FANCM regulates repair pathway choice at stalled replication forks

Abstract

Conservative repair of stalled replication forks is important for the maintenance of a stable genome. However, the mechanisms that regulate repair pathway "choice" at stalled mammalian forks remain poorly understood. The Fanconi anemia complementation group M gene, FANCM, encodes a multi-domain scaffolding and motor protein that interacts with several distinct repair protein complexes at stalled forks. Here we use a chromosomally integrated reporter of stalled fork repair, in combination with defined mutations engineered within the endogenous Fancm gene in primary mammalian cells, to study how Fancm regulates stalled fork repair. We identify separation-of-function Fancm mutants, which reveal that distinct repair functions of FANCM are enacted by modular, molecularly separable scaffolding domains. These findings define FANCM as a key mediator of repair pathway choice at stalled replication forks and reveal its molecular mechanism. Notably, a mutation that inactivates the ATPase function of FANCM disables all FANCM-mediated repair functions and appears to "trap" FANCM at stalled forks. We find that Fancm null cells do not survive genetic inactivation of Brca1. This synthetic lethal interaction is recapitulated in Fancm ATPase-defective mutants. The ATPase function of FANCM may therefore represent a promising "druggable" target for therapy of BRCA1 mutant cancers.