Tissue-Specific Tumorigenesis in Multiple Endocrine Neoplasia Type I

While a germline heterozygous mutation in the MEN1 gene predisposes tumor formation in the endocrine pancreas, parathyroid glands and anterior pituitary, this tissue-specific tumorigenesis is not dependent on MEN1 mutations alone. In fact, a homozygous deletion of Men1 in the entire pancreas of a mouse results in tumor formation only in the endocrine pancreas, not in the exocrine pancreas, suggesting an endocrine tissue-specific mechanism. The MEN1 gene encodes the menin protein, which interacts with chromatin associated protein complexes, therefore engaging in epigenetic control mechanisms. Recognizing menin’s participation in epigenetic regulation led to an investigation of whether the pathogenesis of MEN1 syndrome may be related to epigenetic changes in the affected endocrine tissues. Indeed, MEN1-associated endocrine cell types exhibit various menin-dependent epigenetic mechanisms. In fact, a significant increase in methylated DNA loci was observed in MEN1 human parathyroid tumors when compared to human parathyroid adenomas and carcinomas without known MEN1 mutations. Subsequent studies revealed that loss of menin results in increased activity of DNA methyltransferase 1 (Dnmt1). Our studies have shown that Dnmt1 is transcriptionally regulated by the menin-interacting protein Rbbp5. While menin normally functions to suppress Rbbp5 activity, loss of menin activates Rbbp5, thus upregulating Dnmt1 expression, causing global DNA hypermethylation and subsequent tumorigenesis in MEN1-target endocrine tissues. In order to assess the behavior of Rbbp5 in both MEN1-target tissues and non-target tissues, Rbbp5 protein expression was analyzed in both MEN1-target tissues (endocrine pancreas, anterior pituitary, parathyroid) and non-MEN1-target tissues (kidney, lung, liver, brain, heart) of wild-type (WT) mice. We confirmed that Rbbp5 protein expression is ubiquitous throughout all of these WT mouse tissues. Since Rbbp5 is a transcriptional activator responsible for enhanced Dnmt1 gene expression, and the loss of menin causes Dnmt1 overexpression solely in MEN1-target tissues, we assessed whether Rbbp5 binds preferentially in a tissue-specific manner to the Dnmt1 promoter. We determined the presence of Rbbp5 on the Dnmt1 promoter in MEN1-target tissues (WT mouse endocrine pancreas, normal human parathyroid, WT mouse pituitary) and the absence of Rbbp5 on the Dnmt1 promoter in non-MEN1-target tissues (WT mouse liver, WT mouse kidney, WT mouse lung). These results confirmed that Rbbp5 does exhibit MEN1-target-tissue-specific occupancy at the Dnmt1 promoter. This endocrine-specific localization of Rbbp5 to the Dnmt1 promoter suggests the presence of additional tissue-specific factors (with tissue-specific expression or interactions/activity) that must be validated and tested further.