One gene, many phenotypes: altered molecular and cellular mechanisms in KIF5A-associated neurodegenerative and neurodevelopmental disorders

Mutations targeting different domains of the neuron-specific kinesin KIF5A are linked to distinct neurodegenerative or neurodevelopmental disorders, but the molecular bases of this clinical heterogeneity are still unknown. We characterised and compared in a homogeneous experimental setting five key mutants covering the whole spectrum of KIF5A-related phenotypes: R17Q and R280C KIF5A, linked to spastic paraplegia (SPG); R864* KIF5A, linked to Charcot-Marie-Tooth (CMT) disease; N999Vfs*40 KIF5A, linked to amyotrophic lateral sclerosis (ALS); and C975Vfs*73 KIF5A, linked to neonatal intractable myoclonus (NEIMY). Based on our analyses, the CMT-related R864* and ALS-related N999Vfs*40 KIF5A mutants showed defective autoinhibition and peripheral localisation accompanied by altered mitochondrial distribution, suggesting a disruption of their transport competence. N999Vfs*40 KIF5A also formed SQSTM1/p62-positive inclusions sequestering wild-type KIF5A, indicating a gain of toxic function and a dominant-negative behaviour. Our data also indicate that the ubiquitin-proteasome system plays a central role in mutant KIF5A degradation. Indeed, the SPG-related R17Q KIF5A mutant and N999Vfs*40 KIF5A both displayed a faster turnover compared to the wild-type protein and accumulated into detergent-insoluble aggregates upon proteasomal blockage. Moreover, the SPG-related R280C KIF5A mutant and N999Vfs*40-KIF5A both competed for degradation with proteasome-specific substrates. Finally, the NEIMY-related C975Vfs*73 KIF5A mutant was characterised by a similar, but more severe aberrant behaviour compared to N999Vfs*40 KIF5A. Interestingly, these mutants share an abnormal tail but cause disorders on the opposite end of KIF5A-linked phenotypic spectrum. Altogether, our observations support the pathogenicity of newly identified KIF5A mutants, highlight previously unknown defects of recurrent variants, and demonstrate that both shared and unique mechanisms underpin KIF5A-related diseases.