Clinical variability associated with intronic FGF14 GAA repeat expansion in Japan

Background and Objectives The GAA repeat expansion within the fibroblast growth factor 14 (FGF14) gene has been found to be associated with late‐onset cerebellar ataxia. This study aimed to investigate the genetic causes of cerebellar ataxia in patients in Japan. Methods We collected a case series of 940 index patients who presented with chronic cerebellar ataxia and remained genetically undiagnosed after our preliminary genetic screening. To investigate the FGF14 repeat locus, we employed an integrated diagnostic strategy that involved fluorescence amplicon length analysis polymerase chain reaction (PCR), repeat‐primed PCR, and long‐read sequencing. Results Pathogenic FGF14 GAA repeat expansions were detected in 12 patients from 11 unrelated families. The median size of the pathogenic GAA repeat was 309 repeats (range: 270–316 repeats). In these patients, the mean age of onset was 66.9 ± 9.6 years, with episodic symptoms observed in 56% of patients and parkinsonism in 30% of patients. We also detected FGF14 repeat expansions in a patient with a phenotype of multiple system atrophy, including cerebellar ataxia, parkinsonism, autonomic ataxia, and bilateral vocal cord paralysis. Brain magnetic resonance imaging (MRI) showed normal to mild cerebellar atrophy, and a follow‐up study conducted after a mean period of 6 years did not reveal any significant progression. Discussion This study highlights the importance of FGF14 GAA repeat analysis in patients with late‐onset cerebellar ataxia, particularly when they exhibit episodic symptoms, or their brain MRI shows no apparent cerebellar atrophy. Our findings contribute to a better understanding of the clinical variability of GAA‐FGF14‐related diseases.