MUC5AC hypomethylation is a predictor of microsatellite instability independently of clinical factors associated with colorectal cancer

Colorectal cancers (CRC) with microsatellite instability (MSI) display unique clinicopathologic features including a mucinous pattern with frequent expression of the secreted mucins MUC2 and MUC5AC. The mechanisms responsible for this altered pattern of expression remain largely unknown. We quantified DNA methylation of mucin genes (MUC2, MUC5AC, MUC4) in colonic cancers and examined the association with clinicopathological characteristics and molecular (MSI, KRAS, BRAF, and TP53 mutations) features. A control cohort was used for validation. We detected frequent hypomethylation of MUC2 and MUC5AC in CRC. MUC2 and MUC5AC hypomethylation was associated with MUC2 and MUC5AC protein expression (p=0.004 and p<0.001, respectively), poor differentiation (p=0.001 and p=0.007, respectively) and MSI status (p<0.01 and p<0.001, respectively). Interestingly, MUC5AC hypomethylation was specific to MSI cancers. Moreover, it was significantly associated with BRAF mutation and CpG island methylator phenotype (p<0.001 and p<0.001, respectively). All these results were confirmed in the control cohort. In the multivariate analysis, MUC5AC hypomethylation was a highly predictive biomarker for MSI cancers. MUC5AC demethylation appears to be a hallmark of MSI in CRC. Determination of MUC5AC methylation status may be useful for understanding and predicting the natural history of CRC. What's new? Colorectal cancers (CRC) with microsatellite instability (MSI) display unique clinicopathological features including a mucinous pattern with frequent expression of MUC2 and MUC5AC. The mechanisms underlying such altered pattern of expression, however, remain unclear. This is the first study investigating the methylation profile of mucin genes in different CRC subgroups. It shows that MUC2 and MUC5AC expression in CRC is partially regulated by DNA methylation, and that such MUC5AC regulation is specific for MSI tumors. MUC5AC hypomethylation is a strong predictor of MSI in CRC independently of other clinicopathological characteristics and may serve as a specific marker of the serrated pathway.