Cystic liver diseases. Genetics and cell biology

In 50% of cases, polycystic liver disease is associated with autosomal dominant polycystic kidney disease, which is caused by mutations in the PKD1 and PKD2 genes that encode polycystin-1 and -2, respectively. These proteins form a polycystin-1/2 complex on the plasma membrane, including that localized on the surface of primary cilia, where they act as mechanosensors. Polycystin-1 acts as a (mechano)receptor of environmental signals, and polycystin-2 as a calcium channel mediating intracellular transduction. Isolated autosomal dominant polycystic liver disease is caused by mutations in PRKCSH that encodes hepatocystin, a protein of the endoplasmic reticulum, which may participate in the N-glycosylation and maturation of proteins addressed to the cell surface. Congenital hepatic fibrosis whether it is accompanied by bile duct dilatations (Caroli's syndrome) or not, may be associated with autosomal recessive polycystic kidney disease, which is caused by mutations in PKHD1 that encodes fibrocystin, a protein of primary cilia. Genetic defects in fibrocystin cause ciliary dysfunction, presently considered as a major pathogenic event in cystogenesis. Excessive cell proliferation, a hallmark of cystic biliary epithelium, occurs in combination with deregulation of the epidermal growth factor (EGF) and probably also estrogen receptors. EGF receptor antagonists inhibit kidney and liver cyst development in animal models, and are currently under investigation in phase I and II clinical trials in patients with autosomal dominant polycystic kidney disease.