Intrafamilial Phenotypic Variation in Spinocerebellar Ataxia Type 23

Spinocerebellar ataxia type 23 (SCA23) is an autosomal dominant cerebellar ataxia caused by pathogenic variants in the prodynorphin gene (PDYN). The frequency of PDYN variants is reportedly very low (~ 0.1%) in several ataxia cohorts screened to date. We found five cases of SCA23 in two families (mean age at onset: 37.8 ± 5.5 years; mean age at examination: 64.2 ± 12.3 years) with a novel PDYN variant (c.644G > A:p.R215H). We identified marked heterogeneity in the clinical features in Family 1: the proband showed clinical and neuroimaging features suggestive of multiple system atrophy with predominant parkinsonism (MSA-P). Conversely, the proband’s mother with the PDYN p.R215H variant had no subjective symptoms; she had not come to medical attention before our survey, although she showed apparent cerebellar atrophy on brain magnetic resonance imaging (MRI). The other two patients in Family 1 and a patient in Family 2 showed slowly progressive cerebellar ataxia. We here report two Japanese families with SCA23, one of which showed considerable phenotypic variation in affected members. Our findings support that SCA23 can phenotypically overlap with MSA.