The Interaction of UGT1A, HO1 and α-Thalassemia Variants with Bilirubin Levels and Gallstones in Sickle Cell Disease.

Chronic hyperbilirubinemia is common in patients with sickle cell disease (SCD), frequently resulting in the formation of gallstones. This hyperbilirubinemia (predominantly unconjugated) is attributed to hemolysis that exceeds the conjugating capacity of the hepatic UDP-glucuronosyltransferase (UGT1A) enzyme. Previous studies have shown that genetic variants ([TA]_n repeats) in the promoter region of the UGT1A gene have a major influence on the levels of bilirubin and gallstones. Alpha-thalassemia, which is associated with reduced hemolysis, has also been shown to affect bilirubin levels. Another potential modulating factor is heme-oxygenase, a rate-limiting enzyme in the heme catabolic pathway that results in the production of bilirubin. While the severity of jaundice and cholelithiasis in patients with SCD is predisposed by the inheritance of certain variants of the UGT1A gene, inconsistencies have been observed. We propose that some of these inconsistencies may be explained by the modulating effects of genetic variants of HO1 and α-thalassemia. A total of 263 patients with SCD attending specialist clinics in two hospitals were studied: King’s 116 SS, 5 Sβ⁰ and 59 SC; St Thomas’ 83 SS. 81 ethnically matched subjects were recruited as controls (HbAA). Groups were age and sex matched. Cholelithiasis data, ascertained by liver ultrasound, was available for a subset of SCD patients (76 SS, 4 SC). Samples were genotyped for variants of UGT1A, HO1 and α-thalassemia. The different genetic allele distributions were statistically similar between groups. Data was analysed according to the sum of [TA] repeats on both alleles of UGT1A. A range of 10–15 [TA] repeats was observed. For HO1, the median sum of repeats on both alleles was 63, so samples were grouped as <63 or ≥ 63. α-thalassemia genotypes were as follows: SS, 127 αα/αα, 55 αα/α-, 11 α-/α-; SC, 37 αα/αα, 20 αα/α-; controls 51 αα/αα, 22 αα/α-, 4 α-/α-. Median bilirubin levels varied as expected between groups according to β-globin genotype and were as follows: King’s SS 42 (15–269.5); St Thomas’ SS 52.5 (15.5–696.5); King’s SC 22 (10–81.8); AA Controls 10 (5–24), median (range) mmol/L. Regression analysis showed that serum bilirubin levels were strongly associated with UGT1A repeat length in all subjects (p<0.0001). Furthermore, the increase in serum bilirubin (21.3% mean for SS/Sβ⁰, 20.5% for SC) and cholelithiasis risk odds (86.5% mean for SS/Sβ⁰, 67.6% for SC) could be quantified per [TA] repeat. HO1 genotype did not affect serum bilirubin in SCD patients or the control cohort. The presence of α- thalassemia correlated (negatively) with serum bilirubin in SCD patients (p<0.0001) but not controls. This is the first time the relationship between UGT1A [TA] repeat length, serum bilirubin and cholelithiasis has been shown quantitatively. Additionally, co-existing α-thalassemia appears to moderate bilirubin levels but HO1 variants do not.