Whole-exome sequencing expands the roles of novel mutations of organic anion transporting polypeptide, ATP-binding cassette transporter, and receptor genes in intrahepatic cholestasis of pregnancy

Intrahepatic Cholestasis of Pregnancy: A Single-Centre Whole-Exome Sequencing Study in a Maltese Cohort

Intrahepatic cholestasis of pregnancy (ICP) has a multifactorial pathophysiology involving genetic, endocrine, and environmental factors. It is associated with maternal distress and adverse foetal outcomes. The monogenic aetiology of ICP remains unexplored in the Maltese population. We apply whole exome sequencing in 20 unrelated index cases to assess the molecular spectrum of variants in genes linked to bile acid transport. We shortlisted five unique heterozygous variants. Three ABCB4 variants, including a novel likely pathogenic stop-gain variant were detected. Three genealogically unrelated cases carried an ABCB4 p.Asn510Ser variant, suggestive of a founder through shared haplotype analysis. This study provides preliminary insight into the monogenic aetiology of ICP from an unstudied population. It expands the spectrum of genetic variants associated with ICP and provides evidence for a founder effect in the Maltese population.

Association between ABCB4 variants and intrahepatic cholestasis of pregnancy

The ABCB4 gene encodes multidrug resistance protein 3(MDR3), which is a phosphatidylcholine(PC) transfer enzyme that transfers lecithin from the inner part of the phospholipid bilayer to the extracellular bile. The occurrence of intrahepatic cholestasis of pregnancy(ICP) is closely related to ABCB4 variants, but there is limited research on this topic in southern Anhui, China. We sequenced ABCB4 in pregnant women with ICP and healthy pregnant women to explore the relationship. A total of 30 patients diagnosed with ICP were selected as the study objects and 90 healthy pregnant women were selected as the control group. DNA was extracted from peripheral blood of ICP patients and healthy pregnant women, 27 exons were sequencing by Sanger sequencing. Polymerase chain reaction (PCR) was used to amplify those exons. PolyPhen2, Mutation Taster, Provean, SIFT and Mutpred2 were used to predict protein structure, and Pymol software was used to predict the impact of missense variant c.1954 A > G(p.Arg652Gly) on proteins. Four exonic variants of ABCB4 gene were detected in ICP patients and healthy pregnant women, including synonymous variants c.175 C > T, c.504 C > T,c.711 A > T and missense variant c.1954 A > G(p.Arg652Gly). The incidence of the missense variant c.1954 A > G(p.Arg652Gly) was 6/90 in healthy pregnant womenand 8/30 in ICP patients.In healthy pregnant women with the missense variant c.1954 A > G(p.Arg652Gly), no other exonic variants were found. In ICP patients with missense variant c.1954 A > G(p.Arg652Gly), other exonic variants were found. PolyPhen2, Mutation Taster, Provean, SIFT and Mutpred2 were used to predict that the four exonic variants were benign, while Pymol was used to showed that the missense variant was located in the linker region of MDR3 and had a slight impact on protein function. Among ICP patients with missense variant c.1954 A > G(p.Arg652Gly), patients with three exonic variants(c.504 C > T, c.711 A > T, c.1954 A > G) had higher γ-GT, TBA, ALT and AST than those with two exonic variants. ABCB4 missense variant c.1954 A > G(p.Arg652Gly) requires the combination of other variants(c.175 C > T, c.504 C > T,c.711 A > T) to cause ICP symptoms, and when combined with other variants, it has a superimposed effect.

A novel heterozygous deletion in ABCB4 gene in a Chinese family with intrahepatic cholestasis of pregnancy, neonatal hyperbilirubinemia, and cholelithiasis: Case reports and literature review

BACKGROUND

ABCB4 gene (OMIM *171060) variant is associated with a wide clinical spectrum of hepatobiliary diseases, including familial intrahepatic cholestasis of pregnancy (ICP), progressive familial intrahepatic cholestasis type 3 (PFIC3), and neonatal hyperbilirubinemia due to impaired protection of the bile duct. The majority of reported cases, however, were missense or nonsense variants, with few deletion variant findings in the Chinese population.

METHOD

We performed whole genome sequencing and confirmed it with Sanger sequencing of the proband infant and his families. Clinical courses and laboratory results were documented and collected from the proband infant and his mother. We also reviewed other published cases related to genetic variants in ABCB4 in the Chinese population.

RESULTS

A 26-year-old Chinese female (II.2) who had recurrent intrahepatic cholestasis of pregnancy and her 49-day-old son (III.4) who had hyperbilirubinemia, both presented with extremely elevated total bile acid, cholestatic dominant pattern liver function abnormalities. They were able to stay relatively stable with mild pruritus on ursodeoxycholic acid treatment. After ruling out other possibilities, genetic sequencing revealed a diagnosis of heterozygous deletion variant NM_018849.3:c.1452_1454del (NP_061337.1:p.Thr485del) in ABCB4, which was not reported before, in the symptomatic mother (II.2), index patient (III.4), and the symptomatic grandmother (I.2). This variant resulted in clinical spectrums of ICP, neonatal hyperbilirubinemia, and cholelithiasis in our pedigree.

CONCLUSION

We reported a novel heterozygous deletion variant of the ABCB4 gene in a Chinese family, as well as a literature review of ABCB4-related disorders. We aim to facilitate healthcare professionals to better understand genetic factors as an uncommon cause of hepatobiliary diseases, as well as improve therapeutic strategies in challenging clinical situations such as pregnancy and neonatal care.

Analysis of various ATP-binding cassette transporters revealed quantification of ABCB4 as a potential diagnostic tool in primary sclerosing cholangitis (PSC)

AIMS

ATP-binding cassette transporters are important proteins in regulating bile constituent transport between hepatocytes and the bile canalicular system. Dysfunctional transporters lead to accumulation of toxic bile components within hepatocytes or the biliary system, known as cholestasis, resulting in liver damage. It has been previously reported that two particular ATP-binding cassette transporters, ABCB4 and ABCB11, have altered expression in patients with primary sclerosing cholangitis (PSC). Interested in further analysis of expression patterns of ATP-binding cassette transporters in PSC patients, we investigated liver samples from 201 patients, including 43 patients with PSC and 51 patients with primary biliary cholangitis patients (PBC). In addition to ABCB4 and ABCB11, we also included other ATP-binding cassette transporters, to determine if upregulation of ABCB4 and ABCB11 is specifically found in the liver of patients with PSC.

METHODS AND RESULTS

Retrospectively, formalin-fixed and paraffin-embedded liver biopsies, resections, and explants were selected to investigate the expression of ABCB1, ABCB4, ABCB11, ABCG5/8, and FXR1 using nanoString nCounter and immunohistochemistry for validation of differently expressed transporters seen in PSC liver samples in comparison to non-PSC liver specimens. Strikingly, ABCB4 was the only ATP-binding cassette transporter showing increased gene and protein expression in hepatocytes of PSC livers when compared to non-PSC liver specimens. Furthermore, ABCB4 protein expression also correlated with disease stage in PSC.

CONCLUSION

Our study concluded that altered ABCB4 expression is specifically seen in liver specimens of PSC patients. Therefore, quantitative ABCB4 analysis may be an additional useful tool for the histopathological diagnosis of PSC to distinguish this entity from other cholangiopathies.