RNA-directed transposable element silencing in the mammalian germline

In mammals, genome methylation is erased and reset during germ cell development. The PIWI-interacting (piRNA) pathway directs de novo DNA methylation of young, active transposable elements (TEs). This is a highly precise process that safeguards the genomic integrity of the germline. piRNAs tether the PIWI protein MIWI2 to nascent TE transcripts, triggering events that culminate in DNA methylation. The underlying mechanisms have remained mostly unknown because they occur in a tiny population of developing germ cells and the methylation process is not conserved in other model organisms. Our technical advances have overcome these challenges, leading to our recent discovery of the first nuclear effectors of MIWI2 function. We will harness these advances to discover the factors and mechanisms that mediate piRNA- directed TE co-transcriptional silencing, transcriptional silencing, and DNA methylation. Our recent data suggest that specific licencing steps and multi- factor authentication are required for TE methylation. We will define how constituents of the pathway work together to discover the basis of its exacting precision. In summary, our goal is to provide a mechanistic understanding of this previously intractable process that lies at the heart of mammalian germline immortality.