Wang Cheng, Liang Dongni, Wang Wei, Kuang Wei, Zou Juan, Zeng Jing, Feng Min
Department of Pathology, West China Second University Hospital, Sichuan University, Chengdu, Sichuan, China.
Key Laboratory of Birth Defects and Related Diseases of Women and Children, Ministry of Education, Chengdu, Sichuan, China.
Front Oncol. 2025 May 21;15:1520500. doi: 10.3389/fonc.2025.1520500. eCollection 2025.
Microsatellite instability (MSI)/mismatch repair (MMR) protein testing is important for Lynch syndrome (LS) identification, prognostic stratification, and immune checkpoint inhibitor screening in many solid malignancies. MSH6, an MMR protein, is less studied in LS, and the exact mechanism of inconsistent MSI and MMR results among endometrial cancer (EC) patients who are carriers of MSH6 mutations remains unclear. The aim of this study was to identify the molecular patterns and clinicopathological characteristics of MSH6 protein-deficient LS-related EC and to further investigate possible causes of discordant MSI and IHC results in variant carriers.
Twenty-seven patients who were diagnosed with EC with only MSH6 protein deficiency from 2021 to 2023 at West China Second University Hospital were enrolled. PCR capillary electrophoresis (PCR-CE) was performed in all cases and further next-generation sequencing (NGS) was performed in non-MSI-high cases. Data on immunohistochemistry (IHC) markers, microsatellite shift patterns, and molecular profiles were further reviewed by an experienced molecular pathologist.
Among the 27 patients, 14 (51.9%) cases were found to be non-MSI-high, while only 8 of 14 (57%) cases successfully underwent NGS and ultimately incorporated into our study. All patients who were MSH6 protein negative were diagnosed with early-stage endometrioid carcinoma (EC), with a median age of 55 years (range 48-67 years). We reanalyzed the shift of all microsatellite loci and found one case with an additional unstable locus. Minimal microsatellite shifts (one to three nucleotide shift) were observed in all cases (100%), which occurred in mononucleotide markers from BAT 25 or BAT 26. Nevertheless, 3 of the 8 patients (37.5%) displayed MSI-H by NGS, which revealed truncating mutations in the MSH6 gene in exon 4 in 62.5% (5/8) of the patients, including nonsense mutations (37.5%), frameshift insertions (12.5%), and frameshift deletions (12.5%). The proportion of cases correctly classified (as determined via IHC markers) by MMR genomic status was greater (100%) than that correctly classified by PCR-CE (12.5%) in cases of MSH6 truncating variation. In addition, NGS (37.5%) had a higher MSI-H detection rate than PCR-CE (12.5%) in evaluating MSI status.
Carriers of a germline pathogenic MSH6 variant are more likely to develop EC at an advanced age, and a non-MSI-H phenotype with minimal microsatellite shift is frequently observed only when the MSH6 protein is lost. This atypical MSI pattern is often overlooked, potentially increasing the risk of underdiagnosis of LS.
微卫星不稳定性（MSI）/错配修复（MMR）蛋白检测对于许多实体恶性肿瘤的林奇综合征（LS）识别、预后分层和免疫检查点抑制剂筛查至关重要。MMR蛋白MSH6在LS中的研究较少，MSH6突变携带者的子宫内膜癌（EC）患者中MSI与MMR结果不一致的确切机制仍不清楚。本研究的目的是确定MSH6蛋白缺陷型LS相关EC的分子模式和临床病理特征，并进一步研究变异携带者中MSI和免疫组化（IHC）结果不一致的可能原因。
纳入2021年至2023年在华西第二医院被诊断为仅MSH6蛋白缺陷的EC患者27例。所有病例均进行聚合酶链反应毛细管电泳（PCR-CE），非MSI高的病例进一步进行二代测序（NGS）。由经验丰富的分子病理学家进一步复查免疫组化（IHC）标志物、微卫星移位模式和分子谱数据。
27例患者中，14例（51.9%）为非MSI高，而14例中的8例（57%）成功进行了NGS并最终纳入本研究。所有MSH6蛋白阴性的患者均被诊断为早期子宫内膜样癌（EC），中位年龄为55岁（范围48 - 67岁）。我们重新分析了所有微卫星位点的移位情况，发现1例有额外的不稳定位点。所有病例（100%）均观察到最小微卫星移位（一至三个核苷酸移位），发生在BAT 25或BAT 26的单核苷酸标志物中。然而，8例患者中有3例（37.5%）通过NGS显示为MSI-H，其中62.5%（5/8）的患者在MSH6基因外显子4中存在截短突变，包括无义突变（37.5%）、移码插入（12.5%）和移码缺失（12.5%）。在MSH6截短变异的病例中，通过MMR基因组状态正确分类（通过IHC标志物确定）的病例比例（100%）高于通过PCR-CE正确分类的比例（12.5%）。此外，在评估MSI状态时，NGS（37.5%）的MSI-H检测率高于PCR-CE（12.5%）。
种系致病性MSH6变异的携带者在老年时更易发生EC，仅当MSH6蛋白缺失时才经常观察到微卫星移位最小的非MSI-H表型。这种非典型的MSI模式常被忽视，可能增加LS漏诊的风险。
