The human CYP3A subfamily plays a dominant role in the metabolic elimination of more
drugs than any other biotransformation enzyme. CYP3A enzyme is localized in the liver
and small intestine and thus contributes to first-pass and systemic metabolism. CYP3A
expression varies as much as 40-fold in liver and small intestine donor tissues. CYP3A-dependent
in vivo drug clearance appears to be unimodally distributed which suggests multi-genic
or complex gene-environment causes of variability. Interindividual differences in
enzyme expression may be due to several factors including: variable homeostatic control
mechanisms, disease states that alter homeostasis, up- or down-regulation by environmental
stimuli (such as smoking, drug intake, or diet), and genetic mutations. This review
summarizes the current understanding and implications of genetic variation in the
CYP3A enzymes. Unlike other human P450s (CYP2D6, CYP2C19) there is no evidence of
a 'null' allele for CYP3A4. More than 30 SNPs (single nucleotide polymorphisms) have
been identified in the CYP3A4 gene. Generally, variants in the coding regions of CYP3A4
occur at allele frequencies <5% and appear as heterozygous with the wild-type allele.
These coding variants may contribute to but are not likely to be the major cause of
inter-individual differences in CYP3A-dependent clearance, because of the low allele
frequencies and limited alterations in enzyme expression or catalytic function. The
most common variant, CYP3A4*1B, is an A-392G transition in the 5'-flanking region
with an allele frequency ranging from 0% (Chinese and Japanese) to 45% (African-Americans).
Studies have not linked CYP3A4*1B with alterations in CYP3A substrate metabolism.
In contrast, there are several reports about its association with various disease
states including prostate cancer, secondary leukemias, and early puberty. Linkage
disequilibrium between CYP3A4*1B and another CYP3A allele (CYP3A5*1) may be the true
cause of the clinical phenotype. CYP3A5 is polymorphically expressed in adults with
readily detectable expression in about 10-20% in Caucasians, 33% in Japanese and 55%
in African-Americans. The primary causal mutation for its polymorphic expression (CYP3A5*3)
confers low CYP3A5 protein expression as a result of improper mRNA splicing and reduced
translation of a functional protein. The CYP3A5*3 allele frequency varies from approximately
50% in African-Americans to 90% in Caucasians. Functionally, microsomes from a CYP3A5*3/*3
liver contain very low CYP3A5 protein and display on average reduced catalytic activity
towards midazolam. Additional intronic or exonic mutations (CYP3A5*5, *6, and *7)
may alter splicing and result in premature stop codons or exon deletion. Several CYP3A5
coding variants have been described, but occur at relatively low allelic frequencies
and their functional significance has not been established. As CYP3A5 is the primary
extrahepatic CYP3A isoform, its polymorphic expression may be implicated in disease
risk and the metabolism of endogenous steroids or xenobiotics in these tissues (e.g.,
lung, kidney, prostate, breast, leukocytes). CYP3A7 is considered to be the major
fetal liver CYP3A enzyme. Although hepatic CYP3A7 expression appears to be significantly
down-regulated after birth, protein and mRNA have been detected in adults. Recently,
increased CYP3A7 mRNA expression has been associated with the replacement of a 60-bp
segment of the CYP3A7 promoter with a homologous segment in the CYP3A4 promoter (CYP3A7*1C
allele). This mutational swap confers increased gene transcription due to an enhanced
interaction between activated PXR:RXRalpha complex and its cognate response element
(ER-6). The genetic basis for polymorphic expression of CYP3A5 and CYP3A7 has now
been established. Moreover, the substrate specificity and product regioselectivity
of these isoforms can differ from that of CYP3A4, such that the impact of CYP3A5 and
CYP3A7 polymorphic expression on drug disposition will be drug dependent. In addition
to genetic variation, other factors that may also affect CYher factors that may also
affect CYP3A expression include: tissue-specific splicing (as reported for prostate
CYP3A5), variable control of gene transcription by endogenous molecules (circulating
hormones) and exogenous molecules (diet or environment), and genetic variations in
proteins that may regulate constitutive and inducible CYP3A expression (nuclear hormone
receptors). Thus, the complex regulatory pathways, environmentally susceptible milieu
of the CYP3A enzymes, and as yet undetermined genetic haplotypes, may confound evaluation
of the effect of individual CYP3A genetic variations on drug disposition, efficacy
and safety.