A functional classification of ABCB4 variations causing progressive familial intrahepatic cholestasis type 3

Abstract

Progressive familial intrahepatic cholestasis type 3 is caused by biallelic variations of ABCB4, most often ( 70%) missense. In this study, we examined the effects of 12 missense variations identified in progressive familial intrahepatic cholestasis type 3 patients. We classified these variations on the basis of the defects thus identified and explored potential rescue of trafficking-defective mutants by pharmacological means. Variations were reproduced in the ABCB4 complementary DNA and the mutants, thus obtained, expressed in HepG2 and HEK293 cells. Three mutants were either fully (I541F and L556R) or largely (Q855L) retained in the endoplasmic reticulum, in an immature form. Rescue of the defect, i.e., increase in the mature form at the bile canaliculi, was obtained by cell treatments with cyclosporin A or C and, to a lesser extent, B, D, or H. Five mutations with little or no effect on ABCB4 expression at the bile canaliculi caused a decrease (F357L, T775M, and G954S) or almost absence (S346I and P726L) of phosphatidylcholine secretion. Two mutants (T424A and N510S) were normally processed and expressed at the bile canaliculi, but their stability was reduced. We found no defect of the T175A mutant or of R652G, previously described as a polymorphism. In patients, the most severe phenotypes appreciated by the duration of transplant-free survival were caused by ABCB4 variants that were markedly retained in the endoplasmic reticulum and expressed in a homozygous status. Conclusion: ABCB4 variations can be classified as follows: nonsense variations (I) and, on the basis of current findings, missense variations that primarily affect the maturation (II), activity (III), or stability (IV) of the protein or have no detectable effect (V); this classification provides a strong basis for the development of genotype-based therapies.

Diagnosis of monogenic liver diseases in childhood by next‐generation sequencing

“…Patient has also heterozygous frequent variant c.523A>G, p.(Thr175Ala) in ABCB4 (highest observed allele frequency: 3.38% in Finnish Population, average allele frequency: 1.06%, Exome Aggregation Consortium). Variant is known to be associated with intrahepatic cholestasis of pregnancy, low phospholipid‐associated cholelithiasis syndrome and has been found in compound heterozygous state in a patient with progressive familial intrahepatic cholestasis . However, variant is without effect of localization or function of ABCB4 protein in in vitro assays and has been found in homozygous state in 11 controls (Exome Aggregation Consortium).…”

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

“…However, current medical therapy is not satisfactory for the majority of patients in the long term. Only recently, cell culture‐based studies demonstrated that a disturbed ABCB4 function in selected missense variants leading to defective membrane trafficking can be restored by treatment with chaperone drugs . In the series, the vast majority of patients harbored missense mutations.…”

Functional defect of variants in the adenosine triphosphate–binding sites of ABCB4 and their rescue by the cystic fibrosis transmembrane conductance regulator potentiator, ivacaftor (VX‐770)

“…Two other chemical chaperones, 4‐phenylbutyric acid and curcumin, have been recently proposed to rescue ABCB4 variants that impaired traffic . However, the majority of variations affect PC secretion activity of ABCB4, and no pharmacological treatment has been proposed for these mutations yet.…”