In vivo analysis of L1 retrotransposition in Huntington’s disease mouse brain

Transposable elements (TEs) are mobile genetic elements that constitute a large fraction of eukaryotic genomes. TEs co-evolved with their host genomes, providing powerful tools of genome plasticity and regulation. Long Interspersed Nuclear Elements 1 (L1) are the most numerous TEs in mouse and human genomes. They mobilize via a “copy and paste” mechanism that requires an RNA intermediate. Although most L1s have lost their activity during evolution, the remaining subset continues to move both in the germline and in adult somatic tissues. Mounting evidence suggests that L1s are active in somatic cells of the mammalian brain and that dysregulated activation of L1s is associated with neuropathology, such as schizophrenia, Rett syndrome and Ataxia telengectasia. Huntington’s disease (HD) is an autosomal dominant disorder that manifests in mid-life and is characterized by neuronal loss, prominently in striatum and deep layers of the cerebral cortex. Typical HD-associated phenotypes include somatic genomic instability, epigenetic and transcriptional dysregulations, impaired neurogenesis and altered DNA damage response. At the same time, in HD, the origin and the role of many genetic and epigenetic modifiers acting on disease onset and progression remain largely unknown. In this scenario, I investigated whether L1 retrotransposition might be altered in HD and if it could have a role in HD pathogenesis. To address this question, using a novel Taqman qPCR technique, I characterized endogenous L1 retrotransposition events in the brains of a precise genetic mouse model of HD, considering both pre-symptomatic and symptomatic developmental stages. From this study, I showed that similar levels of L1 genomic copies are present between HD and control brains. Moreover, differences in full length L1 transcript levels have been reported in HD brains. Interestingly, in HD striatum, at 12 months of age, expression of full length L1s was consistently impaired, whereas in the cortex, L1 mRNA levels were increased in HD mice at 3 months and 24 months of age. The dysregulation of L1 expression in the striatum of 12 months old mice did not appear to be linked to differential deposition of H3K4me3, H3K9me3, H3K27me3 and MeCP2 on L1 promoter in HD conditions. Nonetheless, L1 transcriptional alterations might involve a piRNA-mediated regulation. Indeed, in both cortex and striatum of adult HD and control mice I detected appreciable levels of MILI protein, the crucial factor of piRNA biogenesis, suggesting its role not only in the germline but also in adult mammalian brains, as recently proposed by other two independent works. Additionally, I showed that in a subset of neurons of the adult mouse pre-frontal cortex, endogenous L1 transcription is accompanied by expression of L1-encoded ORF2 protein. Finally, by characterizing the transcription of active murine full length L1 elements in a broad range of developmental stages (from E10 up to 24 months), I described the expression profiles of endogenous L1 elements during the entire mouse development. From this study, I showed that a wave of L1 transcription takes place between E12 and P0 in both striatum and cerebral cortex and it is concomitant with telencephalic neurogenesis. In post-natal stages, L1 transcription is maintained at basal levels.