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A Comprehensive Review of Progressive Familial Intrahepatic Cholestasis (PFIC): Genetic Disorders of Hepatocanalicular Transporters

Progressive familial intrahepatic cholestasis or PFIC is a general term used to describe a group of genetic disorders involving the hepatocanalicular transporters. These diseases are characterized by persistent cholestasis, pruritus and jaundice. Type I PFIC is characterized by defect in the gene that codes for aminophospholipid translocase protein and maintains canalicular membrane stability. Types 2 and 3 are caused by defect in genes that code for bile acid transporter and a phospholipid translocase, respectively. This review summarizes the genetics, clinical features, diagnosis and treatment of the three types of PFIC.


Introduction
Progressive familial intrahepatic cholestasis (PFIC) is a heterogeneous group of rare, genetic autosomal recessive disorders, resulting from defects in the mechanisms involved in bile formation with typical clinical, biochemical and histological features. These patients typically present with intrahepatic cholestasis in infancy or childhood. The course of the disease involves portal hypertension, liver failure, cirrhosis, hepatocellular carcinoma along with several ex-tra hepatic manifestations. Three types of PFIC have been described: 1) PFIC type 1, caused by mutations in ATP8B1 gene (also called FIC1); 2) PFIC type 2, caused by mutations in ABCB11 gene (also called BSEP) and 3) PFIC type 3, caused by mutations in ABCB4 gene (also called MDR3). Each mutation is related to the hepatocellular transport system genes involved in bile formation [1,2]. Cholestasis is a major clinical sign in all three types [3]. Table 1 summarizes the typical features of the three types of PFIC.

PFIC Type 1
Genetics PFIC1, an autosomal recessive disease is caused by the mutation in ATP8B1 gene, which encodes a P-type ATPase, located on human chromosome 18. It is found in several tissues such a small intestine, bladder, stomach, liver and pancreas. It is localized on the apical membrane of epithelial cells, including the canalicular membrane of hepatocytes [4,5]. ATP8B1 is hypothesized to be an aminophospholipid flippase, translocating phospholipids such as phosphatidylserine from outer to the inner leaflet of plasma membrane [6]. Deficiency of ATP8B1 in the hepatocyte may result in loss of asymmetric distribution of the phospholipids in the canalicular membrane, thus reducing membrane stability and function of transmembrane transporters like ABCB11, impairing bile salt transport and decreasing hepatocyte resistance to bile salts, ultimately leading to reduced transport of bile salts in liver and gut resulting in cholestasis and watery diarrhea.

Clinical features
In newborns afflicted with type 1 PFIC is characterized by recurrent episodes of intrahepatic cholestasis, intractable pruritus, diarrhea and failure to thrive in first several months of life. It is generally progressive leading to cirrhosis and eventually liver failure in the first decade of life. Other features include short stature, diarrhea, hepatosplenomegaly, malabsorption and occasionally hearing loss.

Diagnosis
The diagnostic features include elevated serum bile salts, normal serum GGT, electron microscopy showing coarse granular bile and light microscopy showing bland cholestasis. Genetic testing is the only reliable way to distinguish between PFIC 1 and PFIC 2.

Treatment
Pharmacological treatment includes ursodeoxycholic acid (UDCA) to promote biliary flow [7], but its effect on disease progression is uncertain. Rifampin (accelerates hepatic detoxification and excretion of bilirubin and bile salts) and cholestyramine (a hydrophilic, water insoluble anion exchange resin that binds bile salts, preventing their reabsorption in enterohepatic circulation) ease pruritus. These patients may also need supplements of fat soluble vitamins. Partial biliary diversion may be beneficial in some patients, significantly decreasing their symptoms [8]. It may even delay or interrupt hepatocyte injury if performed early in the course of the disease. Liver transplantation is the most effective form of treatment especially in those with progressive disease or established cirrhosis, usually within the first decade of life.

PFIC Type 2
Genetics PFIC 2, an autosomal recessive disease is caused by mutation in ABCB11 gene on chromosome 2q24, which encodes the canalicular bile salt export protein (BSEP), a liver specific adenosine triphosphate (ATP)-binding cassette transporter [9,10]. BSEP is a major exporter of primary bile acids against extreme concentration gradients. There are many mutations that have been identified, but the most common mutations are the missense mutations E297G and D482G. Defective BSEP can lead to impaired bile salt secretion, accumulation of bile salts in hepatocytes and subsequent hepatocellular injury, apoptosis or necrosis. Missense mutations cause less severe disease, whereas the risk of hepatobiliary malignancy is increased with trunacating mutations.

Clinical features
Patients usually present with cholestatic jaundice in neonatal period. Pruritus is usually the dominant feature. Clinical features and disease progression may be more severe in PFIC 2 than in PFIC 1. Other features may include growth failure, deficiency of fat soluble vitamins. These patients are at considerable risk of hepatobiliary malignancy, hepatocellular carcinoma or cholangiocarcinoma , thus close surveillance is important in these patients [11].

Diagnosis
Diagnostic features include elevated bile salts, normal serum GGT, electron microscopy showing amorphous or filamentous bile. Light microscopy shows nonspecific giant cell hepatitis. Immunohistochemical staining for canalicular BSEP is negative in majority of patients.

Treatment
Patients with PFIC 2 have poor response to ursodeoxycholic acid. Partial external biliary diversion (PBED) (it interrupts the enterohepatic circulation by partially diverting bile from the gallbladder through a loop of jejunum connecting the gallbladder to abdominal skin) may be useful and if performed early enough may delay or interrupt hepatic injury. Histological changes may show some improvement.
Liver transplantation is usually successful in this group of patients although recurrence after transplantation has been reported [12]. This can be resolved with aggressive immunosuppressive therapy.

PFIC Type 3
Genetics PFIC 3, an autosomal recessive disease is caused by mutations in ABCB4, which encodes the multidrug resistance-3 gene (MDR3), an ATP-binding cassette transporter. It is located on chromosome 7q21. It acts as a phospholipid translocator involved in biliary phospholipid (phosphatiylcholine) excretion. It is mainly expressed in the hepatocyte canalicular membrane. ABCB4 defects cause impaired excretion of phophatidycholine, which is an important component in formation of mixed micelles, which protect the membrane of cells facing biliary tree from the detergent activity of bile salts. Functional deficiency of ABCB4 causes the detergent action of bile salts to solubilize the cell membrane of the biliary epithelium inducing inflammation and cell death.
Several different types of mutation have been identified and those with missense mutation have less severe phenotype, with later onset of disease, slower progression and better response to treatment than those with trunacating mutations.
Heterozygous mutations in ABCB4 gene can cause conditions such as intrahepatic cholestasis of pregnancy, cholesterol gall stone formation, drug induced cholestasis and adult biliary cirrhosis [13].

Clinical features
Patients with ABCB4 deficiency usually have jaundice, pale stools, hepatosplenomegaly or pruritus. Age at onset varies from infancy to adulthood. Adolescent and young adult patients are more likely to have cirrhotic symptoms owing to portal hypertension. No association with malignancy has been described in PFIC 3 patients [14].

Diagnosis
The diagnostic features include elevated serum bile salts, elevated serum GGT, electron microscopy showing presence of cholesterol crystals and loss of microvilli, light microscopy showing ductular proliferation and inflammatory infiltrate in early stages, portal and peri-portal fibrosis, cirrhosis and cholestasis which are seen in the later stages.

Treatment
UDCA is useful in almost half of the patients. It is more likely to work in those who have an MDR3 missense mutation as they have some residual protein activity. Those who do not respond to UDCA treatment are likely to have a truncated protein. In these patients, liver transplantation is the primary treatment. Table 2 lists the differential diagnosis of the three types of PFIC that an astute clinical should be mindful of.

Conclusion
The review aims to summarize the genetics, clinical features, diagnostic and therapeutic strategies for the three types of PFIC. Further research is needed to elucidate the disease mechanism in these disorders. There is also an urgent need to develop mutation specific pharmacological therapies.