L. Arcangeli et al., SITE-SPECIFIC AND STRAND-SPECIFIC MISMATCH REPAIR OF HUMAN H-RAS GENOMIC DNA IN A MAMMALIAN-CELL LINE, Carcinogenesis, 18(7), 1997, pp. 1311-1318
Defective mismatch repair has recently been implicated as the major co
ntributor towards the mutator phenotype tumour cell lines derived from
patients hereditary non-polyposis colon cancer (HNPCC). Cell lines fr
om other cancer-prone syndromes, such as xeroderma pigmentosum, have b
een found to be defective in nucleotide excision repair of damaged bas
es, Some genetic complementation groups are defective specifically in
transcription-coupled excision repair, although this type of repair de
fect has not been associated with cancer proneness. Mechanisms contrib
uting to the high incidence of activating point mutations in oncogenes
(such as H-ras codon 12) are not understood, It is possible that nove
l mechanisms of misrepair or misreplication occur at these sites in ad
dition to the above DNA repair mechanisms, In this study, we have comp
ared the rate of strand-directed mismatch repair of four mispairs (G:A
, A:C, T:C and G:T) at the H-ras codon 12, middle G:C position, Our re
sults indicate that, although this location is not a 'hot spot' for ba
cterial mismatch repair, it is a 'hot spot' for decreased repair of sp
ecific mismatched bases within NIH 3T3 cells, NIH 3T3, unlike Escheric
hia coli, have an extremely low repair rate of the G:A mispair (35%),
as well as the A:C mispair (58%) at this location, NIH 3T3 also have a
moderately low repair rate of the T:C mispair (80%) at the codon 12 l
ocation, Conversely, NIH 3T3 repair of G:T (100%) is comparable to E.
coli repair (94%) of this mismatch, These results demonstrate that a m
ismatch containing an incorrect adenine on either strand at the H-ras
codon 12 middle base pair location is most likely to undergo a mutatio
nal event in NIH 3T3 cells, Conversely, a mismatch containing an incor
rect thymine in the transcribed strand is least likely to undergo a mu
tational event.