Gilbert's syndrome: genetic polimorphisms implications and diagnostic methods

Gilbert’s syndrome (GS) is an hereditary unconjugated hyperbilirubinemia resulting from impaired hepatic bilirubin clearance. It is generally a non-symptomatic disease, manifested with intermittent episodes of jaundice as result of exceeding the normal bilirubin level. GS plays a role in contributing to an accelerated onset of neonatal jaundice; in newborns the disease does not cause severe hyperbilirubinemia by itself, but it could lead to kernicterus if in cooperation with other factors. The clinical phenotype is determined by genetic variations on the uridine 5'-diphospho-glucuronosyltransferases (UGT) gene complex, that lead to polymorphic forms of the corresponding enzymes, displaying reduced activity toward bilirubin glucuronidation and metabolization of several drugs. Mutations in the TATA-box of the UGT1A1 gene, whose wildtype sequence is A(TA)6TAA, must be considered as the genetic basis for GS. A uniqueness of GS is the strong association that is established between certain polymorphisms, this synergy is reflected in the reciprocal influence of enzymes and their metabolizing activity. In order for a faster and more precise diagnosis to be possible, it is essential to develop genotyping techniques that replace the currently non-specific laboratory tests. The best strategy seems to be to use molecular methods calibrated for the single polymorphisms class. Recent studies have assessed this demand by proposing Real-Time Fluorescent PCR for genotyping UGT1A1 (TA)n polymorphism using hybridization probes on the LightCycler™. If this technique is sensitive to all the known combinations, concerning the use of High Resolution Melting curve analysis, it must be considered that the melting profiles of the different sequences are not always distinguishable. Similarly, Real-Time PCR with TaqMan Minor Groove Binder probes is more suitable for certain genotypes. As a general rule, it is advisable to consider the prevalence of polymorphism in the target population, as well as concomitant illnesses that can affect the patients, the availability of laboratory equipment and obviously the costs of the tests to be performed as determining factors in the choice of the most fitting genotyping method.