| Knowledge-base field | Summary |
|---|---|
| Disease definition / synonyms / identifiers | Ultra-rare autosomal-recessive GM2 gangliosidosis caused by deficiency of the GM2 activator protein (GM2AP), clinically often indistinguishable from Tay-Sachs disease. Common names: **Tay-Sachs disease AB variant**, **GM2 gangliosidosis AB variant**, **GM2 activator deficiency**, **GM2 activator protein deficiency**. OMIM disease entry reported as **272750**; GM2A gene OMIM **\*613109**; MONDO **Tay-Sachs disease AB variant = MONDO_0010099**; broader **GM2 gangliosidosis = MONDO_0017720**. Fewer than 30 cases were reported in the literature by 2022; older reviews counted 9-10 molecularly confirmed cases worldwide (pqac-00000021, pqac-00000022, pqac-00000023, pqac-00000057, pqac-00000000). |
| Causal gene and inheritance | Caused by **biallelic GM2A variants** (gene encodes ganglioside GM2 activator, a non-enzymatic lysosomal cofactor required to present GM2 to Hex A for hydrolysis). Inheritance is **autosomal recessive**. Mechanistically distinct from classic Tay-Sachs (HEXA) and Sandhoff (HEXB) despite overlapping phenotype (pqac-00000001, pqac-00000002, pqac-00000006, pqac-00000007). |
| Hallmark diagnostic pattern | Key clue: **GM2 gangliosidosis phenotype with normal Hex A and Hex B enzyme activity** in leukocytes/blood. Recommended workup includes **GM2A sequencing**, with **copy-number analysis** because exon-level deletions can be missed by routine sequencing; supportive/confirmatory tools include plasma **GM2 LC-MS/MS**, fibroblast GM2 studies, EM/IF, and RNA/cDNA analyses in unresolved cases (pqac-00000003, pqac-00000004, pqac-00000033, pqac-00000034, pqac-00000035, pqac-00000036). |
| Main phenotypes by onset: infantile | Usually onset in the **first year** (often ~3-12 months). Features include **developmental delay/regression, hypotonia, hyperacusis/exaggerated startle, seizures, poor visual attention/nystagmus, bilateral cherry-red maculae**, and progressive neurodegeneration. MRI may show **putaminal hyperintensity, thalamic hypointensity, delayed/unmyelinated white matter**. Reported outcome: severe disability and **early death around 4-5 years**, sometimes earlier from respiratory complications (pqac-00000001, pqac-00000002, pqac-00000006, pqac-00000048, pqac-00000049, pqac-00000050, pqac-00000051). |
| Main phenotypes by onset: juvenile | Reported onset roughly **2-10 years** with **ataxia, psychomotor deterioration/regression, spasticity, seizures** and progressive neurologic decline; historically death before adulthood in described juvenile GM2 gangliosidosis summaries. AB-variant-specific juvenile cases are rare and sparsely characterized (pqac-00000001, pqac-00000048). |
| Main phenotypes by onset: late / adult | First late-onset AB-variant case reported in 2022. Phenotype may include **gait disorder beginning around age 10, lower motor neuron involvement, spinocerebellar ataxia, muscular atrophy, mild cognitive/executive deficits, psychiatric features**, and subtle cerebellar vermis atrophy with normal Hex A/B activity. Prognosis appears better than infantile disease but data are very limited (pqac-00000004, pqac-00000022, pqac-00000048, pqac-00000057). |
| Representative pathogenic variants reported | Reported variants include **c.472G>T (p.Glu158Ter / p.E158X)**, **homozygous exon 2 deletion**, **c.79A>T (p.Lys27Ter)**, **c.415C>T (p.Pro139Ser; evaluated as likely hypomorphic / VUS in trans with truncating allele)**, and earlier literature variants such as **c.160G>T (p.Glu54Ter)**, **c.164C>T (p.Pro55Leu)**, **c.412T>C**, **c.506G>C (p.Arg169Pro)**, **c.522T>G (p.Leu174Arg)** plus frameshift/deletion alleles. Population data are sparse; some variants were absent from gnomAD/ExAC/TOPMed and one missense allele had a single prior Latin American report (pqac-00000002, pqac-00000003, pqac-00000004, pqac-00000006, pqac-00000053). |
| Epidemiology / population notes | **Rarest** GM2 gangliosidosis subtype. Literature-based counts: **9 cases / 7 mutations** in older review, **10 molecularly proven cases** by 2016 review table, and **<30 cases** by 2022 review. Reported ancestries/populations include **Indian, Saudi, Spanish, US Black, Laotian/Hmong**, and later **Portuguese**; consanguinity is reported in some infantile cases but no robust carrier-frequency estimate exists for AB variant (pqac-00000021, pqac-00000022, pqac-00000052, pqac-00000056, pqac-00000057, pqac-00000058). |
| 2023-2024 translational development: AAV9-GM2A preclinical | **2023 AB-variant-specific gene therapy proof-of-concept**: systemic **ssAAV9-GM2A** in **Gm2a-/-** mice at **1 × 10^14 vg/kg** (reported also as **1 × 10^11 vg/mouse**) given at **postnatal day 1 or 6 weeks** produced long-term vector persistence in brain/liver, reduced CNS GM2 accumulation, and improved rotarod performance especially in 6-week-treated animals; long-term biochemical correction was partial, suggesting need for higher dose/optimization. Separate **2023 intrathecal scAAV9.hGM2A** study showed dose-responsive biochemical correction with **0.5, 1.0, 2.0 × 10^11 vg/mouse**, broad CNS distribution, persistence up to **104 weeks**, and no severe adverse events (pqac-00000009, pqac-00000010, pqac-00000011, pqac-00000012, pqac-00000013, pqac-00000014, pqac-00000015, pqac-00000016, pqac-00000017). |
| 2023-2024 translational development: severe double-KO model | **2023 Gm2a-/-Neu3-/- double knockout** mouse established as a more severe and translationally relevant AB-variant model. Rationale: single **Gm2a-/-** mice are mild because murine **NEU3** provides an alternative GM2 catabolic route. Double KO causes **marked CNS GM2 accumulation (~3-4-fold vs Gm2a-/-), ataxia, reduced mobility/coordination, weight loss, vacuolization, onset of deficits by ~12-16 weeks, and shortened lifespan (~27 weeks vs ~92 weeks for Gm2a-/-)**, better approximating severe human disease for preclinical testing (pqac-00000041, pqac-00000042, pqac-00000043, pqac-00000044, pqac-00000045). |
| Relevant clinical trials for GM2 gangliosidoses: TSHA-101 | **NCT04798235**; **TSHA-101** bicistronic **AAV9-HEXA/HEXB** gene therapy; **intrathecal**, one-time; target **infantile-onset GM2 gangliosidosis**; **ACTIVE_NOT_RECRUITING**; start date **2021-03-12**; phase **1/2**; planned enrollment **3**. Primary focus: safety/tolerability; secondary outcomes include survival, Hex A activity, motor/neurologic measures (pqac-00000032). |
| Relevant clinical trials for GM2 gangliosidoses: AXO-AAV-GM2 interventional | **NCT04669535**; **AXO-AAV-GM2** dual-vector gene therapy for Tay-Sachs/Sandhoff; delivery described as **bilateral intraparenchymal thalamic plus intracisternal/intrathecal**; pediatric target population; **TERMINATED** in trial search output; start year **2021**; phase **1**; enrollment reported as **9** in trial search output (review chapter listed 18 planned). No posted efficacy results in the retrieved context (pqac-00000031). |
| Relevant clinical trials for GM2 gangliosidoses: AXO-AAV-GM2 long-term follow-up | **NCT06614569**; long-term follow-up of previously treated AXO-AAV-GM2 subjects; parent intervention route **bilateral intraparenchymal thalamic and intracisternal/intrathecal**; **ACTIVE_NOT_RECRUITING**; actual start **2024-09-17**; estimated enrollment **7**; main purpose is delayed safety plus longitudinal neurocognitive/motor follow-up for up to 5 years (pqac-00000030). |
| Relevant clinical trials for GM2 gangliosidoses: natural history | **NCT00668187**; **A Natural History Study of the Gangliosidoses**; observational, no intervention; includes Tay-Sachs/Sandhoff/GM1; **RECRUITING**; start **2010-12**; estimated enrollment **52**. Important for defining progression and outcome measures for future AB-variant and broader GM2 trials (pqac-00000029). |


*Table: This table compiles the core knowledge-base fields for Tay-Sachs disease AB variant (GM2A deficiency), including disease definition, genetics, phenotype, diagnostics, and 2023-2024 translational advances. It also summarizes relevant GM2 gangliosidosis clinical trials to support curation and therapeutic landscape review.*