Seeking new meiotic genes.

H uman infertility is a major worldwide health problem for individuals and their partners (1). Because genetic defects are thought to underlie many of the unexplained pathologies in infertility cases, animal models are expected to provide valuable clues to the underlying defects. More than 200 infertile or subfertile genetic mouse models have already been generated, defining key DNA repair and signaling pathways and other processes involved in mammalian reproduction (2). Many of these models were generated by targeted disruption of known genes or were fortuitously identified as spontaneously arising mutants. Because of the complexity of reproduction, however, this number almost certainly represents only a small fraction of the genes controlling this process (3). To discover new genes in gametogenesis, John Schimenti and colleagues have undertaken an ambitious phenotypebased screen in mice based on chemical mutagenesis of either embryonic stem cells (ethylmethanesulfonate) or whole animals (ethylnitrosourea). In a pilot study, 11 mouse fertility mutants have thus far been generated that affect several different stages of gametogenesis in one or both sexes, including meiosis (4). In this issue of PNAS, Libby et al. (5) report the positional cloning of the first of the mutant genes, Mei1 (meiosis defective 1). Mei1 is expressed almost exclusively in the gonads, in particular, in the testis of prepubertal and adult males and in the ovary of late embryonic females (i.e., embryonic day 17.5), the time of meiotic prophase. The human MEI1 gene is predicted to encode a protein with 79% identity to the mouse protein, and other vertebrate homologs have also been identified. However, the encoded protein contains no significant homology to previously described proteins, and homologs are not evident in yeast, worms, or flies. Thus, with this forward genetic approach, a novel vertebrate meiosis gene has been identified, highlighting the power of the chemical mutagenesis screen for infertility studies. Despite being a novel gene, precise characterization of the Mei1 mutant phenotype in relation to that of other mouse meiotic mutants has provided insight into its function (5, 6). Central to meiosis is recombination between homologous chromosomes resulting in crossover recombinants. In conjunction with sister-chromatid cohesion, crossovers maintain the physical connections between homologs necessary for chromosome congression at the metaphase plate to permit segregation of homologs at the first meiotic division. The events of meiotic recombination, as described for Saccharomyces cerevisiae (7), are as follows (Fig. 1): double-strand breaks (DSBs) are introduced to initiate recombination (step I); 5 terminal strands at the DSBs are degraded to yield 3 single-stranded tails (step II); the tails invade the intact, homologous chromosome (step III); repair synthesis ensues and double-Holliday junction intermediates are formed (step IV); and resolution results in mature crossover products (step V). The catalytic activity for DSB formation appears to reside in the Spo11 protein, which presumably acts as a transesterase rather than as an endonuclease (Fig. 1, step I) (7). In the mouse, 250 DSBs are inferred to be introduced by the Spo11 protein (see, e.g., ref. 8). Because there are only 24 crossovers, most DSBs do not give rise to crossovers but may instead be repaired by recombination without crossing over. DSB formation leads to a DNA damage response, including phosphorylation of histone H2AX ( H2AX) (9), and repair of the Spo11-generated DSBs involves the strand invasion proteins Rad51 and Dmc1, which form foci at the DSB sites (Fig. 1, step III), and other repair proteins (7, 8, 10, 11). Spo11 does not act alone, however, because at least nine other proteins are required in S. cerevisiae for DSB formation (Fig. 1, step I), such that null mutations in any one of those genes confer the same meiotic phenotype as the Spo11 null mutation (7). Four of these proteins, like Spo11, are conserved in other organisms: