高频度微卫星不稳定性大肠癌“修饰型”微卫星变化与MLH1及KRAS基因突变密切相关

本研究通过方法学的改良和观察方式的创新试图阐明这种现象的原因。微卫星非传统的检测方法仅能实现微卫星定性检测,我所在的研究组开发了自动片段分析双荧光标识技术,提高了微卫星检测的感度和重复性。并实现了微卫星片段变化长度的定量。小于6碱基的微卫星变化被定义为修饰型微卫星不稳定,大于8碱基的变化被定义为跳跃型微卫星不稳定．它们的电泳谱截然不同。前者表现为在非肿瘤来源微卫星位点基础上的增加或减少,后者表现为距离非肿瘤微卫星片段远隔部位的新波形的出现。通过研究我们发现,在DNA错配修复缺陷细胞系及基因敲除大鼠自发肿瘤样本,仅有修饰型微卫星不稳定性检出;在人类DNA错配修复缺陷细胞系连续80次传代也没有检出跳跃型变化。跳跃型变化不能通过简单重复序列不稳定基础上的增加或减少的累加而获得。在76例散发大肠癌,我们检测了微卫星不稳定性,KRAS基因突变,并对高频度微卫星不稳定性病例的两个主要DNA错配修复基因MSH2和MLH1进行了全长测序。我们发现,在大肠癌,按频度的传统分类与按波形变化的分类有高度的一致性,高频度微卫星不稳定性病例均检测到跳跃型表现,低频度微卫星不稳定性都表现为修饰型变化。在12例高频度微卫星不稳定病例,有三例检出了跳跃型和修饰型同时存在微卫星不稳定的特殊表型,这3例均检出KRAS的突变,更有趣的是该3例病例也同时检出了DNA错配修复基因MLH1的变异。而在其他9例高频度微卫星不稳定病例,KRAS突变及MLH1、MSH2突变未检出。通过对突变谱的分析我们还发现,修饰型微卫星不稳定与KRAS基因12号密码子的转换型突变高度相关,而微卫星稳定的病例检出的KRAS基因12号密码子突变多为颠换型突变。修饰型微卫星不稳定表型检出的高频度转换?     

Exclusive KRAS mutation in microsatellite-unstable human colorectal carcinomas with sequence alterations in the DNA mismatch repair gene, MLH1

Microsatellite instability (MSI) is regarded as reflecting defective DNA mismatch repair (MMR). MMR defects lead to an increase in point mutations, as well as repeat instability, on the genome. However, despite the highly unstable microsatellites, base substitutions in representative oncogenes or tumor suppressors are extremely infrequent in MSI-positive tumors. Recently, the heterogeneity in MSI-positive colorectal tumors is pointed out, and the 'hereditary' and 'sporadic settings' are proposed. Particularly in the former, base substitution mutations in KRAS are regarded as relatively frequent. We sequenced the KRAS gene in a panel of 76 human colorectal carcinomas in which the MSI status has been determined. KRAS mutations were detected in 22 tumors (28.9%). Intriguingly, all of the KRAS-mutant MSI-H (high) tumors harbored sequence alterations in an essential MMR gene, MLH1, which implies that KRAS mutation more frequently and almost exclusively occurs in MMR gene-mutant MSI-H tumors. Furthermore, in contrast with the prevailing viewpoint, some of these tumors are derived from sporadic colorectal cancer patients. The tight connection between MMR gene mutation and KRAS mutation may suggest previously unrecognized complexities in the relationship between MSI and the mutator phenotype derived from defective MMR.     

Impaired DNA repair and genomic stability in hereditary tyrosinemia type 1

The autosomal recessive disorder, hereditary tyrosinemia type 1 (HT1), is caused by a defective fumarylacetoacetate hydrolase enzyme. Consequently intermediate metabolites such as fumarylacetoacetate, succinylacetone and p-hydroxyphenylpyruvic acid accumulate. Characteristic to HT1 is the development of hepatocellular carcinoma, irrespective of dietary intervention or pharmacological treatment. Carcinogenesis may occur through a chromosomal instability mutator phenotype or a microsatellite instability phenotype, and deficient DNA repair may be a contributing factor thereof. The purpose of this study was to investigate the expression of DNA repair proteins, and the possible occurrence of microsatellite instability in HT1. Gene expression analyses show low expression of hOGG1 and ERCC1 in HT1 patient lymphocytes. Results from microsatellite instability analyses show allelic imbalance on chromosome 7 of the fah^−/− mouse genome, and instability of the D2S123, D5S346 and (possibly) D17S250 microsatellite markers, in HT1 patient lymphocytes.