DR. NAMITA ROY-CHOWDHURY

Department of Medicine 
Department of Molecular Genetics

ULLMANN BLDG. - ROOM 523

718 430-2265 

Glucuronidation is essential for hepatic excretion of bilirubin, the toxic end product of heme catabolism.  Bilirubin glucuronidation is catalyzed by specific isoforms of a family of UDP-glucuronosyltransferase (UGT), particularly by one form, B-UGT1.  B-UGT1 and at least three other UGT isoforms are expressed from a gene with unique organization, ugt1.  Four consecutive exons at the 3' domain of the gene are used in all isoforms expressed from these genes, and encode the carboxy terminal region of the enzymes that are identical in all isoforms.  Upstream to these is a series of unique exons, only one of which is used in a specific UGT isoform.  Each unique region exon encodes the variable NH2 terminal domain of one UGT isoform that imparts aglycone substrate specificity to the isoform.  The unique region exons are driven by independent 5' promoters and are therefore capable of independent regulation.

       Three grades of inherited B-UGT1 deficiency lead to three syndromes in humans, all of which are characterized by unconjugated hyperbilirubinemia.  A near complete deficiency of the enzyme results in the potentially lethal disorder, Crigler-Najjar syndrome type I (CN-1).  Incomplete loss of the enzyme activity causes Crigler-Najjar syndrome type II (CN-II).  Milder forms of B-UGT deficiency cause the common benign disorder, Gilbert syndrome.  Our laboratory is pursuing two related areas of research.  One involves the correlation of molecular structure and catalytic function of B-UGT1.  The second is aimed at characterization of the mechanism of regulation of ugt1 gene expression.

       1.  Structure-function relationship of B-UGT1:  relationship with inherited jaundice:  We plan to amplify and determine the nucleotide sequence of each of the five exons encoding the B-UGT from patients with CN-I and CN-II.  To determine the effect of the observed structural mutations on the catalytic activity of B-UGT1, we will reproduce the mutations by site-directed mutagenesis, express the plasmids in COS cells and determine the kinetic parameters of the expressed enzyme.  Because a large variety of mutations have been associated with CN syndromes, by correlating identified mutations with specific functional abnormalities, we expect to determine the function of various structural domains of this enzyme.  To directly examine the binding sites for bilirubin and UDP-glucuronic acid, we will perform U.V. crosslinking studies.  For this purpose, a large amount of the recombinant enzyme will be produced in E. coli and by expression in the baculovirus system.

       2.  Analysis of the regulatory mechanisms of UGT isoforms.  We have identified a promoter and several putative enhancer elements upstream to the first exon of B-UGT1.  A variant promoter that has an abnormal interaction with a specific upstream enhancer may be responsible for reduced expression of B-UGT1, resulting in mild hyperbilirubinemia in a relatively large segment of the population (Gilbert syndrome).  We plan to produce constructs using the B-UGT UTRs from normal subjects and patients with CN-II or Gilbert syndrome to drive a reporter gene, e.g. firefly luciferase, to determine the significance of the putative regulatory regions and their role in regulation of bilirubin glucuronidation in neonates and following ingestion of various dietary components or drugs.

References NCBI PubMed search of "N. Roy-Chowdhury"

Roy Chowdhury, N., Hays R.M., Bommineni V.R., Franki N., Roy Chowdhury J., Wu C.H., and Wu G.Y. Microtubular distribution prolongs the expression of  human bilirubin-uridine diphosphoglucuronate - glucuronosyltransferase-1 gene transferred into gunn rat livers.  J. Biol. Chem. 271:2341-2346, 1996.

Jauregui HO, Roy Chowdhury N, Roy Chowdhury J.  Use of mammalian liver cells for artificial liver support.  Cell Transplantation.  5:353-357, 1996.

Gantla S,  Deocharan B,  Sengupta K,  Thummala NR, Zweiner J,  Bosma PJ, Roy Chowdhury J and  Roy Chowdhury N.  Intronic mutation: a novel genetic mechanism of Crigler-Najjar syndrome type I. Hepatology, 24:250, 1996

Deocharan B, Gantla S, Morton DH, Rizack L, Roy Chowdhury J and Roy Chowdhury N.  Interaction of  a Crigler-Najjar syndrome type I mutation and a Gilbert type promoter defect results in two grades of hyperbilirubinemia in members of an Amish and a Mennonite kindred of Lancaster County, Pennsylvannia.  Gastroenterology, in press, 1997.

Roy Chowdhury N, Attvar P, Roy Chowdhury J.Hereditary disorders of the liver and biliary system..  Principles and Practice of Medical Genetics, Third Edition, Rimoin DL, Connor JM, Pyeritz RE, editors.  Churchill Livingstone, London, 1997, Pp. 1555-1578.