PTGER2 overexpression in colorectal cancer is associated with microsatellite instability, independent of CpG island methylator phenotype

Abstract

Background: Prostaglandin-endoperoxide synthase 2 (PTGS2, the HUGO Gene Nomenclature Committee-approved official symbol for cycloxygenase-2, COX-2) and its enzymatic product prostaglandin E2 have critical roles in inflammation and carcinogenesis through the G protein-coupled receptor PTGER2 (EP2). The PTGS2 (COX-2) pathway is a promising target for cancer therapy and chemoprevention. PTGS2 (COX-2) expression in colon cancer has been inversely associated with survival as well as tumoral microsatellite instability (MSI) and the CpG island methylator phenotype (CIMP). However, the prognostic significance of PTGER2 expression or its relationship with MSI, CIMP, LINE-1 hypomethylation, or PTGS2 (COX-2) remains uncertain.

Methods: Using the database of 516 colorectal cancers in two prospective cohort studies with clinical outcome data, we detected PTGER2 overexpression in 169 (33%) tumors by immunohistochemistry. We analyzed MSI using 10 microsatellite markers; CIMP by MethyLight (real-time methylation-specific PCR) on an eight-marker panel [CACNA1G, CDKN2A (p16), CRABP1, IGF2, MLH1, NEUROG1, RUNX3 and SOCS1]; BRAF, KRAS, PIK3CA, and methylation in LINE-1 by Pyrosequencing; and CTNNB1 (beta-catenin) and TP53 (p53) by immunohistochemistry.

Results: PTGER2 overexpression was positively associated with the mucinous component (P = 0.0016), signet ring cells (P = 0.0024), CIMP-high (P = 0.0023), and MSI-high (P < 0.0001). In multivariate analysis, the significant relationship between PTGER2 and MSI-high persisted (adjusted odds ratio, 2.82; 95% confidence interval, 1.69-4.72; P < 0.0001). PTGER2 was not significantly associated with PTGS2 (COX-2), TP53, or CTNNB1 expression, patient survival, or prognosis.

Conclusion: PTGER2 overexpression is associated with MSI-high in colorectal cancer.

Impact: Our data imply potential roles of inflammatory reaction by PTGER2 upregulation in carcinogenic process to MSI-high colorectal cancer.

Figure 1

PTGER2 expression in colorectal cancer.

1. PTGER2 expression in colon cancer cells (white arrowheads).

2. No PTGER2 overexpression in colon cancer cells (black arrowheads).

3. Frequencies of MSI/CIMP subtypes of colorectal cancer according to PTGER2 expression status. Note the relationship between MSI and PTGER2 regardless of CIMP status.

4. Schematic representation of the MSI-high pathway and non-MSI-high pathway. We hypothesize that PTGER2 expression plays a role in the MSI-high pathway whereas PTGS2 (COX-2) expression plays a role in the non-MSI-high pathway.

CIMP, CpG island methylator phenotype; MSI, microsatellite instability; MSS, microsatellite stable.

Figure 2

Kaplan-Meier curves for colorectal cancer-specific survival (upper panel) and overall survival (lower panel) according to PTGER2 status in colorectal cancer. The bottom table shows the number of patients who were alive and at risk of death at each time point after the diagnosis of colorectal cancer.