Watanabe N, Okochi E, Hirayama Y, Shimada Y, Yanagihara K, Yoshida M C, Takahashi S, Mochizuki M, Sugimura T, Nagao M, Ushijima T
Carcinogenesis Division, National Cancer Center Research Institute, Tokyo, Japan.
Cancer Res. 2001 Mar 15;61(6):2632-40.
Mutation frequencies (MnFs) of the lacI transgene and mutation rates (MRs) of the endogenous hprt gene were analyzed in two mammary carcinoma cell lines that we established from mammary carcinomas that had been induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in female lacI-transgenic rats. Using the lacI transgene, corrected MnF, which is the number of independent lacI mutations that occurred while 102 cells expanded into 10(7) cells and which reflect the dynamic increase of point mutations, was measured. The corrected MnFs in the two mammary carcinoma cell lines (59 x 10(-6) and 72 x 10(-6) mutations) were significantly higher than that in the primary culture of normal mammary epithelium (4.7 x 10(-6)). MRs of the hprt gene in the two mammary carcinoma cell lines (8.2 x 10(-7) and 11 x 10(-7) mutations/hprt/cell division) were also higher than the same control (1.4 x 10(-7)). A:T to C:G transversion was observed at significantly higher frequencies in the two cell lines (6 of 24 and 6 of 25 for lacI; 10 of 67 and 19 of 92 for hprt) than in the control (0 of 6 for lacI; 0 of 4 for hprt). Taking advantage of the lacI transgene, high frequencies of A:T to C:G transversion (6 of 38 and 8 of 33, respectively) was also confirmed in the primary carcinomas of the two cell lines, which indicated the presence of a common abnormality in the cell lines and in the primary carcinomas. Both the established cell lines and their primary carcinomas were negative for microsatellite instability, which is known to be caused mainly by mismatch repair insufficiency and to increase point mutations, and for p53 mutations. These findings showed that the two cell lines, and possibly their primary carcinomas, had increases in the MRs of point mutations attributable to a mechanism(s) different from mismatch repair insufficiency, and we would suggest that such a state be designated as single nucleotide instability (SNI).
在我们从2-氨基-1-甲基-6-苯基咪唑并[4,5-b]吡啶（PhIP）诱导的雌性lacI转基因大鼠的乳腺癌中建立的两种乳腺癌细胞系中，分析了lacI转基因的突变频率（MnFs）和内源性hprt基因的突变率（MRs）。使用lacI转基因，测量了校正后的MnF，即10²个细胞扩增到10⁷个细胞时发生的独立lacI突变的数量，其反映了点突变的动态增加。两种乳腺癌细胞系中的校正后MnF（59×10⁻⁶和72×10⁻⁶个突变）显著高于正常乳腺上皮原代培养中的校正后MnF（4.7×10⁻⁶）。两种乳腺癌细胞系中hprt基因的MRs（8.2×10⁻⁷和11×10⁻⁷个突变/hprt/细胞分裂）也高于相同对照（1.4×10⁻⁷）。在两种细胞系中，观察到A:T到C:G的颠换频率显著高于对照（lacI：24个中有6个和25个中有6个；hprt：67个中有10个和92个中有19个）（对照中lacI：6个中有0个；hprt：4个中有0个）。利用lacI转基因，在两种细胞系的原发性癌中也证实了高频率的A:T到C:G颠换（分别为38个中有6个和33个中有8个），这表明细胞系和原发性癌中存在共同的异常。建立的细胞系及其原发性癌均无微卫星不稳定性（已知主要由错配修复不足引起并增加点突变）和p53突变。这些发现表明，这两种细胞系及其原发性癌可能由于不同于错配修复不足的机制导致点突变的MRs增加，我们建议将这种状态称为单核苷酸不稳定性（SNI）。
