UC Berkeley

Endogenous retroviruses (ERVs) constitute a significant fraction of mammalian genomes, but their impact on host biology remains poorly understood. One group of ERVs, murine ERV with leucine tRNA primer (MERVL), is highly expressed during the 2-cell (2C) stage of mouse preimplantation development but is silenced thereafter. While active, MERVL-derived cis-regulatory elements drive expression of hundreds of host genes, including “chimeric” isoforms with exonized MERVL sequences. Remarkably, loss of a single miRNA miR-34a in pluripotent stem cells is sufficient to derepress MERVL and imbue expanded fate potential reminiscent of totipotent 2C blastomeres. Using bioinformatic prediction and reporter assays, I identified gata2 as the primary target of miR-34a that mediates MERVL derepression in pluripotent stem cells. While miR-34a is required for MERVL silencing in pluripotent stem cells, it is dispensable for repressing MERVL during preimplantation development. To evaluate the role of MERVL in vivo, I applied a candidate approach to assess the role of one MERVL-driven chimeric gene tead4:MT2B1. Loss of tead4:MT2B1 results in compensatory upregulation of canonical tead4 transcripts, suggesting that the chimeric isoform functions redundantly in development. In order to facilitate efficient in vivo screening of additional candidates, I developed a high-throughput electroporation-based genome editing technique called CRISPR RNP Electroporation of Zygotes (CRISR-EZ). Compared to previous methodologies, CRISPR-EZ offers significant advantages in throughput, cost, and simplicity. Altogether, I have elucidated a molecular axis involved in the regulation of MERVL and fate potency, setting the stage for further in vivo characterization using improved genome editing tools.