FANCJ mutations are associated with breast cancer and genetically linked to the bone marrow disease Fanconi anemia (FA). The genomic instability of FA-J mutant cells suggests that FANCJ helicase functions in the replicational stress response. A putative helicase with sequence similarity to FANCJ in Caenorhabditis elegans (DOG-1) and mouse (RTEL) is required for poly(G) tract maintenance, suggesting its involvement in the resolution of alternate DNA structures that impede replication. Under physiological conditions, guaninerich sequences spontaneously assemble into four-stranded structures (G quadruplexes [G4]) that influence genomic stability. FANCJ unwound G4 DNA substrates in an ATPase-dependent manner. FANCJ G4 unwinding is specific since another superfamily 2 helicase, RECQ1, failed to unwind all G4 substrates tested under conditions in which the helicase unwound duplex DNA. Replication protein A stimulated FANCJ G4 unwinding, whereas the mismatch repair complex MSH2/MSH6 inhibited this activity. FANCJ-depleted cells treated with the G4-interactive compound telomestatin displayed impaired proliferation and elevated levels of apoptosis and DNA damage compared to small interfering RNA control cells, suggesting that G4 DNA is a physiological substrate of FANCJ. Although the FA pathway has been classically described in terms of interstrand cross-link (ICL) repair, the cellular defects associated with FANCJ mutation extend beyond the reduced ability to repair ICLs and involve other types of DNA structural roadblocks to replication.The identification of FANCJ mutations in early-onset breast cancer patients (3, 34) and Fanconi anemia (FA) group J patients (22,23,25) implicates FANCJ as a tumor suppressor caretaker that ensures genomic stability. FANCJ interacts with the tumor suppressor BRCA1 (3) and is a DNA helicase that catalytically unwinds duplex DNA in an ATP hydrolysis-dependent manner (2, 14). FA-J cells are hypersensitive to interstrand cross-linking (ICL) agents (1, 25); untreated FA-J cells exhibit diminished BRCA1 foci, and the cells show delayed formation of ionizing radiation-induced BRCA1 foci (31). In response to DNA damage or replicational stress, FANCJ colocalizes with the single-stranded DNA (ssDNA) binding protein replication protein A (RPA), which serves as an auxiliary factor for the unwinding function of FANCJ (15). FANCJ interacts with the mismatch repair complex MutL␣, and this interaction is required for the correction of the ICL response in FA-J cells (32). The activation of FANCJ helicase activity is required for timely progression through S phase of the cell cycle (20); however, the precise functions of the FANCJ helicase in S-phase progression remain to be understood.Although a role for FANCJ in DNA replication has been proposed previously, definitive evidence for a role of the FANCJ helicase in preventing genomic instability is lacking. One source of genomic instability is alternate DNA structures that may impede the replication fork. Guanine-rich nucleic acids have the potential to f...

It has been proposed that guanine-rich DNA forms four-stranded structures in vivo called G-quadruplexes or G4 DNA. G4 DNA has been implicated in several biological processes, but tools to study G4 DNA structures in cells are limited. Here we report the development of novel murine monoclonal antibodies specific for different G4 DNA structures. We show that one of these antibodies designated 1H6 exhibits strong nuclear staining in most human and murine cells. Staining intensity increased on treatment of cells with agents that stabilize G4 DNA and, strikingly, cells deficient in FANCJ, a G4 DNA-specific helicase, showed stronger nuclear staining than controls. Our data strongly support the existence of G4 DNA structures in mammalian cells and indicate that the abundance of such structures is increased in the absence of FANCJ. We conclude that monoclonal antibody 1H6 is a valuable tool for further studies on the role of G4 DNA in cell and molecular biology.