Department of Medical Genetics, Capital Institute of Pediatrics, Beijing 100020, China.
Chin Med J (Engl). 2011 Feb;124(3):385-9.
Infantile proximal spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder. Approximately 90% - 95% cases of SMA result from homozygous deletion of survival motor neuron gene 1 (SMN1) and 5% cases are caused by compound heterozygous mutation (a SMN1 deletion on one allele and a subtle mutation on the other allele).
In this research, two unrelated patients were clinically diagnosed according to the criteria of proximal SMA. Genetic diagnosis was performed to detect the homozygous deletion of exon 7 of SMN1 by PCR-restriction fragment length polymorphism (RFLP) and genomic sequencing. Multiplex ligation-dependent probe amplification (MLPA) analysis was carried out to measure copy numbers of SMN1, SMN2 and neuronal apoptosis inhibitor protein (NAIP) in the patients. Further sequencing of SMN1 allele-specific PCR (AS-PCR) and SMN1 clones were also performed to analyze the point mutation of SMN1 gene. Additionally, the pedigree analysis of these two families was carried out to identify the transmission of the mutation.
The inconsistent results using PCR-RFLP and genomic sequencing showed homozygous deletion of exon 7 of SMN1 and heterozygous deletion accompanied with a suspicious mutation in SMN1 gene, respectively. MLPA analysis of these two cases exhibited one SMN1 copy deletion. One identical c.863G > T (p.Arg288Met) mutation was found in two cases by sequencing the SMN1 clones, which confirmed that both cases were SMA compound heterozygotes. One case showed partial conversion to form hybrid SMN (SMN2 I7/SMN1 E8) identified by clones sequencing and another case carrying 3 SMN2 implied complete conversion from SMN1 to SMN2.
p.Arg288Met is more a disease-causing mutation than a polymorphism variation, and children with this mutation may have more severe phenotypes.
婴儿型近端型脊肌萎缩症（SMA）是一种常见的常染色体隐性神经肌肉疾病。约 90%-95%的 SMA 病例是由生存运动神经元基因 1（SMN1）的纯合缺失引起的，5%的病例是由复合杂合突变（一个等位基因上的 SMN1 缺失和另一个等位基因上的微妙突变）引起的。
本研究中，根据近端型 SMA 的标准，对 2 例无亲缘关系的患者进行临床诊断。通过 PCR-限制性片段长度多态性（RFLP）和基因组测序检测 SMN1 外显子 7 的纯合缺失，进行基因诊断。对患者进行多重连接依赖性探针扩增（MLPA）分析，以测量 SMN1、SMN2 和神经元凋亡抑制蛋白（NAIP）的拷贝数。还对 SMN1 等位基因特异性 PCR（AS-PCR）和 SMN1 克隆进行了进一步测序，以分析 SMN1 基因的点突变。此外，对这两个家系进行了系谱分析，以确定突变的传递。
PCR-RFLP 和基因组测序的不一致结果分别显示 SMN1 外显子 7 的纯合缺失和杂合缺失伴有 SMN1 基因的可疑突变。这两例的 MLPA 分析均显示 1 个 SMN1 拷贝缺失。通过对 SMN1 克隆进行测序，发现两例均存在相同的 c.863G>T（p.Arg288Met）突变，证实两例均为 SMA 复合杂合子。1 例通过克隆测序发现部分转化形成杂合 SMN（SMN2 I7/SMN1 E8），另 1 例携带 3 个 SMN2 提示 SMN1 完全转化为 SMN2。
p.Arg288Met 更像是一种致病突变，而不是多态性变异，携带这种突变的儿童可能有更严重的表型。
