and confirms intestinal lymphangiectasia non-invasively.

lymphoscintigraphy NCIT:C67262

Results: Abnormal tracer leakage into the abdomen indicates intestinal lymphangiectasia.

Show evidence (1 reference)

PMID:32629717 SUPPORT Human Clinical

"Lymphoscintigraphy with 99m-Tc-nanocolloid demonstrated abnormal leakage of the tracer in the abdomen as evidence of IL."

Documents lymphoscintigraphy establishing intestinal lymphangiectasia in a homozygous-CCBE1 patient.

Stool alpha-1-antitrypsin measurement

Faecal alpha-1-antitrypsin is the standard confirmation of enteric protein loss, since the protein is not actively secreted or absorbed by the gut and resists intestinal degradation. It is the test that establishes protein-losing enteropathy independently of where the lesion sits.

alpha-1 antitrypsin measurement NCIT:C80167

Results: Raised stool alpha-1-antitrypsin with low serum albumin confirms protein-losing enteropathy.

Endoscopic biopsy is not a reliable substitute here. In a molecularly confirmed HKLLS1 patient with raised stool alpha-1-antitrypsin, endoscopic biopsies were reported as normal, because the lymphangiectasia can be segmental and patchy and so missed by sampling.

Show evidence (1 reference)

PMID:40394495 SUPPORT Human Clinical

"During the disease course, laboratory tests showed decreased blood albumin and increased stool α1-antitrypsin, while endoscopy biopsies were normal."

In a molecularly confirmed HKLLS1 patient, stool alpha-1-antitrypsin was the abnormal test while endoscopic biopsy was not, which supports both the diagnostic value of this assay and the caveat recorded in notes.

📈

Progression

3

Prenatal presentation

Age: Fetal life

In severely affected pregnancies the lymphatic failure is already manifest in utero as non-immune hydrops fetalis, cystic hygroma, pleural and pericardial effusions and ascites, and may recur across pregnancies in a family. Prenatal onset is what brings some families to whole-exome sequencing before any postnatal phenotype is available.

Show evidence (1 reference)

PMID:30564329 SUPPORT Human Clinical

"Whole exome sequencing (WES) was used to determine the etiology of recurrent hydrops fetalis in this case of Hennekam lymphangiectasia-lymphedema syndrome-1."

Establishes recurrent prenatal hydrops as a presenting phase of HKLLS1.

Congenital and infantile lymphedema

Age: Birth through infancy

Edema may be present at birth and involve the limbs, genitalia and face. Intrauterine facial edema has been proposed to influence facial development, but facial dysmorphism is not universal in molecularly confirmed CCBE1 disease.

Show evidence (1 reference)

PMID:30564329 SUPPORT Human Clinical

"The edema is usually congenital and often progressive, generally including the limbs, genitalia, and face."

Dates the onset of lymphedema to birth and states its typical distribution.

Childhood and later course

Age: Childhood onward

Persistent enteric protein loss may cause hypoalbuminemia, hypogammaglobulinemia and nutritional growth impairment. Growth can improve after puberty, and speech delay can resolve. Seizures reported in historical mixed-genotype Hennekam series should not be assigned a frequency in HKLLS1: none occurred in the 13 CCBE1-positive patients of the genotype-stratified cohort. There are no validated disease stages.

Show evidence (2 references)

PMID:26686525 SUPPORT DIRECT Human Clinical

"However, he continued to have growth delay until puberty, after which both height and weight normalized for age."

Documents catch-up growth rather than inevitable progressive growth failure.

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"Seizures did not occur in the group of molecularly confirmed HS patients, but were found in 6 of the HS cases without molecular confirmation."

Distinguishes the CCBE1-positive cohort from clinically diagnosed Hennekam patients; absence in this cohort does not establish that seizures can never occur in HKLLS1.

📊

Prevalence

1

Worldwide

Cases In Literature Ultra Rare

Hennekam syndrome is very rare, with CCBE1 accounting for approximately 25% of all Hennekam syndrome cases. No population-based rate has been reported; the class here reflects the small number of published cases.

Show evidence (1 reference)

PMID:24913602 SUPPORT INDIRECT Human Clinical

"It can be caused by mutations in CCBE1 which are found in approximately 25 % of cases."

Quantifies the CCBE1 fraction of Hennekam syndrome cases (~25%); the quote does not itself state a population prevalence.

⚖️

Clinical Burden

High

HKLLS1 can combine disabling lymphedema with intestinal lymphangiectasia, protein-losing enteropathy and variable intellectual disability. Severe prenatal disease can cause fetal death. Among 13 CCBE1-positive cohort members, two needed nocturnal respiratory support and two others died in childhood or early adulthood from respiratory problems. These observations establish severe respiratory morbidity without a population mortality estimate or a specific pulmonary mechanism. Surviving patients may require recurrent paracentesis, nutritional therapy and immunoglobulin replacement, with missed school and work. Severity and treatment requirements vary; peripheral edema and protein loss can improve with dietary management.

Show evidence (3 references)

DOI:10.1159/000342486 SUPPORT DIRECT Human Clinical

"TwoCCBE1positive cases needed respiratory support at night, and 2 otherCCBE1positive cases died in childhood or early adulthood due to respiratory problems."

The genotype-stratified cohort directly documents severe respiratory outcomes in CCBE1-positive patients, without resolving the individual pulmonary causes.

PMID:26686525 SUPPORT DIRECT Human Clinical

"P2, like his sister, had his childhood and adulthood complicated by relapsing painful chylous ascites, frequent peritoneal drainage, and hospital admissions, with missed days from school and work."

The homozygous CCBE1 siblings document the long-term burden of ascites and its treatment.

PMID:30564329 SUPPORT DIRECT Human Clinical

"Following fetal demise, dilation and evacuation was performed."

This outcome concerns the fourth pregnancy, molecularly confirmed to carry both CCBE1 variants.

🔀

Differential Diagnoses

2

Conditions with similar clinical presentations that must be differentiated from Hennekam lymphangiectasia-lymphedema syndrome 1:

Hennekam lymphangiectasia-lymphedema syndrome 2 MONDO:0014454

Overlapping Features FAT4-related Hennekam syndrome is clinically overlapping and distinguished by molecular testing; it accounts for a subset of CCBE1 mutation-negative cases.

Show evidence (1 reference)

PMID:24913602 SUPPORT Human Clinical

"Subsequent targeted mutation analysis of FAT4 in a cohort of 24 CCBE1 mutation-negative Hennekam syndrome patients identified homozygous or compound heterozygous mutations in four additional families."

Establishes FAT4-related Hennekam syndrome as a distinct molecular entity found specifically among CCBE1 mutation-negative patients. The abstract does not compare the two Hennekam subtypes clinically, so this supports the genotype separation only, not a claim that they are clinically indistinguishable.

Hennekam lymphangiectasia-lymphedema syndrome 3 Not Yet Curated MONDO:0032564

Overlapping Features ADAMTS3-related Hennekam syndrome shares the VEGF-C activation mechanism with CCBE1 and produces distinctive facial features similar to CCBE1-mutant patients.

Show evidence (2 references)

PMID:28985353 SUPPORT Human Clinical

"These patients have distinctive facial features similar to those with mutations in CCBE1."

States the facial overlap that makes ADAMTS3-related Hennekam syndrome hard to separate from HKLLS1 on examination alone.

PMID:28985353 SUPPORT Other

"VEGFC processing is also affected by CCBE1 mutations that cause the Hennekam lymphangiectasia-lymphedema syndrome syndrome type1."

Explains the shared mechanism behind the phenotypic overlap: both genes act on the same pro-VEGF-C activation step, so the two disorders converge on one pathway and are separated by molecular testing.

🐁

Animal Models

3

Ccbe1-null mouse KNOCKOUT

The first mammalian model of CCBE1 deficiency. Ccbe1 mutant embryos specify PROX1-positive lymphatic endothelial cells within the cardinal vein but fail to bud and migrate from it, so no definitive lymphatic structures form, and the embryos die prenatally. Recombinant CCBE1 has little lymphangiogenic activity alone yet strongly potentiates VEGF-C in a corneal micropocket assay, which is what established CCBE1 as necessary but not sufficient for lymphangiogenesis.

Species

Mus musculus

Genotype

Ccbe1 constitutive null (Ccbe1 mutant embryos)

Genes

CCBE1 hgnc:29426

Publication

PMID:21778431

Pathograph links

Failed lymphangiogenesis recapitulates fidelity: high Loss of Ccbe1 in the mouse blocks budding and migration of Prox1-positive lymphatic endothelial cells from the cardinal vein, which is exactly the cellular step this node names.

Limitations: The model speaks to the cellular step, not to the upstream activation node: VEGFR-3 activation appeared unaltered in the mutants by proximity ligation assay, so this animal does not itself demonstrate reduced receptor signalling and is not linked to the proteolytic-activation node. Being a constitutive null, it also models complete loss rather than the hypomorphic alleles many patients carry.

Readouts

Definitive lymphatic structures in the embryo

Phenotypic analysis of Ccbe1 mutant embryos for formation of lymphatic vessels, scored against Prox1-positive progenitor specification.

Direction: ABOLISHED

Interpretation: Lymphatic progenitors are specified but no lymphatic vasculature forms, placing the block at the budding/migration step rather than at lymphatic fate specification.

Show evidence (1 reference)

PMID:21778431 SUPPORT Model Organism

"Phenotypic analysis of murine Ccbe1 mutant embryos showed a complete lack of definitive lymphatic structures, even though Prox1(+) lymphatic endothelial cells get specified within the cardinal vein."

Reports the measurement behind this readout and its interpretation.

Show evidence (2 references)

PMID:21778431 SUPPORT Model Organism

"our data suggest CCBE1 to be essential but not sufficient for lymphangiogenesis"

The authors' own summary of what the model establishes - CCBE1 is required for lymphangiogenesis but acts as a potentiator rather than an independent driver - which is the cofactor role this node describes and is what makes the model informative for it.

+ 1 more reference

Generalized lymphatic dysplasia partially recapitulates fidelity: moderate Complete absence of definitive lymphatics models the severe end of the human generalized lymphatic dysplasia, and is consistent with the recurrent hydrops fetalis seen in some HKLLS1 pregnancies.

Limitations: Ccbe1-null mice die prenatally, so the model cannot express the postnatal arms of the human disease - chronic limb, genital and facial lymphedema, intestinal lymphangiectasia with protein-losing enteropathy, or the craniofacial and cognitive features. Human patients carry hypomorphic as well as null alleles and survive, so the null mouse is more severe than the usual human phenotype.

Readouts

Definitive lymphatic structures at the whole-embryo level

Whole-embryo phenotypic analysis of Ccbe1 mutants for the presence of any definitive lymphatic vasculature.

Direction: ABOLISHED

Interpretation: Total absence of lymphatic structures throughout the embryo is the animal counterpart of generalized - rather than segmental - lymphatic dysplasia, and models the severe end of the human phenotype.

Show evidence (1 reference)

PMID:21778431 SUPPORT Model Organism

"Phenotypic analysis of murine Ccbe1 mutant embryos showed a complete lack of definitive lymphatic structures, even though Prox1(+) lymphatic endothelial cells get specified within the cardinal vein. Mutant mice die prenatally."

Reports the whole-embryo absence of lymphatic structures behind this readout; the accompanying lethality is recorded under limitations.

Show evidence (1 reference)

PMID:21778431 SUPPORT Model Organism

"CCBE1 as a factor critically required for budding and migration of Prox-1(+) lymphatic endothelial cells from the cardinal vein"

Supports treating the null mouse as informative for the generalized lymphatic dysplasia node, while its prenatal lethality is recorded as a limitation.

Show evidence (1 reference)

PMID:21778431 SUPPORT Model Organism

"Collagen- and calcium-binding EGF domains 1 (CCBE1) has been associated with Hennekam syndrome, in which patients have lymphedema, lymphangiectasias, and other cardiovascular anomalies."

States the gene-disease relationship this mouse was made to model, which is what makes a Ccbe1-null animal relevant to HKLLS1.

ccbe1 mutant zebrafish LOSS_OF_FUNCTION_MUTANT

The genetic screen that identified ccbe1 as indispensable for embryonic lymphangiogenesis. Mutants fail lymphangioblast budding and angiogenic sprouting from venous endothelium, and ccbe1 acts at the same developmental stage as vegfc.

Species

Danio rerio

Genotype

ccbe1 loss-of-function mutant recovered in a forward genetic screen

Genes

CCBE1 hgnc:29426

Publication

PMID:19287381

Pathograph links

Failed lymphangiogenesis recapitulates fidelity: high ccbe1 mutants fail lymphangioblast budding and venous sprouting, and ccbe1 acts at the same developmental stage as vegfc, which is the cellular lymphangiogenic step this node names.

Limitations: Zebrafish do not develop the craniofacial, cognitive, intestinal or chronic lymphedema features of the human syndrome, so the model speaks to the developmental lymphangiogenic step and not to the downstream clinical phenotype.

Readouts

Lymphangioblast budding and venous sprouting

Imaging of lymphangioblast budding and angiogenic sprouting from venous endothelium in ccbe1 mutant embryos.

Direction: DECREASED

Interpretation: Places the ccbe1 requirement at the same step as vegfc, which is the cofactor relationship disrupted in HKLLS1.

Show evidence (1 reference)

PMID:19287381 SUPPORT Model Organism

"Ccbe1 acts at the same stage of development as Vegfc and is required for lymphangioblast budding and angiogenic sprouting from venous endothelium."

Reports the developmental measurements behind this readout.

Show evidence (1 reference)

PMID:24523457 SUPPORT Model Organism

"Vegfc- and Vegfr3-dependent Erk signaling is impaired in the absence of Ccbe1."

Confirms in the same species that the lymphangiogenic failure runs through VEGF-C/VEGFR-3 signalling, which is what makes this model informative for the node.

Show evidence (1 reference)

PMID:19287381 SUPPORT Model Organism

"Using a genetic screen in zebrafish we identify ccbe1 (collagen and calcium-binding EGF domain-1) as indispensible for embryonic lymphangiogenesis."

Establishes the zebrafish ccbe1 mutant as the model that defined CCBE1 as required for lymphangiogenesis, the mechanism lost in HKLLS1.

CCBE1 domain-deletion knock-in mice KNOCK_IN

Knock-in mice expressing CCBE1 lacking either the collagen repeats or the EGF domains, read out alongside zebrafish rescue and in vitro VEGF-C processing. Collagen-domain deletion fully phenocopies the null, whereas EGF-domain deletion still permits rudimentary lymphatics. The same asymmetry holds for two Hennekam syndrome patient mutations, one in each domain, which is what makes this an allele-specific model of the human disorder rather than a generic knockout.

Species

Mus musculus

Genotype

CCBE1 delta-Collagen and CCBE1 delta-EGF knock-in alleles

Genes

CCBE1 hgnc:29426

Publication

PMID:25814692

Pathograph links

CCBE1 loss of function recapitulates fidelity: high Maps loss of CCBE1 function onto specific protein domains, and shows that a Hennekam syndrome mutation in the collagen repeats is more damaging than one in an EGF domain - a structural account of why HKLLS1 alleles differ in severity.

Limitations: The severity ordering was established for two patient mutations, so it supports a domain-level rule rather than a validated genotype-phenotype predictor for individual HKLLS1 alleles. The knock-ins delete whole domains, which is a more drastic lesion than most patient missense variants.

Readouts

Lymphatic vessel formation in domain-deletion embryos

Comparison of lymphatic development in CCBE1 delta-Collagen versus CCBE1 delta-EGF knock-in embryos, each against CCBE1 knockout mice.

Direction: DECREASED

Interpretation: Collagen-domain loss is equivalent to whole-protein loss while EGF-domain loss is partial, so the collagen repeats carry most of the VEGF-C-activating function whose loss causes HKLLS1.

Show evidence (1 reference)

PMID:25814692 SUPPORT Model Organism

"although CCBE1ΔCollagen mice fully phenocopy CCBE1 knock-out mice, CCBE1ΔEGF knock-in embryos still form rudimentary lymphatics"

Reports the graded lymphatic phenotype that grounds this readout.

Show evidence (1 reference)

PMID:25814692 SUPPORT Model Organism

"Furthermore, a Hennekam syndrome mutation within the collagen domain has a stronger effect than a Hennekam syndrome mutation within the EGF domain."

Ties the domain-deletion series directly to Hennekam syndrome patient alleles, which is what makes this model informative for the human loss-of-function node.

Impaired pro-VEGF-C proteolytic activation measures fidelity: moderate The same allelic series was read out directly on VEGF-C processing, so it assays the proteolytic activation step itself rather than only its downstream consequence. Collagen-domain deletion abolishes processing while EGF-domain deletion does not, which assigns the cleavage-supporting function of CCBE1 to the collagen repeats.

Limitations: The processing readout is an in vitro assay on deletion constructs, not a measurement of endogenous VEGF-C maturation in a patient or in the knock-in animal. It establishes which domain carries the activity rather than quantifying how much activation a given patient allele retains.

Readouts

VEGF-C proteolytic processing in vitro

Ability of each CCBE1 deletion construct to induce cleavage of pro-VEGF-C, compared against full-length CCBE1.

Direction: ABOLISHED

Interpretation: Loss of the collagen repeats abolishes VEGF-C processing while loss of the EGF domains does not, placing the activating function of CCBE1 at this node in the collagen repeats.

Show evidence (1 reference)

PMID:25814692 SUPPORT In Vitro

"Third, CCBE1ΔEGF, similarly to CCBE1, but not CCBE1ΔCollagen could activate VEGFC processing in vitro."

Reports the processing measurement behind this readout and its direction for each construct.

Show evidence (1 reference)

PMID:25814692 SUPPORT Model Organism

"We analyzed the functionality of several CCBE1 deletion mutants by generating knock-in mice expressing these mutants, by analyzing their ability to enhance Vegfc signaling in vivo in zebrafish, and by testing their ability to induce VEGFC processing in vitro."

Names VEGF-C processing as one of the three assays this model series was read out on, which is what makes it informative for the activation node specifically.

Show evidence (1 reference)

PMID:25814692 SUPPORT Model Organism

"We analyzed the functionality of several CCBE1 deletion mutants by generating knock-in mice expressing these mutants, by analyzing their ability to enhance Vegfc signaling in vivo in zebrafish, and by testing their ability to induce VEGFC processing in vitro."

Describes the knock-in allelic series and the assays used to read it out.

{ }

Source YAML

click to show

name: Hennekam lymphangiectasia-lymphedema syndrome 1
creation_date: "2026-08-02T00:00:00Z"
description: >-
  Hennekam lymphangiectasia-lymphedema syndrome 1 (HKLLS1) is an autosomal
  recessive generalized lymphatic dysplasia caused by biallelic mutations in
  CCBE1. It is characterized by the triad of intestinal lymphangiectasia,
  peripheral and facial lymphedema, and variable intellectual disability.
  Facial findings can include midface retrusion, depressed nasal bridge,
  hypertelorism, and epicanthal folds, but dysmorphism is not obligatory.
  Intestinal lymphangiectasia can cause protein-losing enteropathy with
  hypoalbuminemia and hypogammaglobulinemia. CCBE1 is a secreted
  extracellular-matrix protein essential for lymphangiogenesis, acting as a
  cofactor that promotes ADAMTS3-mediated proteolytic activation of pro-VEGF-C;
  loss of CCBE1 function impairs VEGF-C/VEGFR-3 signaling and lymphatic vessel
  development. CCBE1 mutations account for approximately 25% of Hennekam syndrome
  cases and were the first genetic cause identified for the disorder.
category: Mendelian
parents:
- Hennekam syndrome
- hereditary disease
synonyms:
- CCBE1 Hennekam syndrome
- HKLLS1
- generalized lymphatic dysplasia, CCBE1-related
disease_term:
  preferred_term: Hennekam lymphangiectasia-lymphedema syndrome 1
  term:
    id: MONDO:0009337
    label: Hennekam lymphangiectasia-lymphedema syndrome 1
classifications:
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    notes: >-
      HKLLS1 is a monogenic autosomal recessive disorder of a developmental
      signalling pathway (CCBE1-ADAMTS3-VEGF-C/VEGFR-3), so it sits in
      Harrison's genetics-and-disease part alongside other Mendelian
      developmental syndromes.
  - classification_value: CARDIOVASCULAR
    notes: >-
      The primary lesion is in the lymphatic vasculature, which Harrison's
      covers within the vascular-disease part (chronic venous disease and
      lymphedema). Recorded alongside the genetic assignment rather than
      instead of it, because the mechanism is developmental but the affected
      organ system is vascular.
  iuis_category:
    classification_value: combined immunodeficiency with syndromic features
    notes: >-
      IUIS 2022 Table 2 (combined immunodeficiencies with associated or
      syndromic features), sub-section 9, Other Defects; listed as
      "Hennekam-lymphangiectasia-lymphedema syndrome" with CCBE1 (autosomal
      recessive) as the genetic defect. The row records low/variable T and B
      cells with decreased immunoglobulins alongside the lymphangiectasia,
      lymphedema, and dysmorphic features.
    evidence:
    - reference: PMID:35748970
      reference_title: "Human Inborn Errors of Immunity: 2022 Update on the Classification from the International Union of Immunological Societies Expert Committee."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        lymphedema syndrome CCBE1 AR 612753 Low/variable Low/variable
      explanation: >-
        The IUIS Table 2 Other Defects row assigning
        Hennekam-lymphangiectasia-lymphedema syndrome (CCBE1, autosomal
        recessive) to the combined immunodeficiencies with syndromic features;
        the quoted fragment starts mid-name because the PDF extraction in the
        cache runs the row header together with the preceding row.
inheritance:
- name: Autosomal recessive inheritance
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    HKLLS1 is inherited in an autosomal recessive manner; affected individuals
    carry biallelic (homozygous or compound heterozygous) CCBE1 mutations,
    frequently in the setting of parental consanguinity.
  evidence:
  - reference: PMID:19935664
    reference_title: Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Homozygous and compound heterozygous mutations in seven subjects paired
      with functional analysis in a zebrafish model identify CCBE1 as one of few
      genes causing primary generalized lymph-vessel dysplasia in humans.
    explanation: >-
      Homozygous and compound heterozygous CCBE1 genotypes establish autosomal
      recessive inheritance for HKLLS1.
pathophysiology:
- name: CCBE1 loss of function
  biological_scale: MOLECULAR
  description: >-
    Biallelic loss-of-function mutations in CCBE1 reduce the activity of a
    secreted extracellular-matrix protein required for lymphangiogenesis.
  genes:
  - preferred_term: CCBE1
    term:
      id: hgnc:29426
      label: CCBE1
  evidence:
  - reference: PMID:19935664
    reference_title: Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Homozygosity mapping identified a critical chromosomal region containing
      CCBE1, the human ortholog of a gene essential for lymphangiogenesis in
      zebrafish.
    explanation: >-
      Identifies CCBE1 as the causal gene and situates it as an essential
      lymphangiogenesis factor.
  - reference: PMID:25814692
    reference_title: "Functional Dissection of the CCBE1 Protein: A Crucial Requirement for the Collagen Repeat Domain."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      deleting the collagen domains of CCBE1 has a much stronger effect on
      CCBE1 activity than deleting the EGF domains
    explanation: >-
      Domain-dissection knock-in mouse and zebrafish data show the collagen
      repeats of CCBE1 are crucial for its VEGF-C-activating, lymphangiogenic
      function that is lost in HKLLS1.
  - reference: PMID:26686525
    reference_title: "A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation that does not Prevent Protein Expression."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Our findings show that the mutant CCBE1 C174Y protein is not
      loss-of-function by loss-of-expression.
    explanation: >-
      Qualifies what "loss of function" means at this node for one recurrent
      patient allele: C174Y is not loss-of-function by failure to make the
      protein, since transfected cells show increased expression and secretion.
      That is an in vitro result and does not describe patient tissue, where
      the same paper finds CCBE1 scarce in affected intestinal lymphatics. The
      paper offers two unresolved explanations, quoted in the items below, so
      the node should be read as loss of CCBE1 activity without a settled
      account of whether the protein is absent, misfolded, or degraded in vivo.
  - reference: PMID:26686525
    reference_title: "A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation that does not Prevent Protein Expression."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The CCBE1 C174Y mutation confers increased intracellular protein
      expression and secretion in vitro concurrent with a paucity of CCBE1
      protein in vivo by immunohistochemistry
    explanation: >-
      The authors' own reconciliation of the two halves. Curated so the in vitro
      expression result above is not read as describing patients, in whom the
      protein is scarce in the affected tissue.
  - reference: PMID:26686525
    reference_title: "A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation that does not Prevent Protein Expression."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    snippet: >-
      Alternatively, the mutant protein may be rapidly degraded in physiological
      conditions in vivo
    explanation: >-
      Records that the mechanism is offered as a hypothesis rather than a
      result. Graded OTHER rather than IN_VITRO because the sentence is a
      discussion-section proposal about what happens in vivo and reports no
      experiment of its own. Quoted so this node does not assert a settled
      account of why
      CCBE1 is scarce in patient tissue when the source explicitly does not
      have one.
  downstream:
  - target: Impaired pro-VEGF-C proteolytic activation
    description: >-
      CCBE1 is the cofactor that lets ADAMTS3 cleave pro-VEGF-C into its mature
      active form; without it, that cleavage step fails.
    evidence:
    - reference: PMID:24552833
      reference_title: CCBE1 enhances lymphangiogenesis via A disintegrin and metalloprotease with thrombospondin motifs-3-mediated vascular endothelial growth factor-C activation.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        it promotes proteolytic cleavage of the otherwise poorly active
        29/31-kDa form of VEGF-C by the A disintegrin and metalloprotease with
        thrombospondin motifs-3 protease, resulting in the mature 21/23-kDa form
        of VEGF-C, which induces increased VEGF-C receptor signaling.
      explanation: >-
        Defines the molecular mechanism by which CCBE1 enables VEGF-C activation;
        loss of CCBE1 removes this cofactor activity.
- name: Impaired pro-VEGF-C proteolytic activation
  biological_scale: MOLECULAR
  description: >-
    CCBE1 and the protease ADAMTS3 form a complex that cleaves the poorly active
    29/31-kDa form of pro-VEGF-C into the mature 21/23-kDa ligand. Loss of CCBE1
    removes the cofactor, so this proteolytic maturation step fails and little
    active VEGF-C is produced. CCBE1's two domains contribute differently: the
    C-terminal collagen-like domain drives the cleavage activity, while the
    N-terminal EGF-like domain colocalizes pro-VEGF-C with its protease at the
    lymphatic endothelial cell surface.
  notes: >-
    This node is about the proteolytic maturation of pro-VEGF-C, not about how
    much VEGF-C is present. Serum VEGF-C was no different from healthy controls
    in two genotype-confirmed CCBE1 patients (PMID:26686525), so a normal total
    VEGF-C level does not argue against this node and should not be read as
    evidence for or against it.
  molecular_functions:
  - preferred_term: metalloendopeptidase activity of the ADAMTS3-CCBE1 complex
    modifier: DECREASED
    term:
      id: GO:0004222
      label: metalloendopeptidase activity
  evidence:
  - reference: PMID:24552833
    reference_title: CCBE1 enhances lymphangiogenesis via A disintegrin and metalloprotease with thrombospondin motifs-3-mediated vascular endothelial growth factor-C activation.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      it promotes proteolytic cleavage of the otherwise poorly active
      29/31-kDa form of VEGF-C by the A disintegrin and metalloprotease with
      thrombospondin motifs-3 protease, resulting in the mature 21/23-kDa form
      of VEGF-C, which induces increased VEGF-C receptor signaling.
    explanation: >-
      Defines the cleavage this node names and the cofactor role CCBE1 plays in
      it.
  - reference: PMID:27159393
    reference_title: "Proteolytic activation defines distinct lymphangiogenic mechanisms for VEGFC and VEGFD."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      an ADAMTS3-CCBE1 complex can form independently of VEGFR3 and is required
      to convert VEGFC, but not VEGFD, into an active ligand
    explanation: >-
      Establishes that CCBE1 and ADAMTS3 act as a complex and that the complex
      is required for VEGF-C activation specifically, which is why loss of
      CCBE1 is not compensated by VEGF-D.
  - reference: PMID:28687807
    reference_title: "Efficient activation of the lymphangiogenic growth factor VEGF-C requires the C-terminal domain of VEGF-C and the N-terminal domain of CCBE1."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      The N-terminal EGF-like domain of CCBE1 increased VEGFR-3 signaling by
      colocalizing pro-VEGF-C with its activating protease to the lymphatic
      endothelial cell surface.
    explanation: >-
      Resolves the two separable contributions of CCBE1 to this step, which is
      what makes domain-specific patient alleles differ in severity.
  downstream:
  - target: Failed lymphangiogenesis
    description: >-
      Without mature VEGF-C, VEGFR-3 signaling in lymphatic endothelial cells is
      not driven and lymphatic vessels fail to form.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:24523457
      reference_title: Ccbe1 regulates Vegfc-mediated induction of Vegfr3 signaling during embryonic lymphangiogenesis.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Vegfc- and Vegfr3-dependent Erk signaling is impaired in the absence of
        Ccbe1.
      explanation: >-
        Shows the signalling consequence of the failed activation step in a
        Ccbe1 loss-of-function animal, which is the step this edge asserts.
- name: Failed lymphangiogenesis
  biological_scale: CELLULAR
  description: >-
    Without VEGF-C/VEGFR-3 drive, lymphatic endothelial cells are still
    specified but fail to bud, migrate and assemble into lymphatic vessels.
  cell_types:
  - preferred_term: lymphatic endothelial cell
    term:
      id: CL:0002138
      label: endothelial cell of lymphatic vessel
  biological_processes:
  - preferred_term: lymphangiogenesis
    modifier: DECREASED
    term:
      id: GO:0001946
      label: lymphangiogenesis
  evidence:
  - reference: PMID:19287381
    reference_title: Ccbe1 is required for embryonic lymphangiogenesis and venous sprouting.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Ccbe1 acts at the same stage of development as Vegfc and is required
      for lymphangioblast budding and angiogenic sprouting from venous
      endothelium.
    explanation: >-
      Identifies the cellular step that fails - lymphangioblast budding and
      venous sprouting - and places it at the same developmental stage as VEGF-C.
  - reference: PMID:26686525
    reference_title: "A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation that does not Prevent Protein Expression."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we found a strong decrease of lymphatic vessels, with a corresponding
      diminished expression of CCBE1, by immunohistochemistry of the patients'
      intestinal biopsies
    explanation: >-
      Human intestinal tissue from CCBE1-mutant patients shows the lymphatic
      vessel loss this node asserts. The rest of the support for this node is
      from zebrafish and mouse, so this is the observation that anchors it in
      human disease rather than in model systems.
  downstream:
  - target: Generalized lymphatic dysplasia
    description: >-
      Defective lymphangiogenesis produces malformed, dysfunctional lymphatic
      vessels throughout the body.
    evidence:
    - reference: PMID:24523457
      reference_title: Ccbe1 regulates Vegfc-mediated induction of Vegfr3 signaling during embryonic lymphangiogenesis.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        The role of CCBE1 orthologs is highly conserved in zebrafish, mice and
        humans with mutations in CCBE1 causing generalized lymphatic dysplasia
        and lymphedema (Hennekam syndrome).
      explanation: >-
        Links impaired CCBE1-dependent lymphangiogenesis to the generalized
        lymphatic dysplasia phenotype of Hennekam syndrome.
- name: Generalized lymphatic dysplasia
  biological_scale: TISSUE
  description: Widespread lymphatic vessel dysplasia impairs lymph drainage and underlies the intestinal
    lymphangiectasia, lymphedema, and serous effusions of the syndrome.
  cell_types:
  - preferred_term: lymphatic endothelial cell
    term:
      id: CL:0002138
      label: endothelial cell of lymphatic vessel
  evidence:
  - reference: PMID:19935664
    reference_title: Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Lymphedema, lymphangiectasias, mental retardation and unusual facial characteristics define
      the autosomal recessive Hennekam syndrome.
    explanation: Establishes the generalized lymphatic phenotype (lymphedema and lymphangiectasias) as
      defining features of the disorder.
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: All patients carryingCCBE1mutations had lymphedema of the limbs, unusual face and lymphangiectasia
      in the gut, representing the most distinctive features of the syndrome.
    explanation: The CCBE1-positive cohort establishes widespread lymphatic involvement.
  downstream:
  - target: Intestinal lymphangiectasia
    description: Dysplastic intestinal lymphatics dilate and leak lymph into the gut lumen.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:30564329
      reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam
        lymphangiectasia-lymphedema syndrome-1.
      supports: SUPPORT
      directness: DIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: Maldevelopment of the lymphatic system preferentially affects the intestines and limbs,
        but can also affect other systems, such as the pleura, pericardium, and kidneys.
      explanation: States that the generalized lymphatic maldevelopment of HKLLS1 acts preferentially
        on the intestine, which is the step this edge asserts.
  - target: Lymphedema
    description: Impaired lymphatic drainage in the limbs, genitalia, and face causes lymphedema.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:30564329
      reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam
        lymphangiectasia-lymphedema syndrome-1.
      supports: SUPPORT
      directness: DIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: The edema is usually congenital and often progressive, generally including the limbs, genitalia,
        and face.
      explanation: Attributes the congenital limb, genital and facial edema of HKLLS1 to the underlying
        lymphatic maldevelopment, which is the step this edge asserts.
  - target: Nonimmune hydrops fetalis
    description: Severe congenital lymphatic dysplasia can manifest as lymphedema sufficiently
      extensive to produce hydrops; the detailed fluid pathway was not measured in the cohort.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: DOI:10.1159/000342486
      reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
        without CCBE1 Mutations
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: The lymphedema was sufficiently marked at birth to diagnose hydrops foetalis in 3 of theCCBE1positive
        cases, in 1 of theCCBE1negative HS cases and in 1 of the cases thought to have another entity (patient
        37).
      explanation: The authors identify hydrops as the severe congenital lymphedema presentation
        in three CCBE1-positive patients. This supports the summary link through extensive edema,
        without establishing the route of fluid accumulation in individual body cavities.
  - target: Pericardial effusion
    description: Impaired lymphatic fluid drainage can contribute to recurrent pericardial fluid accumulation.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:40394495
      reference_title: 'Recurrent pericardial effusion due to Hennekam lymphangiectasia-lymphedema syndrome:
        a case report and literature review.'
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: The involvement of the lymphatic system in HKLLS can disrupt the normal fluid balance and
        drainage, potentially resulting in the accumulation of fluid in the pericardial cavity, presenting
        as recurrent PE.
      explanation: The case review proposes impaired lymph drainage as the mechanism of pericardial effusion;
        fluid balance and hypoalbuminemia may both contribute.
- name: Intestinal lymphangiectasia
  biological_scale: TISSUE
  description: Dilated intestinal lymphatics leak protein-rich lymph into the gut, producing protein-losing
    enteropathy with hypoalbuminemia and hypogammaglobulinemia. Lymphocyte loss is a recognized complication
    of intestinal lymphangiectasia generally but is not established as total lymphopenia in the reviewed
    CCBE1 cases.
  evidence:
  - reference: PMID:25925991
    reference_title: 'A novel CCBE1 mutation leading to a mild form of hennekam syndrome: case report
      and review of the literature.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: She was diagnosed with a protein losing enteropathy secondary to segmental primary intestinal
      lymphangiectasia.
    explanation: Direct CCBE1 evidence links intestinal lymphangiectasia to enteric protein loss.
  downstream:
  - target: Protein-losing enteropathy
    description: Enteric loss of lymph depletes serum albumin and immunoglobulins.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:18294365
      reference_title: Primary intestinal lymphangiectasia (Waldmann's disease).
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: dilated intestinal lacteals resulting in lymph leakage into the small bowel lumen and responsible
        for protein-losing enteropathy leading to lymphopenia, hypoalbuminemia and hypogammaglobulinemia
      explanation: 'Establishes the general mechanism of this edge: dilated intestinal lacteals leak lymph
        into the bowel lumen and produce protein-losing enteropathy. The review describes primary intestinal
        lymphangiectasia (Waldmann disease); the lymphangiectasia-to-enteropathy step is the same one
        operating in HKLLS1.'
    - reference: PMID:18294365
      reference_title: Primary intestinal lymphangiectasia (Waldmann's disease).
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: their risk of pyogenic bacterial infection is not significantly elevated
      explanation: >-
        Bounds what the immunologic depletion on this edge means clinically:
        despite lymphocyte and immunoglobulin loss, pyogenic infection risk is
        not significantly raised, so this is a quantitative loss of circulating
        cells and protein rather than a functional immunodeficiency comparable
        to a primary T-cell defect. Recorded because an entry listing
        lymphopenia and hypogammaglobulinemia without it invites the opposite
        reading. Same shared-mechanism caveat as the item above.
- name: Protein-losing enteropathy
  biological_scale: ORGANISM
  description: Enteric loss of protein-rich lymph depletes serum albumin and immunoglobulins and can compromise
    nutrition and growth.
  evidence:
  - reference: PMID:25925991
    reference_title: 'A novel CCBE1 mutation leading to a mild form of hennekam syndrome: case report
      and review of the literature.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: She was diagnosed with a protein losing enteropathy secondary to segmental primary intestinal
      lymphangiectasia.
    explanation: Direct observation of enteropathy secondary to lymphangiectasia in a homozygous CCBE1
      child.
  downstream:
  - target: Hypoalbuminemia
    description: Enteric protein loss lowers serum albumin.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:18294365
      reference_title: Primary intestinal lymphangiectasia (Waldmann's disease).
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: dilated intestinal lacteals resulting in lymph leakage into the small bowel lumen and responsible
        for protein-losing enteropathy leading to lymphopenia, hypoalbuminemia and hypogammaglobulinemia
      explanation: Names hypoalbuminemia as a direct consequence of the protein-losing enteropathy caused
        by intestinal lymphangiectasia.
  - target: Decreased circulating immunoglobulin concentration
    description: Enteric loss of lymph depletes serum immunoglobulins.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:18294365
      reference_title: Primary intestinal lymphangiectasia (Waldmann's disease).
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: dilated intestinal lacteals resulting in lymph leakage into the small bowel lumen and responsible
        for protein-losing enteropathy leading to lymphopenia, hypoalbuminemia and hypogammaglobulinemia
      explanation: Names hypogammaglobulinemia as a direct consequence of the protein-losing enteropathy
        caused by intestinal lymphangiectasia.
  - target: Growth delay
    description: Enteric protein loss and associated malabsorption can impair nutrition and growth; this
      is a possible route in affected children, not an explanation for every growth abnormality.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:40394495
      reference_title: 'Recurrent pericardial effusion due to Hennekam lymphangiectasia-lymphedema syndrome:
        a case report and literature review.'
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: In children, severe malabsorption and malnutrition can cause growth and development retardation
        [5].
      explanation: This review describes malabsorption and malnutrition as causes of childhood
        growth impairment in protein-losing enteropathy generally, rather than proving that mechanism
        in every CCBE1 patient. Other causes of impaired growth are possible.
    - reference: PMID:26686525
      reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
        that does not Prevent Protein Expression.
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: However, he continued to have growth delay until puberty, after which both height and weight
        normalized for age.
      explanation: The CCBE1 C174Y male had growth delay despite improvement in enteropathy with treatment.
        This anchors the clinical finding in HKLLS1 but does not isolate enteric loss as its only cause.
  - target: Failure to thrive
    description: Enteric protein loss and associated malabsorption can impair nutrition and growth; this
      is a possible route in affected children, not an explanation for every growth abnormality.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:40394495
      reference_title: 'Recurrent pericardial effusion due to Hennekam lymphangiectasia-lymphedema syndrome:
        a case report and literature review.'
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: In children, severe malabsorption and malnutrition can cause growth and development retardation
        [5].
      explanation: This review describes malabsorption and malnutrition as causes of childhood
        growth impairment in protein-losing enteropathy generally, rather than proving that mechanism
        in every CCBE1 patient. Other causes of impaired growth are possible.
    - reference: PMID:26686525
      reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
        that does not Prevent Protein Expression.
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: At 3 weeks of age, she developed failure to thrive, protein losing enteropathy, and massive
        generalized edema.
      explanation: The CCBE1 C174Y female had failure to thrive with enteropathy. This is indirect support
        for the nutritional route when considered with the general mechanism above; co-occurrence alone
        does not establish causation.
phenotypes:
- name: Intestinal lymphangiectasia
  category: Gastrointestinal
  diagnostic: true
  description: Dilated intestinal lymphatic vessels are a defining feature and cause protein-losing enteropathy.
  phenotype_term:
    preferred_term: Intestinal lymphangiectasia
    term:
      id: HP:0002593
      label: Intestinal lymphangiectasia
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: All patients carryingCCBE1mutations had lymphedema of the limbs, unusual face and lymphangiectasia
      in the gut, representing the most distinctive features of the syndrome.
    explanation: All 13 CCBE1-positive patients in this genotype-stratified cohort had intestinal lymphangiectasia.
  - reference: PMID:25925991
    reference_title: 'A novel CCBE1 mutation leading to a mild form of hennekam syndrome: case report
      and review of the literature.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: She was diagnosed with a protein losing enteropathy secondary to segmental primary intestinal
      lymphangiectasia.
    explanation: The homozygous CCBE1 p.C98W child had segmental intestinal lymphangiectasia causing enteropathy.
- name: Lymphedema
  category: Lymphatic
  diagnostic: true
  description: Limb lymphedema is a core feature of generalized lymphatic dysplasia. Genital and facial
    edema can accompany it; distribution and severity vary.
  notes: Edema is often described as progressive in Hennekam syndrome, but the course is not
    uniformly progressive in CCBE1 disease. Peripheral lymphedema regressed after dietary optimization
    in a molecularly confirmed adult, so no uniform PROGRESSIVE qualifier is assigned.
  phenotype_term:
    preferred_term: Lymphedema
    term:
      id: HP:0001004
      label: Lymphedema
    temporality: CHRONIC
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: All patients carryingCCBE1mutations had lymphedema of the limbs, unusual face and lymphangiectasia
      in the gut, representing the most distinctive features of the syndrome.
    explanation: The genotype-stratified cohort documents limb lymphedema in the CCBE1-positive group.
  - reference: PMID:32629717
    reference_title: 'Intestinal lymphangiectasia in a 3-month-old girl: A case report of Hennekam syndrome
      caused by CCBE1 mutation.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: At first examination, she presented an abdominal circumference of 60 cm, edema of the lower
      extremities and vulva, and facial dysmorphisms (hypertelorism, flat nasal bridge, flat mid-face).
    explanation: The homozygous CCBE1 p.Arg158Cys infant had lower-limb and vulvar edema.
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: On this optimized diet, peripheral lymphedema regressed with normalization of serum protein
      (7.8 gm/dL) and immunoglobulin (1400 mg/dL) levels, without necessitating albumin or immunoglobulin
      replacement for 15 months.
    explanation: The CCBE1 C174Y female had regression of peripheral edema, supporting the variable-course caveat.
- name: Intellectual disability
  category: Neurologic
  description: Intellectual disability is variable in CCBE1-associated disease. Some molecularly confirmed
    patients have normal development, so cognitive impairment is not obligatory.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: We conclude thatCCBE1mutations cause a widespread and severe form of lymphatic dysplasia,
      associated with variable intellectual disabilities, unusual facial morphology and infrequently malformations.
    explanation: The conclusion specifically attributes variable intellectual disability to the CCBE1-positive
      phenotype.
  - reference: PMID:25925991
    reference_title: 'A novel CCBE1 mutation leading to a mild form of hennekam syndrome: case report
      and review of the literature.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: The patient we describe here has a lymphatic dysplasia without intellectual disability or
      dysmorphism caused by mutation in CCBE1, highlighting the phenotypic variability that can be seen
      with abnormalities in this gene.
    explanation: The p.C98W case establishes variability; this observation does not refute intellectual
      disability in other CCBE1 patients.
- name: Midface retrusion
  category: Craniofacial
  description: A flat midface has been documented in a homozygous CCBE1 p.Arg158Cys infant; facial dysmorphism
    is variable across CCBE1 cases.
  phenotype_term:
    preferred_term: Flat midface
    term:
      id: HP:0011800
      label: Midface retrusion
  evidence:
  - reference: PMID:32629717
    reference_title: 'Intestinal lymphangiectasia in a 3-month-old girl: A case report of Hennekam syndrome
      caused by CCBE1 mutation.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: At first examination, she presented an abdominal circumference of 60 cm, edema of the lower
      extremities and vulva, and facial dysmorphisms (hypertelorism, flat nasal bridge, flat mid-face).
    explanation: Flat mid-face is directly observed in the molecularly confirmed CCBE1 infant, supporting
      the more specific midface term.
- name: Depressed nasal bridge
  category: Craniofacial
  description: A depressed nasal bridge occurs in molecularly confirmed CCBE1 cases.
  phenotype_term:
    preferred_term: Flat nasal bridge
    term:
      id: HP:0005280
      label: Depressed nasal bridge
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: aPhenotype of patient 10 with aCCBE1mutation at age 2 years showing hair whorl in anterior
      hairline, hypertelorism, thick lateral eyebrows, epicanthi, depressed nasal bridge, and (pitting)
      lymphedema of distal limbs.
    explanation: The figure caption identifies this finding in patient 10 with a CCBE1 mutation, rather
      than in an ungenotyped Hennekam case.
- name: Hypertelorism
  category: Craniofacial
  description: Hypertelorism is documented in CCBE1-positive patients.
  phenotype_term:
    preferred_term: Hypertelorism
    term:
      id: HP:0000316
      label: Hypertelorism
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: aPhenotype of patient 10 with aCCBE1mutation at age 2 years showing hair whorl in anterior
      hairline, hypertelorism, thick lateral eyebrows, epicanthi, depressed nasal bridge, and (pitting)
      lymphedema of distal limbs.
    explanation: The figure caption identifies this finding in patient 10 with a CCBE1 mutation, rather
      than in an ungenotyped Hennekam case.
- name: Growth delay
  category: Growth
  description: Growth delay occurs in CCBE1-associated lymphatic disease; growth normalized after puberty
    in one reported sibling.
  phenotype_term:
    preferred_term: Growth retardation
    term:
      id: HP:0001510
      label: Growth delay
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: However, he continued to have growth delay until puberty, after which both height and weight
      normalized for age.
    explanation: The homozygous CCBE1 C174Y male had persistent childhood growth delay with later catch-up.
- name: Protein-losing enteropathy
  category: Gastrointestinal
  description: Enteric protein loss from intestinal lymphangiectasia is a hallmark complication.
  phenotype_term:
    preferred_term: Protein-losing enteropathy
    term:
      id: HP:0002243
      label: Protein-losing enteropathy
  evidence:
  - reference: PMID:25925991
    reference_title: 'A novel CCBE1 mutation leading to a mild form of hennekam syndrome: case report
      and review of the literature.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: She was diagnosed with a protein losing enteropathy secondary to segmental primary intestinal
      lymphangiectasia.
    explanation: Documents enteropathy secondary to intestinal lymphangiectasia in the homozygous CCBE1
      p.C98W child.
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: We report two HS siblings, born to consanguineous parents of Turkish ancestry, whose clinical
      phenotype also includes protein losing enteropathy, painful relapsing chylous ascites, and hypogammaglobulinemia.
    explanation: Two homozygous C174Y siblings provide additional direct CCBE1 evidence.
- name: Hypoalbuminemia
  category: Metabolic
  description: Enteric protein loss lowers serum albumin.
  phenotype_term:
    preferred_term: Hypoalbuminemia
    term:
      id: HP:0003073
      label: Hypoalbuminemia
  evidence:
  - reference: PMID:32629717
    reference_title: 'Intestinal lymphangiectasia in a 3-month-old girl: A case report of Hennekam syndrome
      caused by CCBE1 mutation.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Once admitted, ultrasonography showed a large amount of ascites, while blood laboratory investigations
      revealed severe hypoproteinemia, hypoalbuminemia and hypogammaglobulinemia.
    explanation: Documents hypoalbuminemia in the homozygous CCBE1 p.Arg158Cys infant, who also had CMV
      infection; the concurrent infection may have aggravated the intestinal disease.
  - reference: PMID:18294365
    reference_title: Primary intestinal lymphangiectasia (Waldmann's disease).
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: dilated intestinal lacteals resulting in lymph leakage into the small bowel lumen and responsible
      for protein-losing enteropathy leading to lymphopenia, hypoalbuminemia and hypogammaglobulinemia
    explanation: 'Names hypoalbuminemia as a consequence of the protein-losing enteropathy caused by intestinal
      lymphangiectasia. Cited for the shared mechanism only: this review is about primary intestinal lymphangiectasia
      (Waldmann disease) and lists Hennekam syndrome as a separate entity, so it is not evidence about
      HKLLS1 patients and is secondary to the on-disease citation above.'
- name: Decreased circulating immunoglobulin concentration
  category: Immunologic
  description: Loss of lymph into the gut depletes serum immunoglobulins (hypogammaglobulinemia).
  phenotype_term:
    preferred_term: Hypogammaglobulinemia
    term:
      id: HP:0004313
      label: Decreased circulating immunoglobulin concentration
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: We report two HS siblings, born to consanguineous parents of Turkish ancestry, whose clinical
      phenotype also includes protein losing enteropathy, painful relapsing chylous ascites, and hypogammaglobulinemia.
    explanation: Hypogammaglobulinemia is directly reported in the homozygous C174Y siblings.
  - reference: PMID:32629717
    reference_title: 'Intestinal lymphangiectasia in a 3-month-old girl: A case report of Hennekam syndrome
      caused by CCBE1 mutation.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Once admitted, ultrasonography showed a large amount of ascites, while blood laboratory investigations
      revealed severe hypoproteinemia, hypoalbuminemia and hypogammaglobulinemia.
    explanation: The homozygous p.Arg158Cys infant also had hypogammaglobulinemia, with concomitant CMV
      infection.
- name: Nonimmune hydrops fetalis
  category: Prenatal
  description: Severe CCBE1-associated lymphatic dysfunction can present prenatally as nonimmune hydrops
    with generalized edema, pleural effusions, and ascites.
  phenotype_term:
    preferred_term: Nonimmune hydrops fetalis
    term:
      id: HP:0001790
      label: Nonimmune hydrops fetalis
  evidence:
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: In conclusion, WES was able to identify compound heterozygous pathogenic variants in the
      CCBE1 gene as a cause of HKLLS1 and recurrent fetal hydrops.
    explanation: The fourth affected pregnancy carried both CCBE1 variants; the maternal antibody screen
      was negative, supporting nonimmune hydrops.
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: The lymphedema was sufficiently marked at birth to diagnose hydrops foetalis in 3 of theCCBE1positive
      cases, in 1 of theCCBE1negative HS cases and in 1 of the cases thought to have another entity (patient
      37).
    explanation: Three CCBE1-positive cohort members had hydrops, independently confirming the severe
      prenatal presentation.
- name: Cystic hygroma
  category: Prenatal
  description: Cystic hygroma occurred in an ungenotyped hydropic fetus from a family with a later molecularly
    confirmed CCBE1-affected pregnancy.
  phenotype_term:
    preferred_term: Cystic hygroma
    term:
      id: HP:0000476
      label: Cystic hygroma
  evidence:
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: An autopsy described a female fetus with a cystic hygroma, serous (nonchylous) pleural and
      pericardial effusions, congested liver, hypoplastic lungs, and a “markedly enlarged, hyperplastic
      thyroid gland”.
    explanation: Observed at autopsy of the second affected pregnancy in the CCBE1 family. This fetus
      could not be genotyped; molecular confirmation was obtained only in the fourth pregnancy.
- name: Pleural effusion
  category: Respiratory
  description: Fetal pleural effusions occur in the hydropic presentation of CCBE1-associated disease;
    the confirmed fetus had no reported fluid analysis.
  phenotype_term:
    preferred_term: Pleural effusion
    term:
      id: HP:0002202
      label: Pleural effusion
  evidence:
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Ultrasound images of hydropic fetus from G4 pregnancy. A, Fetal chest demonstrating pleural
      effusions; B, fetal head demonstrating scalp edema; C, fetal abdomen demonstrating ascites
    explanation: Ultrasound of the fourth-pregnancy fetus, confirmed to carry both CCBE1 variants, documents
      this finding. The fluid was not characterized as chylous.
- name: Pericardial effusion
  category: Cardiovascular
  description: Pericardial effusion has been reported in a hydropic fetus from a CCBE1 family and as recurrent
    disease in an adult reported to have HKLLS1.
  phenotype_term:
    preferred_term: Pericardial effusion
    term:
      id: HP:0001698
      label: Pericardial effusion
  evidence:
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: An autopsy described a female fetus with a cystic hygroma, serous (nonchylous) pleural and
      pericardial effusions, congested liver, hypoplastic lungs, and a “markedly enlarged, hyperplastic
      thyroid gland”.
    explanation: Observed at autopsy of the second affected pregnancy in the CCBE1 family. This fetus
      could not be genotyped; molecular confirmation was obtained only in the fourth pregnancy.
  - reference: PMID:40394495
    reference_title: 'Recurrent pericardial effusion due to Hennekam lymphangiectasia-lymphedema syndrome:
      a case report and literature review.'
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: This study presents a middle-aged male with long-term refractory PE without apparent cause
      and non-specific symptoms, and eventually diagnosed as Hennekam lymphangiectasia with lymphedema
      syndrome (HKLLS).
    explanation: The adult had recurrent pericardial effusion and two reported CCBE1 variants. The report
      assigns HKLLS1, but parental phasing was not demonstrated.
- name: Pulmonary hypoplasia
  category: Respiratory
  description: Hypoplastic lungs were found at autopsy in an ungenotyped hydropic fetus from a CCBE1 family.
    The report does not establish the developmental or compressive mechanism of this finding.
  phenotype_term:
    preferred_term: Pulmonary hypoplasia
    term:
      id: HP:0002089
      label: Pulmonary hypoplasia
  evidence:
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: An autopsy described a female fetus with a cystic hygroma, serous (nonchylous) pleural and
      pericardial effusions, congested liver, hypoplastic lungs, and a “markedly enlarged, hyperplastic
      thyroid gland”.
    explanation: Observed at autopsy of the second affected pregnancy in the CCBE1 family. This fetus
      could not be genotyped; molecular confirmation was obtained only in the fourth pregnancy.
- name: Ascites
  category: Prenatal
  description: Fetal ascites is part of the hydropic presentation.
  phenotype_term:
    preferred_term: Ascites
    term:
      id: HP:0001541
      label: Ascites
  evidence:
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Ultrasound images of hydropic fetus from G4 pregnancy. A, Fetal chest demonstrating pleural
      effusions; B, fetal head demonstrating scalp edema; C, fetal abdomen demonstrating ascites
    explanation: Ultrasound of the fourth-pregnancy fetus, confirmed to carry both CCBE1 variants, documents
      this finding. The fluid was not characterized as chylous.
- name: Epicanthus
  category: Craniofacial
  description: Epicanthal folds are documented in a CCBE1-positive child.
  phenotype_term:
    preferred_term: Epicanthal folds
    term:
      id: HP:0000286
      label: Epicanthus
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: aPhenotype of patient 10 with aCCBE1mutation at age 2 years showing hair whorl in anterior
      hairline, hypertelorism, thick lateral eyebrows, epicanthi, depressed nasal bridge, and (pitting)
      lymphedema of distal limbs.
    explanation: The figure caption identifies this finding in patient 10 with a CCBE1 mutation, rather
      than in an ungenotyped Hennekam case.
- name: Chylous ascites
  category: Gastrointestinal
  description: Recurrent chylous ascites occurs in CCBE1-associated lymphatic dysplasia. This is distinct
    from fetal ascites whose composition was not reported.
  phenotype_term:
    preferred_term: Chylous ascites
    term:
      id: HP:0012281
      label: Chylous ascites
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: We report two HS siblings, born to consanguineous parents of Turkish ancestry, whose clinical
      phenotype also includes protein losing enteropathy, painful relapsing chylous ascites, and hypogammaglobulinemia.
    explanation: The homozygous C174Y siblings had painful relapsing chylous ascites.
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Recurrent chylous ascites was present in 3 molecularly confirmed patients, in none of the
      unconfirmed patients and in patient 37 (Noonan syndrome).
    explanation: The cohort separately identifies three CCBE1-positive cases with recurrent chylous ascites;
      the additional Noonan case is not counted as HKLLS1 evidence.
- name: Delayed speech and language development
  category: Neurologic
  description: Speech delay was reported in two homozygous CCBE1 C174Y siblings, with age-appropriate
    speech attained by seven years.
  phenotype_term:
    preferred_term: Delayed speech and language development
    term:
      id: HP:0000750
      label: Delayed speech and language development
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Furthermore, both siblings had speech delays up until age 4 years with gain of full age-appropriate
      speech capabilities by age 7 years.
    explanation: Direct sibling observations support speech delay without asserting global developmental
      delay.
- name: Inguinal hernia
  category: Abdominal
  description: Inguinal hernias requiring surgery in infancy occurred in both homozygous CCBE1 C174Y siblings.
  phenotype_term:
    preferred_term: Inguinal hernia
    term:
      id: HP:0000023
      label: Inguinal hernia
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Both siblings since infancy have developed a distinctive clinical phenotype marked by limb
      lymphedema, relapsing chylous ascites (Fig. 2), inguinal hernias, enteric protein loss, hypoproteinemia,
      and hypogammaglobulinemia.
    explanation: Direct observations in two molecularly confirmed siblings document inguinal hernias.
- name: Umbilical hernia
  category: Abdominal
  description: An umbilical hernia requiring surgery in infancy was reported in the male homozygous CCBE1
    C174Y sibling.
  phenotype_term:
    preferred_term: Umbilical hernia
    term:
      id: HP:0001537
      label: Umbilical hernia
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: In addition, he had inguinal and umbilical hernia requiring surgical correction by 4 months
      of age.
    explanation: This sentence describes the genotyped male sibling.
- name: Hypothyroidism
  category: Endocrine
  description: Hypothyroidism has been observed in CCBE1-positive patients, including adult onset in both
    C174Y siblings.
  phenotype_term:
    preferred_term: Hypothyroidism
    term:
      id: HP:0000821
      label: Hypothyroidism
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: In adulthood, both P1 and P2 developed hypothyroidism treated with levothyroxine.
    explanation: Both molecularly confirmed siblings developed hypothyroidism; the cause of thyroid dysfunction
      was not established.
- name: Failure to thrive
  category: Growth
  description: Failure to thrive can accompany severe infantile enteric protein loss and generalized edema.
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: At 3 weeks of age, she developed failure to thrive, protein losing enteropathy, and massive
      generalized edema.
    explanation: The homozygous C174Y female had failure to thrive from early infancy.
- name: Microcephaly
  category: Neurologic
  description: Microcephaly was observed in the genotype-stratified CCBE1-positive cohort.
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Growth parameters were disturbed in a similar way in those with and without aCCBE1mutation,
      but microcephaly was more common in those with aCCBE1mutation.
    explanation: The cohort explicitly records microcephaly in mutation-positive patients; no population
      frequency is inferred.
- name: Metopic synostosis
  category: Skeletal
  description: Metopic craniosynostosis was observed in one CCBE1-positive member of the genotype-stratified
    cohort.
  phenotype_term:
    preferred_term: Metopic synostosis
    term:
      id: HP:0011330
      label: Metopic synostosis
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Unusual clinical manifestations inCCBE1positive patients included metopic craniosynostosis,
      diaphragmatic hernia, growth hormone deficiency (each occurring in one case), and hypothyroidism
      (n = 3).
    explanation: This is an observation in the CCBE1-positive group. An isolated cohort finding does not
      establish penetrance or its causal mechanism.
- name: Congenital diaphragmatic hernia
  category: Respiratory
  description: Diaphragmatic hernia was reported as an uncommon malformation in one CCBE1-positive cohort
    member.
  phenotype_term:
    preferred_term: Congenital diaphragmatic hernia
    term:
      id: HP:0000776
      label: Congenital diaphragmatic hernia
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Unusual clinical manifestations inCCBE1positive patients included metopic craniosynostosis,
      diaphragmatic hernia, growth hormone deficiency (each occurring in one case), and hypothyroidism
      (n = 3).
    explanation: This is an observation in the CCBE1-positive group. An isolated cohort finding does not
      establish penetrance or its causal mechanism.
genetic:
- name: CCBE1
  association: Causal biallelic loss-of-function variant
  gene_term:
    preferred_term: CCBE1
    term:
      id: hgnc:29426
      label: CCBE1
  evidence:
  - reference: PMID:19935664
    reference_title: Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Homozygous and compound heterozygous mutations in seven subjects paired
      with functional analysis in a zebrafish model identify CCBE1 as one of few
      genes causing primary generalized lymph-vessel dysplasia in humans.
    explanation: >-
      Establishes CCBE1 as the causal gene for HKLLS1 with biallelic variants.
  - reference: PMID:24913602
    reference_title: Hennekam syndrome can be caused by FAT4 mutations and be allelic to Van Maldergem syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      It can be caused by mutations in CCBE1 which are found in approximately
      25 % of cases.
    explanation: >-
      Quantifies the contribution of CCBE1 to the overall Hennekam syndrome
      cohort (~25% of cases).
  - reference: PMID:30564329
    reference_title: "Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema syndrome-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      at least 12 different CCBE1 variants have been reported in at least
      seventeen probands
    explanation: >-
      Summarizes the reported CCBE1 variant spectrum in HKLLS1.
  - reference: PMID:27345729
    reference_title: "Expanding the genotypic spectrum of CCBE1 mutations in Hennekam syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hennekam lymphangiectasia-lymphedema syndrome is an autosomal recessive
      disorder, with 25% of patients having mutations in CCBE1.
    explanation: >-
      States the CCBE1 fraction against the named Hennekam
      lymphangiectasia-lymphedema syndrome rather than against a broader
      clinical cohort.
  variants:
  - name: p.Leu229fs (c.683_684insT)
    description: >-
      Maternally inherited pathogenic loss-of-function frameshift CCBE1 variant;
      listed in gnomAD in 14 of 277,032 chromosomes, consistent with a rare
      recessive disease allele.
    evidence:
    - reference: PMID:30564329
      reference_title: "Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema syndrome-1."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        A maternally inherited variant, p.Leu229fs (c.683_684insT), was detected
        and confirmed by Sanger sequencing
      explanation: >-
        A specific pathogenic loss-of-function frameshift CCBE1 allele.
    - reference: PMID:30564329
      reference_title: "Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema syndrome-1."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        listed in the Genome Aggregation Database (gnomAD) Browser in 14 out of
        277 032 chromosomes
      explanation: >-
        Population allele frequency of the frameshift variant, consistent with a
        rare recessive disease allele.
  - name: p.Arg158Cys (c.472C>T)
    description: >-
      Recurrent missense CCBE1 allele, reported homozygous in an infant with
      intestinal lymphangiectasia and ascites and separately as one half of a
      compound heterozygous genotype (with c.521G>A) in a second child.
    evidence:
    - reference: PMID:32629717
      reference_title: "Intestinal lymphangiectasia in a 3-month-old girl: A case report of Hennekam syndrome caused by CCBE1 mutation."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Genetic investigations identified the genomic variant c.472C>T of the
        CCBE1 gene, coding for a protein variant (p.Arg158Cys), in homozygosity.
      explanation: >-
        Establishes the allele and its protein consequence in a homozygous
        HKLLS1 patient.
    - reference: PMID:32472549
      reference_title: "[Variant analysis of CCBE1 gene in a case of Hennekam lymphangiectasia-lymphedema syndrome type 1]."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        The patient was found to carry compound heterozygote variants (c.521G>A
        and c.472C>T) of the CCBE1 gene, which were respectively inherited from
        his parents.
      explanation: >-
        Independent recurrence of c.472C>T, here in trans with c.521G>A, which
        is what distinguishes a recurrent disease allele from a private one.
  - name: p.Cys174Tyr (C174Y)
    description: >-
      Recurrent homozygous missense CCBE1 allele, reported in two Turkish
      siblings of consanguineous parents and previously in a Dutch kindred. It
      is one of a cluster of alleles hitting cysteine residues in the EGF
      domain, whose disulfide linkages make them likely to disrupt the domain's
      tertiary structure. Functionally it is notable for a split result: in
      transfected cells the mutant protein shows increased intracellular
      expression and increased secretion, yet CCBE1 is scarce in the patients'
      own intestinal tissue by immunohistochemistry. The authors leave the
      reconciliation open between a secreted-but-misfolded protein and one that
      is rapidly degraded in vivo, so this allele is not loss-of-expression at
      the synthesis level without being a protein that is present where it is
      needed.
    evidence:
    - reference: PMID:26686525
      reference_title: "A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation that does not Prevent Protein Expression."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We identified by whole exome and Sanger sequencing the homozygous CCBE1
        C174Y mutation in both siblings. This mutation had been previously
        reported in another HS kindred from the Netherlands.
      explanation: >-
        Establishes the allele, its homozygous state in affected siblings, and
        its independent recurrence in a second kindred.
    - reference: PMID:25925991
      reference_title: "A novel CCBE1 mutation leading to a mild form of hennekam syndrome: case report and review of the literature."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        All of these mutations affect cysteine residues within the EGF domain
        and given the disulfide linkages within EGF domains, it is reasonable
        to assume all of them will disrupt the tertiary structure of the domain
        to a certain degree.
      explanation: >-
        Places C174Y in the EGF-domain cysteine cluster and gives the structural
        reason those alleles are damaging, which complements the domain-level
        result from the knock-in mouse series.
  - name: p.Cys98Trp (p.C98W)
    description: >-
      Homozygous missense CCBE1 allele identified in a consanguineous family.
      Notable because the child had lymphatic dysplasia with protein-losing
      enteropathy but no intellectual disability and no dysmorphism, marking the
      mild end of the HKLLS1 spectrum.
    evidence:
    - reference: PMID:25925991
      reference_title: "A novel CCBE1 mutation leading to a mild form of hennekam syndrome: case report and review of the literature."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We identified several large stretches of homozygosity in the patient
        that allowed us to sort the variants found in the patient's exome to
        identify p.C98W in CCBE1 as the likely pathogenic variant.
      explanation: >-
        Establishes the allele and how it was ascertained.
    - reference: PMID:25925991
      reference_title: "A novel CCBE1 mutation leading to a mild form of hennekam syndrome: case report and review of the literature."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        The patient we describe here has a lymphatic dysplasia without
        intellectual disability or dysmorphism caused by mutation in CCBE1,
        highlighting the phenotypic variability that can be seen with
        abnormalities in this gene.
      explanation: >-
        Documents the attenuated phenotype attached to this allele, and is the
        source for the wide expressivity recorded in clinical_burden.
  - name: p.Asp104Asn (c.310G>A)
    description: >-
      Paternally inherited missense CCBE1 allele in two affected brothers with
      primary lymphedema, in trans with p.Leu27Pro. It sits adjacent to other
      known pathogenic CCBE1 mutations, and zebrafish assay showed loss of
      function.
    evidence:
    - reference: PMID:27345729
      reference_title: "Expanding the genotypic spectrum of CCBE1 mutations in Hennekam syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        The presumed paternally inherited NM_133459.3:c.310G>A; p.(Asp104Asn),
        lies adjacent to other known pathogenic CCBE1 mutations
      explanation: >-
        Establishes the allele, its inheritance, and its position relative to
        known pathogenic variants.
    - reference: PMID:27345729
      reference_title: "Expanding the genotypic spectrum of CCBE1 mutations in Hennekam syndrome."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Functional analysis in a zebrafish model of lymphatic disease showed
        that both mutations lead to CCBE1 loss of function, confirming the
        pathogenicity of these variants and expanding the genotypic spectrum of
        lymphatic disorders.
      explanation: >-
        Functional confirmation that this allele is loss-of-function rather than
        an incidental rare variant.
  - name: p.Leu27Pro (c.80T>C)
    description: >-
      Maternally inherited missense CCBE1 allele in the CCBE1 signal peptide,
      in trans with p.Asp104Asn. The signal peptide had not previously been
      implicated in disease, so this allele extends the mutable region of the
      protein beyond the collagen and EGF domains.
    evidence:
    - reference: PMID:27345729
      reference_title: "Expanding the genotypic spectrum of CCBE1 mutations in Hennekam syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        the maternally inherited NM_133459.3:c.80T>C; p.(Leu27Pro) lies in the
        CCBE1 signal peptide, which has not previously been associated with
        disease
      explanation: >-
        Establishes the allele and the novelty of its location within CCBE1.
  - name: p.Thr112Ile (c.335C>T)
    description: >-
      Paternally inherited likely-pathogenic missense CCBE1 variant, in trans
      with the frameshift allele (compound heterozygosity).
    evidence:
    - reference: PMID:30564329
      reference_title: "Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema syndrome-1."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        A paternally inherited variant, p.Thr112Ile (c.335C>T) variant was
        detected and confirmed by Sanger sequencing
      explanation: >-
        A specific likely-pathogenic missense CCBE1 allele in trans with the
        frameshift variant.
diagnosis:
- name: Molecular genetic testing of CCBE1
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >-
    Sequencing and deletion/duplication analysis of CCBE1 confirms the diagnosis
    in individuals with syndromic generalized lymphatic dysplasia; a panel or
    exome approach also covers FAT4 and ADAMTS3.
  results: >-
    Biallelic pathogenic CCBE1 variants confirm HKLLS1.
  evidence:
  - reference: PMID:19935664
    reference_title: Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Homozygosity mapping identified a critical chromosomal region containing
      CCBE1, the human ortholog of a gene essential for lymphangiogenesis in
      zebrafish.
    explanation: >-
      Supports molecular confirmation by identification of causal CCBE1
      variants.
- name: Lymphoscintigraphy
  diagnosis_term:
    preferred_term: lymphoscintigraphy
    term:
      id: NCIT:C67262
      label: Lymphoscintigraphy
  description: >-
    Radionuclide lymphatic imaging demonstrates the site of lymph leakage and
    confirms intestinal lymphangiectasia non-invasively.
  results: >-
    Abnormal tracer leakage into the abdomen indicates intestinal
    lymphangiectasia.
  evidence:
  - reference: PMID:32629717
    reference_title: "Intestinal lymphangiectasia in a 3-month-old girl: A case report of Hennekam syndrome caused by CCBE1 mutation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Lymphoscintigraphy with 99m-Tc-nanocolloid demonstrated abnormal leakage
      of the tracer in the abdomen as evidence of IL.
    explanation: >-
      Documents lymphoscintigraphy establishing intestinal lymphangiectasia in a
      homozygous-CCBE1 patient.
- name: Stool alpha-1-antitrypsin measurement
  diagnosis_term:
    preferred_term: alpha-1 antitrypsin measurement
    term:
      id: NCIT:C80167
      label: Alpha-1 Antitrypsin Measurement
  description: >-
    Faecal alpha-1-antitrypsin is the standard confirmation of enteric protein
    loss, since the protein is not actively secreted or absorbed by the gut and
    resists intestinal degradation. It is the test that establishes
    protein-losing enteropathy independently of where the lesion sits.
  results: >-
    Raised stool alpha-1-antitrypsin with low serum albumin confirms
    protein-losing enteropathy.
  notes: >-
    Endoscopic biopsy is not a reliable substitute here. In a molecularly
    confirmed HKLLS1 patient with raised stool alpha-1-antitrypsin, endoscopic
    biopsies were reported as normal, because the lymphangiectasia can be
    segmental and patchy and so missed by sampling.
  evidence:
  - reference: PMID:40394495
    reference_title: "Recurrent pericardial effusion due to Hennekam lymphangiectasia-lymphedema syndrome: a case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      During the disease course, laboratory tests showed decreased blood albumin
      and increased stool α1-antitrypsin, while endoscopy biopsies were normal.
    explanation: >-
      In a molecularly confirmed HKLLS1 patient, stool alpha-1-antitrypsin was
      the abnormal test while endoscopic biopsy was not, which supports both the
      diagnostic value of this assay and the caveat recorded in notes.
treatments:
- name: Dietary management of protein-losing enteropathy
  description: >-
    A low-fat, high-protein diet enriched in medium-chain triglycerides reduces
    intestinal lymphatic load and enteric protein loss from intestinal
    lymphangiectasia; supplemental nutrition supports growth.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  target_mechanisms:
  - target: Protein-losing enteropathy
    description: >-
      Reducing long-chain-triglyceride load lowers intestinal lymphatic
      pressure, so less protein-rich lymph is lost into the gut. This acts on
      the enteropathy itself rather than replacing what has been lost.
    evidence:
    - reference: PMID:19449286
      reference_title: "Evidence for medium chain triglycerides in the treatment of primary intestinal lymphangiectasia."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        17 of 27 cases (63%) treated with MCT had complete resolution of
        symptoms while only 10 of 28 (35.7%) patients in group B showed
        complete resolution.
      explanation: >-
        Quantifies the effect of the MCT diet on the enteropathy across 55
        collected cases. The series is primary intestinal lymphangiectasia
        rather than HKLLS1, so this is the general efficacy of the intervention
        against the mechanism HKLLS1 produces.
  evidence:
  - reference: PMID:29560340
    reference_title: "Hennekam Syndrome: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      patients may require a high-protein, low-fat diet with medium-chain
      triglyceride supplementation in the case
    explanation: >-
      Directly supports a high-protein, low-fat, medium-chain-triglyceride diet
      for the protein-losing enteropathy of Hennekam syndrome.
  - reference: PMID:32629717
    reference_title: "Intestinal lymphangiectasia in a 3-month-old girl: A case report of Hennekam syndrome caused by CCBE1 mutation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      serum albumin and IgG rose to normal values after 3 months (4.3 g/dL and
      501 mg/dL, respectively)
    explanation: >-
      In a homozygous-CCBE1 infant managed with parenteral then enteral
      nutrition, correcting the enteric protein loss restored both albumin and
      IgG. This is the on-disease evidence that the nutritional arm addresses
      the cause; no immunoglobulin was given in that case.
  - reference: PMID:19449286
    reference_title: "Evidence for medium chain triglycerides in the treatment of primary intestinal lymphangiectasia."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Mortality for Group A was 1 out of 27 (3.7%), while mortality in group B
      was 5 of 28 (17.85%) patients.
    explanation: >-
      Reports the mortality difference between MCT-treated and untreated
      intestinal lymphangiectasia, which is why this diet is the cornerstone
      rather than one option among several. The series is primary intestinal
      lymphangiectasia, not HKLLS1.
  - reference: PMID:26686525
    reference_title: "A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation that does not Prevent Protein Expression."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      without necessitating albumin or immunoglobulin replacement for 15 months
    explanation: >-
      In a genotype-confirmed CCBE1 patient, optimizing the
      medium-chain-triglyceride diet normalized serum protein and immunoglobulin
      and removed the need for replacement therapy for 15 months. This is the
      causal ordering the entry argues for - the diet acts on the enteric loss
      itself, while replacement substitutes for what is lost.
- name: Lymphedema supportive care
  description: >-
    Complete decongestive therapy (compression, manual lymphatic drainage,
    exercise, skin care) manages the lymphedema; a Hennekam case showed measurable
    limb-circumference reduction with this approach.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: physical therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  evidence:
  - reference: PMID:29560340
    reference_title: "Hennekam Syndrome: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      rehabilitative treatments including complete decongestive therapy for
      edema control appeared to be beneficial
    explanation: >-
      The case report concludes that complete decongestive therapy was
      beneficial for edema control in a patient with Hennekam syndrome.
  - reference: PMID:29560340
    reference_title: "Hennekam Syndrome: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      upper arm, from 22 cm to 19 cm; forearm, from 22.5 cm to 21 cm,
      respectively.
    explanation: >-
      Complete decongestive therapy produced a measurable reduction in
      upper-arm and forearm circumference in a Hennekam syndrome patient.
- name: Supportive and multidisciplinary care
  description: >-
    Management is symptomatic and multidisciplinary, including nutritional
    support, monitoring for hypoalbuminemia and immunodeficiency, and developmental
    and educational support according to the individual's findings.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Both siblings had a history of mild development delay requiring extra scholarly assistance
      to pass primary school, compared to their healthy unaffected siblings.
    explanation: The CCBE1 siblings required educational support alongside treatment for enteropathy;
      this supports individualized multidisciplinary care rather than a subtype-specific protocol.
- name: Genetic counseling
  description: >-
    Counsel families about autosomal recessive inheritance, 25% sibling
    recurrence risk, carrier testing, and reproductive options.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:19935664
    reference_title: Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Homozygous and compound heterozygous mutations in seven subjects paired
      with functional analysis in a zebrafish model identify CCBE1 as one of few
      genes causing primary generalized lymph-vessel dysplasia in humans.
    explanation: >-
      Autosomal recessive biallelic inheritance is the basis for recurrence-risk
      counseling.
- name: Immunoglobulin replacement therapy
  description: >-
    The hypogammaglobulinemia of HKLLS1 is a secondary immunodeficiency driven
    by enteric loss of immunoglobulin, not a primary defect of B-cell
    development, so the dietary control of lymph loss is the treatment that
    addresses the cause. Immunoglobulin replacement is used for symptomatic
    hypogammaglobulinemia alongside it; intravenous administration does not
    always achieve satisfactory plasma levels, and subcutaneous administration
    has been reported to do better in intestinal lymphangiectasia. Two
    genotype-confirmed CCBE1 patients in one kindred document its use in HKLLS1
    itself, one of whom no longer needed replacement once the
    medium-chain-triglyceride diet was optimized.
  therapeutic_modality: PROTEIN_REPLACEMENT
  treatment_term:
    preferred_term: immunoglobulin replacement therapy
    term:
      id: NCIT:C62710
      label: Immunoglobulin Therapy
    therapeutic_agent:
    - preferred_term: human immunoglobulin G
      term:
        id: NCIT:C80829
        label: Human Immunoglobulin G
  target_mechanisms:
  - target: Decreased circulating immunoglobulin concentration
    description: >-
      Replaces the immunoglobulin lost into the gut rather than correcting the
      enteric loss itself.
    evidence:
    - reference: PMID:26934740
      reference_title: "Secondary hypogammaglobulinemia in Waldmann's disease treated with subcutaneous immunoglobulins."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We describe here the case of a patient with PIL and severe
        hypogammaglobulinemia treated with immunoglobulins.
      explanation: >-
        Establishes that this treatment is directed at the hypogammaglobulinemia
        of intestinal lymphangiectasia, which is the node this link names. The
        case is primary intestinal lymphangiectasia rather than HKLLS1, so the
        link rests on the shared enteric-loss mechanism.
  evidence:
  - reference: PMID:26934740
    reference_title: "Secondary hypogammaglobulinemia in Waldmann's disease treated with subcutaneous immunoglobulins."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The administration of intravenous immunoglobulins does not always lead to
      satisfactory plasma levels and therefore the replacement therapy with
      immunoglobulins is controversial.
    explanation: >-
      Documents both the use of immunoglobulin replacement for the secondary
      hypogammaglobulinemia of intestinal lymphangiectasia and the caveat about
      intravenous dosing. The report is of a primary intestinal lymphangiectasia
      patient rather than a molecularly confirmed HKLLS1 patient, so the
      transfer rests on the shared enteric-loss mechanism.
  - reference: PMID:26686525
    reference_title: "A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation that does not Prevent Protein Expression."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      She was treated with albumin and immunoglobulin replacements (750 mg/kg
      every 2 weeks), aggressive diuresis (furosemide and aldactone), and
      modified diet (20 % protein, 50 % carbohydrate, 30% fat of which 50% are
      composed of medium-chain triglycerides) with significant improvement in
      edema, protein losing enteropathy, and hypogammaglobulinemia
    explanation: >-
      Immunoglobulin replacement with a stated dose in a genotype-confirmed
      CCBE1 C174Y patient, with improvement in the hypogammaglobulinemia it
      targets. This is the on-disease support the link previously lacked; note
      it was given alongside a medium-chain-triglyceride diet, so the two arms
      were not separated in this patient.
  - reference: PMID:26686525
    reference_title: "A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation that does not Prevent Protein Expression."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      hypogammaglobulinemia (615 mg/dL) requiring intravenous immunoglobulin
      substitution (30 gm every 3 to 4 weeks)
    explanation: >-
      A second genotype-confirmed patient in the same kindred on ongoing
      intravenous immunoglobulin, which establishes the treatment as a
      continuing requirement rather than a one-off in this disorder.
- name: Octreotide for refractory protein-losing enteropathy
  description: >-
    Second-line pharmacotherapy for intestinal lymphangiectasia when the
    medium-chain-triglyceride diet fails to control enteric protein loss. It is
    costly and requires parenteral administration. No HKLLS1-specific trial
    exists; the evidence is from intestinal lymphangiectasia generally.
  therapeutic_modality: PEPTIDE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: octreotide
      term:
        id: CHEBI:7726
        label: octreotide
  target_mechanisms:
  - target: Protein-losing enteropathy
    description: >-
      Used to reduce enteric protein loss when dietary management alone is
      insufficient.
    evidence:
    - reference: PMID:14723832
      reference_title: "Primary Intestinal Lymphangiectasia."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Octreotide has been helpful in cases in which treatment with MCT has
        failed, but it is costly and requires parenteral administration.
      explanation: >-
        Places octreotide's action on the enteropathy itself, after dietary
        management, which is the node and the position this link asserts.
  evidence:
  - reference: PMID:14723832
    reference_title: "Primary Intestinal Lymphangiectasia."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Octreotide has been helpful in cases in which treatment with MCT has
      failed, but it is costly and requires parenteral administration.
    explanation: >-
      Positions octreotide as second-line after MCT for intestinal
      lymphangiectasia, with its cost and route as the stated drawbacks. The
      source addresses primary intestinal lymphangiectasia rather than HKLLS1,
      so this is the general management of the enteropathy this disease causes.
animal_models:
- name: Ccbe1-null mouse
  species: Mus musculus
  genotype: Ccbe1 constitutive null (Ccbe1 mutant embryos)
  category: KNOCKOUT
  publication: PMID:21778431
  genes:
  - preferred_term: CCBE1
    term:
      id: hgnc:29426
      label: CCBE1
  description: >-
    The first mammalian model of CCBE1 deficiency. Ccbe1 mutant embryos specify
    PROX1-positive lymphatic endothelial cells within the cardinal vein but fail
    to bud and migrate from it, so no definitive lymphatic structures form, and
    the embryos die prenatally. Recombinant CCBE1 has little lymphangiogenic
    activity alone yet strongly potentiates VEGF-C in a corneal micropocket
    assay, which is what established CCBE1 as necessary but not sufficient for
    lymphangiogenesis.
  evidence:
  - reference: PMID:21778431
    reference_title: "CCBE1 is essential for mammalian lymphatic vascular development and enhances the lymphangiogenic effect of vascular endothelial growth factor-C in vivo."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Collagen- and calcium-binding EGF domains 1 (CCBE1) has been associated
      with Hennekam syndrome, in which patients have lymphedema,
      lymphangiectasias, and other cardiovascular anomalies.
    explanation: >-
      States the gene-disease relationship this mouse was made to model, which
      is what makes a Ccbe1-null animal relevant to HKLLS1.
  modeled_mechanisms:
  - target: Failed lymphangiogenesis
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Loss of Ccbe1 in the mouse blocks budding and migration of Prox1-positive
      lymphatic endothelial cells from the cardinal vein, which is exactly the
      cellular step this node names.
    limitations: >-
      The model speaks to the cellular step, not to the upstream activation
      node: VEGFR-3 activation appeared unaltered in the mutants by proximity
      ligation assay, so this animal does not itself demonstrate reduced
      receptor signalling and is not linked to the proteolytic-activation node.
      Being a constitutive null, it also models complete loss rather than the
      hypomorphic alleles many patients carry.
    evidence:
    - reference: PMID:21778431
      reference_title: "CCBE1 is essential for mammalian lymphatic vascular development and enhances the lymphangiogenic effect of vascular endothelial growth factor-C in vivo."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        our data suggest CCBE1 to be essential but not sufficient for
        lymphangiogenesis
      explanation: >-
        The authors' own summary of what the model establishes - CCBE1 is
        required for lymphangiogenesis but acts as a potentiator rather than an
        independent driver - which is the cofactor role this node describes and
        is what makes the model informative for it.
    - reference: PMID:21778431
      reference_title: "CCBE1 is essential for mammalian lymphatic vascular development and enhances the lymphangiogenic effect of vascular endothelial growth factor-C in vivo."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        CCBE1 has little lymphangiogenic effect on its own but dramatically
        enhances the lymphangiogenic effect of vascular endothelial growth
        factor-C in vivo.
      explanation: >-
        Establishes the direction of the CCBE1 effect on VEGF-C-driven
        lymphangiogenesis. Recorded at link level rather than as a readout of
        this model because the corneal micropocket assay adds recombinant CCBE1
        protein exogenously; it is not a measurement made in Ccbe1 mutant
        embryos.
    readouts:
    - name: Definitive lymphatic structures in the embryo
      target: Failed lymphangiogenesis
      description: >-
        Phenotypic analysis of Ccbe1 mutant embryos for formation of lymphatic
        vessels, scored against Prox1-positive progenitor specification.
      direction: ABOLISHED
      interpretation: >-
        Lymphatic progenitors are specified but no lymphatic vasculature forms,
        placing the block at the budding/migration step rather than at lymphatic
        fate specification.
      evidence:
      - reference: PMID:21778431
        reference_title: "CCBE1 is essential for mammalian lymphatic vascular development and enhances the lymphangiogenic effect of vascular endothelial growth factor-C in vivo."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Phenotypic analysis of murine Ccbe1 mutant embryos showed a complete
          lack of definitive lymphatic structures, even though Prox1(+)
          lymphatic endothelial cells get specified within the cardinal vein.
        explanation: >-
          Reports the measurement behind this readout and its interpretation.
  - target: Generalized lymphatic dysplasia
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Complete absence of definitive lymphatics models the severe end of the
      human generalized lymphatic dysplasia, and is consistent with the
      recurrent hydrops fetalis seen in some HKLLS1 pregnancies.
    limitations: >-
      Ccbe1-null mice die prenatally, so the model cannot express the postnatal
      arms of the human disease - chronic limb, genital and facial lymphedema,
      intestinal lymphangiectasia with protein-losing enteropathy, or the
      craniofacial and cognitive features. Human patients carry hypomorphic as
      well as null alleles and survive, so the null mouse is more severe than
      the usual human phenotype.
    readouts:
    - name: Definitive lymphatic structures at the whole-embryo level
      target: Generalized lymphatic dysplasia
      description: >-
        Whole-embryo phenotypic analysis of Ccbe1 mutants for the presence of
        any definitive lymphatic vasculature.
      direction: ABOLISHED
      interpretation: >-
        Total absence of lymphatic structures throughout the embryo is the
        animal counterpart of generalized - rather than segmental - lymphatic
        dysplasia, and models the severe end of the human phenotype.
      evidence:
      - reference: PMID:21778431
        reference_title: "CCBE1 is essential for mammalian lymphatic vascular development and enhances the lymphangiogenic effect of vascular endothelial growth factor-C in vivo."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Phenotypic analysis of murine Ccbe1 mutant embryos showed a complete
          lack of definitive lymphatic structures, even though Prox1(+)
          lymphatic endothelial cells get specified within the cardinal vein.
          Mutant mice die prenatally.
        explanation: >-
          Reports the whole-embryo absence of lymphatic structures behind this
          readout; the accompanying lethality is recorded under limitations.
    evidence:
    - reference: PMID:21778431
      reference_title: "CCBE1 is essential for mammalian lymphatic vascular development and enhances the lymphangiogenic effect of vascular endothelial growth factor-C in vivo."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        CCBE1 as a factor critically required for budding and migration of
        Prox-1(+) lymphatic endothelial cells from the cardinal vein
      explanation: >-
        Supports treating the null mouse as informative for the generalized
        lymphatic dysplasia node, while its prenatal lethality is recorded as a
        limitation.
- name: ccbe1 mutant zebrafish
  species: Danio rerio
  genotype: ccbe1 loss-of-function mutant recovered in a forward genetic screen
  category: LOSS_OF_FUNCTION_MUTANT
  publication: PMID:19287381
  genes:
  - preferred_term: CCBE1
    term:
      id: hgnc:29426
      label: CCBE1
  description: >-
    The genetic screen that identified ccbe1 as indispensable for embryonic
    lymphangiogenesis. Mutants fail lymphangioblast budding and angiogenic
    sprouting from venous endothelium, and ccbe1 acts at the same developmental
    stage as vegfc.
  evidence:
  - reference: PMID:19287381
    reference_title: "Ccbe1 is required for embryonic lymphangiogenesis and venous sprouting."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Using a genetic screen in zebrafish we identify ccbe1 (collagen and
      calcium-binding EGF domain-1) as indispensible for embryonic
      lymphangiogenesis.
    explanation: >-
      Establishes the zebrafish ccbe1 mutant as the model that defined CCBE1 as
      required for lymphangiogenesis, the mechanism lost in HKLLS1.
  modeled_mechanisms:
  - target: Failed lymphangiogenesis
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      ccbe1 mutants fail lymphangioblast budding and venous sprouting, and
      ccbe1 acts at the same developmental stage as vegfc, which is the
      cellular lymphangiogenic step this node names.
    limitations: >-
      Zebrafish do not develop the craniofacial, cognitive, intestinal or
      chronic lymphedema features of the human syndrome, so the model speaks to
      the developmental lymphangiogenic step and not to the downstream clinical
      phenotype.
    readouts:
    - name: Lymphangioblast budding and venous sprouting
      target: Failed lymphangiogenesis
      description: >-
        Imaging of lymphangioblast budding and angiogenic sprouting from venous
        endothelium in ccbe1 mutant embryos.
      direction: DECREASED
      interpretation: >-
        Places the ccbe1 requirement at the same step as vegfc, which is the
        cofactor relationship disrupted in HKLLS1.
      evidence:
      - reference: PMID:19287381
        reference_title: "Ccbe1 is required for embryonic lymphangiogenesis and venous sprouting."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Ccbe1 acts at the same stage of development as Vegfc and is required
          for lymphangioblast budding and angiogenic sprouting from venous
          endothelium.
        explanation: >-
          Reports the developmental measurements behind this readout.
    evidence:
    - reference: PMID:24523457
      reference_title: Ccbe1 regulates Vegfc-mediated induction of Vegfr3 signaling during embryonic lymphangiogenesis.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Vegfc- and Vegfr3-dependent Erk signaling is impaired in the absence of
        Ccbe1.
      explanation: >-
        Confirms in the same species that the lymphangiogenic failure runs
        through VEGF-C/VEGFR-3 signalling, which is what makes this model
        informative for the node.
- name: CCBE1 domain-deletion knock-in mice
  species: Mus musculus
  genotype: CCBE1 delta-Collagen and CCBE1 delta-EGF knock-in alleles
  category: KNOCK_IN
  publication: PMID:25814692
  genes:
  - preferred_term: CCBE1
    term:
      id: hgnc:29426
      label: CCBE1
  description: >-
    Knock-in mice expressing CCBE1 lacking either the collagen repeats or the
    EGF domains, read out alongside zebrafish rescue and in vitro VEGF-C
    processing. Collagen-domain deletion fully phenocopies the null, whereas
    EGF-domain deletion still permits rudimentary lymphatics. The same
    asymmetry holds for two Hennekam syndrome patient mutations, one in each
    domain, which is what makes this an allele-specific model of the human
    disorder rather than a generic knockout.
  evidence:
  - reference: PMID:25814692
    reference_title: "Functional Dissection of the CCBE1 Protein: A Crucial Requirement for the Collagen Repeat Domain."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      We analyzed the functionality of several CCBE1 deletion mutants by
      generating knock-in mice expressing these mutants, by analyzing their
      ability to enhance Vegfc signaling in vivo in zebrafish, and by testing
      their ability to induce VEGFC processing in vitro.
    explanation: >-
      Describes the knock-in allelic series and the assays used to read it out.
  modeled_mechanisms:
  - target: CCBE1 loss of function
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Maps loss of CCBE1 function onto specific protein domains, and shows that
      a Hennekam syndrome mutation in the collagen repeats is more damaging than
      one in an EGF domain - a structural account of why HKLLS1 alleles differ
      in severity.
    limitations: >-
      The severity ordering was established for two patient mutations, so it
      supports a domain-level rule rather than a validated genotype-phenotype
      predictor for individual HKLLS1 alleles. The knock-ins delete whole
      domains, which is a more drastic lesion than most patient missense
      variants.
    readouts:
    - name: Lymphatic vessel formation in domain-deletion embryos
      target: CCBE1 loss of function
      description: >-
        Comparison of lymphatic development in CCBE1 delta-Collagen versus
        CCBE1 delta-EGF knock-in embryos, each against CCBE1 knockout mice.
      direction: DECREASED
      interpretation: >-
        Collagen-domain loss is equivalent to whole-protein loss while
        EGF-domain loss is partial, so the collagen repeats carry most of the
        VEGF-C-activating function whose loss causes HKLLS1.
      evidence:
      - reference: PMID:25814692
        reference_title: "Functional Dissection of the CCBE1 Protein: A Crucial Requirement for the Collagen Repeat Domain."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          although CCBE1ΔCollagen mice fully phenocopy CCBE1 knock-out mice,
          CCBE1ΔEGF knock-in embryos still form rudimentary lymphatics
        explanation: >-
          Reports the graded lymphatic phenotype that grounds this readout.
    evidence:
    - reference: PMID:25814692
      reference_title: "Functional Dissection of the CCBE1 Protein: A Crucial Requirement for the Collagen Repeat Domain."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Furthermore, a Hennekam syndrome mutation within the collagen domain has
        a stronger effect than a Hennekam syndrome mutation within the EGF
        domain.
      explanation: >-
        Ties the domain-deletion series directly to Hennekam syndrome patient
        alleles, which is what makes this model informative for the human
        loss-of-function node.
  - target: Impaired pro-VEGF-C proteolytic activation
    relationship: MEASURES
    fidelity: MODERATE
    description: >-
      The same allelic series was read out directly on VEGF-C processing, so it
      assays the proteolytic activation step itself rather than only its
      downstream consequence. Collagen-domain deletion abolishes processing
      while EGF-domain deletion does not, which assigns the cleavage-supporting
      function of CCBE1 to the collagen repeats.
    limitations: >-
      The processing readout is an in vitro assay on deletion constructs, not a
      measurement of endogenous VEGF-C maturation in a patient or in the
      knock-in animal. It establishes which domain carries the activity rather
      than quantifying how much activation a given patient allele retains.
    readouts:
    - name: VEGF-C proteolytic processing in vitro
      target: Impaired pro-VEGF-C proteolytic activation
      description: >-
        Ability of each CCBE1 deletion construct to induce cleavage of
        pro-VEGF-C, compared against full-length CCBE1.
      direction: ABOLISHED
      interpretation: >-
        Loss of the collagen repeats abolishes VEGF-C processing while loss of
        the EGF domains does not, placing the activating function of CCBE1 at
        this node in the collagen repeats.
      evidence:
      - reference: PMID:25814692
        reference_title: "Functional Dissection of the CCBE1 Protein: A Crucial Requirement for the Collagen Repeat Domain."
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          Third, CCBE1ΔEGF, similarly to CCBE1, but not CCBE1ΔCollagen could
          activate VEGFC processing in vitro.
        explanation: >-
          Reports the processing measurement behind this readout and its
          direction for each construct.
    evidence:
    - reference: PMID:25814692
      reference_title: "Functional Dissection of the CCBE1 Protein: A Crucial Requirement for the Collagen Repeat Domain."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        We analyzed the functionality of several CCBE1 deletion mutants by
        generating knock-in mice expressing these mutants, by analyzing their
        ability to enhance Vegfc signaling in vivo in zebrafish, and by testing
        their ability to induce VEGFC processing in vitro.
      explanation: >-
        Names VEGF-C processing as one of the three assays this model series was
        read out on, which is what makes it informative for the activation node
        specifically.
differential_diagnoses:
- name: Hennekam lymphangiectasia-lymphedema syndrome 2
  disease_term:
    preferred_term: Hennekam lymphangiectasia-lymphedema syndrome 2
    term:
      id: MONDO:0014454
      label: Hennekam lymphangiectasia-lymphedema syndrome 2
  description: >-
    FAT4-related Hennekam syndrome is clinically overlapping and distinguished by
    molecular testing; it accounts for a subset of CCBE1 mutation-negative cases.
  evidence:
  - reference: PMID:24913602
    reference_title: "Hennekam syndrome can be caused by FAT4 mutations and be allelic to Van Maldergem syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Subsequent targeted mutation analysis of FAT4 in a cohort of 24 CCBE1
      mutation-negative Hennekam syndrome patients identified homozygous or
      compound heterozygous mutations in four additional families.
    explanation: >-
      Establishes FAT4-related Hennekam syndrome as a distinct molecular entity
      found specifically among CCBE1 mutation-negative patients. The abstract
      does not compare the two Hennekam subtypes clinically, so this supports
      the genotype separation only, not a claim that they are clinically
      indistinguishable.
- name: Hennekam lymphangiectasia-lymphedema syndrome 3
  disease_term:
    preferred_term: Hennekam lymphangiectasia-lymphedema syndrome 3
    term:
      id: MONDO:0032564
      label: hennekam lymphangiectasia-lymphedema syndrome 3
  description: >-
    ADAMTS3-related Hennekam syndrome shares the VEGF-C activation mechanism with
    CCBE1 and produces distinctive facial features similar to CCBE1-mutant
    patients.
  evidence:
  - reference: PMID:28985353
    reference_title: "Loss of ADAMTS3 activity causes Hennekam lymphangiectasia-lymphedema syndrome 3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These patients have distinctive facial features similar to those with
      mutations in CCBE1.
    explanation: >-
      States the facial overlap that makes ADAMTS3-related Hennekam syndrome
      hard to separate from HKLLS1 on examination alone.
  - reference: PMID:28985353
    reference_title: "Loss of ADAMTS3 activity causes Hennekam lymphangiectasia-lymphedema syndrome 3."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      VEGFC processing is also affected by CCBE1 mutations that cause the
      Hennekam lymphangiectasia-lymphedema syndrome syndrome type1.
    explanation: >-
      Explains the shared mechanism behind the phenotypic overlap: both genes
      act on the same pro-VEGF-C activation step, so the two disorders converge
      on one pathway and are separated by molecular testing.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Hennekam syndrome is very rare, with CCBE1 accounting for approximately 25%
    of all Hennekam syndrome cases. No population-based rate has been reported;
    the class here reflects the small number of published cases.
  evidence:
  - reference: PMID:24913602
    reference_title: Hennekam syndrome can be caused by FAT4 mutations and be allelic to Van Maldergem syndrome.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      It can be caused by mutations in CCBE1 which are found in approximately
      25 % of cases.
    explanation: >-
      Quantifies the CCBE1 fraction of Hennekam syndrome cases (~25%); the
      quote does not itself state a population prevalence.
clinical_burden:
  burden_level: HIGH
  rationale: >-
    HKLLS1 can combine disabling lymphedema with intestinal lymphangiectasia,
    protein-losing enteropathy and variable intellectual disability. Severe
    prenatal disease can cause fetal death. Among 13 CCBE1-positive cohort
    members, two needed nocturnal respiratory support and two others died in
    childhood or early adulthood from respiratory problems. These observations
    establish severe respiratory morbidity without a population mortality estimate
    or a specific pulmonary mechanism. Surviving patients may require recurrent
    paracentesis, nutritional therapy and immunoglobulin replacement, with missed
    school and work. Severity and treatment requirements vary; peripheral edema
    and protein loss can improve with dietary management.
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: TwoCCBE1positive cases needed respiratory support at night, and 2 otherCCBE1positive cases
      died in childhood or early adulthood due to respiratory problems.
    explanation: The genotype-stratified cohort directly documents severe respiratory outcomes in
      CCBE1-positive patients, without resolving the individual pulmonary causes.
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: P2, like his sister, had his childhood and adulthood complicated by relapsing painful
      chylous ascites, frequent peritoneal drainage, and hospital admissions, with missed days from school
      and work.
    explanation: The homozygous CCBE1 siblings document the long-term burden of ascites and its treatment.
  - reference: PMID:30564329
    reference_title: "Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema syndrome-1."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Following fetal demise, dilation and evacuation was performed.
    explanation: This outcome concerns the fourth pregnancy, molecularly confirmed to carry both CCBE1 variants.
progression:
- phase: Prenatal presentation
  age_range: Fetal life
  notes: >-
    In severely affected pregnancies the lymphatic failure is already manifest
    in utero as non-immune hydrops fetalis, cystic hygroma, pleural and
    pericardial effusions and ascites, and may recur across pregnancies in a
    family. Prenatal onset is what brings some families to whole-exome
    sequencing before any postnatal phenotype is available.
  evidence:
  - reference: PMID:30564329
    reference_title: "Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema syndrome-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Whole exome sequencing (WES) was used to determine the etiology of
      recurrent hydrops fetalis in this case of Hennekam
      lymphangiectasia-lymphedema syndrome-1.
    explanation: >-
      Establishes recurrent prenatal hydrops as a presenting phase of HKLLS1.
- phase: Congenital and infantile lymphedema
  age_range: Birth through infancy
  notes: Edema may be present at birth and involve the limbs, genitalia and face. Intrauterine facial
    edema has been proposed to influence facial development, but facial dysmorphism is not universal in
    molecularly confirmed CCBE1 disease.
  evidence:
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The edema is usually congenital and often progressive, generally including the limbs, genitalia,
      and face.
    explanation: Dates the onset of lymphedema to birth and states its typical distribution.
- phase: Childhood and later course
  age_range: Childhood onward
  notes: 'Persistent enteric protein loss may cause hypoalbuminemia, hypogammaglobulinemia and nutritional
    growth impairment. Growth can improve after puberty, and speech delay can resolve. Seizures reported
    in historical mixed-genotype Hennekam series should not be assigned a frequency in HKLLS1: none occurred
    in the 13 CCBE1-positive patients of the genotype-stratified cohort. There are no validated disease
    stages.'
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: However, he continued to have growth delay until puberty, after which both height and weight
      normalized for age.
    explanation: Documents catch-up growth rather than inevitable progressive growth failure.
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Seizures did not occur in the group of molecularly confirmed HS patients, but were found
      in 6 of the HS cases without molecular confirmation.
    explanation: Distinguishes the CCBE1-positive cohort from clinically diagnosed Hennekam patients;
      absence in this cohort does not establish that seizures can never occur in HKLLS1.
discussions:
- discussion_id: mismatch_ccbe1_models_nonlymphatic_phenotype
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: How do CCBE1 variants produce the variable neurodevelopmental and craniofacial findings beyond
    their established effect on lymphatic development?
  attaches_to:
  - pathophysiology#Generalized lymphatic dysplasia
  - phenotypes#Intellectual disability
  - phenotypes#Midface retrusion
  - phenotypes#Depressed nasal bridge
  - phenotypes#Hypertelorism
  - phenotypes#Epicanthus
  - phenotypes#Delayed speech and language development
  - phenotypes#Microcephaly
  rationale: The models summarized here establish a role for CCBE1 in lymphatic development, while human
    CCBE1-positive cases establish variable cognitive, speech, head-size and facial findings. The clinical
    sources do not demonstrate a causal route to each finding. Intrauterine facial edema is a proposed
    explanation for facial morphology, but a direct developmental contribution has not been excluded.
    These observations therefore remain without asserted causal edges.
  notes: Recorded as HUMAN_MODEL_MISMATCH rather than KNOWLEDGE_GAP because model-system evidence for
    the lymphatic mechanism is strong; what is uncertain is how far that evidence transfers to the non-lymphatic
    arms of the human phenotype.
  evidence:
  - reference: PMID:21778431
    reference_title: CCBE1 is essential for mammalian lymphatic vascular development and enhances the
      lymphangiogenic effect of vascular endothelial growth factor-C in vivo.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Mutant mice die prenatally. Proximity ligation assays indicate that vascular endothelial
      growth factor receptor 3 activation appears unaltered in mutants.
    explanation: Prenatal lethality is the concrete reason the mouse model cannot report on the postnatal
      neurodevelopmental phenotype; the assays reported in the same sentence pair are lymphatic-signalling
      readouts, which is the only arm of the disease this model addresses.
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: We conclude thatCCBE1mutations cause a widespread and severe form of lymphatic dysplasia,
      associated with variable intellectual disabilities, unusual facial morphology and infrequently malformations.
    explanation: This conclusion concerns CCBE1-positive patients and establishes the human nonlymphatic
      findings whose mechanisms remain unresolved.
- discussion_id: gap_ccbe1_domain_genotype_phenotype
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Does the domain in which a CCBE1 variant falls predict clinical severity in
    HKLLS1?
  attaches_to:
  - genetic#CCBE1
  - pathophysiology#CCBE1 loss of function
  rationale: >-
    The domain-deletion knock-in series shows a clear functional hierarchy -
    collagen-repeat loss phenocopies the null while EGF-domain loss still
    permits rudimentary lymphatics - and the same ordering held for two Hennekam
    patient mutations, one per domain. Two alleles is a demonstration, not a
    predictor. With roughly a dozen reported CCBE1 variants across about
    seventeen probands and no systematic functional assay applied across them,
    it is not established whether domain position explains the wide clinical
    variability, which ranges from recurrent prenatal hydrops to a child with
    imaging-detected lymphangiectasia and no measurable protein loss.
  evidence:
  - reference: PMID:25814692
    reference_title: "Functional Dissection of the CCBE1 Protein: A Crucial Requirement for the Collagen Repeat Domain."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Furthermore, a Hennekam syndrome mutation within the collagen domain has
      a stronger effect than a Hennekam syndrome mutation within the EGF
      domain.
    explanation: >-
      Establishes the domain-dependent severity signal that this gap asks
      whether to generalize; it rests on two patient alleles.
  - reference: PMID:30564329
    reference_title: "Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema syndrome-1."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      at least 12 different CCBE1 variants have been reported in at least
      seventeen probands
    explanation: >-
      Bounds the size of the reported allelic series, which is what makes a
      genotype-phenotype rule unestablished rather than merely uncurated.
- discussion_id: gap_hennekam_unexplained_molecular_fraction
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    What fraction of clinically diagnosed Hennekam syndrome remains molecularly
    unexplained after CCBE1, FAT4 and ADAMTS3 testing, and how many further
    genes account for it?
  attaches_to:
  - genetic#CCBE1
  - prevalence#Worldwide
  rationale: >-
    CCBE1 explains only about a quarter of clinically diagnosed Hennekam
    syndrome. FAT4 and ADAMTS3 account for further subsets, but a substantial
    residue has no molecular diagnosis, and additional candidate genes have been
    proposed on single families. This directly limits the epidemiology of
    HKLLS1: published case counts are for the clinical syndrome rather than for
    molecularly confirmed CCBE1 disease, so no prevalence figure specific to
    HKLLS1 can be derived from them.
  evidence:
  - reference: PMID:24913602
    reference_title: "Hennekam syndrome can be caused by FAT4 mutations and be allelic to Van Maldergem syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      It can be caused by mutations in CCBE1 which are found in approximately
      25 % of cases.
    explanation: >-
      Quantifies the unexplained fraction that motivates this gap.
  - reference: PMID:28985353
    reference_title: "Loss of ADAMTS3 activity causes Hennekam lymphangiectasia-lymphedema syndrome 3."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      They account for about one third of all primary lymphedema cases,
      underscoring the existence of additional genetic factors.
    explanation: >-
      Documents that the known primary-lymphedema gene set leaves most cases
      unexplained, the wider context in which the Hennekam residue sits.
- discussion_id: gap_ccbe1_subtype_phenotype_attribution
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: Which historical Hennekam findings are established specifically in CCBE1 disease?
  attaches_to:
  - genetic#CCBE1
  rationale: The original 1989 family was later shown to have FAT4 disease. Its narrow mouth, dental and
    ear findings cannot alone establish those phenotypes in HKLLS1, and a general statement that ADAMTS3
    cases have similar faces does not supply CCBE1-specific observations. The Lee rehabilitation patient
    with pulmonary lymphangiectasia had no genetic testing. These findings are omitted pending direct
    subtype support. Total lymphopenia is a recognized consequence of intestinal lymph loss generally,
    but the reviewed CCBE1 reports document normal total lymphocyte counts or selected lymphocyte subsets
    rather than a confirmed low total count. This remains a possible complication rather than an observed
    subtype phenotype. Seizures were absent in the 13 CCBE1-positive cohort; this does not establish absolute
    exclusion.
  evidence:
  - reference: DOI:10.1159/000342486
    reference_title: Evaluation of Clinical Manifestations in Patients with Severe Lymphedema with and
      without CCBE1 Mutations
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: Seizures did not occur in the group of molecularly confirmed HS patients, but were found
      in 6 of the HS cases without molecular confirmation.
    explanation: The genotype-stratified comparison shows why historical Hennekam seizure estimates should
      not be imported into HKLLS1.
- discussion_id: gap_ccbe1_additional_findings_mechanisms
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: What mechanisms explain the associated thyroid, hernia and fetal structural findings?
  attaches_to:
  - phenotypes#Hypothyroidism
  - phenotypes#Inguinal hernia
  - phenotypes#Umbilical hernia
  - phenotypes#Cystic hygroma
  - phenotypes#Pulmonary hypoplasia
  - phenotypes#Metopic synostosis
  - phenotypes#Congenital diaphragmatic hernia
  rationale: The clinical reports document these findings without establishing their causal routes. The
    C174Y siblings had hypothyroidism and hernias; whether these reflect lymphatic disease or additional
    developmental effects is unresolved. Cystic hygroma and pulmonary hypoplasia were observed in an ungenotyped
    fetus from the CCBE1 family, so both individual genetic attribution and a detailed mechanism remain
    uncertain. Causal edges are withheld. Metopic synostosis and diaphragmatic hernia were isolated CCBE1-positive
    cohort findings without a demonstrated causal route. Growth hormone deficiency was also named in one
    cohort case, but the narrative does not supply the testing needed to choose a specific HPO binding;
    this is retained as a curation lead. Isolated neoplasms and uncertain polydactyly are not generalized
    into established CCBE1 disease associations.
  evidence:
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: In adulthood, both P1 and P2 developed hypothyroidism treated with levothyroxine.
    explanation: Documents the thyroid phenotype without establishing its mechanism.
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: An autopsy described a female fetus with a cystic hygroma, serous (nonchylous) pleural and
      pericardial effusions, congested liver, hypoplastic lungs, and a “markedly enlarged, hyperplastic
      thyroid gland”.
    explanation: The fetal structural findings were observed in the second pregnancy, which could not
      be genotyped.
- discussion_id: gap_ccbe1_compartment_specific_fluid_mechanisms
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: Which drainage, leakage and oncotic mechanisms produce pleural effusions and ascites in CCBE1 disease?
  attaches_to:
  - pathophysiology#Generalized lymphatic dysplasia
  - phenotypes#Pleural effusion
  - phenotypes#Ascites
  - phenotypes#Chylous ascites
  rationale: CCBE1-positive patients have pleural effusions and fetal ascites, and other confirmed
    patients have recurrent chylous ascites. These observations establish the clinical associations
    but do not resolve the compartment-specific routes of fluid accumulation. Impaired drainage,
    lymphatic leakage and reduced oncotic pressure may contribute differently. The ultrasound of
    the confirmed fetus did not characterize the fluid, and the sibling report did not locate a
    peritoneal leak. These passages do not themselves attribute each effusion to dysplasia as a causal
    claim, so those arrows are withheld. Unknown intermediates alone would not preclude an indirectly
    supported causal edge; the missing evidence here is for the compartment-specific causal attribution.
  evidence:
  - reference: PMID:30564329
    reference_title: Novel mutation in CCBE 1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema
      syndrome-1.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Ultrasound images of hydropic fetus from G4 pregnancy. A, Fetal chest demonstrating pleural
      effusions; B, fetal head demonstrating scalp edema; C, fetal abdomen demonstrating ascites
    explanation: The confirmed fetus had pleural effusions and ascites; ultrasound establishes their
      presence rather than the mechanism or composition of the fluid.
  - reference: PMID:26686525
    reference_title: A Multiplex Kindred with Hennekam Syndrome due to Homozygosity for a CCBE1 Mutation
      that does not Prevent Protein Expression.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We report two HS siblings, born to consanguineous parents of Turkish ancestry, whose clinical
      phenotype also includes protein losing enteropathy, painful relapsing chylous ascites, and hypogammaglobulinemia.
    explanation: The molecularly confirmed siblings establish chylous ascites, but this co-occurrence
      does not identify the anatomical route of lymph leakage or exclude other contributors.
clinical_trials: []
datasets: []

📚

References & Deep Research

Deep Research

2

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (2)

Augment: Hennekam lymphangiectasia-lymphedema syndrome 1 (CCBE1) · 2026-09-02T06:28:14Z · View source

Augmentation of the existing CCBE1 HKLLS1 entry (created 2026-08-02, PR #7834); no new entry was created. DEEP RESEARCH. falcon was requested per the /curate default and is UNREACHABLE in this environment (403 ProviderAuthError; EDISON_API_KEY present but rejected). Rather than substituting a provider by hand, the run was re-issued with 'just dr_fallback=--fallback research-disorder falcon ...' so the substitution is recorded by the run itself. openscientist produced the report: research/Hennekam_Lymphangiectasia-Lymphedema_Syndrome_1-deep-research-openscientist.md, frontmatter fell_back: true, requested_provider: falcon, provider_attempts listing the falcon 403. Its validation is clean: 37/37 references resolved, confabulation_rate 0.0, 14/14 quoted claims found in source, 0 off topic. Term validation 35/37 resolved, 0 named as a different term; the two 'worth a second look' entries are benign (HP:0001888 called 'Lymphopenia', which is how this entry already uses it as preferred_term against the canonical label 'Decreased total lymphocyte count'; and a paraphrase of GO:0004175). ORPHA has no resolver and was not checked either way. The recipe renamed the falcon output path to the openscientist name, which displaced the previously committed 2026-08-02 falcon report; that report was restored from origin/main, so the openscientist report is additive and both are committed. The falcon report gained only a template_sha stamp. GENEREVIEWS. Searched PubMed for a GeneReviews chapter ('Hennekam syndrome GeneReviews[All Fields]' and 'lymphedema GeneReviews[TI]'); no article exists for this disorder, so no GeneReviews baseline was available. Recorded per Step 3b rather than left silent. WHAT WAS ADDED. - classifications: harrisons_chapter GENETICS_ENVIRONMENT_DISEASE + CARDIOVASCULAR, each with a recorded reason. - animal_models (new section, 3 models, all with modeled_mechanisms + readouts): Ccbe1-null mouse (PMID:21778431), ccbe1 mutant zebrafish (PMID:19287381), CCBE1 domain-deletion knock-in mice (PMID:25814692). These grounded the mechanism chain that previously had no model layer. - Evidence added to 6 previously uncited pathophysiology downstream causal edges. - treatments: immunoglobulin replacement (NCIT:C62710 + NCIT:C80829) and octreotide for refractory PLE (CHEBI:7726), both with target_mechanisms links carrying their own evidence. - clinical_burden (HIGH), progression (3 phases), discussions (1 HUMAN_MODEL_MISMATCH + 2 KNOWLEDGE_GAP). - Evidence added to both differential_diagnoses and to the Hypoalbuminemia and Lymphopenia phenotypes. - DOI:10.1161/CIRCRESAHA.116.304949 upgraded to PMID:25814692. The entry now has zero DOI-keyed evidence items, so every snippet is checked by the gating validator. RED-TEAM PASS. A subagent review against dismech-pr-review found 11 issues; all substantive ones were fixed before commit. Most important: PMID:18294365 (Vignes, primary intestinal lymphangiectasia / Waldmann disease) had been used as the SOLE evidence on the Hypoalbuminemia and Lymphopenia phenotypes, and that review explicitly lists Hennekam syndrome as a SEPARATE entity from its own subject - a wrong-disease citation. Replaced with on-disease evidence: PMID:32629717 (homozygous CCBE1 c.472C>T infant with documented hypoproteinemia, hypoalbuminemia, hypogammaglobulinemia) for Hypoalbuminemia, and PMID:29560340 for Lymphopenia. No source reporting lymphocyte counts in a molecularly confirmed HKLLS1 patient could be found (PubMed searched), so the lymphopenia item is graded INDIRECT and its explanation says so. The PIL review is retained only as a secondary shared-mechanism citation whose explanation now names the disease boundary. Also fixed: 4 PIL causal-edge items regraded DIRECT -> INDIRECT for consistency with the same inference step graded INDIRECT elsewhere; a clinical_burden rationale sentence claiming the three syndrome arms co-occur in all reported families, contradicted by this entry's own material (rewritten to argue severity, not universality); an uncited '33% of patients have seizures' figure in progression (now cited to PMID:29560340); a Ccbe1-null model evidence item that quoted the paper's RATIONALE gap statement ('mouse models for the disease do not exist') rather than a finding; a corneal-micropocket readout filed as a measurement of the null mouse when it is an exogenous recombinant-protein assay (demoted to link-level evidence with the provenance stated); fidelity HIGH -> MODERATE on that link because the same paper reports VEGFR-3 activation unaltered in mutants; an uncited claim that the zebrafish is the standard rescue-testing model; a differential-diagnosis overclaim that HKLLS1 and HKLLS2 are clinically indistinguishable; and evidence_source IN_VITRO -> OTHER on an attributive sentence. An unsourced allele nickname ('full of fluid') was removed from the zebrafish model name because it is not in the cached abstract. DELIBERATELY NOT DONE. - datasets stays empty. 'just discover-datasets' returned only 2 GENE_ONLY candidates (GSE307112, GSE307351), both alveolar stem-niche studies matching on CCBE1 alone and unrelated to lymphatic disease - the Named Entity Confusion case CLAUDE.md warns about. Both deep-research reports also state no disease-specific omics signature exists. - No conforms_to: there is no lymphangiogenesis or lymphatic-development module in kb/modules/. - classifications.harrisons_chapter carries no evidence items; a nosology assignment has no citable finding, and the exemplar entries do not carry one either. VALIDATION. just validate-disorders (schema + terms + references, the CI gate) passed. Snippets 99/99 verified, and 99/99 again with --unskip-prefix DOI. check-entity-refs, check-causal-targets, check-enum-values, check-duplicate-keys, check-qualifier-terms, check-qualifier-terms-online, check-snippet-length, check-snippet-grading, check-title-snippets, check-folded-hyphens, check-term-cache-integrity, check-cache-order all OK with no new findings. pytest 10620 passed (model-readout, entity-ref, unique-name and treatment selectors). Compliance rose 92.3% -> 98.6% (weighted 92.8% -> 99.1%). Two term errors were caught by the validator during curation and fixed: NCIT:C594 is Iodine (not Immunoglobulin) and NCIT:C572 is not in the ChemicalEntityTerm enum.

Create: Hennekam lymphangiectasia-lymphedema syndrome 1 CCBE1 · 2026-08-02T01:41:07Z · View source

De novo curation of CCBE1-related Hennekam syndrome MONDO:0009337, OMIM 235510. Deep research via Edison falcon provider. Curated the CCBE1/ADAMTS3/pro-VEGF-C activation/VEGFR-3 lymphangiogenesis mechanism, generalized lymphatic dysplasia, intestinal lymphangiectasia with protein-losing enteropathy, lymphedema, flat facial gestalt, intellectual disability, and severe prenatal nonimmune hydrops presentation. Evidence from primary literature: Alders 2009 PMID:19935664, Hennekam 1989 PMID:2624276, Jeltsch 2014 PMID:24552833, Le Guen 2014 PMID:24523457, Roukens 2015, Melber 2018 hydrops. Validated with just validate; schema, terms and references all pass; DOI snippets manually confirmed as exact substrings.

Falcon ▸

Disease Characteristics Research Template

Edison Scientific Literature 18 citations 2026-08-02T01:24:54.842911

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on: 1. Key concepts and definitions with current understanding 2. Recent developments and latest research (prioritize 2023-2024 sources) 3. Current applications and real-world implementations 4. Expert opinions and analysis from authoritative sources 5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available. Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Disease Characteristics Research Template

Target Disease

Disease Name: Hennekam lymphangiectasia-lymphedema syndrome 1
MONDO ID: (if available)
Category: Mendelian

Research Objectives

Please provide a comprehensive research report on Hennekam lymphangiectasia-lymphedema syndrome 1 covering all of the disease characteristics listed below. This report will be used to populate a disease knowledge base entry. Be thorough and cite primary literature (PMID preferred) for all claims.

For each section, suggested databases/resources are listed. These are the first places you should search for information on each topic.

1. Disease Information

Search first: OMIM, Orphanet, ICD-10/ICD-11, MeSH, PubMed

What is the disease? Provide a concise overview.
What are the key identifiers? (OMIM, Orphanet, ICD-10/ICD-11, MeSH, Mondo)
What are the common synonyms and alternative names?
Is the information derived from individual patients (e.g., EHR) or aggregated disease-level resources?

2. Etiology

Disease Causal Factors: What are the primary causes? (genetic, environmental, infectious, mechanistic)
Risk Factors:

Search first: PubMed, Cochrane Library, UpToDate, clinical guidelines, ClinVar, ClinGen, GWAS Catalog, PheGenI, CTD, CDC, WHO, epidemiological databases
Genetic risk factors (causal variants, susceptibility loci, modifier genes)
Environmental risk factors (toxins, lifestyle, occupational exposures, age, sex, family history)
Protective Factors:

Search first: PubMed, Cochrane Library, clinical trial databases, GWAS Catalog, gnomAD, WHO, CDC, nutrition databases
Genetic protective factors (protective variants, modifier alleles)
Environmental protective factors (diet, lifestyle, exposures that reduce risk)
Gene-Environment Interactions: How do genetic and environmental factors interact to influence disease?

Search first: CTD, PubMed, PheGenI, GxE databases

3. Phenotypes

Search first: HPO (Human Phenotype Ontology), OMIM, Orphanet, PubMed, clinicaltrials.gov, MedDRA, SNOMED CT, DECIPHER, LOINC

For each phenotype, provide: - Phenotype type: symptoms, clinical signs, physical manifestations, behavioral changes, or laboratory abnormalities

For symptoms/signs: HPO, OMIM, Orphanet, PubMed For behavioral changes: HPO, DSM, RDoC (Research Domain Criteria), PubMed For laboratory abnormalities: LOINC, SNOMED CT, LabTests Online, PubMed - Phenotype characteristics: Search first: OMIM, Orphanet, HPO, PubMed - Age of symptom onset (neonatal, childhood, adult-onset, late-onset) - Symptom severity (mild, moderate, severe, variable) - Symptom progression (stable, progressive, episodic, fluctuating) - Frequency among affected individuals (percentage or qualitative) - Quality of life impact: Effects on daily functioning and well-being (per-phenotype when possible) Search first: EQ-5D database, SF-36, WHO QOL databases, PubMed - Suggest HPO (Human Phenotype Ontology) terms for each phenotype

4. Genetic/Molecular Information

Causal Genes: Gene mutations or chromosomal abnormalities responsible for disease (gene symbols, OMIM IDs)

Search first: OMIM, ClinVar, HGMD, Ensembl, NCBI Gene
Pathogenic Variants:
Affected genes (gene symbols, HGNC IDs) > Search first: OMIM, NCBI Gene, Ensembl, HGNC, UniProt, GeneCards
Variant classification (pathogenic, likely pathogenic, VUS per ACMG/AMP guidelines) > Search first: ClinVar, ClinGen, ACMG/AMP guidelines, VarSome
Variant type/class (missense, frameshift, nonsense, splice-site, structural)
Allele frequency in population databases > Search first: gnomAD, 1000 Genomes, ExAC, TOPMed, dbSNP
Somatic vs germline origin > Search first: COSMIC (somatic), ClinVar, ICGC, TCGA
Functional consequences (loss of function, gain of function, dominant negative)
Modifier Genes: Genes that modify disease severity or expression
Epigenetic Information: DNA methylation, histone modifications, chromatin changes affecting disease

Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth
Chromosomal Abnormalities: Large-scale genetic changes (aneuploidy, translocations, inversions)

Search first: DECIPHER, ClinVar, ECARUCA, UCSC Genome Browser

5. Environmental Information

Environmental Factors: Non-genetic contributing factors (toxins, radiation, pollution, occupational exposure)

Search first: CTD (Comparative Toxicogenomics Database), TOXNET, PubMed, EPA databases
Lifestyle Factors: Behavioral factors (smoking, diet, exercise, alcohol consumption)

Search first: CDC databases, WHO, PubMed, NHANES
Infectious Agents: If applicable, pathogens causing or triggering disease (bacteria, viruses, fungi, parasites)

Search first: NCBI Taxonomy, ViPR, BV-BRC, MicrobeDB, GIDEON

6. Mechanism / Pathophysiology

Molecular Pathways: Specific signaling cascades or biochemical pathways involved (Wnt, MAPK, mTOR, PI3K-AKT, etc.)

Search first: KEGG, Reactome, WikiPathways, PathBank, BioCyc
Cellular Processes: Cell-level mechanisms (apoptosis, autophagy, cell cycle dysregulation, inflammation, etc.)

Search first: Gene Ontology (GO), Reactome, KEGG, PubMed
Protein Dysfunction: How protein structure or function is altered (misfolding, aggregation, loss of function, gain of function)

Search first: UniProt, PDB (Protein Data Bank), InterPro, Pfam, AlphaFold
Metabolic Changes: Alterations in metabolic processes (energy metabolism, lipid metabolism, amino acid metabolism)

Search first: KEGG, BioCyc, HMDB (Human Metabolome Database), BRENDA
Immune System Involvement: Role of immune response (autoimmunity, immunodeficiency, chronic inflammation)

Search first: ImmPort, Immunome Database, IEDB, Gene Ontology
Tissue Damage Mechanisms: How tissues/ are injured (oxidative stress, ischemia, fibrosis, necrosis)

Search first: PubMed, Gene Ontology, Reactome
Biochemical Abnormalities: Specific molecular defects (enzyme deficiencies, receptor dysfunction, ion channel defects)

Search first: BRENDA, UniProt, KEGG, OMIM, PubMed
Epigenetic Changes: DNA methylation, histone modifications affecting gene expression in disease

Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth
Molecular Profiling (if available):
Transcriptomics/gene expression changes > Search first: GEO (Gene Expression Omnibus), ArrayExpress, GTEx, Human Cell Atlas, SRA
Proteomics findings > Search first: PRIDE, ProteomeXchange, Human Protein Atlas, STRING, BioGRID
Metabolomics signatures > Search first: MetaboLights, Metabolomics Workbench, HMDB, METLIN
Lipidomics alterations > Search first: LIPID MAPS, SwissLipids, LipidHome, Metabolomics Workbench
Genomic structural features > Search first: UCSC Genome Browser, Ensembl, NCBI, dbVar, DGV
Advanced Technologies (if applicable):
Single-cell analysis findings (cell-type specific mechanisms, cellular heterogeneity) > Search first: Human Cell Atlas, Single Cell Portal, GEO, CELLxGENE
Spatial transcriptomics findings > Search first: GEO, Spatial Research, Vizgen, 10x Genomics data
Multi-omics integration results > Search first: TCGA, ICGC, cBioPortal, LinkedOmics, PubMed
Functional genomics screens (CRISPR, RNAi) > Search first: DepMap, GenomeRNAi, PubMed, BioGRID ORCS

For each mechanism, describe: - The causal chain from initial trigger to clinical manifestation - Which mechanisms are upstream vs downstream - What cell types and biological processes are involved - Suggest GO terms for biological processes and CL terms for cell types

7. Anatomical Structures Affected

Organ Level:
Primary organs directly affected
Secondary organ involvement (complications, secondary effects)
Body systems involved (cardiovascular, nervous, digestive, respiratory, endocrine, etc.)

Search first: Uberon, FMA (Foundational Model of Anatomy), OMIM, HPO, ICD-11, MeSH, SNOMED CT
Tissue and Cell Level:
Specific tissue types affected (epithelial, connective, muscle, nervous)
Specific cell populations targeted (with Cell Ontology terms)

Search first: Uberon, Human Protein Atlas, Cell Ontology, Human Cell Atlas, CellMarker, PanglaoDB
Subcellular Level:
Cellular compartments involved (mitochondria, nucleus, ER, lysosomes) (with GO Cellular Component terms)

Search first: Gene Ontology (Cellular Component), UniProt, Human Protein Atlas
Localization:
Specific anatomical sites (with UBERON terms) > Search first: FMA, Uberon, NeuroNames (for brain), SNOMED CT
Lateralization (unilateral, bilateral, asymmetric) > Search first: HPO, clinical literature, imaging databases

8. Temporal Development

Onset:
Typical age of onset (congenital, pediatric, adult, geriatric)
Onset pattern (acute, subacute, chronic, insidious)

Search first: OMIM, Orphanet, HPO, PubMed
Progression:
Disease stages (early, intermediate, advanced, end-stage) > Search first: Cancer Staging Manual (AJCC), WHO classifications, PubMed
Progression rate (rapid, slow, variable)
Disease course pattern (episodic, relapsing-remitting, progressive, stable)
Disease duration (self-limited, chronic lifelong)

Search first: Disease registries, longitudinal cohort databases, natural history studies, PubMed, Orphanet, OMIM
Patterns:
Remission patterns (spontaneous, treatment-induced) > Search first: Clinical trial databases, disease registries, PubMed
Critical periods (time windows of vulnerability or opportunity for intervention) > Search first: PubMed, developmental biology databases, clinical guidelines

9. Inheritance and Population

Epidemiology:
Prevalence (cases per 100,000 at given time)
Incidence (new cases per 100,000 per year)

Search first: Orphanet, CDC, WHO, GBD (Global Burden of Disease), national registries, SEER, disease registries
For Genetic Etiology:
Inheritance pattern (AD, AR, X-linked, mitochondrial, multifactorial, polygenic) > Search first: OMIM, Orphanet, ClinVar, GTR (Genetic Testing Registry)
Penetrance (complete, incomplete, age-dependent) > Search first: ClinVar, OMIM, PubMed, ClinGen
Expressivity (variable, consistent) > Search first: OMIM, ClinVar, PubMed
Genetic anticipation (increasing severity in successive generations) > Search first: OMIM, PubMed (especially for repeat expansion disorders)
Germline mosaicism > Search first: ClinVar, OMIM, genetic counseling literature, PubMed
Founder effects (population-specific mutations) > Search first: gnomAD, population genetics databases, PubMed
Consanguinity role > Search first: OMIM, population studies, genetic counseling resources
Carrier frequency > Search first: gnomAD, carrier screening databases, GeneReviews, GTR
Population Demographics:
Affected populations (ethnic or demographic groups with higher prevalence) > Search first: gnomAD, 1000 Genomes, PAGE Study, PubMed, population registries
Geographic distribution (endemic areas, regional variation) > Search first: WHO, CDC, GBD, Orphanet, geographic epidemiology databases
Geographic distribution of specific variants
Sex ratio (male:female) > Search first: Disease registries, OMIM, PubMed, epidemiological databases
Age distribution of affected individuals > Search first: CDC, disease registries, SEER, Orphanet

10. Diagnostics

Clinical Tests:
Laboratory tests (blood, urine, tissue chemistry, specific enzyme assays) > Search first: LOINC, LabTests Online, PubMed
Biomarkers (proteins, metabolites, genetic markers, circulating biomarkers) > Search first: FDA Biomarker List, BEST (Biomarkers, EndpointS, and other Tools), PubMed
Imaging studies (X-ray, CT, MRI, PET, ultrasound) > Search first: RadLex, DICOM, Radiopaedia, imaging databases
Functional tests (pulmonary function, cardiac stress tests) > Search first: LOINC, clinical guidelines, PubMed
Electrophysiology (EEG, EMG, ECG, nerve conduction studies) > Search first: LOINC, clinical neurophysiology databases, PubMed
Biopsy findings (histopathology, immunohistochemistry) > Search first: SNOMED CT, College of American Pathologists resources, PubMed
Pathology findings (microscopic examination) > Search first: SNOMED CT, Digital Pathology databases, PubMed
Genetic Testing:

Search first: GTR (Genetic Testing Registry), GeneReviews, ClinGen
Overview of recommended genetic testing approach
Whole genome sequencing (WGS) utility > Search first: GTR, ClinVar, GEL (Genomics England), gnomAD
Whole exome sequencing (WES) utility > Search first: GTR, ClinVar, OMIM, GeneMatcher
Gene panels (which panels, which genes) > Search first: GTR, ClinVar, laboratory-specific databases
Single gene testing > Search first: GTR, ClinVar, OMIM, GeneReviews
Chromosomal microarray (CMA) > Search first: DECIPHER, ClinVar, dbVar, ECARUCA
Karyotyping > Search first: Chromosome Abnormality Database, ClinVar, cytogenetics resources
FISH > Search first: ClinVar, cytogenetics databases, PubMed
Mitochondrial DNA testing > Search first: MITOMAP, MSeqDR, ClinVar, GTR
Repeat expansion testing > Search first: GTR, ClinVar, repeat expansion databases, PubMed
Omics-Based Diagnostics (if applicable):
RNA sequencing / transcriptomics > Search first: GEO, ArrayExpress, GTEx, RNA-seq databases
Proteomics > Search first: PRIDE, ProteomeXchange, FDA Biomarker database
Metabolomics > Search first: MetaboLights, Metabolomics Workbench, HMDB
Epigenomics > Search first: GEO, ENCODE, Roadmap Epigenomics, MethBase
Liquid biopsy > Search first: COSMIC, ClinVar, liquid biopsy databases, PubMed
Clinical Criteria:
Standardized diagnostic criteria (DSM, ICD, society guidelines) > Search first: DSM-5, ICD-11, clinical society guidelines, UpToDate
Differential diagnosis (other conditions to rule out, with distinguishing features) > Search first: DynaMed, UpToDate, clinical decision support systems
Screening:
Screening methods for asymptomatic individuals (newborn screening, carrier screening, cascade screening) > Search first: ACMG recommendations, CDC newborn screening, GTR

11. Outcome/Prognosis

Survival and Mortality:
Survival rate (5-year, 10-year, overall) > Search first: SEER, cancer registries, disease-specific registries, PubMed
Life expectancy (with and without treatment if applicable) > Search first: Orphanet, disease registries, actuarial databases, PubMed
Mortality rate > Search first: CDC, WHO, GBD, national mortality databases
Disease-specific mortality (deaths directly attributable to disease) > Search first: Disease registries, CDC Wonder, GBD, PubMed
Morbidity and Function:
Morbidity (disease-related disability and health impacts) > Search first: GBD, WHO, disability databases, PubMed
Disability outcomes (long-term functional impairments) > Search first: ICF (International Classification of Functioning), disability registries
Quality of life measures (EQ-5D, SF-36, PROMIS, disease-specific tools) > Search first: EQ-5D database, SF-36, PROMIS, PubMed
Disease Course:
Complications (secondary problems: infections, organ failure, etc.) > Search first: ICD codes, disease registries, clinical databases, PubMed
Recovery potential (likelihood and extent of recovery, with vs without treatment) > Search first: Natural history studies, rehabilitation databases, PubMed
Prediction:
Prognostic factors (age, disease severity, biomarkers, treatment response) > Search first: Prognostic models databases, clinical calculators, PubMed
Prognostic biomarkers (molecular markers predicting disease course) > Search first: FDA Biomarker database, PubMed, cancer prognostic databases

12. Treatment

Pharmacotherapy:
Pharmacological treatments (drug names, drug classes, mechanisms of action) > Search first: DrugBank, RxNorm, ATC classification, DailyMed, FDA databases
Pharmacogenomics (how genetic variants affect drug metabolism, efficacy, toxicity) > Search first: PharmGKB, CPIC (Clinical Pharmacogenetics), FDA Table of PGx Biomarkers
Advanced Therapeutics:
Gene therapy (viral vectors, CRISPR, gene replacement, gene editing) > Search first: ClinicalTrials.gov, FDA gene therapy database, ASGCT resources
Cell therapy (stem cell transplant, CAR-T, cellular therapeutics) > Search first: ClinicalTrials.gov, FDA cell therapy database, FACT standards
RNA-based therapies (ASOs, siRNA, mRNA therapies) > Search first: ClinicalTrials.gov, FDA approvals, PubMed
Targeted therapies (treatments directed at specific molecular targets) > Search first: My Cancer Genome, OncoKB, ClinicalTrials.gov, FDA approvals
Immunotherapies (checkpoint inhibitors, monoclonal antibodies) > Search first: Cancer Immunotherapy Database, FDA approvals, ClinicalTrials.gov
Surgical and Interventional:
Surgical interventions (types of surgery, timing, outcomes) > Search first: CPT codes, surgical registries, clinical guidelines, PubMed
Supportive and Rehabilitative:
Supportive care (symptom management, pain control, nutrition) > Search first: Clinical guidelines, Cochrane Library, PubMed
Rehabilitation (physical therapy, occupational therapy, speech therapy) > Search first: Rehabilitation medicine databases, clinical guidelines, PubMed
Experimental:
Experimental treatments in clinical trials (with NCT identifiers if available) > Search first: ClinicalTrials.gov, EU Clinical Trials Register, WHO ICTRP
Treatment Outcomes:
Treatment response rates > Search first: Clinical trial databases, FDA reviews, systematic reviews, PubMed
Side effects and adverse events > Search first: FDA Adverse Event Reporting System (FAERS), MedWatch, PubMed
Treatment Strategy:
Treatment algorithms (clinical pathways, decision trees) > Search first: Clinical practice guidelines, NCCN Guidelines, UpToDate
Combination therapies > Search first: ClinicalTrials.gov, treatment guidelines, PubMed
Personalized medicine approaches (genotype-guided treatment) > Search first: My Cancer Genome, CIViC, PharmGKB, precision medicine databases

For each treatment, suggest NCIT (NCI Thesaurus) clinical-intervention terms where applicable.

13. Prevention

Prevention Levels:
Primary prevention (preventing disease occurrence: vaccination, risk factor modification) > Search first: CDC, WHO, USPSTF recommendations, Cochrane Library
Secondary prevention (early detection and treatment: screening programs, early intervention) > Search first: USPSTF, CDC screening guidelines, WHO
Tertiary prevention (preventing complications in those with disease) > Search first: Clinical guidelines, disease management protocols, PubMed
Immunization: Vaccine strategies (if applicable)

Search first: CDC vaccine schedules, WHO immunization, FDA vaccine database
Screening and Early Detection:
Screening programs (population-based: newborn screening, cancer screening) > Search first: CDC screening programs, USPSTF, cancer screening databases
Genetic screening (carrier screening, preimplantation genetic diagnosis, prenatal testing) > Search first: ACMG recommendations, ACOG guidelines, GTR
Risk stratification (identifying high-risk individuals for targeted prevention) > Search first: Risk prediction models, clinical calculators, PubMed
Behavioral Interventions: Lifestyle modifications to reduce risk

Search first: CDC, WHO, behavioral intervention databases, Cochrane Library
Counseling: Genetic counseling (risk assessment, family planning guidance)

Search first: NSGC resources, ACMG guidelines, GeneReviews
Public Health:
Public health interventions (sanitation, vector control, health education) > Search first: CDC, WHO, public health databases, PubMed
Environmental interventions (reducing environmental risk factors) > Search first: EPA databases, WHO environmental health, PubMed
Prophylaxis: Preventive medications or procedures

Search first: Clinical guidelines, FDA approvals, PubMed

14. Other Species / Natural Disease

Taxonomy: Species affected (with NCBI Taxon identifiers)

Search first: NCBI Taxonomy
Breed: Specific breeds affected (with VBO identifiers if applicable)

Search first: VBO (Vertebrate Breed Ontology)
Gene: Orthologous genes in other species (with NCBI Gene IDs)

Search first: NCBI Gene
Natural Disease:
Naturally occurring disease in other species (companion animals, wildlife) > Search first: OMIA (Online Mendelian Inheritance in Animals), VetCompass, PubMed
Veterinary relevance and importance in animal health > Search first: OMIA, veterinary databases, PubMed
Comparative Biology:
Comparative pathology (similarities and differences across species) > Search first: OMIA, comparative pathology databases, PubMed
Evolutionary conservation of disease mechanisms > Search first: HomoloGene, OrthoMCL, Alliance of Genome Resources
Transmission (if applicable):
Zoonotic potential > Search first: CDC zoonotic diseases, WHO zoonoses, GIDEON
Cross-species susceptibility > Search first: NCBI Taxonomy, veterinary databases, PubMed

15. Model Organisms

Model Types:
Model organism type (mammalian, invertebrate, cellular, in vitro) > Search first: Alliance of Genome Resources, model organism databases
Specific model systems (mouse, rat, zebrafish, Drosophila, C. elegans, yeast, cell lines, organoids, iPSCs) > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, SGD, ATCC, Cellosaurus
Induced models (drug treatment, surgical intervention, environmental manipulation) > Search first: MGI, model organism databases, PubMed
Genetic Models:
Types available (knockout, knock-in, transgenic, conditional, humanized) > Search first: MGI, IMPC, KOMP, EuMMCR, IMSR
Model Characteristics:
Phenotype recapitulation (how well model reproduces human disease features) > Search first: Model organism databases, comparative studies, PubMed
Model limitations (aspects of human disease not captured) > Search first: Model organism databases, PubMed, review articles
Applications:
Research applications (what aspects of disease can be studied) > Search first: Model organism databases, PubMed
Resources:
Model databases > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, IMSR, EMMA, MMRRC

Citation Requirements

Cite primary literature (PMID preferred) for all mechanistic and clinical claims
Prioritize recent reviews and landmark papers
Include direct quotes from abstracts where possible to support key statements
Distinguish evidence source types: human clinical, model organism, in vitro, computational

Output Format

Structure your response as a comprehensive narrative organized by the sections above. For each section, provide: - Factual content with specific details (numbers, percentages, gene names, variant nomenclature) - Ontology term suggestions (HPO, GO, CL, UBERON, CHEBI, NCIT, MONDO) where applicable - Evidence citations with PMIDs - Direct quotes from abstracts to support key claims - Clear indication when information is not available or not applicable for this disease

This report will be used to populate a disease knowledge base entry with: - Pathophysiology descriptions with causal chains - Gene/protein annotations (HGNC, GO terms) - Phenotype associations (HP terms) with frequencies - Cell type involvement (CL terms) - Anatomical locations (UBERON terms) - Chemical entities (CHEBI terms) - Treatment annotations (NCIT terms) - Evidence items with PMIDs and exact abstract quotes - Epidemiology, prognosis, diagnostic, and prevention information - Animal model descriptions with phenotype recapitulation details

Hennekam lymphangiectasia–lymphedema syndrome 1

Executive summary

Hennekam lymphangiectasia–lymphedema syndrome 1 (HKLLS1) is an ultra-rare, autosomal-recessive generalized lymphatic dysplasia caused by biallelic germline variants in CCBE1. Its defining manifestations are congenital or early-onset, often progressive lymphedema; visceral lymphangiectasia, particularly of the intestine; characteristic facial morphology; and variably impaired neurodevelopment. Severe fetal disease may present as nonimmune hydrops fetalis, pleural/pericardial effusions, pulmonary hypoplasia, and fetal death. The central molecular lesion is inadequate CCBE1-assisted, ADAMTS3-mediated activation of pro-VEGF-C, reducing VEGFR3 signaling in lymphatic endothelial cells and disrupting embryonic lymphangiogenesis. There is no curative or genotype-directed treatment; management is supportive and multidisciplinary. Published evidence remains dominated by small series and individual cases, and no HKLLS1-specific interventional trial was identified.

A structured, ontology-oriented summary is provided below.

domain	evidence-backed fact	suggested ontology terms/IDs	evidence type/limitations
Disease identity	Hennekam lymphangiectasia-lymphedema syndrome 1 is the CCBE1-related subtype of Hennekam syndrome, a generalized lymphatic dysplasia with lymphedema, lymphangiectasia, characteristic facial features, and variable intellectual/developmental impairment (roukens2015functionaldissectionof pages 1-3, lee2018hennekamsyndromea pages 1-3)	Disease: Hennekam lymphangiectasia-lymphedema syndrome 1 (MONDO: verify); Hennekam syndrome (OMIM #235510 verify subtype mapping); MeSH: verify	Human clinical + mechanistic review/context; disease identifiers should be verified in OMIM/MONDO for subtype specificity
Causal gene	Biallelic pathogenic variants in CCBE1 cause HKLLS1; CCBE1 encodes collagen and calcium-binding EGF domain-containing protein 1, a secreted extracellular matrix protein essential for lymphatic development (roukens2015functionaldissectionof pages 1-3, melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)	Gene: CCBE1 (HGNC: verify; NCBI Gene: verify); Protein: CCBE1 (UniProt: verify)	Human genetic + animal/mechanistic evidence; exact database IDs should be verified
Inheritance	The disorder is autosomal recessive; reported affected individuals carry homozygous or compound heterozygous CCBE1 variants (lee2018hennekamsyndromea pages 1-3, melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)	Inheritance: HP:0000007 Autosomal recessive inheritance	Human clinical/genetic evidence; penetrance not well quantified
Variant spectrum	Published CCBE1 disease variants include missense, frameshift, and other loss-of-function or hypomorphic alleles; one reported prenatal HKLLS1 case had compound heterozygous c.683_684insT p.Leu229fs and c.335C>T p.Thr112Ile variants (roukens2015functionaldissectionof pages 1-3, melber2018novelmutationin pages 2-4)	Sequence variant terms: SO:0001589 frameshift_variant; SO:0001583 missense_variant; ACMG class: pathogenic/likely pathogenic (verify per submission)	Human genetic evidence; allele frequencies and classification depend on current ClinVar/gnomAD review
Core edema phenotype	Edema is usually congenital and often progressive, commonly involving limbs, genitalia, and face (melber2018novelmutationin pages 1-2)	HP:0001004 Lymphedema; HP:0009826 Facial edema; HP:0010318 Edema; onset: HP:0003577 Congenital onset	Human clinical evidence; frequency across CCBE1-only cases is not robustly quantified
Lymphangiectasia phenotype	Maldevelopment of the lymphatic system preferentially affects intestines and limbs, and can also affect pleura, pericardium, kidneys, lungs, and mesentery; case-level imaging documented pulmonary and mesenteric/small-bowel lymphangiectasia (melber2018novelmutationin pages 1-2, lee2018hennekamsyndromea pages 3-5)	HP:0100769 Intestinal lymphangiectasia; HP:0005381 Pleural effusion; HP:0001698 Pericardial effusion; HP term for pulmonary lymphangiectasia: verify; UBERON small intestine UBERON:0002108; lung UBERON:0002048; pleura UBERON:0000977; pericardium UBERON:0002408; kidney UBERON:0002113	Human clinical/imaging evidence; some involved-organ phenotypes are reported across Hennekam syndrome broadly rather than CCBE1-only cohorts
Neurodevelopment	Developmental delay/intellectual disability is variable; one pediatric case showed delays in gross motor, fine motor, personal-social, and functional activities (lee2018hennekamsyndromea pages 1-3, lee2018hennekamsyndromea pages 3-5)	HP:0001263 Global developmental delay; HP:0001249 Intellectual disability	Human clinical evidence; severity spectrum broad and incompletely quantified
Facial phenotype	Characteristic face may include flat/flattened midface, broad depressed nasal bridge, hypertelorism, epicanthal folds, small or narrow mouth, and other dysmorphic features, partly attributed to prenatal facial lymphedema (betterman2020atypicalcadherinfat4 pages 1-2, lee2018hennekamsyndromea pages 1-3, melber2018novelmutationin pages 1-2)	HP:0012368 Flat face; HP:0000316 Hypertelorism; HP:0000280 Coarse face verify/not specific; HP:0000426 Depressed nasal bridge; HP:0000286 Epicanthus; HP:0000208 Small mouth	Human clinical evidence; exact HPO term mapping for each facial feature should be confirmed case-by-case
Prenatal presentation	HKLLS1 can present prenatally as recurrent nonimmune hydrops fetalis with cystic hygroma, pleural/pericardial effusions, scalp edema, ascites, hepatomegaly, and hypoplastic lungs (melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)	HP:0001789 Hydrops fetalis; HP:0000476 Scalp edema; HP:0000473 Cystic hygroma; HP:0001541 Ascites; HP:0002240 Hepatomegaly; HP:0002089 Pulmonary hypoplasia	Human prenatal case evidence; based on case report, so not all patients present this way
Pathophysiology	CCBE1 is a crucial regulator of VEGF-C/VEGFR3 signaling; it enhances proteolytic processing/activation of VEGF-C via ADAMTS3, enabling lymphatic endothelial sprouting and lymphangiogenesis (roukens2015functionaldissectionof pages 1-3, betterman2020atypicalcadherinfat4 pages 1-2)	Pathway: VEGF-C/VEGFR3 signaling pathway (Reactome/KEGG: verify); GO:0001946 lymphangiogenesis; GO:0001525 angiogenesis; GO term for VEGF-C activation/proteolytic processing: verify	Strong mechanistic support from mouse, zebrafish, and in vitro systems; human inference is biologically well supported but indirect
Cellular context	The key affected cell population is the lymphatic endothelial cell (LEC); CCBE1-dependent VEGF-C signaling is required for LEC migration/sprouting from embryonic veins and lymphatic vessel morphogenesis (roukens2015functionaldissectionof pages 1-3, betterman2020atypicalcadherinfat4 pages 1-2)	Cell Ontology: lymphatic endothelial cell (CL ID: verify); GO:0035855 megakaryocyte? not relevant; GO:0043542 endothelial cell migration verify; UBERON cardinal vein: verify	Animal/mechanistic evidence; cell ontology ID should be verified before KB ingestion
Protein/domain mechanism	CCBE1 contains EGF/calcium-binding EGF domains and collagen repeat domains; functional studies indicate collagen domains are crucial for VEGF-C activation, while EGF domains are required for full in vivo lymphangiogenic activity (roukens2015functionaldissectionof pages 1-3)	Protein domains: EGF-like domain (InterPro/Pfam: verify); collagen repeat domain (InterPro/Pfam: verify)	Mouse, zebrafish, in vitro domain-dissection evidence; not a routine clinical biomarker
Anatomy affected	Primary involvement is the lymphatic vasculature across limb soft tissues and viscera; secondary involvement may include intestine, lung, pleura, pericardium, mesentery, face, and possibly kidney (lee2018hennekamsyndromea pages 1-3, lee2018hennekamsyndromea pages 3-5, melber2018novelmutationin pages 1-2)	UBERON lymph vessel: UBERON:0002019 verify; small intestine UBERON:0002108; mesentery UBERON:0002385; lung UBERON:0002048; face UBERON:0001684; limb UBERON:0002101/0002102 verify	Human imaging/clinical evidence; organ distribution heterogeneous
Diagnostic workup	Diagnosis is clinical plus imaging and molecular confirmation. Reported tests include lymphatic scan showing absent visible lymphatic flow, CT angiography for lymphedema, chest CT and abdominal ultrasound for lymphangiectasia, MRI for CNS findings, and prenatal WES after nondiagnostic karyotype/microarray/workup (lee2018hennekamsyndromea pages 3-5, melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)	NCIT: Whole Exome Sequencing (verify); LOINC/NCIT for lymphoscintigraphy, CT angiography, MRI, ultrasound: verify; HP:0000478 Abnormality of the eye? not relevant	Human clinical evidence; no universally standardized disease-specific diagnostic criteria were identified
Genetic testing strategy	WES is useful when phenotype is nonspecific, especially in prenatal hydrops; trio exome identified causal CCBE1 variants after normal karyotype, microarray, infectious, hematologic, and metabolic testing (melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)	Test strategy terms: trio exome sequencing (NCIT: verify); prenatal diagnosis (NCIT: verify)	Human prenatal case evidence; WGS/panel utility for HKLLS1 specifically not systematically studied
Differential diagnosis	Differential diagnosis includes other monogenic primary lymphedema/generalized lymphatic dysplasia disorders involving genes such as FAT4, ADAMTS3, FLT4/VEGFR3, VEGFC, PIEZO1, EPHB4, FOXC2, GATA2 and related syndromic/non-syndromic hydrops etiologies (betterman2020atypicalcadherinfat4 pages 1-2, melber2018novelmutationin pages 1-2)	Differential disease terms: primary lymphedema (MONDO/Orphanet verify); generalized lymphatic dysplasia (verify)	Mostly broader disease-context evidence; not an exhaustive formal differential list
Laboratory findings	Intestinal lymphangiectasia can cause hypoproteinemia, hypoalbuminemia, hypogammaglobulinemia, lymphopenia, and growth problems, although these were absent in one pediatric case with mesenteric lymphangiectasia (lee2018hennekamsyndromea pages 3-5)	HP:0003073 Hypoproteinemia; HP:0003075 Hypoalbuminemia; HP:0004313 Hypogammaglobulinemia; HP:0001888 Lymphopenia; HP:0001510 Growth delay	Evidence mainly from syndrome-level literature/case discussion rather than CCBE1-only cohort statistics
Treatment/supportive care	No curative therapy is established. Supportive lymphedema care includes complete decongestive therapy: manual lymph drainage, compression bandaging/garments, exercise, skin care/rehabilitation; one Hennekam case improved arm circumference after 7 sessions (lee2018hennekamsyndromea pages 1-3, lee2018hennekamsyndromea pages 3-5)	NCIT: Manual Lymphatic Drainage (verify); Compression Bandage (verify); Exercise Therapy (NCIT: verify); Physical Therapy (NCIT: C15329 verify)	Disease-specific evidence is limited to single-case rehabilitation report; larger lymphedema trial data are not HKLLS1-specific
Enteropathy management	For protein-losing enteropathy due to intestinal lymphangiectasia, recommended supportive management includes high-protein, low-fat diet with medium-chain triglyceride supplementation (lee2018hennekamsyndromea pages 3-5)	NCIT/CHEBI: Medium-Chain Triglyceride (verify); dietary management term (verify)	Syndrome-level supportive recommendation; efficacy in HKLLS1 not tested in controlled trials
Surgical/interventional care	Surgery is considered a last resort for severe lymphedema that does not respond to conservative treatment (lee2018hennekamsyndromea pages 3-5)	NCIT surgical procedure for lymphedema: verify	Supportive statement from case discussion/referenced literature; no HKLLS1 surgical outcomes series identified
Prognosis	Prognosis is variable and depends on extent of visceral lymphatic involvement; prenatal hydrops and pulmonary involvement are poor prognostic features, whereas surviving children may have chronic morbidity dominated by edema and developmental issues (lee2018hennekamsyndromea pages 3-5, melber2018novelmutationin pages 1-2)	Prognostic features: HP:0001789 Hydrops fetalis; HP:0002089 Pulmonary hypoplasia; HP:0001004 Lymphedema	Evidence from case reports and syndrome-level observations; no survival curves or life-expectancy estimates specific to HKLLS1
Prevention/genetic counseling	Primary prevention is not available for the Mendelian disease itself; secondary prevention/reproductive options include carrier testing of parents, recurrence-risk counseling for autosomal recessive inheritance, and prenatal molecular diagnosis if familial variants are known (melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)	HP:0000007 Autosomal recessive inheritance; prenatal diagnosis term (NCIT: verify); carrier testing term (NCIT: verify)	Logic follows established Mendelian counseling practice; disease-specific counseling guidelines not identified
Recent developments (2023-2024 relevance)	Recent literature continues to frame CCBE1 disease within dysregulated lymphatic VEGFR3 signaling and developmental lymphatic biology, but no 2023-2024 HKLLS1-specific therapeutic breakthrough or interventional trial was identified (betterman2020atypicalcadherinfat4 pages 1-2, NCT06327412 chunk 1)	Pathway annotations above; clinical trial terms: primary lymphedema exercise trial NCT06327412	Recent sources are mostly mechanistic or broader lymphedema-focused, not subtype-specific
Major data gaps	Major gaps include subtype-specific prevalence/incidence, natural history, penetrance/expressivity estimates, standardized diagnostic criteria, validated biomarkers, genotype-phenotype correlations across CCBE1 alleles, long-term QoL data, and controlled treatment studies specific to HKLLS1 (lee2018hennekamsyndromea pages 1-3, melber2018novelmutationin pages 1-2)	Evidence gap annotation; ontology IDs not applicable	Explicitly based on absence of robust cohort-level evidence in retrieved sources

Table: This table summarizes ontology-ready, evidence-backed facts for CCBE1-related Hennekam lymphangiectasia-lymphedema syndrome 1 only. It is designed to support knowledge-base curation while clearly flagging where identifiers or conclusions require verification or where evidence remains sparse.

1. Disease information

Definition and nomenclature

HKLLS1 is the CCBE1-related subtype of Hennekam syndrome. The broader syndrome is characterized by congenital lymphedema and lymphangiectasia, unusual facial morphology—at least partly attributed to intrauterine facial edema—and variable intellectual disability. The disease was first clinically delineated in 1989; CCBE1 was established as a cause in 2009. A 2018 clinical report stated that fewer than 50 Hennekam cases had then been reported, although that number included genetically heterogeneous Hennekam syndrome rather than CCBE1-confirmed HKLLS1 alone. Approximately 25% of clinically suspected Hennekam cases have been reported to carry CCBE1 variants. These historical estimates should not be interpreted as contemporary population prevalence. (betterman2020atypicalcadherinfat4 pages 1-2, lee2018hennekamsyndromea pages 1-3, melber2018novelmutationin pages 1-2)

Identifiers and synonyms

OMIM: 235510, Hennekam lymphangiectasia–lymphedema syndrome 1/Hennekam syndrome. The retrieved literature explicitly associates OMIM 235510 with the CCBE1-related disorder. (betterman2020atypicalcadherinfat4 pages 1-2)
MONDO: a subtype-specific MONDO identifier was not securely established from the retrieved primary literature and should be verified against the current MONDO release before ingestion.
Orphanet: use the current Orphanet entry for Hennekam syndrome/generalized lymphatic dysplasia; subtype mapping should likewise be verified in the live database.
ICD-10/ICD-11: no uniquely specific HKLLS1 code was identified. Coding generally falls under congenital lymphatic malformations or hereditary lymphedema.
MeSH: no disease-specific MeSH heading was identified; “Lymphedema,” “Lymphangiectasis,” and “Lymphatic Abnormalities” are appropriate broader concepts.
Synonyms include Hennekam syndrome type 1, CCBE1-related Hennekam syndrome, Hennekam lymphangiectasia–lymphedema syndrome, and historically lymphedema–lymphangiectasia–intellectual disability syndrome.

This report synthesizes aggregated disease-level literature plus individual published patients and experimental models. It is not derived from an electronic-health-record cohort.

Important disambiguation: HKLLS1 is distinct from FAT4-related Hennekam syndrome type 2 and ADAMTS3-related type 3. It is also entirely distinct from Menke–Hennekam syndrome, a CREBBP/EP300-associated neurodevelopmental disorder.

2. Etiology, risk, protection, and environment

Causal factor

The primary cause is biallelic germline pathogenic or likely pathogenic variation in CCBE1, usually homozygous or compound heterozygous. CCBE1 encodes a secreted extracellular-matrix-associated protein required for lymphatic development. Disease alleles include missense and truncating/frameshift variants affecting its cysteine-rich, EGF-like, calcium-binding EGF-like, or collagen-repeat regions. (roukens2015functionaldissectionof pages 1-3, melber2018novelmutationin pages 1-2)

No infectious, toxic, dietary, occupational, radiation, smoking, or other lifestyle cause is established. Maternal exposures do not cause the Mendelian disorder. Environmental factors may, however, alter complications: trauma, obesity, immobility, heat, skin injury, or infection can aggravate lymphedema generally, but HKLLS1-specific interaction studies are unavailable.

Risk and protective factors

Genetic risk: having two disease-causing CCBE1 alleles. For two confirmed heterozygous parents, the expected risk per pregnancy is 25% affected, 50% carrier, and 25% unaffected/non-carrier.
Family history/consanguinity: consanguinity increases the probability that both parents carry the same rare allele, but compound-heterozygous disease also occurs in nonconsanguineous families.
Sex: no established sex bias.
Modifier genes, protective variants, or protective alleles: none validated.
Gene–environment interactions: no formal HKLLS1 G×E evidence.

3. Phenotypic spectrum

The available literature does not provide reliable CCBE1-only percentages for most manifestations. Frequencies below are therefore qualitative unless an explicit statistic is available.

Core lymphatic phenotypes

Lymphedema—usually congenital, chronic, and often progressive; limbs, face, and genitalia are common sites. It may be generalized, asymmetric, or occasionally concentrated in one limb. Functional effects include heaviness, restricted motion, impaired mobility or hand use, recurrent skin infection risk, and psychosocial burden. Suggested HPO: HP:0001004 Lymphedema, HP:0009826 Facial edema, and HP:0003577 Congenital onset. (lee2018hennekamsyndromea pages 1-3, melber2018novelmutationin pages 1-2)
Intestinal lymphangiectasia can produce protein-losing enteropathy, diarrhea, malabsorption, edema, growth failure, hypoalbuminemia, hypoproteinemia, lymphopenia, and hypogammaglobulinemia. Some radiologically affected patients retain normal protein and albumin, demonstrating variable functional severity. Suggested HPO: HP:0100769 Intestinal lymphangiectasia, HP:0003075 Hypoalbuminemia, HP:0001888 Lymphopenia, and HP:0004313 Hypogammaglobulinemia. (lee2018hennekamsyndromea pages 3-5)
Other visceral sites include lungs, pleura, pericardium, mesentery, and kidneys. Pleural or pericardial effusion and congenital pulmonary lymphangiectasia can cause respiratory compromise. (melber2018novelmutationin pages 2-4, lee2018hennekamsyndromea pages 3-5, melber2018novelmutationin pages 1-2)

Craniofacial and neurodevelopmental phenotypes

Reported facial findings include a flat midface, broad/depressed nasal bridge, hypertelorism, epicanthal folds, bulbous nasal tip, small or narrow mouth, low-set or dysplastic ears, smooth philtrum, and dental anomalies. Suggested HPO terms include HP:0012368 Flat face, HP:0000426 Depressed nasal bridge, HP:0000316 Hypertelorism, HP:0000286 Epicanthus, and HP:0000208 Small mouth. (lee2018hennekamsyndromea pages 1-3, lee2018hennekamsyndromea pages 3-5, melber2018novelmutationin pages 1-2)

Development ranges from near-normal to global developmental delay or intellectual disability. Motor, fine-motor, adaptive, and personal-social domains may be affected. Seizures were reported in approximately 33% of the broader historical Hennekam literature, but this is not a reliable CCBE1-specific estimate. Suggested HPO: HP:0001263 Global developmental delay, HP:0001249 Intellectual disability, and HP:0001250 Seizure. (lee2018hennekamsyndromea pages 3-5)

Prenatal and severe presentations

HKLLS1 can present as recurrent nonimmune hydrops fetalis. In one molecularly confirmed family, affected pregnancies manifested at 15–18 weeks with scalp edema, ascites, cystic hygroma, pleural/pericardial effusions, hepatomegaly, and pulmonary hypoplasia. Lymphatic dysplasias account for an estimated 5–15% of nonimmune hydrops generally, and nonimmune hydrops has reported fetal mortality of 50–98%; neither statistic is specific to HKLLS1. Suggested HPO: HP:0001789 Hydrops fetalis, HP:0000473 Cystic hygroma, HP:0001541 Ascites, HP:0005381 Pleural effusion, and HP:0002089 Pulmonary hypoplasia. (melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)

4. Genetic and molecular information

Gene and inheritance

Gene: CCBE1, collagen and calcium-binding EGF domains 1.
Origin: constitutional/germline; no somatic disease mechanism is established.
Inheritance: autosomal recessive, suggested HPO HP:0000007.
Functional class: loss-of-function or hypomorphic alleles impair CCBE1’s lymphangiogenic activity.

A prenatal case carried maternally inherited NM_133459.3:c.683_684insT, p.Leu229fs and paternally inherited c.335C>T, p.Thr112Ile. At publication, the frameshift appeared in 14/277,032 gnomAD chromosomes and p.Thr112Ile in 1/246,160; neither was observed homozygously in the reported data. The first was classified as pathogenic loss-of-function and the second as likely pathogenic on rarity, conservation, domain location, segregation, and computational evidence. (melber2018novelmutationin pages 2-4)

By 2015, nine causal CCBE1 variants had been described in the analyzed literature. Most affected the N-terminal cysteine-rich/EGF regions; two involved collagen repeats. This is historical rather than a complete current ClinVar/HGMD count. No validated modifier gene, epigenetic signature, recurrent chromosomal abnormality, anticipation, or germline-mosaicism rate is known. (roukens2015functionaldissectionof pages 1-3)

5. Mechanism and pathophysiology

Causal chain

Upstream genetic lesion: biallelic CCBE1 dysfunction reduces functional extracellular CCBE1.
Growth-factor processing defect: CCBE1 normally promotes ADAMTS3-mediated proteolytic activation of pro-VEGF-C.
Signaling deficit: reduced mature VEGF-C lowers activation of VEGFR3/FLT4 on lymphatic endothelial cells.
Developmental failure: embryonic lymphatic endothelial progenitors are specified but migrate/sprout inadequately from cardinal veins; lymphatic vessels become sparse, malformed, or functionally insufficient.
Clinical consequences: interstitial-fluid accumulation causes lymphedema; visceral lymphatic dilation/leakage causes intestinal protein loss, effusions, pulmonary disease, and—in severe fetal cases—hydrops. (betterman2020atypicalcadherinfat4 pages 1-2, roukens2015functionaldissectionof pages 1-3)

The domain-dissection study’s abstract states: “deleting the collagen domains of CCBE1 has a much stronger effect on CCBE1 activity than deleting the EGF domains” and concludes that the collagen domains are crucial for VEGF-C activation, whereas EGF domains are needed for full in-vivo lymphangiogenic activity. This is supported by knock-in mice, zebrafish rescue/signaling experiments, and in-vitro VEGF-C processing assays. (roukens2015functionaldissectionof pages 1-3)

Suggested annotations include GO:0001946 lymphangiogenesis, endothelial-cell migration, growth-factor proteolytic processing, VEGF receptor signaling, and extracellular-matrix organization. The principal cell type is the lymphatic endothelial cell; relevant compartments include extracellular space/matrix and the lymphatic endothelial plasma membrane. Downstream VEGFR3 signaling engages PI3K–AKT and MAPK–ERK, but the primary HKLLS1 defect lies upstream at VEGF-C maturation rather than constitutive activation of those pathways.

No reproducible disease-specific transcriptomic, proteomic, metabolomic, lipidomic, single-cell, spatial-transcriptomic, or epigenomic patient signature has been established.

6. Anatomy and temporal development

Primary disease resides in the lymphatic vasculature, especially lymphatic capillaries and collecting pathways of limb soft tissue and viscera. Relevant anatomical concepts include lymph vessels, skin/subcutis, small intestine, mesentery, lung, pleura, pericardium, kidney, face, external genitalia, and limbs. Affected edema may be bilateral/generalized or markedly asymmetric. (lee2018hennekamsyndromea pages 1-3, lee2018hennekamsyndromea pages 3-5, melber2018novelmutationin pages 1-2)

Onset is prenatal or congenital in many patients. The course is chronic and lifelong; peripheral edema may progress, whereas visceral manifestations may be stable, intermittent, or severe from birth. There are no validated disease stages. Critical periods include fetal lymphatic development, the neonatal respiratory period in pulmonary disease, and early childhood for nutrition and neurodevelopmental intervention.

7. Epidemiology and population genetics

No reliable prevalence, incidence, carrier-frequency, sex-ratio, or life-table estimate exists for molecularly confirmed HKLLS1. The statement “fewer than 50 cases” in 2018 refers to Hennekam syndrome broadly and reflects ascertainment and publication, not population prevalence. Cases occur across multiple ancestries; no geographically restricted endemic population or robust founder effect is established. Expressivity is markedly variable, while penetrance among individuals carrying two clearly damaging alleles is presumed high but has not been formally quantified. (lee2018hennekamsyndromea pages 1-3)

8. Diagnosis

Clinical and laboratory evaluation

Suspect HKLLS1 with congenital/early lymphedema plus visceral lymphangiectasia, characteristic facial morphology, developmental differences, or unexplained nonimmune hydrops. Evaluation may include:

Limb circumference/volume and bioimpedance.
Lymphoscintigraphy or other lymphatic imaging; one case showed no visible lymphatic flow in the affected arm.
Ultrasound, CT, or MRI to assess intestinal/mesenteric, pulmonary, pleural, pericardial, renal, and CNS involvement.
CBC with lymphocyte count; total protein, albumin, immunoglobulins, electrolytes, fat-soluble vitamins, and growth/nutritional status.
Stool α1-antitrypsin clearance where protein-losing enteropathy is suspected.
Pulmonary and cardiac assessment when effusions or respiratory symptoms occur. (lee2018hennekamsyndromea pages 1-3, lee2018hennekamsyndromea pages 3-5)

Molecular testing

A practical sequence is a primary-lymphedema/generalized-lymphatic-dysplasia panel including CCBE1, FAT4, ADAMTS3, FLT4, VEGFC, PIEZO1, EPHB4, FOXC2, GATA2, and other phenotype-driven genes, or trio WES/WGS when presentation is nonspecific. Confirm candidate variants and parental phase by Sanger sequencing or equivalent. Exon-level deletion/duplication analysis should be included if the assay does not detect copy-number variants.

In recurrent fetal hydrops, trio WES diagnosed HKLLS1 after normal karyotype, microarray, infectious studies, hematologic testing, and metabolic evaluation. The report’s key message was: “WES is a useful approach for diagnosing rare single-gene conditions with nonspecific phenotypes and should be considered early in the diagnostic process of investigating fetal abnormalities.” (melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)

CMA, karyotyping, and FISH do not diagnose typical sequence-level CCBE1 disease, although they may exclude chromosomal differentials. Mitochondrial and repeat-expansion testing are not routinely indicated. No validated RNA, proteomic, metabolomic, epigenomic, or liquid-biopsy diagnostic exists.

Differential diagnosis

Key differentials include FAT4- and ADAMTS3-related Hennekam syndromes; PIEZO1-related generalized lymphatic dysplasia; FLT4/VEGFR3- or VEGFC-related primary lymphedema; FOXC2 lymphedema-distichiasis; GATA2 deficiency/Emberger syndrome; EPHB4-related lymphatic anomalies; primary intestinal lymphangiectasia; CD55-deficient CHAPLE syndrome; Noonan-spectrum disorders; chromosomal syndromes; and infectious, cardiac, hematologic, metabolic, or structural causes of fetal hydrops. (betterman2020atypicalcadherinfat4 pages 1-2, melber2018novelmutationin pages 1-2)

There are no universally accepted HKLLS1-specific clinical diagnostic criteria. Molecular confirmation is strongly preferred because Hennekam syndrome is genetically heterogeneous.

9. Outcome and prognosis

Natural-history data, five- or ten-year survival, and life expectancy are unavailable. Prognosis varies with visceral involvement:

Hydrops, severe pulmonary lymphangiectasia, pulmonary hypoplasia, or large effusions may be fatal prenatally or neonatally.
Intestinal lymphangiectasia can cause chronic malnutrition, immunoglobulin/lymphocyte loss, infection susceptibility, and growth impairment.
Chronic limb edema can cause reduced function, tissue fibrosis, recurrent cellulitis, pain or heaviness, and impaired quality of life.
Developmental outcome ranges from nearly normal to significant disability.

One review cited in the rehabilitation report noted that pulmonary complications are directly relevant to survival. No validated molecular prognostic biomarker or genotype-based risk calculator exists. (lee2018hennekamsyndromea pages 3-5, melber2018novelmutationin pages 1-2)

10. Treatment and real-world implementation

Lymphedema management

No curative therapy exists. Standard care is complete decongestive therapy, combining compression, exercise, skin care, and usually manual lymph drainage, followed by maintenance garments. In one 28-month-old Hennekam patient, seven sessions—including manual drainage, ≥20-hour/day bandaging, exercise, activities-of-daily-living training, and low-level laser—reduced upper-arm circumference from 22 to 19 cm and forearm circumference from 22.5 to 21 cm. This is disease-specific but only single-patient evidence. (lee2018hennekamsyndromea pages 3-5)

A broader randomized lymphedema trial, NCT01748604, enrolled 194 patients and compared manual drainage plus pneumatic compression and multilayer bandaging with regimens omitting manual drainage. It was not HKLLS1-specific and its registry text does not establish subtype-specific efficacy. (NCT01748604 chunk 1)

Suggested NCIT intervention concepts: physical therapy, manual lymphatic drainage, compression bandaging/garments, exercise therapy, skin care, and intermittent pneumatic compression.

Visceral and supportive management

Protein-losing enteropathy is managed with a high-protein, low-fat diet enriched in medium-chain triglycerides, with replacement of vitamins, minerals, albumin, or immunoglobulin when clinically indicated. Respiratory effusions, chylothorax, infection, seizures, dental problems, developmental delay, and feeding difficulties require organ-specific care. Surgery is reserved for severe, function-limiting lymphedema refractory to conservative management; no controlled HKLLS1 surgical series exists. (lee2018hennekamsyndromea pages 3-5)

There is no approved CCBE1 replacement, VEGF-C therapy, gene therapy, RNA therapy, cell therapy, or pharmacogenomic algorithm. Although VEGF-C/VEGFR3 biology is an attractive target, augmenting lymphangiogenesis in humans raises safety and delivery concerns and remains experimental.

Trials and 2023–2024 developments

No HKLLS1-specific interventional trial was found. A 2024 study of aerobic exercise in primary lower-extremity lymphedema, NCT06327412, planned 35 adults and assessed limb volume, function, VO₂max, and quality of life; its registry status was unknown and it was not genotype-specific. It therefore cannot be taken as evidence of efficacy in HKLLS1. (NCT06327412 chunk 1)

Recent 2023–2024 research has refined the broader VEGFR3 signaling and lymphatic-development framework, but no disease-specific therapeutic breakthrough, large natural-history cohort, or prospective treatment trial was identified. The most authoritative mechanistic interpretation remains that CCBE1 and ADAMTS3 regulate VEGF-C activation upstream of VEGFR3.

11. Prevention

Because HKLLS1 is inherited, lifestyle-based primary prevention is not available. Prevention focuses on:

Genetic counseling and cascade testing after identifying familial variants.
Carrier testing of reproductive partners or at-risk adult relatives.
Prenatal diagnosis by chorionic-villus sampling or amniocentesis for known familial variants.
Preimplantation genetic testing for monogenic disease where desired and legally available.
Early fetal imaging in at-risk pregnancies, recognizing that a normal early scan does not exclude disease.
Tertiary prevention through compression, meticulous skin care, prompt treatment of cellulitis, nutritional surveillance, and developmental intervention.

No vaccine, medication, or environmental intervention prevents the underlying disorder.

12. Other species and model organisms

No well-established naturally occurring veterinary counterpart was identified, and the disorder is not infectious or zoonotic.

Zebrafish

Loss of ccbe1 disrupts lymphovenous sprouting from the cardinal vein, formation of parachordal lymphangioblasts, and major lymphatic vessels. Zebrafish provide rapid in-vivo assays of VEGF-C signaling and variant/domain rescue. Their principal limitation is that they do not reproduce the full human craniofacial, cognitive, intestinal, and chronic edema phenotype. (schultemerker2011lymphaticvascularmorphogenesis pages 7-9, roukens2015functionaldissectionof pages 1-3)

Mouse

Ccbe1-null embryos specify PROX1-positive lymphatic progenitors but fail to achieve normal migration and lymphatic-network formation. Mice lacking CCBE1 collagen domains phenocopy knockout animals, whereas deletion of EGF domains permits rudimentary lymphatics. These models strongly support developmental causality but severe embryonic phenotypes limit long-term treatment studies. (roukens2015functionaldissectionof pages 1-3)

In-vitro systems

VEGF-C-processing assays show that CCBE1’s collagen domains are required to enhance ADAMTS3-mediated VEGF-C activation. Human lymphatic endothelial cells are appropriate for downstream signaling and migration studies, although no validated HKLLS1 patient-derived organoid or iPSC platform was identified. (roukens2015functionaldissectionof pages 1-3)

13. Evidence limitations and expert assessment

The evidence base is constrained by extreme rarity, genetic heterogeneity, historical grouping of CCBE1-, FAT4-, ADAMTS3-, and genetically unresolved cases, and publication bias toward severe or unusual presentations. Consequently, syndrome-wide percentages should not automatically be assigned to HKLLS1. Current expert interpretation supports a high-confidence causal chain from biallelic CCBE1 dysfunction to deficient VEGF-C activation and developmental lymphatic failure, but clinical genotype–phenotype prediction remains weak. Priority research needs are an international molecular registry, longitudinal natural-history and quality-of-life studies, standardized outcome measures, systematic ClinVar/functional curation, and genotype-specific therapeutic development.

Key publications and URLs

Alders M, et al. Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans. Nature Genetics. December 2009;41:1272–1274. PMID: 19935664. https://doi.org/10.1038/ng.484. This is the landmark human gene-discovery study cited by subsequent clinical literature. (lee2018hennekamsyndromea pages 3-5)
Roukens MG, et al. Functional Dissection of the CCBE1 Protein: A Crucial Requirement for the Collagen Repeat Domain. Circulation Research. May 2015;116:1660–1669. https://doi.org/10.1161/CIRCRESAHA.116.304949. (roukens2015functionaldissectionof pages 1-3)
Lee YG, et al. Hennekam Syndrome: A Case Report. Annals of Rehabilitation Medicine. February 2018;42:184–188. https://doi.org/10.5535/arm.2018.42.1.184. (lee2018hennekamsyndromea pages 1-3, lee2018hennekamsyndromea pages 3-5)
Melber DJ, et al. Novel mutation in CCBE1 as a cause of recurrent hydrops fetalis from Hennekam lymphangiectasia-lymphedema syndrome-1. Clinical Case Reports. Published October 2018;6:2358–2363. https://doi.org/10.1002/ccr3.1804. (melber2018novelmutationin pages 1-2, melber2018novelmutationin pages 2-4)
Betterman KL, et al. Atypical cadherin FAT4 orchestrates lymphatic endothelial cell polarity in response to flow. Journal of Clinical Investigation. Published May 18, 2020;130:3315–3328. https://doi.org/10.1172/JCI99027. Although centered on FAT4, it authoritatively contextualizes the CCBE1–ADAMTS3–VEGF-C/VEGFR3 axis and distinguishes Hennekam genetic subtypes. (betterman2020atypicalcadherinfat4 pages 1-2)
ClinicalTrials.gov. NCT01748604: Physical Therapies in the Decongestive Treatment of Lymphedema. First posted December 12, 2012. https://clinicaltrials.gov/study/NCT01748604. (NCT01748604 chunk 1)
ClinicalTrials.gov. NCT06327412: Effects of Aerobic Exercise in Primary Lower Extremity Lymphedema. First posted March 25, 2024. https://clinicaltrials.gov/study/NCT06327412. (NCT06327412 chunk 1)

References

(roukens2015functionaldissectionof pages 1-3): M. Guy Roukens, Josi Peterson-Maduro, Yvonne Padberg, Michael Jeltsch, Veli-Matti Leppänen, Frank L. Bos, Kari Alitalo, Stefan Schulte-Merker, and Dörte Schulte. Functional dissection of the ccbe1 protein: a crucial requirement for the collagen repeat domain. Circulation Research, 116:1660-1669, May 2015. URL: https://doi.org/10.1161/circresaha.116.304949, doi:10.1161/circresaha.116.304949. This article has 72 citations and is from a highest quality peer-reviewed journal.
(lee2018hennekamsyndromea pages 1-3): Yeong Guk Lee, Seung Chan Kim, Si-Bog Park, and Mi Jung Kim. Hennekam syndrome: a case report. Annals of Rehabilitation Medicine, 42:184-188, Feb 2018. URL: https://doi.org/10.5535/arm.2018.42.1.184, doi:10.5535/arm.2018.42.1.184. This article has 11 citations.
(melber2018novelmutationin pages 1-2): Dora J. Melber, Tara S. Andreasen, Rong Mao, Tatiana Tvrdik, Christine E. Miller, Thomas R. Moore, Douglas A. Woelkers, and Leah M. Lamale‐Smith. Novel mutation in ccbe 1 as a cause of recurrent hydrops fetalis from hennekam lymphangiectasia‐lymphedema syndrome‐1. Clinical Case Reports, 6:2358-2363, Oct 2018. URL: https://doi.org/10.1002/ccr3.1804, doi:10.1002/ccr3.1804. This article has 7 citations.
(melber2018novelmutationin pages 2-4): Dora J. Melber, Tara S. Andreasen, Rong Mao, Tatiana Tvrdik, Christine E. Miller, Thomas R. Moore, Douglas A. Woelkers, and Leah M. Lamale‐Smith. Novel mutation in ccbe 1 as a cause of recurrent hydrops fetalis from hennekam lymphangiectasia‐lymphedema syndrome‐1. Clinical Case Reports, 6:2358-2363, Oct 2018. URL: https://doi.org/10.1002/ccr3.1804, doi:10.1002/ccr3.1804. This article has 7 citations.
(lee2018hennekamsyndromea pages 3-5): Yeong Guk Lee, Seung Chan Kim, Si-Bog Park, and Mi Jung Kim. Hennekam syndrome: a case report. Annals of Rehabilitation Medicine, 42:184-188, Feb 2018. URL: https://doi.org/10.5535/arm.2018.42.1.184, doi:10.5535/arm.2018.42.1.184. This article has 11 citations.
(betterman2020atypicalcadherinfat4 pages 1-2): Kelly L. Betterman, Drew L. Sutton, Genevieve A. Secker, Jan Kazenwadel, Anna Oszmiana, Lillian Lim, Naoyuki Miura, Lydia Sorokin, Benjamin M. Hogan, Mark L. Kahn, Helen McNeill, and Natasha L. Harvey. Atypical cadherin fat4 orchestrates lymphatic endothelial cell polarity in response to flow. Journal of Clinical Investigation, 130:3315-3328, May 2020. URL: https://doi.org/10.1172/jci99027, doi:10.1172/jci99027. This article has 71 citations and is from a highest quality peer-reviewed journal.
(NCT06327412 chunk 1): Havva Talay Çalış. The Effects of Aerobic Exercise in Patients With Primary Lower Extremity Lymphedema. Kayseri City Hospital. 2024. ClinicalTrials.gov Identifier: NCT06327412
(NCT01748604 chunk 1): Isabel Forner-Cordero, MD, PhD.. Physical Therapies in the Decongestive Treatment of Lymphedema. Isabel Forner-Cordero, MD, PhD.. 2004. ClinicalTrials.gov Identifier: NCT01748604
(schultemerker2011lymphaticvascularmorphogenesis pages 7-9): Stefan Schulte-Merker, Amélie Sabine, and Tatiana V. Petrova. Lymphatic vascular morphogenesis in development, physiology, and disease. Jun 2011. URL: https://doi.org/10.1084/jem2086oia15, doi:10.1084/jem2086oia15. This article has 532 citations.

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Hennekam Lymphangiectasia–Lymphedema Syndrome Type 1 (HKLLS1): Comprehensive Disease Characteristics Report

openscientist-autonomous 37 citations 2026-09-02T06:02:19.204886

Hennekam Lymphangiectasia–Lymphedema Syndrome Type 1 (HKLLS1): Comprehensive Disease Characteristics Report

Summary

Hennekam lymphangiectasia–lymphedema syndrome type 1 (HKLLS1; OMIM 235510) is an ultra-rare, autosomal recessive form of generalized lymphatic dysplasia (GLD) caused by biallelic loss-of-function variants in CCBE1 (Collagen- and Calcium-Binding EGF domain-containing protein 1; chromosome 18q21.32). CCBE1 is a secreted extracellular matrix protein that, in concert with the metalloprotease ADAMTS3, proteolytically activates the pro-form of VEGFC into mature VEGFC. Mature VEGFC is the principal ligand that drives VEGFR3 (FLT4)-dependent lymphangiogenesis through downstream PI3K and Ras/ERK signaling. When CCBE1 function is lost, mature VEGFC production falls, VEGFR3 signaling in lymphatic endothelial cells is impaired, and lymphatic vessels develop abnormally throughout the body — producing the widespread lymphatic dysplasia that defines the disease.

Clinically, HKLLS1 presents with congenital peripheral lymphedema, intestinal (and often serosal/pulmonary) lymphangiectasia causing protein-losing enteropathy (PLE), chylous effusions, characteristic facial dysmorphism (hypertelorism, flat nasal bridge, flat midface), and variable intellectual disability. Secondary consequences of intestinal lymph leakage — hypoproteinemia, hypoalbuminemia, lymphopenia, and hypogammaglobulinemia — produce a secondary (combined) immunodeficiency that in some patients has presented as pediatric-onset common variable immune deficiency (CVID). Expressivity is highly variable, ranging from mild isolated lymphatic dysplasia without intellectual disability or dysmorphism, to severe recurrent non-immune hydrops fetalis detected prenatally. CCBE1 accounts for roughly 25% of all Hennekam syndrome patients; the syndrome is genetically heterogeneous, with FAT4 (HKLLS2) and ADAMTS3 (HKLLS3) causing clinically overlapping disease, and FBXL7 proposed as a candidate fourth gene.

There is no curative or disease-specific therapy. Management is supportive and multidisciplinary: a low-long-chain-triglyceride, high-protein diet supplemented with medium-chain triglycerides (MCT) is the cornerstone for the protein-losing enteropathy, supplemented as needed by albumin and immunoglobulin replacement, octreotide, diuretics, drainage of effusions, surgical resection of localized lesions, and complex decongestive therapy/compression for lymphedema. Diagnosis is confirmed by whole-exome or whole-genome sequencing, which is applicable prenatally in cases of recurrent hydrops fetalis. The disease is lifelong, and prognosis depends primarily on the severity of protein-losing enteropathy, effusions, and hydrops.

Key Findings

Finding 1 — HKLLS1 is caused by biallelic loss-of-function CCBE1 variants that impair VEGFC processing

Multiple independent families with HKLLS1 carry biallelic (homozygous or compound heterozygous) variants in CCBE1. Reported pathogenic variants include the homozygous missense c.472C>T (p.Arg158Cys), the compound heterozygous combination c.310G>A (p.Asp104Asn) + c.80T>C (p.Leu27Pro), and additional missense substitutions such as C174Y, C98W, and L27P. Functional assays in the zebrafish lymphatic-disease model demonstrated that these patient variants are loss-of-function. Mechanistically, CCBE1 is capable of upregulating the levels of fully processed, mature VEGFC in vitro, and overexpression of mature VEGFC rescues ccbe1 loss-of-function phenotypes in zebrafish; genetically, ccbe1 interacts with vegfc and vegfr3, and Vegfc/Vegfr3-dependent ERK signaling is impaired in the absence of Ccbe1.

The landmark study establishing this mechanism states that "CCBE1 is capable of upregulating the levels of fully processed, mature VEGFC in vitro and the overexpression of mature VEGFC rescues ccbe1 loss-of-function phenotypes in zebrafish" and directly links "mutations in CCBE1 causing generalized lymphatic dysplasia and lymphedema (Hennekam syndrome)" (PMID: 24523457). A separate cohort confirmed that patient CCBE1 variants act by loss of function: "Functional analysis in a zebrafish model of lymphatic disease showed that both mutations lead to CCBE1 loss of function" (PMID: 27345729).

Finding 2 — Hennekam syndrome is genetically heterogeneous: CCBE1 (type 1), FAT4 (type 2), ADAMTS3 (type 3)

HKLLS is an autosomal recessive disorder caused by biallelic variants in at least three genes: CCBE1 (HKLLS1; OMIM 235510), FAT4 (HKLLS2; OMIM 616006), and ADAMTS3 (HKLLS3). CCBE1 mutations account for approximately 25% of Hennekam patients. Critically, CCBE1 and ADAMTS3 act in the same molecular pathway: CCBE1 acts via ADAMTS3 to enhance VEGFC signaling, and ADAMTS3 mutations abolish proteolytic activation of pro-VEGFC. FBXL7 has been proposed as a candidate fourth gene, acting through the FAT4 pathway.

A comprehensive review states: "It is an autosomal recessive condition caused by biallelic mutations in CCBE1 ... (HKLLS1; OMIM 235510) or FAT4 (HKLLS2; OMIM 616006). CCBE1 acts via ADAMTS3 ... to enhance vascular endothelial growth factor C signaling" (PMID: 30450763). The 25% CCBE1 contribution is documented in PMID: 27345729: "Hennekam lymphangiectasia-lymphedema syndrome is an autosomal recessive disorder, with 25% of patients having mutations in CCBE1." ADAMTS3 as the HKLLS3 gene was established through the discovery of "bi-allelic missense mutations in ADAMTS3" (PMID: 28985353). A recent report of a novel biallelic FAT4 splice variant confirms HKLLS2 and the three-gene model: "Hennekam lymphangiectasia-lymphedema syndrome (HKLLS) is an autosomal recessive disorder, caused by biallelic variants in CCBE1, FAT4, and ADAMTS3 genes" (PMID: 41992670).

Finding 3 — Core clinical phenotype: congenital lymphedema, intestinal lymphangiectasia/PLE, facial dysmorphism, variable ID, and secondary immunodeficiency

The HKLLS1 phenotype comprises congenital peripheral lymphedema, generalized lymphatic dysplasia, intestinal lymphangiectasia causing protein-losing enteropathy with severe hypoproteinemia/hypoalbuminemia and hypogammaglobulinemia, ascites and chylous effusions, recurrent pericardial effusion, characteristic facial dysmorphism (hypertelorism, flat nasal bridge, flat midface), and variable intellectual disability. Some CCBE1 cases present as pediatric-onset common variable immune deficiency (CVID). The phenotype is notably variable: mild cases may lack intellectual disability and dysmorphism entirely.

A CCBE1-confirmed pediatric case documented the lymphedema and facial features: "she presented an abdominal circumference of 60 cm, edema of the lower extremities and vulva, and facial dysmorphisms (hypertelorism, flat nasal bridge, flat mid-face)" and "blood laboratory investigations revealed severe hypoproteinemia, hypoalbuminemia and hypogammaglobulinemia" (PMID: 32629717). The core clinical tetrad is summarized as "characterized by congenital lymphedema, intestinal lymphangiectasia, facial dysmorphism, and variable intellectual disability" (PMID: 31633297). The phenotypic variability at the mild end of the spectrum is illustrated by a CCBE1 (p.C98W) patient with "lymphatic dysplasia without intellectual disability or dysmorphism" (PMID: 25925991).

Finding 4 — Mechanistic axis: CCBE1/ADAMTS3 → mature VEGFC → VEGFR3 (FLT4) → PI3K/Ras-ERK in lymphatic endothelium

VEGFR3 is the principal receptor tyrosine kinase in lymphatic endothelial cells (LECs). Binding of mature VEGFC (or VEGFD) stimulates VEGFR3 autophosphorylation and downstream PI3K and Ras/ERK cascades that drive LEC proliferation, migration, and survival. CCBE1, together with ADAMTS3, is required to proteolytically generate mature, active VEGFC. In HKLLS1, loss of CCBE1 function reduces the supply of mature VEGFC and thereby lowers VEGFR3 signaling output — an upstream lesion producing downstream failure of lymphatic vessel formation.

A recent review of VEGFR3 biology states that "VEGFR3 primarily promotes downstream signaling through the phosphoinositide 3-kinase (PI3K) and Ras signaling cascades that promote functions including cell proliferation and migration," and that "The importance of VEGFR3 cascades in lymphatic physiology is underscored by identification of dysfunctional VEGFR3 signaling across several lymphatic-related diseases" (PMID: 40046235). The specific coupling of CCBE1 loss to reduced VEGFR3-ERK signaling is demonstrated by "Vegfc- and Vegfr3-dependent Erk signaling is impaired in the absence of Ccbe1" (PMID: 24523457).

Finding 5 — CCBE1 function and disease mechanism are conserved and validated in zebrafish and mouse models

CCBE1 orthologs are highly conserved across zebrafish, mice, and humans, and CCBE1 mutations cause generalized lymphatic dysplasia/Hennekam syndrome across species. In zebrafish, ccbe1 genetically interacts with vegfc and vegfr3 (flt4), and patient variants (e.g., L27P, D104N, C98W) recapitulate loss of function. In mice, the biochemical mechanism has been dissected: an ADAMTS3-CCBE1 complex forms independently of VEGFR3 and is required to convert VEGFC (but not VEGFD) into an active ligand. Inducible deletion of Ccbe1 in mice impairs postnatal meningeal lymphatic development and reduces macromolecule drainage, establishing a role for CCBE1 in both development and maintenance of lymphatics.

Conservation and disease relevance are stated directly: "The role of CCBE1 orthologs is highly conserved in zebrafish, mice and humans with mutations in CCBE1 causing generalized lymphatic dysplasia and lymphedema (Hennekam syndrome)" (PMID: 24523457). The biochemical complex is established in "an ADAMTS3-CCBE1 complex can form independently of VEGFR3 and is required to convert VEGFC, but not VEGFD, into an active ligand" (PMID: 27159393). Meningeal lymphatic requirement is documented in PMID: 38141283: "inducible deletion of CCBE1 leads to impaired postnatal development of the meningeal lymphatics and decreased macromolecule drainage to deep cervical lymph nodes."

Finding 6 — Immunodeficiency in HKLLS1 is secondary to protein-losing enteropathy, not a primary immune defect

Intestinal lymphangiectasia causes leakage of protein- and lymphocyte-rich lymph into the gut lumen, producing hypoalbuminemia, lymphopenia, and hypogammaglobulinemia — a secondary (combined) immunodeficiency rather than an intrinsic defect in immune-cell development. HKLLS1/CCBE1 cases have presented as pediatric-onset CVID. Immunoglobulin replacement (including subcutaneous) can be used, but the MCT diet, which reduces lymph loss, is the cornerstone that addresses the root cause.

The causal chain is stated as "it is responsible for protein losing enteropathy leading to lymphopenia, hypoalbuminemia and hypogammaglobulinemia" (PMID: 26934740), and the corresponding laboratory abnormalities in a CCBE1 patient are documented in PMID: 32629717: "blood laboratory investigations revealed severe hypoproteinemia, hypoalbuminemia and hypogammaglobulinemia."

Finding 7 — Diagnosis relies on exome/genome sequencing; prenatal presentation is recurrent non-immune hydrops fetalis

Because HKLLS1 has nonspecific, overlapping features, whole-exome sequencing (WES) is the recommended diagnostic approach and can be applied prenatally. WES diagnosed HKLLS1 as the cause of recurrent hydrops fetalis. Generalized lymphatic dysplasia — lymphedema, lymphangiectasia, chylothorax, and pleural/pericardial effusions — can present prenatally as non-immune hydrops fetalis. The differential diagnosis includes other autosomal recessive GLD genes (PIEZO1, GDF2) and syndromic lymphedema genes. Supportive diagnostics include hypoproteinemia/hypoalbuminemia, low immunoglobulins, elevated stool alpha-1-antitrypsin, lymphoscintigraphy showing abnormal tracer leakage, and endoscopic/biopsy evidence of dilated intestinal lymphatics.

WES utility and prenatal presentation are documented: "Whole exome sequencing (WES) was used to determine the etiology of recurrent hydrops fetalis in this case of Hennekam lymphangiectasia-lymphedema syndrome-1. WES is a useful approach for diagnosing rare single-gene conditions with nonspecific phenotypes" (PMID: 30564329). Supportive laboratory findings appear in PMID: 40394495: "laboratory tests showed decreased blood albumin and increased stool α1-antitrypsin." The GLD spectrum and prenatal hydrops context are defined in PMID: 26333996: "Generalized lymphatic dysplasia (GLD) is a rare form of primary lymphoedema characterized by a uniform, widespread lymphoedema affecting all segments of the body, with systemic involvement such as intestinal and/or pulmonary lymphangiectasia, pleural effusions, chylothoraces and/or pericardial effusions. This may present prenatally as non-immune hydrops."

Finding 8 — Epidemiology: ultra-rare autosomal recessive disorder; consanguinity is a major risk context

Hennekam syndrome is autosomal recessive; CCBE1 mutations account for ~25% of patients. It is ultra-rare, with only ~50–60 total reported patients across all subtypes (one patient described as the 51st case worldwide). Many CCBE1 families are consanguineous with homozygous variants. Recurrence risk is 25% per pregnancy, and expressivity is highly variable (from mild lymphatic dysplasia to severe recurrent non-immune hydrops fetalis).

The rarity scale is noted in PMID: 38028107: "this is the 51st case of HS worldwide." Consanguinity as a recurring context is documented in PMID: 26686525: "born to consanguineous parents of Turkish ancestry," and in the mild-form report of a consanguineous family of Pakistani descent (PMID: 25925991).

Finding 9 — Management is supportive: MCT/low-LCT high-protein diet, albumin/immunoglobulin, drainage, and multidisciplinary care

No disease-specific or curative therapy exists. Intestinal lymphangiectasia/PLE is managed with a low-long-chain-triglyceride, high-protein diet supplemented with medium-chain triglycerides (MCT), sometimes with (total) parenteral nutrition and fat-soluble vitamin supplementation. In a pooled review of primary intestinal lymphangiectasia (PIL), 63% (17/27) of MCT-treated patients had complete symptom resolution versus 35.7% (10/28) untreated, with lower mortality (3.7% vs 17.85%). Octreotide is used when MCT fails; diuretics, albumin transfusions, and surgical resection of localized lesions or effusion drainage are adjuncts. Lymphedema is managed with complex decongestive therapy/compression.

The quantitative benefit of MCT is documented in PMID: 19449286: "17 of 27 cases (63%) treated with MCT had complete resolution of symptoms while only 10 of 28 (35.7%) patients in group B showed complete resolution. Mortality for Group A was 1 out of 27 (3.7%), while mortality in group B was 5 of 28 (17.85%)." MCT diet as standard management is reinforced by PMID: 20571826: "In patients with lymphangiectasia, a low fat with medium chain triglycerides (MCT) diet should be prescribed."

Section-by-Section Report

1. Disease Information

Overview. HKLLS1 is a Mendelian, autosomal recessive generalized lymphatic dysplasia — a systemic disorder of lymphatic vessel development. It combines lymphedema with lymphangiectasia (dilated, dysfunctional lymphatic vessels) affecting the intestine and other tissues, characteristic facial dysmorphism, and variable intellectual disability. It is the CCBE1-defined subtype of Hennekam syndrome.

Key identifiers. - OMIM: 235510 (Hennekam lymphangiectasia-lymphedema syndrome 1) - Gene: CCBE1 (HGNC:29426), chromosome 18q21.32 - Related subtypes: HKLLS2 (FAT4; OMIM 616006), HKLLS3 (ADAMTS3) - Orphanet: Hennekam syndrome (ORPHA:2136 for the clinical entity) - Suggested MONDO: the MONDO class mapping to OMIM 235510 (Hennekam lymphangiectasia-lymphedema syndrome 1) - ICD-10: Q82.0 (hereditary lymphedema) as closest mapping; ICD-11: hereditary lymphedema / lymphatic malformation category (LD-block) - MeSH: Lymphangiectasis; Lymphedema (no dedicated Hennekam MeSH descriptor)

Synonyms / alternative names: Hennekam syndrome; Hennekam lymphangiectasia-lymphedema syndrome; generalized lymphatic dysplasia of Hennekam; intestinal lymphangiectasia–lymphedema–intellectual disability syndrome; lymphedema–lymphangiectasia–intellectual disability syndrome.

Information source. The knowledge base for HKLLS1 is derived from aggregated disease-level resources (OMIM, Orphanet) and from individual patient case reports and small case series in the primary literature, given the disease's rarity (~50–60 reported patients).

2. Etiology

Causal factors. HKLLS1 is a purely genetic disorder caused by biallelic (homozygous or compound heterozygous) loss-of-function variants in CCBE1. There is no infectious or environmental cause. The primary lesion disrupts proteolytic maturation of pro-VEGFC (Findings 1, 4).

Genetic risk factors. The causal factor is biallelic pathogenic CCBE1 variation. Consanguinity substantially increases risk of homozygous disease and is a recurring context in reported families (Finding 8; PMID: 26686525, PMID: 25925991). Heterozygous carriers are unaffected. Genes in the same pathway (FAT4, ADAMTS3, and candidate FBXL7) cause clinically overlapping but distinct HKLLS subtypes.

Environmental risk factors / protective factors. None are established. The disorder is fully penetrant given a biallelic loss-of-function genotype; no protective alleles or environmental modifiers have been described. Diet (MCT) modifies the complication (protein-losing enteropathy) but does not alter the underlying genetic cause.

Gene–environment interactions. No specific GxE interactions are documented for HKLLS1.

3. Phenotypes

Phenotype	Type	Suggested HPO	Onset	Frequency/severity	Notes
Congenital lymphedema (limbs, genitalia)	Physical manifestation	HP:0001004 (Lymphedema)	Congenital/neonatal	Common; variable	Peripheral, often bilateral; may persist after generalized edema resolves (PMID: 25925991)
Intestinal lymphangiectasia	Clinical sign / pathology	HP:0002593 (Intestinal lymphangiectasia)	Infancy/childhood	Common	Cause of PLE (PMID: 32629717)
Protein-losing enteropathy	Lab/clinical	HP:0002243 (Protein-losing enteropathy)	Infancy/childhood	Common	Hypoproteinemia, edema
Hypoproteinemia / hypoalbuminemia	Lab abnormality	HP:0003075 (Hypoproteinemia); HP:0003073 (Hypoalbuminemia)	Infancy	Common	Secondary to PLE
Hypogammaglobulinemia	Lab abnormality	HP:0004313 (Decreased circulating antibody level)	Childhood	Common	Secondary immunodeficiency (PMID: 26934740)
Lymphopenia	Lab abnormality	HP:0001888 (Lymphopenia)	Childhood	Common	Lymph loss into gut
Chylous ascites / pleural / pericardial effusion	Clinical sign	HP:0001541 (Ascites); HP:0002202 (Pleural effusion); HP:0001698 (Pericardial effusion)	Congenital–childhood	Variable	Recurrent pericardial effusion reported (PMID: 40394495)
Facial dysmorphism (hypertelorism, flat nasal bridge, flat midface)	Physical manifestation	HP:0000316 (Hypertelorism); HP:0005280 (Depressed nasal bridge); HP:0011800 (Midface retrusion)	Congenital	Common in classic form	(PMID: 32629717)
Intellectual disability	Neurodevelopmental	HP:0001249 (Intellectual disability)	Childhood	Variable; absent in mild forms	(PMID: 31633297, PMID: 25925991)
Non-immune hydrops fetalis	Prenatal clinical sign	HP:0001790 (Nonimmune hydrops fetalis)	Prenatal	Severe end of spectrum	Recurrent presentation (PMID: 30564329)

Quality-of-life impact. Lymphedema and recurrent effusions impair mobility and require lifelong compression/decongestive therapy. Protein-losing enteropathy necessitates lifelong dietary management and predisposes to recurrent infections (secondary immunodeficiency), edema, and growth impairment. Severe/prenatal presentations carry high perinatal morbidity and mortality (hydrops). Formal EQ-5D/SF-36 data are not available for this ultra-rare disease.

4. Genetic/Molecular Information

Causal gene: CCBE1 (Collagen- and Calcium-Binding EGF domain-containing protein 1), 18q21.32; OMIM gene 612753; HGNC:29426; NCBI Gene 147372. The protein is a secreted extracellular-matrix factor with an N-terminal EGF-like (calcium-binding) domain and C-terminal collagen-repeat domains.

Pathogenic variants (reported in HKLLS1):

Variant (cDNA)	Protein	Zygosity	Class	Evidence
c.472C>T	p.Arg158Cys	Homozygous / compound het	Missense, LoF	PMID: 32472549
c.521G>A	(compound het partner)	Compound het	Missense	PMID: 32472549
c.310G>A	p.Asp104Asn	Compound het	Missense, LoF	PMID: 24523457
c.80T>C	p.Leu27Pro	Compound het	Missense, LoF	PMID: 24523457
—	p.Cys98Trp (C98W)	Homozygous	Missense, LoF (mild form)	PMID: 25925991
—	p.Cys174Tyr (C174Y)	—	Missense	(cohort) PMID: 27345729

Variant classification. Reported variants are classified pathogenic/likely pathogenic under ACMG/AMP criteria, supported by functional zebrafish loss-of-function assays (PS3-level functional evidence), segregation, and rarity in population databases. In silico analyses predict additional deleterious CCBE1 nsSNPs (e.g., G330E, C102S, C174R, G107D) as candidate pathogenic variants, several affecting conserved cysteines/EGF domains and ligand-binding residues (PMID: 34234628).

Variant type/class: Predominantly missense variants (many affecting conserved cysteine residues critical for domain folding), plus splice and nonsense variants; large deletions are not the dominant mechanism for CCBE1.

Allele frequency: Pathogenic CCBE1 variants are individually rare/absent in gnomAD, consistent with a recessive ultra-rare disorder. One kindred carried a homozygous CCBE1 variant that does not prevent protein expression, indicating that some pathogenic variants act by disrupting function rather than abolishing protein (PMID: 26686525).

Somatic vs germline: Germline (biallelic, inherited) in HKLLS1. (Note: CCBE1 is also somatically dysregulated/overexpressed in colorectal cancer where it promotes tumor lymphangiogenesis, but this is unrelated to the germline disease — PMID: 32089745, PMID: 36781122.)

Functional consequence: Loss of function — reduced proteolytic maturation of pro-VEGFC and impaired VEGFR3 signaling (Findings 1, 4, 5).

Modifier genes: Not formally established. Genes in the same pathway (ADAMTS3, VEGFC, FLT4/VEGFR3, VEGFD) are biologically plausible modifiers; VEGFD can partially compensate for VEGFC loss in zebrafish facial lymphatics (PMID: 24903752).

Epigenetic information / chromosomal abnormalities: No disease-specific epigenetic signature or recurrent chromosomal abnormality is defined for HKLLS1. (Zebrafish work shows copper stress can epigenetically regulate ccbe1 expression, a mechanistic curiosity rather than a human disease mechanism — PMID: 35034208.)

5. Environmental Information

No environmental, lifestyle, or infectious factors cause or trigger HKLLS1 — it is a monogenic, congenital disorder. Environmental factors are relevant only insofar as diet (long-chain triglyceride intake) exacerbates protein-losing enteropathy, which is why MCT substitution is therapeutic (Finding 9). No infectious agents are implicated; recurrent infections in patients are a consequence of secondary immunodeficiency, not a cause.

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation):

Biallelic loss-of-function CCBE1 variants (germline) lead to a dysfunctional or reduced-activity CCBE1 protein in the extracellular matrix. (demonstrated — patient variants shown LoF in zebrafish)
Loss of CCBE1 function impairs formation of the ADAMTS3–CCBE1 complex and reduces colocalization of pro-VEGFC with its activating protease at the lymphatic endothelial cell surface. (demonstrated — mouse/in vitro; PMID: 27159393, PMID: 28687807)
Reduced proteolytic activation results in decreased levels of mature (fully processed) VEGFC. (demonstrated — PMID: 24523457)
Decreased mature VEGFC leads to reduced VEGFR3 (FLT4) autophosphorylation on lymphatic endothelial cells. (inferred from ligand loss + established VEGFR3 biology)
Reduced VEGFR3 signaling results in impaired downstream PI3K and Ras/ERK cascades. (demonstrated — impaired Vegfc/Vegfr3-dependent ERK signaling without Ccbe1; PMID: 24523457)
Impaired PI3K/ERK signaling leads to defective LEC proliferation, migration, sprouting, and survival. (demonstrated in zebrafish/mouse lymphangiogenesis models)
Defective lymphangiogenesis results in generalized lymphatic dysplasia — hypoplastic/dysfunctional, dilated lymphatic vessels (lymphangiectasia) throughout the body. (demonstrated clinically/histologically)
Branch A: dysplastic peripheral lymphatics cause congenital lymphedema.
Branch B: dilated/leaky intestinal lymphatics cause intestinal lymphangiectasia → protein-losing enteropathy, leaking protein- and lymphocyte-rich lymph into the gut lumen.
PLE leads to hypoproteinemia, hypoalbuminemia, lymphopenia, and hypogammaglobulinemia → secondary (combined) immunodeficiency and edema. (demonstrated — PMID: 26934740)
Branch C: dysplastic serosal/pulmonary lymphatics cause chylous ascites, pleural and pericardial effusions, and — at the severe end — prenatal non-immune hydrops fetalis. (demonstrated — PMID: 26333996, PMID: 30564329)
Facial dysmorphism and variable intellectual disability arise via incompletely understood developmental effects (possibly abnormal facial/cranial lymphatic development and fluid dynamics). (inferred)

CCBE1 (LoF, biallelic)
│  (with ADAMTS3)
▼
  pro-VEGFC  ──X──►  mature VEGFC   [reduced]
          │
          ▼
    VEGFR3 / FLT4   [reduced signaling]
     ┌────┴────┐
   PI3K      Ras/ERK
     └────┬────┘
          ▼
 LEC proliferation/migration/survival  [impaired]
          ▼
     Generalized lymphatic dysplasia
     ┌───────────┼───────────────┐
     ▼           ▼               ▼
   peripheral        intestinal      serosal/pulmonary
   lymphedema      lymphangiectasia   lymphatics
        │                  │
        ▼                  ▼
  protein-losing      ascites/pleural/
  enteropathy         pericardial effusion;
        │             prenatal hydrops fetalis
        ▼
hypoproteinemia, lymphopenia,
hypogammaglobulinemia
→ secondary immunodeficiency

Molecular pathways: VEGFC–VEGFR3 (FLT4) lymphangiogenic signaling; downstream PI3K-AKT and Ras/MAPK-ERK. Suggested GO terms: GO:0001945 (lymph vessel development), GO:0001946 (lymphangiogenesis), GO:0048010 (vascular endothelial growth factor receptor signaling pathway), GO:0004175 (endopeptidase activity, for ADAMTS3), GO:0043542 (endothelial cell migration), GO:0001936 (regulation of endothelial cell proliferation). Cellular processes: LEC proliferation, migration, survival; extracellular matrix–mediated growth-factor processing. Protein dysfunction: loss of CCBE1 co-factor/protease-enhancing function (reduced ADAMTS3 cleavage activity and reduced VEGFC/ADAMTS3 colocalization). Cell types: lymphatic endothelial cells (CL:0002138, endothelial cell of lymphatic vessel); supporting sources of Vegfc/Ccbe1 include fibroblasts and neuronal/mural cells in model systems (PMID: 32483144, PMID: 35316177). CHEBI: medium-chain triglyceride / triacylglycerol (CHEBI:17855) relevant to therapy.

7. Anatomical Structures Affected

Organ / system level: - Primary: lymphatic vasculature (systemic) — UBERON:0001473 (lymphatic vessel); lymphatic system (UBERON:0006558). - Intestine — intestinal lymphatics/lacteals (UBERON:0000160 intestine); site of lymphangiectasia and PLE. - Skin/subcutis of limbs and genitalia (lymphedema) — UBERON:0002097 (skin). - Serous cavities: peritoneal (ascites — UBERON:0002358), pleural (effusion — UBERON:0002402), pericardial (effusion — UBERON:0002407). - Lungs (pulmonary lymphangiectasia) — UBERON:0002048. - Face/craniofacial (dysmorphism). - Secondary/complication: immune system (secondary immunodeficiency), cardiovascular (pericardial effusion), and — prenatally — generalized fetal edema (hydrops).

Tissue and cell level: endothelial tissue of lymphatic vessels; the targeted cell population is the lymphatic endothelial cell (CL:0002138). Connective-tissue/fibroblast and mural/neuronal sources of pathway ligands are involved in normal development.

Subcellular level: CCBE1 is a secreted / extracellular matrix protein (GO:0005576 extracellular region; GO:0031012 extracellular matrix). VEGFR3 signaling occurs at the plasma membrane (GO:0005886) with downstream cytoplasmic kinase cascades.

Localization / lateralization: Systemic and generally bilateral; lymphedema of limbs and genitalia is typically bilateral, though asymmetric involvement occurs across the broader lymphangiectasia spectrum.

8. Temporal Development

Onset: Congenital — lymphedema is present at birth or in the neonatal period; the most severe presentations occur prenatally as recurrent non-immune hydrops fetalis (PMID: 30564329). Intestinal lymphangiectasia/PLE typically manifests in infancy/early childhood (PMID: 32629717).
Onset pattern: Chronic/congenital (structural developmental defect present from birth).
Progression: Lifelong (chronic). Effusions and PLE may follow a fluctuating/episodic course (e.g., recurrent pericardial effusion — PMID: 40394495). Some patients stabilize with dietary management; the underlying lymphatic dysplasia is non-remitting.
Disease duration: Chronic, lifelong.
Critical periods: The prenatal/perinatal period is the key window of vulnerability (hydrops-related mortality); early diagnosis and initiation of the MCT diet in infancy is the main window of therapeutic opportunity for PLE.

9. Inheritance and Population

Inheritance: Autosomal recessive (biallelic CCBE1). Recurrence risk 25% per pregnancy; carrier parents unaffected.
Penetrance: Effectively complete for the biallelic loss-of-function genotype, but expressivity is highly variable — from mild isolated lymphatic dysplasia to lethal recurrent hydrops (Findings 3, 8).
Epidemiology: Ultra-rare; ~50–60 total reported patients of Hennekam syndrome across all subtypes, with CCBE1 accounting for ~25% (PMID: 27345729, PMID: 38028107). Precise prevalence/incidence figures are not established; Orphanet lists Hennekam syndrome as <1/1,000,000.
Consanguinity / founder effects: Consanguinity is a major risk context; multiple homozygous CCBE1 families of Turkish, Pakistani, and other ancestries reported (PMID: 26686525, PMID: 25925991). No specific founder mutation is established.
Sex ratio: No strong sex bias reported (autosomal recessive; both sexes affected). Carrier frequency is not precisely defined but is expected to be low.
Geographic distribution: Worldwide; case clusters reflect consanguineous populations rather than a true endemic distribution.

10. Diagnostics

Genetic testing (definitive): Whole-exome sequencing (WES) or whole-genome sequencing is the recommended diagnostic approach because the phenotype is nonspecific and overlaps other GLD syndromes; WES is applicable prenatally (PMID: 30564329). Targeted CCBE1 sequencing or a primary-lymphedema/GLD gene panel (CCBE1, FAT4, ADAMTS3, PIEZO1, GDF2, FLT4, VEGFC, FOXC2, SOX18, etc.) is appropriate. Sanger confirmation of candidate variants and parental segregation should follow (PMID: 32472549). Chromosomal microarray/karyotype are used to exclude aneuploidy in hydrops workups but do not diagnose HKLLS1.

Supportive laboratory tests: hypoproteinemia, hypoalbuminemia, low immunoglobulins (hypogammaglobulinemia), lymphopenia, and elevated stool alpha-1-antitrypsin (marker of enteric protein loss) (PMID: 40394495, PMID: 32629717).

Imaging / functional: lymphoscintigraphy demonstrates abnormal lymphatic drainage/tracer leakage; ultrasound/MRI/CT characterize effusions, ascites, and lymphatic malformations; prenatal ultrasound detects hydrops, subcutaneous edema, and effusions.

Endoscopy / biopsy: upper GI endoscopy shows dilated intestinal lymphatics; duodenal/jejunal biopsy confirms dilated mucosal/submucosal lymphatic vessels (lymphangiectasia).

Clinical criteria / differential diagnosis: Diagnosis is clinical (generalized lymphedema + lymphangiectasia + dysmorphism ± ID) plus molecular confirmation. Differential diagnosis includes other autosomal recessive GLD genes — PIEZO1 (recessive GLD/hydrops, may have skeletal features; PMID: 34371190, PMID: 26333996, PMID: 33227434) and GDF2 (lymphatic dysplasia with hydrothorax and NIHF; PMID: 32618121) — as well as FAT4 (HKLLS2 / Van Maldergem syndrome overlap; PMID: 37551355) and ADAMTS3 (HKLLS3). Note that "Menke-Hennekam syndrome" (CREBBP/EP300, exon 30/31) is an unrelated dominant disorder despite the similar name.

Screening: No newborn screening exists. Cascade/carrier testing of at-risk relatives and prenatal/preimplantation genetic testing are appropriate once the familial variants are known.

11. Outcome / Prognosis

Survival/mortality: No formal survival statistics exist for HKLLS1 specifically. Prognosis is driven by severity of PLE, effusions, and prenatal hydrops; the most severe (hydrops) cases can be lethal in utero or perinatally (PMID: 30564329). For the related complication of primary intestinal lymphangiectasia, MCT treatment is associated with markedly lower mortality (3.7% vs 17.85% untreated) (PMID: 19449286).
Morbidity: Chronic lymphedema, recurrent effusions, malnutrition/growth impairment, recurrent infections from secondary immunodeficiency, and (in classic form) intellectual disability contribute substantial lifelong disability.
Complications: protein-losing enteropathy, chylous effusions, recurrent pericardial effusion (potential tamponade risk), infections, edema.
Recovery potential: The lymphatic dysplasia is structural and non-reversible; however, complications (PLE, effusions) are frequently controllable with supportive care, and some patients achieve complete symptom resolution of PLE on MCT diet (PMID: 19449286).
Prognostic factors: severity/onset of lymphatic dysplasia (prenatal hydrops = worst prognosis), degree of protein loss, presence of intellectual disability, and responsiveness to dietary therapy.

12. Treatment

No curative or disease-specific therapy exists. Management is supportive and multidisciplinary (Finding 9). Suggested NCIT concepts noted in brackets.

Dietary (cornerstone): low-long-chain-triglyceride, high-protein diet with medium-chain triglyceride (MCT) supplementation [NCIT: dietary intervention / medium-chain triglyceride]. Improves PLE; 63% complete resolution vs 35.7% untreated (PMID: 19449286, PMID: 20571826, PMID: 34587695). Total parenteral nutrition and fat-soluble vitamin supplementation when needed.
Pharmacologic: octreotide [NCIT:C1611 Octreotide] when MCT fails (PMID: 14723832); diuretics for effusions/edema; albumin transfusions for severe hypoproteinemia; immunoglobulin replacement (including subcutaneous) for symptomatic hypogammaglobulinemia (PMID: 26934740).
Lymphedema care: complex decongestive therapy, manual lymphatic drainage, and compression garments [NCIT: compression therapy / lymphedema therapy].
Surgical/interventional: drainage of pleural/pericardial effusions and ascites; pleurodesis; surgical resection of localized/segmental lymphatic lesions (PMID: 14723832).
Experimental / future directions: Because the mechanism converges on the VEGFC–VEGFR3 axis, VEGF-C–pathway–directed therapy is a conceptual future target. Preclinical work shows small molecules (e.g., notoginsenoside R1 via cAMP/PKA/CREB) can upregulate VEGF-C and promote lymphangiogenesis (PMID: 40020630); mTOR inhibitors (sirolimus) and VEGFR-3 inhibitors are used in other lymphatic malformations (PMID: 40414533). None are established HKLLS1 therapies. No genotype-guided pharmacogenomic approach exists.

13. Prevention

Primary prevention: Not possible for an established genetic disease. Genetic counseling for consanguineous couples and families with an affected child (25% recurrence risk) is central.
Secondary prevention: Prenatal testing (WES/targeted variant testing) and preimplantation genetic diagnosis once familial CCBE1 variants are known; prenatal ultrasound surveillance for hydrops in at-risk pregnancies. Cascade/carrier screening of relatives; expanded carrier screening can retrospectively identify pathway-gene disease in families (PMID: 37551355).
Tertiary prevention (complication prevention): early MCT dietary therapy to prevent malnutrition and immunodeficiency; immunoglobulin replacement and infection prophylaxis; monitoring/drainage of effusions to prevent tamponade; nutritional and fat-soluble vitamin support.
Immunization / public health: routine vaccination is advisable given secondary immunodeficiency risk, but no disease-specific immunization exists. No environmental public-health intervention applies.

14. Other Species / Natural Disease

Taxonomy / orthologs: CCBE1 orthologs are highly conserved across humans (Homo sapiens, NCBI:txid9606), mouse (Mus musculus, txid10090), and zebrafish (Danio rerio, txid7955) (PMID: 24523457). Human CCBE1 (NCBI Gene 147372); mouse Ccbe1; zebrafish ccbe1.
Natural disease in other species: No well-documented naturally occurring HKLLS1-equivalent Mendelian disease in companion animals or wildlife is established; disease models are experimentally induced (see Section 15).
Comparative biology: The VEGFC–CCBE1/ADAMTS3–VEGFR3 lymphangiogenesis mechanism is evolutionarily conserved across vertebrates, making cross-species models highly informative (PMID: 27159393, PMID: 28129845).
Transmission / zoonosis: Not applicable (genetic, non-transmissible).

15. Model Organisms

Zebrafish (Danio rerio): The principal functional-validation model. ccbe1 loss-of-function reproduces lymphatic sprouting defects; ccbe1 genetically interacts with vegfc and vegfr3 (flt4); patient CCBE1 variants (L27P, D104N, C98W) tested for loss of function; mature VEGFC overexpression rescues the phenotype (PMID: 24523457, PMID: 27345729). Zebrafish's superior live imaging has clarified the cellular sources (fibroblasts, neuronal/mural cells) of Vegfc-processing components (PMID: 32483144, PMID: 35316177). Resource: ZFIN.
Mouse (Mus musculus): Ccbe1 is required for lymphatic development identically to Vegfc and Adamts3; biochemistry established that the ADAMTS3–CCBE1 complex forms independently of VEGFR3 and activates VEGFC (not VEGFD) (PMID: 27159393); the N-terminal EGF domain of CCBE1 colocalizes pro-VEGFC with ADAMTS3 (PMID: 28687807). Inducible/conditional Ccbe1 deletion impairs postnatal meningeal lymphatic development and macromolecule drainage and causes age-related regression (PMID: 38141283). Resources: MGI, IMPC.
In vitro: lymphatic endothelial cell cultures and transgenic reporter assays for VEGFC processing and VEGFR3 signaling.
Phenotype recapitulation: Models faithfully reproduce the lymphatic developmental defect and the molecular mechanism (VEGFC maturation/VEGFR3 signaling). Limitations: they less directly model the full human syndrome (facial dysmorphism, intellectual disability, in-situ protein-losing enteropathy), and species differences in lymphatic anatomy exist.

Mechanistic Model / Interpretation

The findings converge on a single, coherent, well-supported mechanistic narrative: HKLLS1 is a lymphangiogenesis-signaling deficiency disease. CCBE1 is not itself a growth factor or receptor; it is an accessory extracellular protein whose job is to make the ADAMTS3 protease efficiently convert inactive pro-VEGFC into the mature VEGFC ligand and to concentrate that reaction at the lymphatic endothelial cell surface. Remove CCBE1 function and the entire VEGFC → VEGFR3 → PI3K/ERK signaling axis is throttled at its source. Because this axis is the master pathway for lymphatic vessel growth, its partial failure produces a generalized — not localized — lymphatic dysplasia, which then manifests organ-by-organ: skin (lymphedema), gut (lymphangiectasia → protein-losing enteropathy → secondary immunodeficiency), and serosal/pulmonary lymphatics (effusions, and, at the extreme, prenatal hydrops).

This model explains the disease's key features. The genetic heterogeneity (CCBE1, ADAMTS3, FAT4) is unified by the pathway: CCBE1 and ADAMTS3 are literally partners in the same enzymatic reaction, so mutating either produces the same phenotype. The variable expressivity — from a mild lymphatic dysplasia to lethal recurrent hydrops — is consistent with a dosage-sensitive signaling pathway in which residual VEGFC-activation capacity (variant-specific) sets the severity. The secondary immunodeficiency is fully accounted for as a downstream leak phenomenon, not a primary immune-cell defect, which correctly predicts that reducing enteric lymph loss (MCT diet) is the causal therapy rather than immunosuppression. And the therapeutic logic follows directly: absent a way to restore CCBE1/VEGFC signaling in patients, the only available levers are downstream and symptomatic — dietary reduction of lymphatic load, protein/immunoglobulin replacement, and mechanical management of edema and effusions.

The most important conceptual gap between the molecular model and the clinical picture is the non-lymphatic features — facial dysmorphism and intellectual disability — which are not obviously explained by lymphatic signaling and remain mechanistically inferred rather than demonstrated.

Layer	Upstream ← → Downstream
Genetic lesion	Biallelic LoF CCBE1 variants (upstream initiator)
Biochemical	↓ ADAMTS3–CCBE1 complex → ↓ pro-VEGFC cleavage → ↓ mature VEGFC
Signaling	↓ VEGFR3 (FLT4) activation → ↓ PI3K & Ras/ERK
Cellular	↓ LEC proliferation/migration/survival
Tissue	Generalized lymphatic dysplasia / lymphangiectasia
Clinical (downstream)	Lymphedema; PLE → secondary immunodeficiency; effusions; hydrops; dysmorphism; ID

Evidence Base

PMID	Title (abbrev.)	How it supports the findings
24523457	Ccbe1 regulates Vegfc-mediated induction of Vegfr3 signaling	Core mechanism: CCBE1 upregulates mature VEGFC; rescue in zebrafish; ERK impaired without Ccbe1; conservation. Supports F1, F4, F5.
27345729	Expanding the genotypic spectrum of CCBE1 mutations	AR inheritance; ~25% CCBE1; patient variants are LoF in zebrafish. Supports F1, F2, F8, F9.
30450763	ADAMTS3 loss-of-function in HKLLS	Three-gene heterogeneity; CCBE1→ADAMTS3→VEGFC axis. Supports F2, F5.
28985353	Loss of ADAMTS3 activity causes HKLLS3	Establishes ADAMTS3 as HKLLS3. Supports F2.
41992670	Biallelic FAT4 splice variant causes HKLLS2	Confirms FAT4 (HKLLS2) and the CCBE1/FAT4/ADAMTS3 model. Supports F2.
27159393	Proteolytic activation ... VEGFC and VEGFD	ADAMTS3–CCBE1 complex activates VEGFC. Supports F5.
28687807	CCBE1 N-terminal domain and VEGF-C activation	CCBE1 colocalizes pro-VEGFC with ADAMTS3; enhances cleavage. Supports F4, F5.
40046235	Regulation of VEGFR3 signaling in LECs	Defines PI3K/Ras cascades; links VEGFR3 dysfunction to lymphatic disease. Supports F4.
38141283	CCBE1 regulates meningeal lymphatics	Inducible Ccbe1 deletion impairs lymphatic development/maintenance. Supports F5.
32629717	Intestinal lymphangiectasia — CCBE1	Clinical phenotype: lymphedema, dysmorphism, PLE labs. Supports F3, F6, F7.
31633297	FBXL7 — novel form of Hennekam	Core clinical tetrad; candidate 4th gene. Supports F3, F2.
25925991	Mild form of Hennekam — CCBE1 (C98W)	Phenotypic variability; consanguinity. Supports F3, F8.
26686525	Multiplex kindred — CCBE1	Consanguinity; variant with retained protein expression. Supports F8.
26934740	Secondary hypogammaglobulinemia (Waldmann's)	PLE → lymphopenia, hypoalbuminemia, hypogammaglobulinemia. Supports F6.
30564329	Novel mutation — WES for recurrent hydrops	WES diagnoses HKLLS1; prenatal hydrops. Supports F7.
26333996	PIEZO1 recessive GLD with NIHF	Defines GLD spectrum & prenatal hydrops; differential dx. Supports F7.
40394495	Recurrent pericardial effusion — HKLLS	Supportive diagnostics (albumin, stool α1-antitrypsin); pericardial effusion. Supports F3, F7.
19449286	MCT for primary intestinal lymphangiectasia	Quantifies MCT benefit and mortality reduction. Supports F9.
20571826	Protein-losing enteropathy in children	MCT diet as standard PLE management. Supports F9.
38028107	Newfound features — Hennekam ("51st case")	Ultra-rare scale. Supports F8.
34234628	In silico deleterious CCBE1 nsSNPs	Predicts additional pathogenic variants/residues. Supports F1 (genetics).

Evidence spans human clinical case reports/series (e.g., PMIDs 32629717, 25925991, 26686525, 40394495, 30564329, 36748365), model-organism functional studies (zebrafish/mouse; PMIDs 24523457, 27345729, 27159393, 38141283), and in vitro/biochemical work (PMIDs 28687807, 27159393), with computational prediction (PMID 34234628).

Limitations and Knowledge Gaps

Ultra-rare, case-report-based evidence. With only ~50–60 total Hennekam patients reported (a minority genotyped as CCBE1), quantitative frequencies, penetrance/expressivity estimates, survival statistics, and prevalence figures are imprecise or unavailable. Phenotype frequencies in Section 3 are qualitative, not derived from a systematic cohort.
Unexplained non-lymphatic features. The mechanistic link from CCBE1/VEGFC signaling to facial dysmorphism and intellectual disability is inferred, not demonstrated. Whether these reflect craniofacial lymphatic effects, a moonlighting CCBE1 function, or secondary metabolic/nutritional effects is unknown.
Genotype–phenotype correlation is weak. It is not established which CCBE1 variants predict severe (hydrops) vs mild disease; the concept that residual VEGFC-activation capacity sets severity is plausible but not quantitatively demonstrated.
No approved targeted therapy. VEGFC-pathway-directed treatment is conceptually attractive but entirely preclinical; efficacy/safety in HKLLS1 patients is unknown.
Model limitations. Zebrafish/mouse models reproduce the lymphatic developmental defect and molecular mechanism but do not fully capture the syndromic human phenotype (dysmorphism, ID, in-situ PLE).
Ontology term verification needed. The suggested HPO/GO/CL/UBERON/NCIT/CHEBI identifiers above are indicative and should be reconciled against the current ontology releases before knowledge-base ingestion.

Proposed Follow-up Experiments / Actions

Genotype–phenotype database. Aggregate all reported CCBE1 (and FAT4/ADAMTS3) HKLLS variants with paired phenotype severity into a curated table; correlate variant type/domain (e.g., conserved cysteines, EGF vs collagen domains) with severity and prenatal-hydrops risk.
Functional severity assay. Systematically test additional CCBE1 patient/candidate variants (including in-silico-predicted G330E, C102S, C174R, G107D) in the zebrafish rescue assay and quantitative VEGFC-processing biochemistry to build a variant-to-residual-activity map that could inform prognosis.
VEGFC-pathway therapeutic proof-of-concept. Evaluate VEGF-C/VEGFR3 agonism (e.g., recombinant mature VEGFC or small molecules that upregulate VEGF-C) in Ccbe1-deficient mouse/zebrafish models for rescue of lymphatic function and PLE-equivalent phenotypes.
Natural-history / registry study. Establish an international HKLLS registry to capture survival, complication rates, effusion recurrence, growth, immune outcomes, and MCT-diet response prospectively — currently the single greatest evidence gap.
Mechanism of non-lymphatic features. Investigate CCBE1/VEGFC-pathway effects on craniofacial and neurodevelopment (e.g., conditional models, patient iPSC-derived neural cells) to explain dysmorphism and intellectual disability.
Diagnostic pathway standardization. Define a formal diagnostic algorithm (GLD gene panel → WES/WGS → functional confirmation) and prenatal work-up (recurrent hydrops → exome) as a clinical guideline, with elevated stool α1-antitrypsin and lymphoscintigraphy as supportive first-line tests.
Ontology verification. Reconcile all suggested HPO/GO/CL/UBERON/NCIT/CHEBI terms against current ontology releases prior to knowledge-base ingestion.

Report compiled from a 5-iteration autonomous investigation: 9 confirmed findings, 62 papers reviewed. Evidence sources span human clinical case reports/series, model-organism (zebrafish, mouse) functional studies, and in vitro biochemistry, as noted per claim.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome	Count
References checked	37
Resolved	37
Unresolved (possible confabulation)	0
Unverifiable	0
Quoted claims checked	14
Quoted claims found in source	14
Quoted claims not found in source	0
References weighed for topical relevance	37
On topic	17
Off topic	0

All extracted references resolved successfully.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome	Count
Terms checked	37
Resolved	35
Unresolved (possible confabulation)	0
Obsolete	0
Unverifiable	2
Terms whose name was checked	13
Terms named correctly	11
Terms named as a different term	0
Terms whose name is worth a second look	2

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

HP:0001888 (1 mention) - the report calls it "Lymphopenia"; HP calls it Decreased total lymphocyte count, and lists "Lymphopenia" among its other names
GO:0004175 (1 mention) - the report calls it "endopeptidase activity, for ADAMTS3"; GO calls it endopeptidase activity

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA.

35 of 37 terms resolved to a current term; the rest could not be looked up either way.

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Classifications

Inheritance

Discussions and Knowledge Gaps

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Medical Actions

Diagnosis

Progression

Prevalence

Clinical Burden

Differential Diagnoses

Animal Models

Source YAML

References & Deep Research

Deep Research

Disease Characteristics Research Template

Target Disease

Research Objectives

1. Disease Information

2. Etiology

3. Phenotypes

4. Genetic/Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome/Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Citation Requirements

Output Format

Hennekam lymphangiectasia–lymphedema syndrome 1

Executive summary

1. Disease information

Definition and nomenclature

2. Etiology, risk, protection, and environment

Causal factor

Risk and protective factors

3. Phenotypic spectrum

Core lymphatic phenotypes

Craniofacial and neurodevelopmental phenotypes

Prenatal and severe presentations

4. Genetic and molecular information

Gene and inheritance

5. Mechanism and pathophysiology

Causal chain

6. Anatomy and temporal development

7. Epidemiology and population genetics

8. Diagnosis

Clinical and laboratory evaluation

Molecular testing

Differential diagnosis

9. Outcome and prognosis

10. Treatment and real-world implementation

Lymphedema management

Visceral and supportive management

Trials and 2023–2024 developments

11. Prevention

12. Other species and model organisms

Zebrafish

Mouse

In-vitro systems

13. Evidence limitations and expert assessment

Key publications and URLs

Artifacts

Hennekam Lymphangiectasia–Lymphedema Syndrome Type 1 (HKLLS1): Comprehensive Disease Characteristics Report

Summary

Key Findings

Finding 1 — HKLLS1 is caused by biallelic loss-of-function CCBE1 variants that impair VEGFC processing

Finding 2 — Hennekam syndrome is genetically heterogeneous: CCBE1 (type 1), FAT4 (type 2), ADAMTS3 (type 3)

Finding 3 — Core clinical phenotype: congenital lymphedema, intestinal lymphangiectasia/PLE, facial dysmorphism, variable ID, and secondary immunodeficiency

Finding 4 — Mechanistic axis: CCBE1/ADAMTS3 → mature VEGFC → VEGFR3 (FLT4) → PI3K/Ras-ERK in lymphatic endothelium