Novel CAPN1 mutations extend the phenotypic heterogeneity in combined spastic paraplegia and ataxia

OBJECTIVE: Recessive mutations in the CAPN1 gene have recently been identified in spastic paraplegia 76 (SPG76), a complex hereditary spastic paraplegia (HSP) that is combined with cerebellar ataxia, resulting in an ataxia‐spasticity disease spectrum. This study aims to assess the influence of CAPN1 variants on the occurrence of SPG76 and identify factors potentially contributing to phenotypic heterogeneity. METHODS: We screened a cohort of 240 unrelated HSP families for variants in CAPN1 using high‐throughput sequencing analysis. We described in detail the clinical and genetic features of the SPG76 patients in our cohort and summarized all reported cases. RESULTS: Six unreported CAPN1‐associated families containing eight patients with or without cerebellar ataxia were found in our cohort of HSP cases. These patients carried three previously reported homozygous truncating mutations (p.V64Gfs(*)103, c.759+1G>A, and p.R285(*)), and three additional novel compound heterozygous missense mutations (p.R481Q, p.P498L, and p.R618W). Lower limbs spasticity, hyperreflexia, and Babinski signs developed in about 94% of patients, with ataxia developing in 63% of cases. In total, 33 pathogenic mutations were distributed along the three reported functional domains of calpain‐1 protein, encoded by CAPN1, with no hotspot region. A comparison of gender distribution between the two groups indicated that female SPG76 patients were significantly more likely to present with complicated HSP than male patients (P = 0.015). INTERPRETATION: Our study supports the clinically heterogeneous inter‐ and intra‐family variability of SPG76 patients, and demonstrates that gender and calpain‐1 linker structure may contribute to clinical heterogeneity in SPG76 cases.