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Levine Lab Research Overview
Researchers in the Levine Lab investigate bone and mineral metabolism, including disorders of parathyroid development, vitamin D homeostasis, and mechanisms for G-protein-coupled signal transduction.
The Role of G Proteins in Human Disease
Dr. Levine elucidated the pathophysiology of parathyroid hormone (PTH) resistance in pseudohypoparathyroidism (PHP) through his discovery that levels of Gas, the alpha chain of the heterotrimeric G protein Gs that couples heptahelical receptors to activation of adenylyl cyclase, were reduced in cells from patients with PHP who had features of Albright Hereditary Osteodystrophy (AHO), but normal in PHP patients who lacked these features. This led to the dichotomization of PHP type I into two subtypes: PHP type Ia and PHP type Ib. Dr. Levine showed that patients with PHP type Ia were resistant to multiple hormones that required normal levels of Gas for receptor coupling to adenylyl cyclase, whereas patients with PHP type Ib were resistant only to PTH. These observations led to the first demonstration of mutations in the GNAS gene as the basis for AHO and refined the mechanism for disordered imprinting of GNAS in the pathophysiology of PHP1b.
Molecular Mechanisms for Hypoparathyroidism
Dr. Levine and colleagues were first to show the relationship between hypomethylation of the promoter region of the PTH gene and expression of PTH in the parathyroid gland. He subsequently published the first comprehensive molecular genetic analysis of familial hypoparathyroidism, which applied the term "isolated hypoparathyroidism" to patients with non-syndromic genetic hypoparathyroidism. His careful analysis of pedigrees disclosed various modes of inheritance of isolated hypoparathyroidism, which supported the concept of genetic heterogeneity in this disorder. Moreover, his studies led to the identification of the first mutations in the genes encoding PTH and GCM2 as novel causes of isolated hypoparathyroidism, and he demonstrated the role of mutation in GNA11, which encodes the alpha subunit of the G11 G protein, as a novel basis for hypoparathyroidism with short stature. He has also identified mutations in AIRE and TBX1 as causes of isolated hypoparathyroidism, in addition to their association with more complex syndromic hypoparathyroidism.
Genetic Basis for Uncommon Disorders of Bone and Mineral Metabolism
Dr. Levine and colleagues identified and/or advanced characterization of the molecular basis of a variety of disorders that have extended understanding of parathyroid biology and bone homeostasis. These disorders include familial hypocalciuric hypercalcemia, primary hyperparathyroidism in children and adolescents, cherubism, a novel form of generalized arterial calcification of infancy, and the role of FGF23 and other phosphatonins in tumor-induced osteomalacia. Dr. Levine has identified the molecular basis of novel forms of vitamin D-dependent rickets in which vitamin D deficiency is due to mutation of CYP2R1, the principle 25-hydroxylase, as well as a novel recurrent gain-of-function mutation in CYP3A4 that creates an accessory pathway for inactivation of vitamin D metabolites. In addition, Dr. Levine and his associates have described and characterized short stature due to growth hormone deficiency due to mutation of the GH-releasing hormone receptor, and a novel form of hypercalcemia that is related to the ketogenic diet.
Genetic Control of Vitamin D Metabolism
Dr. Levine and his collaborators were first to identify CYP2R1 as the principal vitamin D 25-hydroxylase, and described genetic defects in the CYP2R1 gene as the basis for vitamin D-dependent rickets type 1b. His laboratory also showed that expression of CYP2R1 declined in models of obesity and aging, providing an alternative explanation for the reduced serum concentrations of 25-hydroxyvitamin D that are present in these conditions. Dr. Levine’s laboratory identified a recurrent gain-of-function mutation in CYP3A4 as the basis for vitamin D-dependent rickets type 3, due to accelerated inactivation of vitamin D metabolites. This observation led to the development of a clinical trial to test the safety and efficacy of repurposing the antibiotic rifampin, a powerful inducer of CYP3A4, as a treatment for hypercalcemia and/or hypercalciuria in patients who have genetic mutations in CYP24A1, which encodes the enzyme that inactivates vitamin D metabolites.