Epigenetic change in pituitary tumorigenesis

Abstract

Throughout the genome CpG dinucleotides are found at one-fifth of their expected frequency and their rarity is further marked by the fact that 70% are methylated. In contrast, CpG islands (CGI), found associated with the promoters of many genes, have maintained their expected frequency of this dinucleotide, and remain unmethylated. Inappropriate methylation of CGIs is associated with histone deacetylation and gene silencing, while methylation of CpGs outside of CGIs is associated with significantly higher mutation rates. Methylation of CGIs is a frequent event in numerous tumour types including those that arise within the pituitary gland. Several studies now show highly frequent methylation of the p16 gene that is significantly associated with loss of cognate protein and that appears to be an early change in pituitary tumorigenesis. Collectively, studies show that somatotrophinomas are an infrequent target for p16 CGI methylation. However, in this pituitary tumour subtype, loss of pRb is associated with either CGI methylation or micro-deletion within the protein-pocket binding domain. As in other tumour types loss of p16 or RB1 appear to be mutually exclusive events in non-functional adenomas and somatotrophinomas respectively. Investigation of the Death Associated Protein Kinase gene shows that loss of its protein (DAPK), a pro-apoptotic molecule, in pituitary tumours is also associated with either methylation or deletion within its associated CGI. In the case of DAPK, however, these changes segregate with invasive pituitary tumours irrespective of tumour subtype. Methylation represents a positive signal that can be detected with exquisite sensitivity; in addition, this change targets multiple genes that show tumour type specificity. Taken together, the detection of DNA methylation changes, using either a panel of predefined marker-islands, or CGI arrays, provides the opportunity to generate "methylation profiles". This new knowledge will increase our understanding of tumour biology and could ultimately aid medical management in these different tumour types, including those of pituitary origin.