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

Abstract

Testis‐expressed X‐linked genes typically evolve rapidly. Here, we report on a testis‐expressed X‐linked micro RNA (mi RNA) 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 mi RNAs encoded by this cluster ( Fx‐mir ) have a predilection for targeting the immediately adjacent gene, Fmr1, an unexpected finding given that mi RNAs usually act in trans, not in cis. Robust repression of Fmr1 is conferred by combinations of Fx‐mir mi RNAs 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 mi RNAs are induced, and that FMRP loss causes SC hyperproliferation and spermatogenic defects. Fx‐mir mi RNAs 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.