Background: The prognostic significance of microsatellite instability (MSI) in colorectal cancers (CRCs) has been addressed in many studies since the initial description of better survival rates in MSI-positive (MSI+) tumors than in MSI-negative (MSI-) tumors. Recent studies have demonstrated that a higher degree of hypomethylation of long interspersed nuclear element-1 (L1) is related to poor prognosis of CRCs and that a wide variation of L1 methylation levels exist within MSI+ CRCs. Our aim was to identify whether L1 and Alu methylation status could predict clinical outcomes within MSI+ CRCs.
Methods: We analyzed 207 MSI+ CRCs for their methylation levels in L1 and Alu repetitive DNA elements using pyrosequencing and correlated them with clinicopathological information including survival data.
Results: Univariate survival analysis showed that low Alu methylation status (<18.60%) and low L1 methylation status (<53.00%) were significantly associated with shorter overall survival time (log-rank test, P = 0.009 and P < 0.001, respectively). Multivariate analysis using nine parameters (Alu methylation status, L1 methylation status, patient's age, disease stage [tumor, node, metastasis staging system], differentiation, Crohn-like lymphoid reaction, KRAS/BRAF mutation status, CpG island methylator phenotype [CIMP] status, and peritumoral lymphocytic infiltration), which were significantly prognostic in MSI+ CRCs, revealed that low L1 methylation status was an independent prognostic factor of MSI+ CRCs (P = 0.009), whereas low Alu methylation status was not.
Conclusions: Clinical outcomes of MSI+ CRCs depend on L1 methylation status, suggesting that lower L1 methylation status serves as a significant prognostic parameter of adverse prognosis in MSI+ CRCs.