Background: Spinal muscular atrophy (SMA) is a degenerative neuromuscular disease that causes progressive muscle weakness and atrophy due to the loss of the motor neurons. Approximately 95% of patients with SMA are homozygous for the deletion of SMN1 exon 7. With an incidence of 1/10.000 and a carrier frequency of 1/40 to 1/50, SMA is the most common genetic cause of death in infants. Purpose: To detect homozygous deletion of SMN1 exon 7 and to analyse the SMN1 copy number by molecular genetic analysis. Materials and Methods: In this study, 3 SMA patients with SMN1 gene homozygous deletion and 17 people of their relatives were included. Molecular genetic analysis was performed in the Central Scientific Research Laboratory of the Institute of Medical Sciences. DNA was extracted from peripheral blood, and its purity was assessed by spectrophotometer. Homozygous deletion of SMN1 gene was analyzed with allele-specific PCR, and the SMN1 gene copy number was evaluated by real-time PCR. Results: Among the five participants diagnosed with SMA by clinical symptom and electromyographic test, three cases were found to have homozygous deletion of exon 7 of the SMN1 gene, while two cases did not exhibit such mutation by the allele specific PCR analysis.
The mean age of study participants was 27.76±16.07 (ranging from 8 months to 52 years). Six of the 7 relatives of the first proband had 1 copy number of SMN1 (0.75±0.29) or were carriers of SMA, while one had 3 copy numbers (2.99) or no deletion of SMN1 gene. Additionally, 6 of the 7 individuals of the second proband had 1 copy number of the SMN1 gene (0.72±0.14), and 1 person had 2 copy numbers. All 3 relatives of the third proband had 1 copy number of SMN1 gene (0.96±0.37). Conclusion We consider that determination of SMN1 gene homozygous deletion and carrier testing can be performed by the PCR method locally. Further, it is necessary to implement the molecular genetic testing method into practice and to study the requirements and needs of early detection of SMA in the newborn screening program of Mongolia.