Partial external biliary diversion in bile salt export pump deficiency: Association between outcome and mutation

To investigate the relation of two different mutations to the outcome of partial external biliary diversion (PEBD) in severe bile salt export pump (BSEP) deficiency. METHODSMutations in the gene encoding BSEP leading to severe BSEP deficiency in two unrelated patients were identified by genomic sequencing. Native liver biopsies and transiently transfected human embryonic kidney (HEK) 293 cells expressing either wild-type or mutated BSEP were subjected to immunofluorescence analysis to assess BSEP transporter localization. Bile acid profiles of patient and control bile samples were generated by ultra-performance liquid chromatography-tandem mass spectrometry. Wild-type and mutant BSEP transport of [3H]-labeled taurocholate (TC) and taurochenodeoxycholate (TCDC) was assessed by vesicular transport assays. RESULTSA girl (at 2 mo) presented with pruritus, jaundice and elevated serum bile salts (BS). PEBD stabilized liver function and prevented liver transplantation. She was heterozygous for the BSEP deletion p.T919del and the nonsense mutation p.R1235X. At the age of 17 years relative amounts of conjugated BS in her bile were normal, while total BS were less than 3% as compared to controls. An unrelated boy (age 1.5 years) presenting with severe pruritus and elevated serum BS was heterozygous for the same nonsense and another missense mutation, p.G1032R. PEBD failed to alleviate pruritus, eventually necessitating liver transplantation. BS concentration in bile was about 5% of controls. BS were mainly unconjugated with an unusual low amount of chenodeoxycholate derivatives (< 5%). The patients’ native liver biopsies showed canalicular BSEP expression. Both BSEP p.T919del and p.G1032R were localized in the plasma membrane in HEK293 cells. In vitro transport assays showed drastic reduction of transport by both mutations. Using purified recombinant BSEP as quantifiable reference, per-molecule transport rates for TC and TCDC were determined to be 3 and 2 BS molecules per wild-type BSEP transporter per minute, respectively. CONCLUSIONIn summary, our findings suggest that residual function of BSEP as well as substrate specificity influence the therapeutic effectiveness of PEBD in progressive familial intrahepatic cholestasis type 2 (PFIC-2).

Current and future therapies for inherited cholestatic liver diseases

Familial intrahepatic cholestasis(FIC) comprises a group of rare cholestatic liver diseases associated with canalicular transport defects resulting predominantly from mutations in ATP8B1, ABCB11 and ABCB4. Phe-notypes range from benign recurrent intrahepatic cholestasis(BRIC), associated with recurrent cholestatic attacks, to progressive FIC(PFIC). Patients often suffer from severe pruritus and eventually progressive cholestasis results in liver failure. Currently, first-line treatment includes ursodeoxycholic acid in patients with ABCB4 deficiency(PFIC3) and partial biliary diversion in patients with ATP8B1 or ABCB11 deficiency(PFIC1 and PFIC2). When treatment fails, liver transplantation is needed which is associated with complications like rejection, post-transplant hepatic steatosis and recurrence of disease. Therefore, the need for more and better therapies for this group of chronic diseases remains. Here, we discuss new symptomatic treatment options like total biliary diversion, pharmacological diversion of bile acids and hepatocyte transplantation. Furthermore, we focus on emerging mutation-targeted therapeutic strategies, providing an outlook for future personalized treatment for inherited cholestatic liver diseases.

Recent updates on progressive familial intrahepatic cholestasis types 1, 2 and 3: Outcome and therapeutic strategies

Recent evidence points towards the role of genotype to understand the phenotype,predict the natural course and long term outcome of patients with progressive familial intrahepatic cholestasis(PFIC).Expanded role of the heterozygous transporter defects presenting late needs to be suspected and identified.Treatment of pruritus,nutritional rehabilitation,prevention of fibrosis progression and liver transplantation(LT)in those with end stage liver disease form the crux of the treatment.LT in PFIC has its own unique issues like high rates of intractable diarrhoea,growth failure;steatohepatitis and graft failure in PFIC1 and antibody-mediated bile salt export pump deficiency in PFIC2.Drugs inhibiting apical sodium-dependent bile transporter and adenovirus-associated vector mediated gene therapy hold promise for future.

Newer variants of progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis(PFIC)is a heterogeneous group of disorders characterized by defects in bile secretion and presentation with intrahepatic cholestasis in infancy or childhood.The most common types include PFIC 1(deficiency of FIC1 protein,ATP8B1 gene mutation),PFIC 2(bile salt export pump deficiency,ABCB11 gene mutation),and PFIC 3(multidrug resistance protein-3 deficiency,ABCB4 gene mutation).Mutational analysis of subjects with normal gamma-glutamyl transferase cholestasis of unknown etiology has led to the identification of newer variants of PFIC,known as PFIC 4,5,and MYO5B related(sometimes known as PFIC 6).PFIC 4 is caused by the loss of function of tight junction protein 2(TJP2)and PFIC 5 is due to NR1H4 mutation causing Farnesoid X receptor deficiency.MYO5B gene mutation causes microvillous inclusion disease(MVID)and is also associated with isolated cholestasis.Children with TJP2 related cholestasis(PFIC-4)have a variable spectrum of presentation.Some have a self-limiting disease,while others have progressive liver disease with an increased risk of hepatocellular carcinoma.Hence,frequent surveillance for hepatocellular carcinoma is recommended from infancy.PFIC-5 patients usually have rapidly progressive liver disease with early onset coagulopathy,high alpha-fetoprotein and ultimately require a liver transplant.Subjects with MYO5 B-related disease can present with isolated cholestasis or cholestasis with intractable diarrhea(MVID).These children are at risk of worsening cholestasis post intestinal transplant(IT)for MVID,hence combined intestinal and liver transplant or IT with biliary diversion is preferred.Immunohistochemistry can differentiate most of the variants of PFIC but confirmation requires genetic analysis.