Infrequent methylation of the DUSP6 phosphatase in endometrial cancer

Objective: Dual-specificity phosphatase six (DUSP6, MKP3, or PYST1) dephosphorylates phosphotyrosine and phosphothreonine residues on ERK-2 (MAPK1) to inactivate the ERK-2 kinase. DUSP6 is a critical regulator of the ERK signaling cascade and has been implicated as a tumor suppressor. DNA methylation in the first intron of DUSP6 abrogates expression in a subset of pancreatic cancers. We sought to determine whether DUSP6 was similarly silenced by methylation in endometrial cancer, a tumor type in which there is frequent activation of the ERK pathway. Methods: One hundred and nine endometrial cancers were analyzed for DUSP6 methylation using combined bisulfite restriction analysis (COBRA). The cohort included 70 primary endometrioid endometrial cancers, 21 primary endometrial tumors of adverse histological types, and 18 endometrial cancer cell lines. Primary tumors, cell lines, and normal endometrial tissues were analyzed for DUSP6 mRNA levels using quantitative RT-PCR and pERK levels by Western blots and/or immunohistochemistry. Results: Methylation of the first intron of the DUSP6 gene was seen in 1/91 primary endometrial cancers investigated. The methylated tumor was also methylated at the more 5' regulatory region of DUSP6. Q-RT-PCR revealed that DUSP6 transcript levels varied widely in primary endometrial tumors. DUSP6 mRNA levels did not correlate with pERK status in primary tumors, consistent with the existence of negative feedback loops activated by pERK that result in transcription of DUSP6. Conclusion: DUSP6 methylation is a rare event in endometrial cancer. Silencing of the DUSP6 phosphatase is unlikely to contribute to constitutive activation of the ERK kinase cascade in endometrial cancer.