Researchers develop mutation database, mouse model for PFIC2
Discoveries can advance understanding of disease, scientists say
Researchers have developed a database of all ABCB11 mutations known to cause progressive familial intrahepatic cholestasis type 2 (PFIC2) and a new mouse model of the liver disease.
Scientists at Rectify Pharmaceuticals, DTR Labs, and Genomenon used Genomenon’s artificial intelligence platform to compile disease-causing ABCB11 mutations and identify a variant associated with PFIC2 and other conditions marked by cholestasis (slowed flow of the digestive fluid bile), suggesting it may be clinically relevant.
The researchers selected the mutation, called E297G, to develop a mouse model that closely mimics PFIC2 in people. Rectify is working on the development of therapies for cholestatic diseases, and has previously helped advance understanding of how certain severe ABCB11 mutations, including E297G, lead to PFIC2.
“Together, these tools can support clinical and translational efforts to advance understanding and treatment of PFIC2,” the researchers wrote in the study, “An ABCB11 variant registry and novel knockin mouse model of PFIC2 based on the clinically relevant ABCB11 E297G variant,” which was published in the Journal of Lipid Research.
Mutation database aims to aid modeling, research
“We’re proud to support meaningful advances in rare disease research,” Mike Klein, Genomenon’s CEO, said in a company press release. “By indexing and curating the full breadth of the scientific literature on ABCB11, we reinforced the research team’s ability to prioritize the most clinically relevant [mutation] for modeling that ultimately demonstrated a treatment response that mirrors what’s observed in patients.”
PFIC is a group of genetic diseases marked by cholestasis, or impaired flow of bile from the liver, where it is produced, to the intestines. When bile builds up, it can damage the liver and leak into the bloodstream, causing itching and other cholestasis symptoms.
PFIC2, which typically develops in the first years of life, is caused by mutations in the ABCB11 gene. These result in low levels of a protein, called bile salt export pump (BSEP) that normally moves bile acids — the main component of bile — out of liver cells. Without enough BSEP, bile acids build up to toxic levels in the liver.
“To date, approximately 200 variants have been associated with PFIC2 yet the majority of variants lack full characterization, limiting the interpretation of genetic tests designed to inform diagnosis and treatment,” the researchers wrote.
They began by compiling all known ABCB11 mutations linked to PFIC2 and a few other cholestatic liver diseases, using both Genomenon’s artificial intelligence platform and manual review of the scientific literature.
They found 476 disease-associated mutations, 240 of which (84%) were linked to PFIC2. More than half (60%) were missense mutations, meaning that a single change in the DNA results in a swap of one of the building blocks of the BSEP protein.
Two of them — E297G and D482G — were particularly common and found in PFIC2 and other cholestatic diseases, including benign recurrent intrahepatic cholestasis type 2 and intrahepatic cholestasis of pregnancy.
Earlier studies suggested that E297G makes it difficult for the BSEP protein to reach the surface of liver cells, where it exerts its action, which likely affects bile flow. However, this hasn’t been directly tested in an appropriate animal model.
Mice with PFIC2
To explore how E297G contributes to cholestasis, researchers at Rectify developed a PFIC2 mouse model, which carried that mutation in both copies of the ABCB11 gene. Unlike their healthy counterparts, these mice showed signs similar to human PFIC2.
Normally, BSEP undergoes a process called glycosylation, where sugar molecules are added to the protein. This helps it fold and reach the surface of liver cells. In the mouse model, glycosylation was incomplete, and BSEP failed to reach its proper location, reducing bile acid flow out of liver cells.
The mice also showed clear signs of cholestasis early on, with high levels of bile acids in their livers. Elevated blood levels of liver enzymes suggested damage, although one particular liver enzyme (GGT) remained normal — similar to what is seen in people with PFIC2. The liver was enlarged, and tissue analysis showed mild inflammation.
Model mice also had fewer toxic bile acids, likely reflecting the liver’s attempt to protect itself. These bile acid changes closely matched what is seen in BSEP-deficient mouse models and people with PFIC2.
To validate the mouse model for translational research, the team tested Ipsen’s PFIC medication Bylvay (odevixibat), which blocks the ileal bile acid transporter (IBAT), a protein in the intestine that reabsorbs bile acids. By blocking IBAT, more bile acids are passed in the stool, reducing toxic buildup.
Mice treated with Bylvay for 14 days showed less liver enlargement and lower levels of bile acids and other liver damage biomarkers, suggesting that the mouse model “responds faithfully to [bile acid-targeted] therapeutics and can serve as a viable translational model for pre-clinical assessment of novel drug products,” the researchers wrote.
The mouse model and the mutation database are “two translational tools to support further research into BSEP-related primary cholestasis and enable the discovery and development of novel therapeutics to treat high unmet need patient populations, such as PFIC2,” the team concluded.
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