Over many years of research on pharmacogenetics and pharmacogenomics, we are now aware that the genetic variability among individuals is able to explain the remarkable diversity of either the therapeutic or the adverse effects observed during drug treatment. The term pharmacogenomics refers to the study of variations in the characteristics of DNA and RNA related to the drug response, while the pharmacogenetics is a subset of pharmacogenomics, and is defined as the influence of variations in DNA sequence on the drug response.1 The response to drug treatment, as well as the occurrence of adverse events, is complex, involving multiple genetic and environmental factors and their interactions. Indeed, it is conditioned either by non-genetic factors, such as environment, diet, age, sex, lifestyle, or socio-economic rank, or by genetic variations in genes codifying proteins involved in drug metabolism and delivery. Thus, personalized medicine is based on the understanding of genomic, epigenomic, environmental and pathophysiological factors as well as on drug interactions.
In the past years, genome-wide association studies (GWAS) have identified a lot of common genetic variants underlying susceptibility, most of them with only small effects, to the complex response to drug treatment. The availability of genetic and genomic data is considered an important advance towards characterizing and explaining the variability in treatment response. This knowledge is mainly translated into clinical application by the introduction of pharmacogenomic labels into product descriptions, which state whether predictive genotyping of the patient’s DNA is mandatory, highly recommended or informative. At present, 176 drugs approved by the US Food and Drug Administration contain pharmacogenomics information, while for the European Medicines Agency, the current number of approximately 130 labels is increasing rapidly.2
Since the drug response is complex, no single molecular event is expected to fully unravel the mechanism of its variability among individuals. Conversely, multiple molecular levels can contribute to define different situations. In this context, the development of high-throughput technologies to detect genetic variation and gene expression certainly marked the opening of a new era for genetic and clinical research. Massively parallel DNA sequencing technologies, termed next generation sequencing (NGS), have rendered the whole-genome resequencing of each individual more and more practical. The increased knowledge of genetic and genomic differences among individuals has enhanced the attention given to the individual response, leading to a transition from population-based dosing and prescriptions to patient individualization both in drug development and in clinical practice (Figure 1). This could improve the probability of achieving desired treatment outcomes and/or reduce the risk of adverse effects.3 However, there is still a need for new integrative approaches that are able to combine data from the different ‘-omics’ technologies, in order to infer the causality relationship between genotype and phenotype.
Anemia affects approximately 1.6 billion people, 25% of the world population, mainly due to iron deficiency.4 However, secondary anemia, i.e. the anemia of chronic disease or anemia due to surgical blood loss, is the most common form of anemia in hospitalized patients and the second most prevalent worldwide after iron deficiency.5 Moreover, anemia is the most common side effect of a wide range of pharmacological treatments for very different conditions. A paradigmatic example of this concept is represented by glucose-6-phosphate dehydrogenase (G6PD) deficiency, which is associated with development of acute hemolytic anemia (AHA) induced by a number of drugs, the best known of which is the antimalarial primaquine. Recently, the list of drugs leading to AHA in G6PD-deficient subjects has been extended and the effects of each drug have been classified into two groups: i) predictable hemolysis, which means that AHA can be expected in a G6PD-deficient patient; ii) possible hemolysis, which means that AHA may or may not take place, depending on dosage, other concomitant drugs, co-morbidity and other factors.6
Another example is represented by the treatment with ribavirin (RBV), which is the standard of care therapy for chronic hepatitis C virus (HCV) infection in combination with pegylated interferon (peg-IFN)-alpha, and which is associated with a range of treatment-limiting adverse effects. One of the most important of these is RBV-induced AHA, which affects most HCV patients and often requires dose modification in up to 15% of patients. It has been demonstrated that genetic variants leading to inosine triphosphatase (ITPA) deficiency, which seems to be a benign condition, are a major protective factor against RBV-induced AHA in HCV-infected patients receiving standard care therapy.7 This knowledge is particularly important if we consider that various genetic predictors of response to standard treatment of chronic HCV infection has been already described, for example IL28B and SOCS3 single nucleotide polymorphisms (SNPs).98 Moreover, since ITPA deficiency seems to be a benign condition, it would be possible to envisage protection against RBV-induced anemia by pharmacological intervention against ITPA.
Genetic variations play a major role also in phenotypic variability in Mendelian diseases. The overwhelming majority of these are caused by rare mutations that affect the function of individual proteins. While the identification of such genetic variants has informed our knowledge of the etiological bases of diseases, there continues to be a sub stantial gap in our understanding of the factors that modify disease severity. Monogenic diseases provide an opportunity to identify modifiers as they have uniform etiology, detailed phenotyping of affected individuals, and familial clustering.10
Hereditary anemias represent a substantial and heterogeneous group of anemias in which the identification of genetic variants is crucial for appropriate treatment. Examples of studies of pharmacogenetics in hereditary anemias have been conducted on sickle cell disease (SCD) and β-thalassemia major (Table 1). Hydroxyurea therapy has been shown to improve signs and symptoms of SCD, primarily by increasing the level of fetal hemoglobin (HbF). Hydroxyurea, especially when escalated to the maximum tolerated dose (MTD, defined by moderate suppression of circulating neutrophils and reticulocytes), can consistently and significantly increase the percentage of HbF (%HbF) level in infants, children and adults with SCD.11 In spite of the clinical benefits from hydroxyurea treatment, extensive phenotypic variation is observed. For example, the %HbF achieved in young patients at hydroxyurea MTD ranges from a low %HbF (10–15%) to a high %HbF that occasionally exceeds 40%.12 Pharmacogenetics analysis highlighted the role of several SNPs influencing either the base-line %HbF (including 4 SNPs within BCL11A, one in Hb Epsilon and one in XmnI γ-globin promoter region) or the variation of %HbF between baseline and MTD; this is the case of the 2 SNPs located in ARG1 and ARG2 genes.
Another example of pharmacogenetics analysis in SCD is the study conducted for the opioid medications codeine and hydrocodone, commonly prescribed for the treatment of mild to moderate vaso-occlusive crisis. Both drugs require metabolic conversion by cytochrome P450 2D6 (CYP2D6) to morphine and hydromorphone, respectively, to exert their analgesic effects. The CYP2D6 gene is highly polymorphic, with variant alleles that result in decreased, absent, rapid or ultra-rapid enzyme activity. Several variants have been analyzed in SCD patients showing correlation with increased, decreased or absent enzyme activity (Table 1).13
In β-thalassemia the only treatment to sustain life is regular transfusions which inevitably lead to iron overload, thus iron chelation is necessary to prevent the multiple organ dysfunction and/or failure and decrease the mortality. For this reason, oral iron chelators like deferoxamine, deferiprone and deferasirox have been introduced into clinical practice. UGT1A6 is responsible for glucuronidation of deferiprone. In the study by Gunaseeli et al., serum ferritin levels were estimated periodically in β-thalassemia patients to evaluate iron overload and the patients were grouped into responders and non-responders depending on the ferritin levels. A significant difference in the genotypic distribution and allelic frequencies of UGT1A6*2 Thr181Ala (rs2070959) between responders and non-responders was observed. Conversely, there was no significant difference in the genotypic distribution of UGT1A6*2 Ser7Ala (rs6759892) between responders and non-responders; however, this SNP showed a significant difference between responders with adverse drug reactions and responders without adverse drug reactions.14
Currently, there are no pharmacogenetics studies exploring the correlation of drug responsiveness to genetic variants for many other hereditary anemias. Undoubtedly, several data will come from NGS studies on genetic variations in modifier genes that affect the clinical phenotype and are thus relevant for prognosis, follow up and for personalized treatment. Furthermore, new avenues are now opening up in the study of human induced pluripotent stem cells (iPS), derived from human somatic cells through the ectopic forced expression of OCT4 and SOX2 with either the combinations of KLF4 and MYC or NANOG and LIN28. The potential applications of iPS are enormous. Indeed, since these cells are derived from the patients themselves, they represent a true in vitro model of that particular disease, such as hereditary anemias, and reflect the same pathological features as in in vivo conditions. Thus, iPS could be considered the best candidate model to use in pharmacogenetics studies.
- Financial and other disclosures provided by the author using the ICMJE (www.icmje.org) Uniform Format for Disclosure of Competing Interests are available with the full text of this paper at www.haematologica.org.
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- Fellay J, Thompson AJ, Ge D. ITPA gene variants protect against anaemia in patients treated for chronic hepatitis C. Nature. 2010; 464(7287):405-8. PubMedhttps://doi.org/10.1038/nature08825Google Scholar
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