Analysis Of UGT1A1 Gene In Unconjugated Hyperbilirubinemia And Its Correlation With NAFLD

Background and Objectives Unconjugated hyperbilirubinemia is a common clinical condition,which is present in 5-10% of the world’s population,but its genetic mechanism is not yet clear.Uridine diphosphate glucuronyltransferase 1A1(UGT1A1)is the only enzyme involved in the conjugation of bilirubin in hepatocytes.Various UGT1A1 gene structural variants decrease the activity of UGT enzyme,causing insufficient indirect bilirubin glucuronidation,thereby increasing bilirubin levels.Previous studies have focused on the effect of single point variant of UGT1A1 gene on unconjugated hyperbilirubinemia,and there is still a lack of comprehensive understanding of the impact of UGT1A1 gene on hyperbilirubinemia.Based on the whole UGT1A1 gene sequencing and from the perspective of the single point variant and compound heterozygous variants of UGT1A1 gene,this study will thoroughly explore the genetic basis of unconjugated hyperbilirubinemia such as Gilbert syndrome and Crigler-Najjar syndrome,providing theoretical basis for the genetic diagnosis of hyperbilirubinemia.Non-alcoholic fatty liver disease(NAFLD)is the most common liver disease worldwide,and previous studies suggest that oxidative stress plays an important role in its development and progression.As a powerful endogenous antioxidant,bilirubin can reduce the risk of oxidative stress-related diseases by combating oxidative stress damage.Retrospective studies suggest that elevated total bilirubin levels are associated with a reduced risk of NAFLD.However,it is not clear whether the association reflects the true biological protective effect of bilirubin or is a confounding of other factors.In this study,we analyzed the effect of bilirubin level and common UGT1A1 gene variants on NAFLD from the perspective of Mendelian randomization.Methods A total of 418 health checker,3 patients with Crigler-Najjar syndrome type II and 9 family members were included in this study.The clinical datas and the EDTA anticoagulation blood were collected.UGT1A1 gene sequencing was performed on all subjects.To explore the association between single point variant and compound heterozygous variants of UGT1A1 gene and unconjugated hyperbilirubinemia,and to analyze the influence of bilirubin level and common UGT1A1 vairnts on NAFLD from the perspective of Mendelian randomization.Results 1.In the analysis of UGT1A1 gene of Gilbert syndrome,compared with the wild-type,the heterozygous variant of UGT1A1-A(TA)6/7TAA can increase the risk of hyperbilirubinemia by 3.11 times(P = 0.036),while the heterozygous variant of UGT1A1-c.-3275T>G is not an independent risk factor of hyperbilirubinemia(P=0.810),but the compound heterozygous variants of c.-3275T>G and A(TA)6/7TAA in UGT1A1 gene can increase the risk of hyperbilirubinemia by 4.34 times(P <0.001).2.In the analysis of UGT1A1 gene of health checkers,five variants in UGT1A1 gene [A(TA)6/7TAA,c.211G>A,c.-3275T>G,c.1091C>T and c.1456T>G] were significantly correlated with TB,IB and DB levels.As the number of variants increased,the three types of bilirubin levels gradually increased.Genetype analysis indicated that the homozygous and compound heterozygous variants of UGT1A1 gene had the highest bilirubin level,followed by heterozygous variants and the lowest was wild-type of UGT1A1 gene.3.In the family study of Crigler-Najjar syndrome type II(CNS-II),three common heterozygous variants of UGT1A1 gene(c.211G>A,c.-3275T>G and c.1456T>G)were found in all three CNS-II patients,and no other pathogenic gene mutation affecting bilirubin metabolism was found.4.In studies on the correlation between bilirubin and NAFLD,the variants of A(TA)6/7TAA and c.211G>A in UGT1A1 gene were strongly associated with increased total bilirubin(TB),direct bilirubin(DB)and indirect bilirubin(IB)levels(each P < 0.001).In the binary logistic regression model,there was no significant association between A(TA)6/7TAA or c.211G>A and NAFLD risk(P = 0.43;P = 0.19).In addition,there was no significant association between TB,IB,or DB and NAFLD(P > 0.30).,Conclusions 1.In the analysis of UGT1A1 gene of unconjugated hyperbilirubinemia,there is an additive effect of UGT1A1 gene common variants on bilirubin level.UGT1A1-c.-3275T>G,do not an independent risk factor,may serve as an auxiliary factor to increase the serum bilirubin levels.2.In the CNS-II family study,a new compound heterozygote triple variant in UGT1A1 gene(c.-3279T>G,c.211G>A and c.1456T>G)in UGT1A1 gene was reported for the first time.3.In the Mendelian randomized study,the A(TA)6/7TAA and c.211G>A variants in UGT1A1 gene were associated with elevated levels of bilirubin.However,neither UGT1A1 gene variants nor serum bilirubin level was associated with NAFLD risk.