Promoter hypermethylation profile of cell cycle regulator genes in pituitary adenomas

Abstract

Aberrant hypermethylation of CpG islands in the promoter region plays a causal role in the inactivation of various key genes involved in the cell cycle regulatory cascade, which could result in a loss of cell cycle control. The aim of the present study was to examine in more detail the prevalence and role of the promoter methylation of genes with a proven involvement in the cell cycle regulation of pituitary adenomas, since their tumorigenesis has not yet been clearly defined. We profiled the CpG island methylation status of a series of well-characterized cell cycle regulation genes: the RB1, p14(ARF), p15(INK4b), p16(INK4a), p21(Waf1/Cip1), p27(Kip1), and p73 genes, in 34 pituitary adenomas as determined by a methylation-specific polymerase chain reaction assay. Promoter hypermethylation of the RB1, p14(ARF), p15(INK4b), p16(INK4a), p21(Waf1/Cip1), p27(Kip1), and p73 genes was detected in 12 (35%), 2 (6%), 11 (32%), 20 (59%), 1 (3%), 0 (0%), and 4 (12%) of the adenomas, respectively. In total, 88% (30 of 34) of the adenomas displayed methylation of at least one of such cell cycle regulatory genes, especially methylation of the member genes of the RB1 pathway (29 of 34; 85%). Promoter hypermethylation of p15(INK4b) coincided with RB1 and/or p16(INK4a) methylation, whereas RB1 and p16(INK4a) methylations tended to be mutually exclusive (p = 0.0048). Furthermore, promoter hypermethylations of p14(ARF), p21(Waf1/Cip1), and p73 (not belonging to the member genes of the RB1 pathway) were also coincident with RB1 and/or p16(INK4a) methylation except in one p73 methylated case. In contrast, none of the clinicopathological features, including the cell proliferation index, was significantly correlated with any particular methylation status. Our results suggested that aberrant hypermethylation of the key cell cycle regulatory genes occurs at a relatively high frequency in pituitary adenomas, especially in RB1 pathway genes with promoter hypermethylation of the p16(INK4a) gene being the most common deregulation. We further obtained evidence to indicate that RB1 and p16(INK4a) methylations tended to be mutually exclusive, but did occasionally coincide with other cell cycle regulation gene methylations.

Cell cycle control of pituitary development and disease

“…A detailed analysis suggested that CDKN2A methylation was confined to particular adenoma subtypes (Simpson et al 1999) and these findings were subsequently confirmed by several other groups concluding that hypermethylation of the CDKN2A is the most common epigenetic deregulation in these neoplasias (Morris et al 2005, Ogino et al 2005, Yoshino et al 2007 INK4a is able to inhibit cell proliferation in pituitary tumor cells in correlation with a shift in the phosphorylation status of pRB, suggesting the relevance of this CDK inhibitor in the activation of pRB and pituitary tumor suppression (Frost et al 1999).…”

“…Cip1 are not commonly observed, this inhibitor may also be downregulated through epigenetic modifications in pituitary neoplasias (Yoshino et al 2007, Zhu et al 2008.…”

Role of the high mobility group A proteins in the regulation of pituitary cell cycle

“…INK4A by methylation is frequent in pituitary, mainly NFPA (Woloschak et al 1997, Simpson et al 1999a, Morris et al 2005, Ogino et al 2005, Yoshino et al 2007). Downregulation of p27 kip1 protein expression, likely due to a JAB1-mediated increased proteolysis (Korbonits et al 2002), is frequent in pituitary carcinomas and ACTH-secreting adenomas (Bamberger et al 1999, Lidhar et al 1999.…”

The Genetics of Pituitary Adenomas

“…Hyper-methylation of the promoter region also accounts for the loss of p16 INK4 protein expression, which is relatively frequent in PA (Simpson et al, 1999). RB and p16 INK4a methylations tended to be mutually exclusive (Yoshino et al, 2007).…”

“…As for pRB and p16 INK4a , down-regulation of p21 Cip1 and p27 kip1 in pituitary adenomas may also be due to epigenetic modifications, including DNA and/or histone methylation (Yoshino et al, 2007, Zhu et al, 2008.…”