Missense mutation of SPAST protein (I344K) results in loss of ATPase activity and prolonged the half-life, implicated in autosomal dominant hereditary spastic paraplegia

The spastin protein (SPAST) contains an ATPase with diverse cellular activities (AAA) domain and regulates microtubule dynamics. Missense mutations of the SPAST gene are frequently detected in patients with hereditary spastic paraplegias (HSPs) and represent the main reason of loss of SPAST function; however, the pathogenicity of mutant SPAST is heterogeneous. Here, SPAST variant with an I344K mutation (I344K-SPAST) was identified in a Korean family with autosomal dominant-type HSP. We investigated the role of the I344K-SPAST in HSP to provide a therapeutic mechanism. The I344K-SPAST mutation prolonged the half-life of the protein compared to wild-type SPAST (WT-SPAST) in cells by modulating post-translational modifications for proteasomal degradation. I344K-SPAST was localized in microtubule but defective in microtubule severing and ATPase activity compared to WT-SPAST in vitro and in cells. Mutant M87 isoform harboring the same mutation with I344K-M1 SPAST also increased protein stability and loss of MT severing activity, but the pathogenicity was not stronger than I344K-M1 SPAST in neurite outgrowth. Overexpression of I344K-SPAST resulted in microtubule accumulation following inhibited neurite growth in neuroblastoma, neural progenitor cells and mouse primary cortical neurons. Conversely, these pathogenic effects of I344K-SPAST were reduced by overexpression of WT-M1 SPAST in a dose dependent manner since WT-SPAST could interact with I344K-SPAST. Our data therefore provide proof-of-concept that gene transfer of WT-M1 SPAST may serve as a valid therapeutic option for HSPs.