Survival of the Replication Checkpoint Deficient Cells Requires MUS81-RAD52 Function
Figure 8
Model for processing of stalled forks in replication checkpoint-deficient cells.
Inactivation of CHK1 determines destabilisation of stalled replication forks and accumulation of ssDNA gaps, likely at both the leading and the lagging strand. Stalled forks with ssDNA gaps (1) may undergo extensive extrusion of the newly-synthesized strands by fork regression (2) leading to a preferential engagement of RAD52 (A). RAD52, through its ssDNA annealing activity, would produce a D-loop intermediate (3) and possibly helps recruiting MUS81/EME1 complex by protein-protein interaction. Alternatively, RAD52 may assemble a D-loop intermediate from the ssDNA gap, either at the leading or the lagging strand behind the stalled fork (1.1). The D-loop intermediate is targeted by MUS81 resulting in DSBs and fork collapse. The BIR event that follows may involve subsequent requirement for viability of another SSE, GEN1. In the absence of a functional checkpoint (i.e. inactive CHK1), the RAD52-dependent pathway is a favourite, but inefficient, way of ensuring proliferation at the expense of genome stability. In the absence of RAD52, a RAD51-dependent mechanism (B) may be forcedly engaged. Viability of RAD52-deficient cells would require MUS81 and GEN1 to process the branched intermediates generated. This latter option, would limit genome instability at the cost of reduced survival, and would result in excessive lethality if MUS81 is also depleted. In contrast, MUS81 down-regulation, would stimulate a RAD51-mediated mechanism (C), but at the expense of both reduced cell viability and genome stability. Further details are discussed in the text.