Se. Tobi et al., Sequence-dependent mutations in a shuttle vector plasmid replicated in a mismatch repair deficient human cell line, CARCINOGENE, 20(7), 1999, pp. 1293-1301
We utilized a shuttle vector plasmid (pLSC) to assess the role of DNA seque
nce and mismatch repair on mutagenesis in human cells. pLSC contains an int
errupted 29 bp mononucleotide poly(G) run within a bacterial suppressor tRN
A gene, which acts as a highly sensitive mutagenic target for detection of
base substitution and frameshift mutations. The frequency of spontaneous mu
tations in pLSC was found to be similar after replication in either the hMS
H6 (GT binding protein) mismatch repair-deficient MT1 line or its parental,
mismatch repair-proficient line, TK6, However, the classes of plasmid muta
tions showed distinct differences in the two cell lines. Single base deleti
ons comprised 48% of the mutations in the 56 independent pLSC plasmids sequ
enced from MT1 cells while these represented only 18% of the 40 independent
pLSC mutants sequenced from the wild-type TK6 cells (P = 0.001). Virtually
all the deletions included the mononucleotide run. In contrast, in pSP189,
which contains the unmodified supF tRNA without the mononucleotide sequenc
e, no single base deletions were observed for either cell line (P < 0.001),
UV treatment of pLSC and pSP189 resulted in a 12-140-fold increase in muta
tions in TK6 and MT1 cells. These were predominately single base substituti
on mutations without a large increase in deletion mutations in the mononucl
eotide run in pLSC, These data indicate that a mononucleotide poly(G) run p
romotes single base deletion mutations. This effect is enhanced in a hMSH6
mismatch repair-deficient cell line and is independent of UV-induced mutage
nesis.