The proper interplay between the expression of Spo11 splice isoforms and the structure of the pseudoautosomal region promotes XY chromosomes recombination

Abstract

XY chromosome missegregation is relatively common in humans and can lead to sterility or the generation of aneuploid spermatozoa. A leading cause of XY missegregation in mammals is the lack of formation of double-strand breaks (DSBs) in the pseudoautosomal region (PAR), a defect that may occur in mice due to faulty expression of Spo11 splice isoforms. Using a knock-in (ki) mouse that expresses only the single Spo11β splice isoform, here we demonstrate that by varying the genetic background of mice, the length of chromatin loops extending from the PAR axis and the XY recombination proficiency varies. In spermatocytes of C57Spo11βki/− mice, in which loops are relatively short, recombination/synapsis between XY is fairly normal. In contrast, in cells of C57/129Spo11βki/− males where PAR loops are relatively long, formation of DSBs in the PAR (more frequently the Y-PAR) and XY synapsis fails at a high rate, and mice produce sperm with sex-chromosomal aneuploidy. However, if the entire set of Spo11 splicing isoforms is expressed by a wild type allele in the C57/129 background, XY recombination and synapsis is recovered. By generating a Spo11αki mouse model, we prove that concomitant expression of SPO11β and SPO11α isoforms, boosts DSB formation in the PAR. Based on these findings, we propose that SPO11 splice isoforms cooperate functionally in promoting recombination in the PAR, constraining XY asynapsis defects that may arise due to differences in the conformation of the PAR between mouse strains.

Recent advances in mechanisms ensuring the pairing, synapsis and segregation of XY chromosomes in mice and humans

“…Using a knock-in mouse model that expresses only SPO11β under its physiological promoter ( Spo11βki -only mouse) we confirmed that DSB frequency in the PAR changes with genetic background. Moreover, we observed that it fails more frequently in the Y-PAR than the X-PAR [ 30 ]. Furthermore, we demonstrated using the Spo11αki mouse model, that concomitant expression of SPO11α with SPO11β strongly boost formation of DSBs in the Y-PAR, regardless of the genetic background [ 30 ].…”

“…Moreover, we observed that it fails more frequently in the Y-PAR than the X-PAR [ 30 ]. Furthermore, we demonstrated using the Spo11αki mouse model, that concomitant expression of SPO11α with SPO11β strongly boost formation of DSBs in the Y-PAR, regardless of the genetic background [ 30 ]. Therefore, it can be concluded that the expression of the single SPO11β isoform increases the risk of XY asynapsis.…”

“…Previous studies have demonstrated that Ewsr1 −/− spermatocytes are deficient in synapsis of the autosomes and XY chromosomes, indicating their essential function during meiosis [ 101 , 102 ]. By using the Spo11βki -only and Spo11αki -only models [ 30 ], we demonstrated that EWSR1 co-immunoprecipitates with SPO11β, SPO11α and REC114 [ 103 ]. Given the significance of SPO11α in DSB formation at the PAR [ 30 ], this observation supports a role for EWS1 in XY recombination.…”

“…By using the Spo11βki -only and Spo11αki -only models [ 30 ], we demonstrated that EWSR1 co-immunoprecipitates with SPO11β, SPO11α and REC114 [ 103 ]. Given the significance of SPO11α in DSB formation at the PAR [ 30 ], this observation supports a role for EWS1 in XY recombination. Intriguingly, homolog synapsis is also defective in spermatocytes lacking FUS/TLS expression [ 104 ].…”

“…A failure in the generation of at least one DSB in the X-PAR and/or Y-PAR results in missegregation of the sex chromosomes. In mice, according to phenotypic penetrance, this can lead to infertility [ 27 ] or sub fertility [ 28 ], with generation of aneuploid sperm for the sex chromosomes [ 29 , 30 ]. In men, infertile, oliogozospermic and oligoasthenoteratozoospermic patients manifest significantly higher levels of XY disomy [ 31 – 33 ], probably due to reduced recombination in the PAR [ 32 , 34 ].…”