Yang A S, Gonzalgo M L, Zingg J M, Millar R P, Buckley J D, Jones P A
Department of Biochemistry and Molecular Biology, University of Southern California, School of Medicine, Los Angeles, 90033, USA.
J Mol Biol. 1996 May 3;258(2):240-50. doi: 10.1006/jmbi.1996.0246.
Cytosine to thymine transition mutations at the CpG dinucleotide are the most common point mutations in cancer and genetic disease. We calculated the in vivo rate of CpG mutation in the primate germline by deriving a primordial consensus sequence for an Alu repetitive element which inserted into intron 6 of the primate p53 gene 35 to 55 million years ago. Comparison of this primordial sequence to the Alu sequence in intron 6 of present-day primates was used to determine the nature and rate of mutations which occurred during evolution. We estimate the half-life of a CpG nucleotide to be 24 to 60 million years, and the rate constant for mutation at this dinucleotide to be 1.2 x 1O(-8) to 2.9 x 1O(-8) years(-1). These results were confirmed by the analysis of a second Alu sequence in intron 10 of the p53 gene. The in vivo mutation rate is at least 1250-fold slower than the in vitro chemical rate of 5-methylcytosine deamination in double-stranded DNA, showing that current estimates of CpG mutation repair have been significantly underestimated. Furthermore, the mutability of the CpG dinucleotide has led to the depletion of this dinucleotide from the vertebrate genome, and calculations in this study suggest that current levels of the CpG dinucleotide in the primate genome are very close to a steady state equilibrium in which the rate of CpG mutation is equal to the rate of CpG formation by random mutation.
在癌症和遗传疾病中，CpG二核苷酸处的胞嘧啶到胸腺嘧啶的转换突变是最常见的点突变。我们通过推导一个约在3500万至5500万年前插入灵长类p53基因内含子6的Alu重复元件的原始共有序列，计算了灵长类种系中CpG突变的体内发生率。将这个原始序列与现代灵长类动物内含子6中的Alu序列进行比较，以确定进化过程中发生的突变的性质和速率。我们估计一个CpG核苷酸的半衰期为2400万至6000万年，该二核苷酸的突变速率常数为1.2×10^(-8)至2.9×10^(-8)年^(-1)。这些结果通过对p53基因内含子10中的第二个Alu序列的分析得到了证实。体内突变率比双链DNA中5-甲基胞嘧啶脱氨的体外化学速率至少慢1250倍，这表明目前对CpG突变修复的估计被大大低估了。此外，CpG二核苷酸的可变性导致了该二核苷酸在脊椎动物基因组中的耗竭，本研究中的计算表明，灵长类基因组中目前的CpG二核苷酸水平非常接近一种稳态平衡，即CpG突变率等于随机突变产生CpG的速率。
