UGT1A1 gene variations in individuals with and without clinical diagnosis of Gilbert Syndrome

Abstract

UGT1A1 gene variations in individuals with and without clinical diagnosis of Gilbert Syndrome Bilirubin is a non-polar metabolite, results from catabolism of haemoglobin and is bound to glucuronic acid in the liver by the uridine diphosphate glucuronosyltransferase (UGT1A1) activity. Molecular studies showed that the presence of two extra bases (TA duplication) in the promoter region of the UGT1A1 gene is responsible for the reduced UGT1A1 glucuronization activity and is the main cause of unconjugated hyperbilirubinemia observed in patients with Gilbert Syndrome (GS). However, individuals with normal bilirubin levels and no clinical symptoms of SG may also present this polymorphism in homozygosity 1,2,3. Consequently, the aim of this work is to determine the presence of other mutations in the UGT1A1gene, downstream of the TA duplication, and how they may contribute towards the inter-individual variation of serum bilirubin levels. This study was carried out in two groups: one comprising 36 individuals without clinical diagnosis of GS (14 with 6/6TA, 11 with 6/7TA, and 11 with 7/7TA repeats); the other group consisting of 36 patients clinically diagnosed with GS. In both, bilirubin levels were determined and direct sequencing of the UGT1A1 was performed. Among the individuals without clinical diagnosis of GS, two new sequence variants were found in heterozygosity (c.643A>G, and c.1156G>A), in the 6/6TA group. No additional mutations were detected in the 6/7 and 7/7 TA groups. In patients clinically diagnosed with GS, 28 were homozygous and 7 heterozygous for the TA duplication, and one with a normal number of repeats. Molecular analysis showed that one (3,6%) of the 7/7TA patients had another mutation in the UGT1A1 gene (c.674T>G). In the 6/7TA group, one additional mutation was also found in three patients (43%), two of which had been previously described (c.674T>G and c.923G>A) and a new one (c.1423C>T). No further mutations were detected in the 6/6TA group. Additionally, 4 polymorphisms were found (c.864+89C>T; c.997-37T>C; c.997-82A>C; c.997-87A>C). In conclusion, we can infer that homozygosity for the TA duplication is associated with GS. In the group without GS, no further mutations were detected in the 6/7 and 7/7 clusters, but in the 6/6 group, two new mutations were found in heterozygosity. These mutations are not associated with increased bilirubin levels. However, they could be associated with GS in the presence of other UGT1A1 mutations. Furthermore, in the GS group with heterozygosity for the TA duplication, we found mutations in 43% of the patients, emphasizing the importance of complete UGT1A1 analysis