Expanding the Genetic Landscape of ATXN2 Variants: Insights From a Biallelic Trinucleotide Repeat Expansion in an Acadian Family.

BACKGROUND AND OBJECTIVES: Spinocerebellar ataxias are a diverse group of autosomal dominant cerebellar ataxias. SCA2 is a complex ataxia with various extracerebellar symptoms, including parkinsonism, dystonia, hyporeflexia, and cognitive impairment. ATXN2 is a modulator of neurologic disease: Expansions of at least 37 CAG (glutamine) repeats are pathogenic for SCA2, while expansions in the intermediate range (30-32) convey risk for the development of neurodegenerative disorders including Parkinson disease and amyotrophic lateral sclerosis. Homozygous variants are exceedingly rare. This study describes a novel ATXN2 presentation identified in an Acadian family from New Brunswick, Canada: a CAG repeat expansion within the fully penetrant range of SCA2, asymptomatic in the heterozygous state and resulting in a neurodegenerative disorder in homozygous patients.

METHODS: Three individuals, 2 siblings and their cousin, were investigated for a neurodegenerative disorder with overlapping phenotypes. The affected individuals and their 5 immediate family members underwent whole-genome sequencing analyzed by ExpansionHunter, repeat-primed PCR and Sanger sequencing.

RESULTS: Sequencing revealed a homozygous 39/39 CAG repeat expansion with 4 CAA interruptions in ATXN2 across all 3 affected individuals. After experiencing childhood intellectual or learning difficulties, the patients developed a pyramidal syndrome with spastic gait and a major neurocognitive disorder characterized by prominent frontal signs during their late twenties. Within a decade, all patients completely lost their autonomy. Shared phenotypic features included ataxia, spasticity, aphasia, dysphagia, myoclonus, atypical parkinsonism, incontinence, diffuse cortical atrophy with frontal predominance, and cerebellar atrophy. The same 39 CAG repeat allele with 4 CAA interruptions was identified in heterozygous state in 4 asymptomatic parents (age 65+) and 1 sibling in their thirties. Three carriers consented to further investigation with a neurologic examination, neuropsychological assessment, and cerebral MRI. Clinical and radiologic signs of disease were absent, despite the carriers' ages and their heterozygous expansion in the fully penetrant range of SCA2.

DISCUSSION: This study describes a novel ATXN2 expansion within the classic pathogenic range for SCA2 that manifests as an early-onset neurodegenerative disorder in the homozygous state, while being asymptomatic into late adulthood in the heterozygous state.