Identification and characterization of CYP3A4*20, a novel rare CYP3A4 allele without functional activity.
BACKGROUND
The major drug-metabolizing enzyme cytochrome P450 (CYP) 3A4 is genetically conserved. One outlier of Brazilian descent was found in a clinical pharmacokinetic trial exhibiting a 6-fold higher exposure than expected to an investigational drug, shown to be a CYP3A4 substrate. We aimed to investigate the genetic background of this finding.


METHODS
The allelic variant of the CYP3A4 gene present in the outlier was sequenced, and the corresponding complementary deoxyribonucleic acid was expressed in yeast and human embryonic kidney cells. The outlier was phenotyped by use of intravenous administration of 1 mg midazolam. Analysis of phenotype and genotype correlation was carried out. The prevalence of the new allele was screened for in a white population.


RESULTS
We identified a subject who heterozygously carried a novel CYP3A4 allele, named CYP3A4*20, with a premature stop codon yielding a truncated protein. Heterologous expression revealed that the CYP3A4.20 enzyme does not incorporate heme and thus is devoid of catalytic activity. CYP3A phenotyping in vivo showed that CYP3A4*20 exhibits a clear genotype-phenotype correlation, demonstrated by the subject's low systemic midazolam clearance (2.99 mL x min(-1) x kg(-1)). Genotyping of a white German population (n = 428) and relatives of the subject, as well as a review of published CYP3A4 sequencing data, suggests that CYP3A4*20 is a rare variant allele (<0.06% in white subjects).


CONCLUSIONS
CYP3A4*20 represents the first CYP3A4 allele to be identified that has been shown to be devoid of functional activity. It causes an intermediate CYP3A4 metabolizer phenotype in a heterozygous carrier. Subjects of this genotype might be susceptible to side effects during drug therapy with substrates or inhibitors of CYP3A4.
Division of Molecular Toxicology, Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden.
Anna
A
Westlind-Johnsson
Robert
R
Hermann
Andreas
A
Huennemeyer
Bernhard
B
Hauns
Gezim
G
Lahu
Nassr
N
Nassr
Karl
K
Zech
Magnus
M
Ingelman-Sundberg
Oliver
O
von Richter
D000328:Adult; D000483:Alleles; D001483:Base Sequence; D001938:Brazil; D002478:Cells, Cultured / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics* / Q000378:metabolism; D005260:Female; D006801:Humans; D008297:Male; D008874:Midazolam; D008969:Molecular Sequence Data; D044465:White People / Q000235:genetics
D003577:Cytochrome P-450 Enzyme System; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D008874:Midazolam