ARPKD: Pathogenesis of Liver Disease and Potential Therapeutic Interventions

Tatyana V Masyuk, Nicholas F LaRusso

Department of Internal Medicine, Mayo Clinic College of Medicine, Rochester, Minnesota 55905

Autosomal Recessive Polycystic Kidney Disease (ARPKD) is a genetic disease with an incidence of 1:10,000 to 1:40,000 in general population affecting ~ 20,000 in the USA alone but with a high mortality rate. While disease presentation is highly variable, genetic linkage studies indicate presence of different mutations in a single gene Polycystic Kidney Hepatic Disease 1 (PKHD1) localized to chromosome region 6p21-23 that encodes the protein fibrocystin/polyductin.

ARPKD is classically associated with massive renal enlargement as a result of fusiform dilatation of collecting ducts leading to neonatal death in 20-30% of cases.  The prognosis of those who survive the first month is much better. In surviving patients, kidneys do not increase in size with age and even a decrease in kidneys size has been reported. However, with age, hepatic lesions become progressively more severe and liver disease is the major cause of morbidity and mortality in older ARPKD patients. Liver involvement in ARPKD is associated with congenital hepatic fibrosis, bile duct dilation and hepatic cystogenesis. Liver symptoms and complications in ARPKD include abdominal distension or discomfort, back pain, cyst infection, fullness, hemorrhage, and rupture, portal hypertension and jaundice. To date, no curative or preventive therapy for polycystic liver disease exists. For symptomatic relief, interventional or surgical options include cyst fenestration, aspiration of cyst fluid, liver resection and isolated liver or combined liver-kidney transplantation.

Recent advances in understanding the mechanisms underlying renal and hepatic cyst progression, identification of genes responsible for disease development and availability of different animals models have facilitated the development of a number of therapeutic approaches. However, while substantial progress has been made regarding the pathogenesis of the renal disease in ARPKD at the molecular and cellular levels and, as a result, therapies of polycystic kidney disease have been emerged, development of treatment options of the polycystic liver received less attention and we still have a lot to learn about factors underlying cyst growth and expansion in the liver.

The overall goal of our research laboratory is to apply the fundamental concepts and broad technologies of cell and molecular biology to understand the pathogenesis of the hepatic manifestations in ARPKD. Revealing the mechanisms involving in growth and expansion of liver cysts is a first step toward designing rational therapies.

To perform our research experiments and to test the effects of different drugs on hepatic and renal cyst progression we choose an animal model of ARPKD, the PCK rat. Several mouse and rat models of ARPKD have been described over the years each varying by age of disease onset, affected portion of renal tissue, or hepatic involvement. Many of the genes causing these disorders have been identified and characterized but only one of these genes (mutated in the PCK rat) correspond to the human ARPKD gene. The PCK rat, a spontaneous mutant, has kidney and liver disease that resembles human ARPKD. Linkage and gene cloning analysis showed that ARPKD and the PCK model are caused by mutation to orthologous genes, PKHD1/Pkhd1. As in the human, polycystic disease in the PCK rat is inherited in an autosomal recessive fashion and severity of liver cystic lesions is age-dependent. Importantly, the PCK rat developed liver fibrosis.  Furthermore, fibrosis seen in this model mirrors that seen in human ARPKD. Importantly, we have developed recently the cultured cholangiocyte cell line, the PCK-CCL, derived from the PCK rat. When grown in 3-D cultures, the PCK-CCL forms cystic structures that expand rapidly and progressively. Thus, these both experimental models - the PCK rat (in vivo model) and the PCL-CCL (in vitro model) are excellent systems to study biliary cystogenesis and to test possible treatment options.

It is well accepted, that genetic defects in ARPKD initiate formation of liver cysts that arise from bile duct epithelial cell, cholangiocytes. The liver cysts continue to further expand due to abnormalities in three major mechanisms: (i) cell proliferation, apoptosis and differentiation; (ii) fluid secretion; and (iii) cell-matrix interactions. Many different factors, individually or in combination, impact these processes and promote cyst growth. One of these potential factors, cAMP, has been shown to play a significant role in two major processes that control cyst expansion - cell proliferation and fluid secretion. Indeed, our recent study demonstrates for the first time that cAMP levels are increased in freshly isolated bile ducts and in serum of the PCK rats. Moreover, activation of cAMP promotes cyst growth while its suppression, in contrast, lead to inhibition of cyst expansion. These observations support the potential importance of cAMP in hepatic disease progression and make cAMP an attractive therapeutic target in ARPKD treatment.

Somatostatin and its analogs, such as octreotide, have been shown to inhibit elevated cAMP levels, and to decrease fluid secretion and cell proliferation in many cell types, including cholangiocytes. Thus, we tested whether the somatostatin analog, octreotide, might be effective in the treatment ARPKD and demonstrated that in vivo, in PCK rats, octreotide suppresses cAMP levels, reduces liver and kidney weights, inhibits hepatic and renal cyst volume and fibrosis; and decreases rate of cell proliferation in hepatic and renal epithelia. Moreover, octreotide has a time- and dose-dependent effects on the measures parameters – longer treatment and higher dose lead to more substantial effects. Furthermore, octreotide had a beneficial effect on cyst growth – even after stopping octreotide treatment hepatic cysts expand much slower compared to non-treated cysts. This pre-clinical study demonstrate that octreotide can inhibit hepatic and renal cystogenesis and attenuate hepatic and renal fibrosis and all together the data provides a strong rationale for assessing the potential value of octreotide in the treatment of polycystic liver and kidney diseases in humans. Importantly, a clinical trial which will evaluate the effect of OctreotideLAR^® on liver cyst volumes in patients with severe polycystic liver diseases is now underway at our Institution.