Survival motor neuron gene deletions among Iranian patients with spinal muscular atrophy

Spinal muscular atrophy (SMA) is a common neuromuscular disorder with progressive paralysis caused by the loss of α–motor neuron in the spinal cord. SMN is encoded by two genes, SMN1 and SMN2, which essentially differ by a single nucleotide in exon 7. The most frequent mutation is biallelic deletion of exon 7 of the SMN1 gene. A small percentage of SMA patients present compound heterozygosity with a point mutation on one allele and deletion on the other. In the remaining cases, the disease is unlikely to be related to SMN1 defects. In spinal muscular atrophy (SMA), SMN2 is not able to compensate for the loss of SMN1 due to exclusion of exon 7. The aim of our study was to estimate the frequency of the common exon 7 SMN1 deletion in the families who referred to our center for carrier detection and prenatal diagnosis. Between March 1999 and March 2006, one hundred sixty seven families with history of at least one affected member were referred to us. We performed detection of deletion exon 7 SMN1 for the patients and carrier detection for their parents, prenatal diagnosis in subsequent pregnancies to couples who previously had an affected child became possible (63 prenatal diagnosis). From 167 families, 139 categorized in type I of the disease, 21 in type II, and 7 in type III. Carrier detection for the parents indicated that in 96 families with history of affected member with type I SMA both parents carried the deletion in exon 7 and in 20 families, one of the parents was carrier. These rates were 16 to 1 for SMA type II, and 3 to 2 for type III SMA. Sixty-four children affected with SMA were studied, 58 of them were found to be homozygous for the loss of exon 7 of the SMN1 gene, except two patients who were heterozygote for exon 7 deletion (frequency of homozygocity: 90.7%). Eleven of sixty-three (17.5%) fetal samples were found to be affected and these pregnancies were terminated. The molecular analysis of the biallelic exon 7 of the SMN1 deletion is a standard and reliable test in cases of SMA.