Prenatal molecular diagnosis of inherited cholestatic diseases

Targeted-Capture Next-Generation Sequencing in Diagnosis Approach of Pediatric Cholestasis

Background: Cholestasis is a frequent and severe condition during childhood. Genetic cholestatic diseases represent up to 25% of pediatric cholestasis. Molecular analysis by targeted-capture next generation sequencing (NGS) has recently emerged as an efficient diagnostic tool. The objective of this study is to evaluate the use of NGS in children with cholestasis. Methods: Children presenting cholestasis were included between 2015 and 2020. Molecular sequencing was performed by targeted capture of a panel of 34 genes involved in cholestasis and jaundice. Patients were classified into three categories: certain diagnosis; suggested diagnosis (when genotype was consistent with phenotype for conditions without any available OMIM or ORPHANET-number); uncertain diagnosis (when clinical and para-clinical findings were not consistent enough with molecular findings). Results: A certain diagnosis was established in 169 patients among the 602 included (28.1%). Molecular studies led to a suggested diagnosis in 40 patients (6.6%) and to an uncertain diagnosis in 21 patients (3.5%). In 372 children (61.7%), no molecular defect was identified. Conclusions: NGS is a useful diagnostic tool in pediatric cholestasis, providing a certain diagnosis in 28.1% of the patients included in this study. In the remaining patients, especially those with variants of uncertain significance, the imputability of the variants requires further investigations.

ABCB4 disease: Many faces of one gene deficiency

ATP-binding cassette (ABC) subfamily B member 4 (ABCB4), also known as multidrug resistance protein 3 (MDR3), encoded by ABCB4, is involved in biliary phospholipid secretion, protecting hepatobiliary system from deleterious detergent and lithogenic properties of the bile. ABCB4 mutations altering canalicular ABCB4 protein function and expression may have variable clinical presentation and predispose to several human liver diseases. Well-established phenotypes of ABCB4 deficit are: progressive familial intrahepatic cholestasis type 3, gallbladder disease 1 (syn. low phospholipid associated cholelithiasis syndrome), high ɣ-glutamyl transferase intrahepatic cholestasis of pregnancy, chronic cholangiopathy, and adult biliary fibrosis/cirrhosis. Moreover, ABCB4 aberrations may be involved in some cases of drug induced cholestasis, transient neonatal cholestasis, and parenteral nutrition-associated liver disease. Recently, genome-wide association studies have documented occurrence of malignant tumours, predominantly hepatobiliary malignancies, in patients with ABCB4/MDR3 deficit. The patient's age at the time of the first presentation of cholestatic disease, as well as the severity of liver disorder and response to treatment are related to the ABCB4 allelic status. Mutational analysis of ABCB4 in patients and their families should be considered in all individuals with cholestasis of unknown aetiology, regardless of age and/or time of onset of the first symptoms.

Genetic Causes of Liver Disease: When to Suspect a Genetic Etiology, Initial Lab Testing, and the Basics of Management

Genetic causes of liver disease lead to a wide range of presentations. This article describes hereditary hemochromatosis, Gilbert syndrome, alpha-1 antitrypsin deficiency, Wilson disease, PFIC, BRIC, and LAL-D. The most common cause of hereditary hemochromatosis is a C282Y mutation in the HFE gene. Gilbert syndrome is a benign cause of indirect hyperbilirubinemia. Alpha-1 antitrypsin deficiency causes both lung and liver disease. Wilson disease can cause neurologic disease and liver disease. Progressive familial intrahepatic cholestasis and benign recurrent intrahepatic cholestasis are rare causes of cholestasis. LAL-D is a rare disease that can appear similar to NAFLD in adults.

Alagille syndrome and pregnancy

Alagille syndrome is an autosomal dominant multisystem disorder with an estimated frequency of 1 in 30 000. Only a small number of pregnancy outcomes have been described in women with this condition. The report details the pregnancy outcomes of two women with Alagille syndrome. We also review the literature pertaining to this syndrome in pregnancy and demonstrate a significant risk of adverse pregnancy outcomes.

Phenotypic spectrum and diagnostic pitfalls of ABCB4 deficiency depending on age of onset

Genetic variants in the adenosine triphosphate-binding cassette subfamily B member 4 (ABCB4) gene, which encodes hepatocanalicular phosphatidylcholine floppase, can lead to different phenotypes, such as progressive familial intrahepatic cholestasis (PFIC) type 3, low phospholipid-associated cholelithiasis, and intrahepatic cholestasis of pregnancy. The aim of this multicenter project was to collect information on onset and progression of this entity in different age groups and to assess the relevance of this disease for the differential diagnosis of chronic liver disease. Clinical and laboratory data of 38 patients (17 males, 21 females, from 29 families) with homozygous or (compound) heterozygous ABCB4 mutations were retrospectively collected. For further analysis, patients were grouped according to the age at clinical diagnosis of ABCB4-associated liver disease into younger age (<18 years) or adult age (≥18 years). All 26 patients diagnosed in childhood presented with pruritus (median age 1 year). Hepatomegaly and splenomegaly were present in 85% and 96% of these patients, respectively, followed by jaundice (62%) and portal hypertension (69%). Initial symptoms preceded diagnosis by 1 year, and 13 patients received a liver transplant (median age 6.9 years). Of note, 9 patients were misdiagnosed as biliary atresia, Alagille syndrome, or PFIC type 1. In the 12 patients with diagnosis in adulthood, the clinical phenotype was generally less severe, including intrahepatic cholestasis of pregnancy, low phospholipid-associated cholelithiasis, or (non)cirrhotic PFIC3. Conclusion: ABCB4 deficiency with onset in younger patients caused a more severe PFIC type 3 phenotype with the need for liver transplantation in half the children. Patients with milder phenotypes are often not diagnosed before adulthood. One third of the children with PFIC type 3 were initially misdiagnosed, indicating the need for better diagnostic tools and medical education. (Hepatology Communications 2018;2:504-514).

Genetic Testing in Liver Disease: What to Order, in Whom, and When

Genetic causes of liver disease lead to a wide range of presentations, from mildly abnormal liver tests to acute liver failure. This article discusses the indications for testing and what to test for hereditary hemochromatosis, progressive familial intrahepatic cholestasis, benign recurrent intrahepatic cholestasis, lysosomal acid lipase deficiency, Gilbert syndrome, alpha-1 antitrypsin deficiency, and Wilson disease.

Molecular genetic characterization of a prenatally detected de novo interstitial deletion of chromosome 20p (20p12-p13) encompassing JAG1 and a literature review of prenatal diagnosis of Alagille syndrome

OBJECTIVE

We present prenatal diagnosis and molecular genetic characterization of a de novo interstitial deletion of chromosome 20p (20p12-p13) and a literature review of prenatal diagnosis of Alagille syndrome (ALGS).

CASE REPORT

A 33-year-old woman underwent amniocentesis at 17 weeks of gestation because of an abnormal result of combined first-trimester screening. Her husband was 35 years old, and there was no family history of congenital malformations. Amniocentesis revealed a karyotype of 46,XY,del(20)(p12p13), and array comparative genomic hybridization analysis on uncultured amniocytes revealed a 3.749-Mb deletion at 20p13-p12.3 and a 1.84-Mb deletion at 20p12.2 encompassing the gene of JAG1. The parental karyotypes were normal. Prenatal ultrasound findings were unremarkable. The fetus postnatally manifested characteristic facial features of ALGS. Postnatal molecular cytogenetic analysis of fetal tissues confirmed the prenatal diagnosis. Polymorphic DNA marker analysis revealed a paternal origin of the deletion.

CONCLUSION

A de novo interstitial 20p deletion can be caused by a paternal effect. Pregnancy with a fetus affected with ALGS may be associated with an abnormal result of combined first-trimester screening and manifest no detectable ultrasound abnormalities.

Early Diagnosis of ABCB11 Spectrum Liver Disorders by Next Generation Sequencing

PURPOSE

The goal of this study was the early diagnosis of ABCB11 spectrum liver disorders, especially those focused on benign recurrent intrahepatic cholestasis and progressive familial intrahepatic cholestasis.

METHODS

Fifty patients presenting neonatal cholestasis were evaluated to identify underlying etiologies. Genetic analysis was performed on patients suspected to have syndromic diseases or ABCB11 spectrum liver disorders. Two families with proven ABCB11 spectrum liver disorders were subjected to genetic analyses to confirm the diagnosis and were provided genetic counseling. Whole exome sequencing and Sanger sequencing were performed on the patients and the family members.

RESULTS

Idiopathic or viral hepatitis was diagnosed in 34%, metabolic disease in 20%, total parenteral nutrition induced cholestasis in 16%, extrahepatic biliary atresia in 14%, genetic disease in 10%, neonatal lupus in 2%, congenital syphilis in 2%, and choledochal cyst in 2% of the patients. The patient with progressive familial intrahepatic cholestasis had novel heterozygous mutations of ABCB11 c.11C>G (p.Ser4^*) and c.1543A>G (p.Asn515Asp). The patient with benign recurrent intrahepatic cholestasis had homozygous mutations of ABCB11 c.1331T>C (p.Val444Ala) and heterozygous, c.3084A>G (p.Ala1028Ala). Genetic confirmation of ABCB11 spectrum liver disorder led to early liver transplantation in the progressive familial intrahepatic cholestasis patient. In addition, the atypically severe benign recurrent intrahepatic cholestasis patient was able to avoid unnecessary liver transplantation after genetic analysis.

CONCLUSION

ABCB11 spectrum liver disorders can be clinically indistinguishable as they share similar characteristics related to acute episodes. A comprehensive genetic analysis will facilitate optimal diagnosis and treatment.

Alagille syndrome: clinical perspectives

Alagille syndrome is an autosomal dominant, complex multisystem disorder characterized by the presence of three out of five major clinical criteria: cholestasis with bile duct paucity on liver biopsy, congenital cardiac defects (with particular involvement of the pulmonary arteries), posterior embryotoxon in the eye, characteristic facial features, and butterfly vertebrae. Renal and vascular abnormalities can also occur. Inter- and intrafamilial variabilities in the clinical manifestations are common. We reviewed the clinical features and management as well as the molecular basis of Alagille syndrome.

Heterozygous ABCB4 mutations in children with cholestatic liver disease

BACKGROUND & AIMS

Monoallelic defects in ABCB4, which encodes the canalicular floppase for phosphatidylcholine MDR3, have been encountered in association with a variety of hepatobiliary disorders, particularly in adult subjects. In this study, we examined the presence of heterozygous ABCB4 variants in a cohort of children with chronic cholestasis and assessed the pathogenicity of the missense changes identified.

METHODS

Sixty-seven children with chronic liver dysfunction were studied by the sequencing of ABCB4 and multiplex ligation-dependent probe amplification analysis. The molecular defects arising from missense variants were analysed in MDCK-II and AD-293 cells.

RESULTS

Defects in a single allele of ABCB4 were identified in nine subjects. They included one small insertion (p.I1242Nfs), one nonsense mutation (p.R144X) and six missense changes (p.T175A, p.G228R, p.A250T, p.S320F, p.P352L and p.A934T). In four children, these defects in ABCB4 co-existed with various medical conditions. In vitro phenotyping of the six missense variants revealed that four (T175A, G228R, S320F and A934T) led to reduced MDR3 protein levels. Two mutations (G228R and A934T) resulted in trapping of the protein in the endoplasmic reticulum. Phosphatidylcholine efflux activity was decreased to 56-18% of reference levels for MDR3 mutants T175A, A250T and S320F. The G228R, P352L and A934T mutants were found to be non-functional.

CONCLUSIONS

These results illustrate the varying effects of ABCB4 missense mutations and suggest that even a modest reduction in MDR3 activity may contribute or predispose to the onset of cholestatic liver disease in the paediatric age.

Analysis of aberrant pre-messenger RNA splicing resulting from mutations in ATP8B1 and efficient in vitro rescue by adapted U1 small nuclear RNA

ATP8B1 deficiency is a severe autosomal recessive liver disease resulting from mutations in the ATP8B1 gene characterized by a continuous phenotypical spectrum from intermittent (benign recurrent intrahepatic cholestasis; BRIC) to progressive familial intrahepatic cholestasis (PFIC). Current therapeutic options are insufficient, and elucidating the molecular consequences of mutations could lead to personalized mutation-specific therapies. We investigated the effect on pre-messenger RNA splicing of 14 ATP8B1 mutations at exon-intron boundaries using an in vitro minigene system. Eleven mutations, mostly associated with a PFIC phenotype, resulted in aberrant splicing and a complete absence of correctly spliced product. In contrast, three mutations led to partially correct splicing and were associated with a BRIC phenotype. These findings indicate an inverse correlation between the level of correctly spliced product and disease severity. Expression of modified U1 small nuclear RNAs (snRNA) complementary to the splice donor sites strongly improved or completely rescued splicing for several ATP8B1 mutations located at donor, as well as acceptor, splice sites. In one case, we also evaluated exon-specific U1 snRNAs that, by targeting nonconserved intronic sequences, might reduce possible off-target events. Although very effective in correcting exon skipping, they also induced retention of the short downstream intron.

CONCLUSION

We systematically characterized the molecular consequences of 14 ATP8B1 mutations at exon-intron boundaries associated with ATP8B1 deficiency and found that the majority resulted in total exon skipping. The amount of correctly spliced product inversely correlated with disease severity. Compensatory modified U1 snRNAs, complementary to mutated donor splice sites, were able to improve exon definition very efficiently and could be a novel therapeutic strategy in ATP8B1 deficiency as well as other genetic diseases.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC) is a group of rare disorders which are caused by defect in bile secretion and present with intrahepatic cholestasis, usually in infancy and childhood. These are autosomal recessive in inheritance. The estimated incidence is about 1 per 50,000 to 1 per 100,000 births, although exact prevalence is not known. These diseases affect both the genders equally and have been reported from all geographical areas. Based on clinical presentation, laboratory findings, liver histology and genetic defect, these are broadly divided into three types-PFIC type 1, PFIC type 2 and PFIC type 3. The defect is in ATP8B1 gene encoding the FIC1 protein, ABCB 11 gene encoding BSEP protein and ABCB4 gene encoding MDR3 protein in PFIC1, 2 and 3 respectively. The basic defect is impaired bile salt secretion in PFIC1/2 whereas in PFIC3, it is reduced biliary phospholipid secretion. The main clinical presentation is in the form of cholestatic jaundice and pruritus. Serum gamma glutamyl transpeptidase (GGT) is normal in patients with PFIC1/2 while it is raised in patients with PFIC3. Treatment includes nutritional support (adequate calories, supplementation of fat soluble vitamins and medium chain triglycerides) and use of medications to relieve pruritus as initial therapy followed by biliary diversion procedures in selected patients. Ultimately liver transplantation is needed in most patients as they develop progressive liver fibrosis, cirrhosis and end stage liver disease. Due to the high risk of developing liver tumors in PFIC2 patients, monitoring is recommended from infancy. Mutation targeted pharmacotherapy, gene therapy and hepatocyte transplantation are being explored as future therapeutic options.

Association of variants of ABCB11 with transient neonatal cholestasis

BACKGROUND

The significance of ABCB11 variants have been studied in some cholestatic diseases, but this is not clear in transient neonatal cholestasis (TNC). The aim of the present study was to explore the association between ABCB11 variants and TNC.

METHODS

This was a case-control study. A total of 192 children with TNC referred to a tertiary referral hospital in eastern China were enrolled as subjects, and 196 healthy children were selected as controls. Part of the promoter and exons of the ABCB11 gene were sequenced directly. The single nucleotide polymorphism (SNP) site of V444A was tested using fluorescent quantitative polymerase chain reaction. Potential consequences of variants were predicted using bioinformatics software. The biochemistry indices were compared between the patients with or without possibly pathogenic variants/mutations.

RESULTS

Twenty-eight variants, including 14 novel ones, were detected. Four novel, possibly pathogenic mutations (I416I, K436N, R928Q and IVS7+5G>A) were detected in six subjects. The γ-glutamyltransferase level of these six was lower than in the others (P = 0.054). The genotype distribution of the four common SNP sites, V444A, A535A, A865V and A1082A, was not significantly different between TNC patients and controls.

CONCLUSIONS

Approximately 3% of TNC cases can be attributed to ABCB11 mutations.

The hepatic phosphatidylcholine transporter ABCB4 as modulator of glucose homeostasis

The hepatic phosphatidylcholine (PC) transporter ATP-binding cassette (ABC) B4 flops PC from hepatocytes into bile, and its dysfunction causes chronic cholestasis and fibrosis. Because a nuclear receptor-dependent PC pathway has been determined to exert antidiabetic effects, we now analyzed the role of ABCB4 in glucose metabolism. We bred congenic Abcb4-knockout (Abcb4(-/-)) mice on the fibrosis-susceptible BALB/cJ background. Knockout mice and wild-type controls were phenotyped by measuring plasma glucose concentrations, intraperitoneal glucose tolerance, hepatic RNA expression profiles, and liver histology. In addition, 4 procholestatic ABCB4 gene variants were correlated with blood glucose levels in 682 individuals from 2 independent European cohorts. Systemic glucose levels differ significantly between Abcb4(-/-) mice and wild-type controls, and knockout mice display improved glucose tolerance with significantly lower area under the curve values on intraperitoneal glucose challenge. Of note, hepatic expression of the antidiabetic nuclear receptor 5A2 (LRH-1) is induced consistently in Abcb4(-/-) mice, and its specific rare PC ligands are detected in liver by mass spectrometry imaging. In humans, serum glucose levels are associated significantly with the common ABCB4 variant c.711A>T. In summary, ABCB4 might play a critical role in glucose homeostasis in mice and humans. We speculate that the effects could be mediated via LRH-1-dependent PC pathways.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC) refers to a heterogeneous group of autosomal-recessive disorders of childhood that disrupt bile formation and present with cholestasis of hepatocellular origin. The exact prevalence remains unknown, but the estimated incidence varies between 1/50,000 and 1/100,000 births. Three types of PFIC have been identified and associated with mutations in hepatocellular transport-system genes involved in bile formation. PFIC1 and PFIC2 usually appear in the first months of life, whereas onset of PFIC3 may arise later in infancy, in childhood or even during young adulthood. The main clinical manifestations include cholestasis, pruritus and jaundice. PFIC patients usually develop fibrosis and end-stage liver disease before adulthood. Serum gamma-glutamyltransferase (GGT) activity is normal in PFIC1 and PFIC2 patients, but is elevated in PFIC3 patients. Both PFIC1 and PFIC2 are caused by impaired bile salt secretion due to defects in ATP8B1 encoding the FIC1 protein and in ABCB11 encoding bile salt export pump (BSEP) protein, respectively. Defects in ABCB4, encoding multidrug resistance 3 protein (MDR3), impair biliary phospholipid secretion, resulting in PFIC3. Diagnosis is based on clinical manifestations, liver ultrasonography, cholangiography and liver histology, as well as on specific tests to exclude other causes of childhood cholestasis. MDR3 and BSEP liver immunostaining, and analysis of biliary lipid composition should help to select PFIC candidates for whom genotyping could be proposed to confirm the diagnosis. Antenatal diagnosis may be proposed for affected families in which a mutation has been identified. Ursodeoxycholic acid (UDCA) therapy should be initiated in all patients to prevent liver damage. In some PFIC1 and PFIC2 patients, biliary diversion may also relieve pruritus and slow disease progression. However, most PFIC patients are ultimately candidates for liver transplantation. Monitoring of liver tumors, especially in PFIC2 patients, should be offered from the first year of life. Hepatocyte transplantation, gene therapy and specific targeted pharmacotherapy may represent alternative treatments in the future.

First description of ABCB4 gene deletions in familial low phospholipid-associated cholelithiasis and oral contraceptives-induced cholestasis

The wide clinical spectrum of the ABCB4 gene (ATP-binding cassette subfamily B member 4) deficiency syndromes in humans includes low phospholipid-associated cholelithiasis (LPAC), intrahepatic cholestasis of pregnancy (ICP), oral contraceptives-induced cholestasis (CIC), and progressive familial intrahepatic cholestasis type 3 (PFIC3). No ABCB4 mutations are found in a significant proportion of patients with these syndromes. In the present study, 102 unrelated adult patients with LPAC (43 patients) or CIC/ICP (59 patients) were screened for ABCB4 mutations using DNA sequencing. Heterozygous ABCB4 point or short insertion/deletion mutations were found in 37% (16/43) of the LPAC patients and in 27% (16/59) of the ICP/CIC patients. High-resolution gene dosage methodologies were used in the 70 negative patients. Here, we describe for the first time ABCB4 partial or complete heterozygous deletions in 7% (3/43) of the LPAC patients, and in 2% (1/59) of the ICP/CIC patients. Our observations urge to systematically test patients with LPAC, ICP/CIC, and also children with PFIC3 for the presence of ABCB4 deletions using molecular tools allowing detection of gross rearrangements. In clinical practice, a comprehensive ABCB4 alteration-screening algorithm will permit the use of ABCB4 genotyping to confirm the diagnosis of LPAC or ICP/CIC, and allow familial testing. An early diagnosis of these biliary diseases may be beneficial because of the preventive effect of ursodeoxycholic acid on biliary complications. Further comparative studies of patients with well-characterized genotypes (including deletions) and phenotypes will help determine whether ABCB4 mutation types influence clinical outcomes.

The Multiple Facets of ABCB4 (MDR3) Deficiency

ABCB4 (MDR3), a lipid translocator, moves phosphatidylcholine from the inner to the outer leaflet of the canalicular membrane. Genetic mutations of ABCB4 lead to three distinct but related hepatobiliary diseases. Progressive familial intrahepatic cholestasis (PFIC) type 3 is a chronic cholestatic syndrome characterized by a markedly elevated gamma-glutamyltranspeptidase. Patients present with jaundice, pruritus, and hepatosplenomegaly. Periportal inflammation progresses to biliary cirrhosis and causes portal hypertension. Ursodeoxycholic acid (UDCA) normalizes liver function tests in approximately one half of treated PFIC type 3 patients. Partial responders or nonresponders eventually will require liver transplantation. Gallstone patients with ABCB4 mutations may have low phospholipid-associated cholelithiasis syndrome, characterized by cholesterol gallstones and intrahepatic microlithiasis, along with recurrent biliary symptoms, despite cholecystectomy. Patients with ABCB4 mutations also may develop intrahepatic brown pigment stones. UDCA may improve biliary symptoms even before the dissolution of stones occurs. Additional therapies such as farnesoid X receptor ligands/agonists and benzfibrates show future therapeutic promise. Intrahepatic cholestasis of pregnancy affects pregnant women with abnormal ABCB4. These women suffer from disabling pruritus and also may experience steatorrhea. Fetuses are at high risk for prematurity and stillbirths. The definitive treatment is delivery of the baby. In the interim, limited fat intake, fat-soluble vitamin supplementation, and UDCA with or without S-adenosylmethionine can provide symptomatic relief. Additional hepatobiliary diseases related to ABCB4 mutations are likely to be identified. This may result in the discovery of additional therapies for PFIC type 3, gallstones, and intrahepatic cholestasis of pregnancy.

Medical management of Alagille syndrome

Alagille syndrome is a highly variable, autosomal dominant disorder that affects the liver, heart, eyes, face, skeleton, kidneys, and vascular system. Much has been learned about the genetics of this disorder, which is caused primarily by mutations in the Notch signaling pathway ligand JAGGED1; however, the medical management of this condition is complex and continues to generate controversy. The significant variability of organ involvement requires the managing physician to have an understanding of the breadth and interplay of the variable manifestations. Furthermore, the liver disease in particular requires an appreciation of the natural history and evolution of the profound cholestasis.

DHPLC screening for mutations in progressive familial intrahepatic cholestasis patients

Progressive familial intrahepatic cholestasis (PFIC) is a group of rare heterogeneous autosomal recessive disorders characterized by metabolic defects in biliary proteins involved in the formation and transfer of bile acids in the liver. The genotype-phenotype correlation is not always clear. Mutations in the ATP8B1, BSEP and MDR3 genes have been associated with PFIC1, PFIC2 and PFIC3, respectively. This study sought to characterize the molecular genetic basis for PFIC subtypes in Israel. It was conducted on 14 children with PFIC and their families; 10 with a PFIC1 or PFIC2 phenotype and 4 with a PFIC3 phenotype. Using denaturing high-performance liquid chromatography (DHPLC), five different mutations were identified in four affected families: three novel mutations in BSEP (G19R-g181c, S226L-c803t and G877R-g2755a), one novel mutation in MDR3 (IVS14+6 t/c) and one heterozygous mutation in ATP8B1 (R600W, in a family with the PFIC1/PFIC2 phenotype). The cause of PFIC was identified in 20% of the families tested. These findings indicate the probable involvement of additional genes in PFIC and the need for further studies to determine whether the abnormality lies on the RNA or protein level. A better understanding of the phenotype-genotype correlation in PFIC will lead to improved diagnoses and treatments.

Liver disease associated with canalicular transport defects: current and future therapies

Bile formation at the canalicular membrane is a delicate process. This is illustrated by inherited liver diseases due to mutations in ATP8B1, ABCB11, ABCB4, ABCC2 and ABCG5/8, all encoding hepatocanalicular transporters. Effective treatment of these canalicular transport defects is a clinical and scientific challenge that is still ongoing. Current evidence indicates that ursodeoxycholic acid (UDCA) can be effective in selected patients with PFIC3 (ABCB4 deficiency), while rifampicin reduces pruritus in patients with PFIC1 (ATP8B1 deficiency) and PFIC2 (ABCB11 deficiency), and might abort cholestatic episodes in BRIC (mild ATP8B1 or ABCB11 deficiency). Cholestyramine is essential in the treatment of sitosterolemia (ABCG5/8 deficiency). Most patients with PFIC1 and PFIC2 will benefit from partial biliary drainage. Nevertheless liver transplantation is needed in a substantial proportion of these patients, as it is in PFIC3 patients. New developments in the treatment of canalicular transport defects by using nuclear receptors as a target, enhancing the expression of the mutated transporter protein by employing chaperones, or by mutation specific therapy show substantial promise. This review will focus on the therapy that is currently available as well as on those developments that are likely to influence clinical practice in the near future.

Missense mutations and single nucleotide polymorphisms in ABCB11 impair bile salt export pump processing and function or disrupt pre-messenger RNA splicing

The gene encoding the human bile salt export pump (BSEP), ABCB11, is mutated in several forms of intrahepatic cholestasis. Here we classified the majority (63) of known ABCB11 missense mutations and 21 single-nucleotide polymorphisms (SNPs) to determine whether they caused abnormal ABCB11 pre-messenger RNA splicing, abnormal processing of BSEP protein, or alterations in BSEP protein function. Using an in vitro minigene system to analyze splicing events, we found reduced wild-type splicing for 20 mutations/SNPs, with normal mRNA levels reduced to 5% or less in eight cases. The common ABCB11 missense mutation encoding D482G enhanced aberrant splicing, whereas the common SNP A1028A promoted exon skipping. Addition of exogenous splicing factors modulated several splicing defects. Of the mutants expressed in vitro in CHO-K1 cells, most appeared to be retained in the endoplasmic reticulum and degraded. A minority had BSEP levels similar to wild-type. The SNP variant A444 had reduced levels of protein compared with V444. Treatment with glycerol and incubation at reduced temperature overcame processing defects for several mutants, including E297G. Taurocholate transport by two assessed mutants, N490D and A570T, was reduced compared with wild-type.

CONCLUSION

This work is a comprehensive analysis of 80% of ABCB11 missense mutations and single-nucleotide polymorphisms at pre-mRNA splicing and protein processing/functional levels. We show that aberrant pre-mRNA splicing occurs in a considerable number of cases, leading to reduced levels of normal mRNA. Thus, primary defects at either the protein or the mRNA level (or both) contribute significantly to BSEP deficiency. These results will help to develop mutation-specific therapies for children and adults suffering from intrahepatic cholestasis due to BSEP deficiency.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC) refers to heterogeneous group of autosomal recessive disorders of childhood that disrupt bile formation and present with cholestasis of hepatocellular origin. The exact prevalence remains unknown, but the estimated incidence varies between 1/50,000 and 1/100,000 births.

Three types of PFIC have been identified and related to mutations in hepatocellular transport system genes involved in bile formation. PFIC1 and PFIC2 usually appear in the first months of life, whereas onset of PFIC3 may also occur later in infancy, in childhood or even during young adulthood. Main clinical manifestations include cholestasis, pruritus and jaundice. PFIC patients usually develop fibrosis and end-stage liver disease before adulthood. Serum gamma-glutamyltransferase (GGT) activity is normal in PFIC1 and PFIC2 patients, but is elevated in PFIC3 patients. Both PFIC1 and PFIC2 are caused by impaired bile salt secretion due respectively to defects in ATP8B1 encoding the FIC1 protein, and in ABCB11 encoding the bile salt export pump protein (BSEP). Defects in ABCB4, encoding the multi-drug resistant 3 protein (MDR3), impair biliary phospholipid secretion resulting in PFIC3.

Diagnosis is based on clinical manifestations, liver ultrasonography, cholangiography and liver histology, as well as on specific tests for excluding other causes of childhood cholestasis. MDR3 and BSEP liver immunostaining, and analysis of biliary lipid composition should help to select PFIC candidates in whom genotyping could be proposed to confirm the diagnosis. Antenatal diagnosis can be proposed for affected families in which a mutation has been identified. Ursodeoxycholic acid (UDCA) therapy should be initiated in all patients to prevent liver damage. In some PFIC1 or PFIC2 patients, biliary diversion can also relieve pruritus and slow disease progression. However, most PFIC patients are ultimately candidates for liver transplantation. Monitoring of hepatocellular carcinoma, especially in PFIC2 patients, should be offered from the first year of life. Hepatocyte transplantation, gene therapy or specific targeted pharmacotherapy may represent alternative treatments in the future.

Prenatal diagnosis of progressive familial intrahepatic cholestasis type 2

BACKGROUND AND AIM

Progressive familial intrahepatic cholestasis type 2 (PFIC2) results from genetic defects of the hepatobiliary bile salt export pump (BSEP, ABCB11) at chromosome 2q24. Patients with progressive cholestasis and liver cirrhosis usually need liver transplantation in the first decade. Mutations in ABCB11 are also associated with benign recurrent intrahepatic cholestasis type 2 and intrahepatic cholestasis of pregnancy in adult patients. We aimed to make the prenatal diagnosis of PFIC2.

METHODS

Genetic diagnosis was performed by genomic DNA analysis. Prenatal genetic diagnosis was made by fetal amniotic DNA and chorionic DNA analysis.

RESULTS

We report on two families of PFIC2 with inherited compound heterozygous mutations of ABCB11 (M183V and R303K in Family 1, V284L and 1145delC in Family 2) from the parents. An infant with heterozygous M183V mutation was later born healthy in Family 1. A fetus with compound heterozygous missense mutation V284L and 1145delC was terminated in Family 2.

CONCLUSION

Prenatal diagnosis of PFIC2 was helpful to prevent further affected children in families with this fatal disease.

Severe bile salt export pump deficiency: 82 different ABCB11 mutations in 109 families

BACKGROUND & AIMS

Patients with severe bile salt export pump (BSEP) deficiency present as infants with progressive cholestatic liver disease. We characterized mutations of ABCB11 (encoding BSEP) in such patients and correlated genotypes with residual protein detection and risk of malignancy.

METHODS

Patients with intrahepatic cholestasis suggestive of BSEP deficiency were investigated by single-strand conformation polymorphism analysis and sequencing of ABCB11. Genotypes sorted by likely phenotypic severity were correlated with data on BSEP immunohistochemistry and clinical outcome.

RESULTS

Eighty-two different mutations (52 novel) were identified in 109 families (9 nonsense mutations, 10 small insertions and deletions, 15 splice-site changes, 3 whole-gene deletions, 45 missense changes). In 7 families, only a single heterozygous mutation was identified despite complete sequence analysis. Thirty-two percent of mutations occurred in >1 family, with E297G and/or D482G present in 58% of European families (52/89). On immunohistochemical analysis (88 patients), 93% had abnormal or absent BSEP staining. Expression varied most for E297G and D482G, with some BSEP detected in 45% of patients (19/42) with these mutations. Hepatocellular carcinoma or cholangiocarcinoma developed in 15% of patients (19/128). Two protein-truncating mutations conferred particular risk; 38% (8/21) of such patients developed malignancy versus 10% (11/107) with potentially less severe genotypes (relative risk, 3.7 [confidence limits, 1.7-8.1; P = .003]).

CONCLUSIONS

With this study, >100 ABCB11 mutations are now identified. Immunohistochemically detectable BSEP is typically absent, or much reduced, in severe disease. BSEP deficiency confers risk of hepatobiliary malignancy. Close surveillance of BSEP-deficient patients retaining their native liver, particularly those carrying 2 null mutations, is essential.