FancJ/Brip1 helicase protects against genomic losses and gains in vertebrate cells

Defects in the FANCJ/BRIP1 helicase gene are associated with genome instability disorders such as familial breast cancer or Fanconi anemia (FA). Although FANCJ has an in vitro activity to resolve G-quadruplex (G4) structures, and FANCJ ortholog in C. elegans prevents G4-associated deletions during replication, how FANCJ loss affects genome integrity in higher organisms remains unclear. Here, we report that FANCJ, but not other FA genes FANCD2 or FANCC, protected against large-scale genomic deletion that occurred frequently at the rearranged immunoglobulin heavy chain (IgH) locus in chicken DT40 cell line, suggesting that FancJ protects the genome independently of the FA ubiquitination pathway. In a more unbiased approach using array-comparative genomic hybridization, we identified de novo deletions as well as amplifications in fancj cells kept in culture for 2 months. A cluster of G4 sequence motifs was found near the breakpoint of one amplified region, but G4 sequence motifs were not detected at the breakpoints of two deleted regions. These results collectively suggest that, unlike in C. elegans, actions of vertebrate FANCJ to promote genome stability may not be limited to protection against the G4-mediated gene deletions.