Ohzeki S, Tachibana A, Tatsumi K, Kato T
Department of Radiation Biology, Osaka University, Medical School, Suita, Japan.
Carcinogenesis. 1997 Jun;18(6):1127-33. doi: 10.1093/carcin/18.6.1127.
Spectra of spontaneous mutations at the hypoxanthine-guanine phosphoribosyltransferase (hprt) locus in colon carcinoma cell lines HCT116 and HCT-15 deficient in mismatch repair and displaying mutator phenotypes were determined. HCT116 and HCT-15 cells, respectively, harbour a mutation in the mismatch repair gene hMLH1 and GTBP. The mutation frequency at the hprt locus in both cell lines was elevated by about two orders, but the microsatellite instability in HCT116 cells was one order higher than in HCT-15 cells. Except for one mutant of HCT-15, all the mutations (114/115) were point mutations; base substitutions of various types and frameshifts (deletions/insertions of less than a few bases, predominantly of +/-1 bp). Base substitutions (57%) and frameshifts (43%) occurred at a comparable rate in HCT116, whereas base substitutions (92%) were the major mutational events in HCT-15. Most frameshifts in HCT116 occurred at sites of monotonous or short tandem repeating sequences, and two of these sites, where there was a run of six Gs and four As, were hot spots. Three hot spot sites of base substitutions were found in HCT-15; two of them at splice acceptor sites, the other at the CpG site shared with HCT116. The distinct mutation spectra of the HCT116 and HCT-15 cell lines may reflect functional differences in the hMLH1 and GTBP gene products in mismatch repair. The gene product GTBP may be involved in the preferential repair of base mismatches, and MLH1 in the repair of both base mismatches and deletions/insertions of less than a few bases. These results suggest that mismatch repair deficiency affects the microsatellite stability as widely reported in colorectal tumour cells, but that it may not severely affect chromosome integrity as the karyotypes of these tumour cells are, unlike other tumour cells, relatively stable.
测定了错配修复缺陷并表现出突变体表型的结肠癌细胞系HCT116和HCT - 15中次黄嘌呤 - 鸟嘌呤磷酸核糖转移酶（hprt）基因座的自发突变谱。HCT116和HCT - 15细胞分别在错配修复基因hMLH1和GTBP中存在突变。两种细胞系中hprt基因座的突变频率均升高了约两个数量级，但HCT116细胞中的微卫星不稳定性比HCT - 15细胞高一个数量级。除了HCT - 15的一个突变体，所有突变（114/115）均为点突变；包括各种类型的碱基替换和移码突变（缺失/插入少于几个碱基，主要为+/-1 bp）。在HCT116中，碱基替换（57%）和移码突变（43%）的发生频率相当，而在HCT - 15中，碱基替换（92%）是主要的突变事件。HCT116中的大多数移码突变发生在单调或短串联重复序列位点，其中有两个位点，一个是六个G的连续序列，另一个是四个A的连续序列，是热点区域。在HCT - 15中发现了三个碱基替换热点位点；其中两个位于剪接受体位点，另一个位于与HCT116共有的CpG位点。HCT116和HCT - 15细胞系不同的突变谱可能反映了hMLH1和GTBP基因产物在错配修复中的功能差异。基因产物GTBP可能参与碱基错配的优先修复，而MLH1则参与碱基错配以及少于几个碱基的缺失/插入的修复。这些结果表明，错配修复缺陷会影响微卫星稳定性，这在结肠肿瘤细胞中已有广泛报道，但与其他肿瘤细胞不同，这些肿瘤细胞的核型相对稳定，因此错配修复缺陷可能不会严重影响染色体完整性。
