Histomorphology and Molecular Genetics in Different Intra-Tumoral Zones and Matched Metastatic Lymph Nodes of Colorectal Cancer with Heterogenous Mismatch Repair Status

Abstract Objective: To better understand the clinicopathological characteristics and molecular alterations in different intra-tumoral components of colorectal cancer (CRC) with heterogeneity of mismatch repair (MMR) protein expression and microsatellite instability (MSI) status. Methods: We identified 4 cases of CRC with heterogenous MMR protein expression and analyzed the histopathological features, MSI status and other molecular alterations in separately microdissected intra-tumoral zones and lymph node metastases by polymerase chain reaction (PCR) -based MSI testing, MLH1 promoter methylation and targeted next-generation sequencing (NGS). Results: Microsatellite instability-high (MSI-H ) was identified in the MLH1/PMS2 deficient zones in Case 1-3, and in the MSH2/MSH6 deficient zone in Case 4, while MSS was in all the intra-tumoral zones and metastatic lymph nodes with proficient MMR (pMMR). Furthermore, heterogeneity of MLH1 promoter methylation and/or other common driving gene mutations of CRC, such as KRAS, PIK3CA and so on, was identified in all the 4 CRCs. In addition, 75% (3/4) of cases showed heterogeneity of histomorphology in intra-tumoral components and metastatic lymph nodes (Case 1, 2, 4), and all the corresponding metastatic lymph nodes were moderate differentiation with MSS/pMMR (Case 2, 3). Conclusions: The heterogeneous MSI status is highly correlated with histomorphological heterogeneity, which is also an important clue for the heterogeneity of drive gene mutations in CRC. These results suggest that it is essential to detect MMR protein expression and other gene mutations in metastases before treatment, especially for the CRCs with heterogenous MMR protein expression or histomorphology.