Determination of CYP2D6 gene copy number by multiplex polymerase chain reaction analysis

Simple and Robust Detection of CYP2D6 Gene Deletions and Duplications Using CYP2D8P as Reference

Genotyping of the CYP2D6 gene is the most commonly applied pharmacogenetic test globally. Significant economic interests have led to the development of a plurality of assays, available for almost any genotyping platform or DNA detection chemistry. Of all the genetic variants, copy number variations are particular difficult to detect by polymerase chain reaction. Here, we present two simple novel approaches for the identification of samples carrying either deletions or duplications of the CYP2D6 gene; by relative quantification using a singleplex 5′nuclease real-time PCR assay, and by high-resolution melting of PCR products. These methods make use of universal primers, targeting both the CYP2D6 and the reference gene CYP2D8P, which is necessary for the analysis. The assays were validated against a reference method using a large set of samples. The singleplex nature of the 5′nuclease real-time PCR ensures that the primers anneal with equal affinity to both the sequence of the CYP2D6 and the reference gene. This facilitates robust identification of gene deletions and duplications based on the cycle threshold value. In contrast, the high-resolution melting assay is an end-point PCR, where the identification relies on variations between the amount of product generated from each of the two genes.

Unique Genotyping Protocol of CYP2D6 Allele Frequency Using Real Time Quantitative PCR from Japanese Healthy Women

CYP2D6 is an important drug-metabolizing enzyme involved in the metabolism of 20-25% of commonly prescribed drugs. Genetic polymorphism of CYP has clinically significant modifications in patients' drug-metabolizing capacities. Since gene copy number variation (CNV) and single nucleotide polymorphism (SNP) frequently occur in the CYP2D6 gene, which the activity of CYP2D6 particularly depend on the genetic factors. This study aimed to investigate the frequencies of CYP2D6 genotypes in a Japanese female subject of 216 healthy volunteers. The volunteers were genotyped for CNV Exon 9 and four CYP2D6 genetic variants (*2, *5, *10, *14, *41) performed by TaqMan^® genotyping assays. The CNV allele frequencies were 82.9% for two copies, 11.6% for one copy, 4.6% for three copies and 0.9% for zero copy, respectively. The frequencies of CYP2D6*1, *2, *5, *10, *14, and *41 were 38.7, 16.7, 6.3, 34.7, 0.2, and 1.2%, respectively. CYP2D6*5 and *14 were the major defective alleles. However, this genotyping is labor intensive, time consuming, and costly. We report an optimized novel protocol for the determination of CNV and SNP in CYP2D6 gene by real-time quantitative PCR. This can lower the cost and accurately determine CNV and SNP in the CYP2D6 gene with a higher output and enabling reliable estimates of disease prediction in large epidemiological samples.

Pharmacogenetics of analgesic drugs

• Individual variability in pain perception and differences in the efficacy of analgesic drugs are complex phenomena and are partly genetically predetermined.

• Analgesics act in various ways on the peripheral and central pain pathways and are regarded as one of the most valuable but equally dangerous groups of medications.

• While pharmacokinetic properties of drugs, metabolism in particular, have been scrutinised by genotype–phenotype correlation studies, the clinical significance of inherited variants in genes governing pharmacodynamics of analgesics remains largely unexplored (apart from the µ-opioid receptor).

• Lack of replication of the findings from one study to another makes meaningful personalised analgesic regime still a distant future.

• This narrative review will focus on findings related to pharmacogenetics of commonly used analgesic medications and highlight authors’ views on future clinical implications of pharmacogenetics in the context of pharmacological treatment of chronic pain.

A novel simple method for determining CYP2D6 gene copy number and identifying allele(s) with duplication/multiplication

Background

Cytochrome P450 2D6 (CYP2D6) gene duplication and multiplication can result in ultrarapid drug metabolism and therapeutic failure or excessive response in patients. Long range polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP) and sequencing are usually used for genotyping CYP2D6 duplication/multiplications and identification, but are labor intensive, time consuming, and costly.

Methods

We developed a simple allele quantification-based Pyrosequencing genotyping method that facilitates CYP2D6 copy number variation (CNV) genotyping while also identifying allele-specific CYP2D6 CNV in heterozygous samples. Most routine assays do not identify the allele containing a CNV. A total of 237 clinical and Coriell DNA samples with different known CYP2D6 gene copy numbers were genotyped for CYP2D6 *2, *3, *4, *6, *10, *17, *41 polymorphisms and CNV determination.

Results

The CYP2D6 gene allele quantification/identification were determined simultaneously with CYP2D6*2, *3, *4, *6, *10, *17, *41 genotyping. We determined the exact CYP2D6 gene copy number, identified which allele had the duplication or multiplication, and assigned the correct phenotype and activity score for all samples.

Conclusions

Our method can efficiently identify the duplicated CYP2D6 allele in heterozygous samples, determine its copy number in a fraction of time compared to conventional methods and prevent incorrect ultrarapid phenotype calls. It also greatly reduces the cost, effort and time associated with CYP2D6 CNV genotyping.

Complexities of CYP2D6 gene analysis and interpretation

Cytochrome P450 2D6 (CYP2D6) plays an important role in the metabolism and bioactivation of about 25% of clinically used drugs including many antidepressants, antipsychotics and opioids. CYP2D6 activity is highly variably ranging from no activity in so-called poor metabolizers to ultrarapid metabolism at the other end of the extreme of the activity distribution. A large portion of this variability can be explained by the highly polymorphic nature of the CYP2D6 gene locus for which > 100 variants and subvariants identified to date. Allele frequencies vary markedly between ethnic groups; some have exclusively or predominantly only been observed in certain populations. Pharmacogenetic testing holds the promise of individualizing drug therapy by identifying patients with CYP2D6 diplotypes that puts them at an increased risk of experiencing dose-related adverse events or therapeutic failure. Inferring a patient's CYP2D6 metabolic capacity, or phenotype, however, is a challenging task due to the complexity of the CYP2D6 gene locus. Allelic variation includes SNPs, small insertions and deletions, gene copy number variation and rearrangements with CYP2D7, a highly related non-functional gene. This review provides a summary of the intricacies of CYP2D6 variation and genotype analysis, knowledge that is invaluable for the translation of genotype into clinically useful information.

Opioid genetics: the key to personalized pain control?

There are now several strong opioids available to choose from for the relief of moderate to severe pain. On a population level, there is no difference in terms of analgesic efficacy or adverse reactions between these drugs; however, on an individual level there is marked variation in response to a given opioid. The genetic influences to this variation are complex, and although current research has shown some promising results, these have not been replicated across larger studies and as such the ultimate aim of personalized prescribing remains elusive. If personalized prescribing could be achieved this would have a major impact at an individual level to facilitate safe, effective and rapid symptom control. This review presents some of the recent positive advances in opioid pharmacogenetic studies, focusing on associations between candidate genes and the three main elements of opioid response: analgesic, upper gastrointestinal and central adverse reactions.

CYP2D6, SULT1A1 and UGT2B17 copy number variation: quantitative detection by multiplex PCR

Aim: Among the genes of drug-metabolizing enzymes, CYP2D6 is notoriously difficult to characterize owing to the complexity of gene deletions, duplications, multiplications and the presence of hybrid genes composed of CYP2D6 and CYP2D7. For SULT1A1 up to five gene copies have been reported, while UGT2B17 is known for gene deletions only. Different platforms exist for copy number variation (CNV) detection; however, there are no gold standards. Robust methods are required that address specific challenges to accurately determine gene CNVs in complex gene loci. Materials & methods: Quantitative multiplex PCR amplification (MPA) was performed on a diverse set of genomic DNA samples. Resulting PCR fragments were separated on an ABI 3730 instrument and analyzed with GeneMapper. CYP2D6 was targeted at four different gene regions and either normalized against CYP2D8 or UGT2B15 and SULT1A2. Inconsistent observations and CNVs contrasting genotype data were further characterized by long-range PCR and/or DNA sequence analysis. UGT2B17 and SULT1A1 were normalized against UGT2B15 and SULT1A2, respectively. Results: MPA detected 0–5, 1–5 and 0–2 copies for CYP2D6, SULT1A1 and UGT2B17, respectively. The interrogation of four CYP2D6 regions resulted in robust copy number assignments that were in agreement with genotype, sequencing and extra long PCR-based data. Gene deletions, duplication, and multiplications among known and novel hybrid genes were reliably identified. Novel findings regarding allelic variation include nonfunctional CYP2D6/2D7 hybrids such as CYP2D6*4N and *68, which were consistently identified on a subset of CYP2D6*4 alleles. In addition, a novel variant, designated CYP2D6*83, was discovered. For SULT1A1, we report the first six-copy case and for UGT2B15 and UGT2B17 we have evidence for rare deletion and duplication events, respectively. Conclusion: This MPA-based copy number platform not only allowed us to determine CNVs, but also served as a tool for allele discovery and characterization in a diverse panel of samples in a fast and reliable manner.

Original submitted 6 July 2011; Revision submitted 24 August 2011

Association of CYP2D6 single-nucleotide polymorphism with response to ophthalmic timolol in primary open-angle Glaucoma--a pilot study

OBJECTIVES

We aimed to investigate whether CYP2D6 polymorphisms were associated with the intraocular pressure (IOP)-lowering effect and systemic complications (especially bradycardia) of ophthalmic timolol.

METHODS

One hundred twenty-three primary open-angle glaucoma subjects (123 eyes) were treated with 0.5% aqueous formulations of ophthalmic timolol. IOP and heart rate were measured before and after timolol administration. DNA was extracted from venous leukocytes of all the subjects. Two single-nucleotide polymorphisms (SNPs) of CYP2D6, for example, rs16947 (2850C>T, R296C) and rs1135840 (4180C>G, S486T), were detected by restriction fragment length polymorphism analysis.

RESULTS

Neither rs16947 (P = 0.339) nor rs1135840 (P = 0.903) genotype was statistically correlated with the IOP-lowering effect of timolol eye drops. The rs16947 genotype was significantly associated with occurrence of bradycardia in primary open-angle glaucoma patients (P = 0.021). The patients with rs16947 CT (P = 0.043) or TT (P = 0.043) were more inclined to bradycardia than those with rs16947 CC, although there was no significant difference between CT and TT (P = 0.177). The analysis of variance showed no significant difference in heart rate (P = 0.559) among GG, GC, and CC groups.

CONCLUSIONS

CYP2D6 SNP rs16947 may confer susceptibility to timolol-induced bradycardia. Patients with CC genotype were unlikely to suffer from timolol-induced bradycardia, whereas those with TT genotype were found to suffer from timolol-induced bradycardia.

Isolation and characterization of the CYP2D6 gene in Felidae with comparison to other mammals

The highly polymorphic CYP2D6 protein metabolizes about 25% of commonly used drugs and underlies a broad spectrum of drug responses among individuals. In contrast to extensive knowledge on the human CYP2D6 gene, little is known about the gene in non-human mammals. CYP2D6 mRNA from 23 cats (Felidae) spanning seven species were compared to available CYPD6 sequences in ten additional mammals and multiple allelic variants in humans. A relatively high mean dN/dS ratio (0.565) was observed, especially within Felidae. Pairwise dN/dS ratios were non-monotonically distributed with respect to evolutionary distance suggesting either positive selection or retention of slightly deleterious mutations. Positive selection on specific codons, most notably in regions involved in substrate recognition and membrane anchoring is supported and the possible influence of diet on specific amino acid changes in substrate binding sites is discussed.