G-quadruplex (G4)
DNA, an alternate structure formed by Hoogsteen hydrogen bonds between guanines in G-rich sequences, threatens
genomic stability by perturbing normal
DNA transactions including replication, repair, and transcription. A variety of G4 topologies (intra- and intermolecular) can form in vitro, but the molecular architecture and cellular factors influencing G4 landscape in vivo are not clear. Helicases that unwind structured
DNA molecules are emerging as an important class of G4-resolving
enzymes. The BRCA1-associated FANCJ helicase is among those helicases able to unwind G4
DNA in vitro, and FANCJ mutations are associated with
breast cancer and linked to
Fanconi anemia. FANCJ belongs to a conserved
iron-
sulfur (Fe S) cluster family of helicases important for
genomic stability including XPD (nucleotide excision repair), DDX11 (sister chromatid cohesion), and RTEL (telomere metabolism), genetically linked to
xeroderma pigmentosum/
Cockayne syndrome, Warsaw breakage syndrome, and
dyskeratosis congenita, respectively. To elucidate the role of FANCJ in
genomic stability, its molecular functions in G4 metabolism were examined. FANCJ efficiently unwound in a kinetic and
ATPase-dependent manner entropically favored unimolecular G4
DNA, whereas other Fe-S helicases tested did not. The G4-specific
ligands Phen-DC3 or Phen-DC6 inhibited FANCJ helicase on unimolecular G4 ∼1000-fold better than bi- or tetramolecular G4
DNA. The G4
ligand telomestatin induced DNA damage in human cells deficient in FANCJ but not DDX11 or XPD. These findings suggest FANCJ is a specialized Fe-S cluster helicase that preserves
chromosomal stability by unwinding unimolecular G4
DNA likely to form in transiently unwound single-stranded genomic regions.