A microRNA cluster in the Fragile‐X region expressed during spermatogenesis targets FMR1

Testis‐expressed X‐linked genes typically evolve rapidly. Here, we report on a testis‐expressed X‐linked microRNA (miRNA) cluster that despite rapid alterations in sequence has retained its position in the Fragile‐X region of the X chromosome in placental mammals. Surprisingly, the miRNAs encoded by this cluster ( Fx‐mir ) have a predilection for targeting the immediately adjacent gene, Fmr1 , an unexpected finding given that miRNAs usually act in trans , not in cis . Robust repression of Fmr1 is conferred by combinations of Fx‐mir miRNAs induced in Sertoli cells (SCs) during postnatal development when they terminate proliferation. Physiological significance is suggested by the finding that FMRP, the protein product of Fmr1 , is downregulated when Fx‐mir miRNAs are induced, and that FMRP loss causes SC hyperproliferation and spermatogenic defects. Fx‐mir miRNAs not only regulate the expression of FMRP, but also regulate the expression of eIF4E and CYFIP1, which together with FMRP form a translational regulatory complex. Our results support a model in which Fx‐mir family members act cooperatively to regulate the translation of batteries of mRNAs in a developmentally regulated manner in SCs. Synopsis The rapidly evolving X‐linked microRNA cluster Fx‐mir is strongly induced during Sertoli cell development. Fx‐mir miRNAs target components of a translational regulatory complex, including the Fragile‐X protein FMRP, the initiation factor eIF4E and the repressor CYFIP1. A large X‐linked miRNA cluster ( Fx‐mir ) is functionally characterized. Members of Fx‐mir have a predilection for targeting the immediately adjacent gene Fmr1 . Fx‐mir miRNAs are postnatally induced in Sertoli cells and act additively to repress Fmr1 . The Fx‐mir cluster is rapidly evolving in sequence but its ability to target Fmr1 is conserved. Graphical Abstract The rapidly evolving X‐linked microRNA cluster Fx‐mir is strongly induced during Sertoli cell development. Fx‐mir miRNAs target components of a translational regulatory complex, including the Fragile‐X protein FMRP, the initiation factor eIF4E and the repressor CYFIP1.