Pathogenic variants in BORCS5 Cause a Spectrum of Neurodevelopmental and Neurodegenerative Disorders with Lysosomal Dysfunction.

BORCS5 encodes a subunit of the BLOC-one-related complex (BORC), which is known to mediate the kinesin-dependent anterograde movement of lysosomes. Using whole-exome sequencing, we identified 12 cases from seven families carrying bi-allelic BORCS5 variants, including four loss-of-function and two missense variants. Carriers of homozygous loss-of-function variants presented with prenatally lethal arthrogryposis multiplex congenita, brain malformations, and neuropathological evidence of diffuse neuroaxonal dystrophy. Individuals with missense variants presented differently, with microcephaly, developmental epileptic encephalopathy, intellectual disability, optic atrophy, spasticity, and progressive movement disorders. In this group, brain MRI showed diffuse hypomyelination and progressive global cerebral atrophy, consistent with neurodegeneration. Borcs5 knockout in zebrafish exhibited microcephaly, motor deficits, and seizures, mirroring the patients' clinical presentation. At the cellular level, BORCS5 loss-of-function but not missense variants, resulted in lower protein expression and impaired BORC assembly, paralleled by perinuclear lysosomal clustering. However, both loss-of-function and missense BORCS5 variants were associated with reduced total lysosomal proteolysis, reduced activity of the lysosomal hydrolases glucocerebrosidase and cathepsin B, and presence of multilamellar bodies, indicating lysosomal dysfunction. Our study reveals a novel role for BORCS5 in the regulation of lysosomal function, in addition to its known role in the anterograde movement of lysosomes, possibly underlying the diverse clinical manifestations in individuals with BORCS5-related disorders.