Current Drug Metabolism - Volume 15, Issue 2, 2014

The relative role of genetic and environmental factors in the pathogenesis of Parkinson’s disease (PD) has been the matter of investigation and debate, especially in the last 30 years. The possible interaction between genetic and environmental factors led to a great number of association studies between single nucleotide polymorphisms (SNPs) of many candidate genes and PD risk. In this study we summarized and critically reviewed the results of studies published on this issue, with especial reference to those reported in the last 5 years. Many studies provided conflicting findings and, when positive associations were identified, associations were weak. Polymorphisms related with activation or detoxification of drugs and xenobiotics, such as CYP1A1, CYP1A2, CYP19A1, CYP1B1, CYP2C9, CYP2C19, CYP2E1, CYP2D6, NAT2, GSTM1, GSTM3, GSTO1, GSTP1, PON1, PON2, ABCB1 and ADH genes have not been demonstrated convincingly a definitive association with the risk of developing PD. Nor did polymorphisms in genes related to dopamine or serotonin DRD, DAT, TH, DDC, DBH, MAO, COMT, SLC6A4, MTR, MTHFR, oxidative stress NOQ1, NOQ2, mEPHX, HFE, GPX, CAT, mnSOD, HFE, HO-1, HO-2, NFE2L2, KEAP1, inflammatory processes, ILs, TNF, ACT, NOS, HNMT, ABP1, HRHs, trophic and growth factors BDNF, FGF, or mitochondrial metabolism and function. In addition we analyzed other putative relations and genes associated with monogenic familial PD.Taking together the results of candidate gene association studies and genome wide association studies, only some SNPs of the MAPT, SNCA, HLA and GBA genes seem to be the most likely associated with PD risk.

Cyclooxygenases (prostaglandin-endoperoxide synthases, (EC 1.14.99.1) 1 and 2 (COX-1 and COX-2)) are key enzymes with a highly functional and pharmacological relevance. Genetic variations in the corresponding genes PTGS1 and PTGS2 are related to diverse human disorders and adverse drug reactions. Although COX-2 is highly inducible, most genetic association studies have focused on coding region gene variants. The aim of this study is to analyze the genetic variants modifying transcription factor binding sites in human PTGS genes based on the combined use of bioinformatics with 1,000 genomes data and replication by next generation sequencing. Updated information on gene sequences and variants was obtained from the 1,000 genomes website and from a replication sequencing study. Of the 570 upstream PTGS1 gene variants, 43 altered binding sites, either by disrupting existing sequences or by creating new binding sites. The most relevant are the SNP rs72769722, which creates a new binding site for NFKB, and the SNPs rs73559017 and rs76403914, both disrupting binding sites for CDX1. Of the 682 upstream PTGS2 gene variants, 31 altered binding sites, the most relevant being rs689466 and rs20417, which disrupt binding sequences for MYB and E2F, respectively; rs689462 which creates a new binding site for POU3F2; and a haplotype combining the SNPs rs34984585+rs10911904, which creates a new binding site for SRY. This study provides a detailed catalog of variant and invariant transcription factor binding sites for PTGS genes and related haplotypes. This information can be useful to identify potential genetic targets for studies related to COX enzymes.

Adverse drug reactions involving a range of prescribed drugs and affecting the skin, liver and other organs show strong associations with particular HLA alleles. For some reactions, HLA typing prior to prescription, so that those positive for the risk allele are not given the drug associated with the reaction, shows high positive and negative predictive values. The best example of clinical implementation relates to the hypersensitivity reaction induced by the anti-HIV drug abacavir. When this reaction is phenotyped accurately, 100% of those who develop it are positive for HLA-B*57:01. Drug regulators worldwide now recommend genotyping for HLA-B*57:01 before abacavir is prescribed. Serious skin rashes including Stevens-Johnson syndrome and toxic epidermal necrosis can be induced by carbamazepine and other anticonvulsant drugs. In certain East Asians, these reactions are significantly associated with HLA-B*15:02, and typing for this allele is now recommended prior to carbamazepine prescription in these populations. Other HLA associations have been described for skin rash induced by carbamazepine, allopurinol and nevirapine and for liver injury induced by flucloxacillin, amoxicillin-clavulanate, lapatanib, lumiracoxib and ticlopidine. However, the predictive values for typing HLA alleles associated with these adverse reactions are lower. Clinical implementation therefore seems unlikely. Performing HLA typing is relatively complex compared with genotyping assays for single nucleotide polymorphisms. With emphasis on HLA-B*57:01, the approaches used commonly, including use of sequence-specific oligonucleotide PCR primers and DNA sequencing are considered, together with their successful implementation. Genotyping single nucleotide polymorphisms tagging HLA alleles is a simpler alternative to HLA typing but appears insufficiently accurate for clinical use.

Pharmacogenetics and Pharmacogenomics areas are currently emerging fields focused to manage pharmacotherapy that may prevent undertreatment while avoiding associated drug toxicity in patients. Large international differences in the awareness and in the use of pharmacogenomic testing are presumed, but not well assessed to date. In the present study we review the awareness of Latin American scientific community about pharmacogenomic testing and the perceived barriers for their clinical application. In order to that, we have compiled information from 9 countries of the region using a structured survey which is compared with surveys previously performed in USA and Spain. The most relevant group of barriers was related to the need for clear guidelines for the use of pharmacogenomics in clinical practice, followed by insufficient awareness about pharmacogenomics among clinicians and the absence of regulatory institutions that facilitate the use of pharmacogenetic tests. The higher ranked pairs were TPMT/thioguanine, TPMT/azathioprine, CYP2C9/warfarin, UGT1A1/irinotecan, CYP2D6/amitriptiline, CYP2C19/citalopram and CYP2D6/clozapine. The lower ranked pairs were SLCO1B1/simvastatin, CYP2D6/metoprolol and GP6D/chloroquine. Compared with USA and Spanish surveys, 25 pairs were of lower importance for Latin American respondents. Only CYP2C19/esomeprazole, CYP2C19/omeprazole, CYP2C19/celecoxib and G6PD/dapsone were ranked higher or similarly to the USA and Spanish surveys. Integration of pharmacogenomics in clinical practice needs training of healthcare professionals and citizens, but in addition legal and regulatory guidelines and safeguards will be needed. We propose that the approach offered by pharmacogenomics should be incorporated into the decision-making plans in Latin America.

The Clinical Pharmacogenetics Implementation Consortium (CPIC) publishes genotype-based drug guidelines to help clinicians understand how available genetic test results could be used to optimize drug therapy. CPIC has focused initially on well-known examples of pharmacogenomic associations that have been implemented in selected clinical settings, publishing nine to date. Each CPIC guideline adheres to a standardized format and includes a standard system for grading levels of evidence linking genotypes to phenotypes and assigning a level of strength to each prescribing recommendation. CPIC guidelines contain the necessary information to help clinicians translate patient-specific diplotypes for each gene into clinical phenotypes or drug dosing groups. This paper reviews the development process of the CPIC guidelines and compares this process to the Institute of Medicine’s Standards for Developing Trustworthy Clinical Practice Guidelines.

The cytochrome P450 2D6 (CYP2D6) enzyme contributes to the metabolism and/or bioactivation of approximately 25% of clinically used drugs. The CYP2D6 gene locus is highly polymorphic and complex, and variants within this gene locus affect CYP2D6 enzymatic function resulting in a wide range of metabolic activity from little to no activity to ultrarapid metabolism. For many of the drugs metabolized by CYP2D6, the variation in metabolic activity is one of the most important factors responsible for interindividual drug response. Therefore, determining an individual’s CYP2D6 phenotype, or metabolic status, will help identify individuals that may benefit from a change in drug or drug dosage. Genotype analysis has become the method of choice to predict a person's metabolic status. Numerous reference laboratories now offer CYP2D6 genotyping; however, there can be substantial differences in the number of genetic variants interrogated as well as test interpretation. Furthermore, there is no standardized process of how a CYP2D6 genotype result is translated into a phenotype assignment. This review summarizes the complexity of CYP2D6 genotyping and highlights the major challenges for phenotype classification. We call for the implementation of a universally accepted system for CYP2D6 phenotype assignment to promote consistency of test interpretation among reference laboratories and medical institutions. We propose a system that utilizes the CYP2D6 activity score system to place individuals into a continuum of activity scores - rather than using the traditional poor, intermediate, extensive and ultra-rapid metabolizer categorizations - and directly translating activity scores into clinically actionable recommendations.

Numerous studies have analyzed the impact of N-acetyltransferase 2 (NAT2) polymorphisms on drug efficacy, side effects as well as cancer risk. Here, we present the state of the art of deriving haplotypes from polymorphisms and discuss the available software. PHASE v2.1 is currently considered a gold standard for NAT2 haplotype assignment. In vitro studies have shown that some slow acetylation genotypes confer reduced protein stability. This has been observed particularly for G191A, T341C and G590A. Substantial ethnic variations of the acetylation status have been described. Probably, upcoming agriculture and the resulting change in diet caused a selection pressure for slow acetylation. In recent years much research has been done to reduce the complexity of NAT2 genotyping. Deriving the haplotype from seven SNPs is still considered a gold standard. However, meanwhile several studies have shown that a two-SNP combination, C282T and T341C, results in a similarly good distinction in Caucasians. However, attempts to further reduce complexity to only one 'tagging SNP' (rs1495741) may lead to wrong predictions where phenotypically slow acetylators were genotyped as intermediate or rapid. Numerous studies have shown that slow NAT2 haplotypes are associated with increased urinary bladder cancer risk and increased risk of anti-tuberculosis drug-induced hepatotoxicity. A drawback of the current practice of solely discriminating slow, intermediate and rapid genotypes for phenotype inference is limited resolution of differences between slow acetylators. Future developments to differentiate between slow and ultra-slow genotypes may further improve individualized drug dosing and epidemiological studies of cancer risk.