Somatic retrotransposition alters the genetic landscape of the human brain

Reshaping the brain's genetic circuits Mobilization of retrotransposons, genetic elements able to move around in the genome where they can become incorporated and start to amplify themselves, is normally suppressed in somatic cells. However, recent reports indicate that L1 retrotransposons can be mobilized in the human brain; this has important consequences for intercellular variation. Using a high-throughput approach, Baillie et al . identify numerous germ-line mutations and putative somatic insertions in the human hippocampus and caudate nucleus, including those of Alu elements. The implication is that retrotransposition-driven somatic mosaicism may reshape the genetic circuitry that underpins normal and abnormal neurobiological processes. Retrotransposons are mobile genetic elements that use a germline ‘copy-and-paste’ mechanism to spread throughout metazoan genomes 1 . At least 50 per cent of the human genome is derived from retrotransposons, with three active families (L1, Alu and SVA) associated with insertional mutagenesis and disease 2 , 3 . Epigenetic and post-transcriptional suppression block retrotransposition in somatic cells 4 , 5 , excluding early embryo development and some malignancies 6 , 7 . Recent reports of L1 expression 8 , 9 and copy number variation 10 , 11 in the human brain suggest that L1 mobilization may also occur during later development. However, the corresponding integration sites have not been mapped. Here we apply a high-throughput method to identify numerous L1, Alu and SVA germline mutations, as well as 7,743 putative somatic L1 insertions, in the hippocampus and caudate nucleus of three individuals. Surprisingly, we also found 13,692 somatic Alu insertions and 1,350 SVA insertions. Our results demonstrate that retrotransposons mobilize to protein-coding genes differentially expressed and active in the brain. Thus, somatic genome mosaicism driven by retrotransposition may reshape the genetic circuitry that underpins normal and abnormal neurobiological processes.