UNIVERSITY OF MALTA
LIFE SCIENCE RESEARCH SEMINARS
Web: http://www.um.edu.mt/events/scisem/
Email: scisem@um.edu.mt
Abstract form
Title: Pharmacogenomics of cytochrome b5 and NADH cytochrome b5
reductase
Presenter: Dr James Sacco
Contact address: University of Wisconsin-Madison, School of Veterinary Medicine,
2015 Linden Drive, Madison, WI 53706, USA
Tel: 001 608 2635520
Fax: 001 608 2658020
Email: saccoj@svm.vetmed.wisc.edu
Presentation date: October 18th, 2010
Abstract
NADH cytochrome b5 reductase (CYB5R3) and cytochrome b5 (CYB5A) catalyze the
reduction of sulfamethoxazole hydroxylamine (SMX-HA) to the parent drug sulfamethoxazole.
SMX-HA and a spontaneous nitroso metabolite are thought to contribute to sulfonamide
hypersensitivity. The aim of this study was to characterize variability in SMX-HA reduction in
111 human livers and investigate its association with single nucleotide polymorphisms (SNPs)
in CYB5A and CYB5R3 cDNA. Hepatic SMX-HA reduction displayed a 19-fold variability
(0.09-1.69 nmol/min/mg protein), which could contribute to the risk of SMX hypersensitivity.
African-American (AA) females had significantly lower SMX-HA reduction activity and b5
protein expression than Caucasian-Americans (CA). While several novel coding SNPs in the
CYB5R3 and CYB5A genes were associated with low activity and expression, these could not
account for all the variability observed. A follow-up study evaluated the contribution of SNPs
in the promoter and 3' untranslated (UTR) regions of these two genes to variation in activity
and protein expression in a subset of 69 livers (54 CA and 15 AA) with intermediate and
outlying activity and expression. Since few AAs were represented in the original liver
population, those SNPs found in livers with outlying activity and/or expression were further
analyzed for allele frequency in a separate population of AA (n=96) using a TaqMan
genotyping assay. Twenty-eight SNPs (15 novel) were found in the non-coding regions of the
two genes. Livers either with the novel promoter SNP CYB5R3 251G>T, CYB5A -389 G>A,
or CYB5A -382T>C promoter were associated with low activity and protein expression. The
minor allele frequencies of these SNPs were 0.09, 0.01 and 0.01 respectively in AAs. The 251G>T SNP was predicted to result in deletion of a VDR/CAR/PXR binding site, while both
the CYB5A promoter SNPs may result in deletion of a NF- B binding site. A *392G>C SNP in
the CYB5R3 3'UTR was associated with decreased activity and a trend towards decreased
CYB5R3 protein expression. This SNP is predicted to be within sequences recognized by
miR-96. In conclusion, in addition to cSNPs, several SNPs in the promoter and 3'UTRs of
CYB5A and CYB5R3 may impair hepatic SMX-HA reduction activity via decreases in protein
expression. Current studies aim to characterize the functional effects of these SNPs on
transcriptional and post-transcriptional gene regulation using site directed mutagenesis and
luciferase reporter constructs.