Point mutation analysis of SMN1 gene in patients with spinal muscular atrophy

Objective To identify the point mutations in survival motor neuron gene 1 (SMN1) gene and confirm the existence of compound heterozygous mutations in Chinese patients with spinal muscular atrophy (SMA). Methods Three unrelated patients were diagnosed and clinically typed according to the criteria of proximal SMA established by the International SMA Consortium. Multiplex ligation-dependent probe amplification (MLPA) analysis was carried out to measure the copy numbers of SMN1, SMN2 and neuronal apoptosis inhibitory protein gene (NAIP) in the patients. The point mutation analysis of SMN1 gene was performed by reversed transcript-polymerase chain reaction (RT-PCR) and cloning sequencing. The MLPA assay and point mutation analysis were also performed in the family members to confirm the transmission of the mutations. Results Two point mutations were identified in the present study, i.e., the p. Leu228X in one patient and p. Arg288Met in two patients. The mutation p. Arg288Met was first reported in Chinese and p. Leu228X was first reported in Mainland Chinese. The case carrying p. Leu228X mutation was diagnosed as SMA Ⅰ with 2 copies of SMN2, and the cases with p. Arg288Met were diagnosed as SMA Ⅰ and SMA Ⅱ, respectively, with 3 copies of SMN2 gene. Conclusion The mutations p. Leu228X and p.Arg288Met caused severe clinical phenotypes, SMA Ⅰ or SMA Ⅱ. This study suggested that the compound heterozygous mutations of SMN1 existed in Chinese SMA patients, which was rarely reported previously in Chinese. It was necessary to detect the point mutation in SMN1 for genetic diagnosis of those patients with heterozygous deletion of SMN1, which would be beneficial to prenatal diagnosis and genetic counseling in these families. Key words: spinal muscular atrophy;  survival motor neuron gene 1;  point mutation;  copy numbers