The Mre11 complex influences DNA repair, synapsis, and crossing over in murine meiosis

Abstract

The Mre11 complex (consisting of MRE11, RAD50, and NBS1/Xrs2) is required for double-strand break (DSB) formation, processing, and checkpoint signaling during meiotic cell division in S. cerevisiae. Whereas studies of Mre11 complex mutants in S. pombe and A. thaliana indicate that the complex has other essential meiotic roles , relatively little is known regarding the functions of the complex downstream of meiotic break formation and processing or its role in meiosis in higher eukaryotes. We analyzed meiotic events in mice harboring hypomorphic Mre11 and Nbs1 mutations which, unlike null mutants, support viability . Our studies revealed defects in the temporal progression of meiotic prophase, incomplete and aberrant synapsis of homologous chromosomes, persistence of strand exchange proteins, and alterations in both the frequency and placement of MLH1 foci, a marker of crossovers. A unique sex-dependent effect on MLH1 foci and chiasmata numbers was observed: males exhibited an increase and females a decrease in recombination levels. Thus, our findings implicate the Mre11 complex in meiotic DNA repair and synapsis in mammals and indicate that the complex may contribute to the establishment of normal sex-specific differences in meiosis.

Figure 1. SC Assembly Defects in Prophase Meiocytes from Mre11 Complex Hypomorphic Mice

Pachytene cells from Mre11^ATLD1/ATLD1 mice exhibit incomplete synapsis (oocyte, [A], arrow), fragmented SCs (spermatocyte, [C], arrow), and SC end associations (oocyte, [D, E]; spermatocyte, [F], arrow). An example of a mutant spermatocyte with normal SC morphology is shown in (B). SC associations result in many overlapping telomere signals ([D], green) and occur at the centromeric ends of SCs ([E], blue). Spermatocytes from Nbs1^δB/δB mice exhibit asynapsis in late prophase ([G], auto, indicates autosomal asynapsis; arrow, XY bivalent).

Figure 2. Persistent RAD51 Localization in Mre11^ATLD1/ATLD1 Mice

Relative to controls (wild-type oocyte, [A]; wild-type spermatocyte, [D], XY bivalent indicated by arrow), pachytene meiocytes from Mre11^ATLD1/ATLD1 mice exhibit prolonged localization of RAD51 foci (mutant oocyte, [B]; mutant spermatocyte, [E]). Persistent RAD51 foci colocalize with γH2AX ([C], same spread as in [B], but overlayed with γH2AX, blue).

Figure 3. A Change in the Position of MLH1 Foci Occurs in Mre11^ATLD1/ATLD1 Spermatocytes

The location of foci relative to the centromere of single exchange bivalents was determined for Mre11^ATLD1/ATLD1 mutants in comparison to wild-type mice. The resultant distributions from individual animals are arrayed horizontally along the x axis, with percent of single exchange bivalents scored represented by height of symbol relative to the y axis. Gray diamonds, wild-type males; open black triangles, Mre11^ATLD1/ATLD1 males.