Impact of Common Polymorphic Variant A503V in POR on Drug Metabolism: Implications for POR Deficiency

Abstract

A broad spectrum of human diseases, including abnormalities in steroidogenesis, is caused by mutations in the NADPH cytochrome P450 oxidoreductase (POR) [1, 2]. Cytochrome P450 proteins perform several reactions, including the metabolism of steroids, drugs, and other xenobiotics. Therefore, genetic variations in POR can impact many different metabolic pathways by changing the activities of cytochromes P450. In 2004 the first human patients with defects in POR were reported with symptoms of ambiguous genitalia and bone malformations, and over 200 variations in POR are known.By analyzing the POR sequences from genomics databases, we focused on the common variant in POR A503V, which is often present in heterozygous form in many PORD patients. We expressed the A503V variant of POR in bacteria and characterized it by functional studies using purified recombinant proteins. The POR, as well as P450 Proteins, were expressed in E. coli BL21(DE3) and purified by ion‐exchange, affinity, and metal chelate chromatography for activity assays as described in our earlier publications [3, 4]. Activities of cytochrome P450 enzymes were tested with lipids into which P450 and P450 reductase proteins were embedded and assayed using fluorogenic substrates on a microplate spectrofluorometer.We found a positive effect on many drug‐metabolizing enzyme activities due to the A503V variant in POR. The activity of CYP2C9 was increased to 180% of the WT POR, and CYP2C19 showed 243% of WT activity with the A503V variant of POR. Most remarkable was the impact of the A503V variant on CYP3A5 activity, which was increased by 421% compared to WT POR.Effect on drug metabolism in patients with PORD requires careful considerations of cytochrome P450 mediated activities. While many PORD patients have low or reduced drug metabolism due to defects in POR, the presence of the A503V allele can alter the overall effect on drug metabolism. Significantly, tacrolimus, a drug given during organ transplant, can be metabolized several folds faster due to the presence of the A503V variant of POR. The presence of the A503V allele is likely beneficial in patients with POR deficiency.Support or Funding InformationThis work was supported by the Swiss National Science Foundation (31003A‐134926), the Novartis Foundation for Medical‐Biological Research (18A053), and a grant from Burgergemeiende Bern to AP.