The autonomous retrotransposon Long Interspersed Element 1 (LINE-1 or L1) comprises approximately 17 and 18% of the human and mouse genomes respectively. Utilising the ‘copy and paste’ mechanism of retrotransposition, a subset of L1 elements have retained the ability to insert new copies of themselves throughout the genome. Doing so results in mutation at insertion sites that can be deleterious to genomic function and stability, sometimes contributing to isolated cases of disease in humans. To successfully pass their genetic information onto the next generation and beyond, retrotransposition competent L1s must generate new insertions in pluripotent embryonic cells prior to germ line specification, or in cells of the germ lineage. DNA methylation of the L1 5’UTR internal promoter is a major determinant of L1 expression. In the mouse germ line, L1 DNA methylation dynamics differ dramatically between males and females. It therefore stands to reason that the developmental timing of heritable L1 retrotransposition events likewise differs between the sexes.
In this project, we are undertaking a systematic characterisation of sex-specific L1 expression dynamics during male and female germ cell development in the mouse. To characterise L1 mRNA expression with cell type specificity and subcellular resolution, we are utilising RNAscope single-molecule RNA fluorescence in situ hybridisation with a probe targeting the L1 TF 5’UTR (Bodea et al. 2022) alongside probes for germ cell specific genes. This method will produce a highly detailed picture of sex-specific LINE-1 expression with spatial and temporal resolution. Our results will provide insight into L1 activity in the germline and inform our understanding of sex-specific L1 mutagenesis.