TEX15

TEX15 is a testis-enriched nuclear protein that functions at two critical junctures of male germ cell development: meiotic recombination and transposon silencing. During meiotic prophase, TEX15 is required for loading the strand-exchange proteins RAD51 and DMC1 onto DNA double-strand break sites; Tex15-deficient spermatocytes form DSBs normally but fail to recruit these repair factors, leading to chromosomal asynapsis and male-specific meiotic arrest (PMID:18283110). TEX15 also serves as a nuclear effector of the piRNA pathway, associating with MILI and MIWI2 to direct de novo DNA methylation and transcriptional silencing of transposable elements in foetal gonocytes; Tex15 mutants phenocopy Mili/Dnmt3c mutants in the extent of transposon hypomethylation, establishing TEX15 as an essential downstream mediator of piRNA-guided epigenetic silencing (PMID:32719317, PMID:32381626).

1. 2008 High

   Establishing that TEX15 has a specific mechanistic role in meiotic recombination — not in DSB formation itself, but in the downstream step of loading RAD51 and DMC1 onto break sites — explained why loss of this testis protein causes male-specific meiotic arrest with intact DSB induction but defective synapsis.

   Evidence Tex15 knockout mouse with immunofluorescence for RAD51/DMC1 on meiotic spreads and cytological synapsis analysis

   PMID:18283110

   Open questions at the time

   • Biochemical mechanism by which TEX15 promotes RAD51/DMC1 loading is unknown
   • Why females are unaffected despite shared meiotic recombination machinery is unexplained
   • No structural or domain-function analysis of TEX15
2. 2020 High

   Two independent studies repositioned TEX15 from a meiotic recombination factor to a dual-function nuclear effector of the piRNA/DNA methylation axis: one study showed TEX15 associates with MIWI2 in foetal gonocytes and is required for piRNA-directed TE methylation without affecting piRNA biogenesis; the other showed TEX15 associates with MILI and phenocopies Mili/Dnmt3c mutants in TE hypomethylation, placing TEX15 at the convergence of both piRNA branches for de novo DNA methylation of transposons.

   Evidence Reciprocal co-immunoprecipitation of TEX15 with MIWI2 and separately with MILI; piRNA sequencing in Tex15 mutants; bisulfite sequencing for TE methylation; genetic epistasis comparing Tex15, Mili, Miwi2, and Dnmt3c mutant methylation profiles

   PMID:32381626 PMID:32719317

   Open questions at the time

   • Whether TEX15 directly recruits DNMT3C or another de novo methyltransferase to target loci is unknown
   • The two studies disagree on whether TEX15 primarily functions via MILI or MIWI2, and the relative contribution of each branch remains unresolved
   • How TEX15 coordinates its meiotic recombination role with its piRNA/methylation role during the different developmental stages is unclear
3. 2025 Medium

   Extending TEX15 function beyond the germline, transient Tex15 expression in vomeronasal sensory neuron precursors was shown to control receptor repertoire diversity; loss of Tex15 distorted V1R/V2R receptor choice, reduced accessory olfactory bulb activation, and abolished stereotyped intermale aggression, suggesting a role in epigenetic regulation of receptor gene selection.

   Evidence Tex15 knockout mouse with VSN receptor RNA profiling, c-Fos mapping of accessory olfactory bulb, and behavioral aggression assays (preprint)

   PMID:41415471

   Open questions at the time

   • Preprint not yet peer-reviewed; independent replication is needed
   • Whether the VSN phenotype reflects the same DNA methylation mechanism as in the germline is untested
   • Direct molecular targets of TEX15 in VSN precursors are unidentified

• The biochemical mechanism by which TEX15 bridges piRNA-loaded Piwi proteins to the de novo DNA methylation machinery, and separately promotes RAD51/DMC1 loading, remains unknown; no enzymatic activity, domain architecture, or structural information has been reported.
• No crystal or cryo-EM structure of TEX15 or any of its domains
• No identified enzymatic or catalytic activity
• Mechanism coupling piRNA target recognition to DNMT recruitment is entirely open