Evaluation of global DNA hypomethylation in human prostate cancer and prostatic intraepithelial neoplasm tissues by immunohistochemistry.

OBJECTIVE

To identify the differences of 5-methylcytosine (5-MC) level between primary prostate cancer tissues (PCTs), prostate cancer-adjacent benign tissues (PCABTs), low-grade prostatic intraepithelial neoplasia (LGPIN), and high-grade prostatic intraepithelial neoplasia (HGPIN), and further analysis the 5-MC alterations in prostate cancer with pathologic grade and clinical prognosis.

MATERIALS AND METHODS

Immunohistochemistry method with a 5-MC monoclonal antibody was used to identify the 5-methylcytosine (5-MC) levels in PCTs, PCABTs, LGPIN, and HGPIN specimens in the present study. Statistical analysis with SPSS software (SPSS Inc., Chicago, IL) was used to compare differences of 5-MC levels in the four groups and evaluate the 5-MC alterations in prostate cancer with pathologic grade and clinical prognosis.

RESULTS

We found that 38 of 48 (79.1%) patients studied showed a decrease in 5-MC staining of PCTs compared with PCABTs. The difference in the methylation levels for the PCTs and the PCABTs was highly statistically significant (P < 0.001). Spearman correlation showed there was no statistically significant association between the average score of 5-MC staining and Gleason score sum. Kaplan-Meier survival analysis showed that patient group with no or weak 5-MC staining compared with group with moderate and strong 5-MC staining was associated with better survival of patients, although there was no statistically significant difference between the 2 groups in predicating prognosis (P = 0.385). The average scores of 5-MC staining for LGPIN, HGPIN, PCABTs, and PCTs groups were 6.91, 1.58, 6.63, and 3.10, respectively. The methylation level of HGPIN group, as well as that of PCTs group, was significantly lower than those of LGPIN (P < 0.001; P < 0.001) and PCABTs groups (P < 0.001; P < 0.001), respectively, with the 5-MC levels of PCABTS group similar to that of LGPIN group (P = 0.476). 5-MC levels of HGPIN group was lower than that PCTs group (P = 0.004).

CONCLUSIONS

We found that global DNA methylation was low in most prostate cancer compared with benign regions from the same patient's sections. None of the DNA hypomethylation changes in primary cancers were associated with pathologic grade and clinical prognosis. In addition, immunohistochemistry showed that the global methylation was lower in HGPIN compared with LGPIN and methylcytosine staining in HGPIN was lower than that of PCTs. The results suggest that global DNA hypomethylation might play an important role in the process of prostate cancer initiation rather than progression.