Homozygous deletion of exon 7 in SMN1 gene without phenotypic features of spinal muscular atrophy

Spinal muscular atrophy (SMA)(OMIM#:253300) is an autosomal recessive disorder, resulting in symmetrical progressive weakness of skeletal and respiratory muscles and atrophy. The corresponding gene for the disease is the survival motor neuron 1 (SMN1) and SMN2 genes. Homozygous deletion of SMN1 exons is the most common underlying cause of the disease, and SMN2 copy numbers modify the disease phenotype. However, homozygous deletion of exon 7 of SMN1 in a completely asymptomatic individual is an extremely rare finding. The present report discusses a case of homozygote deletion of exon 7 of SMN1 in a healthy female. A healthy couple with a family history of affected family members with SMA was referred for genetic counseling. Genomic DNA was extracted from the peripheral blood of the couple and the copy number of exon 7 of the SMN1 gene was assessed for using real-time polymerase chain reaction (PCR) and PCR-Restriction fragment length polymorphism (RFLP). Assessment of SMN1-related ct in the female compared with control samples showed that the female had a homozygous deletion in the SMN1 gene. PCR-RFLP and gel electrophoresis results also confirmed the homozygous deletion of exon 7 in the female SMN1 gene.
Conclusion: According to the results of this study and also other findings in previous studies, the lack of symptoms in the female with biallelic deletion of SMN1 may be related to the presence of SMN2 copies or other modifier genes.