Everything turns upside down: Mammalian RAD51 promotes genome instability in a homology-independent manner
Genetic instability is a hallmark of cancer cells. Rearrangements involving distant DNA partners lead to profound rearrangements (Gross Chromosomal Rearrangements GCR). Remarkably, many GCRs exhibit insertions at the junctions between the distant partners.
DNA double strand break is a prominent source of genetic instability. We have previously reported that distance between two DNA double strand breaks (DSBs) favors the insertions of heterologous ectopic sequences (≥45bp) at the repair junctions and that these events are dependent on CtIP (resection) but are inhibited by 53BP1. Moreover, analysis of the boarders of the insertions often reveals the presence of microhomologies between the insertions and the recipient sequences. We proposed that the Microhomologu-Mediated Insertions (MiMIs) aroused through a process we named Microhomology-Mediated Template Switching (MMTS).
We analyzed the MiMIs at junctions of GCR in breast cancer genomes. About 20% of junctions contain an insertion of at least one nucleotide, MiMIS (≥45bp) are present at 3% of junctions corresponding thus to 15-20% of all insertions. Surprisingly, although they do not involve sequence homology, MiMIs at the GCR junctions are less frequent in BRCA2 deficient tumors. These data are surprising because BRCA2 is a key player in homologous recombination (HR), which requires long sequence homologies and is considered a high fidelity repair mechanism that preserves genome stability.
Using two megadonuclease(I-SceI)-induced breaks at a specific locus we combined locus-specific with High Throughput Genome wide Translocation Sequencing analyses. Although they do not involve sequence homology, the MiMis at the junctions of distant sequences were dependent on RAD51, in 53BP1 depleted human cells. Moreover, reconstituting t(2;5) translocation with CRISP-Cas9 we found that RAD51 was indeed implicated in generating interchromosomal rearrangements that are prevented by 53BP1.
Domain 2 - UMR 3244 - Dynamics of genetic Information
Domain 2 - UMR 3664 - Nuclear Dynamics