Musashi-2: an essential regulator of DNA recombination and repair (#31)
Mammalian spermatogenesis is a complex developmental process requiring the strict regulation of stage-specific gene expression to produce physically mature spermatozoa from undifferentiated germ cells. Crucial to this process are RNA binding proteins responsible for the regulated post-transcriptional control of essential mRNAs. Msi2 is known to act in haemopoietic stem cells via translational repression of key regulators of cell commitment and differentiation and Msi2 has also been found to be overexpressed in human myeloid leukaemia’s correlating with poor prognosis. In prior studies, we have identified that Msi2 is a targets CDKN1C, a cell cycle regulator, for post transcriptional repression, to facilitate the mitosis to meiosis transition in spermatogenesis.
In addition Msi2 was localised to the nucleus of meiotic spermatocytes and post meiotic spermatids. We produced Msi2 transgenic mice designed to ectopically over-express full-length Msi2 cDNA in testicular germ cells. Fertility trials demonstrated complete sterility in Msi2 transgenic males with wildtype females. Histological examination revealed aberrant spermatogenesis and significant apoptotic mediated depletion of meiotic and post meiotic germ cells. Further functional and morphological analysis of Msi2 transgenic sperm in vitro revealed oligo-astheno-teratrozoospermia and a concomitant failure of sperm oocyte interaction. Genomic damage assays, such as TUNEL and COMET, detected significant single and double DNA strand breaks in isolated meiotic and post meiotic germ cells and epididymal spermatozoa from transgenic males.
Immunoprecipitation studies and proximity ligase assays revealed that the Splicing factor, proline- and glutamine-rich (SFPQ) protein and Paraspeckle Protein Component (PSPC1) are novel interacting factors with Msi2 in meiotic germ cells. SFPQ in conjunction with PSPC1 and Rad51 has an established role in DNA strand break repair and DNA recombination. Additionally, Msi2 interacts with GammaH2.AX and colocalises with Rad51 in leptotene and pachytene spermatocytes.
Phenotypic analysis of Msi2 over-expression transgenic mice thus indicates that Msi2 is essential to spermatogenesis particularly during post mitotic development. Over-expression of Msi2 substantially disrupts meiosis and consequently inhibits normal sperm development and function. We predict that Msi2 regulates the SFPQ/PSPC1/RAD51/GammaH2.AX complex to control the resolution of DNA double strand breaks during meiosis and DNA repair, thus indicating a role for Msi2 in the aetiology of cancer.