Spinocerebellar ataxia type 25 (SCA25) is a rare autosomal dominant disorder caused by heterozygous pathogenic variants in the PNPT1 gene, primarily affecting the critical S1 RNA-binding domain. This study reports the first Brazilian and South American family with SCA25. To describe the clinical, genetic, and molecular findings in a family with a novel PNPT1 variant and compare them with previously reported cases. Clinical evaluation, neuroimaging, and genetic testing were performed on affected family members. The proband underwent clinical exome sequencing, with Sanger confirmation of the identified variant. Computational tools, including SpliceAI, were used to predict the molecular consequences of the variant. The proband, a 1-year-8-month-old girl, presented with progressive ataxia, cerebellar atrophy, and sensory neuropathy. Genetic testing identified a novel heterozygous truncating variant in PNPT1 (c.2068del; p.?), inherited from her father, who was mildly affected with polyneuropathy but no ataxia. SpliceAI predicted significant splicing disruptions, including intron retention or exon skipping, leading to a frameshift (p.(Arg690Glyfs*5)) and likely triggering nonsense-mediated decay or post-translational degradation. These findings align with previously reported PNPT1 variants associated with SCA25, which exhibit phenotypic variability and incomplete penetrance. This report expands the clinical and genetic spectrum of SCA25 and highlights the importance of considering this condition in the differential diagnosis of progressive ataxias. Further studies, including RNA and protein analyses, are required to confirm the molecular consequences of the PNPT1:c.2068del variant and to advance our understanding of the pathophysiology of SCA25.

Dominant spinocerebellar ataxias (SCA) are characterized by genetic heterogeneity. Some mapped and named loci remain without a causal gene identified. Here we applied next generation sequencing (NGS) to uncover the genetic etiology of the SCA25 locus. Whole-exome and whole-genome sequencing were performed in families linked to SCA25, including the French family in which the SCA25 locus was originally mapped. Whole exome sequence data were interrogated in a cohort of 796 ataxia patients of unknown etiology. The SCA25 phenotype spans a slowly evolving sensory and cerebellar ataxia, in most cases attributed to ganglionopathy. A pathogenic variant causing exon skipping was identified in the gene encoding Polyribonucleotide Nucleotidyltransferase PNPase 1 (PNPT1) located in the SCA25 linkage interval. A second splice variant in PNPT1 was detected in a large Australian family with a dominant ataxia also mapping to SCA25. An additional nonsense variant was detected in an unrelated individual with ataxia. Both nonsense and splice heterozygous variants result in premature stop codons, all located in the S1-domain of PNPase. In addition, an elevated type I interferon response was observed in blood from all affected heterozygous carriers tested. PNPase notably prevents the abnormal accumulation of double-stranded mtRNAs in the mitochondria and leakage into the cytoplasm, associated with triggering a type I interferon response. This study identifies PNPT1 as a new SCA gene, responsible for SCA25, and highlights biological links between alterations of mtRNA trafficking, interferonopathies and ataxia. ANN NEUROL 2022;92:122-137.