Congenital Lactase Deficiency

Mendelian MONDO:0009115 Pathograph 11 Show in embeddings browser Gastrointestinal Disease

Congenital lactase deficiency (CLD) is an ultra-rare autosomal recessive neonatal disorder caused by biallelic loss-of-function variants in LCT, the gene encoding lactase-phlorizin hydrolase (LPH). Affected infants have essentially absent intestinal brush-border lactase activity from birth and develop severe watery diarrhea from the first lactose-containing feed (breast milk or standard formula). CLD is historically enriched in Finland because of a founder nonsense mutation. CLD is genetically and clinically distinct from the extremely common "adult-type hypolactasia" (primary lactase non-persistence/lactose intolerance, MONDO:0100345), which is a normal polymorphic trait caused by a regulatory (non-coding) variant near LCT/MCM6 that lowers, but does not abolish, lactase expression only after weaning; CLD instead results from coding-region mutations in LCT itself that eliminate LPH protein function from birth. CLD is also distinct from secondary/acquired lactase deficiency due to mucosal injury.

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1
Inheritance
3
Pathophys.
4
Phenotypes
11
Pathograph
1
Genes
1
Medical Actions
9
References
👪

Inheritance

1
Autosomal Recessive HP:0000007
CLD is inherited as an autosomal recessive disorder; most Finnish patients are homozygous for a single founder nonsense mutation, and other populations show homozygous or compound heterozygous LCT loss-of-function genotypes.
autosomal recessive inheritance
Show evidence (2 references)
PMID:9758622 SUPPORT Human Clinical
"Congenital lactase deficiency (CLD) is an autosomal recessive, gastrointestinal disorder characterized by watery diarrhea starting during the first 1-10 d of life, in infants fed lactose-containing milks."
Directly states the autosomal recessive inheritance pattern of CLD.
PMID:16400612 SUPPORT Human Clinical
"Twenty-seven patients out of 32 (84%) were homozygous for a nonsense mutation, c.4170T-->A (Y1390X), designated "Fin(major).""
Documents the homozygous founder nonsense mutation genotype in most Finnish CLD patients.
⚙

Pathophysiology

3
LCT Loss-of-Function and Absent Lactase-Phlorizin Hydrolase Activity
Biallelic loss-of-function variants in the coding region of LCT prevent normal biosynthesis, folding, intracellular trafficking, or catalytic function of lactase-phlorizin hydrolase (LPH), the brush-border enzyme that hydrolyzes lactose. Several characterized CLD variants (e.g. p.Y1390*, p.E1612*, p.S1150Pfs*19, p.S1121L, p.R1587H, p.S688P) produce LPH proteins that are transport-incompetent and enzymatically inactive, so essentially no functional enzyme reaches the apical enterocyte membrane.
enterocyte CL:0000584 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves enterocyte (CL:0000584). CL:0000584 is a cell type from the Cell Ontology.
LCT hgnc:6530 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves LCT (hgnc:6530). hgnc:6530 is a gene from the HUGO Gene Nomenclature Committee.
protein transport GO:0015031 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal protein transport (GO:0015031). GO:0015031 is a biological process from the Gene Ontology. ⚠ ABNORMAL
lactase activity GO:0000016 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves absent lactase activity (GO:0000016). GO:0000016 is a molecular function from the Gene Ontology. ∅ ABSENT
Downstream
Failure to Hydrolyze Lactose into Glucose and Galactose Absent brush-border lactase activity prevents hydrolysis of dietary lactose into its absorbable monosaccharides, glucose and galactose.
Show evidence (2 references)
PMID:35007711 SUPPORT In Vitro
"Our data unequivocally demonstrate that these mutants are absolutely transport incompetent, some of which are readily degraded, and are enzymatically inactive."
Cell biology study demonstrates that characterized CLD LCT mutants fail to traffic to the membrane and lack catalytic activity.
PMID:30813293 SUPPORT Other
"CLD is associated with mutations in the translated region of the LPH gene that elicit loss-of-function of LPH."
Review confirms translated-region LCT mutations cause loss of LPH function in CLD.
Failure to Hydrolyze Lactose into Glucose and Galactose
With little or no functional LPH at the brush border, ingested lactose is not cleaved into glucose and galactose and cannot be absorbed by the normal monosaccharide transport route; it instead remains intact within the intestinal lumen.
enterocyte CL:0000584 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves enterocyte (CL:0000584). CL:0000584 is a cell type from the Cell Ontology.
carbohydrate metabolic process GO:0005975 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased carbohydrate metabolic process (GO:0005975). GO:0005975 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Osmotic Load and Unabsorbed Lactose in the Intestinal Lumen Undigested lactose remaining in the intestinal lumen is an osmotically active solute that draws water into the bowel lumen.
Show evidence (1 reference)
PMID:30813293 SUPPORT Other
"This disorder is diagnosed already during the first few days of the newborn's life due to the inability to digest lactose, the main carbohydrate in mammalian milk."
Confirms the neonatal-onset inability to digest lactose that defines CLD.
Osmotic Load and Unabsorbed Lactose in the Intestinal Lumen
The osmotic load created by unabsorbed lactose in the small and large bowel drives water secretion into the lumen, producing severe watery diarrhea beginning with the first lactose-containing feed, with dehydration, metabolic acidosis, and, in a subset of infants, secondary hypercalcemia and nephrocalcinosis.
enterocyte CL:0000584 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves enterocyte (CL:0000584). CL:0000584 is a cell type from the Cell Ontology.
Downstream
Severe Watery Diarrhea Osmotic water secretion produces severe watery diarrhea upon lactose exposure.
Hypercalcemia and Nephrocalcinosis A substantial fraction of infants with CLD develop hypercalcemia and medullary nephrocalcinosis, which resolve after starting a lactose-free diet.
Show evidence (1 reference)
PMID:9758622 SUPPORT Human Clinical
"Congenital lactase deficiency (CLD) is an autosomal recessive, gastrointestinal disorder characterized by watery diarrhea starting during the first 1-10 d of life, in infants fed lactose-containing milks."
Establishes the neonatal-onset watery diarrhea phenotype triggered by lactose-containing feeds.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Referential integrity issues (1):
  • Target 'Hypercalcemia and Nephrocalcinosis' (from 'Osmotic Load and Unabsorbed Lactose in the Intestinal Lumen') not found in named elements
Pathograph: causal mechanism network for Congenital Lactase Deficiency Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

4
Digestive 1
Severe Watery Diarrhea HP:0002014 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is watery diarrhea, annotated with Diarrhea (HP:0002014), qualified as temporality acute. HP:0002014 is a phenotype from the Human Phenotype Ontology.
Temporal: ACUTE
Show evidence (2 references)
PMID:9758622 SUPPORT Human Clinical
"Congenital lactase deficiency (CLD) is an autosomal recessive, gastrointestinal disorder characterized by watery diarrhea starting during the first 1-10 d of life, in infants fed lactose-containing milks."
Establishes severe neonatal-onset watery diarrhea as the defining CLD phenotype.
PMID:19161632 SUPPORT Human Clinical
"Congenital lactase deficiency (CLD) is a severe gastrointestinal disorder of newborns."
Independent source confirms CLD as a severe newborn gastrointestinal disorder.
Metabolism 2
Dehydration HP:0001944 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is dehydration (HP:0001944). HP:0001944 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:19432082 SUPPORT Human Clinical
"Untreated disease leads to dehydration that usually requires hospitalization."
Directly states that untreated CLD leads to dehydration requiring hospitalization.
Hypercalcemia HP:0003072 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is hypercalcemia (HP:0003072). HP:0003072 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:8523189 SUPPORT Human Clinical
"At the time of admission, 7 of 10 infants had hypercalcemia."
Directly documents hypercalcemia in a majority of a CLD infant case series.
PMID:40536626 SUPPORT Human Clinical
"the authors present the case of an infant referred to them for hypercalcemia, who had been experiencing watery diarrhea, abdominal distension, and failure to thrive since early neonatal age. The eventual diagnosis was congenital lactase deficiency."
Case report directly links CLD to infantile hypercalcemia presenting with watery diarrhea from early neonatal age.
Other 1
Nephrocalcinosis HP:0000121 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is nephrocalcinosis (HP:0000121). HP:0000121 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:8523189 SUPPORT Human Clinical
"Five of the seven infants for whom renal ultrasonography was performed at the time of diagnosis had medullary nephrocalcinosis."
Directly documents medullary nephrocalcinosis in CLD infants at diagnosis.
🧬

Genetic Associations

1
LCT loss-of-function variants (Causative)
Gene: LCT hgnc:6530 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is LCT (hgnc:6530). hgnc:6530 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (3 references)
PMID:16400612 SUPPORT Human Clinical
"our data demonstrate that, in contrast to common adult-type hypolactasia (lactose intolerance) caused by a variant of the regulatory element, the severe infancy form represents the outcome of mutations affecting the structure of the protein inactivating the enzyme."
Directly contrasts the CLD coding-region loss-of-function mechanism with the adult-type hypolactasia regulatory-variant mechanism, supporting the NEC disambiguation.
PMID:19161632 SUPPORT Human Clinical
"Both Finnish patients were heterozygous for the Finnish founder mutation Y1390X."
Confirms the Y1390X founder mutation recurs even in non-Finnish patients of partial Finnish ancestry.
PMID:9758622 SUPPORT Human Clinical
"the CLD mutation was found to be enriched in sparsely populated eastern and northern Finland, because of a founder effect."
Establishes the Finnish founder-effect basis for CLD enrichment.
💊

Medical Actions

1
Lifelong Lactose-Free Diet
Action: dietary interventionNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is dietary intervention (NCIT:C15447). NCIT:C15447 is a clinical intervention from the NCI Thesaurus. Ontology label: Dietary Intervention NCIT:C15447
Strict elimination of lactose (lactose-free formula in infancy, and a lactose-free/lactose-restricted diet thereafter) is the definitive first-line management of CLD and rapidly resolves diarrhea and associated hypercalcemia.
Mechanism Target:
MODULATES Osmotic Load and Unabsorbed Lactose in the Intestinal Lumen — Removing dietary lactose eliminates the osmotically active substrate that produces malabsorptive watery diarrhea, since the enzymatic block itself cannot be corrected by diet.
Show evidence (1 reference)
PMID:8523189 SUPPORT Human Clinical
"Hypercalcemia ceased within a week of the start of a lactose-free diet."
Rapid resolution of downstream hypercalcemia after lactose withdrawal supports that removing the substrate interrupts the osmotic-malabsorption mechanism.
Target Phenotypes: watery diarrhea HP:0002014 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets watery diarrhea, annotated with Diarrhea (HP:0002014). HP:0002014 is a phenotype from the Human Phenotype Ontology. dehydration HP:0001944 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets dehydration (HP:0001944). HP:0001944 is a phenotype from the Human Phenotype Ontology. hypercalcemia HP:0003072 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets hypercalcemia (HP:0003072). HP:0003072 is a phenotype from the Human Phenotype Ontology. nephrocalcinosis HP:0000121 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets nephrocalcinosis (HP:0000121). HP:0000121 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:17345962 SUPPORT Other
"results in severe clinical consequences if not properly diagnosed and treated by lactose avoidance."
Directly identifies lactose avoidance as the treatment that prevents severe clinical consequences of CLD.
PMID:8523189 SUPPORT Human Clinical
"Hypercalcemia ceased within a week of the start of a lactose-free diet."
Directly documents resolution of hypercalcemia after starting a lactose-free diet.
🔬

Biochemical Markers

1
Absent to Near-Absent Intestinal Lactase Activity (DECREASED)
Context: Jejunal/intestinal biopsy shows an almost total lack of lactase-phlorizin hydrolase activity in CLD, in contrast to the partial (5-10% of normal) residual activity typical of adult-type hypolactasia.
Pathograph Readouts
Readout Of LCT Loss-of-Function and Absent Lactase-Phlorizin Hydrolase Activity Negative Diagnostic
Near-total absence of jejunal lactase activity directly measures loss of LPH enzyme function caused by biallelic LCT coding mutations.
Show evidence (1 reference)
PMID:9758622 SUPPORT Human Clinical
"CLD is the most severe form of lactase deficiency, with an almost total lack of lactase-phlorizin hydrolase (LPH) activity on jejunal biopsy. In adult-type hypolactasia, the most common genetic enzyme deficiency in humans, this enzyme activity is reduced to 5%-10%."
Directly contrasts the near-total loss of jejunal LPH activity in CLD against the partial reduction in adult-type hypolactasia, supporting the NEC disambiguation.
Show evidence (1 reference)
PMID:9758622 SUPPORT Human Clinical
"CLD is the most severe form of lactase deficiency, with an almost total lack of lactase-phlorizin hydrolase (LPH) activity on jejunal biopsy."
Establishes near-total loss of jejunal lactase activity as the core biochemical finding in CLD.
🔬

Diagnosis

2
Intestinal Biopsy Disaccharidase Assay
Measurement of lactase activity in a jejunal/duodenal mucosal biopsy is the classic diagnostic assay, showing an almost total lack of lactase-phlorizin hydrolase activity.
Biopsy Procedure NCIT:C15189 NCI Thesaurus (NCIT)
Results: Near-absent lactase activity on jejunal biopsy.
Show evidence (1 reference)
PMID:9758622 SUPPORT Human Clinical
"CLD is the most severe form of lactase deficiency, with an almost total lack of lactase-phlorizin hydrolase (LPH) activity on jejunal biopsy."
Confirms intestinal biopsy lactase assay as the diagnostic finding.
Molecular LCT Genetic Testing
Sequencing the coding region of LCT identifies biallelic loss-of-function variants (e.g., the Finnish founder Y1390X mutation or other nonsense, frameshift, or missense variants), enabling molecular diagnosis and genetic counseling.
molecular genetic testing NCIT:C19770 NCI Thesaurus (NCIT)
Results: Homozygous or compound heterozygous LCT coding-region loss-of-function variants.
Show evidence (1 reference)
PMID:16400612 SUPPORT Human Clinical
"These findings facilitate genetic testing in clinical practice and enable genetic counseling for this severe disease."
Directly supports molecular LCT testing as a clinical diagnostic and counseling tool.
📊

Prevalence

2
Finland
Cases In Literature Not yet documented
CLD is historically enriched in Finland due to a founder nonsense mutation (Y1390X, "Fin(major)"); 42 patients were diagnosed in Finland between 1966 and the 1998 report, with the founder mutation traced to sparsely populated eastern and northern Finland.
Show evidence (1 reference)
PMID:9758622 SUPPORT Human Clinical
"Since 1966, 42 patients have been diagnosed in Finland."
Direct historical case count for the Finnish founder population.
Worldwide (outside Finland)
Cases In Literature Ultra Rare
CLD has increasingly been reported outside Finland (Central Europe, Japan, Italy, Turkey), with LCT mutations distinct from or in addition to the Finnish founder variant, suggesting the disease is more common than previously appreciated but remains ultra-rare worldwide.
Show evidence (1 reference)
PMID:19161632 SUPPORT Human Clinical
"This is the first report of CLD mutations in patients living outside Finland. It seems that disease is more common than previously thought."
Documents CLD cases and causative LCT mutations outside the Finnish founder population.
{ }

Source YAML

click to show 
name: Congenital Lactase Deficiency
creation_date: "2026-07-06T00:00:00Z"
category: Mendelian
synonyms:
- CLD
- lactase-phlorizin hydrolase deficiency, congenital
description: >-
  Congenital lactase deficiency (CLD) is an ultra-rare autosomal recessive
  neonatal disorder caused by biallelic loss-of-function variants in LCT, the
  gene encoding lactase-phlorizin hydrolase (LPH). Affected infants have
  essentially absent intestinal brush-border lactase activity from birth and
  develop severe watery diarrhea from the first lactose-containing feed
  (breast milk or standard formula). CLD is historically enriched in Finland
  because of a founder nonsense mutation. CLD is genetically and clinically
  distinct from the extremely common "adult-type hypolactasia" (primary
  lactase non-persistence/lactose intolerance, MONDO:0100345), which is a
  normal polymorphic trait caused by a regulatory (non-coding) variant near
  LCT/MCM6 that lowers, but does not abolish, lactase expression only after
  weaning; CLD instead results from coding-region mutations in LCT itself that
  eliminate LPH protein function from birth. CLD is also distinct from
  secondary/acquired lactase deficiency due to mucosal injury.
disease_term:
  preferred_term: congenital lactase deficiency
  term:
    id: MONDO:0009115
    label: congenital lactase deficiency
parents:
- Gastrointestinal Disease
inheritance:
- name: Autosomal Recessive
  inheritance_term:
    preferred_term: autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    CLD is inherited as an autosomal recessive disorder; most Finnish patients
    are homozygous for a single founder nonsense mutation, and other
    populations show homozygous or compound heterozygous LCT loss-of-function
    genotypes.
  evidence:
  - reference: PMID:9758622
    reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Congenital lactase deficiency (CLD) is an autosomal recessive,
      gastrointestinal disorder characterized by watery diarrhea starting
      during the first 1-10 d of life, in infants fed lactose-containing milks.
    explanation: Directly states the autosomal recessive inheritance pattern of CLD.
  - reference: PMID:16400612
    reference_title: "Mutations in the translated region of the lactase gene (LCT) underlie congenital lactase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Twenty-seven patients out of 32 (84%) were homozygous for a nonsense
      mutation, c.4170T-->A (Y1390X), designated "Fin(major)."
    explanation: Documents the homozygous founder nonsense mutation genotype in most Finnish CLD patients.
genetic:
- name: LCT loss-of-function variants
  gene_term:
    preferred_term: LCT
    term:
      id: hgnc:6530
      label: LCT
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  features: >-
    CLD is caused by mutations in the coding (translated) region of LCT that
    inactivate the lactase-phlorizin hydrolase enzyme, in contrast to the
    common adult-type hypolactasia variant, which lies in a distal regulatory
    element and only reduces developmental transcription of an otherwise
    normal LCT gene. The Finnish founder mutation is a nonsense variant,
    c.4170T>A (Y1390X); other populations carry distinct nonsense, frameshift,
    or missense LCT variants.
  evidence:
  - reference: PMID:16400612
    reference_title: "Mutations in the translated region of the lactase gene (LCT) underlie congenital lactase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      our data demonstrate that, in contrast to common adult-type hypolactasia
      (lactose intolerance) caused by a variant of the regulatory element, the
      severe infancy form represents the outcome of mutations affecting the
      structure of the protein inactivating the enzyme.
    explanation: Directly contrasts the CLD coding-region loss-of-function mechanism with the adult-type hypolactasia regulatory-variant mechanism, supporting the NEC disambiguation.
  - reference: PMID:19161632
    reference_title: "Four novel mutations in the lactase gene (LCT) underlying congenital lactase deficiency (CLD)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Both Finnish patients were heterozygous for the Finnish founder mutation
      Y1390X.
    explanation: Confirms the Y1390X founder mutation recurs even in non-Finnish patients of partial Finnish ancestry.
  - reference: PMID:9758622
    reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      the CLD mutation was found to be enriched in sparsely populated eastern
      and northern Finland, because of a founder effect.
    explanation: Establishes the Finnish founder-effect basis for CLD enrichment.
pathophysiology:
- name: LCT Loss-of-Function and Absent Lactase-Phlorizin Hydrolase Activity
  conforms_to: "diet_induced_osmotic_diarrhea#Loss of a Substrate-Specific Brush-Border Digestive or Absorptive Step"
  description: >-
    Biallelic loss-of-function variants in the coding region of LCT prevent
    normal biosynthesis, folding, intracellular trafficking, or catalytic
    function of lactase-phlorizin hydrolase (LPH), the brush-border enzyme
    that hydrolyzes lactose. Several characterized CLD variants (e.g. p.Y1390*,
    p.E1612*, p.S1150Pfs*19, p.S1121L, p.R1587H, p.S688P) produce LPH proteins
    that are transport-incompetent and enzymatically inactive, so essentially
    no functional enzyme reaches the apical enterocyte membrane.
  genes:
  - preferred_term: LCT
    term:
      id: hgnc:6530
      label: LCT
  cell_types:
  - preferred_term: enterocyte
    term:
      id: CL:0000584
      label: enterocyte
  molecular_functions:
  - preferred_term: lactase activity
    term:
      id: GO:0000016
      label: lactase activity
    modifier: ABSENT
  biological_processes:
  - preferred_term: protein transport
    term:
      id: GO:0015031
      label: protein transport
    modifier: ABNORMAL
  evidence:
  - reference: PMID:35007711
    reference_title: "Hypomorphic variants of lactase-phlorizin hydrolase in congenital lactase deficiency are trafficking incompetent and functionally inactive."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Our data unequivocally demonstrate that these mutants are absolutely
      transport incompetent, some of which are readily degraded, and are
      enzymatically inactive.
    explanation: Cell biology study demonstrates that characterized CLD LCT mutants fail to traffic to the membrane and lack catalytic activity.
  - reference: PMID:30813293
    reference_title: "Congenital Lactase Deficiency: Mutations, Functional and Biochemical Implications, and Future Perspectives."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      CLD is associated with mutations in the translated region of the LPH
      gene that elicit loss-of-function of LPH.
    explanation: Review confirms translated-region LCT mutations cause loss of LPH function in CLD.
  downstream:
  - target: Failure to Hydrolyze Lactose into Glucose and Galactose
    causal_link_type: DIRECT
    description: >-
      Absent brush-border lactase activity prevents hydrolysis of dietary
      lactose into its absorbable monosaccharides, glucose and galactose.
    evidence:
    - reference: PMID:17345962
      reference_title: "Molecular differentiation of congenital lactase deficiency from adult-type hypolactasia."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        some infants are born with an inability to digest lactase (congenital
        lactase deficiency or CLD) due to low levels of LPH activity, which
        results in severe clinical consequences if not properly diagnosed and
        treated by lactose avoidance.
      explanation: Review directly links low LPH activity in CLD to inability to digest lactose.
- name: Failure to Hydrolyze Lactose into Glucose and Galactose
  description: >-
    With little or no functional LPH at the brush border, ingested lactose is
    not cleaved into glucose and galactose and cannot be absorbed by the
    normal monosaccharide transport route; it instead remains intact within
    the intestinal lumen.
  cell_types:
  - preferred_term: enterocyte
    term:
      id: CL:0000584
      label: enterocyte
  chemical_entities:
  - preferred_term: lactose
    term:
      id: CHEBI:17716
      label: lactose
    modifier: INCREASED
  biological_processes:
  - preferred_term: carbohydrate metabolic process
    term:
      id: GO:0005975
      label: carbohydrate metabolic process
    modifier: DECREASED
  evidence:
  - reference: PMID:30813293
    reference_title: "Congenital Lactase Deficiency: Mutations, Functional and Biochemical Implications, and Future Perspectives."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      This disorder is diagnosed already during the first few days of the
      newborn's life due to the inability to digest lactose, the main
      carbohydrate in mammalian milk.
    explanation: Confirms the neonatal-onset inability to digest lactose that defines CLD.
  downstream:
  - target: Osmotic Load and Unabsorbed Lactose in the Intestinal Lumen
    causal_link_type: DIRECT
    description: >-
      Undigested lactose remaining in the intestinal lumen is an osmotically
      active solute that draws water into the bowel lumen.
    evidence:
    - reference: PMID:30813293
      reference_title: "Congenital Lactase Deficiency: Mutations, Functional and Biochemical Implications, and Future Perspectives."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        The symptoms are similar to those in other carbohydrate malabsorption
        disorders, such as congenital sucrase-isomaltase deficiency, and
        include severe osmotic watery diarrhea.
      explanation: Review classifies CLD symptoms as severe osmotic watery diarrhea from unabsorbed carbohydrate, analogous to other disaccharidase deficiencies.
- name: Osmotic Load and Unabsorbed Lactose in the Intestinal Lumen
  conforms_to: "diet_induced_osmotic_diarrhea#Unabsorbed Dietary Solute Retention in the Intestinal Lumen"
  description: >-
    The osmotic load created by unabsorbed lactose in the small and large
    bowel drives water secretion into the lumen, producing severe watery
    diarrhea beginning with the first lactose-containing feed, with
    dehydration, metabolic acidosis, and, in a subset of infants, secondary
    hypercalcemia and nephrocalcinosis.
  cell_types:
  - preferred_term: enterocyte
    term:
      id: CL:0000584
      label: enterocyte
  evidence:
  - reference: PMID:9758622
    reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Congenital lactase deficiency (CLD) is an autosomal recessive,
      gastrointestinal disorder characterized by watery diarrhea starting
      during the first 1-10 d of life, in infants fed lactose-containing milks.
    explanation: Establishes the neonatal-onset watery diarrhea phenotype triggered by lactose-containing feeds.
  downstream:
  - target: Severe Watery Diarrhea
    causal_link_type: DIRECT
    description: Osmotic water secretion produces severe watery diarrhea upon lactose exposure.
    evidence:
    - reference: PMID:9758622
      reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Congenital lactase deficiency (CLD) is an autosomal recessive,
        gastrointestinal disorder characterized by watery diarrhea starting
        during the first 1-10 d of life, in infants fed lactose-containing milks.
      explanation: Directly supports watery diarrhea beginning in the first days of life on lactose-containing feeds.
  - target: Hypercalcemia and Nephrocalcinosis
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Unhydrolyzed lactose reaching the ileum may directly enhance ileal calcium absorption.
    - Chronic diarrhea causes metabolic acidosis, which may promote hypercalcemia and renal calcium deposition.
    description: >-
      A substantial fraction of infants with CLD develop hypercalcemia and
      medullary nephrocalcinosis, which resolve after starting a
      lactose-free diet.
    evidence:
    - reference: PMID:8523189
      reference_title: "Hypercalcemia and nephrocalcinosis in patients with congenital lactase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We describe 11 infants with congenital lactase deficiency, whose age
        at diagnosis varied from 6 to 88 days. At the time of admission, 7 of
        10 infants had hypercalcemia.
      explanation: Clinical case series directly documents hypercalcemia in a majority of CLD infants at diagnosis.
    - reference: PMID:8523189
      reference_title: "Hypercalcemia and nephrocalcinosis in patients with congenital lactase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        The mechanism of hypercalcemia is unclear but may be related to
        metabolic acidosis or may be promoted by the lactose effect (i.e., by
        nonhydrolyzed lactose that has a direct enhancing effect on calcium
        absorption in the ileum).
      explanation: Directly proposes the intermediate mechanisms (acidosis, direct luminal lactose effect on ileal calcium absorption) linking unabsorbed lactose to hypercalcemia.
phenotypes:
- name: Severe Watery Diarrhea
  category: Gastrointestinal
  description: >-
    Severe watery diarrhea with neonatal onset, beginning within the first
    1-10 days of life on the first lactose-containing feed (breast milk or
    standard infant formula).
  phenotype_term:
    preferred_term: watery diarrhea
    term:
      id: HP:0002014
      label: Diarrhea
    temporality: ACUTE
  evidence:
  - reference: PMID:9758622
    reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Congenital lactase deficiency (CLD) is an autosomal recessive,
      gastrointestinal disorder characterized by watery diarrhea starting
      during the first 1-10 d of life, in infants fed lactose-containing milks.
    explanation: Establishes severe neonatal-onset watery diarrhea as the defining CLD phenotype.
  - reference: PMID:19161632
    reference_title: "Four novel mutations in the lactase gene (LCT) underlying congenital lactase deficiency (CLD)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Congenital lactase deficiency (CLD) is a severe gastrointestinal
      disorder of newborns.
    explanation: Independent source confirms CLD as a severe newborn gastrointestinal disorder.
- name: Dehydration
  category: Systemic
  phenotype_term:
    preferred_term: dehydration
    term:
      id: HP:0001944
      label: Dehydration
  evidence:
  - reference: PMID:19432082
    reference_title: "[Congenital lactase deficiency--a more common disease than previously thought?]."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Untreated disease leads to dehydration that usually requires
      hospitalization.
    explanation: Directly states that untreated CLD leads to dehydration requiring hospitalization.
- name: Hypercalcemia
  category: Metabolic
  phenotype_term:
    preferred_term: hypercalcemia
    term:
      id: HP:0003072
      label: Hypercalcemia
  evidence:
  - reference: PMID:8523189
    reference_title: "Hypercalcemia and nephrocalcinosis in patients with congenital lactase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      At the time of admission, 7 of 10 infants had hypercalcemia.
    explanation: Directly documents hypercalcemia in a majority of a CLD infant case series.
  - reference: PMID:40536626
    reference_title: "Congenital Lactase Deficiency, An Unusual Cause of Infantile Hypercalcemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      the authors present the case of an infant referred to them for
      hypercalcemia, who had been experiencing watery diarrhea, abdominal
      distension, and failure to thrive since early neonatal age. The eventual
      diagnosis was congenital lactase deficiency.
    explanation: Case report directly links CLD to infantile hypercalcemia presenting with watery diarrhea from early neonatal age.
- name: Nephrocalcinosis
  category: Renal
  phenotype_term:
    preferred_term: nephrocalcinosis
    term:
      id: HP:0000121
      label: Nephrocalcinosis
  evidence:
  - reference: PMID:8523189
    reference_title: "Hypercalcemia and nephrocalcinosis in patients with congenital lactase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Five of the seven infants for whom renal ultrasonography was performed
      at the time of diagnosis had medullary nephrocalcinosis.
    explanation: Directly documents medullary nephrocalcinosis in CLD infants at diagnosis.
biochemical:
- name: Absent to Near-Absent Intestinal Lactase Activity
  presence: DECREASED
  context: >-
    Jejunal/intestinal biopsy shows an almost total lack of lactase-phlorizin
    hydrolase activity in CLD, in contrast to the partial (5-10% of normal)
    residual activity typical of adult-type hypolactasia.
  readouts:
  - target: LCT Loss-of-Function and Absent Lactase-Phlorizin Hydrolase Activity
    relationship: READOUT_OF
    direction: NEGATIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      Near-total absence of jejunal lactase activity directly measures loss
      of LPH enzyme function caused by biallelic LCT coding mutations.
    evidence:
    - reference: PMID:9758622
      reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        CLD is the most severe form of lactase deficiency, with an almost
        total lack of lactase-phlorizin hydrolase (LPH) activity on jejunal
        biopsy. In adult-type hypolactasia, the most common genetic enzyme
        deficiency in humans, this enzyme activity is reduced to 5%-10%.
      explanation: Directly contrasts the near-total loss of jejunal LPH activity in CLD against the partial reduction in adult-type hypolactasia, supporting the NEC disambiguation.
  evidence:
  - reference: PMID:9758622
    reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      CLD is the most severe form of lactase deficiency, with an almost total
      lack of lactase-phlorizin hydrolase (LPH) activity on jejunal biopsy.
    explanation: Establishes near-total loss of jejunal lactase activity as the core biochemical finding in CLD.
diagnosis:
- name: Intestinal Biopsy Disaccharidase Assay
  description: >-
    Measurement of lactase activity in a jejunal/duodenal mucosal biopsy is
    the classic diagnostic assay, showing an almost total lack of
    lactase-phlorizin hydrolase activity.
  diagnosis_term:
    preferred_term: Biopsy Procedure
    term:
      id: NCIT:C15189
      label: Biopsy Procedure
  results: Near-absent lactase activity on jejunal biopsy.
  evidence:
  - reference: PMID:9758622
    reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      CLD is the most severe form of lactase deficiency, with an almost total
      lack of lactase-phlorizin hydrolase (LPH) activity on jejunal biopsy.
    explanation: Confirms intestinal biopsy lactase assay as the diagnostic finding.
- name: Molecular LCT Genetic Testing
  description: >-
    Sequencing the coding region of LCT identifies biallelic loss-of-function
    variants (e.g., the Finnish founder Y1390X mutation or other nonsense,
    frameshift, or missense variants), enabling molecular diagnosis and
    genetic counseling.
  diagnosis_term:
    preferred_term: molecular genetic testing
    term:
      id: NCIT:C19770
      label: Molecular Analysis
  results: Homozygous or compound heterozygous LCT coding-region loss-of-function variants.
  evidence:
  - reference: PMID:16400612
    reference_title: "Mutations in the translated region of the lactase gene (LCT) underlie congenital lactase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These findings facilitate genetic testing in clinical practice and
      enable genetic counseling for this severe disease.
    explanation: Directly supports molecular LCT testing as a clinical diagnostic and counseling tool.
treatments:
- name: Lifelong Lactose-Free Diet
  description: >-
    Strict elimination of lactose (lactose-free formula in infancy, and a
    lactose-free/lactose-restricted diet thereafter) is the definitive
    first-line management of CLD and rapidly resolves diarrhea and associated
    hypercalcemia.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: dietary intervention
    term:
      id: NCIT:C15447
      label: Dietary Intervention
  evidence:
  - reference: PMID:17345962
    reference_title: "Molecular differentiation of congenital lactase deficiency from adult-type hypolactasia."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      results in severe clinical consequences if not properly diagnosed and
      treated by lactose avoidance.
    explanation: Directly identifies lactose avoidance as the treatment that prevents severe clinical consequences of CLD.
  - reference: PMID:8523189
    reference_title: "Hypercalcemia and nephrocalcinosis in patients with congenital lactase deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypercalcemia ceased within a week of the start of a lactose-free diet.
    explanation: Directly documents resolution of hypercalcemia after starting a lactose-free diet.
  target_mechanisms:
  - target: Osmotic Load and Unabsorbed Lactose in the Intestinal Lumen
    treatment_effect: MODULATES
    description: >-
      Removing dietary lactose eliminates the osmotically active substrate
      that produces malabsorptive watery diarrhea, since the enzymatic
      block itself cannot be corrected by diet.
    evidence:
    - reference: PMID:8523189
      reference_title: "Hypercalcemia and nephrocalcinosis in patients with congenital lactase deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Hypercalcemia ceased within a week of the start of a lactose-free
        diet.
      explanation: Rapid resolution of downstream hypercalcemia after lactose withdrawal supports that removing the substrate interrupts the osmotic-malabsorption mechanism.
  target_phenotypes:
  - preferred_term: watery diarrhea
    term:
      id: HP:0002014
      label: Diarrhea
  - preferred_term: dehydration
    term:
      id: HP:0001944
      label: Dehydration
  - preferred_term: hypercalcemia
    term:
      id: HP:0003072
      label: Hypercalcemia
  - preferred_term: nephrocalcinosis
    term:
      id: HP:0000121
      label: Nephrocalcinosis
prevalence:
- population: Finland
  measure_type: CASES_IN_LITERATURE
  prevalence_class: NOT_YET_DOCUMENTED
  notes: >-
    CLD is historically enriched in Finland due to a founder nonsense
    mutation (Y1390X, "Fin(major)"); 42 patients were diagnosed in Finland
    between 1966 and the 1998 report, with the founder mutation traced to
    sparsely populated eastern and northern Finland.
  evidence:
  - reference: PMID:9758622
    reference_title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Since 1966, 42 patients have been diagnosed in Finland.
    explanation: Direct historical case count for the Finnish founder population.
- population: Worldwide (outside Finland)
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    CLD has increasingly been reported outside Finland (Central Europe,
    Japan, Italy, Turkey), with LCT mutations distinct from or in addition to
    the Finnish founder variant, suggesting the disease is more common than
    previously appreciated but remains ultra-rare worldwide.
  evidence:
  - reference: PMID:19161632
    reference_title: "Four novel mutations in the lactase gene (LCT) underlying congenital lactase deficiency (CLD)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      This is the first report of CLD mutations in patients living outside
      Finland. It seems that disease is more common than previously thought.
    explanation: Documents CLD cases and causative LCT mutations outside the Finnish founder population.
references:
- reference: PMID:16400612
  title: "Mutations in the translated region of the lactase gene (LCT) underlie congenital lactase deficiency."
  found_in: []
  findings: []
- reference: PMID:17345962
  title: "Molecular differentiation of congenital lactase deficiency from adult-type hypolactasia."
  found_in: []
  findings: []
- reference: PMID:19161632
  title: "Four novel mutations in the lactase gene (LCT) underlying congenital lactase deficiency (CLD)."
  found_in: []
  findings: []
- reference: PMID:19432082
  title: "[Congenital lactase deficiency--a more common disease than previously thought?]."
  found_in: []
  findings: []
- reference: PMID:30813293
  title: "Congenital Lactase Deficiency: Mutations, Functional and Biochemical Implications, and Future Perspectives."
  found_in: []
  findings: []
- reference: PMID:35007711
  title: "Hypomorphic variants of lactase-phlorizin hydrolase in congenital lactase deficiency are trafficking incompetent and functionally inactive."
  found_in: []
  findings: []
- reference: PMID:40536626
  title: "Congenital Lactase Deficiency, An Unusual Cause of Infantile Hypercalcemia."
  found_in: []
  findings: []
- reference: PMID:8523189
  title: "Hypercalcemia and nephrocalcinosis in patients with congenital lactase deficiency."
  found_in: []
  findings: []
- reference: PMID:9758622
  title: "Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene."
  found_in: []
  findings: []
📚

References & Deep Research

References

9
PMID:16400612
Mutations in the translated region of the lactase gene (LCT) underlie congenital lactase deficiency.
No top-level findings curated for this source.
PMID:17345962
Molecular differentiation of congenital lactase deficiency from adult-type hypolactasia.
No top-level findings curated for this source.
PMID:19161632
Four novel mutations in the lactase gene (LCT) underlying congenital lactase deficiency (CLD).
No top-level findings curated for this source.
PMID:19432082
[Congenital lactase deficiency--a more common disease than previously thought?].
No top-level findings curated for this source.
PMID:30813293
Congenital Lactase Deficiency: Mutations, Functional and Biochemical Implications, and Future Perspectives.
No top-level findings curated for this source.
PMID:35007711
Hypomorphic variants of lactase-phlorizin hydrolase in congenital lactase deficiency are trafficking incompetent and functionally inactive.
No top-level findings curated for this source.
PMID:40536626
Congenital Lactase Deficiency, An Unusual Cause of Infantile Hypercalcemia.
No top-level findings curated for this source.
PMID:8523189
Hypercalcemia and nephrocalcinosis in patients with congenital lactase deficiency.
No top-level findings curated for this source.
PMID:9758622
Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene.
No top-level findings curated for this source.