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The PIWI-interacting RNA (piRNA) pathway guides the DNA methylation of young, active transposons during germline development in male mice 1 . piRNAs tether the PIWI protein MIWI2 (PIWIL4) to the nascent transposon transcript, resulting in DNA methylation through SPOCD1 (refs. 2-5 ). Transposon methylation requires great precision: every copy needs to be methylated but off-target methylation must be avoided. However, the underlying mechanisms that ensure this precision remain unknown. Here, we show that SPOCD1 interacts directly with SPIN1 (SPINDLIN1), a chromatin reader that primarily binds to H3K4me3-K9me3 (ref. 6 ). The prevailing assumption is that all the molecular events required for piRNA-directed DNA methylation occur after the engagement of MIWI2. We find that SPIN1 expression precedes that of both SPOCD1 and MIWI2. Furthermore, we demonstrate that young LINE1 copies, but not old ones, are marked by H3K4me3, H3K9me3 and SPIN1 before the initiation of piRNA-directed DNA methylation. We generated a Spocd1 separation-of-function allele in the mouse that encodes a SPOCD1 variant that no longer interacts with SPIN1. We found that the interaction between SPOCD1 and SPIN1 is essential for spermatogenesis and piRNA-directed DNA methylation of young LINE1 elements. We propose that piRNA-directed LINE1 DNA methylation requires a developmentally timed two-factor authentication process. The first authentication is the recruitment of SPIN1-SPOCD1 to the young LINE1 promoter, and the second is MIWI2 engagement with the nascent transcript. In summary, independent authentication events underpin the precision of piRNA-directed LINE1 DNA methylation., (© 2024. The Author(s).)

In the male mouse germ line, PIWI-interacting RNAs (piRNAs), bound by the PIWI protein MIWI2 (PIWIL4), guide DNA methylation of young active transposons through SPOCD1. However, the underlying mechanisms of SPOCD1-mediated piRNA-directed transposon methylation and whether this pathway functions to protect the human germ line remain unknown. We identified loss-of-function variants in human SPOCD1 that cause defective transposon silencing and male infertility. Through the analysis of these pathogenic alleles, we discovered that the uncharacterized protein C19ORF84 interacts with SPOCD1. DNMT3C, the DNA methyltransferase responsible for transposon methylation, associates with SPOCD1 and C19ORF84 in fetal gonocytes. Furthermore, C19ORF84 is essential for piRNA-directed DNA methylation and male mouse fertility. Finally, C19ORF84 mediates the in vivo association of SPOCD1 with the de novo methylation machinery. In summary, we have discovered a conserved role for the human piRNA pathway in transposon silencing and C19ORF84, an uncharacterized protein essential for orchestrating piRNA-directed DNA methylation., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)