Genetic mutation associated with meiotic metaphase-specific apoptosis in MRL/MpJ mice

It has been reported that the MRL/MpJ mouse strain shows several unique phenotypes, including rapid wound healing, inherent collagen disease, heat shock-resistant spermatocytes, and metaphase-specific apoptosis (Msa) in the testis. In the present study, we found the genetic mutation associated with Msa by chromosomal mapping with 555 backcross progeny. The Sertoli cell index of abnormal metaphasic spermatocytes was clearly divided into two groups in the first 200 male backcross progeny, which were created by mating female F1 (female C57BL/6 x male MRL/MpJ) with male MRL/MpJ mice, indicating that Msa was caused by only one gene. The result of chromosomal mapping throughout the 555 backcross progeny by using microsatellite markers and single nucleotide polymorphism (SNP) revealed that Msa was mapped on the telomeric region of chromosome 1 and was significantly linked with exonuclease 1 (Exo1) and choroideremia-like (rab escort protein 2) (Chml/Rep2) genes. It was found that the Chml/Rep2 gene was not a candidate for Msa by means of the nucleotide sequences of several inbred strains. On the Exo1 gene in strain MRL/MpJ, but not in other strains, it was surprisingly noted that the truncated forms (tr1-Exo1 and tr2-Exo1) were expressed in all tissues examined as well as normal Exo1 by reverse transcriptase-polymerase chain reaction (RT-PCR). Additionally, the truncated forms of the Exo1 gene were suggested to be transcribed by alternative splicing of the 9th exon, possibly resulting from nucleotide substitution of the branch site existing in the 8th intron. These results suggested that the testicular meiotic Msa in MRL/MpJ mice was a unique phenotype caused by incomplete alternative splicing of the Exo1 gene.