Mechanistic Studies Of A Novel Ppar-Gamma Mutant That Causes Lipodystrophy And Diabetes

PPAR-gamma is a nuclear receptor that plays a central role in metabolic regulation by regulating extensive gene expression networks in adipose, liver, skeletal muscle and many other tissues. Human PPAR-gamma mutations are rare and cause a monogenetic form of severe type II diabetes with metabolic syndrome, known as familiar partial lypodystrophy. The E157D PPAR-gamma mutant causes atypical lipodystrophy in a large Canadian kindred, presenting with multiple musculoskeletal, neurological and hematological abnormalities in addition to the classic lipodystrophy features of insulin-resistant diabetes, hypertension and dyslipidemia. This mutation is localized to the p-box of PPAR-gamma, a small region that interacts directly with the DNA molecule and is required for DNA binding site specificity. Mechanistic analysis revealed that E157D PPAR-gamma binds PPAR-gamma response elements (PPREs), but is mildly, moderately or severely defective at inducing transcription from most promoters, without dominant negative activity. This suppression of transcriptional activity may be mediated by an increased effect of nuclear receptor corepressors on E157D PPAR-gamma. In addition, the mutant binds atypical PPREs in the regulatory regions of a small set of genes outside of the PPAR-gamma-regulated network, and induces transcription of these genes. The loss of transcriptional activity on PPAR-gamma-regulated promoters leads to the metabolic disease in the E157D PPAR-gamma cohort, while the gain of activity on non- PPAR-gamma target promoters may explain the atypical clinical presentation associated with this mutation. The misregulation of target genes by this DNA-contacting mutant highlights a previously under-appreciated importance of the DNA molecule as an allosteric regulator of the transcriptional activity of nuclear receptors. In summary, this dissertation describes a human PPAR-gamma mutation that works through a novel mechanism to cause atypical lipodystrophy, and provides support for a more integrated view of the nuclear receptor-DNA interaction and transcription activation