Genetic Analysis of ABCB4 Mutations and Variants Related to the Pathogenesis and Pathophysiology of Low Phospholipid-Associated Cholelithiasis

Abstract

Clinical studies have revealed that the ABCB4 gene encodes the phospholipid transporter on the canalicular membrane of hepatocytes, and its mutations and variants are the genetic basis of low phospholipid-associated cholelithiasis (LPAC), a rare type of gallstone disease caused by a single-gene mutation or variation. The main features of LPAC include a reduction or deficiency of phospholipids in bile, symptomatic cholelithiasis at <40 years of age, intrahepatic sludge and microlithiasis, mild chronic cholestasis, a high cholesterol/phospholipid ratio in bile, and recurrence of biliary symptoms after cholecystectomy. Needle-like cholesterol crystals, putatively “anhydrous” cholesterol crystallization at low phospholipid concentrations in model and native bile, are characterized in ABCB4 knockout mice, a unique animal model for LPAC. Gallbladder bile with only trace amounts of phospholipids in these mice is supersaturated with cholesterol, with lipid composition plotting in the left two-phase zone of the ternary phase diagram, consistent with “anhydrous” cholesterol crystallization. In this review, we summarize the molecular biology and physiological functions of ABCB4 and comprehensively discuss the latest advances in the genetic analysis of ABCB4 mutations and variations and their roles in the pathogenesis and pathophysiology of LPAC in humans, based on the results from clinical studies and mouse experiments. To date, approximately 158 distinct LPAC-causing ABCB4 mutations and variants in humans have been reported in the literature, indicating that it is a monogenic risk factor for LPAC. The elucidation of the ABCB4 function in the liver, the identification of ABCB4 mutations and variants in LPAC patients, and the exploration of gene therapy for ABCB4 deficiency in animal models can help us to better understand the cellular, molecular, and genetic mechanisms underlying the onset of the disease, and will pave the way for early diagnosis and prevention of susceptible subjects and effective intervention for LPAC in patients.

Molecular Insights of Cholestasis in MDR2 Knockout Murine Liver Organoids

“…Since HMGCR is negatively regulated by AMPK-mediated phosphorylation, it might be speculated that HMGCR activity is enhanced in MDR2KO organoids. It is worth to consider that a reduced phospholipid production concomitant to increased cholesterol levels may lead to cholesterol crystal formation that is one of the common symptoms of PFIC3 patients . Conversely to cholesterol, fatty acid synthesis appears to be downregulated in MDR2KO organoids, as evidenced by the significant downregulation of FASN expression (Figure E).…”

Obeticholic acid as a second‐line treatment for low phospholipid‐associated cholelithiasis syndrome

“…Other drugs could be considered as potential candidates for second‐line therapy of LPAC syndrome. Ezetimibe has been shown to reduce biliary cholesterol saturation and crystallisation in mice and humans and may therefore be suggested as a potential second‐line drug in combination with UDCA 15,16 . Fibrates may also be candidates, as they are known inducers of the MDR3 canalicular transporter and of phospholipid secretion into bile 17,18 .…”

“…Ezetimibe has been shown to reduce biliary cholesterol saturation and crystallisation in mice and humans and may therefore be suggested as a potential secondline drug in combination with UDCA. 15,16 Fibrates may also be candidates, as they are known inducers of the MDR3 canalicular transporter and of phospholipid secretion into bile. 17,18 However, they may also increase biliary cholesterol concentration by inducing the ABCG5/G8 transporter, and some studies have shown an increased risk of gallstones with fibrates.…”

Research progress on the immune microenvironment of the gallbladder in patients with cholesterol gallstones

“…A recent study compared the chemical composition of fresh gallbladder bile between ABCB4 knockout and wild-type mice and found cholesterol supersaturation and the presence of cholesterol crystals in gallbladder bile in the former but not in the latter. The results of this study demonstrate the critical role of ABCB4 in phospholipid transport and the important role of ABCB4 mutations in the formation of cholesterol gallstones[ 54 ]. A strong association between gallstone disease and ABCG8 was shown in a genome-wide association study (GWAS) involving 280 patients with gallstones and 360 controls in 2007[ 55 ].…”