Hypopigmentation, Organomegaly, and Delayed Myelination and Development (HOD)

Comprehensive Disease Knowledge-Base Report

Answer in brief: HOD is an ultra-rare, dominantly-acting Mendelian multisystem disorder caused by de novo gain-of-function missense variants in CLCN7, which encodes the lysosomal 2Cl⁻/1H⁺ antiporter ClC-7 (obligate partner: OSTM1). Excessive ClC-7 activity hyper-acidifies lysosomes and lysosome-related organelles (melanosomes), causing generalized hypopigmentation/oculocutaneous albinism, lysosomal storage with hepatosplenomegaly (organomegaly), and CNS involvement (delayed myelination, developmental delay). It is mechanistically the mirror image of CLCN7 loss-of-function/dominant-negative disease (osteopetrosis), and reported HOD patients characteristically lack osteopetrosis.

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; 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 (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 (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).

• 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, 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 (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 (11741829 39994654 17936098).
• Modifier genes: OSTM1 (obligate β-subunit) is essential for ClC-7 function; unproven as a HOD modifier. Others hypothesized (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)

1. 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; 39056574 31155284).
2. 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"; 39056574).
3. Abnormally low luminal pH disrupts pH-optimal lysosomal hydrolases and membrane trafficking → impairs autophagic/degradative clearance (demonstrated in cell models of GoF variants; 38136669) → results in accumulation of undigested storage material and cytoplasmic vacuolization (demonstrated; 39056574).
4. 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).
5. 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).
6. 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, 15706348, and microglial phagocytic failure, 38294065).
7. Branch D — Immune: impaired lysosomal/endosomal processing in B-lineage/antigen-processing cells → hypogammaglobulinemia (reported; 39056574).
8. Chronic storage/vacuolization in neurons and glia is inferred to drive progressive neurodegeneration (from model organisms; NCL-like subunit-c and lysosomal-enzyme accumulation, 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; 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; 16525474) and the OSTM1 dimer shields luminal ClC-7 (cryo-EM; 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, 33217309), and endolysosomal CLC dysfunction generally produces NCL-like storage and neurodegeneration (review 33708769; CLC-3 deficiency NCL model 12059962).
• Metabolic changes: Accumulation of NCL-type storage (subunit c of mitochondrial ATP synthase) and lysosomal enzymes (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, 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, 19389708). ClC-7 supplies the Cl⁻ counterion that enables V-ATPase-driven organelle acidification (Mindell 2012, 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 (36641477) and V-ATPase-deficiency oculocutaneous albinism in zebrafish (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, 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 (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 (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 (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., 38838776).
• Zebrafish: clcn7 and ostm1 mutants show microglial/macrophage phagocytic failure (impaired clearance of apoptotic cells and β-amyloid) with normal microglial development (38294065) — models the neuroimmune/clearance component.
• In vitro/heterologous: Xenopus oocyte and mammalian-cell expression for electrophysiology and lysosomal-pH assays (functional variant validation; 31155284 38136669 33125761); Arabidopsis vacuole system for CLC-7 transport mechanism (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)

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

Evidence-Source Classification

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