New mouse model could help guide PFIC3 treatment research
Model showed more severe liver damage and response to colesevelam
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Researchers have developed a new mouse model of progressive familial intrahepatic cholestasis type 3 (PFIC3) that more closely mimics key features of the disease in people, which may improve preclinical research on disease progression and treatment response.
“This … PFIC3 model is anticipated to accelerate the development of novel therapies for PFIC3 and potentially other cholestatic liver diseases,” the researchers wrote.
The development process and findings were described in the study, “Bile acid detergency as determinant of liver pathology in a humanized mouse model of progressive familial intrahepatic cholestasis type 3,” published in Cellular and Molecular Gastroenterology and Hepatology.
Human-like bile acids may improve PFIC3 modeling
PFIC3 is marked by persistent cholestasis, a condition in which the flow of bile from the liver, where bile is produced, is slowed or stalled. As bile builds up to toxic levels in the liver, patients are at risk of irreversible liver scarring, known as cirrhosis, and liver failure, for which a liver transplant may be the only therapeutic option. Liver damage also allows bile acids to leak into the bloodstream.
ABCB4 mutations disrupt the function of a protein that moves phosphatidylcholines (PCs), a type of fat molecule, from liver cells into bile. There, PCs combine with bile acids and cholesterol to help protect the cells lining the bile ducts from bile acids’ detergent-like effects. When ABCB4 is not working properly, this protection is weakened.
Mice that lack the equivalent gene, called Abcb4-knockout mice, have been used as an animal model of PFIC3, but their disease progression is notably slower and milder than in patients.
This difference is likely because mice naturally produce a less toxic bile acid pool than humans. In mice, up to 60% of bile acids are hydrophilic, or water-attracting, muricholic acids (MCAs), which are less toxic. Humans, by contrast, have predominantly hydrophobic, or water-repelling, bile acids that can be toxic at high levels.
Mice lacking Cyp2c70, the gene that codes for the enzyme responsible for producing MCAs, show a more hydrophobic bile acid profile that more closely resembles that of humans. These mice provide a more human-like bile acid background for studying PFIC3 and other bile acid-related liver diseases.
To better replicate human PFIC3, a team led by researchers at the University of Groningen in the Netherlands developed and characterized a mouse model in which Cyp2c70 was knocked out and Abcb4 activity was suppressed in the liver.
New model shows more severe liver injury
Initial tests showed that bile flow remained unaffected, but levels of phospholipids, a group of fat molecules that includes PCs, dropped sharply after Abcb4 activity was reduced, both in mice with standard bile acids and in mice with a more human-like bile acid profile.
Tests also confirmed that mice lacking Cyp2c70 were unable to produce MCAs, but had higher levels of chenodeoxycholic acid (CDCA), a hydrophobic bile acid found in humans. This resulted in a potentially more toxic bile acid profile.
Also, mice lacking Cyp2c70 and with reduced Abcb4 activity had significantly heavier livers than those with reduced Abcb4 activity alone, “suggesting the presence of more liver damage and a higher inflammatory state,” the team wrote.
In blood tests, liver damage markers were modestly increased in mice with reduced Abcb4 activity alone, but rose sharply in mice lacking Cyp2c70 and with reduced Abcb4 activity. Genetic markers of liver scarring, or fibrosis, blood bile acid levels, and liver cell death were also more strongly elevated in these mice.
Further statistical analyses showed that lower PC concentrations in bile were significantly associated with higher blood levels of liver damage markers, and that this relationship was most pronounced in mice lacking Cyp2c70 and with reduced Abcb4 activity.
Liver damage worsens over time in new model
After six weeks of reduced Abcb4 activity, mice lacking Cyp2c70 showed even heavier livers, more toxic bile acid profiles, higher liver damage markers, and more advanced liver fibrosis than mice examined earlier. This showed that liver damage progressed over time, resembling what happens in PFIC3.
Reducing the reabsorption of bile acids from the intestines, which limits their return to the liver for reuse, has been reported to ease cholestasis and bile duct injury in mice with reduced Abcb4 activity or Cyp2c70 deficiency.
With this in mind, the team treated mice lacking Cyp2c70 and with reduced Abcb4 activity with colesevelam, a medication that binds bile acids in the intestines and prevents their reabsorption. Six weeks of treatment increased the amount of bile acids excreted in feces and reduced liver abnormalities in the new model.
Specifically, liver weight, blood bile acid levels, signs of liver injury, and signs of fibrosis were all reduced.
“This study demonstrates that suppression of Abcb4 gene [activity] in the livers of Cyp2c70-[knockout] mice with a hydrophobic, human-like [bile acid] pool composition profoundly exacerbates liver [disease],” the researchers concluded. The study also “indicates that modulating the concentration and composition of biliary [bile acids], particularly in the early phases of the disease, may slow down the progression of liver [disease] in patients with PFIC3.”
