Influence of Pharmacogenomics on Disease and Symptom Management

Linda A. Howe, PhD, APRN-BC, CNE and Julie Eggert, PhD, GNP-BC, AOCN

Abstract

Advances in pharmacogenomics, the study of genetic inheritance and drug response, are on the verge of transforming pharmacology. Knowledge of genetic-based differences in a patient's drug pharmacodynamics and pharmacokinetics will be used in the near future to individualize drug therapy. The study of these alterations can help to explain the variations in responses to drugs from individual to individual even of the same ethnicity, sex, age group and family. Utilizing detailed genetic analyses such as microarray technology in individual patients will permit a more precise evaluation of disease processes and identification of patient-specific differences in drug response and metabolism. More positive patient outcomes will result, since individualized drug therapy would improve efficacy and minimize the risk of toxicity. Currently, pharmacogenetics is best defined for drug metabolism by the cytochrome P-450 system and used clinically to predict the potential risk of many drug interactions of clinical significance. However, in the future, testing for genetic-based differences in individual patients will allow a well defined, patient-specific risk assessment to individualized therapy. Many interactions between the drugs and metabolic enzymes or disease pathways are associated with polymorphisms. Many developmental and genetic conditions have a pharmacotherapeutic facet. These are explored and explained in order to improve nursing practice in the post-Human Genome Project health care arena.

Keywords: Pharmacogenomics Polymorphisms CYP-450

INTRODUCTION

Sequencing of the human genome has revealed that 99.9% of the genetic material is identical among individuals. In other words, all of the physical and molecular differences occur due to variations in 0.1% of the DNA. Some of these variations in the nucleotide (A, T, C, and G) sequences have been found to be responsible for alterations in drug responsiveness among individuals. Other changes are found with interactions between the drugs and metabolic enzymes or disease pathways associated with polymorphisms. Being cognizant of common medications and diseases or syndromes that may be affected by these nucleotide variations is an evolving role for nurses in all specialties, including those caring for persons with intellectual and/or developmental disabilities

DEFINITIONS

Genetics:

The study of genes and inheritance.

Genomics:

The determination and study of an organism's entire DNA sequence and the identification and study of genes (disease associated or otherwise) contained therein. Focuses on the function of gene(s).

Pharmacogenomics:

The study of how an individual's genetic inheritance affects the body's response to drugs.

Pharmacogenetics:

The study of genetic factors that influence an organism's reaction to a drug.

Pharmacodynamics:

The study of the concentration of a drug in the serum and the magnitude of the biological or physiological effects.

Pharmacokinetics:

The study of absorption, distribution, metabolism and excretion of drugs.

Drug:

A chemical or biological substance used in the diagnosis, treatment, or prevention of a disease.

Genotype:

The internally coded, heritable information carried by the organism. Variation in genotype represents differences in sequence within a species, such as SNPs, the location or the number of repeats, deletions, or critical splice sites.

Phenotype:

The observable properties of an organism produced by the interaction of the genotype with the environment.

Genetic Polymorphism:

Multiple differences of a DNA sequence found in at least 1% of the population, often yielding variation in a trait, such as the A, B and O blood groups.

Single Nucleotide Polymorphism (SNP):

A variation in DNA sequence occurring when a single nucleotide (A, T, C, or G) DNA sequence differs among members of a population. The incorporation of multiple single nucleotide polymorphisms into each DNA blueprint creates the uniqueness of individuals^{1, 2, 3, 4}.

Since the completion of the Human Genome Project (HGP), the discovery of the influence that genetic inheritance has on the development of diseases and the response to drug therapy has increased dramatically. This has opened new fields of study, such as pharmacogenomics. Pharmacogenomics is an emerging branch of pharmaceutics that deals with the influences of genetic variations and the affects on drug response in individuals through the correlation of gene expression with drug efficacy or toxicity. In the future, the application of pharmacogenomics will enable the administration of drugs with the greatest efficacy and the lowest risk of adverse reactions based on the individual's genotype. This will lead to the advent of individualized medicine, where drugs will be prescribed based on the individual's genetic makeup.

Pharmacokinetics (PK) is the study of the absorption, distribution, metabolism and excretion of drugs from the body. Absorption is simply the movement of the drug into the vascular system for distribution into the cells. Metabolism or biotransformation is an enzyme-mediated process that alters the structure of the drug. Excretion is the removal of the drug and the resulting metabolites from the body. This entire process determines the concentration of the drug at the receptor sites in the body. Genetic differences can alter the rate of metabolism through the enzyme system known as CYP-450, as well as alteration of drug distribution through the p-glycoprotein transport and in differences in receptor sites^{5, 19}.

Metabolism, or biotransformation, is comprised of two types. The majority of Phase I processes are the result of metabolism by cytochrome P-450 (CYP) microsomal enzymes, whereas Phase II is conjugation, where the drug or its metabolite is combined with other chemicals (e.g., sulfate or acetate). Phase I oxidation processes are the result of CYP microsomal enzymes, which cause inactivation of the drug or the production of an active compound (e.g., Codiene conversion to morphine via CYP2D6). Cytochrome P-450 is not a single enzyme but a group of 12 enzyme families that are hemeproteins similar to hemoglobin. Three of these families, CYP1, CYP2 and CYP3, are known to be important in the metabolism of drugs, while the others metabolize various endogenous substances such as lipids, steroids and hormones. Each of the three drug-metabolizing families is composed of multiple members that metabolize specific drugs. A specific nomenclature was designed to identify individual enzymes of the CYP system, such as CYP1A2, CYP2D6 and CYP3A4, which are representatives of the CYP1, CYP2 and CYP3 families. This nomenclature is based on CYP, designating human cytochrome, followed by a numeral designating the family, followed by a capital letter to designate the subfamily, followed by the second numeral designating the gene that encodes for a single the enzyme⁶. In the CYP3 group, CYP3A performs more than 50% of the known oxidation reactions. CYP3A4 is one member of this subgroup that is associated with many drug-drug and drug-food interactions. In addition to functioning in the liver, CYP3A4 is also located in intestinal mucosa where grapefruit juice may inhibit it, limiting deactivation of certain drugs, thus increasing the bioavailability of certain drugs and elevation of blood levels to toxic levels. Some chemical substances increase the activity of an enzyme system, which is called induction, and others can decrease the activity, known as enzyme inhibition. If a chemical is seen as increasing the activity of the enzyme, thus possibly deactivating more of the drug, then the chemical has been responsible for induction. If, however, the chemical prevents, or inhibits, the enzyme activity, the drug may not be metabolized and blood levels of the drug may rise. Induction and inhibition actions are usually associated with drug interactions with foods (such as grapefruit juice, charbroiled foods and cruciferous vegetables), environmental pollutants (such as cigarette smoke) and other drugs⁶. Some of the major substrates of enzymes of the CYP system having important clinical implications are listed in Tables 1 and 2.⁷.

Drug metabolism rates can affect the rate of renal excretion of the drug, inactivation of the drug, increased therapeutic action, activation of "pro-drugs," and increased or decreased toxicity. However, polymorphisms can occur that affect Pharmacokinetics (PK) and pharmacodynamics (PD) at the drug target, the drug uptake or activation pathways, or the drug metabolism pathways². SNPs (pronounced "snips") in the targeted protein can reduce drug efficacy. SNPs in drug uptake or activation could result in the patient being a nonresponder to the drug administered, since an alteration in uptake would prevent the drug from reaching the target cells and a mutation in activation would prevent the drug from having an effect². The primary focus of this discussion, however, deals with the mutations that occur in the metabolism genes that affect the CYP 450 enzyme system.

Research based on the HGP has been able to identify the genes responsible for the production of the isoenzymes in the CYP enzyme system. Pharmacogenomics needs to be taken into account when prescribing known cytochrome substrates. A thorough family history is crucial to determine familial differences in drug responses. In the future, prescription may be based upon genotypes. Using the Online Mendelian Inheritance Map⁸, some of the genetic locations for some of the CYP isoenzymes have been identified on chromosomes 4q35.1, 7q22.1 (isoenzyme CYP3A4), 10q24.1-q24.3 (CYP2C9 & CYP2C19), 19q13.2 (CYP2A6) and 22q13.1 (CYP2D6). With the completion of the HGP, single nucleotide polymorphisms (SNPs) are now known to be associated with what previously was termed "idiosyncratic reactions"⁶. Genetic alterations are evident in various ethnic groups. For instance, a percentage of the population of a particular group may lack one of the isoenzymes of the CYP-450 system, which forces them to utilize other metabolism pathways. It is known that 7% of the Caucasian population does not convert codeine to morphine because they have a defective CYP2D6 enzyme causing them to have no greater pain relief from Tylenol #3 as from plain Tylenol. In addition, in the United States about half of the population is what is termed slow acetylators. These individuals are more prone to develop certain adverse reactions with drugs that are metabolized by acetylation, such as isoniazid and procainamide^{5, 6, 9}.

Currently, scientists are making progress to catalog as many of the genetic variations found within the human genome as possible. These variations or SNPs could be useful in determining a patient's response to a drug. In order for this to happen, an individual's DNA must be sequenced for the presence of specific SNPs. However, this technology is currently very involved and is a significantly lengthy and expensive process. This prevents a widespread use of SNPs as a diagnostic tool for prescription of medication. DNA microarrays (also called DNA chips) are an evolving technology that shows promise in making a more rapid determination for specific SNPs that would be affordable. Currently, a single microarray can screen for 100,000 SNPs in under a day. In 2005, the FDA approved a diagnostic test for genotyping two important drug metabolizing enzymes (CYP450 2D6 and 2C19). This assay identifies functionally relevant SNPs using a DNA microarray⁹. As this technology advances, it would be possible in the future that this could be used in a physician's office prior to drug prescription¹⁰. For now, prescriber and practitioner alike need to be aware of the possibility of individual reactions to drugs and also have the knowledge of the CYP-450 system.

In the future, SNP screenings of patient would benefit the patient, the drug company and the prescriber. Drug developers could exclude individuals whose screenings would identify them as potentially being harmed by the drug or for whom the drug would be ineffective. This could also show the drug to be at a higher efficacy level when tested in individuals who would respond favorably. This could also have potential fiscal benefits since clinical trails could be smaller and less costly, thus decreasing the cost of the resultant drug in the market place. The prescriber would have more confidence in the medical regimen, and the patient would have greater confidence in the medication prescribed. This could lead to better development, prescription, and outcomes in pharmacotherapeutics.

Many diseases and disorders have genetic links when considering pharmacotherapy (See Table 2). The following section identifies these problems and focuses on some major treatment considerations. It is important for healthcare providers to be aware that certain genes or gene products can alter responses to drug therapy.

PSYCHIATRY

A multiplicity of depression and anxiety symptoms are treated with various medications, including the tricyclic antidepressants and serotonin-specific reuptake inhibitors (SSRIs). Approximately 3.5% of persons in the Netherlands have a mutation in the CYP2D6 causing ultra-rapid metabolizing. For this group of individuals certain drugs are broken down so rapidly that they cannot provide a therapeutic effect. One example is amitryptyline. Because it takes 4-6 weeks for many psychiatric agents to offer symptom relief, the knowledge that amitryptyline is metabolized by the CYP2D6 would help healthcare providers to anticipate that this group of patients in the Netherlands would not respond. This would enable early use of an alternate treatment, thus preventing an extended period of depression and diminished quality of life⁴.

SEIZURES

Seizure disorders are common for individuals with development disabilities. Phenytoin (Dilantin) is often a drug of choice for these patients. Phenytoins are inducers for a variety of CYP isoenzymes, including the CYP3A4 isoenzyme. Substrates of that enzyme would be metabolized more quickly and result in less effective treatment. These substrate drugs include carbamazepine (Tegretol), corticosteroids, lovastatin (Mevacor), and diazepam (Valium). Persons receiving these drugs should be carefully evaluated for successful therapy. (See Table 2.)

PROBLEMS WITH ANESTHESIA

Malignant Hyperthermia

The genetic link for malignant hyperthermia post anesthesia is the RYR1 (Ryanodine receptor) gene found on the long arm of chromosome 19, or 19q13 and other variants on chromosomes 17, 7 and 3⁸. In most affected families, it is an autosomal dominant inheritance pattern with an incidence of 1:60,000 and suspected in 1:4000. Malignant hyperthermia is considered a pharmacogenetic disease of the skeletal muscles, with symptoms (tachycardia, hyperthermia, hypercarbia, hypertension and rigidity) occurring only in the presence of inhalation gases used for anesthesia and the depolarizing muscle relaxants succinylcholine and decamethonium. There is a possible association with genetic diseases such as Duchenne's muscular dystrophy, other dystrophies and myopathies, such as myotonia congenita. Because malignant hyperthermia can lead to death if not treated early, individuals with a family incidence could be evaluated for possible phenotypic expressions of this gene with the Halothane Caffeine Contracture Test¹¹.

Serum Cholinesterase and Succinyulcholine Sensitivity

Also called Pseudocholinesterase deficiency, mutant alleles on the BCHE1 locus (gene map 3q26.1-q26.2) are responsible for a prolonged period of apnea (which can be fatal without rapid intervention) following the administration of normal dosages of succinyulcholine. Dantrolene is the drug of choice for treatment. Symptoms are only exhibited in the presence of the drug⁹.

Acetylation Polymorphism

Individuals with mutations in genes encoding a metabolizing protein that reduce the activity of the enzyme are considered poor metabolizers and may have difficulty breaking down the drug or clearing it from the body. These individuals could have toxic levels of the drug in the body and have an adverse reaction or suffer toxicity. The metabolism gene could also mutate and result in increased activity of the enzyme and produce an individual known as an extensive metabolizer. This individual could be a "nonresponder" to the drug, since metabolism would be increased and a greater amount of drug inactivated or cleared from the body much more quickly than anticipated². This explains the variations in responses to drugs from individual to individual even of the same ethnicity, sex, age group and family.

Glucose 6 Phospate Dehydrogenase Deficiency

It is estimated that more than 400 million individuals have a deficiency of the G6PD enzyme. This enzyme protects red blood cells from lysis. The gene for this enzyme is located on the q28 locus on the X chromosome. Since it is a recessive sex-linked gene, it is often not present in females, but is expressed in males. There are over 400 variant alleles that can contribute to this deficiency. Different populations have different variations of the mutation¹⁴. In Egypt, the Mediterranean variant is present, while in Japan, the Japanese variant predominates. Most of the affected individuals reside in Africa, the Middle East and South East Asia. African Americans have a very high incidence that approaches one in every four. This deficiency is also called "favism" since the individuals are also allergic to fava beans (a main staple in the Mid-Eastern diet)¹⁵. This enzyme deficiency also puts the individual at risk of potentially lethal hemolysis when given doses of a very long list of medications. These include aspirin (and other analgesics), probenicid, alpha-methydopa, ascorbic acid, hydroxychloroquine, and other antimalarials, sulfonamides, procainimide and quinidine¹⁶. Persons, or their family members, with this enzyme deficiency should be educated about the concern that these commonly used medications could cause the red blood cells to be destroyed.

Asthma

A person that is homozygous for the Gly 16-genotype will have fewer numbers of beta2-adrenergic receptors. Individuals with this genotype and an asthma diagnosis will need to use steroids because they have fewer receptors for the beta agonists to target⁴.

Alzheimer's Disease (AD)

Some disease-associated polymorphisms can have bearing on how an individual responds to a drug even though the DNA changes have nothing to do with the pharmacological activity of the agent. Alzheimer's disease and the drug tacrine (Cognex) are one example. Apolipoprotein E (APOE) is a genotype that influences how much enzyme activity is available to produce the neurotransmitter acetylcholine. Like several medications used to treat AD, tacrine inhibits the activity of the enzyme cholinesterase. For some undetermined reason, tacrine is more effective in decreasing the symptoms of AD dementia seen in persons with the sub-genotype, APOE4⁴. Even though testing for this sub-genotype is not sensitive at this time, in the future it may have important treatment implications.

Acute Intermittent Porphyria

One example of a gene-environment connection in pharmacogenomics is the disorder called acute intermittent porphyria. This is an autosomal dominant malady caused by a defect in porphobilinogen deaminase activity and is seen more frequently in women. The latent form of the disease exists until an environmental precipitating event, such as an infection, or excessive dieting and medications, like sulfonamides and barbiturates, induces the synthesis of the heme-containing P450 cytochromes. This causes the cellular level of heme to fall, thus affecting a feedback loop leading to a relative heme deficiency. Much about the cause of the most common clinical manifestation, neurological dysfunction, in is unknown. In addition, defects of the central nervous system and peripheral and autonomic neuropathies occur. For an unknown reason, hyponatremia due to inappropriate anti-diuretic hormone may also develop^12,13. This exemplifies how important awareness of environmental events and genes interaction with medications is for the health professional.

CONCLUSION

Pharmacogenomics can take the speculation out of prescribing medication and therefore enhance accurate and timely treatments. Practitioners and caregivers should be familiar with at least the genes or gene products that modify disease or treatment response to drugs (See Table 2). A further responsibility is to stay informed about cutting edge pharmacogenomic discoveries. For an updated site with information about genetic influences and drug responses go to http://medicine.iupui.edu/flockhart/²⁰.

Table 1: Some Inducers and Inhibitors of CYP-450 Isoenzymes^6,17

Isoenzyme	Inducer	Inhibitor
CYP1A2	Antimicrobials: rifampin Anti-epileptics: carbamazepine Antidiabetics: insulin Foods: chargrilled meat Recreational drugs: tobacco	Antimicrobials: ciprofloxacin, enoxacin, erythromycin Antidepressants: fluvoxamine Anitplatelets: ticlopidine H2 Blockers: cimetidine
CYP2C9	Antimicrobials: rifampin Barbiturates: Phenobarbital, secobarbital	Antimicrobials: ofloxacinisonizid. metronidazole, sulfamethoxazole, trimethoprim Antidepressants: fluvoxamine, paroxetine, sertraline Antidysrhythmics: amiodarone Antifungals: fluconazole, miconazole
CYP2C19	Antimicrobials: rifampin Anti-epileptics: carbamazepine Hormones: norethindrone	Anti-epileptics: felbamate Antidepressants: fluoxetine, fluvoxamine, paroxetine Antifungals: ketoconazole Proton Pump Inhibitors: lansoprazole, omeprazole
CYP2D6	Antimicrobials: rifampin Corticosteroids: dexamethasone	Antidepressants: fluoxetine, paroxetine, sertraline Antidysrhythmics: amiodarone, quinidine H1 Blockers: chlorpheniramine, hydroxyzine, promethazine H2 Blockers: cimetidine, ranitidine NSAIDS: celecoxib Recreational: cocaine
CYP2E1	Antimicrobials: isoniazid Recreational: ETOH, tobacco	Anti-Alcohol: disulfram
CYP3A4	Antimicrobials: rifabutin, rifampin Anti-epileptics: carbamazepine, phenytoin Barbiturates: Phenobarbital, secobarbital Corticosteroids: dexamethasone, hydrocortisone, prednisolone, methylprednisone Herbals: St. John's Wort Antivirals: efavirenz, nevirapine Anti-diabetics: piolglitazone, troglitazone	Antimicrobials: ciprofloxacin, clarithromycin, erythromycin, norfloxacin Antidepressants: fluvovoxamine, nefazodone Antidysrhythmics: amiodarone Calcium-channel blockers: diltiazem, verapamil Foods: grapefruit juice H2 blockers: cimetidine Protease inhibitors: idinavir, nelfinivir, ritonavir, saquinavir

Table 2. Genes/Gene Products that Modify Disease or Treatment Response to Drugs¹⁸

Gene or Gene Product	Disease or Response Association	Medication	Influence of Polymorphism
Adducin	Hypertension	Diuretics	MI or Strokes
Apolipoprotein E (APOE)	Progression of atherosclerosis, ischemic cardiovascular events	Statins	Enhanced survival
APOE	Alzheimer's disease	Tacrine	Clinical improvement
HLA	Toxicity	Abacavir	Hypersensitivity reaction
Ion channels (HERG,KvLQT1, Mink, MiRP1)	Congenital long-QT syndrome	Erythromycin, terfenadine, cisapride, clarithromycin, quinidine	Increased risk of drug-induced torsade de pointes
Stromelysin-1	Atherosclerosis progression	Statins (e.g., Pravastatin)	Reduction in cardiovascular events by pravastatin; reduction in risk of multiple angioplasties
CYP 450 2C9	Seizures	Phenytoin	Increased risk of seizure activity