Kouidou Sofia, Malousi Andigoni, Maglaveras Nicos
Laboratory of Biological Chemistry, School of Medicine, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece.
Mutat Res. 2006 Jul 25;599(1-2):167-77. doi: 10.1016/j.mrfmmm.2006.03.002. Epub 2006 Apr 19.
All exonic CG sequences in p53 are methylated; this epigenetic modification is correlated with frequent G:C-->A:T transitions in p53. Recent reports reveal the presence in p53 of non-CG methylation in CC and CCC sequences, complementary to sites of selective guanosine adduct formation (GG and GGG), and the association of genetic instability with methylation at repetitive sequences. We presently investigated the distribution of methylation sites and repetitive elements in silent and nonsense p53 mutations (2051) among the IARC's TP53 somatic mutation database for exons 5-8. Silent mutations are nonrandom, but mostly involve G:C-->A:T transitions (62%); in particular C-->T mutations (39% of all silent mutations) are mostly correlated with CC and CCC sequences, while G-->A mutations with GG sequences. Sequence analysis of all non-G:C-->A:T silent mutations reveals the frequent formation of new methylation sites (CG), new CCC and GGG sequences in the resulting sequence, refinement of symmetry elements at interrupted microsatellite-like sequences and formation of small repeats (55.3%). The G:C-->A:T silent mutations characterize cancers associated with cigarette smoking (e.g. bladder or lung and bronchus cancer versus colorectal cancer); on the contrary, non-G:C-->A:T silent mutations have similar frequencies in most cancers. Nonsense mutations in exons 5-8, all resulting in mutants lacking amino acids 307-393, which are crucial for p53 activity, were also analyzed. The frequency of nonsense mutations is higher at methylated sites or repeats 1-2 nucleotides removed from methylation sites. Frameshift mutations are also more frequent at repeated sequences. The frequent G:C-->A:T silent mutations could indicate that CC and CCC sequences of exons 5-8 are occasionally targets of non-CpG methylation of cytosine. This process of de novo methylation in the presence of microsatellite-like sequences and small repeats might influence the genetic stability of a variety of genes.
p53基因所有外显子中的CG序列均发生甲基化；这种表观遗传修饰与p53基因中频繁出现的G:C→A:T转换相关。最近的报告显示，p53基因中存在CC和CCC序列的非CG甲基化，这些序列与选择性鸟嘌呤加合物形成位点（GG和GGG）互补，并且遗传不稳定性与重复序列处的甲基化有关联。我们目前在国际癌症研究机构（IARC）的外显子5 - 8的TP53体细胞突变数据库中，研究了沉默和无义p53突变（2051个）中甲基化位点和重复元件的分布情况。沉默突变并非随机发生，但大多涉及G:C→A:T转换（62%）；特别是C→T突变（占所有沉默突变的39%）大多与CC和CCC序列相关，而G→A突变与GG序列相关。对所有非G:C→A:T沉默突变的序列分析显示，在所得序列中频繁形成新的甲基化位点（CG）、新的CCC和GGG序列，中断的微卫星样序列处对称元件的细化以及小重复序列的形成（55.3%）。G:C→A:T沉默突变是与吸烟相关癌症（如膀胱癌或肺癌和支气管癌与结直肠癌相比）的特征；相反，非G:C→A:T沉默突变在大多数癌症中的频率相似。我们还分析了外显子5 - 8中的无义突变，所有这些突变均导致缺乏对p53活性至关重要的氨基酸307 - 393的突变体。无义突变在甲基化位点或距甲基化位点1 - 2个核苷酸的重复序列处频率更高。移码突变在重复序列处也更频繁。频繁出现的G:C→A:T沉默突变可能表明外显子5 - 8的CC和CCC序列偶尔是胞嘧啶非CpG甲基化的靶点。在存在微卫星样序列和小重复序列的情况下这种从头甲基化过程可能会影响多种基因的遗传稳定性。
