Two-factor authentication represents a modern approach to securing our digital lives from outside threats; however, research now suggests that this concept may not be so modern. A new study now reveals that male germ cells employ a piRNA-mediated epigenetic “two-factor authentication” strategy to secure the precisely timed DNA methylation-induced silencing of insider threats that impact male fertility – transposons!
Epigenetic security experts headed by Dónal O’Carroll (University of Edinburgh) recognized that the piRNA (PIWI-interacting RNA) pathway ensured transposon methylation and silencing to support germline development in male mice; however, the mechanisms controlling the precision and timing of this epigenetic silencing strategy remained unknown.
Let’s hear more from Mirandela and colleagues on how two-factor authentication secures transposon DNA methylation:
- ChIP-seq reveals that H3K4me3 and H3K9me3, as well as the H3K4me3-K9me3 chromatin reader protein SPIN1, mark transposons before piRNA-directed DNA methylation arises in male mouse fetal gonocytes (spermatogonia precursors)
- The transcriptional activity of active transposons induces elevated H3K4me3 levels, although the mechanisms controlling H3K9me3 deposition remain undetermined
- SPIN1 expression occurs before the appearance of SPOCD1 (connects piRNA and de novo methylation machinery) and MIWI2 (tethered by piRNAs to nascent transposon transcripts, resulting in SPOCD1-mediated DNA methylation)
- These transcriptomic patterns suggest a developmentally timed functional hierarchy
- Further assays reveal that SPOCD1 interacts directly with SPIN1, with this complex then binding cis-H3K4me3-K9me3 at high affinity to specifically target SPOCD1 to transposons to support piRNA-directed DNA methylation
- SPIN1 recruitment to transposons and the SPOCD1–SPIN1 interaction takes place before the nuclear localization of MIWI2 and the process of de novo DNA methylation
- The SPOCD1–SPIN1 association displays conservation from amphibians to mammals
- A Spocd1 separation-of-function allele encoding a SPOCD1 variant that doesn’t interact with SPIN1 reveals the essential nature of a SPIN1-SPOCD1 interaction for spermatogenesis/piRNA-directed DNA methylation of transposons
Overall, these findings demonstrate the existence of an epigenetic two-factor authentication strategy that ensures secure, on-target, and precisely timed DNA methylation, which silences potentially damaging transposon activity and safeguards germ cell development and male fertility.
The first authentication step occurs via SPIN1-SPOCD1 recruitment to active transposons through the recognition of a specific histone modification pattern; subsequently, the second authentication step involves the piRNA-guided MIWI2 engagement with nascent transcripts, which supports SPOCD1-mediated DNA methylation and transposon silencing.
For more on how epigenetic two-factor authentication secures against insider threats, safely make your way to Nature, September 2024.