Handbook of Clinical Anesthesia
Genomic Basis of Perioperative Medicine
Human biological diversity involves interindividual variability in morphology, behavior, physiology, development, susceptibility to disease, and response to stressful stimuli and drug therapy (phenotypes) (Podgoreanu MV, Mathew JP: Pharmacogenomics and proteomics. In Clinical Anesthesia. Edited by Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC. Philadelphia: Lippincott Williams & Wilkins, 2009, pp 115–136). Phenotypic variation is determined, at least in part, by differences in the specific genetic makeup (genotype) of an individual.
- Genetic Basis of Disease
- Many common diseases, such as atherosclerosis, coronary artery disease, hypertension, diabetes, cancer, and asthma and many individual responses to injury, drugs, and non-pharmacologic therapies are genetically complex, characteristically involving an interplay of many genetic variations in molecular and biochemical pathways.
- The perioperative period represents a unique and extreme example of gene–environment interaction.
- A hallmark of perioperative physiology is the striking variability in patient responses to the perturbations induced by events occurring in the operative period.
- This translates into substantial interindividual variability in immediate perioperative adverse events (e.g., mortality, incidence, or severity of organ dysfunction), as well as long-term outcomes.
- Genetic variation is partly responsible for the observed variability in outcomes.
- With increasing evidence suggesting that genetic variation can significantly modulate the risk of adverse perioperative events,
the emerging field of perioperative genomics aims to apply functional genomic approaches to discover underlying biological mechanisms.
- These approaches explain why similar patients have such dramatically different outcomes after surgery. These outcomes are determined by a unique combination of environmental insults and postoperative phenotypes that characterize surgical and critically ill patient populations.
- To integrate this new generation of genetic results into clinical practice, perioperative physicians need to understand the patterns of human genome variation, the methods of population-based genetic investigation, and the principles of gene and protein expression analysis.
- Overview of Human Genetic Variation
- Although the human DNA sequence is 99.9% identical between individuals, the variations may greatly affect a person's disease susceptibility.
- Rare genetic variants (mutations) are responsible for more than 1,500 monogenic disorders (e.g., hypertrophic cardiomyopathy, long-QT syndrome, sickle cell anemia, cystic fibrosis, familial hypercholesterolemia).
- Most of the genetic diversity in the population is attributable to more widespread DNA sequence variations (polymorphisms), typically single nucleotide base substitutions (single nucleotide polymorphisms[SNPs]) (Fig. 6-1).
- About 15 million SNPs are estimated to exist in the human genome, approximately once every 300 base pairs, located in genes as well as in the surrounding regions of the genome.
- Polymorphisms may directly alter the amino acid sequence and therefore potentially alter protein function or alter regulatory DNA sequences that modulate protein expression.
- Sets of nearby SNPs on a chromosome are inherited in blocks, referred to as haplotypes.
- The year 2007 was marked by the realization that DNA differs from person to person much more than previously suspected.
Figure 6-1. Categories of genetic polymorphisms. A. Single nucleotide polymorphisms (SNPs) can be silent or have functional consequences ranging from changes in amino acid sequence or premature termination of protein synthesis. B. Microsatellite polymorphism with varying number of dinucleotide (CA)n repeats. C. Insertion–deletion polymorphism. The locus is the location of a gene or genetic marker in the genome. Alleles are alternative forms of a gene or genetic marker. Genotype is the observed alleles for an individual at a genetic locus. Heterozygous means that two different alleles are present at a locus. Homozygous means that two identical alleles are present at a locus.
III. Methodologic Approaches to Studying the Genetic Architecture of Common Complex Diseases
- Most ongoing research on complex disorders focuses on identifying genetic polymorphisms that enhance susceptibility to given conditions (e.g., candidate gene and genome scans used to identify polymorphisms affecting common diseases).
- Linkage analysisis used identify the chromosomal location of gene variants related to a given disease by studying the distribution of disease alleles in affected individuals throughout a pedigree. The nature of most complex diseases (especially for perioperative adverse events)
precludes the study of extended multigenerational family pedigrees.
- Genetic association studiesexamine the frequency of specific genetic polymorphisms in a population-based sample of unrelated diseased individuals and appropriately matched unaffected controls. The fact that these studies do not require family-based sample collections is the main advantage of this approach over linkage analysis.
- Accumulating evidence from candidate gene association studies also suggests that specific genotypes are associated with a variety of organ-specific perioperative adverse outcomes (e.g., myocardial infarction [MI], neurocognitive dysfunction, renal compromise, vein graft restenosis, postoperative thrombosis, vascular reactivity, severe sepsis, transplant rejection, death).
- Replication of findings across different populations or related phenotypes remains the most reliable method of validating a true relationship between genetic polymorphisms and disease.
- The year 2007 marked the publication of adequately powered and successfully replicated genome-wide association studiesthat identified significant genetic contributors to the risk for common polygenic diseases (e.g., coronary artery disease, MI, types I and II diabetes, atrial fibrillation, obesity, asthma, common cancers, rheumatoid arthritis, Crohn's disease).
- Variants in or near CDKN2A/B(cyclin-dependent kinase inhibitor 2 A/B) have been shown to confer increased risk for both type II diabetes and MI, which may lead to a mechanistic explanation for the link between the two disorders.
- Large-Scale Gene and Protein Expression Profiling: Static Versus Dynamic Genomic Markers of Perioperative Outcomes
- Genomic approaches are anchored in the concept of transcription of messenger RNA (mRNA) from a DNA template, followed by translation of RNA into protein (Fig 6-2).
- Transcription is a key regulatory step that may eventually signal many other cascades of events.
Figure 6-2. Central dogma of molecular biology. Protein expression involves two main processes, RNA synthesis (transcription) and protein synthesis (translation), with many intermediate regulatory steps.
- Although the human genome contains only about 25,000 genes, functional variability at the protein level is far more diverse, resulting from extensive post-transcriptional, translational, and post-translational modifications.
- It is believed that there are approximately 200,000 distinct proteins in humans, which are further modified post-translationally by phosphorylation, glycosylation, oxidation, and disulfide structures.
- Increasing evidence suggests that variability in gene expression levels underlies complex diseases and is determined by regulatory DNA polymorphisms affecting transcription, splicing, and translation efficiency in a tissue- and stimulus-specific manner.
- The main functional categories of genes identified as potentially involved in cardioprotective pathways include a host of transcription factors, proteins, and antioxidant genes.
- Different gene programs appear to be activated in ischemic versus anesthetic preconditioning, resulting in two distinct cardioprotective phenotypes.
- The transcriptome(the complete collection of transcribed elements of the genome) is not fully representative of the
proteome (the complete complement of proteins encoded by the genome) because many transcripts are not targeted for translation, as evidenced recently by the concept of gene silencing by RNA interference.
- Therefore, alternative splicing, a wide variety of post-translational modifications, and protein–protein interactions responsible for biological function would remain undetected by gene expression profiling.
- This has led to the emergence of a new field, proteomics, which studies the sequence, modification, and function of many proteins in a biological system at a given time. Rather than focusing on “static” DNA, proteomic studies examine dynamic protein products with the goal of identifying proteins that undergo changes in abundance, modification, or localization in response to a particular disease state, trauma, stress, or therapeutic intervention.
- Proteomics offers a more global and integrated view of biology, complementing other functional genomic approaches.
- Genomics and Perioperative Risk Profiling
- More than 40 million patients undergo surgery annually in the United States at a cost that totals $450 billion. Each year, approximately 1 million patients sustain medical complications after surgery, resulting in costs of $25 billion annually.
- Perioperative complications are significant, costly, variably reported, and often imprecisely detected and identified. There is a critical need for accurate, comprehensive perioperative outcome databases.
- Presurgical risk profiling is inconsistent and deserves further attention, especially for noncardiac, nonvascular surgery and older patients.
- It is becoming increasingly recognized that perioperative morbidity arises as a direct result of the environmental stress of surgery occurring on a landscape of susceptibility that is determined by an individual's clinical and genetic characteristics and may even occur in otherwise healthy individuals.
- Understanding the role of allotypic variation in pro-inflammatory and pro-thrombotic pathways, the main pathophysiological mechanisms responsible for perioperative complications may contribute to the
development of target-specific therapies, thereby limiting the incidence of adverse events in high-risk patients.
- Genetic Susceptibility to Adverse Perioperative Cardiovascular Outcomes
- Perioperative Myocardial Infarction.Identifying patients at the highest risk of perioperative MI remains difficult.
- Genetic susceptibility to MI has been established.
- In the setting of cardiac surgery, postoperative MI involves three major converging pathophysiological processes, including systemic and local inflammation, “vulnerable” blood, and neuroendocrine stress
- Inflammation Variability and Perioperative Myocardial Outcomes.Inflammatory gene polymorphisms that are independently predictive of postoperative MI after cardiac surgery with cardiopulmonary bypass have been identified.
- Coagulation Variability and Perioperative Myocardial Outcomes.In addition to inflammatory activation, the host response to surgery is also characterized by an increase in fibrinogen concentration, platelet adhesiveness, and plasminogen activator inhibitor-1 production.
- Perioperative thrombotic outcomes after cardiac surgery (e.g., coronary graft thrombosis, MI, stroke, pulmonary embolism) represent one extreme on a continuum of coagulation dysfunction, with coagulopathy at the other end of the spectrum.
- Evidence suggests genetic variability modulates the activation of each of these mechanistic pathways, reflecting a significant heritability of the prothrombotic state.
- Genetic Variability and Perioperative Vascular Reactivity
- Perioperative stress responses are also characterized by sympathetic nervous system activation, known to play a role in the pathophysiology of postoperative MI.
- Patients with coronary artery disease and specific adrenergic receptor genetic polymorphisms may be particularly susceptible to catecholamine toxicity and cardiac complications.
- Perioperative Atrial Fibrillation.New-onset perioperative atrial fibrillation (AF) remains a common complication of cardiac and major noncardiac thoracic surgical procedures (incidence, 27 to 40%)
and is associated with increased morbidity, longer hospital lengths of stay, increased rehospitalization, increased health care costs, and reduced survival.
- Heritable forms of AF occur in the ambulatory nonsurgical population.
- A role for inflammation for perioperative AF is suggested by baseline C-reactive protein levels in male patients and exaggerated postoperative leukocytosis, which both predict perioperative AF; postoperative administration of nonsteroidal antiinflammatory drugs shows a protective effect.
- Cardiac Allograft Rejection.Identification of peripheral blood gene- and protein-based biomarkers to noninvasively monitor, diagnose, and predict perioperative cardiac allograft rejection is an area of rapid scientific growth.
- Genetic Variability and Postoperative Event-Free Survival.Increasing evidence suggests that the ACE gene polymorphism may influence complications after coronary artery bypass graft (CABG) surgery, with carriers of the D allele having higher mortality and restenosis rates after CABG surgery compared with carriers of the I allele.
- Genetic Susceptibility to Adverse Perioperative Neurologic Outcomes
- Despite advances in surgical and anesthetic techniques, significant neurologic morbidity continues to occur after cardiac surgery, ranging in severity from coma and focal stroke (incidence, 1 to 3%) to more subtle cognitive deficits (incidence, ≤69%), with a substantial impact on the risk of perioperative death, quality of life, and resource utilization.
- The pathophysiology of perioperative neurologic injury is thought to involve complex interactions between primary pathways associated with atherosclerosis and thrombosis and secondary response pathways such as inflammation, vascular reactivity, and direct cellular injury.
- Many functional genetic variants have been reported in each of these mechanistic pathways involved in modulating the magnitude and the response to neurologic injury, which may have implications in chronic as well as acute perioperative neurocognitive outcomes. Specific pathways are associated with the development of postoperative complications such as postoperative cognitive dysfunction.
- There is a significant association between the apolipoprotein E genotype and adverse cerebral outcomes in patients undergoing cardiac surgery. The incidence of postoperative delirium after major noncardiac surgery in elderly and critically ill patients is increased in carriers of this genotype.
- Platelet activation may be important in the pathophysiology of adverse neurologic sequelae. The implications for perioperative medicine include identifying populations at risk that might benefit not only from an improved informed consent, stratification, and resource allocation but also from targeted antiinflammatory strategies.
- Genetic Susceptibility to Adverse Perioperative Renal Outcomes
- Acute renal dysfunction is a common, serious complication of cardiac surgery. About 8 to 15% of patients develop moderate renal injury (peak creatinine increase of >1.0 mg/dL), and up to 5% of them develop renal failure requiring dialysis. Acute renal failure is independently associated with in-hospital mortality rates exceeding 60% in patients requiring dialysis.
- Studies have demonstrated that inheritance of genetic polymorphisms are associated with acute kidney injury after CABG surgery.
- Pharmacogenomics and Anesthesia
- The term pharmacogenomicsis used to describe how inherited variations in genes modulating drug actions are related to interindividual variability in drug response.
- Such variability in drug action may be pharmacokineticor pharmacodynamic (Fig. 6-3).
- Pharmacokinetic variabilityrefers to variability in a drug's absorption, distribution, metabolism, and excretion that mediates its efficacy and toxicity. The molecules involved in these processes include drug-metabolizing enzymes (e.g., members of the cytochrome P450 or CYP superfamily) and drug transport molecules that mediate drug uptake into and efflux from intracellular sites.
- Pharmacodynamic variabilityrefers to variable drug effects despite equivalent drug delivery to molecular sites of action. This may reflect variability in the function of the molecular target of the drug or in the
pathophysiological context in which the drug interacts with its receptor–target (affinity, coupling, expression).
Figure 6-3. Pharmacogenomic determinants of individual drug response operate by pharmacokinetic and pharmacodynamic mechanisms.
- Pseudocholinesterase Deficiency.Individuals with an atypical form of pseudocholinesterase resulting in a markedly reduced rate of drug metabolism are at risk for excessive neuromuscular blockade and prolonged apnea. More than 20 variants have since been identified in the butyrylcholinesterase gene. Therefore, pharmacogenetic testing is currently not recommended in the population at large but only as an explanation for an adverse event.
- Genetics of Malignant Hyperthermia
- Malignant hyperthermia (MH) is a rare autosomal dominant genetic disease of skeletal muscle calcium metabolism that is triggered by administration of a volatile anesthetic agent or succinylcholine in susceptible individuals.
- MH susceptibility was initially linked to the ryanodine receptor (RYRI) gene locus on chromosome 19q, but it is becoming increasingly apparent that MH susceptibility results from a complex interaction between multiple genes and environmental factors (e.g., environmental toxins).
- Because of the polygenic determinism and variable penetrance, direct DNA testing in the general population for susceptibility to MH is currently not recommended. In contrast, testing in individuals from families with affected individuals has the potential to greatly reduce mortality and morbidity.
- Genetic Variability and Response to Anesthetic Agents
- Anesthetic potency, defined by the minimum alveolar concentration (MAC) of an inhaled anesthetic that abolishes purposeful movement in response to a noxious stimulus, varies among individuals, with a coefficient of variation (the ratio of standard deviation to the mean) of approximately 10%.
- Evidence of a genetic basis for increased anesthetic requirements is suggested by the observation that desflurane requirements are increased in subjects with red hair versus those with dark hair.
- Genetic Variability and Response to Anesthetic Agents
- Similar to the observed variability in anesthetic potency, the response to painful stimuli and analgesic manipulations varies among individuals.
- Increasing evidence suggests that pain behavior in response to noxious stimuli and its modulation by the
central nervous system in response to drug administration or environmental stress, as well as the development of persistent pain conditions through pain amplification, are strongly influenced by genetic factors.
- Genetic Variability in Response to Other Drugs Used Perioperatively
- A wide variety of drugs used in the perioperative period display significant pharmacokinetic or pharmacodynamic variability that is genetically modulated (Table 6-1).
- The most commonly cited categories of drugs involved in adverse drug reactions include cardiovascular, antibiotic, psychiatric, and analgesic medications, and each category has a known genetic basis for increased risk of adverse reactions.
- Genetic variation in drug targets (receptors) can have profound effect on drug efficacy.
- Carriers of susceptibility alleles have no manifest QT-interval prolongation or family history of sudden death until a QT-prolonging drug challenge is superimposed.
- Predisposition to QT-interval prolongation (considered a surrogate for risk of life-threatening ventricular arrhythmias) has been responsible for more withdrawals of drug from the market than any other category of adverse event.
- Pharmacogenomics is emerging as an additional modifying component to anesthesia along with age, gender, comorbidities, and medication usage. Specific testing and treatment guidelines allowing clinicians to appropriately modify drug utilization (e.g., adjust doses or change drugs) already exist for a few compounds and will likely be expanded to all relevant therapeutic compounds, together with identification of novel therapeutic targets.
- Genomics and Critical Care
- Genetic Variability in Response to Injury
- Systemic injury (including trauma and surgical stress), shock, and infection trigger physiological responses of fever, tachycardia, tachypnea, and leukocytosis that collectively define the systemic inflammatory response syndrome.
- A new paradigm in critical care medicine states that outcomes of critical illness are determined by the interplay
between the injury and repair processes triggered by the initial insults.
Table 6-1 Examples of Genetic Polymorphisms Involved in Variable Responses to Drugs Used in the Perioperative Period
- Negative outcomes are the combined result of direct tissue injury, the side effects of resulting repair processes, and secondary injury mechanisms leading to suboptimal repair.
- This concept forms the basis of the new PIRO (predisposition, infection or insult, response, organ dysfunction) staging system in critical illness.
- Genomic factors play a role along this continuum, from inflammatory gene variants and modulators of pathogen–host interaction to microbial genomics and rapid detection assays that identify pathogens to biomarkers differentiating infection from inflammation to dynamic measures of cellular responses to insult, apoptosis, cytopathic hypoxia, and cell stress.
- The large interindividual variability in the magnitude of response to injury, including activation of inflammatory and coagulation cascades, apoptosis, and fibrosis, suggests the involvement of genetic regulatory factors.
- Functional Genomics of Injury
- At a cellular level, injurious stimuli trigger adaptive stress responses determined by quantitative and qualitative changes in interdigitating cascades of biological pathways interacting in complex, often redundant ways. As a result, numerous clinical trials attempting to block single inflammatory mediators have been largely unsuccessful.
- Organ injury may be defined by patterns of altered gene and protein synthesis.
VII. Future Directions
- Systems Biology Approach to Perioperative Medicine: The “Perioptome.”Systems biology is a conceptual framework within which scientists attempt to correlate massive amounts of apparently unrelated data into a single unifying explanation.
- Targeted Therapeutic Applications: The “5 Ps” of Perioperative Medicine and Pain Management
- Genomic and proteomic approaches are rapidly becoming platforms for all aspects of drug discovery and development, from target identification and validation to individualization of drug therapy.
- The human genome contains about 25,000 genes encoding for approximately 200,000 proteins, which represent potential drug targets.
- Ethical Considerations
- Although one of the aims of the Human Genome Project is to improve therapy through genome-based prediction, the birth of personal genomics opens up a Pandora's box of ethical issues, including privacy and the risk for discrimination against individuals who are genetically predisposed to medical disorders.
- Another ethical concern is the transferability of genetic tests across ethnic groups, particularly in the prediction of adverse drug responses.
- Most polymorphisms associated with variability in drug response show significant differences in allele frequencies among populations and racial groups.
- The patterns of linkage disequilibrium are markedly different between ethnic groups, which may lead to spurious findings when markers, instead of causal variants, are used in diagnostic tests extrapolated across populations.
- With the goal of personalized medicine being the prediction of risk and treatment of disease on the basis of an individual's genetic profile, some have argued that biologic consideration of race will become obsolete.
- The Human Genome Project has revolutionized all aspects of medicine, allowing us to assess the impact of genetic variability on disease taxonomy, characterization, and outcome and of individual responses to various drugs and injuries.
- Mechanistically, information gleaned through genomic approaches is already unraveling long-standing mysteries behind general anesthetic action and adverse responses to drugs used during surgery.
- Using currently available high-throughput molecular technologies, genetic profiling of drug-metabolizing enzymes, carrier proteins, and receptors will enable personalized choice of drugs and dosage regimens tailored to suit a patient's pharmacogenetic profile. At that point, perioperative physicians will have far more robust information to use in designing the most appropriate and safest anesthetic plan for a given patient.
Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine
Title: Handbook of Clinical Anesthesia, 6th Edition
Copyright ©2009 Lippincott Williams & Wilkins
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