麻醉领域的个体化用药,药物基因组学(Evan Kharasch)

Pharmacogenetics in Anesthesia

Evan D. Kharasch, M.D., Ph.D. St. Louis, Missouri 302 Page 1

Pharmacogenetics (or pharmacogenomics) aims to understand the inherited basis for variability in drug response. The promise of pharmacogenetics has been a change from “one drug and dose fits all” to individualized predictive medicine, or “the right drug at the right dose in the right patient”. Anesthesiology as a specialty played a key role in developing pharmacogenetics. Prolonged apnea after succinylcholine, thiopental-induced acute porphyria, and malignant hyperthermia were clinical problems of the 1960’s whose investigation helped craft the new science of pharmacogenetics. Today we perhaps take for granted the knowledge that they are genetically-based problems, due to variants in pseudocholinesterase, heme synthesis and the ryanodine receptor, respectively. This review will address basic principles of pharmacogenetics and their application to drugs used in anesthetic practice.

The term pharmacogenetics was originally defined (1959) as “the role of genetics in drug response”. Since the science of pharmacokinetics (drug absorption, distribution, metabolism, excretion) evolved earlier than pharmacodynamics, early pharmacogenetic studies addressed mainly pharmaco-kinetics. Application (fusion) of the genomic revolution and associated technologies to pharmaco-genetics spawned pharmacogenomics. Pharmacogenetics has been used by some in a more narrow sense, to refer only to genetic factors which influence drug kinetics and dynamics (drug receptor actions), while pharmacogenomics has been used more broadly to refer to the application of genomic technologies (whole-genome or individual gene changes) to drug discovery, pharmacokinetics and pharmacodynamics, pharmacologic response, and therapeutic outcome. Nonetheless, many consider this distinction unimportant and use the two terms interchangeably, as will this review.

BASIC CONCEPTS

A polymorphism is a discontinuous variation in a population (a bimodal or trimodal distribution). It is different than simple continuous variability (i.e. a unimodal population distribution, even if quite wide). A genetic polymorphism is the presence of multiple discrete states (i.e. for a particular trait) within a population, which has an inherited difference. The complete human genome consists of approximately 3 billion base pairs, which encode approximately 30,000 genes. A single nucleotide polymorphism (SNP) is a variation in the DNA sequence which occurs at a specific base. Polymorphisms are relatively common, occurring by definition in ≥1% of the population, while mutations are less common, occurring in <1%. Only 3% of DNA consists of sequences which code for protein (exons). Other portions of the DNA include promoter regions (near the transcription initiation site), enhancer regions (which bind regulatory transcription factors), and introns (DNA sequences which do not code for protein). After exons and introns are transcribed, the intronic mRNA is excised and the exonic mRNA is spliced together to form the final mature mRNA, which then undergoes translation into protein. SNPs are frequent, occurring in approximately 1:100-1:1000 bases. SNPs and mutations may occur in the coding or noncoding regions of the DNA. Since most occur in the latter, they are usually synonymous (or silent, having no effect on proteins), although intronic changes and promoter variants can change protein expression. Non-synonymous SNPs result in a change in an amino acid. A conservative change results in a similar amino acid that does not alter protein function, while a non-conservative change yields an amino acid which alters protein structure or function. These latter SNPs may be clinically significant. SNPs are not the only

events which can cause RNA and protein changes; others are deletions, insertions, duplications, and

splice variants, however these are not inherited.

Multiple SNPs can occur in the DNA which encodes

a particular protein. A haplotype is a set of closely linked alleles or DNA polymorphisms which are inherited together. While SNPs are important, haplotypes are more clinically relevant.

Polymorphisms can be classified at the DNA locus (which depicts the normal “wild-type” and the altered base pair; for example the mu opioid receptor gene polymorphism at base pair 118 which codes for changing an adenine nucleotide to a

guanine is abbreviated as A118G, or 118 A>G); at

polymorphism changes the amino acid at position 40