Chromosome Pairing Through Tensioned DNA Tethers Revealed by BRCA2 Meiotic Domain Deletion

BRCA2 has multiple functional domains that interact with different partners, and is essential for both somatic and meiotic homologous recombination (HR). We created a Brca2Δ12-14 mouse model with an internal deletion of the region which we named "the meiotic domain of BRCA2", as its loss results in complete failure of meiotic HR, while somatic HR is intact. The deletion in the protein includes the HSF2BP-binding motifs (exons 12-13) and the DMC1-binding PhePP domain (exon 14). Brca2Δ12-14 mice showed complete infertility in both males and females, with sexually dimorphic features. Recombinase foci (both RAD51 and DMC1) were completely undetectable in mutant spermatocytes, but while DMC1 foci were also absent in mutant oocytes, RAD51 foci numbers were only partially reduced (up to 60% fewer, 20% less intense). The relevance of the PhePP domain for meiotic HR has been controversial, but both the phenotype of Brca2Δ12-14 , and our biochemical data indicate that, along with the BRC repeats of BRCA2, PhePP is both essential and specific for DMC1 loading in meiotic HR, analogous to the C-terminal RAD51-specific TR2/CTRB. Further investigation of DSB end processing in Brca2Δ12-14 meiocytes and controls, using super-resolution imaging of RPA and SYCP3 led to discovery of two novel features. First, in Brca2Δ12-14 oocytes, but not in the spermatocytes nor wild types, we observed RPA foci as doublets ~200 nm apart, which could represent DSB end separation. Second, we describe RPA structures that are completely HR-dependent and are indicative of long, double-stranded DNA connections between homologs prior to synapsis. The observations led us to a model for chromosome alignment via multiple, tensed DNA tethers that connect the homologous DNA regions. We propose that in combination with dynamic adjustment of chromatin loops by meiotic cohesins, this is a plausible molecular mechanism to bring the homologs closer and initiate synapsis. ### Competing Interest Statement The authors have declared no competing interest.