Genetics and the potential for predictive tests in adverse drug reactions.

Drug hypersensitivity reactions are an immune-mediated reaction to otherwise innocuous antigens derived from drugs. These reactions can affect many different organs, with the skin being the commonest. Skin involvement can range in severity with hypersensitivity syndrome (or DRESS) and the blistering reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis), also termed serious cutaneous adverse drug reactions, being the most severe and most feared. There is increasing evidence for the role of the immune system in the pathogenesis of these reactions, with drug-specific T cells having been identified in many patients. Until recently, very little was known about the predisposition to these reactions. However, the availability of more accurate molecular typing methods, and the ability to analyse the whole genome in an unbiased fashion, has led to some remarkable findings of the role of the HLA genes as genomic biomarkers of predisposition. The 'revolution' started with abacavir where the predisposition to hypersensitivity was linked to HLA-B*57:01, which was confirmed in a clinical trial, and where its implementation has shown to reduce the incidence of hypersensitivity in a cost-effective manner. Since then, associations have also been shown for allopurinol (HLA-B*58:01)- and carbamazepine (HLA-B*1502 and HLA-A*3101)-induced serious cutaneous adverse drug reactions. The latter is interesting since the association with HLA-B*1502 is present in certain South-Eastern Asian populations, and the predisposition is phenotype specific (only for SJS/TEN). The utility of this biomarker has been shown in a prospective cohort study performed in Taiwan. By contrast, the association with HLA-A*3101 is seen in more diverse ethnic groups, and predisposes to mild as well more severe cutaneous reactions associated with carbamazepine. It is important to note that strong HLA associations have also been shown with a number of drugs that cause liver injury including flucloxacillin, lumiracoxib, lapatinib and ximelagatran, indicating that the immune system is also important in the pathogenesis of other forms of drug-induced organ toxicity. The crucial question as to whether these HLA alleles are truly causative or acting as surrogate markers of predisposition, however, is still unclear, and will require further investigations in larger patient cohorts, through the use of bioinformatic techniques, fine mapping using next generation sequencing technologies and functional studies.