Background

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder caused by homozygous deletion or intragenic mutation of the SMN1 gene. It is well-known that high copy number of its homologous gene, SMN2, modifies the phenotype of SMN1-deleted patients. However, in the patients with intragenic SMN1 mutation, the relationship between phenotype and SMN2 copy number remains unclear.

Methods

We have analyzed a total of 515 Japanese patients with SMA-like symptoms (delayed developmental milestones, respiratory failures, muscle weakness etc.) from 1996 to 2019. SMN1 and SMN2 copy numbers were determined by quantitative polymerase chain reaction (PCR) method and/or multiplex ligation-dependent probe amplification (MLPA) method. Intragenic SMN1 mutations were identified through DNA and RNA analysis of the fresh blood samples.

Results

A total of 241 patients were diagnosed as having SMA. The majority of SMA patients showed complete loss of SMN1 (n = 228, 95%), but some patients retained SMN1 and carried an intragenic mutation in the retaining SMN1 (n = 13, 5%). Ten different mutations were identified in these 13 patients, consisting of missense, nonsense, frameshift and splicing defect-causing mutations. The ten mutations were c.275G > C (p.Trp92Ser), c.819_820insT (p.Thr274Tyrfs*32), c.830A > G (p.Tyr277Cys), c.5C > T (p.Ala2Val), c.826 T > C (p.Tyr276His), c.79C > T (p.Gln27*), c.188C > A (p.Ser63*), c.422 T > C (p.Leu141Pro), c.835-2A > G (exon 7 skipping) and c.835-3C > A (exon 7 skipping). It should be noted here that some patients with milder phenotype carried only a single SMN2 copy (n = 3), while other patients with severe phenotype carried 3 SMN2 copies (n = 4).

Conclusion

Intragenic mutations in SMN1 may contribute more significantly to clinical severity than SMN2 copy numbers.

中文翻译：

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保留 SMN1 基因内突变的 SMA 患者的表型

背景

脊髓性肌萎缩症（SMA）是一种常染色体隐性遗传的神经肌肉疾病，由SMN1基因纯合缺失或基因内突变引起。众所周知，其同源基因SMN2的高拷贝数会改变SMN1缺失患者的表型。然而，在基因内SMN1突变的患者中，表型与SMN2拷贝数之间的关系仍不清楚。

方法

我们分析了 1996 年至 2019 年间共有 515 名具有 SMA 样症状（发育里程碑延迟、呼吸衰竭、肌肉无力等）的日本患者。SMN1和SMN2拷贝数通过定量聚合酶链反应 (PCR) 方法和/或多重连接依赖性探针扩增（MLPA）方法。通过新鲜血液样本的 DNA 和 RNA 分析鉴定了基因内 SMN1突变。

结果

共有 241 名患者被诊断为患有 SMA。大多数 SMA 患者表现出SMN1完全缺失（n = 228, 95%），但一些患者保留了SMN1并在保留的 SMN1 中携带基因内突变（ n = 13, 5%）。在这 13 名患者中鉴定出 10 种不同的突变，包括错义、无义、移码和导致剪接缺陷的突变。十个突变是 c.275G > C (p.Trp92Ser), c.819_820insT (p.Thr274Tyrfs*32), c.830A > G (p.Tyr277Cys), c.5C > T (p.Ala2Val), c. 826 T > C (p.Tyr276His), c.79C > T (p.Gln27*), c.188C > A (p.Ser63*), c.422 T > C (p.Leu141Pro), c.835- 2A > G（外显子 7 跳跃）和 c.835-3C > A（外显子 7 跳跃）。SMN2拷贝（n = 3），而其他具有严重表型的患者携带 3 个SMN2拷贝（n = 4）。

结论

SMN1的基因内突变可能比SMN2拷贝数对临床严重程度的贡献更大。