Molecular defects in KIF5A-linked neurodegenerative and neurodevelopmental diseases

Mutations targeting the three domains of the neuron-specific kinesin KIF5A cause distinct neurodegenerative or neurodevelopmental diseases, but the molecular bases of this clinical heterogeneity are still poorly understood. To fill this gap, we functionally compared five KIF5A mutants covering the whole spectrum of KIF5A related disorders: 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). First, we showed that the CMT-related R864* and ALS-related N999Vfs*40 KIF5A mutants do not undergo autoinhibition and mainly localise at cell periphery. These abnormal behaviours are accompanied by an altered mitochondrial distribution in cells that suggests a disruption of mutant KIF5A transport competence. N999Vfs*40 KIF5A also forms SQSTM1/p62-positive inclusions sequestering wild-type (WT) KIF5A, indicating a gain of toxic function and a dominant-negative behaviour. We also demonstrated that the newly identified SPG-related R17Q KIF5A mutant and N999Vfs*40 KIF5A both display a faster turnover compared to WT KIF5A and accumulate into detergent-insoluble aggregates when the proteasome is blocked. Moreover, the SPG-related R280C KIF5A mutant and N999Vfs*40-KIF5A both compete for degradation with proteasome-specific substrates. Finally, we showed that the NEIMY-related C975Vfs*73 KIF5A mutant is characterised by a similar, but more severe aberrant behaviour compared to N999Vfs*40 KIF5A. Notably, the two mutants share an abnormal tail but cause phenotypes on the opposite ends of KIF5A-linked disease spectrum. Together, our data support the pathogenicity of newly identified KIF5A mutants, highlight previously unknown defects of recurrent variants, and demonstrate that both shared and unique molecular defects underpin KIF5A-linked diseases.