Investigating Transcriptional Regulation Within Bone Development: Characterization of HRPT2/CDC73 In Vivo and the Effects of Ascorbic Acid in Osteoblast Differentiation In Vitr

The integrity of the mechanisms that control gene transcription during development and in post-natal life is essential to maintain tissue homeostasis and impede the development of genetic diseases such as cancer. Inheritance of a defective hyperpatahyroidism 2 (HRPT2) allele, an essential regulator of gene transcription, predisposes individuals to a constellation of symptoms ranging from endocrine abnormalities to parathyroid adenomas and jaw bone tumors (HPT-JT). In order to elucidate the function of the HRPT2 gene and the pathogenesis that results upon spontaneous inactivation in familial cases of parathyroid cancer and HPT-JT, mouse models were generated that allow for deletion of Hrpt2 within different stages and tissues during development. We have used the Hrpt2 flox mouse model to delete Hrpt2 in mesenchymal progenitor cells as well as committed, terminally differentiated osteoblasts and osteocytes. Whereas loss of Hrpt2 in mesenchymal progenitors was embryonic lethal, genetic deletion of Hrpt2 in mature bone forming cells led to increased bone mass and bone strength. However, Hrpt2 conditional knockout bones had increased cortical porosity and osteocyte apoptosis associated with increased osteoblast specific gene expression. This work suggests that Hrpt2 is required for cell proliferation and differentiation and acts as a transcriptional repressor in terminally differentiated cell types.;Control of gene transcription defines cell identity and fate. Ascorbic acid (AA, also known as vitamin C) is an essential vitamin for humans and is well known for its role in collagen synthesis. AA acts as a cofactor for TET enzymes, which hydroxylate methylated cytosines. Here, we characterize how 7 days of AA treatment causes changes in gene transcription, 5-hydroxymethylcytosine deposition, and the active chromatin marks H3K4me3 and H3k27ac in MC3T3-E1 murine pre-osteoblasts cells, initiating cell differentiation and expression of the osteoblast phenotype. Though 5hmC deposition was not specific for only highly expressed genes, it was highly enriched at transcriptional start sites and CpG islands. While H3K4me3 was mostly unchanged, H3K27ac was predictive of driving gene expression. This work suggests that AA causes dramatic changes to the epigenome through epigenetic modifiers to impact cell differentiation.