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Warfarin dose and the pharmacogenomics of CYP2C9 and VKORC1 – Rationale and perspectives

Warfarin is extensively prescribed as an oral anticoagulant for the prevention and treatment of thrombosis diseases.  “An insufficient dose may fail to prevent thromboembolism, while an overdose increases the risk of bleeding” (Yin 2). The main genes looked at concerning the effect of warafin dosage are the CYP2C9 and VKORC1 genes.

Warfarin inhibited the vitamin K epoxide reductase (VKOR) which is encoded by the vitamin K epoxide reductase complex subunit 1 (VKORC1) gene.  Demonstrated in Figure 1, Warfarin prevents the VKORC1 to regenerate reduced vitamin K from it’s epoxide form. The reduced version of vitamin K is needed as a cofactor for  GGCX, an enzyme that catalyzes vitamin-K dependent clotting factors; thus having reduced coagulation. “Congential deficiencies in GGCX and VKORC1 have disordered hemostasis,” and “functional abnormalities in VKORC1…” are also Warfarin resistant (Yin 3).

Polymorphisms in CYP2C9

The functional sequences of the human CYP2C9 gene are the CYP2C9*1, the wild-type, and CYP2C9*2 and CYP2C9*3. Analysis showed patients with either “CYP2C9*2 or CYP2C9*2 variant require lower warfarin maintenance dose” (Yin 4). The risk for bleeding doubled in these patients, as they metabolize warfarin slower than the wild-type patient.

S-warfarin and R-warfarin make up the mixture of warfarin. “S-warfarin is a five-fold more potent vitamin K antagonist than R-warfarin…, metabolized primarily by CYP2C9”; this is also shown in Figure 1 (Yin 4).

Table 1 corresponds to the missense mutations mapped on Figure 2.

The table I have constructed below, Table 1.A, shows the variations of CYP2C9 alleles in varying ethnicities. This can give medical doctors a clue on how to better dose patients.

Polymorphisms in VKORC1

The two most commonly looked at polymorphism a inVKORC1 are the “1173C>T in intron 1 and 3730G>A in the 3′ untranslated region.” (Yin 5). The dosage for warfarin was higher in individuals with the VKORC1 1173CC variantion than those with the CT or TT variation. Patients with the 1173CT genotype have a higher risk from bleeding events was shown in a study of 330 patients.

Haplotypes analysis was carried out by constructing 5 haplotypes groups from the 10 most common SNPs, and their relationships to warfarin dose were examined in Caucasians. The table I have constructed, Table 2.A, below can explain “a large degree of the interindividual variations of a warfarin dose” (Yin 5).

Table 3 demonstrates the effects of VKORC1 alleles and haplotypes in varying ethnicities.

Other studies looking at the VKORC1 and CYP2C9 genes support the findings from above “that Asians require a lower average maintenance warfarin dose and African-Americans a higher average dose” (Yin 6).

Algorithms for warfarin dose determination

A dosing algorithm was created based on 297 Caucasian warfarin-treated patients, accounting for 55% of warfarin dose variability.

Dose=(0.628)-(0.0135)*(age, year)-0.240(CYP2C9*2)-0.370(CYP2C9*3)-0.241(VKORC1, -1639G>A)+0.0162*(height, cm)

For patients with homozygous wild-type for CYP2C9 and VKORC1 is below. Those with heterozygous or homozygous for CYP2C9, “the maintenance dose was further reduced by 1.3 and 2.9 mg respectively” (Yin 7).

(mg)=6.6-0.035*(age, year)+0.031*(body weight, kg)

Other alternative warfarin doses were developed by classifying patients into three groups called the “warfarin-response index”.

Perspective

Together the variations between CYP2C9, VKORC1, and “factors such as age, gender, body weight, height..”, and others can predict more than 33% of the variability in warfarin dosage. Ethnicity is an especially important factor too for doctors to use in prescribing warfarin (Yin 5). The author’s of this paper go on to say that genotyping in patients may not necessarily be cost-effective. “Treatment algorithms incorporating pharmacogenomic data must be evaluated prospectively in a randomized controlled clinical trial before incorporating into routine clinical practice” (Yin 8).

References

Yin, T. & Miyata, T. (2006). Warfarin dose and the pharmacogenomics of CYP2C9 and VKORC1 – Rationale and perspectives. Thrombosis Research, 120, 1-10. doi:10.1016/j.thromres.2006.10.021

Serotonin Transporter: Gene, Genetic Disorders, and Pharmacogenetics

Dr. Murphy, et al. published an article in 2004 named Serotonin Transporter: Gene, Genetic Disorders, and Pharmacogenetics; the studies within that journal article are the discussion of this blog. Serotonin transporter helps transport serotonin into neurons, enterchromaffin cells, platelets, and other cells (Murphy, 2004). It re-uptakes and reuses serotonin after neural stimulation. Serotonin is a neurotransmitter that helps maintain mood, digestion and appetite, sleep, memory, and many other things mostly found in the brain and intestines. The serotonin transporter gene spans 37.8 kb on chromosome 17q11.2. Studies primarily look at polymorphisms on the 5HTTLPR region of repeated elements; this variable tandem repeat is about 1.4 kb upstream and near exon two, shown in Figure 1. The most common composition is fourteen (short, S) or sixteen (long, L) repeated elements. The L variant has high antidepressant efficacy. In some cases super long 5HTTLPR causes sudden infant death syndrome and OCD. Mice with genetically reduce or absent SERT  had low antidepressant effectiveness. The S variant is associated with lower expression of the SERT, and so an SS 5HTTLPR genotype has poorer antidepressant responses and more considerable side effects.  Genotyping of a patient before care would help improve drug treatments.

Figure 2 demonstrates the homology present in the SERT gene between ten species. There is also high sequence homology with the dopamine transporter (DAT) and norepinephrine transporter (NET). There is over 90% homology in vertebrates and lesser in insects and worms (Murphy, 2004). In the SERT TM8 region there is an isoleucine 425 valine mutation that is highly conserved of the transmembrane region. This missense mutation was recently detected in family members with obsessive compulsive disorder, and others like Asperger’s syndrome, social phobia, anorexia nervosa, tic disorder, depression, and alcohol abuse (Murphy, 2004). The mutation modifies the alpha helix and therefore function of the transporter. Both mutations in the I425V and LL 5HTTLPR direct towards more significant expression, function, and interactions. Interaction of two or more mutations should be considered but leads to more complex clinical trials.

Studies of SERT in the brain use positron emission tomography (PET) to visualize SERT activity. SERT is confirmed by a serotonin re-uptake inhibitors (SRIs) antagonist, most popularly used is DASB because of its good penetration of the blood brain barrier. Images of PET displaying SERT function can be seen in Figure 3. SRIs can work as antidepressants and in treatment of OCD. Studies are done on SRI metabolizing enzymes, like CYP2D6. Gene variants of CYP2D6 cause slow or ultra-rapid metabolism of SRIs in patients that needs to be monitored for toxicity and/or under dosing (Murphy, 2004). Overall, genetic polymorphisms in the 5HTTLPR can lead to developmental disorders, or other complex disorders like poly-substance abuse, irritable bowl syndrome, primary pulmonary hypertension, and myocardial infarction.  Variations in length of the 5HTTLPR and other mutations transform the function of SERT and should be taken into consideration when finding treatment for a patient. Also under consideration should be environmental interactions and epistatic genes. These studies will be useful in innovating the pharmacogenomics field.

Citations

Murphy, D. L., et al. (2004, April). Serotonin Transporter: Gene, Genetic Disorders, and Pharmacogenetics. Molecular Interventions, 4(2), 109-123. doi:10.1124/mi.4.2.8

Pharmacogenomics studies gene variations among individuals and groups of people that affect the responses to medication. The modern view of pharmacogenetics is it will help provide a “personalized medicine” to patients depending on their genome composition. The goal is to reduce adverse effects to medications, lower the cost of therapies, and improve efficacy (Meyer 2004). Doctors will have a better idea of what drug and dose to safely prescribe patients. Ways of doing this are looking at polymorphisms in genes that may encode for determinants of drug effects. These include transport, enzymes metabolism of drugs, receptors or ion channels (Meyer 2004).

     In the 1900s Sir Archibald Garrod came up with the concept of “chemical individuality” of man and carried out the first study on common genetic polymorphisms associated with response to chemicals. His experiment tested patient’s ability to taste a foreign chemical. Garrod was testing for the inability to taste (taste blindness) to phenylthiocarbamide (PTC). It was found that taste blindness is inherited as an autosomal-recessive trait. These were clues that varying genetic components, as well as race and ethnicity being recorded, respond differently to chemicals (Meyer 2004).

To establish a drug-profile of individuals, single nucleotide polymorphisms (SNPs) are sequenced and their frequencies are used for markers of genetic variation. High density genomics maps of SNPs that are inherited together are available online, like at International HapMap Project. This provides information about the phenotypic response of the gene.  Patients can be divided into groups based on their genomic composition and marks to predict treatment (Sadee et al. 2005). Also, entire mRNA products of a genome can be looked at to find nuclear receptors and transcription factors that induce transformation of drugs(Meyer 2004).

Alteration of genes can be caused by imprinting or chromatin remodeling when there are no polymorphisms. Epigenetic modifications can be made on histones and DNA like methylation and acetlyation. Recent studies have shown changes in the epigenome may have a role in diseases and possible therapies. Silencing tumors can be helped along by reverse CpG methylations and increase histone acetylation, in hopes of generating more suppressor genes (Sadee et al. 2005).

Overall, getting pharmacogenetics into doctor’s offices is slow because of the vast amount of encoded and imprinted information. Finding diagnostic genetic markers will help bring about new and better drugs.  As said by Sadee et al, we are shifting “from the mindset of ’one-drug-fits all’ and to ‘the right drug for the right patient at the right dose and time’ “.

Citations

Meyer, U. A. (2004). Pharmacogenetics- five decades of therapeutic lessons from genetic diversity. Nature Reviews Genetics, 5, 669-676. doi:10.1038/nrg1428

Sadee, W., et al. (2005). Pharmacogenetics/genomics and personalized medicine. Human Molecular Genetics, 14(2), 207-214. doi:10.1093/hmg/ddi261

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