DataSheet1_A Compound Heterozygous Mutation in Calpain 1 Identifies a New Genetic Cause for Spinal Muscular Atrophy Type 4 (SMA4).PDF

Spinal Muscular Atrophy (SMA) is a heterogeneous group of neuromuscular diseases characterized by degeneration of anterior horn cells of the spinal cord, leading to muscular atrophy and weakness. Although the major cause of SMA is autosomal recessive exon deletions or loss-of-function mutations of survival motor neuron 1 (SMN1) gene, next generation sequencing technologies are increasing the genetic heterogeneity of SMA. SMA type 4 (SMA4) is an adult onset, less severe form of SMA for which genetic and pathogenic causes remain elusive.Whole exome sequencing in a 30-year-old brother and sister with SMA4 identified a compound heterozygous mutation (p. G492R/p. F610C) in calpain-1 (CAPN1). Mutations in CAPN1 have been previously associated with cerebellar ataxia and hereditary spastic paraplegia. Using skin fibroblasts from a patient bearing the p. G492R/p. F610C mutation, we demonstrate reduced levels of CAPN1 protein and protease activity. Functional characterization of the SMA4 fibroblasts revealed no changes in SMN protein levels and subcellular distribution. Additional cellular pathways associated with SMA remain unaffected in the patient fibroblasts, highlighting the tissue specificity of CAPN1 dysfunction in SMA4 pathophysiology. This study provides genetic and functional evidence of CAPN1 as a novel gene for the SMA4 phenotype and expands the phenotype of CAPN1 mutation disorders.

脊髓性肌萎缩症（Spinal Muscular Atrophy）是一组异质性神经肌肉疾病，以脊髓前角细胞变性为特征，进而引发肌肉萎缩与肌无力。尽管SMA的主要致病原因为运动神经元生存蛋白1（survival motor neuron 1, SMN1）基因的常染色体隐性外显子缺失或功能丧失性突变，但新一代测序技术正不断揭示SMA更为丰富的遗传异质性。4型脊髓性肌萎缩症（SMA type 4, SMA4）是一类成年起病、病情相对轻微的SMA亚型，其具体遗传与致病机制至今仍未明确。本研究对一对确诊为SMA4的30岁兄妹实施全外显子测序，在钙蛋白酶1（calpain-1）基因中发现了复合杂合突变（p.G492R/p.F610C）。既往研究显示，CAPN1基因突变曾与小脑共济失调及遗传性痉挛性截瘫相关。研究人员利用携带该复合杂合突变的患者皮肤成纤维细胞开展功能实验，证实CAPN1蛋白表达水平及蛋白酶活性均显著降低。对该SMA4患者成纤维细胞的功能学分析表明，SMN蛋白的表达水平与亚细胞分布均未出现异常；其余与SMA相关的细胞通路在患者成纤维细胞中亦未受影响，这凸显了CAPN1功能异常在SMA4病理生理过程中的组织特异性。本研究为CAPN1作为SMA4表型的新型致病基因提供了遗传学与功能学双重证据，同时拓展了CAPN1突变相关疾病的表型谱。