Assessment of Adenosine Triphosphatase Phospholipid Transporting 8B1 (ATP8B1) Function in Patients With Cholestasis With ATP8B1 Deficiency by Using Peripheral Blood Monocyte-Derived Macrophages

Activation and substrate specificity of the human P4-ATPase ATP8B1

Asymmetric distribution of phospholipids in eukaryotic membranes is essential for cell integrity, signaling pathways, and vesicular trafficking. P4-ATPases, also known as flippases, participate in creating and maintaining this asymmetry through active transport of phospholipids from the exoplasmic to the cytosolic leaflet. Here, we present a total of nine cryo-electron microscopy structures of the human flippase ATP8B1-CDC50A complex at 2.4 to 3.1 Å overall resolution, along with functional and computational studies, addressing the autophosphorylation steps from ATP, substrate recognition and occlusion, as well as a phosphoinositide binding site. We find that the P4-ATPase transport site is occupied by water upon phosphorylation from ATP. Additionally, we identify two different autoinhibited states, a closed and an outward-open conformation. Furthermore, we identify and characterize the PI(3,4,5)P3 binding site of ATP8B1 in an electropositive pocket between transmembrane segments 5, 7, 8, and 10. Our study also highlights the structural basis of a broad lipid specificity of ATP8B1 and adds phosphatidylinositol as a transport substrate for ATP8B1. We report a critical role of the sn-2 ester bond of glycerophospholipids in substrate recognition by ATP8B1 through conserved S403. These findings provide fundamental insights into ATP8B1 catalytic cycle and regulation, and substrate recognition in P4-ATPases.

Clinicopathologic Features, Genetics, Treatment, and Long-Term Outcomes in Japanese Children and Young Adults with Benign Recurrent Intrahepatic Cholestasis: A Multicenter Study

BACKGROUND

Few reports of benign recurrent intrahepatic cholestasis (BRIC) have focused on East Asian patients. We describe the clinicopathologic features, genetics, treatment, and outcomes in Japanese BRIC patients.

METHODS

We recruited patients with BRIC type 1 (BRIC-1) or 2 (BRIC-2) treated at four pediatric centers and one adult center between April 2007 and March 2022. Demographics, clinical course, laboratory results, molecular genetic findings concerning ATP8B1 and ABCB11 genes, histopathology, and treatment response were examined retrospectively.

RESULTS

Seven Japanese patients with BRIC were enrolled (four male, three female; four BRIC-1 and three BRIC-2). The median age at onset for BRIC-1 was 12 years; for BRIC-2, it was 1 month. Intermittent cholestatic attacks numbered from one to eight during the 11 years of median follow-up. Six patients received a mainstream education; only one patient attended special education. None developed cirrhosis. Three with BRIC-1 showed compound heterozygosity for a variant ATP8B1 gene, while one was heterozygous; two BRIC-2 patients showed compound heterozygosity in ABCB11 and one was heterozygous. Liver biopsy specimens obtained during cholestatic attacks showed fibrosis varying from none to moderate; inflammation was absent or mild. Rifampicin administered to three patients for cholestatic attacks was effective in all, as was cholestyramine in two of three.

CONCLUSIONS

To our knowledge, this is the first East Asian multicenter study of BRIC patients. Onset age and number of cholestatic attacks varied. Rifampicin and cholestyramine were effective against attacks. No patient developed cirrhosis; most had normal growth and development. The long-term outcomes were satisfactory.

Genetic alterations and molecular mechanisms underlying hereditary intrahepatic cholestasis

Hereditary cholestatic liver disease caused by a class of autosomal gene mutations results in jaundice, which involves the abnormality of the synthesis, secretion, and other disorders of bile acids metabolism. Due to the existence of a variety of gene mutations, the clinical manifestations of children are also diverse. There is no unified standard for diagnosis and single detection method, which seriously hinders the development of clinical treatment. Therefore, the mutated genes of hereditary intrahepatic cholestasis were systematically described in this review.

Clinical phenotype of adult-onset liver disease in patients with variants in ABCB4, ABCB11, and ATP8B1

Variants in ATP8B1, ABCB11, and ABCB4 underlie the most prevalent forms of progressive familial intrahepatic cholestasis. We aim to describe variants in these genes in a cohort of patients with adult‐onset liver disease, and explore a genotype–phenotype correlation. Patients with onset of liver disease aged above 18 who underwent sequencing of cholestasis genes for clinical purposes over a 5‐year period were identified. Bioinformatic analysis of variants was performed. Liver histology was evaluated in patients with variants. Of the 356 patients tested, at least one variant was identified in 101 (28.4%): 46 ABCB4, 35 ABCB11, and 28 ATP8B1. Patients with ABCB4 variants had chronic liver disease (71.7%) and pregnancy‐associated liver dysfunction (75%), with a younger age of onset in more severe genotypes (p = 0.046). ABCB11 variants presented with pregnancy‐associated liver dysfunction (82.4%) and acute/episodic cholestasis (40%), with no association between age of onset and genotype severity. ATP8B1 variants were associated with chronic liver disease (75%); however, they were commonly seen in patients with an alternate etiology of liver disease and variants were of low predicted pathogenicity. In adults with suspected genetic cholestasis, variants in cholestasis genes were frequently identified and were likely to contribute to the development of liver disease, particularly ABCB4 and ABCB11. Variants were often in heterozygous state, and they should no longer be considered recessive Mendelian traits. Sequencing cholestasis genes in selected patients with adult‐onset disease should be considered, with interpretation in close collaboration with histopathologists and geneticists.

Variants in cholestasis genes were found in patients with adult‐onset liver disease. ABCB4 variants are associated with chronic biliary disease, variants in ABCB11 are seen in patients with acute cholestasis, and the clinical significance of ATP8B1 variants is not clear.

Case Report: A Rare Case of Benign Recurrent Intrahepatic Cholestasis-Type 1 With a Novel Heterozygous Pathogenic Variant of ATP8B1

Benign recurrent intrahepatic cholestasis type 1 (BRIC1) is a rare autosomal recessive disorder that is characterized by intermittent episodes of jaundice and intense pruritus and caused by pathogenic variants of adenosine triphosphatase phospholipid transporting 8B1 (ATP8B1). The presence of genetic heterogeneity in the variants of ATP8B1 is suggested. Herein, we describe a unique clinical course in a patient with BRIC1 and a novel heterozygous pathogenic variant of ATP8B1. A 20-year-old Japanese man experienced his first cholestasis attack secondary to elevated transaminase at 17 years of age. Laboratory examinations showed no evidence of liver injury caused by viral, autoimmune, or inborn or acquired metabolic etiologies. Since the patient also had elevated transaminase and hypoalbuminemia, he was treated with ursodeoxycholic acid and prednisolone. However, these treatments did not relieve his symptoms. Histopathological assessment revealed marked cholestasis in the hepatocytes, Kupffer cells, and bile canaliculi, as well as a well-preserved intralobular bile duct arrangement and strongly expressed bile salt export pump at the canalicular membrane. Targeted next-generation sequencing detected a novel heterozygous pathogenic variant of ATP8B1 (c.1429 + 2T > G). Taken together, the patient was highly suspected of having BRIC1. Ultimately, treatment with 450 mg/day of rifampicin rapidly relieved his symptoms and shortened the symptomatic period.