ABCC12 Gene in Chronic Cholestasis Syndromes

Study authors found an association between pathogenic ABCC12 gene variants with cholestatic phenotypes in pediatric patients with low gamma-glutamyl transferase (GGT) chronic cholestasis, as well as in zebrafish and mice models, according to findings published in the journal of Gastroenterology.

This study examined 93 patients: 4 from the Middle East, 34 from the Cincinnati Children’s Hospital Medical Center, and 55 from the National Institute of Diabetes and Digestive and Kidney Diseases-supported Childhood Liver Disease Research Network. Whole-exome sequencing was conducted on the 4 patients with idiopathic low GGT cholestasis from the Middle East and targeted gene sequencing was performed on the other 89 patients.

Among the 93 patients, 6 pathogenic ABCC12 variants were detected, 2 interpreted to cause premature truncation and 4 of unknown significance. In 1 female patient from the Middle East, a nonsense pathogenic ABCC12 variant was identified that encoded for the multidrug resistance-associated protein 9 (MRP9) transporter. The following is a timeline of events in this specific female patient at different ages:

• 6 months: Seizure and subdural hematoma
• 12 months: Liver enzyme levels were mildly elevated, GGT and direct bilirubin were within normal limits
• 15 months: Pruritus episode, serum total bile acid levels highly elevated (278 µmol/L), direct bilirubin increased, GGT remained within normal limits
• 9 years: Itching improved, mild periportal inflammation, stage 1 fibrosis

In humans, ABCC12 gene expression is abundant in the brain and testis; however, 2 variants were detected in the liver. Both immunohistochemistry and immunofluorescence revealed MRP9 proteins in the intrahepatic bile ducts and on the apical membrane of the interlobular bile ducts. The human MRP9 protein is 52% and 85% identical to its zebrafish and mouse orthologs, respectively. It was noted that MRP9 displayed conserved expression in cholangiocytes in all 3 species.

ABCC12 mutant zebrafish were generated and did not demonstrate obvious defects in the number of cholangiocytes or morphology of the biliary network at the larval stage. However, male mutants were smaller with splenomegaly and had increased total liver bile salt levels. Additionally, cell death and vascular congestion without inflammation or fibrosis was observed.

In contrast, female mutants did not have growth delay, splenomegaly, or liver injury. Study authors observed a significant difference in reduction in cholangiocyte count in mutants at 4 weeks, coinciding with sexual differentiation.

In adult ABCC12 mutant mice, hepatic injury and GGT levels within normal limits were observed. Study authors observed a decreased average number of well-formed bile ducts per portal triad in the mutant mice. Target ABCC12 deletion in mice supports the phenotype of MRP9 loss seen in the female patient and mutant zebrafish. “Partial and complete loss of MRP9 renders bile duct epithelium susceptible to bile acid-induced apoptosis and results in bile duct loss.”

Possible mechanisms of intrahepatic bile duct deficiency in ABCC12 mutants are as follows:

• Failure to maintain cholangiocyte cell identity
• Decreased cholangiocyte proliferation
• Increased cholangiocyte apoptosis

“We identify ABCC12 as a novel candidate gene for chronic cholestasis syndromes. It will be important to validate our findings in an external cohort of both low and high GGT cholestasis to estimate the contribution of pathogenic variants of ABCC12 to chronic liver disease with various phenotypes and perform phenotype-genotype correlation studies,” study authors concluded.

Reference

Pham DH, Kudira R, Xu L, et al. Deleterious variants in ABCC12 are detected in idiopathic chronic cholestasis and cause intrahepatic bile duct loss in model organisms: ABCC12 pathogenic variants lead to cholestasis. Gastroenterol. Published online March 1, 2021. doi: 10.1053/j.gastro.2021.03.026