Hypotonia, Infantile, with Psychomotor Retardation and Characteristic Facies 3

1. Disease Information

• Overview: A severe childhood-onset encephaloneuropathy characterized by infantile hypotonia, psychomotor/developmental delay, intellectual disability, epilepsy, and characteristic facies, with progressive CNS deterioration. Also termed TBCK deficiency disorder (TBCK-DD) or TBCK-related encephaloneuropathy.
• Key identifiers:
• OMIM: #616900 (phenotype); TBCK gene OMIM 616899.
• Mondo: MONDO:0014635 (Hypotonia, infantile, with psychomotor retardation and characteristic facies 3).
• Orphanet: ORPHA:494526 (TBCK-related intellectual disability syndrome).
• ICD-10: best mapped to G71.2/Q87-range congenital/neuromuscular categories; ICD-11: LD24 / 8C (no specific code).
• MeSH: no dedicated term; indexed under Muscle Hypotonia, Intellectual Disability, Neurodevelopmental Disorders.
• HGNC: TBCK (HGNC:28348).
• Synonyms: IHPRF3; TBCK syndrome; TBCK deficiency disorder; TBCK-related encephaloneuropathy; hypotonia, infantile, with psychomotor retardation and characteristic facies-3; (proposed) neuronal ceroid lipofuscinosis type 15 / CLN15.
• Data source type: Aggregated disease-level knowledge derived from individual patient reports and small cohorts (WES-diagnosed families) — not EHR-scale datasets. The founding description reported 13 individuals from 9 families (PMID 27040691); a 2019 review compiled 33 cases (PMID 30103036).

2. Etiology

• Primary cause (genetic): Biallelic (homozygous or compound heterozygous) pathogenic variants in TBCK. "13 individuals from nine unrelated families and affected by likely pathogenic biallelic variants in TBC1-domain-containing kinase (TBCK) were identified through whole-exome sequencing" (PMID 27040691). Autosomal recessive: "an autosomal recessive disorder caused by biallelic pathogenic variants of the TBCK gene" (PMID 36273129).
• Genetic risk factors: The disease is monogenic/Mendelian; the sole causal locus is TBCK. Consanguinity substantially increases risk (multiple homozygous cases in consanguineous/first-cousin unions, e.g., PMID 39865381). No established modifier genes; genotype–phenotype correlation is imperfect (PMID 32363625).
• Environmental risk factors: None established. Onset is congenital/genetic; no toxin, infectious, or lifestyle trigger is implicated.
• Protective factors: None specifically identified. In carriers, a single functional TBCK allele is sufficient (parents asymptomatic, PMID 36273129).
• Gene–environment interactions: The only therapeutically relevant "environmental" modifier is dietary L-leucine, which can reactivate residual mTOR signaling (PMID 27040691) — an interaction exploited for potential treatment rather than a disease cause.

3. Phenotypes

Core phenotypes (with suggested HPO terms, typical onset, and qualitative frequency):

• Severity: Severe to profound; variable across variants. Older children may develop coarse, storage-disorder-like features (PMID 27040691).
• Progression: Progressive/neurodegenerative — "pathogenic mutations of the said gene lead to a progressive neurodegenerative disease" (PMID 30103036).
• Quality-of-life impact: Severe — most affected individuals are non-ambulatory, nonverbal, dependent for all activities of daily living, often requiring gastrostomy feeding and respiratory support.

4. Genetic / Molecular Information

• Causal gene: TBCK (TBC1-domain-containing kinase), 4q24; HGNC:28348; encodes a protein with three conserved domains — an N-terminal Ser/Thr (pseudo)kinase domain, a central TBC (Rab-GAP) domain, and a C-terminal rhodanese-homology domain (RHOD) — with long and short isoforms generated by alternative splicing (PMID 33240423). Biochemical study of purified full-length protein confirms TBCK is a catalytically inactive class I pseudokinase lacking canonical VAIK/HRD/DFG motifs and nucleotide binding (PMID 42546825).
• Representative pathogenic variants (ClinVar/literature):
• c.247C>T, p.Arg83Ter (nonsense) — novel in a Chinese patient (PMID 36273129).
• p.Gln164* (nonsense, exon 6) — LOF (PMID 37876076).
• c.753dup, p.Lys252* (frameshift/truncating) (PMID 30103036).
• c.1532G>A (missense) — recurrent; associated with cardiac involvement (PMID 39865381).
• Early truncating variant with complete absence of TBCK protein and myopathy (PMID 32363625).
• c.376C>T, p.Arg126* (p.R126X) — recurrent founder stop-gain variant predominating in individuals of Puerto Rican/"Boricua" ancestry; defines "TBCK-encephaloneuropathy / Boricua syndrome" (PMID 42563865).
• Variant classification: Pathogenic/likely pathogenic per ACMG/AMP (PMID 36273129). Variant types: predominantly nonsense, frameshift, splice, and structural/duplication (LOF); some missense. Functional consequence: loss of function — "TBCK was absent in cells from affected individuals" (PMID 27040691).
• Allele frequency (gnomAD v4, this analysis): Individual pathogenic alleles are ultrarare. Most pathogenic truncating variants have exome AF <1e-5 (often singletons or absent). The founder p.Arg126* (c.376C>T; 4-106262103-G-A) is the most enriched pathogenic LoF allele: exome AF ≈ 6.0×10⁻⁵ (1/16,569 alleles); genome AF ≈ 2.4×10⁻⁴ (1/4,209 alleles), the higher genome frequency reflecting over-representation in admixed/Latino samples. Historical: c.247C>T ~8.2×10⁻⁶ in ExAC (PMID 36273129).
• Gene constraint (gnomAD v4): TBCK is loss-of-function tolerant — pLI ≈ 0, oe_lof = 0.87 (90% CI 0.74–1.03; LOEUF ~1.03) — consistent with recessive inheritance in which heterozygous carriers are largely unaffected.
• Allelic architecture: ClinVar lists 882 TBCK variants; pathogenic truncating alleles are distributed across the gene (marked allelic heterogeneity) with no mutational hotspot other than the Puerto Rican founder allele.
• Somatic vs germline: Germline, biallelic.
• Modifier genes: None validated. Functional partners PPP1R21 and FERRY3/C12orf4 form a complex with TBCK (PMID 41789809) and are candidate biology-level modifiers.
• Epigenetics/chromosomal abnormalities: No specific methylation signature or recurrent chromosomal aberration; chromosomal microarray is typically normal aside from runs of homozygosity in consanguineous families (PMID 39865381).

Suggested ontology terms: Gene — TBCK (HGNC:28348); MONDO:0014635.

5. Environmental Information

• Environmental factors: None causative. Infectious agents: none. Lifestyle factors: not applicable to disease causation. The only relevant environmental/nutritional variable is L-leucine as a candidate therapeutic modulator (PMID 27040691).

6. Mechanism / Pathophysiology

• Molecular pathways: mTOR / mTORC1 signaling — TBCK normally supports mTOR activity; its loss reduces downstream signaling, evidenced by decreased phospho-S6: "decreased phosphorylation of phospho-ribosomal protein S6 was also observed, a finding suggestive of downregulation of mTOR signaling" (PMID 27040691). TBCK also acts as a RAB5-GAP within the TBCK–PPP1R21–FERRY3/C12orf4 complex governing endo-lysosomal homeostasis (PMID 41789809).
• Cellular processes: Impaired ER-to-Golgi vesicle transport, autophagosome biogenesis, cell-cycle progression, and cell migration in patient iNPCs: "Lack of functional TBCK protein in iNPC is associated with impaired endoplasmic reticulum-to-Golgi vesicle transport and autophagosome biogenesis, as well as altered cell cycle progression and severe impairment in the capacity of migration" (PMID 35095425). Dysregulated autophagy–lysosome flux (enhanced mTORC1-mediated autophagosome formation with reduced Rab-mediated autophagosome–lysosome fusion) drives storage (PMID 30591081).
• Protein dysfunction: Loss of function / absent protein; TBCK's Rab-GAP and scaffolding roles are lost.
• Metabolic/storage changes: Lipofuscin and carbohydrate-rich storage material accumulate — "a widespread and massive accumulation of lipofuscin storage material in neurons of the central nervous system" (PMID 30591081). Lysosomal dysfunction impairs mitochondrial quality control, contributing to neurodegeneration (PMID 34816123).
• RNA/ribosomal dysregulation: Multi-omics of TBCK-deficient cells shows altered ribosomal function, RNA splicing, and miRNA networks (PMID 41040160); patient-derived neurons exhibit compartment-specific mRNA and lysosomal trafficking defects (PMID 40093117), consistent with a role in RNA transport.
• Immune involvement: Not a primary feature; vacuolated lymphocytes are a storage marker (PMID 30591081).
• Tissue damage mechanism: Progressive intraneuronal storage and impaired neurogenesis/neuronal migration → neurodevelopmental deficit plus neurodegeneration.
• Causal chain: Biallelic TBCK LOF → loss of Rab5-GAP activity + reduced mTORC1 (↓pS6) → defective endolysosomal trafficking, secretory transport, and autophagosome–lysosome fusion → lipofuscin/carbohydrate storage + impaired neuroprogenitor proliferation/migration → hypotonia, developmental delay, and progressive neurodegeneration. Upstream: TBCK loss; downstream: mTOR, RAB5/autophagy, storage accumulation, neuronal dysfunction.

Suggested ontology terms: GO:0031929 (TOR signaling); GO:0006914 (autophagy); GO:0016197 (endosomal transport); GO:0005096 (GTPase activator activity); GO:0006888 (ER-to-Golgi vesicle transport); CL:0000031 (neuroblast/neural progenitor), CL:0000540 (neuron), CL:0000127 (astrocyte).

7. Anatomical Structures Affected

• Organ level: Central nervous system (primary) — cerebral cortex, white matter, ventricles, corpus callosum; skeletal muscle (secondary myopathy); heart (rare malformations); systemic storage in spleen and liver (PMID 30591081). Body systems: nervous, musculoskeletal, occasionally cardiovascular, and respiratory (secondary).
• Tissue/cell level: Neurons (lipofuscin storage), astrocytes (storage), lymphocytes (vacuolation); neural progenitor cells (migration/proliferation defects).
• Subcellular level: Lysosome/autophagosome (GO:0005764, GO:0005776), endoplasmic reticulum & Golgi (GO:0005783, GO:0005794), endosome (GO:0005768).
• Localization/lateralization: Bilateral, diffuse CNS involvement (ventriculomegaly, widened subarachnoid space, atrophy, white-matter changes) (PMID 36273129).

Suggested UBERON terms: UBERON:0000955 (brain); UBERON:0002316 (white matter); UBERON:0001851 (cortex); UBERON:0002037 (cerebellum); UBERON:0001134 (skeletal muscle tissue).

8. Temporal Development

• Onset: Congenital/infantile — hypotonia and delay evident in early infancy.
• Onset pattern: Insidious/chronic; developmental delay recognized within the first months to ~15 months (PMID 36273129).
• Progression: Progressive neurodegenerative course (PMID 30103036); older children develop coarsening facies and storage features (PMID 27040691). Course is generally slowly progressive with variable rate; severe forms cause premature death (PMID 30591081).
• Duration: Chronic, lifelong; substantially shortened life expectancy in severe cases.
• Critical periods: Early neurodevelopmental window (neurogenesis/migration) is central; any mTOR-directed intervention would plausibly need to be early, though efficacy is unproven.

9. Inheritance and Population

• Epidemiology: Ultrarare; only tens of cases reported worldwide (33 reviewed by 2019, PMID 30103036). No reliable prevalence/incidence estimates; Orphanet classifies prevalence as <1/1,000,000.
• Inheritance: Autosomal recessive (PMID 36273129). Penetrance: essentially complete in biallelic LOF. Expressivity: variable (severity, muscle, cardiac, imaging features). Heterozygotes: population-level gnomAD data show LoF tolerance (LOEUF ~1), yet targeted study reports a mild neurologic syndrome in some human and mouse heterozygotes (PMID 37353954) — i.e., largely recessive with a subtle possible dosage effect. No anticipation (not a repeat-expansion disorder).
• Consanguinity: Strong contributor — multiple homozygous cases in consanguineous families (PMID 39865381). Founder effect (quantified, gnomAD v4 + 1000 Genomes, this analysis): the p.Arg126* allele (rs575822089) is ~45× enriched in Admixed American/Latino samples (AMR genome AF 1.97×10⁻³, carrier ~1/253) vs Non-Finnish European (AF 4.4×10⁻⁵), and reaches AF ≈ 1.0×10⁻² in 1000 Genomes Puerto Ricans (carrier ~1/49) — a strong "Boricua syndrome" founder signal (PMID 42563865). The allele is essentially absent in African, East Asian, South Asian, Finnish, Ashkenazi Jewish, and Middle Eastern groups, and no homozygotes are seen in gnomAD (consistent with a severe recessive phenotype). Carrier frequency for TBCK deficiency overall is low in general (non-founder) populations, consistent with the ultrarare, mostly private pathogenic alleles.
• Demographics: Originally reported mainly in individuals of European ancestry; subsequently described across populations including the first non-Caucasian (Chinese) patient (PMID 36273129), broadening geographic distribution. A pronounced founder effect in Puerto Rico/Latino (AMR) populations (p.R126, ~45× enriched vs NFE; PUR carrier ~1/49) concentrates cases in the Boricua* population, where prevalence may be higher than expected (4 diagnoses in 6 months at one center; PMID 42563865). Consanguineous populations (e.g., Middle East, South Asia) are also enriched via private homozygous alleles. No strong sex bias (autosomal); age distribution centers on infants/children.

10. Diagnostics

• Genetic testing (definitive): Whole-exome sequencing is the primary diagnostic route (PMID 27040691, PMID 39865381); WGS, multigene hypotonia/neurodevelopmental panels including TBCK, or targeted single-gene testing (e.g., for known founder variants) are alternatives. Chromosomal microarray is typically normal (may show runs of homozygosity) (PMID 39865381). Variants classified per ACMG/AMP (PMID 36273129).
• Biochemical/functional: Absent TBCK protein and reduced phospho-S6 on Western blot in patient cells (research assay; PMID 27040691). Vacuolated lymphocytes and tissue storage material on biopsy support a storage phenotype (PMID 30591081). Muscle biopsy may show nonspecific myopathic changes (PMID 32363625).
• Imaging: Brain MRI — cerebral/cerebellar atrophy, white-matter changes, ventriculomegaly, widened subarachnoid space, occasional corpus callosum agenesis (PMID 27040691, PMID 36273129, PMID 39865381), and characteristically periventricular leukomalacia (PVL)-like changes in the absence of prematurity — a diagnostic clue prompting TBCK testing (PMID 36522252).
• RNA-seq diagnostics: Transcriptome sequencing (fibroblasts > blood) can diagnose TBCK-associated encephalopathy missed by conventional ES/CMA (PMID 33001864). Fetal echocardiography/ultrasound may detect cardiac anomalies and cystic hygroma in affected pregnancies (PMID 39865381).
• Clinical criteria / differential diagnosis: No formal criteria; diagnosis is molecular. Differentials: other causes of infantile hypotonia with DD/ID and coarse facies — lysosomal storage disorders (mucopolysaccharidoses, NCLs), congenital myopathies, other IHPRF forms (IHPRF1/NALCN, IHPRF2/UNC80), Prader-Willi, congenital disorders of glycosylation, and mitochondrial disease. Storage-disease workup is typically negative except for TBCK (PMID 30591081).
• Screening: Not part of standard newborn screening. Carrier/cascade screening and prenatal/preimplantation genetic testing are informative in known families (utilized for prenatal diagnosis in PMID 39865381).

11. Outcome / Prognosis

• Survival/mortality: Guarded; severe forms feature premature death (PMID 30591081), commonly from respiratory failure/recurrent infections related to hypotonia and neurodegeneration. Central hypoventilation/respiratory insufficiency is a major, under-recognized complication — 5/6 in one cohort required respiratory support (PMID 36522252), warranting early respiratory surveillance. No formal survival statistics exist given rarity.
• Morbidity/function: Profound disability — most patients are non-ambulatory, nonverbal, and fully dependent, often requiring feeding-tube and respiratory support.
• Disease course: Progressive; complications include epilepsy, scoliosis, contractures, aspiration, feeding failure, and infections.
• Prognostic factors: Variant severity (complete LOF/absent protein associated with severe multisystem presentations, e.g., myopathy PMID 32363625; cardiac malformation with c.1532G>A, PMID 39865381). Neurorehabilitation showed limited benefit in a reported case (PMID 36273129).

12. Treatment

• No approved disease-modifying therapy. Management is supportive and multidisciplinary (PMID 37876076): antiseizure medications for epilepsy, physical/occupational/speech therapy, nutritional support (gastrostomy), respiratory care, orthopedic management of scoliosis/contractures.
• Candidate directed therapy — L-leucine: Rationale from retained mTOR responsiveness — "activation of the mTOR pathway in response to L-leucine supplementation was retained, suggesting a possible avenue for directed therapies for this condition" (PMID 27040691). Efficacy remains unproven clinically.
• Rehabilitation: Neurorehabilitation training reported with poor response in at least one case (PMID 36273129).
• Experimental/future: mTOR-pathway modulation and endolysosomal/autophagy-targeted approaches are conceptually motivated by the RAB5-GAP/FERRY complex biology (PMID 41789809); no registered disease-specific trials established. Genetic counseling and reproductive options are central to family management.

Suggested NCIT terms: Supportive Care; Anticonvulsant Therapy; Physical Therapy; Nutritional Support; Leucine (dietary supplement). CHEBI: L-leucine (CHEBI:15603); sirolimus/rapamycin (CHEBI:9168) is conceptually contraindicated as an mTOR inhibitor given baseline mTORC1 downregulation.

13. Prevention

• Primary prevention: Not applicable (monogenic). Genetic counseling for at-risk (especially consanguineous) families; 25% recurrence risk per pregnancy for carrier couples.
• Reproductive/secondary prevention: Carrier screening, prenatal diagnosis, and preimplantation genetic testing in known families (demonstrated prenatally in PMID 39865381). Cascade testing of relatives. Population-targeted carrier screening for the p.Arg126* founder allele is high-yield in individuals of Puerto Rican/Latino ancestry (PUR carrier frequency ~1/49; this analysis, PMID 42563865), and equitable access to genetic testing improves ascertainment (PMID 42563865).
• Tertiary prevention: Anticipatory management of seizures, aspiration/respiratory infections, feeding difficulties, and orthopedic complications to reduce morbidity.

14. Other Species / Natural Disease

• Taxonomy/orthologs: TBCK is conserved across vertebrates — human (NCBI Taxon 9606), mouse Tbck (NCBI Taxon 10090, Gene ID 271981), zebrafish tbck (NCBI Taxon 7955). Functional conservation demonstrated: mouse Tbck loss causes leukoencephalopathy (PMID 42182390) and zebrafish tbck is required for heart development (PMID 36574072).
• Natural disease in animals: No well-characterized naturally occurring animal counterpart is established in OMIA; the disorder is defined in humans and studied via engineered models.
• Comparative biology: Evolutionary conservation of the TBC (Rab-GAP), rhodanese, and pseudokinase domains supports cross-species modeling; heterozygous phenotypes are conserved between mice and humans (PMID 37353954). Zoonotic potential: none (non-infectious, genetic).

15. Model Organisms

• Human cellular models: Patient-derived iPSCs → induced neuroprogenitor cells (iNPCs) demonstrate impaired ER-to-Golgi transport, autophagosome biogenesis, cell-cycle, and migration (PMID 35095425). Patient-derived neurons show compartment-specific mRNA and lysosomal trafficking defects (PMID 40093117); patient fibroblasts/lymphoblasts show absent TBCK, reduced pS6, and autophagic vesicle accumulation (PMID 27040691, 42563865). Multi-omics in patient cells reveal ribosomal/RNA-splicing/miRNA dysregulation (PMID 41040160).
• Mouse models (mammalian): A novel mouse model of TBCK Syndrome recapitulates a degenerative leukoencephalopathy with multisystem involvement, including neurodevelopmental, craniofacial, and pulmonary features and a unique enamel/dentin/alveolar-bone mineralization defect (PMID 42182390, 41756984). Heterozygous (haploinsufficient) Tbck variants cause a mild neurologic syndrome in both mice and humans (PMID 37353954) — relevant to carrier counseling.
• Zebrafish (invertebrate/vertebrate screen): tbck knockdown identified it as essential for heart development (atrioventricular canal formation) (PMID 36574072), providing a mechanistic basis for the emerging human cardiac phenotype.
• Phenotype recapitulation: Mouse models capture CNS leukoencephalopathy, craniofacial, and pulmonary components with good face validity; zebrafish captures cardiac requirement.
• Limitations: Cellular models incompletely capture progressive in vivo neurodegeneration; cross-species differences in lifespan/lysosomal storage may limit modeling of the human storage/CLN15 phenotype.
• Resources: MGI (mouse Tbck), ZFIN (zebrafish tbck); NCBI Gene orthologs — mouse Tbck (Gene ID 271981), zebrafish tbck.

Supported vs. Refuted Hypotheses

Supported: 1. IHPRF3 is autosomal recessive, caused by biallelic LOF in TBCK (PMID 27040691, 36273129). 2. Pathophysiology involves mTOR downregulation plus RAB5/endolysosomal–autophagy dysfunction (PMID 27040691, 35095425, 41789809). 3. The disorder is progressive/neurodegenerative with lysosomal storage (lipofuscin; proposed CLN15) (PMID 30591081, 30103036). 4. The phenotype extends beyond CNS to muscle and (rarely) heart (PMID 32363625, 39865381). 5. Residual mTOR responsiveness to L-leucine offers a therapeutic rationale (PMID 27040691).

Refuted / not supported: - Hypotonia is purely neurogenic — refuted; primary muscle disease can contribute (PMID 32363625). - It is a classical single-substrate lysosomal storage disease — refuted; >20 known LSDs excluded, storage is heterogeneous (PMID 30591081).

Limitations and Future Directions

• Rarity: Only tens of cases; no robust prevalence, survival, or genotype–phenotype statistics.
• Therapeutics: Leucine benefit is theoretical; no controlled trials. Need for validated in vivo models and natural-history studies.
• Mechanism: Relationship between mTOR downregulation and endolysosomal/RAB5 dysfunction (upstream vs. parallel) requires clarification (PMID 35095425, 41789809).
• Diagnosis: Broader awareness needed so hypotonia + coarse facies + storage-negative workup prompts TBCK sequencing.

Key References (PMID)

27040691 (Bhoj 2016, foundational cohort & mTOR/leucine); 30591081 (Beck-Wödl 2018, CLN15/storage neuropathology); 30103036 (Zapata-Aldana 2019, 33-case review, progressive neurodegeneration); 35095425 (Moreira 2021, iNPC secretory/autophagy defects); 32363625 (Saredi 2020, myopathy); 36273129 (Tan 2022, first non-Caucasian case, ACMG); 37876076 (Chand 2023, LOF nonsense case); 39865381 (Mastromoro 2025, cardiac involvement, prenatal); 41789809 (Chen 2026, TBCK–PPP1R21–FERRY3 RAB5-GAP complex); 42563865 (De Luca-Ramirez 2023, Puerto Rican p.R126*/Boricua founder); 36522252 (Sabanathan 2023, hypoventilation & PVL-like MRI); 33240423 (Wu 2021, three-domain structure/isoforms); 42546825 (Maurya 2026, class I pseudokinase); 34816123 (Tintos-Hernández 2021, lysosomal–mitochondrial quality control); 41040160 (Diaz-Rosado 2025, ribosome/RNA-splicing/miRNA multi-omics); 40093117 (Flores-Mendez 2025, mRNA & lysosomal trafficking in patient neurons); 37353954 (Nair 2023, heterozygote mild syndrome, mouse); 42182390 (Melendez-Perez 2026, mouse model); 41756984 (Katsura 2026, mouse mineralization/leukoencephalopathy); 36574072 (Ma 2022, zebrafish tbck heart development); 33001864 (Murdock 2021, RNA-seq diagnostics).

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