PRDM9 is a major determinant of meiotic recombination hotspots in humans and mice

Meiotic recombination events cluster into narrow segments of the genome, defined as hotspots. Here, we demonstrate that a major player for hotspot specification is the Prdm9 gene. First, two mouse strains that differ in hotspot usage are polymorphic for the zinc finger DNA binding array of PRDM9. Second, the human consensus PRDM9 allele is predicted to recognize the 13-mer motif enriched at human hotspots; this DNA binding specificity is verified by in vitro studies. Third, allelic variants of PRDM9 zinc fingers are significantly associated with variability in genome-wide hotspot usage among humans. Our results provide a molecular basis for the distribution of meiotic recombination in mammals, in which the binding of PRDM9 to specific DNA sequences targets the initiation of recombination at specific locations in the genome.

Recombinational DNA double-strand breaks in mice precede synapsis

In Saccharomyces cerevisiae, meiotic recombination is initiated by Spo11-dependent double-strand breaks (DSBs), a process that precedes homologous synapsis. Here we use an antibody specific for a phosphorylated histone (gamma-H2AX, which marks the sites of DSBs) to investigate the timing, distribution and Spo11-dependence of meiotic DSBs in the mouse. We show that, as in yeast, recombination in the mouse is initiated by Spo11-dependent DSBs that form during leptotene. Loss of gamma-H2AX staining (which in irradiated somatic cells is temporally linked with DSB repair) is temporally and spatially correlated with synapsis, even when this synapsis is 'non-homologous'.

Meiotic recombination: Making and breaking go hand in hand

Accurate segregation of homologous chromosomes at the first meiotic division requires the tight coordination of DNA replication, homologous recombination and chromosome organization. Recent studies suggest that the initiation of meiotic recombination is mechanistically coupled to premeiotic DNA replication.

Chromosome synapsis defects and sexually dimorphic meiotic progression in mice lacking Spo11

Spo11, a protein first identified in yeast, is thought to generate the chromosome breaks that initiate meiotic recombination. We now report that disruption of mouse Spo11 leads to severe gonadal abnormalities from defective meiosis. Spermatocytes suffer apoptotic death during early prophase; oocytes reach the diplotene/dictyate stage in nearly normal numbers, but most die soon after birth. Consistent with a conserved function in initiating meiotic recombination, Dmc1/Rad51 focus formation is abolished. Spo11(-/-) meiocytes also display homologous chromosome synapsis defects, similar to fungi but distinct from flies and nematodes. We propose that recombination initiation precedes and is required for normal synapsis in mammals. Our results also support the view that mammalian checkpoint responses to meiotic recombination and/or synapsis defects are sexually dimorphic.

[Mechanisms and control of meiotic recombination in the yeast Saccharomyces cerevisiae]

Recent studies in Saccharomyces cerevisiae have provided new insights in our understanding of the molecular mechanisms of meiotic recombination. Meiosis-specific DNA double-strand breaks have been detected and have been shown to be the lesions that initiate recombination events. These are located mostly in promoter regions where the chromatin is in an open configuration, and cluster in domains along the chromosome. They are likely to be made by a topoisomerase II-like protein encoded by the SPO11 gene. Several DNA intermediates in the meiotic double strand-break repair pathway have been characterised and several multi-protein complexes have been identified and shown to be involved at different steps in the repair pathway. The conservation of these protein complexes in higher eukaryotes suggests that the meiotic recombination mechanism could be conserved. With the application of the well characterised genetical, molecular, cytological and biochemical techniques and the recently developed technology for genomic studies (biochips), we can expect a rapid increase in our comprehension of the meiotic recombination process.

A mouse homolog of the Saccharomyces cerevisiae meiotic recombination DNA transesterase Spo11p

The Saccharomyces cerevisiae Spo11 protein is thought to catalyze formation of the DNA double-strand breaks that initiate meiotic recombination. We have cloned cDNA and genomic DNA for a mouse gene encoding a protein with significant sequence similarity to conserved domains found in proteins of the Spo11p family. This putative mouse Spo11 gene maps to the distal region of chromosome 2 (homologous to human chromosome 20q13.2-q13.3) and comprises at least 12 exons, spanning approximately 15-18 kb. Strong expression of the Spo11 message is seen in juvenile and adult testis by RNA in situ hybridization, RT-PCR, and Northern blot, with much weaker expression in thymus and brain. In situ hybridization detects expression in oocytes of embryonic ovary, but not of adult ovary. RT-PCR and in situ hybridization analyses of a time course of juvenile testis development indicate that Spo11 expression begins in early meiotic Prophase I, prior to the pachytene stage, with increasing accumulation of mRNA through the pachytene stage. Taken together, these results strongly suggest that this gene encodes the functional homolog of yeast Spo11p, which in turn suggests that the mechanism of meiotic recombination initiation is conserved between yeast and mammals.

Clustering of meiotic double-strand breaks on yeast chromosome III

In the yeast Saccharomyces cerevisiae, meiotic recombination is initiated by transient DNA double-strand breaks (DSBs) that are repaired by interaction of the broken chromosome with its homologue. To identify a large number of DSB sites and gain insight into the control of DSB formation at both the local and the whole chromosomal levels, we have determined at high resolution the distribution of meiotic DSBs along the 340 kb of chromosome III. We have found 76 DSB regions, mostly located in intergenic promoter-containing intervals. The frequency of DSBs varies at least 50-fold from one region to another. The global distribution of DSB regions along chromosome III is nonrandom, defining large (39-105 kb) chromosomal domains, both hot and cold. The distribution of these localized DSBs indicates that they are likely to initiate most crossovers along chromosome III, but some discrepancies remain to be explained.

Initiation of recombination in Saccharomyces cerevisiae haploid meiosis

In most eukaryotes during prophase I of meiosis, homologous chromosomes pair and recombine by coordinated molecular and cellular processes. To directly test whether or not the early steps of the initiation of recombination depend on the presence of a homologous chromosome, we have examined the formation and processing of DNA double-strand breaks (DSBs, the earliest physical landmark of recombination initiation) in various haploid Saccharomyces cerevisiae strains capable of entering meiosis. We find that DSBs occur in haploid meiosis, showing that the presence of a homolog is not required for DSB formation. DSBs occur at the same positions in haploid and diploid meioses. However, these two types of meiosis exhibit subtle differences with respect to the timing of formation and levels of DSBs. In haploid meiosis, a slower rate of DSB formation and a reduction in the frequency of DSB (at one of the three sites analyzed) were observed. These results might indicate that interactions between homologs play a role in the formation of meiotic DSBs. Furthermore, haploid strains exhibit a pronounced delay in the disappearance of meiotic DSBs compared to diploid strains, which suggests that sister chromatid interactions for DSB repair are inhibited in haploid meiosis.