Bioinformatics and Functional Genomics Center, Department of Biomedical Sciences, Cedars-Sinai Cancer, Cedars-Sinai Medical Center, Los Angeles, California.
Department of Medicine (Oncology), Stanford University, Stanford, California.
J Natl Compr Canc Netw. 2023 Jul;21(7):743-752.e11. doi: 10.6004/jnccn.2023.7020.
Most mismatch repair-deficient (MMRd) colorectal cancer (CRC) cases arise sporadically, associated with somatic MLH1 methylation, whereas approximately 20% have germline mismatch repair pathogenic variants causing Lynch syndrome (LS). Universal screening of incident CRC uses presence of MLH1 methylation in MMRd tumors to exclude sporadic cases from germline testing for LS. However, this overlooks rare cases with constitutional MLH1 methylation (epimutation), a poorly recognized mechanism for LS. We aimed to assess the frequency and age distribution of constitutional MLH1 methylation among incident CRC cases with MMRd, MLH1-methylated tumors.
In retrospective population-based studies, we selected all CRC cases with MMRd, MLH1-methylated tumors, regardless of age, prior cancer, family history, or BRAF V600E status, from the Columbus-area HNPCC study (Columbus) and Ohio Colorectal Cancer Prevention Initiative (OCCPI) cohorts. Blood DNA was tested for constitutional MLH1 methylation by pyrosequencing and real-time methylation-specific PCR, then confirmed with bisulfite-sequencing.
Results were achieved for 95 of 98 Columbus cases and all 281 OCCPI cases. Constitutional MLH1 methylation was identified in 4 of 95 (4%) Columbus cases, ages 34, 38, 52, and 74 years, and 4 of 281 (1.4%) OCCPI cases, ages 20, 34, 50, and 55 years, with 3 showing low-level mosaic methylation. Mosaicism in blood and normal colon, plus tumor loss of heterozygosity of the unmethylated allele, demonstrated causality in 1 case with sample availability. Age stratification showed high rates of constitutional MLH1 methylation among younger patients. In the Columbus and OCCPI cohorts, respectively, these rates were 67% (2 of 3) and 25% (2 of 8) of patients aged <50 years but with half of the cases missed, and 75% (3 of 4) and 23.5% (4 of 17) of patients aged ≤55 years with most cases detected.
Although rare overall, a significant proportion of younger patients with MLH1-methylated CRC had underlying constitutional MLH1 methylation. Routine testing for this high-risk mechanism is warranted in patients aged ≤55 years for a timely and accurate molecular diagnosis that will significantly alter their clinical management while minimizing additional testing.
大多数错配修复缺陷（MMRd）结直肠癌（CRC）病例是散发性的，与体细胞 MLH1 甲基化有关，而大约 20%的病例存在导致林奇综合征（LS）的种系错配修复致病性变异。对新发病例 CRC 的普遍筛查使用 MMRd 肿瘤中 MLH1 甲基化来排除 LS 种系检测的散发性病例。然而，这忽略了具有固有 MLH1 甲基化（表观遗传突变）的罕见病例，这是 LS 的一种认识不佳的机制。我们旨在评估新发病例 CRC 中 MMRd、MLH1 甲基化肿瘤中固有 MLH1 甲基化的频率和年龄分布。
在回顾性基于人群的研究中，我们从哥伦布地区 HNPCC 研究（哥伦布）和俄亥俄州结直肠癌预防倡议（OCCPI）队列中选择了所有 MMRd、MLH1 甲基化肿瘤的 CRC 病例，无论年龄、既往癌症、家族史或 BRAF V600E 状态如何。通过焦磷酸测序和实时甲基化特异性 PCR 对血液 DNA 进行固有 MLH1 甲基化检测，然后用亚硫酸氢盐测序进行确认。
对 98 例哥伦布病例中的 95 例和 281 例 OCCPI 病例中的所有病例进行了结果分析。在 95 例哥伦布病例中有 4 例（4%）、年龄分别为 34、38、52 和 74 岁，281 例 OCCPI 病例中有 4 例（1.4%）、年龄分别为 20、34、50 和 55 岁，发现了固有 MLH1 甲基化，其中 3 例显示低水平镶嵌甲基化。在有样本可供分析的情况下，血液和正常结肠中的镶嵌性以及未甲基化等位基因的杂合性丢失证明了因果关系。年龄分层显示年轻患者中固有 MLH1 甲基化的发生率较高。在哥伦布和 OCCPI 队列中，年龄<50 岁的患者中分别有 67%（2 例）和 25%（2 例），但有一半病例被遗漏，而年龄≤55 岁的患者中分别有 75%（3 例）和 23.5%（4 例），大多数病例都被检测到。
尽管总体上很少见，但相当一部分 MLH1 甲基化 CRC 的年轻患者存在潜在的固有 MLH1 甲基化。对于年龄≤55 岁的患者，应进行这种高风险机制的常规检测，以便及时、准确地进行分子诊断，这将显著改变他们的临床管理，同时最大限度地减少额外的检测。
