GM2 gangliosidosis AB variant (GM2AB) is a rare neurodegenerative lysosomal storage disorder with clinical features resembling Tay-Sachs disease but characterized by normal lysosomal β-hexosaminidase A enzyme activity. To date, only 14 cases of the acute infantile form have been reported. To the best of our knowledge, this is the first case of GM2AB in a Portuguese patient reported in the literature. We describe the case of a girl with GM2AB, whose clinical presentation and pathological findings were critical for diagnosis. Post-mortem genetic sequencing identified a pathogenic mutation in homozygosity in the GM2A gene, confirming the diagnosis. This case highlights the importance of considering GM2AB in patients with severe neurodegenerative phenotypes and typical pathological findings, even when enzymatic studies are normal. Preserving genetic material post-mortem may allow for diagnosis even years after death, providing critical insights into rare disorders. Acute infantile GM2 activator deficiency is a neurodegenerative disorder in which infants, who are generally normal at birth, have progressive weakness and slowing of developmental progress between ages four and 12 months. An ensuing developmental plateau is followed by progressively rapid developmental regression. By the second year of life decerebrate posturing, difficulty in swallowing, and worsening seizures lead to an unresponsive vegetative state. Death usually occurs between ages two and three years. The diagnosis of GM2 activator deficiency is established in a proband with suggestive findings of GM2 gangliosidosis, normal beta-hexosaminidase A (HEX A) enzyme activity levels, and biallelic pathogenic (or likely pathogenic) variants in GM2A identified by molecular genetic testing. Treatment of manifestations: There is no cure for GM2 activator deficiency. Supportive care to provide adequate nutrition and hydration, manage infectious disease, protect the airway, and control seizures involves multidisciplinary care by specialists in relevant fields. Surveillance: Periodic multidisciplinary evaluations to monitor existing disease manifestations and identify new manifestations requiring modification of supportive care. Agents/circumstances to avoid: Positioning that increases aspiration risk during feedings and seizure medication dosages that result in excessive sedation. GM2 activator deficiency is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for a GM2A pathogenic variant, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of inheriting neither of the familial pathogenic variants. Once the GM2A pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible.

AB variant is the rarest form of GM2 gangliosidosis, neurodegenerative diseases caused by lysosomal accumulation of GM2 gangliosides. Less than thirty cases are referenced in the literature, and to date, no late-onset form has been described. Our proband is a 22-year-old male with spinocerebellar ataxia and lower limbs motor deficiency. His symptoms started at the age of 10. A genetic analysis revealed two mutations in the GM2A gene encoding the GM2 activator protein (GM2-AP), an essential co-factor of hexosaminidase A. Both mutations, GM2A:c.79A > T:p.Lys27* and GM2A:c.415C > T:p.Pro139Ser, were inherited respectively from his father and his mother. The nonsense mutation was predicted to be likely pathogenic, but the missense mutation was of unknown significance. To establish the pathogenicity of this variant, we studied GM2 accumulation and GM2A gene expression. Electron microscopy and immunofluorescence performed on patient's fibroblasts did not reveal any lysosomal accumulation of GM2. There was also no difference in GM2A gene expression using RT-qPCR, and both mutations were found on cDNA Sanger sequencing. Measurement of plasma gangliosides by liquid-phase chromatography-tandem mass spectrometry showed an accumulation of GM2 in our patient's plasma at 83.5 nmol/L, and a GM2/GM3 ratio at 0.066 (median of negative control at 30.2 nmol/L [19.7-46.8] and 0.019 respectively). Therefore, the association of both p.Lys27* and p.Pro169Ser mutations leads to a GM2-AP functional deficiency. Whereas the first mutation is more likely to be linked with infantile form of GM2 gangliosidosis, the hypomorphic p.Pro169Ser variant may be the first associated with a late-onset form of AB variant.

GM2 gangliosidosis, AB variant, is a very rare form of GM2 gangliosidosis due to a deficiency of GM2 activator protein. We report on two patients with typical clinical features suggestive of GM2 gangliosidosis, but normal results for hexosaminidase A and hexosaminidase B as well as their corresponding genes. Genetic analysis of the gene encoding the activator protein, the GM2A gene, elucidated the cause of the disease, adding a novel mutation to the spectrum of GM2 AB variant. This report points out that in typical clinical constellations with normal enzyme results, genetic diagnostic for activator protein defects should be performed.

GM2 gangliosidosis-AB variants a rare autosomal recessive neurodegenerative disorder occurring due to deficiency of GM2 activator protein resulting from the mutation in GM2A gene. Only seven mutations in nine cases have been reported from different population except India. Present case is a one year old male born to 3rd degree consanguineous Indian parents from Maharashtra. He was presented with global developmental delay, hypotonia and sensitive to hyperacusis. Horizontal nystagmus and cherry red spot was detected during ophthalmic examination. MRI of brain revealed putaminal hyperintensity and thalamic hypointensity with some unmyelinated white matter in T2/T1 weighted images. Initially he was suspected having Tay-Sachs disease and finally diagnosed as GM2 gangliosidosis, AB variant due to truncated protein caused by nonsense mutation c.472 G > T (p.E158X) in GM2Agene. Children with phenotypic presentation as GM2 gangliosidosis (Tay-Sachs or Sandhoff disease) and normal enzyme activity of β-hexosaminidase-A and -B in leucocytes need to be investigated for GM2 activator protein deficiency.

GM2-gangliosidosis, AB variant is a very rare form of GM2 gangliosidosis due to a deficiency of GM2 activator protein, associated with autosomal recessive mutations in GM2A. Less than ten patients, confirmed by molecular analysis, have been described in the literature.A 12-month-old Hmong girl presented to the neurometabolic clinic for evaluation of global developmental delay, hypotonia, and cherry red spots. The parents were not known to be consanguineous. Her examination was remarkable for hypotonia with hyperreflexia and excessive startling. The head circumference was normal. An extensive neurometabolic evaluation was negative.Developmental regression began at 14 months of age. Retinal examination at 16 months of age disclosed 4+ cherry red/black spots with "heaped up" ring of whitish infiltrate surrounding both foveae but no evidence of optic atrophy or peripheral retinal abnormalities. Repeat magnetic resonance imaging (MRI) scan at 17 months of age revealed delayed but interval myelination associated with abnormal signal intensity of the bilateral thalami presenting as T2 hyperintensity of the posterior thalami in the region of the pulvinar nuclei and T2 hypointensity in the anterior thalami. Sequencing of the GM2A gene revealed a homozygous c.160 G>T mutation, predicted to result in a premature protein termination p. Glu54*.