Spinocerebellar ataxia type 2 followed by amyotrophic lateral sclerosis due to a pure CAG repeat expansion in ATXN2: a case report and literature review.

backgroundSpinocerebellar ataxia type 2 (SCA2) is an autosomal dominant cerebellar ataxia caused by abnormal CAG expansions (≥ 34 repeats) in the ATXN2 gene (ATXN2), whereas intermediate CAG expansions (27-33 repeats) have been linked to amyotrophic lateral sclerosis (ALS). CASE DESCRIPTION: A 53-year-old woman with longstanding cerebellar ataxia developed progressive upper limb weakness and muscle atrophy at the age of 51 years. On neurological examination, she was found to have ataxic dysarthria, slow saccadic eye movements, tongue atrophy with fasciculations, muscle atrophy and weakness in both upper limbs, hyperreflexia with Babinski's sign, and limb and gait ataxia. Brain magnetic resonance imaging (MRI) showed brainstem and cerebellar atrophy. Genetic analysis identified an expanded CAG-repeat of 39/22 in ATXN2, and screening for other known ALS-related gene mutations was negative, leading to a diagnosis of both SCA2 and ALS associated with ATXN2.

conclusionsSCA2 is typically associated with uninterrupted CAG-repeat expansions, whereas ALS-related ATXN2 expansions usually contain at least one CAA triplet. However, despite carrying an uninterrupted CAG-repeat expansion, this patient developed ALS. This case shows that ALS can emerge several decades after SCA2 onset, even in patients with pure CAG-repeats, underscoring the need for long-term monitoring in SCA2 patients. Further research is needed to clarify the roles of repeat length, CAA interruptions, and other factors in ATXN2-related ALS.