MLH1 gene promoter methylation status partially overlaps with CpG methylator phenotype (CIMP) in colorectal adenocarcinoma

Background: RAS/BRAF mutations, mismatch DNA repair complex deficiency (MMRd)/microsatellite instability (MSI), and CpG methylator phenotype (CIMP) are key molecular actors in colorectal carcinogenesis. To date, conflicting evidence about the correlations between these molecular features has been reported.

Materials and methods: A retrospectively selected cohort of 123 CRCs was divided into 3 groups based on the molecular characteristics: MMR proficient (MMRp)/BRAF p.V600E mutated (BRAF^mut), MMRd/BRAF^mut, and MMRd/BRAF wild type (BRAF^wt). MLH1 promoter (pMLH1) methylation status was assessed by pyrosequencing. For 82 samples the CIMP phenotype was evaluated using the EpiTect® MethyLight kit.

Results: The MMRd/BRAF^mut group showed a higher pMLH1 methylation rate compared to both the MMRd/BRAF^wt and the MMRp/BRAF^mut groups. Overall, the two MMRd groups had a higher methylation rate compared to the MMRp cases independently from the mutational status of BRAF (p-value <0.0001). The MMRd/BRAF^mut group was characterized by a 90.0 % of CIMP high (CIMP-H) tumors of which 97.2 % were pMLH1 methylated. Instead, the MMRd/BRAF^wt group presented 50.0 % of CIMP-H adenocarcinomas.

Conclusions: Our study demonstrates that pMLH1 hypermethylation, MMRd, BRAF^mut and CIMP phenotype do not completely overlap in CRC. These findings further refine the knowledge on the molecular landscape of CRC and may have critical implications also for the clinical management of the disease.