ATP8B1 gene expression is driven by a housekeeping-like promoter independent of bile acids and farnesoid X receptor

Sequencing of FIC1, BSEP and MDR3 in a large cohort of patients with cholestasis revealed a high number of different genetic variants

BACKGROUND & AIMS

The bile salt export pump (BSEP, ABCB11), multidrug resistance protein 3 (MDR3, ABCB4) and the ATPase familial intrahepatic cholestasis 1 (FIC1, ATP8B1) mediate bile formation. This study aimed to determine the contribution of mutations and common variants in the FIC1, BSEP and MDR3 genes to cholestatic disorders of differing disease onset and severity.

METHODS

Coding exons with flanking intron regions of ATP8B1, ABCB11, and ABCB4 were sequenced in cholestatic patients with assumed genetic cause. The effects of new variants were evaluated by bioinformatic tools and 3D protein modeling.

RESULTS

In 427 patients with suspected inherited cholestasis, 149 patients carried at least one disease-causing mutation in FIC1, BSEP or MDR3, respectively. Overall, 154 different mutations were identified, of which 25 were novel. All 13 novel missense mutations were disease-causing according to bioinformatics analyses and homology modeling. Eighty-two percent of patients with at least one disease-causing mutation in either of the three genes were children. One or more common polymorphism(s) were found in FIC1 in 35.3%, BSEP in 64.3% and MDR3 in 72.6% of patients without disease-causing mutations in the respective gene. Minor allele frequencies of common polymorphisms in BSEP and MDR3 varied in our cohort compared to the general population, as described by gnomAD. However, differences in ethnic background may contribute to this effect.

CONCLUSIONS

In a large cohort of patients, 154 different variants were detected in FIC1, BSEP, and MDR3, 25 of which were novel. In our cohort, frequencies for risk alleles of BSEP (p.V444A) and MDR3 (p.I237I) polymorphisms were significantly overrepresented in patients without disease-causing mutation in the respective gene, indicating that these common variants can contribute to a cholestatic phenotype.

LAY SUMMARY

FIC1, BSEP, and MDR3 represent hepatobiliary transport proteins essential for bile formation. Genetic variants in these transporters underlie a broad spectrum of cholestatic liver diseases. To confirm a genetic contribution to the patients' phenotypes, gene sequencing of these three major cholestasis-related genes was performed in 427 patients and revealed 154 different variants of which 25 have not been previously reported in a database. In patients without a disease-causing mutation, common genetic variants were detected in a high number of cases, indicating that these common variants may contribute to cholestasis development.

Promoting expression of transporters for treatment of progressive familial intrahepatic cholestasis disease

Progressive familial intrahepatic cholestasis (PFIC) is a heterogeneous group of autosomal recessive genetic diseases with a major clinical manifestation of severe intrahepatic cholestasis and an incidence rate of 1/10000 to 1/5000. PFIC is usually first diagnosed in infancy or childhood and eventually develops into liver failure and death. Based on clinical manifestations, laboratory tests, and genetic defects in liver tissue, PFIC is roughly divided into three types: PFIC-1, PFIC-2 and PFIC-3. Studies have demonstrated that all three types of PFIC are associated with the mutations of bile transport system genes in the liver. Promoting transporter expression has important clinical significance for the treatment of PFIC. In this paper, we summarize the etiology and treatment status of PFIC and discuss the treatment of PFIC by promoting the expression of transporters.