piRNAs, transposon silencing, and germline genome integrity.

Integrity of the germline genome is essential for the production of viable gametes and successful reproduction. In mammals, the generation of gametes involves extensive epigenetic changes (DNA methylation and histone modification) in conjunction with changes in chromosome structure to ensure flawless progression through meiotic recombination and packaging of the genome into mature gametes. Although epigenetic reprogramming is essential for mammalian reproduction, reprogramming also provides a permissive window for exploitation by transposable elements (TEs), autonomously replicating endogenous elements. Expression and propagation of TEs during the reprogramming period can result in insertional mutagenesis that compromises genome integrity leading to reproductive problems and sporadic inherited diseases in offspring. Recent work has identified the germ cell associated PIWI Interacting RNA (piRNA) pathway in conjunction with the DNA methylation and histone modification machinery in silencing TEs. In this review we will highlight these recent advances in piRNA mediated regulation of TEs in the mouse germline, as well as mention the repercussions of failure to properly regulate TEs.