Keywords
JAG1
-
NOTCH2
- cholestasis - liver transplant
Alagille syndrome (ALGS) is a multisystem autosomal dominant disorder with highly
variable expression that was first described in 1969 by Alagille et al.[1] ALGS was initially called arteriohepatic dysplasia due to having the identifiable
features of congenital hypoplasia and stenosis of the pulmonary arteries, neonatal
cholestatic liver disease, and various other congenital anomalies, including characteristic
facies.[2]
[3] Since its initial description, additional disease features have emerged, including
vertebral, cardiac, vascular, renal, and ocular phenotypes.
The reported incidence rates of ALGS are 1:70,000 to 100,000 live births but with
the advent of molecular diagnosis, the true incidence is probably closer to 1 in 30,000.[4] Mutations in JAGGED1 (JAG1), a gene encoding a ligand in the Notch signaling pathway, were first identified
in 1997. It is estimated that nearly 95% of individuals with ALGS have a pathogenic
variant in JAG1.[5]
[6]
[7] More recently, a smaller subset of patients with ALGS have been found to have a
variant in NOTCH2 (∼2.5%) which encodes a Notch signaling receptor.[7]
[8]
[9]
[10] JAG1 and NOTCH2 are both transmembrane proteins, and their direct interaction results
in the cleavage and nuclear translocation of an intracellular region of NOTCH2 which
then interacts with transcription factors to influence gene expression. As the number
of identified variants in JAG1 has increased over the years (now amounting to over 600 published variants), it has
been well established that loss-of-function variants are the most frequent variant
type, and a disease pathomechanism of haploinsufficiency has been proposed.[6]
[10]
[11]
[12] Despite our strong understanding of disease-causing variants for ALGS, it is not
understood why different phenotypic characteristics and severity occur, even among
family members who have the same pathogenic variants.[9]
[10]
[12]
[13] One hypothesis has been that a second genetic change may exist outside of JAG1 and NOTCH2 that modifies the disease phenotype, and this remains an active area of research.[14]
[15]
[16]
[17]
In this review, we will provide a comprehensive background of the clinical features
of ALGS, followed by an overview on the genetics of this disease. We will conclude
with an in-depth discussion on treatments for ALGS, both those that are currently
available and active areas of research. Current treatment for ALGS is focused on alleviation
of clinical symptoms while ongoing and future research include the development of
therapeutics that directly affect JAG1 and NOTCH2 gene expression and protein interaction.
Clinical Features
There is a wide spectrum of clinical variability in ALGS ranging from life-threatening
liver or cardiac disease to only subclinical manifestations. Such highly variable
expressivity is seen even among individuals from the same family with the same pathogenic
variant. Those with severe liver or cardiac manifestations are most often diagnosed
with ALGS in infancy versus those with subclinical or mild hepatic involvement who
may not have an established diagnosis until later in life, even as adults. In a study
of 53 JAG1 mutation-positive relatives of 34 ALGS probands, only 21% of mutation-positive relatives
had clinical features that would have led to a diagnosis of ALGS.[4] The frequency of cardiac and liver disease was notably lower in the relatives than
in the probands. The clinical feature with the highest penetrance was the characteristic
facies of ALGS, occurring almost universally in mutation-positive probands and relatives.[4]
[Table 1] summarizes the clinical features and overall frequency associated with ALGS.[11]
[18]
[19]
Table 1
Multisystem involvement, clinical features, and frequency in Alagille syndrome, based
on reports by references[18]
[19]
[20]
|
Involved system
|
Clinical features
|
Overall frequency[a] (%)
|
|
Hepatic
|
• Bile duct paucity
• Conjugated hyperbilirubinemia
• Chronic cholestasis
Pruritus, xanthomas, and/or fat-soluble vitamin deficiencies
•End-stage liver disease
|
Up to 100
|
|
Cardiac
|
•Structural abnormalities, most common:
Pulmonary artery stenosis/hypoplasia
The Tetralogy of Fallot
|
90–97[b]
|
|
Skeletal
|
•Vertebral anomalies
Hemivertebra
Butterfly vertebra
|
33–93
|
|
Facial
|
•Prominent forehead
•Deep-set eyes with moderate hypertelorism
•Pointed chin
•Saddle or straight nose with bulbous tip
|
20–97
|
|
Ocular
|
•Posterior embryotoxon
|
78–89
|
|
Vascular
|
•Intracranial bleeds, including:
Epidural, subdural, subarachnoid, and intraparenchymal
•Systemic vascular anomalies, including:
Aorta, renal, celiac, superior mesenteric, and subclavian arteries
•Other: Moyamoya syndrome
|
15–30
|
|
Renal
|
•Renal abnormalities, most common:
Renal dysplasia
Renal tubular acidosis
•Ureteropelvic obstruction
|
39
|
a Percentages based on probands either with documented JAG1 or NOTCH2 variants or meeting clinical criteria for Alagille syndrome.
b Includes those with cardiac murmur only.
Hepatic
Liver disease typically presents in the neonatal period with direct hyperbilirubinemia,
and varies in severity. Mild liver disease often improves during early childhood,
and progressive liver disease does not develop outside of early childhood.[18]
[20] Reports indicate cholestasis varying from mild to severe in 89% of patients, making
it one of the most common phenotypes seen in ALGS.[19]
It is difficult to predict who will improve versus progress, but in children under
5 years of age, total bilirubin > 6.5 mg/dL, conjugated bilirubin > 4.5 mg/dL, and
cholesterol > 520 mg/dL are predictors of sustained and more severe liver disease.[21] A recent large, multicenter natural history study of cholestasis in ALGS demonstrated
a previously underappreciated burden of liver disease.[22] This study found early profound cholestasis, progression of portal hypertension
later in childhood, and < 25% of patients reaching young adulthood with their native
liver.[22] Chronic cholestasis is associated with many complications with some of the most
bothersome being pruritus and xanthomas. Pruritus develops early, is associated with
serum bile salt level elevation occurring independently of bilirubin level, and can
often become the most debilitating symptom of ALGS. However, pruritus is not always
associated with an elevated bile salt level. Impaired bile secretion is also associated
with reduced secretion of cholesterol and can lead to significantly elevated cholesterol
levels often resulting in the formation of xanthomas which are skin lesions that typically
develop early on and resolve as cholestasis improves.[23]
Histopathology
The most consistently reported feature of ALGS syndrome is bile duct paucity ([Fig. 1A]) which is defined as having a bile duct to portal tract ratio < 0.5 (compared with
normal ratio ranges from 0.9 to 1.8).[24] Bile duct paucity may not be detectable prior to 6 months of age ([Fig. 1B]), with one ALGS study showing bile duct paucity in 60 versus 95% of liver biopsies
completed prior to 6 months compared with after 6 months of age, respectively.[18] The progression of bile duct paucity over the first months of life could be related
to continued bile duct development postnatally, or alternatively to an inability of
compromised bile ducts to keep up with rapid hepatic growth that occurs after birth,
but the exact mechanisms are unknown. Additional insights have been provided from
animal studies. Mouse models have shown that JAG1 expression in hepatoblasts is dispensable for bile duct development, while conditional
deletion of Jag1 in the portal vein mesenchyme results in failure of ductal plate remodeling into
tubular structures in early mouse liver development.[25]
[26] These results indicate that JAG1 expression in specific cell types and at specific times during development is necessary
to activate the Notch signaling for normal biliary tract remodeling.
Fig. 1 Variability in the clinical features of Alagille syndrome (ALGS). (A) Hematoxylin & eosin staining of a liver biopsy from a 7-week-old infant with ALGS
shows a portal tract with a branch of the portal vein and hepatic artery but no bile
duct, indicative of bile duct paucity. (B) Liver biopsy from an infant (< 6 months of age) with ALGS shows presence of bile
ducts. (C) Facial features in children with ALGS include a triangular face, deeply set eyes,
and straight nose with a bulbous nasal tip. (D) ALGS facies evolve from childhood to adulthood with characteristic facial features
in adults being a square jaw and prominent chin. Facies pictures used from Kamath
et al,[59] with permission.
In addition to undetectable bile duct paucity prior to 6 months of age, other histologic
features, including ductular proliferation and giant cell hepatitis, can be present
in infancy, further complicating the diagnosis with biliary atresia. ALGS patients
who undergo Kasai's hepatoportoenterostomy have no added benefit after surgery and
rather may have worsened outcomes (early transplantation or mortality); therefore,
the Kasai procedure should be avoided for patients with ALGS.[18]
[27] In circumstances in which a liver biopsy cannot distinguish infantile ALGS from
biliary atresia, it is of paramount importance to identify extrahepatic syndromic
features or a family history to help guide additional evaluations (see Diagnostic
Testing in Management).
Hepatic Lesions
Hepatic lesions are a reportedly rare complication of ALGS, although their true incidence
is unknown.[28] Lesions typically manifest as either regenerative nodules which are benign, homogeneous
masses with relative preservation of interlobular bile ducts that usually do not necessitate
intervention, or as hepatocellular carcinoma (HCC) which are malignant, single, or
multifocal heterogeneous tumors prone to necrosis, hemorrhage, and metastasis that
are largely resistant to chemotherapy but also often not appropriate for resection
or liver transplantation due to extensive invasion of healthy liver tissue.[29]
[30]
[31]
[32]
[33]
[34]
[35] HCC has been documented in children and adults with ALGS phenotypes ranging from
mild to severe with differing degrees of liver involvement, though little is known
about its incidence, etiology, or typical manifestation age among affected individuals.[36] Case reports of adults with mild ALGS presenting with HCC and no overt ALGS liver
phenotype have caused some to argue that ALGS-causing variants could predispose affected
individuals to HCC by interfering with the Notch signaling.[30]
[36]
[37]
[38] Dysfunctional Notch signaling has been implicated in HCC and intrahepatic cholangiocarcioma.[39] Somatic variants in JAG1 and NOTCH2 have been identified in a variety of human tumor tissues; however, the exact role
of the Notch signaling in HCC remains unclear.[39]
[40]
[41] The risk for developing HCC in ALGS regardless of phenotypic severity highlights
the need for a cancer screening protocol that would enable early detection and treatment
in this at-risk population.[36]
Cardiovascular
Congenital cardiac disease is common in ALGS. Structural abnormalities are seen in
up to 94% of patients with the most common being stenosis/hypoplasia of the branch
pulmonary arteries followed by the tetralogy of Fallot.[42] Pulmonary artery disease in ALGS is likely underestimated because invasive cardiac
imaging is not routinely performed. Survival is significantly impacted by structural
congenital heart disease with 6-year survival decreased to 40% compared with 95% in
those with versus without intracardiac disease, respectively.[18] The presence and type of cardiac involvement does not seem to be correlated with
JAG1 mutation type.[42]
[43]
JAG1 is known to play a crucial role in cardiac and vascular development. While the exact
mechanisms behind disrupted development are not known, genetic mouse models offer
some insights. Endothelial expression of Jag1 is essential for vascular smooth muscle development, and disruption of Jag1 expression in endothelium in a conditional knockout mouse model leads to vascular
and congenital heart defects similar to those seen in ALGS.[44]
[45]
Vascular
Vascular anomalies are highly prevalent in ALGS and can lead to significant morbidity
and mortality with noncardiac vascular anomalies responsible for 34% of mortality
in patients.[46] Up to 25% of ALGS patients have intracranial bleeds with the vast majority having
no other risk factors.[18]
[47] Vascular anomalies are frequently (> 30%) identified on screening brain magnetic
resonance imaging (MRI) in asymptomatic patients with both arterial and venous anomalies
reported, with many detected within the first decade of life.[48]
[49] Screening brain MRI/magnetic resonance angiography (MRA) is recommended in all ALGS
patients when they reach an age where they do not require sedation for the examination.
In the case of trauma or neurological symptoms, there must be a low threshold to repeat
imaging. As with cardiovascular anomalies, there is no correlation between vascular
abnormality phenotype and ALGS genotype.[50]
Renal
Renal abnormalities can be a disease-defining criterion in ALGS. Renal involvement
has been reported in 39% of ALGS patients with the most common being renal dysplasia
(58.9%) followed by renal tubular acidosis (9.5%).[51] Renal insufficiency is rare but patients should have a full renal functional and
structural evaluation at the time of ALGS diagnosis. A renal evaluation should be
repeated if a patient is under consideration for liver transplantation. Evidence of
renal disease is not a contraindication for liver transplantation. Knowledge of underlying
renal disease is important to help the transplant team manage medications and limit
nephrotoxic therapies.
Skeletal
The most common skeletal abnormality in ALGS is butterfly vertebrae with a wide range
in frequency of 33 to 66% of patients.[52] There is no structural significance to this finding and patients are asymptomatic,
but it can aid in diagnosis, although its presence does not confirm ALGS. Various
other skeletal abnormalities can occur, including temporal bone abnormalities and
middle ear bone defects, which increase the risk for chronic otitis media and hearing
loss.[53]
[54] There is an increased risk of fragility fracture in ALGS, causing significant morbidity
and this can be an indication for liver transplantation.[55]
[56] The etiology of increased fracture risk is unknown but likely multifactorial, relating
to both clinical factors including chronic cholestasis, malabsorption, and fat-soluble
vitamin deficiencies, as well as the underlying genetics of the disease, given the
known role of JAG1 and the Notch signaling pathway in skeletal development.[57] In a recent cross-sectional study, deficits in cortical bone size and trabecular
bone microarchitecture were evident in ALGS children compared with healthy controls.[58] Further investigation is needed to determine how these deficits contribute to increased
fracture risk.[58]
Facial Features
The typical ALGS facies are often described as triangular and can include a prominent
forehead, deeply set eyes, moderate hypertelorism, pointed chin, and bulbous tip of
the nose ([Fig. 1C]).[59]
[60] These features change over time to a predominant lower face and mandible and less
prominent upper face and forehead ([Fig. 1D]).[59]
[60] Notably, the characteristic facies of ALGS has been shown to be the clinical feature
with the highest penetrance, with almost universal occurrence in JAG1 mutation-positive probands and relatives.[4]
JAG1 has been implicated in the Notch signaling pathway of cranial neural crest cells
which perhaps play a role in the formation of these characteristic facial features.[61]
Ocular
Ocular abnormalities in ALGS can be helpful for diagnosis but do not typically affect
vision. Posterior embryotoxon is the most common ocular feature found in 56 to 95%
of patients with ALGS, although it is also commonly detected in nonaffected children
with a reported prevalence of 22.5% in children (18 months–20 years) seen in a general
ophthalmic clinic.[62] This term describes a prominent, centrally positioned ring or line at the Schwalbe
ring which is the point when the corneal endothelium and the uveal trabecular meshwork
join.[62] Ophthalmic evaluation for optic disc drusen may be more specific for diagnosis reported
in up to 95% of ALGS children versus only 0.3 to 2% of the general population.[63] There is no clear association between ocular abnormalities and fat-soluble vitamin
A and E levels, nor are there genotype–phenotype correlations.[64]
Growth
Growth failure is well established in ALGS, with more than half of patients falling
below the 5th percentile for height and/or weight.[65] The underlying etiology is likely multifactorial including inadequate caloric intake,
contribution from chronic disease and fat malabsorption from cholestasis, and a possible
role of JAG1 in growth deficiency.[66]
[67] Another proposed mechanism is growth hormone (GH) insensitivity by which growth-retarded
children with ALGS fail to increase insulin-like growth factor-1 (IGF-1) concentrations
in response to GH.[68] This has been supported by a mouse model of ALGS showing reduced expression of IGF-1.[69] These studies imply that children with ALGS and growth failure may benefit from
IGF-1 treatment rather than GH.[68] In a recent ALGS cohort study, total bilirubin showed modest negative correlation
with height and weight z-scores, while the potential contribution of cardiac defect status was not significant,
suggesting that growth impairments in ALGS may be intrinsic and related to the underlying
genetic defects rather than secondary to the clinical manifestations of the disease.[22] There is a need for an ALGS-specific growth chart to help better understand the
growth patterns and interpret the impact of liver transplantation and/or new therapies.
Neurodevelopment
Neurocognitive deficits and the need for special education are seen in almost half
of patients with ALGS.[70] While improvements in nutritional status and cholestasis may help combat this, it
has shown that patients with ALGS, particularly those with evidence of progression
of liver disease, appear to be at higher risk of intelligence quotient (IQ) impairment
compared with other cholestatic diseases, possibly implicating a role for JAG1 in neurodevelopment.[71] Mental health issues can also occur due not only to the challenges of dealing with
a chronic disease, but also because of ALGS-specific issues, such as intractable pruritus
and xanthomas, that may affect quality of life.[70] One study showed that health-related quality of life (HRQOL) is impaired in children
with ALGS similar to children with other causes of chronic intrahepatic cholestasis,
and that it is associated with growth failure, a potentially treatable determinant
of HRQOL.[72] Therefore, psychological support should be incorporated into all stages of ALGS
management, from infancy to adulthood.
Genetics
As mentioned above, ALGS is an autosomal dominant disorder with pathogenic variants
predominantly occurring in JAG1 (almost 95% of affected individuals). Variant type is variable and includes primarily
protein-truncating variants (insertion-deletions, nonsense, splice, full gene deletions,
and partial gene deletions; [Table 2]), suggesting a disease mechanism of reduced gene dosage resulting in JAG1 haploinsufficiency.[6]
[7]
[10]
[11]
[12] Biallelic variants in JAG1 have not been described and are likely lethal. Functional studies performed on a
handful of JAG1 missense variants have corroborated the proposed mechanism of haploinsufficiency,
with evidence showing that mutant proteins are retained intracellularly where they
are unable to interact with the NOTCH2 receptor and/or are defective in their physical
ability to interact with NOTCH2 and propagate a signal.[10]
[73]
[74]
[75]
[76]
[77] Pathogenic NOTCH2 variants, which are far less common than JAG1 variants (∼2.5% of affected individuals), tend to be predominantly missense ([Table 1]), and the mechanism for pathogenicity has not been confirmed through functional
studies.[7]
[8]
[10]
[78] To date, 696 pathogenic JAG1 variants and 20 pathogenic NOTCH2 variants have been reported for ALGS.[10]
[79]
Table 2
Distribution and frequency of mutation type for JAG1 and NOTCH2 variants in Alagille syndrome
|
Gene
|
|
JAG1
|
NOTCH2
|
|
n = 696
|
n = 20
|
|
%
|
n
|
%
|
n
|
|
Frameshift
|
43.5
|
303
|
10.0
|
2
|
|
Nonsense
|
16.2
|
113
|
15.0
|
3
|
|
Missense
|
15.0
|
104
|
65.0
|
13
|
|
Splice site
|
12.8
|
89
|
5.0
|
1
|
|
Structural variants[a]
|
11.1
|
77
|
5.0
|
1
|
|
In frame deletion
|
1.3
|
9
|
0
|
0
|
|
Promoter variant
|
0.1
|
1
|
0
|
0
|
a including multi–exon deletions/duplications, full gene deletions/duplications, translocations,
and inversions.[7]
[10]
The overall diagnostic yield of ALGS in patients who meet diagnostic criteria has
been reported to be 96.6% following standard genetic testing.[7] Genetic testing for ALGS typically follows a sequential testing strategy that involves
JAG1 sequencing and deletion/duplication analysis (which identifies up to ∼94% of pathogenic
variants) followed by NOTCH2 sequencing analysis, although depending on the clinical testing laboratory, these
tests may be offered as panels that include both sequencing and deletion/duplication
analysis for both JAG1 and NOTCH2.[80] Until recently, deletion/duplication variants in NOTCH2 have not been reported, and therefore copy number analysis of NOTCH2 is not always included; however, a recent report identifying a multi–exon deletion
of NOTCH2 in an individual with ALGS suggests that copy number analysis of NOTCH2 should not be overlooked.[7]
Despite the high diagnostic yield obtained by standard genetic testing of JAG1 and NOTCH2 for ALGS, there remains a very small percentage (3.4%) of individuals in whom a molecular
diagnosis is not identified despite a confident clinical diagnosis.[7] In a recent publication, genome sequencing (GS) was performed in a cohort of 14
individuals without a molecular diagnosis for their ALGS, and it resulted in the identification
of four novel variants, three involving JAG1 and one involving NOTCH2.[7] The JAG1 pathogenic variants included a promoter single nucleotide variant (SNV), a balanced
inversion with breakpoints located within intron 3 of JAG1 and within an upstream gene desert, and a four-exon deletion that was missed by a
prior multiplex ligation-dependent probe amplification (MLPA) assay due to limitations
of the MLPA probe design.[7] The report also identified a multi–exon deletion within the NOTCH2 gene which was previously missed since copy number analysis of NOTCH2 was not performed for that individual.[7] This study showed that utilization of GS increases the diagnostic yield in ALGS
by 0.9%, bringing it to 97.5%, and reducing the percentage of individuals in whom
a molecular diagnosis is not identified to 2.5%.[7] Although prior reports have revealed cases where additional genetic testing has
yielded alternative diagnoses in individuals thought to have ALGS,[81]
[82]
[83] it is also possible, particularly for individuals with a strong clinical indication
of ALGS, that variants in as-of-yet unidentified regulatory regions for JAG1 or NOTCH2 may exist, and additional testing strategies, such as RNA sequencing, or improved
understanding of GS data analysis may further increase the diagnostic yield for this
disease.
As mentioned throughout that the overview on clinical disease features, there have
been no observed genotype–phenotype correlations in ALGS; conversely, there have been
multiple reports of individuals with the same pathogenic variant and divergent clinical
presentations.[4]
[12]
[13]
[18]
[84]
[85]
[86]
[87]
[88]
[89] This has led to the hypothesis that a variant in a separate gene may influence or
modify the effects of the JAG1 pathogenic variants, further defining patient phenotype, and whether the clinical
course is likely to be mild or severe. Multiple groups have provided evidence for
various genes to function as genetic modifiers in ALGS. Putative modifiers have been
identified in proteins that are responsible for posttranslational glycosylation of
JAG1,[15]
[16] Thrombospondin 2 (THBS2), a protein that is capable of interacting with NOTCH2 and
interfering with the downstream Notch signaling,[14] and more recently the transcription factor SOX9, with a study that showed reduced
expression led to a worsened liver phenotype in a mouse model of ALGS.[17]
Management
Diagnostic Testing
If a diagnosis of ALGS is suspected, initial clinical evaluation should include routine
laboratories (liver function tests including gamma-glutamyl transferase, serum cholesterol
and triglycerides, serum bile acids, complete blood cell count, and coagulation studies)
and imaging (liver ultrasound). Depending on the clinical scenario, a liver biopsy
may or may not be performed. For example, it is immediately necessary to perform a
liver biopsy in a cholestatic infant in whom a diagnosis of biliary atresia must be
excluded in a timely manner and where awaiting the results of genetic testing is not
an option. In this clinical scenario, the range of findings in infantile ALGS need
to be considered to avoid misdiagnosis with biliary atresia and unnecessary hepatic
surgery. There must be a high level of suspicion for ALGS in cholestatic infants as
the initial di-isopropyl iminodiacetic acid (DISIDA) scan (hepatobiliary scan), cholangiogram,
and liver biopsy results may mimic those of biliary atresia. Further, even with a
nonexcreting DISIDA, histological bile duct proliferation, and an operative cholangiogram
that fails to show the intrahepatic biliary tree, ALGS is still a possibility. Therefore,
it is important to identify syndromic features or a family history of ALGS to help
guide additional evaluations such as cardiology (echocardiogram), renal (renal ultrasound
and renal function tests), skeletal (anteroposterior spine radiograph), ocular (ophthalmic
exam), nutritional assessment (including fat-soluble vitamins), and genetic testing
(previously discussed).
Outside of the clinical scenario of cholestatic infants, a liver biopsy may not be
necessary at all, especially given advancements in genomic diagnostics that have shown
ALGS diagnosis may occur even in those not meeting all of the classic clinical criteria.
Cholestatic liver disease next generation sequencing panels, which include testing
for JAG1 and NOTCH2, may identify a pathogenic variant in one of these two ALGS disease genes in the
absence of or before a clinical presentation of ALGS fully emerges. This scenario
may be more common in NOTCH2-related disease; however, studies have also shown that a JAG1 mutation does not always lead to the classical presentation of ALGS either.[4]
[8]
[78] Genomic diagnostics typically includes sequence and deletion/duplication testing
of JAG1 and NOTCH2.
Liver disease assessment: Use of transient elastography (FibroScan) for measurement of liver stiffness is
of growing importance in the assessment of liver disease, and although pediatric experiences
with this technique are limited, this is a growing area of research. Shneider et al
recently published a prospective investigation of FibroScan in pediatric cholestatic
liver disease showing that nonfasted liver stiffness correlates with liver disease
parameters and portal hypertension, although to a lesser degree in children with ALGS
compared with biliary atresia, suggesting a disease-specific pattern exists. Use of
FibroScan for this purpose in children with ALGS remains investigational and is not
yet part of routine clinical care.[90]
Medical Management
Management of ALGS requires a multidisciplinary approach depending on the organ system
involvement as described below.
Pruritus: Initial measures to help minimize itching and excoriations include skin emollients,
cutting nails short, and avoiding bathing in hot water. Ursodeoxycholic acid (UDCA),
a choleretic which stimulates bile flow, is commonly used as a treatment for cholestasis,
but there are no definitive studies of efficacy.[91] Other medications include cholestyramine or colesevelam, bile acid-binding resins;
naltrexone, an opioid-antagonist which has been effective in other pediatric cholestatic
diseases; rifampin, an antibiotic whose mechanism to treat refractory pruritus may
involve modulation of pruritogen modulating factors being targeted by the Pregnane
X Receptor (PXR, present in both hepatocytes and enterocytes); and antihistamines
which are helpful when dosed at night when pruritus interferes with sleep.[92]
[93] More recently, a prospective, multicenter study tested the safety and efficacy of
a serotonin reuptake inhibitor, sertraline, in treating children with refractory cholestatic
pruritus, including in those related to ALGS. After 3 months of sertraline therapy,
pruritus, median itching score, skin scratch marks, and sleep quality improved with
a tolerable side-effect profile.[94] Combinations of these medications are often required and should be added in a step-wise
fashion ([Table 3]).[60]
[95] When medical therapy fails, surgical biliary diversion or ileal resection can be
considered prior to liver transplantation.[96]
[97]
Table 3
Medication management of cholestatic pruritus in Alagille syndrome (ALGS)
|
Medication
|
Dose
|
Adverse effects
|
|
Choleretics
|
|
|
|
Ursodeoxycholic acid
|
10–20 mg/kg/day, divided in 2 doses
|
Diarrhea, abdominal pain, worsening pruritus, or cholestasis at high dose
|
|
Antibiotics
|
|
|
|
Rifampin
|
10 mg/kg/day, divided in 2 doses (max 600 mg/day)
|
Red discoloration of body fluids, hypersensitivity reactions, hepatitis, and altered
metabolism of other drugs via induction of cytochrome P450 3A
|
|
Bile-salt binding agents
|
|
|
|
Cholestyramine
|
240 mg/kg/day divided in 3 doses (max 8 g/day)
|
Constipation, abdominal pain, and hyperchloremic metabolic acidosis, malabsorption
(including fat-soluble vitamins)
|
|
Colesevelam
|
Limited pediatric data (adult dose 625 mg daily)
|
|
Antihistamines
|
|
|
|
Diphenhydramine
|
5 mg/kg/day, divided in 3–4 doses
|
Drowsiness
|
|
Hydroxyzine
|
2 mg/kg/day, divided in 3–4 doses
|
|
Opioid antagonists
|
|
|
|
Naltrexone
|
0.25–0.5 mg/kg once daily, max 50 mg (adult dose)
|
Symptoms of opioid withdrawal
|
|
Serotonin reuptake inhibitor
|
|
|
|
Sertraline
|
1–4 mg/kg/day (maximum: 200 mg/day)
|
Behavior disorders, skin reactions, vomiting, and transient arterial hypertension
|
|
Lipid-lowering agents
|
|
|
|
Atorvastatin
|
Limited pediatric data, 10 mg once daily (for children ages 10–17 years)
|
Headache, increased transaminases
|
|
Drugs under investigation: apical sodium-dependent bile acid transporter inhibitors
[98]
[99]
[100]
|
|
|
|
Maralixibat
(Expanded Access Program for patients with cholestatic pruritus associated with ALGS—NCT04530994)
|
As per study protocol
|
Gastrointestinal symptoms
|
|
Odevixibat
(Efficacy and Safety in patients with ALGS—NCT04674761)
|
As per study protocol
|
Gastrointestinal symptoms
|
Notes: These agents are used empirically and generally have not been tested.
Adapted from references.[60]
[95]
A new class of drugs under investigation for the treatment of refractory pruritus
in ALGS and other cholestatic liver disorders are the ileal bile acid transporter
(IBAT) inhibitors. These drugs work by interrupting the enterohepatic circulation
of bile acids, in effect acting as a medical biliary diversion. The evaluation of
LUM001 (maralixibat) in the Reduction of Pruritus in Alagille Syndrome (ITCH; NCT02057692)
multicenter, randomized, placebo-controlled trial was recently completed for the IBAT
inhibitor, maralixibat, for the treatment of refractory pruritus in ALGS.[98] The data suggest that maralixibat is safe and may reduce pruritus in ALGS.[98] The Safety and Efficacy Study of LUM001 With a Drug Withdrawal Period in Participants
with Alagille Syndrome (ICONIC; NCT02160782) phase-2 randomized withdrawal trial showed
that 4 years of maralixibat treatment in patients with ALGS was associated with significant
and durable improvement in pruritus, serum bile acids, quality of life, cholesterol,
xanthoma, and height growth.[99] Results of a phase-2 study of the IBAT inhibitor odevixibat also showed reduction
in serum bile acids and pruritus in patients with ALGS and other cholestatic liver
disorders.[100] Other long-term follow-up studies of maralixibat and odevixibat are ongoing (NCT04530994
and NCT04674761).
Xanthomas: These lesions can form on areas with high friction when cholesterol levels are > 500 mg/dL
and can be cosmetically disfiguring for patients. No specific treatment is required
as they typically resolve as cholestasis improves; however, as per personal communication,
statins have reportedly been used in severe, debilitating cases with beneficial effect.[95] Importantly, the hypercholesterolemia of ALGS is mainly due to lipoprotein X (LpX),
which is rich in phospholipids, albumin, and free cholesterol, having a density similar
to low-density lipoprotein (LDL), however, unlike LDL, LpX has no apoB-100 and is
not removed from circulation via the LDL receptor and is not thought to be atherogenic.
As such, treatment of LpX-dependent hypercholesterolemia with conventional hypolipidemic
drugs is frequently ineffective, and definitive treatment relies on correction of
the underlying cause of cholestasis.[101]
Fat-soluble vitamin deficiency: Vitamin levels should be checked routinely and replaced as needed. Fixed ratio multivitamins,
such as ADEKs or DEKAs, are frequently used but may result in excessive intake of
some vitamins to treat insufficiency of others. In that case, vitamins may need to
be dosed individually.
Malnutrition and growth failure: High-calorie supplements should be used as needed to optimize oral nutrition. Nasogastric
feeds or gastrostomy tube placement may be required to reach calorie intake goals.
Liver Transplantation
In a recent ALGS cohort study of children presenting with cholestasis, survival to
early adulthood with native liver occurred in only 24% of children at 18.5 years.[22] This study further showed that despite improvement of cholestasis, progression of
hepatic disease occurred in later childhood in ALGS. This phase was associated with
clinically significant portal hypertension, and ultimate need for liver transplantation
later in childhood.[22] Typical indications for liver transplant in ALGS include severe pruritus, synthetic
dysfunction, portal hypertension, bone fractures, and growth failure. Pretransplant
evaluation must include cardiac and renal function assessments. ALGS patients rarely
require a cardiac or renal transplant at the time of liver transplant. Cardiac disease
should be repaired prior to liver transplant when possible. The extent of renal disease
involvement may impact choice of immunosuppression regimen. Other considerations include
head and abdominal imaging to identify vascular anomalies which could impact bleeding
risk and technical aspects of the transplant procedure.
Patients do well after liver transplant with improved nutritional and growth status.[102] Patient survival 1-year after liver transplant is 80% with similar survival rates
seen in those receiving a living related donor graft.[103]
[104] All potential living related donors should undergo genetic testing. Any potential
donor with a JAG1 mutation should be eliminated from consideration, even if no overt liver disease
is present.[104]
Prognosis
A retrospective study of 92 individuals with ALGS determined the factors that contributed
significantly to mortality were complex congenital heart disease, intracranial bleeding,
and hepatic disease or hepatic transplantation. A mortality of 17% was reported with
major causes of death being hepatic death (average age, 7.5 years), intracranial bleed
(average age, 2.9 years), and multisystem/cardiac failure (average age, 1.6 years).[18] The 20-year predicted life expectancy was 75% for all patients, 80% for those not
requiring liver transplant, and 60% for those requiring liver transplant.[18] In a more recent longitudinal study of 293 individuals with ALGS with native liver,
11 (4%) died with native liver during the study follow-up (median = 2.7 years, range:
0–10 years).[22] Only three deaths were directly related to liver disease with the remainder due
to cardiac involvement, pulmonary hemorrhage, and “other.”[22]
Future Directions/Novel Therapeutics/Research
Future Directions/Novel Therapeutics/Research
To date, therapeutic interventions in ALGS have been supportive and directed to address
specific clinical manifestations, rather than the underlying pathophysiology of disordered
Notch signaling pathway. Theoretically, it should be possible to develop novel therapeutics
to augment the Notch signaling in ALGS, thereby ameliorating the signs and symptoms
of the disease. Notch signaling pathway activation would need to be limited to specific
cell types and most likely for a defined period of time, since unregulated Notch signaling
can lead to development of cancer. Both bile duct and vascular development continue
postnatally and could be amenable to such an approach.
Depending on the individual patient, clinical scenario, and specific pathogenic variant
in JAG1 or NOTCH2, several different therapeutic approaches may be appropriate. One approach would
be to increase expression of the normal JAG1 or NOTCH2 allele by delivering mRNA or by altering transcriptional regulation. Direct delivery
of the Jag1 ligand has been shown to effectively improve fracture healing in a mouse
model and may be possible in some circumstances.[105] Depending on the individual pathogenic variant in a particular patient, another
approach might be to promote read-through of a nonsense variant or to correct a missense
variant either in vitro or in vivo.
Genetic modifiers of the Notch signaling also represent potential therapeutic targets.
For example, glycosylation status of the Notch receptor affects binding affinity for
the ligands. Deletion of the O-glucosyltransferase, Poglut1, rescues the phenotype of bile duct paucity in a Jag1 mouse model, while loss of the Fringe glycosyltransferases has the opposite effect.[15]
[16] Altering the Notch receptor glycosylation to increase affinity of JAG1 ligand binding
could result in a subtle increase in the Notch signaling pathway in the setting of
JAG1 haploinsufficiency. Another recently identified genetic modifier of liver disease
severity in ALGS is the extracellular matrix protein THBS2.[14] THBS2 has been shown to directly inhibit JAG1–NOTCH2 interactions in vitro, and
a polymorphism leading to increased expression of THBS2 was associated with severe liver disease in a well-characterized ALGS cohort. Targeting
THBS2 expression could therefore lead to more efficient Notch signaling pathway.
While still largely theoretical, these approaches to augment the Notch signaling pathway
([Fig. 2]) are under investigation in both in vitro and animal models and could lead to viable
therapeutic interventions in the not too distant future.
Fig. 2 Schematic of putative Notch signaling therapeutics for Alagille syndrome (ALGS).
Dysregulated Notch signaling is depicted at the top with a pathogenic JAG1 variant shown in red. Putative therapeutics include (a) increasing expression of
the wild type JAG1 allele, (b) correcting the pathogenic variant in the mutant JAG1 allele, (c) augmenting expression of putative genetic modifiers (reducing THBS2 or increasing SOX9), and (d) altering NOTCH2 glycosylation.
Conclusion
ALGS is a complex, multisystem, autosomal dominant disease with variable penetrance
caused predominantly by pathogenic variants in JAG1 but also by pathogenic variants in NOTCH2. Although cardiac and vascular involvements are a major cause of mortality, liver
disease accounts for significant morbidity in the ALGS population, and despite a clear
genetic etiology, no genotype–phenotype correlations have been elucidated for any
organ system. Clinical management remains largely supportive; however, multiple avenues
of active research have identified promising drugs for refractory pruritus. Research
is ongoing to discover targeted interventions to augment the Notch signaling in involved
tissues.
Major Concepts and Learning Points
Major Concepts and Learning Points
-
Alagille syndrome (ALGS) is a rare, debilitating, autosomal dominant disorder caused
by pathogenic variants in one of two disease-causing genes, JAGGED1 (JAG1) or NOTCH2, leading to disruption of the Notch signaling pathway.
-
The syndrome is primarily characterized by intrahepatic bile duct paucity and cholestasis,
but has multiple clinical features.
-
Typical indications for liver transplant in ALGS include severe pruritus, synthetic
dysfunction, portal hypertension, bone fractures, and growth failure.
-
There are no observed genotype–phenotype correlations in ALGS, leading to the hypothesis
that a variant in a separate gene may modify the effects of JAG1 or NOTCH2 pathogenic variants.
-
Management of ALGS aims to support nutrition and alleviate pruritus, and while there
are currently no approved treatments, ileal bile acid transporter (IBAT)/apical sodium-dependent
bile acid transporter (ASBT) inhibition has demonstrated promising clinical results.
