Cells from Fanconi anemia (FA) patients have defective DNA repair and
are hypersensitive to DNA crosslinking agents such as mitomycin C (MMC
). We examined the possibility that topoisomerase I is involved in the
DNA crosslink repair system and is deficient in FA group A cells. FA
cells and control cells were exposed to MMC with or without camptothec
in (CPT), a topoisomerase I inhibitor. The cells did not show any incr
eased sensitivity to killing by MMC with CPT, suggesting that the topo
isomerase I is not involved in MMC-damaged DNA repair. However, FA cel
ls showed increased sensitivity to CPT in comparison to control cells,
raising the possibility of altered topoisomerase I in FA cells. There
fore, a mutation analysis was performed on topoisomerase I cDNA from F
A cells by using chemical cleavage mismatch scanning and nucleotide se
quencing. No mutation was detected from GM1309, a group A FA cell line
. A base transition (C to T) at position 241, causing an amino acid ch
ange (His to Tyr), was found in GM2061, a FA cell line of unknown comp
lementation group. However, allele-specific oligonucleotide hybridizat
ion analysis showed that this is a gene polymorphism. We conclude that
FA cells have normal gene structure for topoisomerase I.