Guanine-rich DNA strands can fold in vitro into non-canonical DNA structures called G-quadruplexes. These structures may be very stable under physiological conditions. Evidence suggests that G-quadruplex structures may act as ‘knots’ within genomic DNA, and it has been hypothesized that proteins may have evolved to remove these structures. The first indication of how G-quadruplex structures could be unfolded enzymatically came in the late 1990s with reports that some well-known duplex DNA helicases resolved these structures in vitro. Since then, the number of studies reporting G-quadruplex DNA unfolding by helicase enzymes has rapidly increased. The present review aims to present a general overview of the helicase/G-quadruplex field.

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, guanine-rich 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.

Helicase-dependent DNA damage response and repair mechanisms help cells to cope with endogenous or exogenous stress to prevent chromosomal instability and maintain cellular homeostasis. Inactivating mutations in DNA helicase genes are linked to genetic disorders that are frequently associated with various cancers. However, the expression of many DNA helicases is upregulated in transformed or neoplastic cells and tissues and is required for cancer cell proliferation or resistance to DNA damage imposed by chemotherapies. The RecQ and iron-sulphur (Fe-S) families of DNA helicases have prominent roles in the maintenance of genomic stability through their catalytic functions and protein interactions in telomere maintenance and DNA repair pathways including nucleotide-excision repair (NER), homologous recombination (HR)-mediated repair of double-strand breaks (DSBs), interstrand crosslink (ICL) repair, and base-excision repair (BER). Specialized DNA helicases efficiently unwind G-quadruplex (G4) DNA and other forms of alternative DNA structures such as telomeric displacement loops (T-loops). Emerging evidence suggests that DNA helicases that unwind non-conventional DNA structures have important roles in the replication or repair of telomeres. Replication forks in rapidly dividing cancer cells are likely to encounter DNA lesions that perturb fork progression. Evidence suggests that certain DNA helicases help cells to cope with replicative lesions by remodelling the fork, restoring the integrity of broken replication forks, or having a role in the signalling mechanism for the intra-S-phase checkpoint. Elevated expression of DNA helicases in rapidly proliferating cells and tumours suggests that they have a role in resistance to DNA-damaging agents and may represent good biomarkers for response to chemotherapies. High-throughput screening of chemical libraries may prove to be beneficial for the discovery of small molecules that modulate helicase function in vivo. Such compounds may be useful in synthetic lethal approaches used in anticancer strategies that target tumours with existing DNA repair deficiencies.

Helicases are molecular motor proteins that couple the hydrolysis of NTP to nucleic acid unwinding. The growing number of DNA helicases implicated in human disease suggests that their vital specialized roles in cellular pathways are important for the maintenance of genome stability. In particular, mutations in genes of the RecQ family of DNA helicases result in chromosomal instability diseases of premature aging and/or cancer predisposition. We will discuss the mechanisms of RecQ helicases in pathways of DNA metabolism. A review of RecQ helicases from bacteria to human reveals their importance in genomic stability by their participation with other proteins to resolve DNA replication and recombination intermediates. In the light of their known catalytic activities and protein interactions, proposed models for RecQ function will be summarized with an emphasis on how this distinct class of enzymes functions in chromosomal stability maintenance and prevention of human disease and cancer.

Alternate DNA structures that deviate from B‐form double‐stranded DNA such as G‐quadruplex (G4) DNA can be formed by sequences that are widely distributed throughout the human genome. G‐quadruplex secondary structures, formed by the stacking of planar quartets composed of four guanines that interact by Hoogsteen hydrogen bonding, can affect cellular DNA replication and transcription, and influence genomic stability. The unique metabolism of G‐rich chromosomal regions that potentially form quadruplexes may influence a number of biological processes including immunoglobulin gene rearrangements, promoter activation and telomere maintenance. A number of human diseases are characterized by telomere defects, and it is proposed that G‐quadruplex structures which form at telomere ends play an important role in telomere stability. Evidence from cellular studies and model organisms suggests that diseases with known defects in G4 DNA helicases are likely to be perturbed in telomere maintenance and cellular DNA replication. In this minireview, we discuss the connections of G‐quadruplex nucleic acids to human genetic diseases and cancer based on the recent literature.

Werner syndrome (WS) is a human premature aging disorder characterized by chromosomal instability. The cellular defects of WS presumably reflect compromised or aberrant function of a DNA metabolic pathway that under normal circumstances confers stability to the genome. We report a novel interaction of the WRN gene product with the human 5′ flap endonuclease/5′–3′ exonuclease (FEN‐1), a DNA structure‐specific nuclease implicated in DNA replication, recombination and repair. WS protein (WRN) dramatically stimulates the rate of FEN‐1 cleavage of a 5′ flap DNA substrate. The WRN–FEN‐1 functional interaction is independent of WRN catalytic function and mediated by a 144 amino acid domain of WRN that shares homology with RecQ DNA helicases. A physical interaction between WRN and FEN‐1 is demonstrated by their co‐immunoprecipitation from HeLa cell lysate and affinity pull‐down experiments using a recombinant C‐terminal fragment of WRN. The underlying defect of WS is discussed in light of the evidence for the interaction between WRN and FEN‐1.