Highlights d Single-cell genomic analysis of hippocampal neurons reveals a somatic L1 insertion d The donor L1 is slightly 5ʹ truncated and lacks a conserved YY1 binding site d Young L1s with truncated or mutated YY1 binding sites are globally hypomethylated d L1 is able to mobilize in the brain because of locus-specific exceptions to repression

Long INterspersed Element class 1 (LINE‐1) elements are a type of abundant retrotransposons active in mammalian genomes. An average human genome contains ~100 retrotransposition‐competent LINE‐1s, whose activity is influenced by the combined action of cellular repressors and activators. TREX1, SAMHD1 and ADAR1 are known LINE‐1 repressors and when mutated cause the autoinflammatory disorder Aicardi‐Goutières syndrome (AGS). Mutations in RNase H2 are the most common cause of AGS, and its activity was proposed to similarly control LINE‐1 retrotransposition. It has therefore been suggested that increased LINE‐1 activity may be the cause of aberrant innate immune activation in AGS. Here, we establish that, contrary to expectations, RNase H2 is required for efficient LINE‐1 retrotransposition. As RNase H1 overexpression partially rescues the defect in RNase H2 null cells, we propose a model in which RNase H2 degrades the LINE‐1 RNA after reverse transcription, allowing retrotransposition to be completed. This also explains how LINE‐1 elements can retrotranspose efficiently without their own RNase H activity. Our findings appear to be at odds with LINE‐1‐derived nucleic acids driving autoinflammation in AGS.

LINE-1 (L1) retrotransposons are a source of insertional mutagenesis in tumor cells. However, the clinical significance of L1 mobilization during tumorigenesis remains unclear. Here, we applied retrotransposon capture sequencing (RC-seq) to multiple single-cell clones isolated from five ovarian cancer cell lines and HeLa cells and detected endogenous L1 retrotransposition in vitro. We then applied RC-seq to ovarian tumor and matched blood samples from 19 patients and identified 88 tumor-specific L1 insertions. In one tumor, an intronic de novo L1 insertion supplied a novel cis-enhancer to the putative chemoresistance gene STC1. Notably, the tumor subclone carrying the STC1 L1 mutation increased in prevalence after chemotherapy, further increasing STC1 expression. We also identified hypomethylated donor L1s responsible for new L1 insertions in tumors and cultivated cancer cells. These congruent in vitro and in vivo results highlight L1 insertional mutagenesis as a common component of ovarian tumorigenesis and cancer genome heterogeneity.

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Transposable elements (TEs) are a reservoir of new transcription factor binding sites for protein-coding genes. Developmental programs that activate TE-derived regulatory elements could, in principle, manifest in lineage-specific TE mobility. While somatic LINE-1 (L1) retrotransposon insertions have been detected in human neurons, the impact of L1 insertions on neurodevelopmental gene regulation, and whether L1 mobility is restricted to certain neuronal lineages, is unknown. Here, we reveal programmed L1 activation by SOX6, a transcription factor critical for parvalbumin (PV+) interneuron development. PV+ neurons harbor unmethylated and euchromatic L1 promoters, express L1 mRNA, and permit L1 transgene mobilization in vivo. Elevated L1 expression in adult dentate gyrus PV+ neurons is however attenuated by environmental enrichment. Nanopore sequencing of PV+ neurons identifies unmethylated L1 loci providing alternative promoters to core PV+ neuron genes, such as CAPS2. These data depict SOX6-mediated L1 activation as an ingrained component of the mammalian PV+ neuron developmental program.