Tudor domain containing 12 (TDRD12) is essential for secondary piRNA biogenesis in mice

Piwi-interacting RNAs (piRNAs) are gonad-specific small RNAs that provide defence against transposable genetic elements called transposons. Our knowledge of piRNA biogenesis is sketchy, partly due to an incomplete inventory of the factors involved. Here, we identify Tudor domain-containing 12 (TDRD12; also known as ECAT8) as a novel piRNA biogenesis factor in mice. TDRD12 is detected in complexes containing MILI (PIWIL2), its associated primary piRNAs, and TDRD1, all of which are already implicated in secondary piRNA biogenesis. Male mice carrying either a nonsense point mutation (repro23 mice) or a targeted deletion in the Tdrd12 locus are infertile, and de-repress retrotransposons. We find that TDRD12 is dispensable for primary piRNA biogenesis but essential for production of secondary piRNAs that enter MIWI2 (PIWIL4). Cell culture studies with the insect orthologue of TDRD12 suggest a role for the multi-domain protein in mediating complex formation with other participants during secondary piRNA biogenesis. Total small RNA and immunoprecipitated small RNA were purified from mouse testis extract and Bmn4 cells for preparation of high-throughput sequencing libraries.