Molecular Mechanism of Intrahepatic Cholestasis in Sickle Cell Disease

Sickle cell disease (SCD) is an autosomal recessive genetic disorder that affects over three million people worldwide. Sickle Cell Anemia, the most common form of SCD is caused by the substitution of hydrophilic glutamic acid with hydrophobic valine at the sixth position in β-globin chain. The mutated hemoglobin (HbS) polymerizes under hypoxic conditions to promote erythrocyte sickling. Sickling promotes vaso-occlusion (ischemia) and hemolysis, which are two major pathophysiological events contributing to end organ damage. SCD patients can develop Sickle Cell Intrahepatic Cholestasis (SCIC), which is a rare but lethal form of hepatopathy. Besides SCIC, cholethiasis (gallstone formation), viral hepatitis and choledocholithiasis (stones in bile duct) have also been reported in SCD patients. The morbidity associated with sickle hepatopathy is expected to rise with growing life expectancy of SCD patients. Rescue therapies to halt the progression of sickle hepatopathy will be highly beneficial, however, development of such therapies is stalled by the lack of understanding of the molecular mechanism underlying sickle hepatopathy.

We assessed the liver pathophysiology in a humanized mouse model of SCD in Townes SS mice homozygous for Hba^tm1(HBA)Tow and homozygous for Hbb^{tm2(HBG1,HBB*)Tow}). Littermate sickle cell trait mice (Townes AS mice homozygous for Hba^tm1(HBA)Tow and compound heterozygous for Hbb^{tm2(HBG1,HBB*)Tow}/Hbb^{tm3(HBG1,HBB)Tow}) were used as control mice. Our findings reveal that SCD but not control mice developed progressive intrahepatic cholestasis with age characterized by increased level of serum bilirubin, alanine aminotransferase (ALT), aspartate aminotransferase (AST) and cholesterol. Immunohistochemical analysis of SCD mice liver revealed increased ductular reaction, inflammation, and fibrosis, which is hallmark of cholestatic liver disease. Immunoelectron microscopy revealed damage to bile canalicular structure and loss of several bile transporters from hepatocyte membrane. Remarkably, biochemical analysis revealed that intrahepatic cholestasis in SCD mice was associated with loss of Wnt signaling in hepatocytes. Our preliminary findings suggest that further studies are needed to elucidate the molecular mechanism underlying progressive cholestasis in SCD mice. Profound understanding of the regulation of cholestasis and hepatobiliary injury in SCD could potentially benefit the development of new therapies to attenuate sickle hepatopathy in SCD patients.