The linear effects of α‐thalassaemia, the UGT1A1 and HMOX1 polymorphisms on cholelithiasis in sickle cell disease

Summary Serum bilirubin levels and predisposition to gallstones in sickle cell disease (SCD) are influenced by genetic variation in the hepatic uridine diphosphate (UDP)‐glucuronosyltransferase ( UGT1A1 ) gene, but the association is not consistent. This study investigated whether variation in the gene encoding haem oxygenase ( HMOX1 ), a rate‐limiting enzyme upstream of UGT1A in the haem catabolic pathway, and α ‐thalassaemia could explain some of the inconsistent effects. The UGT1A1 [TA] n and HMOX1 [GT] n promoter polymorphisms and α globin genotypes were determined in 263 SCD patients (199 HbSS, 5 HbS/ β 0 , 59 HbSC). Detection of gallstones was based on ultrasound of the liver/biliary tree. Regression analysis showed that serum bilirubin levels and the incidence of gallstones were strongly associated with the number of UGT1A1 [TA] repeats in all subjects ( P < 0·0001 and P < 0·01, respectively). While HMOX1 genotype had no effect, co‐inheritance of α ‐thalassaemia reduced serum bilirubin levels in all SCD patients independently of the number of UGT1A1 [TA] repeats. Each additional [TA] repeat is associated with an increase in mean serum bilirubin levels of 21% and cholelithiasis risk of 87% in SCD.