First characterized by Daniel Alagille in 1969, Alagille syndrome (ALGS) encompasses a constellation of pathologic findings that includes cholestasis secondary to biliary duct paucity along with extrahepatic manifestations such as structural heart defects, peripheral pulmonic stenosis, a characteristic facial appearance, ocular abnormalities such as posterior embryotoxon, and skeletal anomalies including butterfly vertebrae.
Recently, an article published in Translational Gastroenterology and Hepatology from researchers at the University of Pennsylvania Perelman School of Medicine in Philadelphia set out to review the genetic underpinnings of ALGS. The review also highlights the improvements in disease modeling afforded by a molecular understanding of ALGS and touches upon the potential for targeted gene-based and protein-based therapies.
The Molecular Basis of ALGS
The autosomal dominant inheritance pattern of ALGS is well-defined, according to the review authors. Pathogenic variations in JAGGED1 (JAG1) and NOTCH2 genes have been identified in up to 97% of ALGS cases. Normally, JAG1 and NOTCH2 genes encode a ligand-receptor pair that participates in signaling pathways integral to the formation of bile ducts. Faulty JAG1 and NOTCH2 expression leads to a failure to activate this signaling, resulting in bile duct paucity — one of the cardinal features of ALGS.
The majority of JAG1 pathogenic variations, the review authors note, are due to protein-truncating errors which result in partial or whole gene deletion. Missense variants are found in a smaller percentage of individuals with ALGS, occurring in approximately 13% of cases. Interestingly, phenotypes appear to be indistinguishable between individuals with whole gene mutations and those with intragenic variants, including missense variants. This observation has led researchers to hypothesize that disease pathogenesis is likely due to haploinsufficiency of JAG1.
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Pathogenic variants arising from NOTCH2 are less commonly seen in individuals with ALGS, occurring in only 2.5% of individuals. Unlike with JAG1, 68% of NOTCH2 variants are missense errors. Furthermore, the review authors note, the few NOTCH2 missense variants that have been functionally studied appear to show a reduction in the ability of the resultant protein to undergo activation by JAG1 when analyzed with luciferase reporter assays.
Phenotypic Variability
One of the hallmarks of ALGS is variability in clinical phenotype, according to the review authors. Traditional diagnostic criteria have required finding 3 out of 5 major disease features. However, through advances in JAG1/NOTCH2 gene testing, individuals who have ALGS but fewer than 3 clinical features have been identified, suggesting that clinical criteria alone are insufficient to identify the full spectrum of ALGS.
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The severity of disease features can also vary greatly among individuals with ALGS. The review authors note that “variability extends to familial studies where related individuals with the same pathogenic variants have been found to have highly disparate disease presentations.”
This variable expressivity in ALGS disease features has prompted researchers to try to identify genetic factors capable of modifying pathogenic variants in JAG1 and NOTCH2 genes. Interestingly, genes involved in the post-translational addition of sugar moieties to JAG1 proteins have demonstrated an ability to influence JAG1-mediated Notch signaling, according to the review.
Hepatic Manifestations
Similar to extrahepatic disease manifestations, the severity of liver involvement in ALGS can be highly variable. Clinical findings can range from asymptomatic biochemical abnormalities to profound cholestasis associated with malnutrition, refractory pruritus, and xanthomas. The review authors note that progression to liver transplantation is expected in approximately 20% to 30% of cases and may be higher in individuals who present with cholestatic liver disease earlier in life.
The timing of interventions, such as liver transplantation, is also problematic in younger patients presenting with chronic cholestasis. As the review notes, cholestasis may resolve on its own in a small percentage of patients.
Defining who may need early intervention has prompted research into clinical biomarkers such as bilirubin, conjugated bilirubin, and cholesterol as predictors of severe outcomes. The presence of fibrosis on liver biopsy before 5 years of age, presence of xanthomas on physical exam, and total bilirubin levels >3.8 mg/dL between ages 1 and 2 years have all been suggested as potential biomarkers of severe hepatic outcomes in ALGS.
Read more about Alagille syndrome symptoms
In a separate study published in 2010 in the Journal of Pediatric Gastroenterology and Nutrition, researchers performed a retrospective review of laboratory data from 33 pediatric patients with ALGS. Individuals were stratified into mild and severe hepatic outcome groups. Total bilirubin levels above 6.5 mg/dL, conjugated bilirubin above 4.5 mg/dL, and cholesterol levels greater than 520 mg/dL in patients aged less than 5 years were found to be associated with severe liver disease later in life.
The study authors stated, “These values may assist the clinician in guiding optimal management strategies. A child with laboratory values above these values should be considered at greater probability of having a severe hepatic outcome later in life and therefore may benefit from earlier transplantation evaluation.”
Current and Future Therapies
Supportive care is the mainstay of treatment for ALGS-related liver disease, according to the researchers from Perelman School of Medicine. They note that pruritus of cholestasis is particularly difficult to treat. Traditional pharmacological agents such as antihistamines, choleretics, rifampin, and opioid antagonists such as naltrexone are often utilized. In refractory cases, surgical biliary diversion may provide symptomatic improvement. Sodium-dependent bile acid transporter (ASBT) inhibitors, a new class of drugs that interrupt enterohepatic circulation through chemical inhibition of the ileal bile acid transporters, have also shown promise as a future treatment option.
As in vitro disease models improve, novel therapeutic approaches are also being devised that target the Notch signaling pathway. Unlike congenital cardiac defects which occur early on in embryogenesis, intrahepatic bile duct development continues well into the postnatal period. By targeting this process, the researchers hypothesize, Notch signaling could potentially be augmented, allowing for the development of new bile ducts and thereby ameliorating clinical manifestations of cholestatic disease.
“Having a molecular understanding of ALGS has allowed for the development of numerous in vitro and in vivo disease models, which have provided hope and promise for the future generation of gene-based and protein-based therapies,” the review authors wrote.
“Generation of these disease models has offered scientists a mechanism to study the dynamics of bile duct development and regeneration, and in doing so, produced tools that are applicable to the understanding of other congenital and acquired liver diseases.”
References
Gilbert M, Loomes K. Alagille syndrome and non-syndromic paucity of the intrahepatic bile ducts. Transl Gastroenterol Hepatol. 2021;6:22. doi:10.21037/tgh-2020-03
Kamath B, Munoz P, Bab N, Baker A, Chen Z, Spinner N, Piccoli D. A longitudinal study to identify laboratory predictors of liver disease outcome in Alagille syndrome. J Pediatr Gastroenterol Nutr. 2010;50(5):526-30. doi:10.1097/MPG.0b013e3181cea48d
