Localization of Alagille syndrome to 20p11.2-p12 by linkage analysis of a three-generation family

The DSL domain in mutant JAG1 ligand is essential for the severity of the liver defect in Alagille syndrome

Alagille syndrome (AGS) is a congenital multi-system anomaly mainly characterized by paucity of intrahepatic bile ducts caused by haploinsufficiency of the Jagged 1 gene (JAG1). To explore the relationship between genotype and phenotype, we analyzed the JAG1 gene in 25 Japanese AGS families at the genomic DNA level and identified 15 point mutations and one large deletion. Analysis of the genotype and phenotype strongly indicated that the Delta/Serrate/Lag-2 (DSL) domain in JAG1 protein played an essential role in determining the severity of the liver disorder. In four sporadic cases, missing an entire DSL domain in mutant JAG1 resulted in progressive liver failure and all 4 patients needed a liver transplant at a very young age. This correlation was further confirmed by statistical analysis (chi2=9.143, p<0.001). Our finding demonstrated that the DSL domain in JAG1 appears to be essential for normal liver development and function.

Genetic basis of syndromes associated with congenital heart disease

Numerous syndromes affecting patients have phenotypes that include congenital heart defects (CHDs). These disorders have fascinated physicians for many years, raising questions about how seemingly disparate aspects of human development can be perturbed together in striking, but consistent, ways. Paralleling the major advances in human genetics during recent decades, we have come to understand that some of these syndromes arise from gross defects in chromosomal number, some from subtler alterations in genomic regions, and still others from point mutations in specific genes. These disorders, largely mendelian in nature, have provided researchers with the wherewithal to discover disease genes underlying CHD. Although some of these medical conditions are relatively rare, their solution has often provided insights that could be applied toward understanding the basis of nonsyndromic CHD. In this review, recent progress toward uncovering the molecular basis of several forms of syndromic CHD is discussed.

Parental mosaicism of JAG1 mutations in families with Alagille syndrome

The Alagille syndrome (AGS), a congenital disorder affecting liver, heart, skeleton and eye in association with a typical face, is an autosomal dominant disease with nearly complete penetrance and variable expression. AGS is caused by mutations in the developmentally important JAG1 gene. In our mutation screening, where 61 mutations in JAG1 were detected, we identified five cases where mosaicism is present. Our results point to a significant frequency of mosaicism for JAG1 mutations in AGS of more than 8.2%. Because mosaicism may be associated with a very mild phenotype, the appropriate diagnosis of AGS and consequently the determination of the recurrence risk can be complicated.

Alagille syndrome. The widening spectrum of arteriohepatic dysplasia

Alagille syndrome was described more than 35 years ago as a genetic entity characterized by five major features: chronic cholestasis resulting from paucity of interlobular bile ducts, peripheral pulmonary stenosis, butterflylike vertebral arch defect, posterior embryotoxon, and peculiar facies. Recently, JAGGED1 has been identified as a responsible gene by demonstration of mutations in AGS patients. Studies of the JAGGED1 expression pattern demonstrate that minor features and almost all the elements in the long list of manifestations described in AGS patients are not coincidental. This finding suggests that the definition of AGS may be reconsidered in the light of JAGGED1 mutations.

Current status of pediatric liver transplantation

Increased survival for young liver transplant recipients has greatly improved. Increasing success has led to broader indications, thereby increasing the number of potential recipients. Pediatric liver centers are developing new strategies to cope with the ever-increasing demands for suitable size appropriate grafts. UNOS is in the process of updating guidelines to regulate the sharing of organs which become available from new surgical techniques. In the future, alternative therapies, such as artificial liver assist devices and techniques of cellular transplantation and genetic modification of hepatocytes, may decrease the number of children who die while waiting for a suitable organ or even obviate the need for the liver transplantation.

Mutations in JAGGED1 gene are predominantly sporadic in Alagille syndrome

BACKGROUNDS & AIMS

Mutations in the JAGGED1 gene are responsible for the Alagille syndrome, an autosomal dominant disorder characterized by neonatal jaundice, intrahepatic cholestasis, and developmental disorders affecting the liver, heart, vertebrae, eyes, and face. We screened a large group of patients for mutations in JAGGED1 and studied transmission of the mutations.

METHODS

The coding sequence of the JAGGED1 gene was searched by single-strand conformation polymorphism and sequence analysis for mutations in 109 unrelated patients with the Alagille syndrome and their family if available.

RESULTS

Sixty-nine patients (63%) had intragenic mutations, including 14 nonsense mutations, 31 frameshifts, 11 splice site mutations, and 13 missense mutations. We identified 59 different types of mutation of which 54 were previously undescribed; 8 were observed more than once. Mutations were de novo in 40 of 57 probands.

CONCLUSIONS

Most of the observed mutations other than the missense mutations in JAGGED1 are expected to give rise to truncated and unanchored proteins. All mutations mapped to the extracellular domain of the protein, and there appeared to be regional hot spots, although no clustering was observed. Thus, the sequencing of 7 exons of JAGGED1 would detect 51% of the mutations. Transmission analysis showed a high frequency of sporadic cases (70%).

Bile acid transport

Bile acids undergo a unique enterohepatic circulation, which allows them to be efficiently reused with minimal loss. With the cloning of key bile acid transporter genes in the liver and intestine, clinicians now have a detailed understanding of how the different components in the enterohepatic circulation operate. These advances in basic knowledge of this process have directly led to a rapid and highly detailed understanding of rare genetic disorders of bile acid transport, which usually present as pediatric cholestatic disorders. Mutations in specific bile acid or lipid transporters have been identified within specific cholestatic disorders, which allows for genetic tests to be established for specific diseases and provides a unique opportunity to understand how these genes operate together. These same transporters may also prove useful for development of novel drug delivery systems, which can either enhance intestinal absorption of drugs or be used to target delivery to the liver or biliary system. Knowledge gained from these transporters will provide new therapeutic modalities to treat cholestatic disorders caused by common diseases.

Spectrum and frequency of jagged1 (JAG1) mutations in Alagille syndrome patients and their families

Alagille syndrome (AGS) is a dominantly inherited disorder characterized by liver disease in combination with heart, skeletal, ocular, facial, renal, and pancreatic abnormalities. We have recently demonstrated that Jagged1 (JAG1) is the AGS gene. JAG1 encodes a ligand in the Notch intercellular signaling pathway. AGS is the first developmental disorder to be associated with this pathway and the first human disorder caused by a Notch ligand. We have screened 54 AGS probands and family members to determine the frequency of mutations in JAG1. Three patients (6%) had deletions of the entire gene. Of the remaining 51 patients, 35 (69%) had mutations within JAG1, identified by SSCP analysis. Of the 35 identified intragenic mutations, all were unique, with the exceptions of a 5-bp deletion in exon 16, seen in two unrelated patients, and a C insertion at base 1618 in exon 9, also seen in two unrelated patients. The 35 intragenic mutations included 9 nonsense mutations (26%); 2 missense mutations (6%); 11 small deletions (31%), 8 small insertions (23%), and 1 complex rearrangement (3%), all leading to frameshifts; and 4 splice-site mutations (11%). The mutations are spread across the coding sequence of the gene within the evolutionarily conserved motifs of the JAG1 protein. There is no phenotypic difference between patients with deletions of the entire JAG1 gene and those with intragenic mutations, which suggests that one mechanism involved in AGS is haploinsufficiency. The two missense mutations occur at the same amino acid residue. The mechanism by which these missense mutations lead to the disease is not yet understood; however, they suggest that mechanisms other than haploinsufficiency may result in the AGS phenotype.

Linkage analysis and identification of deletion in Alagille syndrome gene

Alagille syndrome (AGS) is a genetic disease and the responsible gene has already been mapped at 20p12. To more accurately detect the region of the AGS gene on the linkage map of chromosome 20p, 14 yeast artificial chromosome (YAC) clones were screened to construct a YAC contig in the candidate region and 13 locus markers and 2 sequence-tagged sites (STS) were ordered. Combining all of the analyses, a 1.3 Mb critical region from D20S507 to D20S61 for the AGS gene was identified. As the human Jagged 1 gene (JAG1) lies just in this region and is responsible for the AGS disease, the genomic DNA in an AGS family without a visible deletion were analyzed by single-strand conformational polymorphism (SSCP) and direct DNA sequencing, and a 2-bp (CT) deletion mutation at exon 26 of the JAG1 was identified.

Identification and cloning of the human homolog (JAG1) of the rat Jagged1 gene from the Alagille syndrome critical region at 20p12

Notch proteins are a family of closely related transmembrane receptors proven to be instrumental in cell fate decisions. Recently, Notch ligands Delta and Jagged have been identified in Drosophila and rat, respectively. We have isolated the human homolog of the rat Jagged1 gene, JAG1, from a CpG island in a YAC clone covering the Alagille syndrome critical region at chromosome 20p12 (tel-SNAP-D20S186-cen). Alagille syndrome is an autosomal dominant disorder characterized by neonatal jaundice, paucity of intrahepatic bile ducts, and abnormalities of the heart, skeleton, and eyes. The human Jagged1 (JAG1), therefore, appears to be a strong candidate gene for this disease. Here we describe the identification, full-length cDNA cloning, expression patterns, and precise physical location of this gene within the Alagille syndrome critical region.

Alagille syndrome

Alagille syndrome (OMIM 118450) is an autosomal dominant disorder associated with abnormalities of the liver, heart, eye, skeleton, and a characteristic facial appearance. Also referred to as the Alagille-Watson syndrome, syndromic bile duct paucity, and arteriohepatic dysplasia, it is a significant cause of neonatal jaundice and cholestasis in older children. In the fully expressed syndrome, affected subjects have intrahepatic bile duct paucity and cholestasis, in conjunction with cardiac malformations (most frequently peripheral pulmonary stenosis), ophthalmological abnormalities (typically of the anterior chamber with posterior embryotoxon being the most common), skeletal anomalies (most commonly butterfly vertebrae), and characteristic facial appearance. Inheritance is autosomal dominant, but expressivity is highly variable. Sibs and parents of probands are often found to have mild expression of the presumptive disease gene, with abnormalities of only one or two systems. The frequency of new mutations appears relatively high, estimated at between 15 and 50%. The disease gene has been mapped to chromosome 20 band p12 based on multiple patients described with cytogenetic or molecular rearrangements of this region. However, the frequency of detectable deletions of 20p12 is low (less than 7%). Progress has been made in the molecular definition of an Alagille syndrome critical region within the short arm of chromosome 20. We will review the clinical, genetic, cytogenetic, and molecular findings in this syndrome.