Functional characterization of human UDP-glucuronosyltransferase 1A9 variant, D256N, found in Japanese cancer patients.

Abstract

SN-38 (7-ethyl-10-hydroxycamptothecin), an active metabolite of the antitumor prodrug irinotecan, is conjugated and detoxified to SN-38 10-O-beta-d-glucuronide by hepatic UDP-glucuronosyltransferase (UGT) 1A1. Recent studies have revealed that other UGT1A isoforms, UGT1A7 and UGT1A9, also participate in SN-38 glucuronidation. Although several genetic polymorphisms are reported for UGT1A1 and UGT1A7 that affect the SN-38 glucuronidation activities, no such polymorphisms have been identified for UGT1A9. In the present study, UGT1A9 exon 1 and its flanking regions were sequenced from 61 Japanese cancer patients who were all treated with irinotecan. A novel nonsynonymous single nucleotide polymorphism was identified in UGT1A9 exon 1, heterozygous 766G>A resulting in the amino acid substitution of D256N. The wild-type and D256N UGT1A9s were transiently expressed at similar protein levels in COS-1 cells, and their membrane fractions were characterized in vitro for the glucuronidation activities toward SN-38. The apparent Km values were 19.3 and 44.4 microM, and the Vmax values were 2.94 and 0.24 pmol/min/mg of membrane protein for the wild-type and D256N variant, respectively. The SN-38 glucuronidation efficiency (normalized Vmax/Km) of D256N was less than 5% that of wild-type UGT1A9. These results clearly indicate that the D256N variant is essentially nonfunctional with regard to SN-38 glucuronidation. These findings highlight the importance of further studies into the potential influence of UGT1A9 D256N variant to irinotecan metabolism in vivo.

• Authors: Hideto Jinno, Mayumi Saeki, Yoshiro Saito, Toshiko Tanaka-Kagawa, Nobumitsu Hanioka, Kimie Sai, Nahoko Kaniwa, Masanori Ando, Kuniaki Shirao, Hironobu Minami, Atsushi Ohtsu, Teruhiko Yoshida, Nagahiro Saijo, Shogo Ozawa, Jun-ichi Sawada
• Journal: The Journal of pharmacology and experimental therapeutics (J Pharmacol Exp Ther) Vol. 306 Issue 2 Pg. 688-93 (Aug 2003) ISSN: 0022-3565 [Print] United States
• PMID: 12730278 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)

Chemical References

• 7-ethyl-10-hydroxycamptothecin beta-glucuronide
• Antineoplastic Agents
• Glucuronates
• UGT1A9 protein, human
• Aspartic Acid
• Asparagine
• Irinotecan
• Glucuronic Acid
• Glucuronosyltransferase
• UDP-Glucuronosyltransferase 1A9
• Camptothecin

Topics

• Amino Acid Substitution
• Animals
• Antineoplastic Agents (adverse effects, therapeutic use)
• Asparagine (genetics)
• Aspartic Acid (genetics)
• COS Cells
• Camptothecin (adverse effects, analogs & derivatives, metabolism, pharmacology, therapeutic use)
• Exons
• Genetic Variation
• Glucuronates (metabolism, pharmacology)
• Glucuronic Acid (metabolism)
• Glucuronosyltransferase (drug effects, genetics, metabolism)
• Humans
• Irinotecan
• Japan
• Mutation (drug effects)
• Neoplasms (drug therapy, genetics, pathology)
• Polymorphism, Single Nucleotide
• Transfection
• UDP-Glucuronosyltransferase 1A9