INTRODUCTION: EMAST is a poorly understood form of microsatellite instability (MSI) in colorectal cancer (CRC) for which loss of MSH3 has been proposed as the underlying mechanism, based on experimental studies. We aimed to evaluate whether MSH3 loss is associated with EMAST in CRC. METHODS: A consecutive cohort of patients with stage I-III CRC. Digital image analysis using heatmap-derived hot spots investigated MSH3 expression by immunohistochemistry. Fragment analysis of multiplex PCR was used to assess MSI and EMAST, and results cross-examined with MSH3 protein expression. RESULTS: Of 152 patients, EMAST was found in 50 (33%) and exclusively in the colon. Most EMAST-positive cancers had instability at all 5 markers, and EMAST overlapped with MSI-H in 42/50 cases (84%). The most frequently altered tetranucleotide markers were D8S321 (38.2% of tumors) and D20S82 (34.4%). Subjective evaluation of MSH3 expression by IHC in tumor found ≤10% negative tumor cells in all samples, most being ≤5% negative. Digital analysis improved the detection but showed a similar spread of MSH3 loss (range 0.1–15.7%, mean 2.2%). Hotspot MSH3 negativity ranged between 0.1 to 95.0%, (mean 8.6%) with significant correlation with the whole slide analysis (Spearman's rho = 0.677 P < .001). Loss of MSH3 expression did not correlate with EMAST. CONCLUSIONS: In a well-defined cohort of patients with CRC, loss of MSH3 was not associated with EMAST. Further investigation into the mechanisms leading to EMAST in CRC is needed.

简介： EMAST 是结直肠癌 (CRC) 中一种人们知之甚少的微卫星不稳定性 (MSI) 形式，根据实验研究，MSH3 的缺失被认为是其潜在机制。我们的目的是评估 MSH3 丢失是否与 CRC 中的 EMAST 相关。方法：连续队列 I-III 期 CRC 患者。使用热图衍生的热点进行数字图像分析，通过免疫组织化学研究 MSH3 表达。使用多重 PCR 片段分析来评估 MSI 和 EMAAST，并将结果与​​ MSH3 蛋白表达进行交叉检验。结果：在 152 名患者中，50 名患者（33%）发现 EMAST，且仅存在于结肠中。大多数 EMAAST 阳性癌症在所有 5 个标志物上均不稳定，并且 EMAAST 与 MSI-H 在 42/50 例 (84%) 的病例中重叠。最常改变的四核苷酸标记是 D8S321（占肿瘤的 38.2%）和 D20S82（占肿瘤的 34.4%）。通过 IHC 对肿瘤中 MSH3 表达的主观评估发现，所有样本中≤10% 阴性肿瘤细胞，大多数≤5% 阴性。数字分析改进了检测，但显示 MSH3 损失的分布相似（范围 0.1-15.7%，平均 2.2%）。热点 MSH3 阴性范围为 0.1 至 95.0%（平均 8.6%），与整个载玻片分析显着相关（Spearman's rho = 0.677 P  < .001）。MSH3 表达的丧失与 EMAAST 不相关。结论：在明确的 CRC 患者队列中，MSH3 缺失与 EMAAST 无关。需要进一步研究导致 CRC 中 EMAST 的机制。