RAB24 Missense Variant in Dogs with Cerebellar Ataxia

Hereditary ataxias are a highly heterogenous group of diseases characterized by loss of coordination. In this study, we investigated a family of random-bred dogs, in which two siblings were affected by a slowly progressive ataxia. They presented with clinical signs of progressive cerebellar ataxia, hypermetria, and absent menace response. The MRI revealed generalized brain atrophy, reduced cortical demarcation, hypoplastic corpus callosum, and cerebellar folia thinning, highly suggestive of a neurodegenerative disorder. We sequenced the genomes of the two affected dogs and their unaffected parents. Filtering for protein-changing variants that had homozygous alternate genotypes in the affected dogs, heterozygous genotypes in the parents, and homozygous reference genotypes in 1576 control genomes yielded a single missense variant in the RAB24 gene, XM_038534663.1:c.239G>T or XP_038390591.1:p.(Gly80Val). Genotypes at this variant showed the expected co-segregation with the ataxia phenotype in the investigated family. The predicted amino acid affects the conserved RabF4 motif. Glycine-80 resides at the protein surface and the introduction of a hydrophobic isopropyl side chain of the mutant valine might impede solvent accessibility. Another missense variant in RAB24, p.Glu38Pro, was previously reported to cause a clinically similar form of cerebellar ataxia in Gordon Setters and Old English Sheepdogs. Taken together, the available data suggest that RAB24:p.Gly80Val represents the causal variant in the studied dogs. To the best of our knowledge, this is only the second report of a potentially pathogenic RAB24 variant in any species and further supports that RAB24 should be considered a candidate gene in human ataxia patients with unclear molecular etiology.