Genetic Modifiers of the Spinal Muscular Atrophy Phenotype

Abstract

Proximal spinal muscular atrophy (SMA) is an autosomal recessive neurodegenerative disease caused by homozygous deletion in the seventh exon of the SMN1 gene. The aim of this work is to analyze the association of the allelic polymorphism of telomeric genes SMN1 and NAIP and the centromeric gene SMN2 of the 5q13 region with the clinical phenotype of SMA. It was shown that the homozygous genotype, which contains a telomeric deletion, covering both SMN1 and NAIP, is significantly more often observed in patients with the most severe type of SMA. Three or more copies of SMN2 are associated with a milder phenotype; the number of SMN2 copies affects the SMA phenotype more heavily than the length of the telomeric deletion. It was shown that one SMN2 copy is significantly more frequent than three or more copies of this gene in SMA-patients with homozygous deletion of SMN1 and NAIP. This fact may indicate the presence of a large deletion of all the three studied genes in SMA genotypes associated with the most severe type of SMA. It is noted that congenital SMA (type 0) is significantly less common in female patients, which may indicate the presence of SMA modifier genes on the X-chromosome.