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Unit of Rheumatology
Differences in susceptibility to diseases, as well as variations in disease activity and in the level of response to treatment, have long been believed to be genetically determined. Nowadays, several remarkable examples of diversity in the metabolic rates of common drugs, and polymorphisms in the genes underlying these processes, have opened new fields for pharmacogenetics/pharmacogenomics.
Genotyping for prediction of dose and the mode of response to drugs has become a reality for hospital practice as well as for clinical trials. This is an important issue for public healthcare in order to rationalise treatment costs.
Recent findings in the genetics of responses to the treatment of rheumatoid arthritis (RA) show possibilities for fine tuning of the therapy for this relatively common human disease.
Therapeutic approaches to RA
RA is a multifactorial inflammatory disease with a prevalence of 0.5–1% in different populations. The main manifestations of the disease at the tissue level are joint-specific inflammation, proliferation of synovial tissue and bone erosion in the joint. The proinflammatory cytokine TNF (tumour necrosis factor) was shown to play one of the central roles in the cascade of inflammatory reactions preceding and resulting in RA. There are several therapeutic approaches now available for the treatment of arthritis, including use of nonsteroidal anti-inflammatory drugs (NSAIDs), glucocorticoids, disease-modifying antirheumatic drugs (DMARDs) and TNF-blocking agents. Among the DMARDs, methotrexate, sulfasalazine and azathioprine are the most frequently used.
Despite their profound effects, especially during the early stages of the disease, some patients remain nonresponsive to common doses of these drugs or develop side-effects because of toxicity.
Genetics of drug response
Genetic variation, polymorphism, is one of the common features of the human genome. Distinct alleles of the same gene may differ in only one nucleotide that is crucial for the structure or level of production of the protein. There are also numerous silent differences and genetic markers in the genome with as yet unknown function. Each polymorphism’s frequency can be estimated in a population and may differ between ethnic groups or in geographically or historically isolated areas. Several studies of the genes of enzymes involved in drug metabolism show that certain alleles, usually corresponding to isoforms with lower enzyme activity, are associated with poor response to therapy or toxicity effects in RA patients.
One of the most studied examples is thiopurine S-methyltransferase (TPMT), an enzyme in the purine metabolic pathway.(1) Metabolism of thiopurine-based drugs (eg, azathioprine) was shown to be associated with activity of TPMT, and this activity could be decreased or even not detectable in individuals with rare alleles of the TPMT gene. Several studies show a higher frequency of side-effects of azathioprine in RA patients carrying these rare alleles. In the analysis of 111 RA patients treated with azathioprine, all carriers of the rare allele of the TPMT gene developed moderate or severe side-effects.(2) Estimation of cost benefits from preliminary genotyping of individuals rather than empirical selection of azathioprine doses shows a positive effect in different populations and healthcare settings.(3,4)
Arylamine N-acetyltransferases (NAT) are a group of enzymes that catalyse the acetyl transfer from acetylcoenzyme A to an aromatic amine or a hydrazine compound. Acetylation is an important route of conversion and inactivation of many common drugs (eg, sulfasalazines).(5) Recently, remarkable differences in the frequency of adverse effects of sulfasalazine were reported in a group of 144 Japanese RA patients with different NAT2 (isoform 2 of arylamine N-acetyltransferase) genotypes.(6)
The enzyme 5,10-methylenetetrahydrofolate reductase (MTHFR) plays a central role in folate metabolism. It was shown to interact with methotrexate, exhibiting an inhibitory effect. Two common polymorphisms of MTHFR affect enzyme activity and thermostability, and alter both the efficacy and toxicity of methotrexate in RA patients.(7) In a study of 106 Japanese RA patients a significant difference in the rate of ESR and CRP changes were reported in individuals with different A1298C alleles, while the C677T polymorphism was associated with side-effects of methotrexate treatment.(8) In another study of 236 Dutch RA patients the C677T polymorphism was associated with methotrexate discontinuation because of adverse events, while association of A1298C polymorphism with efficacy was not demonstrated.(9)
Methotrexate has also been shown to inhibit thymidylate synthase (TS), and differential activity of this enzyme may affect toxicity and efficacy of methotrexate treatment. Indeed, in a study of 167 Japanese individuals with RA a significant association of efficacy of methotrexate treatment with polymorphism in the TS gene was confirmed, while no effect for MTHFR polymorphism was evident.(10)
Corticosteroid therapy is known as a rapid and efficient tool for downregulation of synovial inflammation in RA, although induction of tolerance to the treatment, immunosuppression and numerous metabolic effects are rather common. Several rare mutations in the human glucocorticoid receptor could not explain these effects, and the list of candidate genes involved in correspondent metabolic pathways is extremely broad.(11) One interesting finding shows an association of RA with a polymorphism in the 3′ untranslated region of exon 9b of the human glucocorticoid receptor beta,(12) indicating an approach for further analysis of corticosteroid resistance in these patients.
It was established a long time ago that the cytochrome P450 subfamily CYP2C9 is involved in metabolising many common drugs, including NSAIDs. Nevertheless, no clear association between polymorphisms in these genes and side-effects of NSAIDs has been found to date.(13)
Successful introduction of anti-TNF therapies dramatically changed the potential of treatment of inflammatory diseases such as RA. Two main forms of this therapeutic agent – recombinant receptor (etanercept) and monoclonal antibody (infliximab) are currently on the market. Several side-effects have been reported, mainly infections and allergic reactions, and 20–50% of patients with RA do not respond to anti-TNF therapy.
Only two studies of the pharmacogenetics of anti-TNF treatment have been published to date. In one of these, 59 patients with RA treated with infliximab were analysed for G-308A TNFA polymorphism, and a significantly low rate of response was found in carriers of the -308A TNFA allele (TNF2 allele).(14) Our own study of 123 RA patients treated with etanercept showed no single marker, but a combination of G-308A TNFA (TNF2) and G-1087A IL10 polymorphisms to be most predictive for a positive effect of therapy.(15)
Possibly the balance between proinflammatory (TNF) and anti-inflammatory (IL-10) cytokine production rather than the concentration of TNF alone better reflects the conditions of poor response to anti-TNF therapy. Indeed, the analysis of two other genes reflecting this balance – TGFB1 (tissue growth factor beta-1) and IL1RN (interleukin-1 receptor antagonist) – show that poor responders are more likely to have a rare combination of alleles of these two genes.(15)
Further analysis of genetic markers influencing this balance is essential for understanding this phenomenon. For example, treatment of Crohn’s disease patients with infliximab was shown to have different effects in individuals with different alleles of the TNFRSF1B gene (TNFR2, type 2 of the TNF receptor).(16) No published data on genetic markers for the side-effects of anti-TNF therapy are known to date.
It is obvious that efficient and fast genotyping provides the possibility to analyse several genetic markers related to efficacy and/or side-effects of antirheumatic drugs. Despite the fact that not all details are clearly understood, one can predict the imminent emergence of genetic testing of RA patients before prescribing drugs.