Presented as Thesis (Ph. D.)--University of Wisconsin--Madison, 1925. / Includes bibliographical references (p. 227-229).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1967. / Typescript. Vita. Description based on print version record. Includes bibliographical references.

Thesis (MPhil)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

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This thesis describes three studies on mammalian 25-hydroxyvitamin D$ sb3$-24-hyroxylase (24-hydroxylase), the first enzyme in the C24-oxidation pathway, a major catabolic pathway for vitamin D metabolites in kidney and other target tissues for vitamin D hormone. The first study examines the involvement of protein kinase C in the regulation of 24-hydroxylase activity in mouse kidney. Evidence is presented supporting a stimulatory role for protein kinase C in the regulation of constitutive, but not inducible, renal 24-hydroxylase. The kinase is also implicated in the aberrant expression of renal vitamin D metabolism in the mutant X-linked hypophosphatemic (Hyp) mouse. The second study investigates the mechanism(s) by which forskolin, a classic activator of adenylate cyclase, inhibits 24-hydroxylase activity in mouse kidney. Both the traditional cAMP-dependent mechanism and a novel cAMP-independent mode of action are observed. A direct interaction between forskolin and the substrate binding site of 24-hydroxylase is suggested for the latter based on kinetic analyses and structural similarities between the diterpene and the steroid substrate for the hydroxylase. The third study addresses the structural relationship between renal 1-hydroxylase and renal and target cell 24-hydroxylase(s) by assessing 24-hydroxylase activity in patients with vitamin D dependency rickets type I (VDDR-I), a Mendelian disorder of 1-hydroxylase function. Both constitutive renal 24-hydroxylase, indirectly ascertained through measurement of circulating levels of relevant vitamin D metabolites, and inducible target cell 24-hydroxylase, directly measured in cultured skin fibroblasts, are shown to be intact in VDDR-I patients undergoing calcitriol therapy. These findings suggest that the 1- and 24-hydroxylase activities likely represent or contain distinct gene products.

Thesis (Ph. D.)--University of Chicago, 1933. / Lithoprinted. "Private edition, distributed by the University of Chicago libraries, Chicago, Illinois." Includes bibliographical references.

La vitamine D (D3 ou cholécalciférol du règne animal et D2 ou ergostérol du règne végétal) est une hormone pléiotrope qui possède de nombreux effets biologiques incluant la régulation du métabolisme du calcium et du phosphate. Chez l’Homme, ce composé est synthétisé au niveau cutané sous forme inactive. On décrit ainsi le métabolisme de la vitamine D qui conduit à la production de métabolites actifs (par les vitamine D 25- et 1α-hydroxylases codées par les gènes CYP2R1 et CYP27B1) et à leur dégradation par la vitamine D 24-hydroxylase (gène CYP24A1). L’expression des vitamine 1α- et 24-hydroxylases est finement et inversement régulée afin de maintenir l’homéostasie phosphocalcique, grâce à plusieurs boucles de rétrocontrôle impliquant entre autres la forme 1,25-dihydroxylée de la vitamine D et son récepteur VDR, la calcémie et la parathormone, la phosphatémie et le FGF23. La carence en vitamine D et les défauts de son activation sont associés à un phénotype de rachitisme, tandis que les excès en vitamine D sont associés à un phénotype d’hypercalcémie-hypercalciurie par intoxication (surdosage) ou hypersensibilité à la vitamine D (excès d’activation ou défaut de dégradation).L’objectif de ce travail de thèse est d’identifier des causes génétiques de dérégulation du métabolisme de la vitamine D et de préciser leurs mécanismes physiopathologiques par une description précise du phénotype associé. Pour ce faire, nous avons utilisé de façon conjointe les outils de la génétique (séquençage nouvelle génération et Sanger) et de la biochimie (dosage des métabolites) dans une cohorte de patients recrutés grâce au centre de référence maladies rares du métabolisme du calcium et du phosphate.Ce travail a permis de préciser le rôle de deux gènes dans les maladies liées à la dérégulation métabolisme de la vitamine D, CYP2R1 et CYP24A1, par la mise en évidence de mutations perte de fonction chez des patients avec un phénotype de rachitisme à 25-hydroxyvitamine D basse et d’hypersensibilité à la vitamine D respectivement. Notre étude a permis aussi de préciser le phénotype de ces affections. Dans la cohorte des patients étudiés, l’identification de mutations de gènes impactant le métabolisme du phosphate (SLC34A1 et SLC34A3), souligne l’intérêt de l’étude des facteurs régulateurs des activités vitamine D 1α- et 24-hydroxylases.Aucune variation significative dans les régions promotrices proximales de CYP27B1 et CYP24A1 n’a été identifiée. Le peu de connaissances sur l’ensemble des éléments régulateurs chez l’Homme n’a pas permis d’approfondir notre étude. L’identification et l’étude de ces éléments régulateurs distaux permettra de déterminer leur implication dans les maladies rares du métabolisme de la vitamine D. / The vitamin D (D3 or cholecalciferol from animal kingdom and D2 or ergosterol from plan kingdom) is a pleiotropic hormone who has numerous biological effects including the regulation of calcium and phosphate metabolism. In humans, this compound is synthetized in skin in an inactive form. Thus, we call vitamin D metabolism the biological process which leads to the production of active metabolites (by enzymes 25- and 1α-hydroxylases encoded by CYP2R1 and CYP27B1 genes) and its degradation by vitamin D 24-hydroxylase (gene CYP24A1). The expression of 1α- and 24-hydroxylases is tightly and inversely regulated to maintain calcium and phosphate homeostasis, thanks to several feedback loops including 1,25-dihydroxyvitamin D and its receptor VDR, serum calcium and parathormone, serum phosphate and FGF23. Vitamin D deficiency and vitamin D activation deficiency are associated with rickets, while vitamin D excess are associated with hypercalcemia-hypercalciuria due to vitamin D intoxication (overdose) or hypersensitivity to vitamin D (activation excess or degradation deficiency).Our aim is to identify genetic causes of vitamin D metabolism deregulation and to specify pathophysiologic mechanisms describing phenotype. Thus, we jointly used the tools of genetics (next-generation and Sanger sequencing) and biochemistry (vitamin D metabolites assay) in a cohort of human patients ascertained thanks to the national center for rare diseases of calcium and phosphate metabolism.This work allowed us to specify the role of two genes in diseases of vitamin D metabolism, CYP2R1 and CYP24A1, showing loss of function mutations in patients with rickets and low 25-hydroxyvitamin D and hypersensitivity to vitamin D, respectively. Our study brought new phenotypic elements in these affections. In our cohort of patients, the identification of mutations leading to phosphate deregulation (in SLC34A1 and SLC34A3) highlights the putative role of regulators of vitamin D 1α- and 24-hydroxylases activities in pathophysiology.No significant variation have been identified in the proximal promoting regions of CYP27B1 and CYP24A1. We could not go further considering the lack of knowledge in regulating regions and factors in humans. Identifying distal regulators will allow to study their implication in rare diseases of vitamin D metabolism.

A Dissertation Submitted to the Faculty of Medicine, University of the Witwatersrand, Johannesburg for the degree of Master of Medicine. / Disturbed mineral and bone metabolism is frequently found in very low-birth-weight infants fed breast-milk during the first three months of life. The study was designed to assess the prevalence of this disturbed mineral homeostasis in a very low-birth-weight populatiun at Baragwanath Hospital and to determine whether the addition of a preterm infant formula to the feeds reduced the prevalence and increased the rate of weight gain. Fifty three neonates weighing less than 1200g born at Baragwanath Hospital were monitored for weight gain, growth and for biochemical and radiological evidence of metabolic bone disease. The infants were randomized to receive either breast-milk only feeds or a combination of breast-milk and a premature formula in order to assess the effect of the different feeds on the development of bone disease. Weight gain and growth were similar in both groups. Calcium and phosphorus intakes were higher in the mixed feeding group. However, serum calcium and phosphorus values were similar in the two groups throughout the study. The breast-milk group had significantly higher alkaline phosphatase levels. Radiological rickets was uncommun in both groups, although periosteal reactions and osteopenia occurred frequently and with similar prevalence in both groups. Overt rickets is not a major problem in very-low birth- weight infants born at Baragwanath Hospital, although raised serum alkaline phosphatase values occur frequently. Feeding with breast-milk and a premature infant formula in equal proportions (as opposed to breast-milk only) does not appear to have any effect on weight gain and growth in very low-birth-weight infants, but does partially prevent the pathological rise in alkaline phosphatase levels. / Andrew Chakane 2019