An X-Linked Ataxia Syndrome in a Family with Hearing Loss Associated with a Novel Variant in the BCAP31 Gene.

OBJECTIVE: Pathogenic variants in B-cell receptor-associated protein (BCAP31) are associated with X-linked, deafness, dystonia and cerebral hypomyelination (DDCH) syndrome. DDCH is congenital and non-progressive, featuring severe intellectual disability (ID), variable dysmorphism, and sometimes associated with shortened survival. BCAP31 encodes one of the most abundant chaperones, with several functions including acting as a negative regulator of endoplasmic reticulum (ER) calcium ion (Ca(2+)) concentration. Here, we characterize an X-linked syndrome, its underlying genotype, and a functional evaluation of the identified candidate genetic variant. METHODS: Evaluation of motor features, neuroimaging studies, neurophysiological, and cognitive tests. Whole exome sequencing (WES) was applied, a plasmid encoding BCAP31 with and without a candidate variant was transfected into SH-SY5Y cells to assess subcellular location and to measure Ca(2+) levels in the cytoplasm. RESULTS: Adult-onset ataxia, cognitive impairment, and hearing loss leading to deafness are the predominant features. Reduced penetrance, slow progression with preserved ability to walk in advance age, and universal cerebellar atrophy are other features for this syndrome. This condition is associated with the new variant c.22G>A (V8I) in BCAP31 at Xq28. The subcellular location of the V8I BCAP31 protein was not altered but caused significant elevation of cytosolic Ca(2+). CONCLUSIONS: Our findings expand the spectrum of variants in BCAP31 from neurodevelopmental syndromes to include a progressive neurodegenerative disease with variable expressivity. This is the first time ataxia is described in association with a BCAP31 variant and functional evidence of pathogenicity is provided. Additional BCAP31 cases featuring ataxia are needed to establish an association. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.