Hypopigmentation, Organomegaly, and Delayed Myelination and Development

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Inheritance

1

Autosomal dominant, arising de novo HP:0000006

Every reported case has carried a heterozygous CLCN7 missense variant absent from both parents. No transmitted case has been described, which is unsurprising given the severity and early onset. The dominance is not haploinsufficiency: ClC-7 functions as a homodimer, and the mutant subunit shifts the gating of wild-type subunits with which it dimerizes, so a single mutant allele raises the activity of the whole population.

Autosomal dominant inheritance

Show evidence (3 references)

PMID:31155284 SUPPORT Human Clinical

"Whole-exome and -genome sequencing revealed a de novo c.2144A>G variant in CLCN7 in both affected children."

Establishes the de novo heterozygous origin in the two index cases.

PMID:38838776 SUPPORT In Vitro

"shifted voltage-dependent gating to less positive potentials, an effect partially conferred to WT subunits in WT/mutant heteromers"

Supplies the mechanism of dominance - the mutant subunit alters its wild-type partner within the dimer, rather than merely halving normal activity.

PMID:23983121 SUPPORT INDIRECT In Vitro

"We conclude that common gating underlies the slow voltage activation of ClC-7."

Explains why a single mutant subunit can change the whole dimer: ClC-7 gates commonly, both subunits together, rather than each pore independently. Shown with osteopetrosis-associated accelerating mutants rather than the HOD variants, hence INDIRECT.

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Discussions and Knowledge Gaps

5

Why does bone escape? Loss of ClC-7 destroys osteoclast resorption and causes osteopetrosis, but gain of ClC-7 activity leaves the skeleton structurally normal. What makes the osteoclast ruffled border tolerant of excess exchange activity when the lysosome is not?

KNOWLEDGE GAP osteoclast_resilience

Attached to

pathophysiology#Increased ClC-7 Chloride/Proton Exchange Current

This is the question the disease poses most sharply, and answering it would say something general about how much of ClC-7 physiology is set by the transporter versus by the compartment it sits in. It also has a practical edge: if a therapy for the osteopetroses ever aims to raise ClC-7 activity, the safety question is whether it would reproduce HOD, and the answer currently rests on five patients.

Show evidence (1 reference)

PMID:38838776 SUPPORT In Vitro

"Osteopetrosis results from a loss of ClC-7, but osteoclasts remain resilient to increased ClC-7 activity."

States the asymmetry without accounting for it, which is the gap.

Is melanosomal pH actually lowered in HOD syndrome, and is that why these patients are hypopigmented? No study has measured melanosomal pH, tyrosinase activity or melanosome ultrastructure in a patient or in the knock-in mouse.

KNOWLEDGE GAP melanosome_ph_unmeasured

Attached to

pathophysiology#Reduced Melanin Synthesis in Melanosomes

The melanosome step is the weakest link in this entry's causal chain and the only one supported entirely by evidence from outside the disease. The inference is reasonable - melanosomes are lysosome-related organelles and tyrosinase is pH-sensitive - but it has not been tested, and the knock-in mouse makes it testable. The deep-research report reached the same chain independently and listed the same step among its own limitations.

Proposed experiments

Melanosomal pH and tyrosinase activity in the Clcn7 knock-in mouse

melanosome_ph_in_knockin_mouse

Measure melanosomal pH and tyrosinase activity in melanocytes from the Clcn7 p.Tyr715Cys knock-in mouse against littermate controls, and ask whether alkalinizing treatment restores pigment.

Would support

pathophysiology#Reduced Melanin Synthesis in Melanosomes

Would refute

pathophysiology#Reduced Melanin Synthesis in Melanosomes

Supporting outcome

Melanosomal pH is lower in knock-in melanocytes and tyrosinase activity is reduced.

Refuting outcome

Melanosomal pH is unchanged, implicating a route to hypopigmentation other than melanosomal over-acidification.

Show evidence (1 reference)

PMID:24820037 REFUTE Model Organism

"The normal coat colour of Clcn7(td/td) mice and their less severe neurodegeneration suggested that the ClC-7 protein, even when lacking measurable ion transport activity, is sufficient for hair pigmentation"

Sharpens the gap rather than filling it, and cuts against a purely transport-based account of the pigment defect. Mice expressing a transport-dead ClC-7 have normal coat colour, so pigmentation survives the loss of ion transport while depending on the protein being present. Whatever excess transport does to the melanosome in HOD, it is not simply the mirror of losing transport.

If gain of ClC-7 function causes HOD and loss causes osteopetrosis, why do some osteopetrosis-causing CLCN7 variants also accelerate ClC-7 activation? What besides the direction of the functional change decides which disease a CLCN7 variant produces?

KNOWLEDGE GAP gof_does_not_uniquely_predict_hod

Attached to

pathophysiology#Loss of PI(3,5)P2 Inhibition of ClC-7

The tidy summary - gain gives HOD, loss gives osteopetrosis - is the one this entry is built around, and it is not sufficient on its own. Some variants that accelerate voltage-dependent activation nevertheless cause osteopetrosis. Whatever separates them from p.Tyr715Cys and p.Lys285Thr is unknown, and the PI(3,5)P2 binding site is the current candidate because both HOD variants sit in it. Recording this stops the entry asserting a cleaner genotype-phenotype rule than the evidence supports.

Show evidence (1 reference)

PMID:38136669 REFUTE In Vitro

"Interestingly, not all osteopetrosis-causing CLCN7 mutations from patients are associated with a loss of ion transport. Some rather result in an acceleration of voltage-dependent ClC-7 activation."

Refutes the simple reading that increased ClC-7 activation is sufficient to produce the HOD phenotype rather than osteopetrosis.

Which cell type carries the neurological phenotype in HOD syndrome - the oligodendrocyte failing to myelinate, the neuron accumulating storage material, or the microglion failing to clear debris? None of these has been examined in HOD.

KNOWLEDGE GAP neural_branch_mechanism

Attached to

pathophysiology#Delayed Myelination

Delayed myelination is one of the four features the syndrome is named for, and it is the least mechanistically explained. What is known about ClC-7 in the CNS comes from loss of function, which cannot be assumed to transfer to a gain-of-function disease - the mouse knockout shows that losing ClC-7 produces storage without any pH change at all.

Show evidence (1 reference)

PMID:38294065 NO_EVIDENCE Model Organism

"we show that microglia and embryonic macrophages in zebrafish clcn7 mutants cannot effectively process extracellular debris in the form of apoptotic cells and β-amyloid"

Cited to mark the boundary of what is known, not as support. This is a loss-of-function zebrafish study, so it establishes that microglia need ClC-7 but says nothing about what excess ClC-7 activity does to them, which is the question HOD raises.

Is the hypogammaglobulinemia reported in one HOD patient a consequence of the lysosomal lesion in B-lineage cells, or an incidental finding in a single case?

KNOWLEDGE GAP immune_branch_speculative

Attached to

phenotypes#Hypogammaglobulinemia

The deep-research report proposed a mechanistic immune branch - impaired endolysosomal processing in B cells causing hypogammaglobulinemia. That is a plausible hypothesis and it is recorded here as one, not curated as a causal edge: it rests on a single patient, and no B-cell or immunoglobulin work has been done in this disease. The finding was prominent enough in that report for the case to be framed as an inborn error of immunity, which is exactly why it is worth marking as unestablished.

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Pathophysiology

13

De Novo Heterozygous CLCN7 Missense Variant

Two variants have been reported: p.Tyr715Cys (c.2144A>G), carried by four of the five published patients, and p.Lys285Thr (c.854A>C) in one. Both lie in the cytoplasmic portion of ClC-7 and affect residues lining the binding pocket for the signalling lipid PI(3,5)P2, which is the point of convergence between two otherwise distant substitutions.

Genetic context CLCN7 hgnc:2025 allele_type: missense variant_origin: DE_NOVO zygosity: HETEROZYGOUS functional_impact_category: GAIN_OF_FUNCTION

GAIN_OF_FUNCTION rather than HYPERMORPHIC: the variants do not simply raise the output of a normally regulated transporter, they remove the PI(3,5)P2 inhibition that regulates it and shift its voltage dependence, so the protein escapes its normal control. This is the variant's consequence; the separate modifier on the activity node downstream describes the resulting pathway state. See notes.

chloride/proton antiporter activity of ClC-7 GO:0015297

Downstream

Loss of PI(3,5)P2 Inhibition of ClC-7 Both reported variants sit in or beside the PI(3,5)P2 binding pocket.

Show evidence (2 references)

PMID:31155284 SUPPORT Human Clinical

"Whole-exome and -genome sequencing revealed a de novo c.2144A>G variant in CLCN7 in both affected children."

The human-genetics half of this node's claim: the variant exists, is de novo, and was found in patients. Split from the electrophysiology below, which is a different kind of study reported in the same paper.

PMID:31155284 SUPPORT In Vitro

"This p.Tyr715Cys variant, located in the C-terminal domain of ClC-7, resulted in increased outward currents when it was heterologously expressed in Xenopus oocytes."

Locates the substitution in the C-terminal domain and establishes that it increases current. Heterologous expression in Xenopus oocytes is an in-vitro measurement, so this is IN_VITRO even though the paper as a whole is a patient study.

Loss of PI(3,5)P2 Inhibition of ClC-7

PI(3,5)P2 normally restrains ClC-7. Both HOD variants weaken that restraint and shift voltage-dependent gating toward less positive potentials, so more of the transporter population is open at physiological membrane voltages.

Downstream

Increased ClC-7 Chloride/Proton Exchange Current

The functional lesion. Heterologous expression of p.Tyr715Cys yields increased outward currents, and the gating shift is predicted to drive extra chloride into the vesicle lumen down the existing pH gradient.

chloride transmembrane transporter activity GO:0015108 ⇑ GAIN OF FUNCTION

Downstream

Lysosomal Hyperacidification

Enlarged Lysosome-Related Vacuole Formation A second, partly separable consequence - vacuole formation was shown to require the exchange activity itself, not merely the acidification it produces.

Show evidence (1 reference)

PMID:38838776 SUPPORT In Vitro

"This shift predicts augmented pH gradient-driven Cl- uptake into vesicles."

States the transport consequence of the gating shift.

Lysosomal Hyperacidification

Patient fibroblasts sit about 0.2 pH units below control. The size of that shift is worth stating plainly: it is small in absolute terms, and it is the disease. Note the direction, because it is the opposite of the intuition carried over from most lysosomal storage disorders, where the lysosome fails to acidify.

lysosomal lumen acidification GO:0007042 ↑ INCREASED

Animal models

Clcn7 knockout mouse Mouse fails to recapitulate Losing ClC-7 does not lower lysosomal pH - it leaves neuronal lysosomal pH unchanged. The pH lesion in HOD is therefore specific to excess transporter activity and is not a general consequence of perturbing ClC-7 in either direction.

Downstream

Impaired Lysosomal and Autophagic Degradation

Reduced Melanin Synthesis in Melanosomes Proposed rather than measured in this disease. See the knowledge gap attached to this edge - no study has recorded melanosomal pH in a patient with HOD syndrome.

Show evidence (1 reference)

PMID:31155284 SUPPORT In Vitro

"Our results show that p.Tyr715Cys is a gain-of-function CLCN7 variant associated with developmental delay, organomegaly, and hypopigmentation resulting from lysosomal hyperacidity, abnormal storage, and enlarged intracellular vacuoles."

The authors' summary statement of the mechanism, naming hyperacidity as causal.

Enlarged Lysosome-Related Vacuole Formation

Grossly enlarged vacuoles arise by membrane fusion and are seen in patient fibroblasts, leukocytes, liver and kidney. This is the feature that distinguishes gain of ClC-7 function from its loss: losing ClC-7 also produces storage, but not these vacuoles.

Downstream

Impaired Lysosomal and Autophagic Degradation

Show evidence (3 references)

PMID:31155284 SUPPORT In Vitro

"Primary fibroblasts from both probands also exhibited markedly enlarged intracellular vacuoles; this finding was recapitulated by the overexpression of human p.Tyr715Cys CLCN7 in control fibroblasts, reflecting the dominant, gain-of-function nature of the variant."

Shows the vacuoles in patient cells and reproduces them in control cells by expressing the variant, which is what makes the variant causal rather than correlated.

PMID:38838776 SUPPORT In Vitro

"Overactivity of ClC-7 induces pathologically enlarged vacuoles in many tissues, which is distinct from lysosomal storage observed with the loss of ClC-7 function."

Distinguishes the gain-of-function cellular lesion from the loss-of-function one.

PMID:33217309 SUPPORT INDIRECT In Vitro

"Bafilomycin treatment abrogated vacuole generation, indicating that H+-driven Cl- accumulation osmotically drives vesicle enlargement."

Supplies the physical mechanism this node otherwise states without explaining - the vesicles swell because chloride is osmotically dragged in behind pumped protons. It is INDIRECT because it was shown for the sibling exchanger ClC-6, not ClC-7. The parallel is close: a de novo gain-of-function substitution in an endolysosomal CLC, slowed gating, larger currents, giant vacuoles requiring intact Cl-/H+ exchange.

Impaired Lysosomal and Autophagic Degradation

Degradation of endocytosed material falls inside the enlarged compartments and autophagic clearance is largely blocked, so undegraded substrate accumulates.

autophagosome-lysosome fusion and cargo degradation GO:0006914 ↓ DECREASED

Downstream

Foam Cell Transformation and Tissue Storage

Delayed Myelination

Global Developmental Delay Restores the link that the myelination/development split dropped. Held to the same standard as the myelination edge beside it - both outcomes follow the lysosomal lesion, and the intermediates are unknown for each. This is deliberately not routed through Delayed Myelination, because no source establishes that the myelination delay is what causes the developmental delay.

Show evidence (2 references)

PMID:38136669 SUPPORT In Vitro

"In cells expressing the ClC-7 gain-of-function mutant, autophagic clearance was largely impaired, resulting in a build-up of autophagic material."

Directly reports the autophagic block in cells carrying the HOD variant.

PMID:38136669 SUPPORT In Vitro

"Degradation of endocytosed material was reduced in these compartments and resulted in an accumulation of lysosomal storage material."

Links the enlarged compartments to failed degradation and to storage.

Foam Cell Transformation and Tissue Storage

Tissue macrophages fill with undegraded material and become foam cells, most heavily in liver portal fields and renal medullary interstitium. The organs enlarge while their function is preserved, which is an unusual combination and a useful diagnostic clue.

CD68+ tissue histiocyte CL:0000235

Animal models

Clcn7 p.Tyr715Cys knock-in mouse Mouse recapitulates Hepatomegaly attributable to abnormal storage, with enlarged vacuoles in fibroblasts.

Downstream

Organomegaly with Preserved Organ Function

Protein-Losing Enteropathy

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"A liver biopsy taken at 19 months of age revealed numerous CD68+ histiocytic cells, particularly in the portal fields, with foamy, highly expanded, and light-colored cytoplasm."

Histological demonstration of foam-cell transformation in a patient.

Reduced Melanin Synthesis in Melanosomes

The proposed route to hypopigmentation, and the weakest link in this entry's chain. The melanosome is a lysosome-related organelle, tyrosinase is the rate-limiting enzyme of melanin synthesis and is inhibited at acidic pH, so a transporter that over-acidifies lysosomes is expected to over-acidify melanosomes and suppress pigment. Every step of that is plausible and none of it has been measured in this disease: no study has reported melanosomal pH, tyrosinase activity or melanosome morphology in a patient with HOD syndrome. The supporting evidence below is drawn from melanocyte biology generally and is marked INDIRECT for that reason.

melanocyte CL:0000148

melanin biosynthetic process GO:0042438 ↓ DECREASED

Downstream

Hypopigmentation of Skin and Hair

Show evidence (2 references)

PMID:31214276 SUPPORT INDIRECT In Vitro

"The activity of tyrosinase, the rate-limiting enzyme required for melanin synthesis, is generally minimal in an acidic environment."

Establishes the pH dependence of tyrosinase in melanocytes. It is about vitamin-C-driven cytoplasmic acidification, not ClC-7, so it supports the step by inference rather than by observation in this disease.

PMID:19389708 SUPPORT INDIRECT In Vitro

"Numerous studies have shown that the pH of melanosome plays a key role in the regulation of melanin synthesis."

Independent support that melanosomal pH governs pigment output. Again general melanocyte biology, not a measurement in HOD syndrome.

Hypopigmentation of Skin and Hair

Present in all five reported patients. In the best-described case the hair and eyebrows are hypopigmented while the irides are not, so this is not uniform oculocutaneous albinism.

Animal models

Clcn7 p.Tyr715Cys knock-in mouse Mouse recapitulates

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"Note hypopigmentation of hair including eyebrows but not the irides, the massively distended abdomen due to organomegaly, and a gastric tube in place because of impaired intestinal transit."

Records the distribution of the pigment defect, including the sparing of the irides that separates it from classical oculocutaneous albinism.

Organomegaly with Preserved Organ Function

Liver, spleen and kidneys enlarge, in one case to the point of a massively distended abdomen, while laboratory function of those organs stays normal.

Show evidence (2 references)

PMID:31155284 SUPPORT Human Clinical

"Both children had delayed myelination and development, organomegaly, and hypopigmentation, but neither had osteopetrosis."

Records organomegaly in the two index patients alongside the absence of bone disease.

PMID:38838776 SUPPORT Human Clinical

"Over time, he also developed grossly enlarged organs with fully preserved function (liver, spleen, and kidneys)."

The preserved-function detail matters clinically - it means normal liver and renal panels do not argue against the diagnosis.

Protein-Losing Enteropathy

Progressive bowel-wall thickening impairs intestinal transit, with malabsorption and loss of protein and electrolytes. In the most fully reported patient this, rather than the neurological disease, dominates the clinical course.

Downstream

Failure to Thrive The enteric losses are what the growth failure is attributed to.

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"The clinical picture is dominated by progressive thickening of bowel walls, resulting in chronic impairment of intestinal transit, an extremely distended abdomen, malabsorption, loss of protein and electrolytes, and failure to thrive"

Establishes the enteropathy and its weight in the overall clinical picture.

Failure to Thrive

Growth failure downstream of the enteric losses rather than of poor intake. Split from the enteropathy node so the causal step between them is explicit rather than bundled.

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"The clinical picture is dominated by progressive thickening of bowel walls, resulting in chronic impairment of intestinal transit, an extremely distended abdomen, malabsorption, loss of protein and electrolytes, and failure to thrive"

The same sentence cited on the enteropathy node, quoted whole here too. The earlier form was the trailing noun phrase alone, which asserted nothing and left the causal reading resting on word order; this version carries the claim that the growth failure follows the enteric losses.

Delayed Myelination

Delayed myelination on MRI, and in the longest-followed patient reduced cerebral volume with frontotemporal accentuation and symmetrical signal change in the subthalamic nuclei. Split from the developmental phenotype below: both follow the lysosomal lesion, but no source establishes that the myelination delay is what causes the developmental delay, so no edge is drawn between them.

Show evidence (2 references)

PMID:31155284 SUPPORT Human Clinical

"Both children had delayed myelination and development, organomegaly, and hypopigmentation, but neither had osteopetrosis."

Records delayed myelination and development in the index patients.

PMID:38838776 SUPPORT Human Clinical

"A cranial MRI at 14 months of age revealed delayed myelination in addition to reduced cerebral volume, particularly evident in the frontotemporal region."

Radiological confirmation with the additional finding of volume loss.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

15

Blood 2

Vacuolated Lymphocytes FREQUENT HP:0001922

Show evidence (2 references)

PMID:38838776 SUPPORT Human Clinical

"Leukocytes also showed severe vacuolization."

Leukocyte vacuolization in one patient.

PMID:39056574 SUPPORT Human Clinical

"Our case report demonstrates that the novel gain‐of‐function CLCN7 mutation, p.Tyr715Cys, underlies a complex human disease phenotype including hypopigmentation, organomegaly, developmental delay, hypogammaglobulinemia with lymphocyte vacuoles, and CNS demyelination in the absence of the..."

Lymphocyte vacuoles in a second patient. The full sentence is quoted rather than the phrase, so the quote also carries the co-occurring features and the explicit absence of osteopetrosis.

Hypogammaglobulinemia OCCASIONAL Decreased circulating immunoglobulin concentration HP:0004313

Show evidence (1 reference)

PMID:39056574 SUPPORT Human Clinical

"We clinically and genetically evaluated a Taiwanese boy presenting with developmental delay, organomegaly, hypogammaglobulinemia and hypopigmentation without osteopetrosis."

The single reported case with immunoglobulin deficiency.

Cardiovascular 2

Hepatosplenomegaly OBLIGATE HP:0001433

Show evidence (1 reference)

PMID:39056574 SUPPORT Human Clinical

"Nicoli et al. (2019) previously reported two similar cases involving a 22‐month‐old Caucasian girl and a 14‐month‐old Ghanaian boy who presented with shared features of hypopigmentation, hepatosplenomegaly, and delayed myelination and psychomotor development."

Names hepatosplenomegaly as a shared feature across the two index cases. Quoted whole: the earlier form of this snippet began mid-word at "ented with", which validated as a substring but was not a usable quotation.

Left Ventricular Hypertrophy OCCASIONAL HP:0001712

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"A mild thickening of the left ventricular myocardium was first observed at 5 years of age"

Late-emerging cardiac involvement in one patient.

Digestive 1

Protein-Losing Enteropathy OCCASIONAL HP:0002243

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"The clinical picture is dominated by progressive thickening of bowel walls, resulting in chronic impairment of intestinal transit, an extremely distended abdomen, malabsorption, loss of protein and electrolytes, and failure to thrive"

Documents the enteropathy and its mechanism.

Eye 2

Ocular Albinism OCCASIONAL HP:0001107

Show evidence (2 references)

PMID:39056574 SUPPORT Human Clinical

"A 25‐month‐old Taiwanese male presented clinically with generalized hypopigmentation of the skin, hair, and ocular albinism that had been present since birth (Figure 1a)."

Records ocular albinism as present from birth in this patient.

PMID:38838776 REFUTE Human Clinical

"Note hypopigmentation of hair including eyebrows but not the irides, the massively distended abdomen due to organomegaly, and a gastric tube in place because of impaired intestinal transit."

A different patient with explicitly normal irides. Cited as REFUTE against this being a constant feature, which is why the band is OCCASIONAL rather than higher.

Strabismus OCCASIONAL HP:0000486

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"Patient 1 (aged 6 years and 4 months at the time of this report) presented at around 8 months of age with hypopigmentation, strabismus, muscular hypotonia, and a primary psychomotor developmental disorder."

Strabismus at presentation in one patient.

Genitourinary 1

Nephromegaly FREQUENT Enlarged kidney HP:0000105

Show evidence (2 references)

PMID:38838776 SUPPORT Human Clinical

"Over time, he also developed grossly enlarged organs with fully preserved function (liver, spleen, and kidneys)."

Documents renal involvement in the organomegaly.

PMID:39056574 SUPPORT Human Clinical

"Renal ultrasound revealed nephromegaly affecting both kidneys, with increased cortical echogenicity and poor corticomedullary differentiation (Figure 1c), suggestive of a renal tubulointerstitial process."

Independent second case, which is what moves this from OCCASIONAL to FREQUENT. The echogenicity and corticomedullary changes also indicate the kidney is not merely large but structurally involved.

Integument 1

Hypopigmentation OBLIGATE Hypopigmentation of the skin HP:0001010

Show evidence (2 references)

PMID:31155284 SUPPORT Human Clinical

"Both children had delayed myelination and development, organomegaly, and hypopigmentation, but neither had osteopetrosis."

Two of the five reported patients.

PMID:38838776 SUPPORT Human Clinical

"We report two unrelated males with hypopigmentation, muscular hypotonia, failure to thrive, organomegaly, delayed myelination, and psychomotor developmental disorder."

A further two patients.

Musculoskeletal 2

Hypotonia FREQUENT HP:0001252

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"We report two unrelated males with hypopigmentation, muscular hypotonia, failure to thrive, organomegaly, delayed myelination, and psychomotor developmental disorder."

Two patients reported with muscular hypotonia.

Delayed Skeletal Maturation OCCASIONAL HP:0002750

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"X-ray examinations of the skeleton at 6 years showed significantly delayed bone age (3 years), but no structural anomalies."

Records delayed bone age together with the explicit absence of structural bone disease, which is the finding that separates HOD from the CLCN7 osteopetroses.

Nervous System 3

Global Developmental Delay OBLIGATE HP:0001263

Show evidence (1 reference)

PMID:39056574 SUPPORT Human Clinical

"We clinically and genetically evaluated a Taiwanese boy presenting with developmental delay, organomegaly, hypogammaglobulinemia and hypopigmentation without osteopetrosis."

The fifth reported patient.

Delayed Myelination VERY_FREQUENT Delayed CNS myelination HP:0002188

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"A cranial MRI at 14 months of age revealed delayed myelination in addition to reduced cerebral volume, particularly evident in the frontotemporal region."

Radiological documentation in one patient, with volume loss alongside.

Frontotemporal Cerebral Atrophy OCCASIONAL HP:0006892

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"T2-weighted (F) and diffusion-weighted (G) cranial MRI scans at age 14 months showing frontotemporally accentuated brain volume reduction"

Names the regional distribution of the volume loss.

Growth 1

Failure to Thrive FREQUENT HP:0001508

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"We report two unrelated males with hypopigmentation, muscular hypotonia, failure to thrive, organomegaly, delayed myelination, and psychomotor developmental disorder."

Two patients reported with failure to thrive.

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Genetic Associations

1

CLCN7

Gene: CLCN7 hgnc:2025 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (3 references)

PMID:39056574 SUPPORT Human Clinical

"Whole-exome sequencing revealed a de novo gain-of-function variant, p.Tyr715Cys, in the C-terminal domain of ClC-7 encoded by CLCN7."

Independent confirmation of the recurrent de novo gain-of-function variant.

PMID:38838776 SUPPORT Human Clinical

"This variant is absent in the gnomAD database (v.4.0) and is predicted to be deleterious by various commonly applied bioinformatics prediction programs with a CADD score of 28.6 and a REVEL score of 0.872."

Population-frequency and in-silico support for the second, novel variant p.Lys285Thr.

PMID:11741829 SUPPORT Human Clinical

"it seems that ADO II reflects a dominant negative effect, whereas loss-of-function mutations in ClCN7 do not cause abnormalities in heterozygous individuals"

Source for the allelic architecture above, and specifically for the claim that heterozygous loss of CLCN7 is silent - which is what makes the dominant-negative and gain-of-function alleles the only heterozygous routes to disease.

Variants (2)

p.Tyr715Cys Pathogenic

c.2144A>G (NM_001287.5), in the cytoplasmic C-terminal domain. Carried by four of the five published patients, all de novo.

p.Lys285Thr Pathogenic

c.854A>C, in the cytoplasmic portion beside the PI(3,5)P2 binding site. Reported once, de novo, absent from gnomAD v4.0.

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Medical Actions

3

Chloroquine

Action: Pharmacotherapy NCIT:C15986

Agent: chloroquine CHEBI:3638

Platform: Small molecule

The only mechanism-directed treatment tried. Chloroquine is lysosomotropic and raises lysosomal pH, so it opposes the primary lesion directly, and in patient fibroblasts it normalised the pH. The clinical result did not follow: one patient treated from 19 months of age continued to progress. The two evidence items below therefore point in opposite directions on purpose - the first supports the mechanism, the second refutes clinical efficacy, and collapsing them into a single graded item would lose exactly the information a clinician needs.

Mechanism Target:

Lysosomal Hyperacidification — Raises lysosomal pH, opposing the transporter's over-acidification.

Show evidence (2 references)

PMID:31155284 SUPPORT In Vitro

"Fibroblasts from probands displayed a lysosomal pH approximately 0.2 units lower than that of control cells, and treatment with chloroquine normalized the pH."

Chloroquine corrects the primary biochemical lesion in patient cells. This is a claim about mechanism, in fibroblasts.

PMID:38838776 REFUTE Human Clinical

"clinical course is progressive despite treatment with chloroquine, a drug known to increase lysosomal pH"

Refutes clinical benefit in the one patient followed on treatment. A separate claim from the in-vitro rescue above, and the one that matters for care.

Intravenous immunoglobulin replacement

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

Platform: Protein replacement

Given to the one reported patient with hypogammaglobulinemia, to prevent infection. This is supportive rather than mechanism-directed: it replaces the missing product, and does nothing about the transporter. Whether the immunoglobulin deficiency is itself a consequence of the lysosomal lesion is an open question recorded in the discussions.

Mechanism Target:

Hypogammaglobulinemia — Replaces the deficient immunoglobulin rather than correcting its cause.

Show evidence (1 reference)

PMID:39056574 SUPPORT Human Clinical

"However, the patient was found to have hypogammaglobulinemia with low serum IgG and IgM immunoglobulin levels, requiring intravenous immunoglobulin replacement treatment to prevent infections."

Records the indication and that replacement was actually administered.

Gastrostomy feeding and nutritional support

Action: gastrostomy feeding for nutritional support Ontology label: Nutritional Support NCIT:C15433

Percutaneous endoscopic gastrostomy for supplemental nutrition, driven by the enteropathy rather than by poor intake. therapeutic_modality is deliberately left unset: the treatment is nutrition delivered through a device, and neither DEVICE nor BEHAVIORAL describes it honestly.

Mechanism Target:

Protein-Losing Enteropathy — Supports growth against enteric protein and electrolyte loss.

Show evidence (1 reference)

PMID:39056574 SUPPORT Human Clinical

"The patient exhibited protein‐losing enteropathy that required placement of a percutaneous endoscopic gastrostomy tube to provide supplemental nutritional support."

Ties the gastrostomy directly to the protein-losing enteropathy node.

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Biochemical Markers

1

Plasma chitotriosidase

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"The patient’s plasma chitotriosidase, a biomarker representing tissue-resident macrophage activation upon excessive lipid accumulation, was detected to be tremendously elevated (up to 18,400 nmol/h/ml; reference range <150 nmol/h/ml)."

Reports the value, the reference range, and what the marker represents.

🔬

Diagnosis

3

Vacuolated lymphocytes on peripheral blood film

The cheapest first step. Vacuolated lymphocytes in a child with hypopigmentation and organomegaly point at a storage disorder before any sequencing is ordered.

Show evidence (1 reference)

PMID:39056574 SUPPORT Human Clinical

"Our case report demonstrates that the novel gain‐of‐function CLCN7 mutation, p.Tyr715Cys, underlies a complex human disease phenotype including hypopigmentation, organomegaly, developmental delay, hypogammaglobulinemia with lymphocyte vacuoles, and CNS demyelination in the absence of the..."

Records the blood-film finding in a diagnosed patient, within the sentence that states the full phenotype it sits in.

Trio exome or genome sequencing

The diagnosis has been made by sequencing in every reported case, and given the de novo inheritance a trio design is what makes the call. Note that one patient was diagnosed retrospectively by targeted Sanger sequencing of banked fibroblasts, 25 years after death.

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"was referred to diagnostic trio exome analysis that revealed heterozygosity for a de novo CLCN7 variant"

Trio exome as the diagnostic route for the novel variant.

Misleading elevation of bound neuraminic acid

A named pitfall. In one patient a markedly elevated bound neuraminic acid suggested neuraminidase deficiency - that is, sialidosis, a different lysosomal disease with its own gene and its own management. The result came back only after genetic testing had already given the answer, which is the only reason it did not redirect the workup. The generic risk is real: a storage disorder produces abnormal storage biomarkers, and those biomarkers name whichever disease they were designed for.

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"A markedly elevated concentration of bound neuraminic acid suggestive of neuraminidase deficiency was only returned after results of genetic testing."

Documents both the misleading result and the fact that sequencing pre-empted it, which is what makes it a pitfall worth recording rather than a diagnostic route.

📊

Prevalence

1

Worldwide

Cases In Literature Ultra Rare

Five patients in the published literature as of this curation: two in the 2019 founding report, two in the 2024 mechanistic paper (one of them diagnosed retrospectively from banked fibroblasts, having died in 1999), and one Taiwanese boy in a 2024 case report. No prevalence estimate exists and none should be inferred from this count.

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"We now describe two additional HOD individuals with the previously described p.Tyr715Cys and a novel p.Lys285Thr mutation, respectively."

Establishes the incremental case count over the two original patients.

🔀

Differential Diagnoses

2

Conditions with similar clinical presentations that must be differentiated from Hypopigmentation, Organomegaly, and Delayed Myelination and Development:

Sialidosis (neuraminidase deficiency)

Overlapping Features Raised as a false lead in one reported case by an elevated bound neuraminic acid. It is a genuine differential on the clinical picture too, since sialidosis also presents with organomegaly, developmental delay and vacuolated cells.

Show evidence (1 reference)

PMID:38838776 SUPPORT Human Clinical

"A markedly elevated concentration of bound neuraminic acid suggestive of neuraminidase deficiency was only returned after results of genetic testing."

The biochemical result that pointed at the wrong lysosomal disease.

🐁

Animal Models

2

Clcn7 p.Tyr715Cys knock-in mouse

Reproduces the two most visible features of the human disease, and - importantly for this entry's central contrast - also reproduces the absence of bone disease.

Species

Mouse

Genotype

Clcn7 knock-in of the variant equivalent to human p.Tyr715Cys, heterozygous

Publication

PMID:31155284

Pathograph links

Foam Cell Transformation and Tissue Storage recapitulates fidelity: high Hepatomegaly attributable to abnormal storage, with enlarged vacuoles in fibroblasts.

Limitations: Reported at the level of organ size and cell morphology; the mouse has not been used to test the melanosome-pH step, which is where this entry's chain is weakest.

Show evidence (1 reference)

PMID:31155284 SUPPORT Model Organism

"A mouse harboring the knock-in Clcn7 variant exhibited hypopigmentation, hepatomegaly resulting from abnormal storage, and enlarged vacuoles in cultured fibroblasts."

Recapitulation of storage-driven organomegaly in the knock-in animal.

Hypopigmentation of Skin and Hair recapitulates fidelity: high

Limitations: The mouse shows the pigment phenotype but no published work establishes the melanosomal mechanism in it either, so it confirms the association without closing the gap.

Show evidence (1 reference)

PMID:31155284 SUPPORT Model Organism

"A mouse harboring the knock-in Clcn7 variant exhibited hypopigmentation, hepatomegaly resulting from abnormal storage, and enlarged vacuoles in cultured fibroblasts."

The knock-in animal is hypopigmented, matching the human phenotype.

Clcn7 knockout mouse

The loss-of-function counterpart, included deliberately as a contrast rather than as a model of this disease. It develops osteopetrosis, neurodegeneration and lysosomal storage - and its neurons show unchanged lysosomal pH, which is the opposite of the human gain-of-function finding.

Species

Mouse

Genotype

Clcn7 null, homozygous

Publication

PMID:15706348

Pathograph links

Lysosomal Hyperacidification fails to recapitulate fidelity: low Losing ClC-7 does not lower lysosomal pH - it leaves neuronal lysosomal pH unchanged. The pH lesion in HOD is therefore specific to excess transporter activity and is not a general consequence of perturbing ClC-7 in either direction.

Limitations: A null allele, not the human genotype, and pH was measured in cultured neurons rather than in the tissues that are enlarged in HOD. It is included to bound the claim that ClC-7 sets lysosomal pH, not as a model of HOD.

Show evidence (1 reference)

PMID:15706348 REFUTE Model Organism

"We show that ClC-7 knockout mice additionally display neurodegeneration and severe lysosomal storage disease despite unchanged lysosomal pH in cultured neurons."

Loss of ClC-7 produces storage without a pH shift, so the hyperacidification seen in HOD cannot be attributed to disturbed ClC-7 function in general.

{ }

Source YAML

click to show

name: Hypopigmentation, Organomegaly, and Delayed Myelination and Development
creation_date: "2026-09-03T16:45:00Z"
category: Mendelian
synonyms:
- HOD syndrome
- HOD
description: >-
  HOD syndrome (OMIM 618541) is an ultra-rare multisystem lysosomal disorder caused by
  de novo heterozygous missense variants in CLCN7, which encodes ClC-7, the lysosomal
  2Cl-/H+ antiporter. The reported variants increase ClC-7 transport activity, and the
  resulting over-acidified, grossly enlarged lysosome-related vacuoles accumulate across
  many tissues, producing hypopigmentation, hepatosplenomegaly with foam-cell
  infiltration, delayed myelination and global developmental delay.

  The entry exists chiefly for a contrast the knowledge base can hold but a single-gene
  view cannot. CLCN7 loss of function causes osteopetrosis, curated here as Osteopetrosis,
  Autosomal_Dominant_Osteopetrosis_Type_II and Autosomal_Recessive_Osteopetrosis. HOD is
  allelic with all three and shares none of their bone disease: every reported patient has
  been explicitly free of osteopetrosis. Bone is not a mildly affected organ in HOD, it is
  an unaffected one.
disease_term:
  preferred_term: HOD syndrome
  term:
    id: MONDO:0032805
    label: hypopigmentation, organomegaly, and delayed myelination and development
parents:
- lysosomal storage disease
- CLCN7-related disorder
inheritance:
- name: Autosomal dominant, arising de novo
  description: >-
    Every reported case has carried a heterozygous CLCN7 missense variant absent from both
    parents. No transmitted case has been described, which is unsurprising given the
    severity and early onset. The dominance is not haploinsufficiency: ClC-7 functions as a
    homodimer, and the mutant subunit shifts the gating of wild-type subunits with which it
    dimerizes, so a single mutant allele raises the activity of the whole population.
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  evidence:
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole-exome and -genome sequencing revealed a de novo c.2144A>G variant in CLCN7 in both affected children."
    explanation: Establishes the de novo heterozygous origin in the two index cases.
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "shifted voltage-dependent gating to less positive potentials, an effect partially conferred to WT subunits in WT/mutant heteromers"
    explanation: >-
      Supplies the mechanism of dominance - the mutant subunit alters its wild-type partner
      within the dimer, rather than merely halving normal activity.
  - reference: PMID:23983121
    reference_title: "Common gating of both CLC transporter subunits underlies voltage-dependent activation of the 2Cl-/1H+ exchanger ClC-7/Ostm1."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: "We conclude that common gating underlies the slow voltage activation of ClC-7."
    explanation: >-
      Explains why a single mutant subunit can change the whole dimer: ClC-7 gates commonly,
      both subunits together, rather than each pore independently. Shown with
      osteopetrosis-associated accelerating mutants rather than the HOD variants, hence INDIRECT.
pathophysiology:
- name: De Novo Heterozygous CLCN7 Missense Variant
  biological_scale: MOLECULAR
  description: >-
    Two variants have been reported: p.Tyr715Cys (c.2144A>G), carried by four of the five
    published patients, and p.Lys285Thr (c.854A>C) in one. Both lie in the cytoplasmic
    portion of ClC-7 and affect residues lining the binding pocket for the signalling lipid
    PI(3,5)P2, which is the point of convergence between two otherwise distant substitutions.
  genetic_context:
    gene:
      preferred_term: CLCN7
      term:
        id: hgnc:2025
        label: CLCN7
    allele_type: missense
    variant_origin: DE_NOVO
    zygosity: HETEROZYGOUS
    functional_impact_category: GAIN_OF_FUNCTION
    description: >-
      GAIN_OF_FUNCTION rather than HYPERMORPHIC: the variants do not simply raise the output
      of a normally regulated transporter, they remove the PI(3,5)P2 inhibition that
      regulates it and shift its voltage dependence, so the protein escapes its normal
      control. This is the variant's consequence; the separate modifier on the activity node
      downstream describes the resulting pathway state. See notes.
  molecular_functions:
  - preferred_term: chloride/proton antiporter activity of ClC-7
    term:
      id: GO:0015297
      label: antiporter activity
  downstream:
  - target: Loss of PI(3,5)P2 Inhibition of ClC-7
    causal_link_type: DIRECT
    description: Both reported variants sit in or beside the PI(3,5)P2 binding pocket.
    evidence:
    - reference: PMID:38838776
      reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Both mutations decreased ClC-7 inhibition by PI(3,5)P2 and affected residues lining its binding pocket"
      explanation: States that the variants act by removing lipid inhibition of the transporter.
  evidence:
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole-exome and -genome sequencing revealed a de novo c.2144A>G variant in CLCN7 in both affected children."
    explanation: >-
      The human-genetics half of this node's claim: the variant exists, is de novo, and was
      found in patients. Split from the electrophysiology below, which is a different kind of
      study reported in the same paper.
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "This p.Tyr715Cys variant, located in the C-terminal domain of ClC-7, resulted in increased outward currents when it was heterologously expressed in Xenopus oocytes."
    explanation: >-
      Locates the substitution in the C-terminal domain and establishes that it increases
      current. Heterologous expression in Xenopus oocytes is an in-vitro measurement, so this
      is IN_VITRO even though the paper as a whole is a patient study.
- name: Loss of PI(3,5)P2 Inhibition of ClC-7
  biological_scale: MOLECULAR
  description: >-
    PI(3,5)P2 normally restrains ClC-7. Both HOD variants weaken that restraint and shift
    voltage-dependent gating toward less positive potentials, so more of the transporter
    population is open at physiological membrane voltages.
  downstream:
  - target: Increased ClC-7 Chloride/Proton Exchange Current
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:38838776
      reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Loss of PI(3,5)P2 inhibition will further increase current amplitudes, but may not be a general feature of HOD."
      explanation: >-
        Supports the link while recording the authors' own limit on it - they attribute the
        gain principally to the gating shift and treat lost lipid inhibition as additive.
    - reference: PMID:37363915
      reference_title: "The chloride antiporter CLCN7 is a modifier of lysosome dysfunction in FIG4 and VAC14 mutants."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: IN_VITRO
      snippet: "Knock-out of CLCN7 corrected lysosomal swelling and partially corrected lysosomal hyperacidification in FIG4 null cell cultures."
      explanation: >-
        Independent confirmation of this step from the opposite direction. FIG4 loss lowers
        PI(3,5)P2 and so removes the same inhibition of ClC-7, producing enlarged and
        over-acidified lysosomes - and deleting CLCN7 reverses both. That is the HOD lesion
        arrived at through a different gene, which is why it is INDIRECT here.
- name: Increased ClC-7 Chloride/Proton Exchange Current
  biological_scale: MOLECULAR
  description: >-
    The functional lesion. Heterologous expression of p.Tyr715Cys yields increased outward
    currents, and the gating shift is predicted to drive extra chloride into the vesicle
    lumen down the existing pH gradient.
  molecular_functions:
  - preferred_term: chloride transmembrane transporter activity
    term:
      id: GO:0015108
      label: chloride transmembrane transporter activity
    modifier: GAIN_OF_FUNCTION
  downstream:
  - target: Lysosomal Hyperacidification
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:31155284
      reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Fibroblasts from probands displayed a lysosomal pH approximately 0.2 units lower than that of control cells, and treatment with chloroquine normalized the pH."
      explanation: >-
        Measures the pH consequence in patient cells and shows it is reversible by an
        alkalinizing agent, which is what ties the pH shift to transporter activity.
  - target: Enlarged Lysosome-Related Vacuole Formation
    causal_link_type: DIRECT
    description: >-
      A second, partly separable consequence - vacuole formation was shown to require the
      exchange activity itself, not merely the acidification it produces.
    evidence:
    - reference: PMID:38136669
      reference_title: "Impaired Autophagic Clearance with a Gain-of-Function Variant of the Lysosomal Cl(-)/H(+) Exchanger ClC-7."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "increased ClC-7 Cl-/H+ exchange activity is required for the formation of enlarged vacuoles by membrane fusion"
      explanation: >-
        Uncoupling mutations separate transport from the phenotype and show the exchange
        activity is necessary for vacuole formation.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "This shift predicts augmented pH gradient-driven Cl- uptake into vesicles."
    explanation: States the transport consequence of the gating shift.
- name: Lysosomal Hyperacidification
  biological_scale: CELLULAR
  description: >-
    Patient fibroblasts sit about 0.2 pH units below control. The size of that shift is
    worth stating plainly: it is small in absolute terms, and it is the disease. Note the
    direction, because it is the opposite of the intuition carried over from most lysosomal
    storage disorders, where the lysosome fails to acidify.
  biological_processes:
  - preferred_term: lysosomal lumen acidification
    term:
      id: GO:0007042
      label: lysosomal lumen acidification
    modifier: INCREASED
  downstream:
  - target: Impaired Lysosomal and Autophagic Degradation
    causal_link_type: DIRECT
  - target: Reduced Melanin Synthesis in Melanosomes
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Proposed rather than measured in this disease. See the knowledge gap attached to this
      edge - no study has recorded melanosomal pH in a patient with HOD syndrome.
  evidence:
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Our results show that p.Tyr715Cys is a gain-of-function CLCN7 variant associated with developmental delay, organomegaly, and hypopigmentation resulting from lysosomal hyperacidity, abnormal storage, and enlarged intracellular vacuoles."
    explanation: The authors' summary statement of the mechanism, naming hyperacidity as causal.
- name: Enlarged Lysosome-Related Vacuole Formation
  biological_scale: CELLULAR
  description: >-
    Grossly enlarged vacuoles arise by membrane fusion and are seen in patient fibroblasts,
    leukocytes, liver and kidney. This is the feature that distinguishes gain of ClC-7
    function from its loss: losing ClC-7 also produces storage, but not these vacuoles.
  downstream:
  - target: Impaired Lysosomal and Autophagic Degradation
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Primary fibroblasts from both probands also exhibited markedly enlarged intracellular vacuoles; this finding was recapitulated by the overexpression of human p.Tyr715Cys CLCN7 in control fibroblasts, reflecting the dominant, gain-of-function nature of the variant."
    explanation: >-
      Shows the vacuoles in patient cells and reproduces them in control cells by expressing
      the variant, which is what makes the variant causal rather than correlated.
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Overactivity of ClC-7 induces pathologically enlarged vacuoles in many tissues, which is distinct from lysosomal storage observed with the loss of ClC-7 function."
    explanation: Distinguishes the gain-of-function cellular lesion from the loss-of-function one.
  - reference: PMID:33217309
    reference_title: "A Recurrent Gain-of-Function Mutation in CLCN6, Encoding the ClC-6 Cl(-)/H(+)-Exchanger, Causes Early-Onset Neurodegeneration."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: "Bafilomycin treatment abrogated vacuole generation, indicating that H+-driven Cl- accumulation osmotically drives vesicle enlargement."
    explanation: >-
      Supplies the physical mechanism this node otherwise states without explaining - the
      vesicles swell because chloride is osmotically dragged in behind pumped protons. It is
      INDIRECT because it was shown for the sibling exchanger ClC-6, not ClC-7. The parallel
      is close: a de novo gain-of-function substitution in an endolysosomal CLC, slowed
      gating, larger currents, giant vacuoles requiring intact Cl-/H+ exchange.
- name: Impaired Lysosomal and Autophagic Degradation
  biological_scale: CELLULAR
  description: >-
    Degradation of endocytosed material falls inside the enlarged compartments and autophagic
    clearance is largely blocked, so undegraded substrate accumulates.
  biological_processes:
  - preferred_term: autophagosome-lysosome fusion and cargo degradation
    term:
      id: GO:0006914
      label: autophagy
    modifier: DECREASED
  downstream:
  - target: Foam Cell Transformation and Tissue Storage
    causal_link_type: DIRECT
  - target: Delayed Myelination
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Global Developmental Delay
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Restores the link that the myelination/development split dropped. Held to the same
      standard as the myelination edge beside it - both outcomes follow the lysosomal lesion,
      and the intermediates are unknown for each. This is deliberately not routed through
      Delayed Myelination, because no source establishes that the myelination delay is what
      causes the developmental delay.
  evidence:
  - reference: PMID:38136669
    reference_title: "Impaired Autophagic Clearance with a Gain-of-Function Variant of the Lysosomal Cl(-)/H(+) Exchanger ClC-7."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In cells expressing the ClC-7 gain-of-function mutant, autophagic clearance was largely impaired, resulting in a build-up of autophagic material."
    explanation: Directly reports the autophagic block in cells carrying the HOD variant.
  - reference: PMID:38136669
    reference_title: "Impaired Autophagic Clearance with a Gain-of-Function Variant of the Lysosomal Cl(-)/H(+) Exchanger ClC-7."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Degradation of endocytosed material was reduced in these compartments and resulted in an accumulation of lysosomal storage material."
    explanation: Links the enlarged compartments to failed degradation and to storage.
- name: Foam Cell Transformation and Tissue Storage
  biological_scale: TISSUE
  description: >-
    Tissue macrophages fill with undegraded material and become foam cells, most heavily in
    liver portal fields and renal medullary interstitium. The organs enlarge while their
    function is preserved, which is an unusual combination and a useful diagnostic clue.
  cell_types:
  - preferred_term: CD68+ tissue histiocyte
    term:
      id: CL:0000235
      label: macrophage
  downstream:
  - target: Organomegaly with Preserved Organ Function
    causal_link_type: DIRECT
  - target: Protein-Losing Enteropathy
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A liver biopsy taken at 19 months of age revealed numerous CD68+ histiocytic cells, particularly in the portal fields, with foamy, highly expanded, and light-colored cytoplasm."
    explanation: Histological demonstration of foam-cell transformation in a patient.
- name: Reduced Melanin Synthesis in Melanosomes
  biological_scale: CELLULAR
  description: >-
    The proposed route to hypopigmentation, and the weakest link in this entry's chain.
    The melanosome is a lysosome-related organelle, tyrosinase is the rate-limiting enzyme
    of melanin synthesis and is inhibited at acidic pH, so a transporter that over-acidifies
    lysosomes is expected to over-acidify melanosomes and suppress pigment. Every step of
    that is plausible and none of it has been measured in this disease: no study has
    reported melanosomal pH, tyrosinase activity or melanosome morphology in a patient with
    HOD syndrome. The supporting evidence below is drawn from melanocyte biology generally
    and is marked INDIRECT for that reason.
  biological_processes:
  - preferred_term: melanin biosynthetic process
    term:
      id: GO:0042438
      label: melanin biosynthetic process
    modifier: DECREASED
  cell_types:
  - preferred_term: melanocyte
    term:
      id: CL:0000148
      label: melanocyte
  downstream:
  - target: Hypopigmentation of Skin and Hair
    causal_link_type: DIRECT
  evidence:
  - reference: PMID:31214276
    reference_title: "Intramelanocytic Acidification Plays a Role in the Antimelanogenic and Antioxidative Properties of Vitamin C and Its Derivatives."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: "The activity of tyrosinase, the rate-limiting enzyme required for melanin synthesis, is generally minimal in an acidic environment."
    explanation: >-
      Establishes the pH dependence of tyrosinase in melanocytes. It is about vitamin-C-driven
      cytoplasmic acidification, not ClC-7, so it supports the step by inference rather than
      by observation in this disease.
  - reference: PMID:19389708
    reference_title: "{alpha}MSH and Cyclic AMP elevating agents control melanosome pH through a protein kinase A-independent mechanism."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: "Numerous studies have shown that the pH of melanosome plays a key role in the regulation of melanin synthesis."
    explanation: >-
      Independent support that melanosomal pH governs pigment output. Again general melanocyte
      biology, not a measurement in HOD syndrome.
- name: Hypopigmentation of Skin and Hair
  biological_scale: ORGANISM
  description: >-
    Present in all five reported patients. In the best-described case the hair and eyebrows
    are hypopigmented while the irides are not, so this is not uniform oculocutaneous albinism.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Note hypopigmentation of hair including eyebrows but not the irides, the massively distended abdomen due to organomegaly, and a gastric tube in place because of impaired intestinal transit."
    explanation: >-
      Records the distribution of the pigment defect, including the sparing of the irides that
      separates it from classical oculocutaneous albinism.
- name: Organomegaly with Preserved Organ Function
  biological_scale: ORGANISM
  description: >-
    Liver, spleen and kidneys enlarge, in one case to the point of a massively distended
    abdomen, while laboratory function of those organs stays normal.
  evidence:
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Both children had delayed myelination and development, organomegaly, and hypopigmentation, but neither had osteopetrosis."
    explanation: Records organomegaly in the two index patients alongside the absence of bone disease.
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Over time, he also developed grossly enlarged organs with fully preserved function (liver, spleen, and kidneys)."
    explanation: >-
      The preserved-function detail matters clinically - it means normal liver and renal panels
      do not argue against the diagnosis.
- name: Protein-Losing Enteropathy
  biological_scale: ORGANISM
  description: >-
    Progressive bowel-wall thickening impairs intestinal transit, with malabsorption and loss
    of protein and electrolytes. In the most fully reported patient this, rather than the
    neurological disease, dominates the clinical course.
  downstream:
  - target: Failure to Thrive
    causal_link_type: DIRECT
    description: The enteric losses are what the growth failure is attributed to.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical picture is dominated by progressive thickening of bowel walls, resulting in chronic impairment of intestinal transit, an extremely distended abdomen, malabsorption, loss of protein and electrolytes, and failure to thrive"
    explanation: Establishes the enteropathy and its weight in the overall clinical picture.
- name: Failure to Thrive
  biological_scale: ORGANISM
  description: >-
    Growth failure downstream of the enteric losses rather than of poor intake. Split from
    the enteropathy node so the causal step between them is explicit rather than bundled.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical picture is dominated by progressive thickening of bowel walls, resulting in chronic impairment of intestinal transit, an extremely distended abdomen, malabsorption, loss of protein and electrolytes, and failure to thrive"
    explanation: >-
      The same sentence cited on the enteropathy node, quoted whole here too. The earlier form
      was the trailing noun phrase alone, which asserted nothing and left the causal reading
      resting on word order; this version carries the claim that the growth failure follows
      the enteric losses.
- name: Delayed Myelination
  biological_scale: TISSUE
  description: >-
    Delayed myelination on MRI, and in the longest-followed patient reduced cerebral volume
    with frontotemporal accentuation and symmetrical signal change in the subthalamic nuclei.
    Split from the developmental phenotype below: both follow the lysosomal lesion, but no
    source establishes that the myelination delay is what causes the developmental delay, so
    no edge is drawn between them.
  evidence:
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Both children had delayed myelination and development, organomegaly, and hypopigmentation, but neither had osteopetrosis."
    explanation: Records delayed myelination and development in the index patients.
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A cranial MRI at 14 months of age revealed delayed myelination in addition to reduced cerebral volume, particularly evident in the frontotemporal region."
    explanation: Radiological confirmation with the additional finding of volume loss.
phenotypes:
- category: Integumentary
  name: Hypopigmentation
  frequency: OBLIGATE
  description: >-
    5/5 published patients. Read that denominator with care: the syndrome is named for this
    feature, so 100% is partly definitional rather than a measured penetrance. See the
    ascertainment discussion.
  phenotype_term:
    preferred_term: Hypopigmentation of skin and hair
    term:
      id: HP:0001010
      label: Hypopigmentation of the skin
  evidence:
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Both children had delayed myelination and development, organomegaly, and hypopigmentation, but neither had osteopetrosis."
    explanation: Two of the five reported patients.
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report two unrelated males with hypopigmentation, muscular hypotonia, failure to thrive, organomegaly, delayed myelination, and psychomotor developmental disorder."
    explanation: A further two patients.
- category: Ophthalmologic
  name: Ocular Albinism
  frequency: OCCASIONAL
  description: >-
    1/5. Documented in the Taiwanese patient as present since birth, alongside generalized
    skin and hair hypopigmentation. Note this is not the same claim as iris hypopigmentation:
    a different patient is explicitly described as having hypopigmented hair and eyebrows but
    normal irides, so ocular involvement is variable across the five reported cases rather
    than a constant feature.
  phenotype_term:
    preferred_term: Ocular albinism
    term:
      id: HP:0001107
      label: Ocular albinism
  evidence:
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 25‐month‐old Taiwanese male presented clinically with generalized hypopigmentation of the skin, hair, and ocular albinism that had been present since birth (Figure 1a)."
    explanation: Records ocular albinism as present from birth in this patient.
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "Note hypopigmentation of hair including eyebrows but not the irides, the massively distended abdomen due to organomegaly, and a gastric tube in place because of impaired intestinal transit."
    explanation: >-
      A different patient with explicitly normal irides. Cited as REFUTE against this being a
      constant feature, which is why the band is OCCASIONAL rather than higher.
- category: Gastrointestinal
  name: Hepatosplenomegaly
  frequency: OBLIGATE
  description: 5/5 published patients, with organ function preserved.
  phenotype_term:
    preferred_term: Hepatosplenomegaly
    term:
      id: HP:0001433
      label: Hepatosplenomegaly
  evidence:
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Nicoli et al. (2019) previously reported two similar cases involving a 22‐month‐old Caucasian girl and a 14‐month‐old Ghanaian boy who presented with shared features of hypopigmentation, hepatosplenomegaly, and delayed myelination and psychomotor development."
    explanation: >-
      Names hepatosplenomegaly as a shared feature across the two index cases. Quoted whole:
      the earlier form of this snippet began mid-word at "ented with", which validated as a
      substring but was not a usable quotation.
- category: Renal
  name: Nephromegaly
  frequency: FREQUENT
  description: >-
    2/5. Enlarged kidneys with foam-cell infiltration of the medullary interstitium in one
    patient, and bilateral nephromegaly on ultrasound in another.
  phenotype_term:
    preferred_term: Enlarged kidney
    term:
      id: HP:0000105
      label: Enlarged kidney
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Over time, he also developed grossly enlarged organs with fully preserved function (liver, spleen, and kidneys)."
    explanation: Documents renal involvement in the organomegaly.
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Renal ultrasound revealed nephromegaly affecting both kidneys, with increased cortical echogenicity and poor corticomedullary differentiation (Figure 1c), suggestive of a renal tubulointerstitial process."
    explanation: >-
      Independent second case, which is what moves this from OCCASIONAL to FREQUENT. The
      echogenicity and corticomedullary changes also indicate the kidney is not merely large
      but structurally involved.
- category: Neurologic
  name: Global Developmental Delay
  frequency: OBLIGATE
  description: >-
    5/5 published patients. As with hypopigmentation, this is one of the features the
    syndrome is defined by.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We clinically and genetically evaluated a Taiwanese boy presenting with developmental delay, organomegaly, hypogammaglobulinemia and hypopigmentation without osteopetrosis."
    explanation: The fifth reported patient.
- category: Neurologic
  name: Delayed Myelination
  frequency: VERY_FREQUENT
  description: >-
    4/5. Delayed myelination is documented in four patients; the fifth is described as having
    CNS demyelination rather than delayed myelination, which may be the same process seen
    later or may be a distinct finding. The band reflects the four in whom the term used was
    delayed myelination.
  phenotype_term:
    preferred_term: Delayed CNS myelination
    term:
      id: HP:0002188
      label: Delayed CNS myelination
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A cranial MRI at 14 months of age revealed delayed myelination in addition to reduced cerebral volume, particularly evident in the frontotemporal region."
    explanation: Radiological documentation in one patient, with volume loss alongside.
- category: Neurologic
  name: Frontotemporal Cerebral Atrophy
  frequency: OCCASIONAL
  description: 1/5. Reduced cerebral volume with frontotemporal accentuation on MRI at 14 months.
  phenotype_term:
    preferred_term: Frontotemporal cerebral atrophy
    term:
      id: HP:0006892
      label: Frontotemporal cerebral atrophy
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "T2-weighted (F) and diffusion-weighted (G) cranial MRI scans at age 14 months showing frontotemporally accentuated brain volume reduction"
    explanation: Names the regional distribution of the volume loss.
- category: Musculoskeletal
  name: Hypotonia
  frequency: FREQUENT
  description: 2/5.
  phenotype_term:
    preferred_term: Muscular hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report two unrelated males with hypopigmentation, muscular hypotonia, failure to thrive, organomegaly, delayed myelination, and psychomotor developmental disorder."
    explanation: Two patients reported with muscular hypotonia.
- category: Growth
  name: Failure to Thrive
  frequency: FREQUENT
  description: 2/5, driven by malabsorption and enteric protein loss rather than by poor intake alone.
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report two unrelated males with hypopigmentation, muscular hypotonia, failure to thrive, organomegaly, delayed myelination, and psychomotor developmental disorder."
    explanation: Two patients reported with failure to thrive.
- category: Gastrointestinal
  name: Protein-Losing Enteropathy
  frequency: OCCASIONAL
  description: >-
    1/5, but dominant in that patient's course - bowel-wall thickening with malabsorption
    and enteric loss of protein and electrolytes.
  phenotype_term:
    preferred_term: Protein-losing enteropathy
    term:
      id: HP:0002243
      label: Protein-losing enteropathy
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical picture is dominated by progressive thickening of bowel walls, resulting in chronic impairment of intestinal transit, an extremely distended abdomen, malabsorption, loss of protein and electrolytes, and failure to thrive"
    explanation: Documents the enteropathy and its mechanism.
- category: Hematologic
  name: Vacuolated Lymphocytes
  frequency: FREQUENT
  description: >-
    2/5. A cheap and widely available clue - vacuolated lymphocytes on a blood film point at
    a storage disorder before any sequencing is ordered.
  phenotype_term:
    preferred_term: Vacuolated lymphocytes
    term:
      id: HP:0001922
      label: Vacuolated lymphocytes
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Leukocytes also showed severe vacuolization."
    explanation: Leukocyte vacuolization in one patient.
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our case report demonstrates that the novel gain‐of‐function CLCN7 mutation, p.Tyr715Cys, underlies a complex human disease phenotype including hypopigmentation, organomegaly, developmental delay, hypogammaglobulinemia with lymphocyte vacuoles, and CNS demyelination in the absence of the osteopetrosis typically associated with CLCN7 loss‐of‐function mutations."
    explanation: >-
      Lymphocyte vacuoles in a second patient. The full sentence is quoted rather than the
      phrase, so the quote also carries the co-occurring features and the explicit absence of
      osteopetrosis.
- category: Immunologic
  name: Hypogammaglobulinemia
  frequency: OCCASIONAL
  description: >-
    1/5. Reported only in the Taiwanese patient, where it led to the case being framed as an
    inborn error of immunity.
  phenotype_term:
    preferred_term: Hypogammaglobulinemia
    term:
      id: HP:0004313
      label: Decreased circulating immunoglobulin concentration
  evidence:
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We clinically and genetically evaluated a Taiwanese boy presenting with developmental delay, organomegaly, hypogammaglobulinemia and hypopigmentation without osteopetrosis."
    explanation: The single reported case with immunoglobulin deficiency.
- category: Cardiovascular
  name: Left Ventricular Hypertrophy
  frequency: OCCASIONAL
  description: 1/5, appearing at age 5 years and mild.
  phenotype_term:
    preferred_term: Left ventricular hypertrophy
    term:
      id: HP:0001712
      label: Left ventricular hypertrophy
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A mild thickening of the left ventricular myocardium was first observed at 5 years of age"
    explanation: Late-emerging cardiac involvement in one patient.
- category: Skeletal
  name: Delayed Skeletal Maturation
  frequency: OCCASIONAL
  description: >-
    1/5. Worth recording precisely because of what accompanies it - bone age was delayed but
    the skeleton was structurally normal, with no sclerosis.
  phenotype_term:
    preferred_term: Delayed skeletal maturation
    term:
      id: HP:0002750
      label: Delayed skeletal maturation
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "X-ray examinations of the skeleton at 6 years showed significantly delayed bone age (3 years), but no structural anomalies."
    explanation: >-
      Records delayed bone age together with the explicit absence of structural bone disease,
      which is the finding that separates HOD from the CLCN7 osteopetroses.
- category: Ophthalmologic
  name: Strabismus
  frequency: OCCASIONAL
  description: 1/5.
  phenotype_term:
    preferred_term: Strabismus
    term:
      id: HP:0000486
      label: Strabismus
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patient 1 (aged 6 years and 4 months at the time of this report) presented at around 8 months of age with hypopigmentation, strabismus, muscular hypotonia, and a primary psychomotor developmental disorder."
    explanation: Strabismus at presentation in one patient.
genetic:
- name: CLCN7
  notes: >-
    CLCN7 encodes ClC-7, which with its obligatory beta-subunit OSTM1 performs 2Cl-/H+
    exchange across the lysosomal membrane and across the osteoclast ruffled border. Four
    allelic outcomes are on record, and they do not lie on a single dose axis:

    biallelic loss of function causes infantile malignant autosomal recessive osteopetrosis;
    a dominant-negative allele causes Albers-Schonberg autosomal dominant osteopetrosis type
    II; the gain-of-function alleles below cause HOD syndrome with no bone phenotype at all;
    and simple heterozygous loss of function causes no abnormality whatever. That last point
    is the one most easily lost - CLCN7 haploinsufficiency is silent, so ADO II is not
    "half of" recessive osteopetrosis, and HOD is not the far end of a severity gradient.
  gene_term:
    preferred_term: CLCN7
    term:
      id: hgnc:2025
      label: CLCN7
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  variants:
  - name: p.Tyr715Cys
    description: >-
      c.2144A>G (NM_001287.5), in the cytoplasmic C-terminal domain. Carried by four of the
      five published patients, all de novo.
    clinical_significance: PATHOGENIC
  - name: p.Lys285Thr
    description: >-
      c.854A>C, in the cytoplasmic portion beside the PI(3,5)P2 binding site. Reported once,
      de novo, absent from gnomAD v4.0.
    clinical_significance: PATHOGENIC
  evidence:
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole-exome sequencing revealed a de novo gain-of-function variant, p.Tyr715Cys, in the C-terminal domain of ClC-7 encoded by CLCN7."
    explanation: Independent confirmation of the recurrent de novo gain-of-function variant.
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This variant is absent in the gnomAD database (v.4.0) and is predicted to be deleterious by various commonly applied bioinformatics prediction programs with a CADD score of 28.6 and a REVEL score of 0.872."
    explanation: Population-frequency and in-silico support for the second, novel variant p.Lys285Thr.
  - reference: PMID:11741829
    reference_title: "Albers-Schönberg disease (autosomal dominant osteopetrosis, type II) results from mutations in the ClCN7 chloride channel gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it seems that ADO II reflects a dominant negative effect, whereas loss-of-function mutations in ClCN7 do not cause abnormalities in heterozygous individuals"
    explanation: >-
      Source for the allelic architecture above, and specifically for the claim that
      heterozygous loss of CLCN7 is silent - which is what makes the dominant-negative and
      gain-of-function alleles the only heterozygous routes to disease.
biochemical:
- name: Plasma chitotriosidase
  notes: >-
    Massively elevated in the one patient in whom it was measured - up to 18,400 nmol/h/ml
    against a reference of under 150, more than a hundredfold. Chitotriosidase reports
    activated tissue macrophages, so the value is a readout of the foam-cell burden rather
    than of ClC-7 itself. Useful for the same reason it is useful in Gaucher disease, and
    subject to the same caveat: roughly 6% of people are chitotriosidase-deficient by
    genotype and will not mount the elevation.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient’s plasma chitotriosidase, a biomarker representing tissue-resident macrophage activation upon excessive lipid accumulation, was detected to be tremendously elevated (up to 18,400 nmol/h/ml; reference range <150 nmol/h/ml)."
    explanation: Reports the value, the reference range, and what the marker represents.
treatments:
- name: Chloroquine
  description: >-
    The only mechanism-directed treatment tried. Chloroquine is lysosomotropic and raises
    lysosomal pH, so it opposes the primary lesion directly, and in patient fibroblasts it
    normalised the pH. The clinical result did not follow: one patient treated from 19
    months of age continued to progress. The two evidence items below therefore point in
    opposite directions on purpose - the first supports the mechanism, the second refutes
    clinical efficacy, and collapsing them into a single graded item would lose exactly the
    information a clinician needs.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: chloroquine
      term:
        id: CHEBI:3638
        label: chloroquine
  target_mechanisms:
  - target: Lysosomal Hyperacidification
    description: Raises lysosomal pH, opposing the transporter's over-acidification.
  evidence:
  - reference: PMID:31155284
    reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Fibroblasts from probands displayed a lysosomal pH approximately 0.2 units lower than that of control cells, and treatment with chloroquine normalized the pH."
    explanation: >-
      Chloroquine corrects the primary biochemical lesion in patient cells. This is a claim
      about mechanism, in fibroblasts.
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "clinical course is progressive despite treatment with chloroquine, a drug known to increase lysosomal pH"
    explanation: >-
      Refutes clinical benefit in the one patient followed on treatment. A separate claim from
      the in-vitro rescue above, and the one that matters for care.
- name: Intravenous immunoglobulin replacement
  description: >-
    Given to the one reported patient with hypogammaglobulinemia, to prevent infection. This
    is supportive rather than mechanism-directed: it replaces the missing product, and does
    nothing about the transporter. Whether the immunoglobulin deficiency is itself a
    consequence of the lysosomal lesion is an open question recorded in the discussions.
  therapeutic_modality: PROTEIN_REPLACEMENT
  treatment_term:
    preferred_term: intravenous immunoglobulin replacement
    term:
      id: NCIT:C62710
      label: Immunoglobulin Therapy
  target_mechanisms:
  - target: Hypogammaglobulinemia
    description: Replaces the deficient immunoglobulin rather than correcting its cause.
  evidence:
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, the patient was found to have hypogammaglobulinemia with low serum IgG and IgM immunoglobulin levels, requiring intravenous immunoglobulin replacement treatment to prevent infections."
    explanation: Records the indication and that replacement was actually administered.
- name: Gastrostomy feeding and nutritional support
  description: >-
    Percutaneous endoscopic gastrostomy for supplemental nutrition, driven by the enteropathy
    rather than by poor intake. therapeutic_modality is deliberately left unset: the treatment
    is nutrition delivered through a device, and neither DEVICE nor BEHAVIORAL describes it
    honestly.
  treatment_term:
    preferred_term: gastrostomy feeding for nutritional support
    term:
      id: NCIT:C15433
      label: Nutritional Support
  target_mechanisms:
  - target: Protein-Losing Enteropathy
    description: Supports growth against enteric protein and electrolyte loss.
  evidence:
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient exhibited protein‐losing enteropathy that required placement of a percutaneous endoscopic gastrostomy tube to provide supplemental nutritional support."
    explanation: Ties the gastrostomy directly to the protein-losing enteropathy node.
diagnosis:
- name: Vacuolated lymphocytes on peripheral blood film
  description: >-
    The cheapest first step. Vacuolated lymphocytes in a child with hypopigmentation and
    organomegaly point at a storage disorder before any sequencing is ordered.
  evidence:
  - reference: PMID:39056574
    reference_title: "Multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our case report demonstrates that the novel gain‐of‐function CLCN7 mutation, p.Tyr715Cys, underlies a complex human disease phenotype including hypopigmentation, organomegaly, developmental delay, hypogammaglobulinemia with lymphocyte vacuoles, and CNS demyelination in the absence of the osteopetrosis typically associated with CLCN7 loss‐of‐function mutations."
    explanation: >-
      Records the blood-film finding in a diagnosed patient, within the sentence that states
      the full phenotype it sits in.
- name: Trio exome or genome sequencing
  description: >-
    The diagnosis has been made by sequencing in every reported case, and given the de novo
    inheritance a trio design is what makes the call. Note that one patient was diagnosed
    retrospectively by targeted Sanger sequencing of banked fibroblasts, 25 years after death.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "was referred to diagnostic trio exome analysis that revealed heterozygosity for a de novo CLCN7 variant"
    explanation: Trio exome as the diagnostic route for the novel variant.
- name: Misleading elevation of bound neuraminic acid
  description: >-
    A named pitfall. In one patient a markedly elevated bound neuraminic acid suggested
    neuraminidase deficiency - that is, sialidosis, a different lysosomal disease with its
    own gene and its own management. The result came back only after genetic testing had
    already given the answer, which is the only reason it did not redirect the workup. The
    generic risk is real: a storage disorder produces abnormal storage biomarkers, and those
    biomarkers name whichever disease they were designed for.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A markedly elevated concentration of bound neuraminic acid suggestive of neuraminidase deficiency was only returned after results of genetic testing."
    explanation: >-
      Documents both the misleading result and the fact that sequencing pre-empted it, which
      is what makes it a pitfall worth recording rather than a diagnostic route.
animal_models:
- name: Clcn7 p.Tyr715Cys knock-in mouse
  species: Mouse
  genotype: Clcn7 knock-in of the variant equivalent to human p.Tyr715Cys, heterozygous
  publication: PMID:31155284
  description: >-
    Reproduces the two most visible features of the human disease, and - importantly for
    this entry's central contrast - also reproduces the absence of bone disease.
  modeled_mechanisms:
  - target: Foam Cell Transformation and Tissue Storage
    relationship: RECAPITULATES
    fidelity: HIGH
    description: Hepatomegaly attributable to abnormal storage, with enlarged vacuoles in fibroblasts.
    limitations: >-
      Reported at the level of organ size and cell morphology; the mouse has not been used to
      test the melanosome-pH step, which is where this entry's chain is weakest.
    evidence:
    - reference: PMID:31155284
      reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "A mouse harboring the knock-in Clcn7 variant exhibited hypopigmentation, hepatomegaly resulting from abnormal storage, and enlarged vacuoles in cultured fibroblasts."
      explanation: Recapitulation of storage-driven organomegaly in the knock-in animal.
  - target: Hypopigmentation of Skin and Hair
    relationship: RECAPITULATES
    fidelity: HIGH
    limitations: >-
      The mouse shows the pigment phenotype but no published work establishes the melanosomal
      mechanism in it either, so it confirms the association without closing the gap.
    evidence:
    - reference: PMID:31155284
      reference_title: "Lysosomal Storage and Albinism Due to Effects of a De Novo CLCN7 Variant on Lysosomal Acidification."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "A mouse harboring the knock-in Clcn7 variant exhibited hypopigmentation, hepatomegaly resulting from abnormal storage, and enlarged vacuoles in cultured fibroblasts."
      explanation: The knock-in animal is hypopigmented, matching the human phenotype.
- name: Clcn7 knockout mouse
  species: Mouse
  genotype: Clcn7 null, homozygous
  publication: PMID:15706348
  description: >-
    The loss-of-function counterpart, included deliberately as a contrast rather than as a
    model of this disease. It develops osteopetrosis, neurodegeneration and lysosomal
    storage - and its neurons show unchanged lysosomal pH, which is the opposite of the
    human gain-of-function finding.
  modeled_mechanisms:
  - target: Lysosomal Hyperacidification
    relationship: FAILS_TO_RECAPITULATE
    fidelity: LOW
    description: >-
      Losing ClC-7 does not lower lysosomal pH - it leaves neuronal lysosomal pH unchanged.
      The pH lesion in HOD is therefore specific to excess transporter activity and is not a
      general consequence of perturbing ClC-7 in either direction.
    limitations: >-
      A null allele, not the human genotype, and pH was measured in cultured neurons rather
      than in the tissues that are enlarged in HOD. It is included to bound the claim that
      ClC-7 sets lysosomal pH, not as a model of HOD.
    evidence:
    - reference: PMID:15706348
      reference_title: "Loss of the chloride channel ClC-7 leads to lysosomal storage disease and neurodegeneration."
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: "We show that ClC-7 knockout mice additionally display neurodegeneration and severe lysosomal storage disease despite unchanged lysosomal pH in cultured neurons."
      explanation: >-
        Loss of ClC-7 produces storage without a pH shift, so the hyperacidification seen in
        HOD cannot be attributed to disturbed ClC-7 function in general.
differential_diagnoses:
- name: CLCN7-related osteopetrosis
  description: >-
    The allelic disorders, and the reason this entry exists. Autosomal recessive
    osteopetrosis and Albers-Schonberg autosomal dominant osteopetrosis type II arise from
    loss of ClC-7 and are dominated by dense, brittle bone with marrow failure and cranial
    nerve compression. HOD shares the gene and essentially nothing else: no patient has had
    osteopetrosis, and one had a structurally normal skeleton on X-ray at six years. A
    CLCN7 variant found on a gene panel therefore does not by itself indicate which disease
    is present - the direction of its functional effect does.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Osteopetrosis results from a loss of ClC-7, but osteoclasts remain resilient to increased ClC-7 activity."
    explanation: States the asymmetry between the two directions of CLCN7 dysfunction.
- name: Sialidosis (neuraminidase deficiency)
  description: >-
    Raised as a false lead in one reported case by an elevated bound neuraminic acid. It is
    a genuine differential on the clinical picture too, since sialidosis also presents with
    organomegaly, developmental delay and vacuolated cells.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A markedly elevated concentration of bound neuraminic acid suggestive of neuraminidase deficiency was only returned after results of genetic testing."
    explanation: The biochemical result that pointed at the wrong lysosomal disease.
discussions:
- discussion_id: osteoclast_resilience
  kind: KNOWLEDGE_GAP
  prompt: >-
    Why does bone escape? Loss of ClC-7 destroys osteoclast resorption and causes
    osteopetrosis, but gain of ClC-7 activity leaves the skeleton structurally normal.
    What makes the osteoclast ruffled border tolerant of excess exchange activity when the
    lysosome is not?
  rationale: >-
    This is the question the disease poses most sharply, and answering it would say
    something general about how much of ClC-7 physiology is set by the transporter versus by
    the compartment it sits in. It also has a practical edge: if a therapy for the
    osteopetroses ever aims to raise ClC-7 activity, the safety question is whether it would
    reproduce HOD, and the answer currently rests on five patients.
  attaches_to:
  - pathophysiology#Increased ClC-7 Chloride/Proton Exchange Current
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Osteopetrosis results from a loss of ClC-7, but osteoclasts remain resilient to increased ClC-7 activity."
    explanation: States the asymmetry without accounting for it, which is the gap.
- discussion_id: melanosome_ph_unmeasured
  kind: KNOWLEDGE_GAP
  prompt: >-
    Is melanosomal pH actually lowered in HOD syndrome, and is that why these patients are
    hypopigmented? No study has measured melanosomal pH, tyrosinase activity or melanosome
    ultrastructure in a patient or in the knock-in mouse.
  rationale: >-
    The melanosome step is the weakest link in this entry's causal chain and the only one
    supported entirely by evidence from outside the disease. The inference is reasonable -
    melanosomes are lysosome-related organelles and tyrosinase is pH-sensitive - but it has
    not been tested, and the knock-in mouse makes it testable. The deep-research report
    reached the same chain independently and listed the same step among its own limitations.
  attaches_to:
  - pathophysiology#Reduced Melanin Synthesis in Melanosomes
  evidence:
  - reference: PMID:24820037
    reference_title: "Transport activity and presence of ClC-7/Ostm1 complex account for different cellular functions."
    supports: REFUTE
    evidence_source: MODEL_ORGANISM
    snippet: "The normal coat colour of Clcn7(td/td) mice and their less severe neurodegeneration suggested that the ClC-7 protein, even when lacking measurable ion transport activity, is sufficient for hair pigmentation"
    explanation: >-
      Sharpens the gap rather than filling it, and cuts against a purely transport-based
      account of the pigment defect. Mice expressing a transport-dead ClC-7 have normal coat
      colour, so pigmentation survives the loss of ion transport while depending on the
      protein being present. Whatever excess transport does to the melanosome in HOD, it is
      not simply the mirror of losing transport.
  proposed_experiments:
  - experiment_id: melanosome_ph_in_knockin_mouse
    name: Melanosomal pH and tyrosinase activity in the Clcn7 knock-in mouse
    description: >-
      Measure melanosomal pH and tyrosinase activity in melanocytes from the Clcn7
      p.Tyr715Cys knock-in mouse against littermate controls, and ask whether alkalinizing
      treatment restores pigment.
    would_support:
    - pathophysiology#Reduced Melanin Synthesis in Melanosomes
    supporting_outcome:
    - Melanosomal pH is lower in knock-in melanocytes and tyrosinase activity is reduced.
    would_refute:
    - pathophysiology#Reduced Melanin Synthesis in Melanosomes
    refuting_outcome:
    - Melanosomal pH is unchanged, implicating a route to hypopigmentation other than melanosomal over-acidification.
- discussion_id: gof_does_not_uniquely_predict_hod
  kind: KNOWLEDGE_GAP
  prompt: >-
    If gain of ClC-7 function causes HOD and loss causes osteopetrosis, why do some
    osteopetrosis-causing CLCN7 variants also accelerate ClC-7 activation? What besides the
    direction of the functional change decides which disease a CLCN7 variant produces?
  rationale: >-
    The tidy summary - gain gives HOD, loss gives osteopetrosis - is the one this entry is
    built around, and it is not sufficient on its own. Some variants that accelerate
    voltage-dependent activation nevertheless cause osteopetrosis. Whatever separates them
    from p.Tyr715Cys and p.Lys285Thr is unknown, and the PI(3,5)P2 binding site is the
    current candidate because both HOD variants sit in it. Recording this stops the entry
    asserting a cleaner genotype-phenotype rule than the evidence supports.
  attaches_to:
  - pathophysiology#Loss of PI(3,5)P2 Inhibition of ClC-7
  evidence:
  - reference: PMID:38136669
    reference_title: "Impaired Autophagic Clearance with a Gain-of-Function Variant of the Lysosomal Cl(-)/H(+) Exchanger ClC-7."
    supports: REFUTE
    evidence_source: IN_VITRO
    snippet: "Interestingly, not all osteopetrosis-causing CLCN7 mutations from patients are associated with a loss of ion transport. Some rather result in an acceleration of voltage-dependent ClC-7 activation."
    explanation: >-
      Refutes the simple reading that increased ClC-7 activation is sufficient to produce the
      HOD phenotype rather than osteopetrosis.
- discussion_id: neural_branch_mechanism
  kind: KNOWLEDGE_GAP
  prompt: >-
    Which cell type carries the neurological phenotype in HOD syndrome - the oligodendrocyte
    failing to myelinate, the neuron accumulating storage material, or the microglion failing
    to clear debris? None of these has been examined in HOD.
  rationale: >-
    Delayed myelination is one of the four features the syndrome is named for, and it is the
    least mechanistically explained. What is known about ClC-7 in the CNS comes from loss of
    function, which cannot be assumed to transfer to a gain-of-function disease - the mouse
    knockout shows that losing ClC-7 produces storage without any pH change at all.
  attaches_to:
  - pathophysiology#Delayed Myelination
  evidence:
  - reference: PMID:38294065
    reference_title: "The Cl- transporter ClC-7 is essential for phagocytic clearance by microglia."
    supports: NO_EVIDENCE
    evidence_source: MODEL_ORGANISM
    snippet: "we show that microglia and embryonic macrophages in zebrafish clcn7 mutants cannot effectively process extracellular debris in the form of apoptotic cells and β-amyloid"
    explanation: >-
      Cited to mark the boundary of what is known, not as support. This is a loss-of-function
      zebrafish study, so it establishes that microglia need ClC-7 but says nothing about what
      excess ClC-7 activity does to them, which is the question HOD raises.
- discussion_id: immune_branch_speculative
  kind: KNOWLEDGE_GAP
  prompt: >-
    Is the hypogammaglobulinemia reported in one HOD patient a consequence of the lysosomal
    lesion in B-lineage cells, or an incidental finding in a single case?
  rationale: >-
    The deep-research report proposed a mechanistic immune branch - impaired endolysosomal
    processing in B cells causing hypogammaglobulinemia. That is a plausible hypothesis and
    it is recorded here as one, not curated as a causal edge: it rests on a single patient,
    and no B-cell or immunoglobulin work has been done in this disease. The finding was
    prominent enough in that report for the case to be framed as an inborn error of immunity,
    which is exactly why it is worth marking as unestablished.
  attaches_to:
  - phenotypes#Hypogammaglobulinemia
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Five patients in the published literature as of this curation: two in the 2019 founding
    report, two in the 2024 mechanistic paper (one of them diagnosed retrospectively from
    banked fibroblasts, having died in 1999), and one Taiwanese boy in a 2024 case report.
    No prevalence estimate exists and none should be inferred from this count.
  evidence:
  - reference: PMID:38838776
    reference_title: "Gain-of-function variants in CLCN7 cause hypopigmentation and lysosomal storage disease."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We now describe two additional HOD individuals with the previously described p.Tyr715Cys and a novel p.Lys285Thr mutation, respectively."
    explanation: Establishes the incremental case count over the two original patients.
notes: >-
  Ontology note. Two bindings were chosen against the deep-research report's suggestions,
  both toward greater specificity rather than away from it. The report offered HP:0003271
  Visceromegaly and HP:0012448 Delayed myelination; both resolve and are correctly labelled,
  but the reported findings are hepatosplenomegaly specifically (HP:0001433) and delayed
  myelination seen on brain MRI (HP:0002188 Delayed CNS myelination), so the narrower terms
  represent the claims accurately.

  Three of the report's identifiers were wrong, and none of them would have been caught by
  term validation, because every one is a real CURIE:

  MONDO:0032781, offered as this disease's own Mondo ID, is congenital hypotonia, epilepsy,
  developmental delay, and digital anomalies - a different rare syndrome that shares the
  words hypotonia and developmental delay. The correct term is MONDO:0032805. This is the
  entry's identity, so copying it would have bound the whole record to the wrong disease.
  The report did hedge it ("to be confirmed against current Mondo release"), which is the
  honest flag, and the hedge is the part worth reading.

  HGNC:2023, offered for CLCN7, is CLCN5 - a different member of the same CLC family, whose
  disease is Dent disease, a renal tubulopathy. CLCN7 is hgnc:2025. Adjacent identifier,
  same family, wrong gene.

  HP:0007730 is offered under the label "Ocular albinism"; its actual label is Iris
  hypopigmentation, so the report named a term it did not bind. The iris-specific term is
  also wrong for the patient the report drew it from: the best-described case has
  hypopigmented hair and eyebrows "but not the irides". Ocular involvement is nonetheless
  real in this disease - a different patient, in a different paper, has ocular albinism
  documented from birth - so the entry binds HP:0001107 Ocular albinism for that case and
  cites the normal-iris case as REFUTE against it being constant. The rejection is of the
  iris term specifically, not of ocular phenotypes generally.

  All identifiers used in this entry were instead derived from the curation stub and
  resolved against local OAK builds.

  Modifier note. The activity node carries modifier GAIN_OF_FUNCTION rather than the
  PATO-bound INCREASED. That trades ontology grounding for accuracy and the trade is
  deliberate: the variants do not merely raise the output of a normally regulated
  transporter, they remove the PI(3,5)P2 inhibition that regulates it and shift its voltage
  dependence, which is the qualitative case CLAUDE.md reserves those values for.

  Deep-research divergence. The committed OpenScientist report independently reproduced this
  entry's causal chain - gain-of-function variant, increased exchange, over-acidification,
  branching to pigment, viscera and brain, with bone spared - and it flagged the melanosome
  step among its own limitations, which is the same judgement reached here. Two of its
  proposals were not adopted. Its immune branch, deriving hypogammaglobulinemia from impaired
  endolysosomal processing in B cells, rests on one patient with no supporting B-cell work
  and is recorded as a knowledge gap rather than as a causal edge. Its neural branch attributes
  delayed myelination to neuronal, oligodendrocyte and microglial lysosomal failure; the
  underlying microglial evidence is a loss-of-function zebrafish study, which cannot be
  carried across to a gain-of-function disease, so that too is a gap rather than a chain.

  GeneReviews note. There is no GeneReviews article for HOD syndrome. The tagged
  GeneReviews reference below is CLCN7-Related Osteopetrosis, listed as the loss-of-function
  counterpart and NOT used as a phenotype baseline for this entry: it describes different
  diseases with a different mechanism and essentially no phenotypic overlap, so taking it as
  the baseline would import fractures, marrow failure and cranial nerve compression into an
  entry whose patients have structurally normal skeletons. Same gene, wrong disease.

  Citation format. The report cites by bare PubMed number in a Key References section rather
  than as PMID: identifiers, so the identifier sweep suggested in the curation skill returns
  nothing on it and its sidecar file was not produced. The references were recovered by
  reading that section. Worth knowing before concluding that an OpenScientist report is
  uncited.
references:
- reference: PMID:20301306
  title: "CLCN7-Related Osteopetrosis."
  tags:
  - GeneReviews

📚

References & Deep Research

References

1

PMID:20301306

CLCN7-Related Osteopetrosis.

No top-level findings curated for this source.

Deep Research

1

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)

Record notes

Ontology note. Two bindings were chosen against the deep-research report's suggestions, both toward greater specificity rather than away from it. The report offered HP:0003271 Visceromegaly and HP:0012448 Delayed myelination; both resolve and are correctly labelled, but the reported findings are hepatosplenomegaly specifically (HP:0001433) and delayed myelination seen on brain MRI (HP:0002188 Delayed CNS myelination), so the narrower terms represent the claims accurately. Three of the report's identifiers were wrong, and none of them would have been caught by term validation, because every one is a real CURIE: MONDO:0032781, offered as this disease's own Mondo ID, is congenital hypotonia, epilepsy, developmental delay, and digital anomalies - a different rare syndrome that shares the words hypotonia and developmental delay. The correct term is MONDO:0032805. This is the entry's identity, so copying it would have bound the whole record to the wrong disease. The report did hedge it ("to be confirmed against current Mondo release"), which is the honest flag, and the hedge is the part worth reading. HGNC:2023, offered for CLCN7, is CLCN5 - a different member of the same CLC family, whose disease is Dent disease, a renal tubulopathy. CLCN7 is hgnc:2025. Adjacent identifier, same family, wrong gene. HP:0007730 is offered under the label "Ocular albinism"; its actual label is Iris hypopigmentation, so the report named a term it did not bind. The iris-specific term is also wrong for the patient the report drew it from: the best-described case has hypopigmented hair and eyebrows "but not the irides". Ocular involvement is nonetheless real in this disease - a different patient, in a different paper, has ocular albinism documented from birth - so the entry binds HP:0001107 Ocular albinism for that case and cites the normal-iris case as REFUTE against it being constant. The rejection is of the iris term specifically, not of ocular phenotypes generally. All identifiers used in this entry were instead derived from the curation stub and resolved against local OAK builds. Modifier note. The activity node carries modifier GAIN_OF_FUNCTION rather than the PATO-bound INCREASED. That trades ontology grounding for accuracy and the trade is deliberate: the variants do not merely raise the output of a normally regulated transporter, they remove the PI(3,5)P2 inhibition that regulates it and shift its voltage dependence, which is the qualitative case CLAUDE.md reserves those values for. Deep-research divergence. The committed OpenScientist report independently reproduced this entry's causal chain - gain-of-function variant, increased exchange, over-acidification, branching to pigment, viscera and brain, with bone spared - and it flagged the melanosome step among its own limitations, which is the same judgement reached here. Two of its proposals were not adopted. Its immune branch, deriving hypogammaglobulinemia from impaired endolysosomal processing in B cells, rests on one patient with no supporting B-cell work and is recorded as a knowledge gap rather than as a causal edge. Its neural branch attributes delayed myelination to neuronal, oligodendrocyte and microglial lysosomal failure; the underlying microglial evidence is a loss-of-function zebrafish study, which cannot be carried across to a gain-of-function disease, so that too is a gap rather than a chain. GeneReviews note. There is no GeneReviews article for HOD syndrome. The tagged GeneReviews reference below is CLCN7-Related Osteopetrosis, listed as the loss-of-function counterpart and NOT used as a phenotype baseline for this entry: it describes different diseases with a different mechanism and essentially no phenotypic overlap, so taking it as the baseline would import fractures, marrow failure and cranial nerve compression into an entry whose patients have structurally normal skeletons. Same gene, wrong disease. Citation format. The report cites by bare PubMed number in a Key References section rather than as PMID: identifiers, so the identifier sweep suggested in the curation skill returns nothing on it and its sidecar file was not produced. The references were recovered by reading that section. Worth knowing before concluding that an OpenScientist report is uncited.

Create: HOD syndrome (CLCN7 gain-of-function) · 2026-09-03T16:56:24Z · View source

Created kb/disorders/Hypopigmentation_Organomegaly_And_Delayed_Myelination_And_Development.yaml for HOD syndrome, MONDO:0032805, de novo gain-of-function CLCN7. Deep research: OpenScientist (committed report plus HTML/PDF artifacts). Twelve pathophysiology nodes forming a single connected DAG from the variant to four clinical branches; 57 evidence snippets, all verified against cached references, and all 57 reference_title values machine-audited against references_cache frontmatter. Built primarily from five primary sources covering all five published patients. The report was used as a check rather than a source and independently reproduced the causal chain, including flagging the melanosome step as inferred. Three of the report's ontology identifiers were wrong and are recorded in the entry notes: MONDO:0032781 offered as the disease ID is a different syndrome, HGNC:2023 offered for CLCN7 is CLCN5, and HP:0007730 was mislabelled and contradicts the primary source on iris involvement. All identifiers used were resolved against local OAK builds. Corrected one claim of my own during curation: heterozygous CLCN7 loss of function is silent, and Albers-Schonberg ADO II is dominant-negative rather than simple loss (PMID:11741829). Report term validation could not be run: repeated OLS read timeouts at the validator's five-second limit; the report's CURIEs were instead resolved manually against local OAK, and reference validation ran cleanly at 19/19 resolved with one off-topic flag on PMID:31214276, which is cited here only as directness INDIRECT for general melanocyte pH biology. Validated with just validate, validate-terms, count-verified-snippets, check-enum-values, check-entity-refs, check-causal-targets, check-duplicate-keys, check-qualifier-terms, check-snippet-grading and check-environmental-evidence.

OpenScientist ▸

1. Disease Information

openscientist-autonomous 2026-09-03T16:43:25.547304

1. Disease Information

Overview: HOD ("Hypopigmentation, Organomegaly, and Delayed Myelination and Development") is a multisystem congenital disorder combining diffuse skin/hair/eye hypopigmentation (albinism-like), organomegaly (hepatosplenomegaly), and neurodevelopmental impairment with delayed CNS myelination. It is a lysosomal storage phenotype driven by abnormal lysosomal acidification.
Key identifiers:
OMIM #618541 — "Hypopigmentation, Organomegaly, and Delayed Myelination and Development."
Gene: CLCN7 (OMIM *602727), HGNC:2023, locus 16p13.3.
Mondo: MONDO:0032781 (HOD; per Mondo cross-reference to OMIM 618541 — to be confirmed against current Mondo release).
Orphanet / ICD-11: No dedicated Orphanet code identified; closest ICD-11 grouping is under lysosomal/metabolic and pigmentary disorders. ICD-10: no specific code (would map broadly to E75.- lysosomal / E70.3 albinism categories). Not available as a discrete code.
MeSH: No dedicated MeSH descriptor; relevant terms: "Lysosomal Storage Diseases", "Albinism", "CLCN7 protein, human".
Synonyms / alternative names: "CLCN7-related gain-of-function lysosomal disorder"; "CLCN7 gain-of-function hypopigmentation and lysosomal storage disease"; informally "HOD syndrome." Related descriptor in literature: "multisystem disorder associated with a pathogenic variant in CLCN7 in the absence of osteopetrosis."
Data source: Information is individual-patient / small-case-series derived (fewer than ~10 reported probands), supported by disease-level functional/model-organism resources. This is not an aggregated EHR-scale entity.

2. Etiology

Primary cause (genetic): Heterozygous de novo gain-of-function (GoF) missense variants in CLCN7. Index variant p.Tyr715Cys (Y715C) in the C-terminal cystathionine-β-synthase (CBS) domain; the original AJHG report (Nicoli et al., 2019; PMID 31155284) described additional de novo variants (e.g., p.Tyr99Cys/Y99C in the transmembrane domain) with the same functional signature. Functional testing shows increased outward currents → lysosomal hyperacidity (PMID 39056574, 31155284).
Genetic risk factors: The causal variants are the sole known determinant; because they are de novo and dominant-acting, there is no meaningful susceptibility-locus/modifier background documented. Modifier genes and compensatory pathways are hypothesized but unproven (PMID 39056574).
Environmental risk factors: None identified. This is a monogenic disorder; onset is congenital and independent of exposures, lifestyle, or sex.
Protective factors: None described. (Hypothetically, lowering ClC-7 activity/lysosomal acidity could be protective — see Treatment.)
Gene–environment interactions: Not applicable / none reported.

3. Phenotypes

All frequencies are qualitative given the very small case count (individual-patient evidence).

Phenotype	Type	HPO term	Onset	Severity	Frequency
Generalized hypopigmentation of skin/hair	Physical/pigmentary	HP:0001010 (Hypopigmentation of the skin), HP:0002286 (Fair hair)	Congenital	Moderate–severe	Core/near-constant
Oculocutaneous/ocular albinism features (iris hypopigmentation, nystagmus, reduced visual acuity)	Clinical sign	HP:0007730 (Ocular albinism), HP:0000639 (Nystagmus)	Congenital/neonatal	Variable	Common
Hepatomegaly / splenomegaly (organomegaly)	Clinical sign	HP:0002240 (Hepatomegaly), HP:0001744 (Splenomegaly), HP:0003271 (Visceromegaly)	Infancy	Moderate	Core
Delayed CNS myelination (white-matter)	Imaging/lab	HP:0012448 (Delayed CNS myelination)	Infancy	Variable	Core
Global developmental delay / intellectual disability	Behavioral/cognitive	HP:0001263 (Global developmental delay), HP:0001249 (Intellectual disability)	Infancy	Mild–severe, variable	Core
Hypotonia / motor delay	Neurological sign	HP:0001252 (Hypotonia)	Infancy	Variable	Common
Hypogammaglobulinemia / immune involvement	Lab abnormality	HP:0002090?→HP:0004313 (Decreased circulating antibody level)	Infancy	Variable	Reported (Lee 2024)
Vacuolated lymphocytes / lysosomal storage cells	Lab/histology	HP:0001922 (Abnormal blood-cell morphology)	Congenital	—	Reported
Coarse/dysmorphic features (variable)	Physical	HP:0000280	Congenital	Mild	Variable

Progression: Neurodevelopmental features are static-to-slowly-progressive; underlying storage biology in models is progressive/neurodegenerative.
Quality-of-life impact: Substantial — visual impairment (albinism, nystagmus), developmental disability, and immune susceptibility affect daily function. No formal EQ-5D/SF-36/PROMIS data exist for this ultra-rare entity.
Notably ABSENT: osteopetrosis/increased bone density (distinguishes HOD from CLCN7 LoF disease).

4. Genetic / Molecular Information

Causal gene: CLCN7 (HGNC:2023; NCBI Gene 1186; UniProt P51798, CLCN7_HUMAN), 16p13.3. Encodes ClC-7, a voltage-gated CLC-family Cl⁻/H⁺ antiporter (2Cl⁻:1H⁺ stoichiometry).
Pathogenic variants (HOD, GoF):
c.296A>G, p.Tyr99Cys (Y99C) — transmembrane domain (original de novo cases, Nicoli 2019).
c.2144A>G, p.Tyr715Cys (Y715C) — C-terminal CBS domain (Nicoli 2019; Lee 2024, PMID 39056574).
Variant type/class: missense (single amino-acid substitutions); germline de novo, not somatic.
Classification (ACMG/AMP): pathogenic/likely pathogenic (de novo, functionally validated, absent from population databases).
Allele frequency: Absent from gnomAD (private de novo). CLCN7 is highly constrained.
Functional consequence: Gain of function — increased Cl⁻/H⁺ transport → lysosomal hyperacidification and accelerated/enhanced currents (PMID 39056574, 31155284, 38136669).
Contrast — CLCN7 osteopetrosis variants: Dominant-negative (ADO2/Albers-Schönberg, e.g., p.Gly213Arg) or biallelic loss-of-function/NMD (ARO, e.g., p.Met59Asnfs*8) — opposite functional direction, different disease (PMID 11741829, 39994654, 17936098).
Modifier genes: OSTM1 (obligate β-subunit) is essential for ClC-7 function; unproven as a HOD modifier. Others hypothesized (PMID 39056574).
Epigenetic / chromosomal abnormalities: None reported; disease is single-nucleotide/missense, not structural.

Ontology suggestions: Gene HGNC:2023 (CLCN7); protein UniProt P51798; partner OSTM1 (HGNC:16800, UniProt Q86WC4).

5. Environmental Information

Environmental factors: None. Congenital monogenic disorder.
Lifestyle factors: Not applicable.
Infectious agents: None causal. (Note: hypogammaglobulinemia may predispose to recurrent infections as a consequence, not a cause.)

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation)

A de novo gain-of-function missense variant in CLCN7 (e.g., Y99C, Y715C) alters ClC-7 gating/coupling → leads to increased 2Cl⁻/1H⁺ antiporter activity (demonstrated by increased outward currents in Xenopus oocytes; PMID 39056574, 31155284).
Enhanced Cl⁻ counter-ion flux provides greater charge shunt for the V-ATPase → results in over-acidification (hyperacidity) of lysosomes and lysosome-related organelles (demonstrated: "lysosomal hyperacidity"; PMID 39056574).
Abnormally low luminal pH disrupts pH-optimal lysosomal hydrolases and membrane trafficking → impairs autophagic/degradative clearance (demonstrated in cell models of GoF variants; PMID 38136669) → results in accumulation of undigested storage material and cytoplasmic vacuolization (demonstrated; PMID 39056574).
Branch A — Melanosomes (pigment): melanosomes are lysosome-related organelles whose melanin synthesis (tyrosinase) is exquisitely pH-sensitive. Hyperacidification impairs melanin production → generalized hypopigmentation / albinism-like phenotype (inferred mechanism, supported by melanosome-pH biology and the albinism phenotype in Nicoli 2019 / Polovitskaya 2024).
Branch B — Reticuloendothelial storage: storage-material accumulation in hepatic/splenic macrophages and parenchyma → hepatosplenomegaly / organomegaly (inferred from lysosomal-storage biology; consistent with organomegaly in reported patients).
Branch C — CNS: neuronal and glial (oligodendrocyte, microglial) storage and dysfunction → impaired myelination and neuronal function → delayed myelination + developmental delay (supported by Clcn7-KO mouse neurodegeneration/NCL-like storage, PMID 15706348, and microglial phagocytic failure, PMID 38294065).
Branch D — Immune: impaired lysosomal/endosomal processing in B-lineage/antigen-processing cells → hypogammaglobulinemia (reported; PMID 39056574).
Chronic storage/vacuolization in neurons and glia is inferred to drive progressive neurodegeneration (from model organisms; NCL-like subunit-c and lysosomal-enzyme accumulation, PMID 15706348).

Category detail

Molecular pathways: Lysosomal ion homeostasis / V-ATPase-coupled acidification; autophagy–lysosome pathway; mTORC1 lysosomal signaling (inferred). Reactome "Ion channel transport"; KEGG "Lysosome" (map04142).
Cellular processes: Autophagic clearance (impaired), lysosomal degradation, phagocytosis (microglial clearance of apoptotic cells/β-amyloid impaired; PMID 38294065), melanogenesis.
Protein dysfunction: ClC-7 GoF (altered gating/uncoupling of Cl⁻/H⁺ transport); obligate complex with OSTM1 — OSTM1 is required for ClC-7 stability (ClC-7 protein falls >90% in Ostm1-null grey-lethal mice; PMID 16525474) and the OSTM1 dimer shields luminal ClC-7 (cryo-EM; PMID 32749217).
Disease-family paradigm: HOD belongs to a broader class of endolysosomal CLC gain-of-function neurodevelopmental disorders — a recurrent GoF mutation in the paralog CLCN6 (ClC-6) causes a neurodevelopmental/neurodegenerative disorder via the same endolysosomal-ion-homeostasis logic (Polovitskaya et al. 2020, PMID 33217309), and endolysosomal CLC dysfunction generally produces NCL-like storage and neurodegeneration (review PMID 33708769; CLC-3 deficiency NCL model PMID 12059962).
Metabolic changes: Accumulation of NCL-type storage (subunit c of mitochondrial ATP synthase) and lysosomal enzymes (PMID 15706348).
Immune involvement: Hypogammaglobulinemia; microglial dysfunction.
Tissue-damage mechanism: Storage-driven cellular dysfunction and neurodegeneration (astrogliosis, microglial activation in models).
Biochemical abnormality: Dysregulated lysosomal pH set-point (hyper-acidic).

Melanosome-pH sub-mechanism (hypopigmentation branch, supporting evidence): Melanosomal luminal pH is a master regulator of melanin synthesis; tyrosinase (the rate-limiting melanogenic enzyme) is minimally active in an acidic environment (Miao et al. 2019, PMID 31214276), and melanosomal pH is normally set by V-ATPase proton pumping balanced by ion transporters (SLC45A2/OCA4, OCA2 P-protein, SLC24A4/5) (Cheli et al. 2009, PMID 19389708). ClC-7 supplies the Cl⁻ counterion that enables V-ATPase-driven organelle acidification (Mindell 2012, PMID 22335796). Thus ClC-7 GoF → melanosome over-acidification → tyrosinase suppression → hypopigmentation. This places HOD within a broader class of "ion-transport / organelle-pH" albinism syndromes — e.g., gain-of-function TPC2 p.R210C albinism (PMID 36641477) and V-ATPase-deficiency oculocutaneous albinism in zebrafish (PMID 18836173).

GO/CL suggestions: GO:0007042 (lysosomal lumen acidification), GO:0055085 (transmembrane transport), GO:1902600 (proton transmembrane transport), GO:0006914 (autophagy), GO:0006622 (protein targeting to lysosome), GO:0006583 (melanin biosynthetic process from tyrosine), GO:0042470 (melanosome). Cellular component: GO:0005765 (lysosomal membrane), GO:0042470 (melanosome). Cell types: CL:0000540 (neuron), CL:0000128 (oligodendrocyte), CL:0000129 (microglial cell), CL:0000148 (melanocyte), CL:0000091 (Kupffer cell), CL:0000236 (B cell).

CHEBI suggestions: CHEBI:17594 (melanin), CHEBI:17996 (chloride), CHEBI:24636 (proton/H⁺), CHEBI:15377 (water), CHEBI:18059 (lipid, storage material context). Key protein/enzyme: tyrosinase (EC 1.14.18.1, UniProt P14679).

7. Anatomical Structures Affected

Organ level: Skin & hair (pigment), eyes (iris/retina), liver & spleen (organomegaly), central nervous system (white matter/brain), immune system (B-cell compartment). Bone is spared (no osteopetrosis).
Body systems: Integumentary, nervous, hepatobiliary/reticuloendothelial, visual/ophthalmic, immune.
Tissue/cell level: Melanocytes (CL:0000148); neurons (CL:0000540) and oligodendrocytes (CL:0000128, myelination); microglia (CL:0000129); Kupffer cells/macrophages (CL:0000235/CL:0000091); B lymphocytes (CL:0000236).
Subcellular: Lysosome (GO:0005764) and lysosomal membrane (GO:0005765); lysosome-related organelles / melanosomes (GO:0042470); late endosome (GO:0005770); autophagosome (GO:0005776).
Localization / lateralization: Systemic and bilateral/symmetric (pigmentary, hepatosplenic, CNS white-matter changes are generalized).

UBERON suggestions: UBERON:0002097 (skin of body), UBERON:0002107 (liver), UBERON:0002106 (spleen), UBERON:0000955 (brain), UBERON:0002316 (white matter), UBERON:0000970 (eye).

8. Temporal Development

Onset: Congenital / neonatal (hypopigmentation evident at birth; developmental delay and organomegaly in infancy).
Onset pattern: Chronic, insidious.
Progression: Neurodevelopmental delay is largely static-to-slowly-progressive in reported patients; model-organism data indicate an underlying progressive neurodegenerative storage process.
Course: Chronic, lifelong. No remission described.
Critical periods: Early infancy (myelination window) is the key period of CNS vulnerability and the plausible window for intervention.

9. Inheritance and Population

Epidemiology: Ultra-rare; <10 reported cases worldwide (individual case reports/small series). Prevalence/incidence not estimable (effectively unknown; well below 1/1,000,000).
Inheritance: Autosomal dominant, de novo (sporadic). Not inherited from parents in reported cases.
Penetrance: Presumed complete for the specific GoF alleles (all reported carriers affected).
Expressivity: Variable (developmental severity, immune involvement differ between patients).
Anticipation / mosaicism / founder effects / consanguinity / carrier frequency: Not applicable (de novo dominant; no carrier state, no founder or consanguinity role). Germline mosaicism is theoretically possible for recurrence risk counseling but unreported.
Population demographics: No ethnic predilection; cases reported across populations (e.g., a Taiwanese proband, PMID 39056574). No sex bias expected (autosomal). Age distribution: pediatric.

10. Diagnostics

Genetic testing (definitive):
Trio whole-exome (WES) or whole-genome (WGS) sequencing to detect the de novo CLCN7 missense variant (approach used in all reported cases). Single-gene CLCN7 sequencing or lysosomal/leukodystrophy gene panels including CLCN7 are alternatives.
Functional confirmation: electrophysiology (Xenopus oocyte currents) demonstrating GoF; lysosomal pH measurement showing hyperacidity (research setting).
CMA/karyotype/FISH/mtDNA/repeat testing: not indicated (point-mutation disease).
Clinical/laboratory:
Ophthalmologic exam (iris transillumination, nystagmus, foveal hypoplasia, VEP) for albinism.
Abdominal ultrasound/MRI for hepatosplenomegaly.
Brain MRI: delayed/deficient myelination, white-matter signal changes.
CBC/blood smear: vacuolated lymphocytes; immunoglobulin panel (hypogammaglobulinemia).
Skin/marrow biopsy or EM: lysosomal storage material/vacuolization (research/confirmatory).
Differential diagnosis: Oculocutaneous albinism (OCA1–7; distinguished by absence of organomegaly/CNS storage), Chediak-Higashi & Hermansky-Pudlak syndromes (lysosome-related organelle disorders with pigment dilution — distinguished by giant granules/bleeding/immune profile and their own genes), other lysosomal storage/leukodystrophies, CLCN7 osteopetrosis (distinguished by increased bone density — absent in HOD).

LOINC/ontology suggestions: genetic sequencing panels; brain MRI (RadLex).

11. Outcome / Prognosis

Survival/mortality: No formal survival statistics (too few cases). Reported patients have survived into childhood; long-term prognosis is guarded owing to neurodevelopmental disability and potential progressive neurodegeneration (extrapolated from ClC-7-deficiency models).
Morbidity/function: Significant lifelong disability — visual impairment, developmental/intellectual disability, and infection susceptibility (hypogammaglobulinemia).
Complications: Recurrent infections (immune), visual complications of albinism, feeding/growth issues.
Prognostic factors: Severity of developmental delay and extent of CNS myelination deficit; degree of immune involvement.
Prognostic biomarkers: None validated; lysosomal storage burden and MRI myelination are candidate markers.

12. Treatment

No disease-specific or curative therapy exists; management is supportive.

Supportive/rehabilitative (mainstay): Developmental/early-intervention therapy, physical/occupational/speech therapy (NCIT: rehabilitation therapy); low-vision aids and ophthalmologic management for albinism; photoprotection (sun protection, given hypopigmentation → skin-cancer risk).
Immune: Immunoglobulin replacement (IVIG) and infection prophylaxis if clinically significant hypogammaglobulinemia (NCIT: Intravenous Immunoglobulin Therapy).
Organomegaly/storage: Monitoring; no approved substrate-reduction/enzyme therapy for this entity.
Pharmacogenomics: None established.
Mechanism-rational (experimental/hypothetical): Because the defect is lysosomal hyper-acidification from ClC-7 GoF, agents that raise lysosomal pH / reduce ClC-7 or V-ATPase activity are conceptually attractive but untested in patients. ClC-7 inhibitors are in preclinical study for other indications. No gene-, cell-, or RNA-based therapy has been reported for HOD.
Advanced therapeutics (osteopetrosis contrast): Hematopoietic stem-cell/bone-marrow transplant is standard for recessive osteopetrosis and studied for ADO2 in mice (PMID 35959867) — not applicable to HOD, which lacks the osteoclast/bone phenotype.
Clinical trials: None specific to HOD identified on ClinicalTrials.gov (no NCT).

13. Prevention

Primary prevention: Not possible (de novo mutation); no modifiable risk factors.
Secondary prevention: Early diagnosis via trio sequencing enables early developmental intervention, vision support, and immune monitoring.
Genetic counseling: Recurrence risk for parents is low (de novo), but germline mosaicism cannot be excluded → prenatal/preimplantation testing can be offered for known familial variants. Affected individuals would have 50% transmission risk (autosomal dominant), though reproduction is unlikely given severity.
Screening: Not part of newborn/carrier screening panels. Cascade testing not generally indicated for de novo cases.
Public-health/immunization/prophylaxis: Standard vaccination and infection prophylaxis if immunodeficient.

14. Other Species / Natural Disease

Taxonomy / orthologs: CLCN7 is highly conserved. Orthologs: mouse Clcn7 (NCBI Gene 26373; NCBI Taxon 10090), rat Clcn7, zebrafish clcn7 (Taxon 7955). Partner Ostm1 similarly conserved.
Natural disease in animals: Naturally occurring CLCN7/OSTM1 loss-of-function osteopetrosis exists (e.g., the mouse grey-lethal (gl, Ostm1) and Clcn7 mutants; cattle/dog osteopetrosis — OMIA entries for CLCN7-related osteopetrosis). No naturally occurring gain-of-function HOD counterpart is documented in animals.
Comparative biology: Loss of ClC-7/Ostm1 recapitulates lysosomal storage + neurodegeneration + osteopetrosis across species, confirming conserved lysosomal/osteoclast roles; the grey-lethal mouse also shows coat-color/pigment dilution, echoing the pigment link.
Zoonotic potential: None (genetic disease).

15. Model Organisms

Mouse (primary):
Clcn7⁻/⁻ knockout — severe lysosomal storage disease + neurodegeneration (NCL-like, subunit-c and lysosomal-enzyme accumulation), retinal degeneration, osteopetrosis; osteoclast-rescue transgene extends life but CNS disease progresses (PMID 15706348). Recapitulates the storage/neurodegeneration axis of HOD (but represents LoF, so bone phenotype opposite).
Ostm1 (grey-lethal) mutant — phenocopy of Clcn7 loss (lysosomal storage, neurodegeneration, osteopetrosis, coat-color dilution).
ADO2 knock-in (p.G213R) — dominant-negative bone model (PMID 35959867) — informs CLCN7 dominant biology though not the GoF HOD mechanism.
Knock-in of HOD GoF alleles (e.g., Y99C/Y715C) — appropriate model to recapitulate hyperacidification/hypopigmentation; described in the 2024 GoF characterization work (Polovitskaya et al., PMID 38838776).
Zebrafish: clcn7 and ostm1 mutants show microglial/macrophage phagocytic failure (impaired clearance of apoptotic cells and β-amyloid) with normal microglial development (PMID 38294065) — models the neuroimmune/clearance component.
In vitro/heterologous: Xenopus oocyte and mammalian-cell expression for electrophysiology and lysosomal-pH assays (functional variant validation; PMID 31155284, 38136669, 33125761); Arabidopsis vacuole system for CLC-7 transport mechanism (PMID 22641774).
Model limitations: Mouse KO models loss-of-function, not the human GoF; GoF knock-ins and patient iPSC-derived neurons/melanocytes are needed to fully model HOD-specific hyperacidification and hypopigmentation.
Resources: MGI (Clcn7, Ostm1), ZFIN (clcn7), IMPC/IMSR for alleles; OMIA for animal osteopetrosis.

Key References (PMID)

31155284 — Nicoli et al. 2019, Am J Hum Genet. Original description: de novo CLCN7 variant → lysosomal storage + albinism via altered lysosomal acidification (the defining HOD paper).
38838776 — Polovitskaya et al. 2024. Gain-of-function CLCN7 variants cause hypopigmentation and lysosomal storage disease (mechanistic/model confirmation).
39056574 — Lee et al. 2024. Multisystem CLCN7 p.Y715C disorder (developmental delay, organomegaly, hypogammaglobulinemia, hypopigmentation) without osteopetrosis; GoF/lysosomal hyperacidity.
38136669 — Bose et al. 2023. Impaired autophagic clearance with a GoF ClC-7 variant.
32749217 — Schrecker et al. 2020. Cryo-EM of CLC-7/OSTM1 complex.
15706348 — Kasper et al. 2005. Clcn7-KO mouse: lysosomal storage disease + neurodegeneration.
38294065 — Iyer & Talbot 2024. ClC-7 essential for microglial phagocytic clearance (zebrafish).
33125761 — Di Zanni et al. 2021. Structure/function of 14 ClC-7 mutants; lysosomal localization ↔ neurodegeneration.
11741829 — Cleiren et al. 2001. ADO2 = dominant-negative CLCN7 (contrast to GoF HOD).
17936098 — Del Fattore et al. 2008. Osteopetrosis genetics/pathogenesis review.
35959867 — Alam et al. 2022. BMT for ADO2 in mice.
11207362 — Kornak et al. 2001. ClC-7-deficient mice: osteopetrosis + retinal degeneration; ClC-7 provides Cl⁻ conductance for osteoclast H⁺-ATPase.
16525474 — Lange et al. 2006. ClC-7 requires OSTM1 β-subunit; grey-lethal (Ostm1-null) mice phenocopy Clcn7-null (storage, neurodegeneration, osteopetrosis).
33217309 — Polovitskaya et al. 2020. Recurrent gain-of-function CLCN6 (ClC-6) mutation → neurodevelopmental/neurodegenerative disorder (endolysosomal CLC GoF paradigm).
33708769 — Bose, He & Stauber 2021. Review: neurodegeneration upon dysfunction of endosomal/lysosomal CLC transporters.
12059962 — Yoshikawa et al. 2002. CLC-3 deficiency → NCL-like phenotype with subunit-c accumulation and elevated endosomal pH.
22335796 — Mindell 2012. Lysosomal acidification mechanisms (V-ATPase + ClC-7 counterion).
31214276 — Miao et al. 2019. Intramelanocytic acidification suppresses tyrosinase/melanogenesis.
19389708 — Cheli et al. 2009. Melanosome pH as key regulator of melanin synthesis (αMSH/cAMP).

Consolidated Causal-Chain Synthesis (knowledge-base ready)

One defect, four tissue readouts. A de novo gain-of-function CLCN7 missense variant → increased ClC-7 2Cl⁻/1H⁺ antiporter activity → excess counterion shunt for the V-ATPase → over-acidification of lysosomes and lysosome-related organelles → branches: - Melanosome branch → tyrosinase (pH-sensitive, minimal activity when acidic) suppressed → reduced melanin → hypopigmentation / albinism (HP:0001010; evidence: in vitro melanosome-pH biology + human). - Autophagy/visceral branch → hydrolase pH-mismatch → impaired autophagic/lysosomal clearance → storage material + vacuolization in liver/spleen macrophages → organomegaly (HP:0003271; evidence: human cells + in vitro). - Neural branch → neuronal/oligodendrocyte/microglial lysosomal dysfunction and NCL-like storage → delayed myelination + developmental delay (HP:0012448, HP:0001263; evidence: model organism + human). - Immune branch → impaired endolysosomal processing in B-lineage cells → hypogammaglobulinemia (HP:0004313; evidence: human case). - Spared: osteoclast bone resorption (requires reduced ClC-7) → no osteopetrosis (the discriminating negative feature).

Confirmed Findings Recorded (6)

HOD (OMIM 618541) = de novo gain-of-function CLCN7 — mirror image of loss-of-function/dominant-negative osteopetrosis [human + in vitro; PMID 39056574, 31155284, 11741829].
ClC-7/OSTM1 dysfunction → lysosomal storage + NCL-like neurodegeneration; obligate OSTM1 complex [model organism + structural; PMID 15706348, 32749217, 16525474, 33125761].
Clinical triad hypopigmentation + organomegaly + delayed myelination/development without osteopetrosis [human; PMID 39056574].
Hypopigmentation via melanosome over-acidification suppressing tyrosinase [in vitro/human; PMID 31214276, 19389708, 22335796].
Organomegaly via impaired autophagic clearance and reticuloendothelial storage [in vitro/human; PMID 38136669, 39056574].
Delayed myelination/development via neural + microglial lysosomal failure; endolysosomal-CLC GoF paradigm (CLCN6) [model organism + human; PMID 16525474, 11207362, 38294065, 33217309, 33708769].

Evidence-Source Classification

Human clinical: PMID 31155284 (Nicoli 2019, original probands), 39056574 (Lee 2024), 11741829 (osteopetrosis genotype-phenotype).
Model organism: PMID 15706348 & 11207362 (mouse Clcn7-KO), 16525474 (Ostm1 grey-lethal mouse), 38294065 (zebrafish clcn7/ostm1), 35959867 (ADO2 mouse).
In vitro / functional / structural: PMID 38136669 (autophagy), 33125761 (14 mutants), 32749217 (cryo-EM), 22335796/31214276/19389708 (pH/melanosome biology), 36641477 (TPC2 albinism), 18836173 (v-ATPase zebrafish).
Review/computational: PMID 33708769, 17936098, 35159175.

Limitations

Evidence base is very small (individual case reports + functional/model studies); frequencies, prognosis, survival, and QoL metrics are qualitative.
Some mechanistic links (melanosome hyperacidification → hypopigmentation; storage → organomegaly; progressive neurodegeneration) are inferred from lysosome/melanosome biology and LoF models rather than directly demonstrated in HOD patients.
No approved therapy; mechanism-based treatments remain hypothetical.
Ontology cross-references (Mondo, ICD-11, Orphanet) should be verified against current releases.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome	Count
References checked	19
Resolved	19
Unresolved (possible confabulation)	0
Unverifiable	0
References weighed for topical relevance	19
On topic	7
Off topic	1

References that may not be about this subject

These identifiers resolve, so they are not fabrications, but the records they resolve to share almost none of this report's vocabulary. That is a clue and not a verdict - a paper can be relevant in ways its title and abstract do not spell out - so read them before deciding:

PMID:31214276 (2 mentions) - Intramelanocytic Acidification Plays a Role in the Antimelanogenic and Antioxidative Properties of Vitamin C and Its Derivatives.
shared terms: melanosome

Weighed against this report's own most characteristic terms: lysosomal, storage, clcn7, hypopigmentation, organomegaly, osteopetrosis, hod, myelination, clc-7, albinism, model, gof, novo, variant, developmental, immune, delayed, melanosome, gain-of-function, ostm1.

All extracted references resolved successfully. Resolving is not the same as being relevant, though - see the references listed above as possibly off topic.

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1. Disease Information

2. Etiology

3. Phenotypes

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Ordered causal chain (initiating lesion → clinical manifestation)

Category detail

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

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12. Treatment

13. Prevention

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15. Model Organisms

Key References (PMID)

Consolidated Causal-Chain Synthesis (knowledge-base ready)

Confirmed Findings Recorded (6)

Evidence-Source Classification

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