[Mutation analysis of SMN1 gene in patients with spinal muscular atrophy]

Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disorder. About 80% - 90% of SMA patients are missing both copies of SMN1, and 5% - 10% of patients are compound heterozygotes. In the present study, we aimed to analyze survival motor neuron 1 (SMN1) gene mutation in three patients with spinal muscular atrophy and their families to explore the effect of mutation on SMN protein function and the relationship between mutation and clinical phenotype.

According to the international criterion, all patients were diagnosed by a neurologist. Patient 1 is a 5 years old boy with SMA type II. Patient 2, female, 2.5 years old, was SMA type II. Patient 3, female, 9 years old, was SMA type III. The brother of patient 3 was SMA type II, too. The age at last examination was 14 years. Genomic DNA was extracted from peripheral blood leukocytes by using standard phenol/chloroform method and total RNA was extracted from whole blood with QIAamp RNA Blood Mini Kit. PCR/RFLP was used to detect the homozygosis deletion of the SMN1 exon 7, and multiplex ligation-dependent probe amplification (MLPA) were performed to analyze the gene dosage of SMN1 and SMN2 for each patient and his/her family members; reverse transcriptase (RT)-PCR and clone sequencing were conducted for identifying the point mutation of SMN1 in three patients. The sequencing of genomic DNA and MLPA were carried out in the 3 families members to confirm the transition of mutation.

No homozygous deletion of the SMN1 exon 7 was observed in any member of the 3 families. Case 1 and case 2 had one SMN1 copy compound with c.400G > A (p.Glu134Lys) mutation on it and SMN2 was two copies, respectively. Case 3 and her brother also had one copy of SMN1 and two copies of SMN2, and a mutation c.689C > T (p.Ser230Leu) occurred on the retained SMN1. All point mutations were from their fathers and deletion come from their mothers for SMN1 gene.

In this work, p.Glu134Lys and p.Ser230Leu mutations were identified in three unrelated families and p.Glu134Lys from two patients was first discovered in Chinese SMA. The p.Glu134Lys mutation within the SMN Tudor domain prevents the binding of SMN and Sm. The fact that p.Ser230Leu results in a polar amino acid substituted for non-polar amino acid possibly affects the structure of SMN and then damages its function. SMN1 point mutation analysis is not only advantageous to the diagnosis of those patients with heterozygous deletion of SMN1, but will be beneficial to the prenatal diagnosis and genetic counseling for their families.