Novel family of human transposable elements formed due to fusion of the first exon of gene MAST2 with retrotransposon SVA.

We identified a novel human-specific family of transposable elements that consists of fused copies of the CpG-island containing the first exon of gene MAST2 and retrotransposon SVA. We propose a mechanism for the formation of this family termed CpG-SVA, comprising 5'-transduction by an SVA insert. After the divergence of human and chimpanzee ancestor lineages, retrotransposon SVA has inserted into the first intron of gene MAST2 in the sense orientation. Due to splicing of an aberrant RNA driven by MAST2 promoter, but terminally processed using SVA polyadenylation signal, the first exon of MAST2 has fused to a spliced 3'-terminal fragment of SVA retrotransposon. The above ancestor CpG-SVA element due to retrotranspositions of its own copies has formed a novel family represented in the human genome by 76 members. Recruitment of a MAST2 CpG island was most likely beneficial to the hybrid retrotransposons because it could significantly increase retrotransposition frequency. Also, we show that human L1 reverse transcriptase adds an extra cytosine residue to the 3' terminus of the nascent first strand of cDNA.