AB039. SOH23ABS_198. MLH1-deficient colorectal carcinoma with wild type BRAF and MLH1 promoter hypermethylation—an understudied molecular phenotype

Background: Up to 15% of colorectal cancers (CRCs) display microsatellite instability (MSI). MSI is reflective of a deficient mismatch repair (MMR) system (dMMR). dMMR CRC represents a heterogeneous group of diseases with separate tumorigenic pathways. The majority of dMMR CRC is sporadic (>90%) and occurs due to acquired methylation of the MLH1 gene promoter. Sporadic MLH1-deficient CRC is thought to originate in lesions that arise from the serrated neoplasia pathway. Although the presence of the BRAF V600E mutation is characteristic of a sporadic cancer, as many as half of CRCs with MLH1 promoter hypermethylation will lack a BRAF mutation.

Methods: MMR status was determined by immunohistochemistry (IHC) for the MMR proteins hMLH1, hPMS2, hMSH2, and hMSH6. BRAF mutation status of MLH1 +/− PMS2 deficient specimens were evaluated using a mouse monoclonal BRAF V600E mutation-specific monoclonal antibody. The MLH1 promoter methylation status of MLH1-deficient, BRAF wild type CRC was assessed using a direct MLH1 promoter methylation assay.

Results: We report the clinicopathologic and molecular features of MLH1-hypermethylated, BRAF wild-type CRC (n=38) in comparison with MLH1-deficient, BRAF mutated CRC (n=144) and patients with suspected Lynch syndrome-associated CRC (n=56). Both MLH1-hypermethylated, BRAF wild-type CRC and MLH1-deficient, BRAF mutated CRC more commonly occurred in females (P≥0.002), had a tendency for the right colon (86.8% vs. 91.7% vs. 62.5%, P≥0.010) and had an advanced age at presentation (P<0.000) compared with the suspected Lynch group. In the tumours that had KRAS mutation analysis performed 15.7% (3/19) of the MLH1-hypermethylated BRAF wild type CRC harboured a KRAS mutation. There was no significant difference in mucinous differentiation, signet ring cell differentiation, tumour budding, poorly differentiated cluster grade, tumour-infiltrating lymphocytes or Crohn’s-like reaction between the 3 tumour groups. Both MLH1-hypermethylated BRAF, wild-type CRC and MLH1-deficient, BRAF mutated CRC appear to be associated with a poorly differentiated phenotype with loss of CDX2 expression and increased medullary morphology compared to suspected Lynch group (P≥0.046). No significant difference in disease-free survival was observed (P>0.05).

Conclusions: Our results indicate MLH1-hypermethylated, BRAF wild type CRC can harbor KRAS mutations and arise from conventional tubular/tubulovillous adenomas or serrated lesions.

Keywords: Colorectal cancer (CRC); BRAF; Lynch syndrome; microsatellite instability (MSI); mismatch repair (MMR)