Comprehensive Analysis of Retrotransposon Insertions within the Survival Motor Neuron Genes Involved in Spinal Muscular Atrophy

Transposable elements (TEs) are interspersed repetitive DNA sequences with the ability to mobilize in the genome. The recent development of improved tools for evaluating TE-derived sequences in genomic studies has enabled an increasing attention to the contribution of TEs to human development and disease. Spinal muscular atrophy (SMA) is an autosomal recessive motor neuron disease that is caused by deletions or mutations in the Survival Motor Neuron 1 (SMN1) gene. SMN2 gene is a nearly perfect duplication of SMN1. Both genes (collectively known as SMN1/SMN2) are highly enriched in TEs. A comprehensive analysis of TEs insertions in the SMN1/2 loci of SMA carriers, patients and healthy/control individuals was completed to perceive TE dynamics in SMN1/2 and try to establish a link between these elements and SMA. We found an Alu insertion in the promoter region and one L1 element in the 3’UTR that likely play an important role as an alternative promoter and as an alternative terminator to the gene, respectively. Additionally, the several Alu repeats inserted in the genes’ introns influence splicing, giving rise to alternative splicing events that cause RNA circularization and the birth of new alternative exons. These Alu repeats present throughout the genes are also prone to recombination events that can lead to SMN1 exons deletions, that ultimately lead to SMA. The many good and bad implications associated with the presence of TEs inside SMN1/2 make this genomic region ideal for understanding the implications of TEs on genomic evolution as well as on human genomic disease.