Prevalence of exon 7/exon 8 deletion in patients with hypotonia and spinal muscular atrophy

SMN1 exon 7/exon 8 deletion in spinal muscular atrophy

Abstract

Spinal Muscular Atrophy (SMA) is a neuromuscular disease due to degeneration of the anterior horn cells of the spinal cord. The estimated incidence of SMA is 1:6,000-1:10,000. The complete deletion of exon 7 of the SMN1 gene is the hallmark of 95-98% of SMA patients in most population. The first line of investigation for a child or young adult patient suspected to have SMA should be Multiplex ligation-dependent probe amplification (MLPA) testing for homozygous deletion of exons 7 and exon 8 in the SMN1 gene. In this paper, we report the results of SMN 1 exon 7 deletion tests in children who attended the Genetic clinic of a tertiary care hospital in Kerala with one or more of the symptoms especially floppy infants, hypotonia, muscle weakness, tongue fasiculations etc. SMN1 exon 7 and exon 8 deletion was confirmed in 58% cases (19) of the total 33 hypotonia patients. SMA Type I, Type II and Type III were 68.4% (13), 21% (4) and 10.5% (2) respectively among the SMA positive cases. Carrier testing of the non-consanguineous parents showed that all parents were heterozygous carriers. Until 2016, the treatment for this disease was supportive only. Recently Nusinersen, Zolgensma and Risdiplam have become available for SMA patients. The carrier testing in parents with previous SMA child history is essential for the implementation of prenatal diagnosis of this disorder in future pregnancies. The paper emphasizes the importance of this rare neuromuscular disease.