Institute of Pathology, Centre Hospitalier Universitaire Vaudois, University of Lausanne, Bugnon 25, 1011, Lausanne, Switzerland.
Fam Cancer. 2010 Jun;9(2):167-72. doi: 10.1007/s10689-009-9302-4.
Lynch syndrome is one of the most common hereditary colorectal cancer (CRC) syndrome and is caused by germline mutations of MLH1, MSH2 and more rarely MSH6, PMS2, MLH3 genes. Whereas the absence of MSH2 protein is predictive of Lynch syndrome, it is not the case for the absence of MLH1 protein. The purpose of this study was to develop a sensitive and cost effective algorithm to select Lynch syndrome cases among patients with MLH1 immunohistochemical silencing. Eleven sporadic CRC and 16 Lynch syndrome cases with MLH1 protein abnormalities were selected. The BRAF c.1799T> A mutation (p.Val600Glu) was analyzed by direct sequencing after PCR amplification of exon 15. Methylation of MLH1 promoter was determined by Methylation-Sensitive Single-Strand Conformation Analysis. In patients with Lynch syndrome, there was no BRAF mutation and only one case showed MLH1 methylation (6%). In sporadic CRC, all cases were MLH1 methylated (100%) and 8 out of 11 cases carried the above BRAF mutation (73%) whereas only 3 cases were BRAF wild type (27%). We propose the following algorithm: (1) no further molecular analysis should be performed for CRC exhibiting MLH1 methylation and BRAF mutation, and these cases should be considered as sporadic CRC; (2) CRC with unmethylated MLH1 and negative for BRAF mutation should be considered as Lynch syndrome; and (3) only a small fraction of CRC with MLH1 promoter methylation but negative for BRAF mutation should be true Lynch syndrome patients. These potentially Lynch syndrome patients should be offered genetic counselling before searching for MLH1 gene mutations.
林奇综合征是最常见的遗传性结直肠癌（CRC）综合征之一，由 MLH1、MSH2 及罕见的 MSH6、PMS2、MLH3 基因突变引起。虽然 MSH2 蛋白缺失可预测林奇综合征，但 MLH1 蛋白缺失并非如此。本研究旨在开发一种灵敏且经济有效的算法，以筛选 MLH1 免疫组化失活的林奇综合征患者。选择了 11 例散发性 CRC 和 16 例 MLH1 蛋白异常的林奇综合征病例。在对 15 号外显子进行 PCR 扩增后，通过直接测序分析 BRAF c.1799T > A 突变（p.Val600Glu）。通过甲基化敏感单链构象分析确定 MLH1 启动子的甲基化。在林奇综合征患者中，未发现 BRAF 突变，仅 1 例显示 MLH1 甲基化（6%）。在散发性 CRC 中，所有病例均为 MLH1 甲基化（100%），11 例中有 8 例携带上述 BRAF 突变（73%），而只有 3 例 BRAF 野生型（27%）。我们提出以下算法：（1）对于 MLH1 甲基化和 BRAF 突变的 CRC，无需进行进一步的分子分析，应将这些病例视为散发性 CRC；（2）未甲基化 MLH1 且 BRAF 突变阴性的 CRC 应考虑为林奇综合征；（3）只有一小部分 MLH1 启动子甲基化但 BRAF 突变阴性的 CRC 可能是真正的林奇综合征患者。这些潜在的林奇综合征患者在寻找 MLH1 基因突变之前应进行遗传咨询。
