The life cycle of sexually reproducing eukaryotes depends on two specialized chromosome segregation programs: mitosis and meiosis. Whereas mitosis drives cellular proliferation and the stable propagation of the genome, meiosis promotes genetic diversity and the formation of haploid gametes, which combine at fertilization to restore the diploid state. Remarkably, both genome stability and genetic diversity/haploidisation depend on the cell’s ability to repair damaged chromosomes using homologous recombination.
To support the repair of DNA lesions, recombinases promote pairing and strand-exchange between a damaged chromosome and a homologous DNA template, leading to the formation of DNA joint molecules. Such DNA-based chromosomal connections can be disengaged at an early stage by anti-recombinogenic helicases or, under circumstances that are currently undefined, mature to form four-way junction, also known as Holliday junctions (HJs). During meiosis, processing of HJs by structure-selective nucleases primes the formation of reciprocal inter-homolog exchanges (crossovers). In most organisms, crossovers are required for the establishment of cohesin-mediated inter-homolog connections, which provide the mechanical basis for reductional chromosome segregation.
In mitotic cells, homologous recombination plays an important role in the repair of damaged or broken chromosomes, which can occur in cells exposed to DNA damaging agents (e.g. ionizing radiation) and as a consequence of impaired DNA replication. Such breaks represent one of the most threatening forms of chromosome damage as they can lead to mutation, chromosome loss and chromosomal translocations. Therefore, throughout most of our life cycle recombination must ensure efficient DNA repair, which is crucial for genome stability and cancer avoidance, while minimizing genetic exchanges, which could lead to loss of heterozygosity. To this end, mitotic cells disengage the vast majority DNA joint molecules to generate non-crossover recombinants.
Extensive research has focused on the genetic delineation and biochemical characterization of the pathways involved in the metabolism of DNA repair intermediates. Paradoxically, however, a similar set of DNA processing enzymes appears to function during mitotic and meiotic DNA repair, implying that cells must be capable or regulating their actions according to their specialized needs (genome stability vs genome haploidisation).
Our group uses a combination of approaches (proteomics, biochemistry, cell biology and genetics) and model systems (budding yeast and human tissue culture) to investigate how cells rewire the recombination machinery in order to: 1) promote genetic diversity and haploidisation during meiosis; 2) prevent genome instability during mitotic proliferation; 3) ensure the efficient disengagement of all joint molecules prior to chromosome segregation and cell division.
A) Cartoon of a structure-selective endonuclease cleaving a Holliday junction to disengage two recombining chromosomes (illustration by Miguel G. Blanco).
B) Abnormal sister chromatid exchanges (inter-sister crossovers) in a cell line derived from a Bloom’s syndrome patient, caused by the actions of structure-selective nucleases (Wyatt et al., 2013).
C) Chromosome missegregation and aneuploidy in budding yeast cells lacking multiple pathways of Holliday junction resolution (Matos et al., 2013).
D) and E) Foci of the structure-selective endonuclease Mus81-Mms4 (Red) on meiotic chromosome spreads