The accurate and timely DNA replication program is a prerequisite of a stable genome. This proposed
project is built around our discovery that the RAD52 DNA repair protein performs an important and
previously unknown function in supporting DNA replication. RAD52 protects replication forks from
excessive degradation, which depends on fork regression and on the MRE11 nuclease. Several
mechanisms, including a well characterized mechanism ascribed to the activities of SMARCAL1, ZRANB3,
BRCA1, BRCA2 and RAD51, protect replication forks stalled by damage or endogenous roadblocks due
to the replication stress. Distinctly, the function of RAD52 in fork protection is relevant not only after
induction of fork stalling by exogenous stress, but also during an unchallenged cell growth.
Our goal here is to develop a comprehensive mechanistic understanding of the RAD52 function at the
The MRE11-dependent degradation of the replication forks depends on fork regression, i.e. on the
conversion of a three-way junction of stalled replication fork into a four-way junction called “chicken foot”.
We propose that one or both of the following non-mutually exclusive mechanisms contribute(s) to RAD52
function at the replication forks. In the first, RAD52 may serve as a gatekeeper by preventing regression
of stalled, but undamaged forks. In the second, RAD52 may work as a protector of regressed forks either
together with, or in parallel to the BRCA1/BRCA2/RAD51 axis.
In AIM 1 and AIM 2 we will use cell-based analyses, stretched DNA fibers, proximity-ligation assays and
single-molecule total internal reflection fluorescence microscopy (smTIRFM) to test the gatekeeper and
the protector mechanisms. By building a comprehensive mechanistic description of the RAD52-fork
interaction in the cell and in singulo we will discern whether one or both of these mechanisms are applicable
to RAD52 and how RAD52 contributes to replication fork stability.
In AIM 3, to characterize the consequences of the RAD52 deficiency, we will combine the cell-based
assays, stretched DNA fibers, smTIRFM with the analysis of whole genome sequences by MMBIRFinder,
which is a new bioinformatics tool we developed to detect complex mutation events. We will determine the
mechanism(s) by which the aberrant recovery of DNA replication is funneled into different genome
destabilizing mechanisms in the presence and absence of RAD52.
Upon successful completion of the proposed studies we will learn how RAD52 functions at distressed
replication forks, how does it contribute to genome stability and how its deficiency leads to genome
destabilizing events during replication stress.