Iu. De Silva et al., Defining the roles of nucleotide excision repair and recombination in the repair of DNA interstrand cross-links in mammalian cells, MOL CELL B, 20(21), 2000, pp. 7980-7990
The mechanisms by which DNA interstrand cross-links (ICLs) are repaired in
mammalian cells are unclear. Studies in bacteria and yeasts indicate that b
oth nucleotide excision repair (NER) and recombination are required for the
ir removal and that double-strand breaks are produced as repair intermediat
es in yeast cells. The role of NER and recombination in the repair of ICLs
induced by nitrogen mustard (HN2) was investigated using Chinese hamster ov
ary mutant cell lines. XPF and ERCC1 mutants (defective in genes required f
or NER and some types of recombination) and;XRCC2 and XRCC3 mutants (defect
ive in RAD51-related homologous recombination genes) were highly sensitive
to HN2. Cell lines defective in other genes involved in NER (XPB, XPD, and
XPG), together with a mutant defective in nonhomologous end joining (XRCC5)
, showed only mild sensitivity. In agreement with their extreme sensitivity
, the XPF and ERCC1 mutants were defective in the incision or "unhooking" s
tep of ICL repair. In contrast, the other mutants defective in NER activiti
es, the XRCC2 and XRCC3 mutants, and the XRCC5 mutant all showed normal unh
ooking kinetics. Using pulsed-field gel electrophoresis, DNA double-strand
breaks (DSBs) were found to be induced following nitrogen mustard treatment
. DSB induction and repair were normal in all the NER mutants, including XP
F and ERCC1. The XRCC2, XRCC3, and XRCC5 mutants also shelved normal induct
ion kinetics. The XRCC2 and XRCC3 homologous recombination mutants were, ho
wever, severely impaired in the repair of DSBs. These results define a role
for,XPF and ERCC1 in the excision of ICLs, but not in the recombinational
components of cross-link repair. In addition, homologous recombination but
not nonhomologous end joining appears to play an important role in the repa
ir of DSBs resulting from nitrogen mustard treatment.