Pathogenic variants inBORCS5Cause a Spectrum of Neurodevelopmental and Neurodegenerative Disorders with Lysosomal Dysfunction

Abstract 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. Competing Interest Statement The authors have declared no competing interest. Funding Statement National Institutes of Health [grant 1K08NS131581 to N.E.M and grants R37 NS096241 and R35 NS122257 to D.K. and Intramural Program of NICHD project ZIA HD001607 to J.S.B.] Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Individuals and/or their legal guardians recruited for this study gave informed consent for their participation. This study received approval from the Review Boards and Bioethics Committees at University College London Hospital (project 06/N076). Permission for inclusion of their anonymized medical data in this cohort, including photographs, was obtained using standard forms at each local site by the responsible referring physicians. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data Availability All data produced in the present study are available upon reasonable request to the authors