Clinicopathological Characteristics and Outcomes of Colorectal Cancer With Heterogenous Staining of Mismatch Repair Protein.

Background: Scant data are available on heterogenous staining of mismatch repair protein in colorectal cancer.

Objective: This study aimed to improve insights into clinicopathologic features and prognosis of colorectal cancer harboring heterogenous mismatch repair protein staining.

Design: A single-center retrospective observational study.

Setting: This study was conducted in a tertiary referral center in China between 2014 and 2018.

Patients: Colorectal cancers with heterogenous staining of mismatch repair protein were included.

Main outcomes measures: Clinicopathologic and molecular features, and survival outcomes were analyzed.

Results: A total of 151 out of 6721 colorectal cancers (2.2%) exhibited heterogenous staining for at least one mismatch repair protein, with intraglandular heterogeneity being the most common pattern (89.4%). Heterogenous MLH1 staining was significantly associated with distant metastasis (p = 0.03), while heterogenous MSH2 staining was associated with left-sided (p = 0.03) and earlier pT stage tumors (p = 0.02). The rates of microsatellite instability-high, KRAS and BRAF mutation were 12.6%, 47.3% and 3.4%, respectively. Microsatellite instability-high was significantly associated with higher intraglandular MSH6 heterogeneity frequency (p < 0.001) and decreased MSH6 expression level (< 27.5%, p = 0.01). BRAF mutation was associated with the coexistence of intraglandular and clonal heterogeneity (p = 0.003) and decreased PMS2 expression level (p = 0.01). Multivariable analysis revealed that progression-free survival was significantly associated with tumor stage (p = 0.003), stroma fraction (p = 0.004), and heterogenous PMS2 staining (p = 0.02). Overall survival was linked to tumor stage (p = 0.006) and BRAF mutation (p = 0.01).

Limitations: The limitations of this study include the absence of testing for MLH1 promoter methylation and mismatch repair gene mutations, its retrospective design, and insufficient data related to direct comparison with deficient mismatch repair and proficient mismatch repair colorectal cancer.

Conclusions: Heterogenous mismatch repair protein staining in colorectal cancer exhibits distinct associations with tumor location, stage, microsatellite instability, BRAF mutation and prognosis. It is recommended to report MSH6 heterogeneity as it may indicate microsatellite instability-high. See Video Abstract.