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.

Human cytomegalovirus UL97 kinase prevents the deposition of mutant protein aggregates in cellular models of Huntington's disease and ataxia.

The presence of aggregates of abnormally expanded polyglutamine (polyQ)-containing proteins are a pathological hallmark of a number of neurodegenerative diseases including Huntington's disease (HD) and spinocerebellar ataxia-3 (SCA3). Previous studies in cellular, Drosophila, and mouse models of HD and SCA have shown that neurodegeneration can be prevented by manipulations that inhibit polyQ aggregation. We have shown that the UL97 kinase of the human cytomegalovirus (HCMV) prevents aggregation of the pp71 and pp65 viral tegument proteins. To explore whether UL97 may act as a general antiaggregation factor, we examined whether UL97 prevents aggregation of cellular non-polyQ and polyQ proteins. We report that UL97 prevents the deposition of aggregates of two non-polyQ proteins: a protein chimera (GFP170*) composed of the green fluorescent protein and a fragment of the Golgi Complex protein (GCP-170) and a chimera composed of the red fluorescent protein (RFP) fused to the Werner syndrome protein (WRN), a RecQ helicase and exonuclease involved in DNA repair. Furthermore, we show that UL97 inhibits aggregate deposition in cellular models of HD and SCA3. UL97 prevents the deposition of aggregates of the mutant huntingtin exon 1 containing 82 glutamine repeats (HttExon1-Q82) or full length ataxin-3 containing a 72 polyQ track (AT3-72Q). The kinase activity of UL97 appears critical, as the kinase-dead UL97 mutant (K335M) fails to prevent aggregate formation. We further show that UL97 disrupts nuclear PML bodies and decreases p53-mediated transcription. The universality of the antiaggregation effect of UL97 suggests that UL97 targets a key cellular factor that regulates cellular aggregation mechanisms. Our results identify UL97 as a novel means to modulate polyQ aggregation and suggest that UL97 can serve as a novel tool to probe the cellular mechanisms that contribute to the formation of aggregates in polyglutamine disorders.