Progesterone receptor B gene inactivation and CpG hypermethylation in human uterine endometrial cancer.

The expressions of two isoforms of human progesterone receptor (PR) are under the control of the two different promoters. Recent studies revealed differences between these isoforms, PRA and PRB, in their expression and function in endometrial cells. Aberrant methylation of normally unmethylated CpG islands has been associated with inactivation of several genes in human cancers. In this study, we investigated the methylation status and the expression of the two different PR isoforms, PRA and PRB, in uterine endometrial carcinoma (UEC) using methylation-specific PCR (MSP), reverse transcription-PCR (RT-PCR), the 5' rapid amplification of cDNA ends method (5'RACE), and immunohistochemical staining. The results of RT-PCR and 5'RACE suggest that only PRB is inactivated, although PRA is activated in all UEC cell lines. Treatment with a demethylating agent, 5-aza-2'-deoxycytidine, restored PRB expression in all cell lines, suggesting that inactivation of this gene is through methylation. By MSP and direct DNA sequencing, PRB was methylated, whereas PRA was unmethylated in all of the cell lines. To determine the methylation status of PRB in UEC patients, we investigated 83 cancerous and 33 normal samples. Sixty-two of 83 cancer samples had only methylated alleles of PRB, although all cancer samples had only unmethylated PRA alleles. Seventy-one of 83 cancer samples were negative for PRB expression. All 62 cancer samples that had only methylated PRB alleles were negative for PRB expression. No significant changes were observed in PRA methylation status or immunohistochemistry positivity in normal and cancer samples. To determine whether de novo methylation of PRB occurred in UEC patients, we studied 32 pairs of cancer and normal samples from the same patient. Twenty of 32 cancer samples had only methylated PRB alleles, although all 32 normal samples had only unmethylated PRB alleles. The loss of unmethylated alleles was well correlated with negativity in immunohistochemical staining for PRB. This is the first report of the selective methylation and the subsequent silencing of PRB in uterine endometrial cancer.