[Molecular and clinical characteristics of a family with Alagille syndrome]. / Características clínicas y moleculares de una familia con síndrome de Alagille.

BACKGROUND AND OBJECTIVE: The Alagille syndrome (AS) is characterized by biliary ductopenia and abnormalities of heart, eyes, face, bones, kidneys and brain with a dominant inheritability. Mutations of Jagged 1 gene are observed in individuals with the full syndrome and/or relatives with little or no phenotypic features. Prognosis of patients depends on the hepatic and cardiovascular involvement. PATIENTS AND METHOD: We present the cases of a woman and her 2 male nephews with AS. We performed a molecular study of the Jagged 1 gene in family members with and without the syndrome. RESULTS: The molecular study detected mutations in the position 2785+2 of TAAG (intron 19) of the Jagged 1 gene in 3 relatives with the full syndrome and in 2 other members with a partial syndrome. Other relatives, without mutation, have some of the phenotypic features of it. CONCLUSIONS: We comment on the clinical forms of AS in this family and the detected mutation. Molecular diagnosis allows to make a genetic counsel.

Prenatal molecular diagnosis of inherited cholestatic diseases.

OBJECTIVES: Progressive familial intrahepatic cholestasis (PFIC) and to a lesser extent, Alagille syndrome, often lead to end-stage liver disease during childhood. We report our experience of DNA-based prenatal diagnosis of PFIC1-3 and Alagille syndrome. PATIENTS AND METHODS: Four molecular antenatal diagnoses were performed in 3 PFIC families and 17 in 11 Alagille syndrome families. DNA was isolated from chorionic villus or cultured amniocyte samples from women, without pregnancy complications. RESULTS: All four foetuses with a family history of PFIC1, 2, or 3 were heterozygous for an ATP8B1, ABCB11, or ABCB4 mutation and pregnancies were continued. Three of the infants were healthy after birth, and 1 premature infant, who had an ABCB4 mutation, experienced transient neonatal cholestasis. Among the families with a history of de novo JAG1 mutation, none of the foetuses was mutated, versus 40% of those with a history of familial mutation. Of 4 pregnant women with a JAG1-mutated foetus, 3 cut short their pregnancy and 1 gave birth to a child with overt Alagille syndrome. CONCLUSIONS: Molecular antenatal diagnosis of PFIC1-3 and Alagille syndrome is reliable because clinical outcome after birth corresponded to molecular foetal data.

Biological function of mutant forms of JAGGED1 proteins in Alagille syndrome: inhibitory effect on Notch signaling.

Heterozygous mutations in JAGGED1, encoding a single-pass transmembrane ligand for the Notch receptors, cause Alagille syndrome (AGS), a polymalformative disorder affecting the liver, heart, eyes and skeleton and characterized by a peculiar facies. Most of the JAGGED1 mutations generate premature termination codons, and as a result, two pathogenic mechanisms causing AGS have been proposed: haploinsufficiency or a dominant-negative effect of putative truncated proteins. To determine whether missense or protein-truncating mutations in JAGGED1 can lead to the synthesis and function of abnormal proteins, we performed cell culture experiments. We showed that human JAGGED1 undergoes a metalloprotease-dependent cleavage resulting in the shedding of its extracellular domain and that this domain seems able to fulfill a biological function in vitro, probably by antagonizing Notch signaling. Moreover, the soluble form of JAGGED1 was able to compete with the transmembrane ligand. Mutant proteins with missense or nonsense mutations were synthesized and gave rise to a chord-like phenotype and a migration defect when expressed by stably transfected cells. These chord-like structures were similar to the phenotype exhibited by fibroblasts isolated from a fetus with a protein-truncating mutation. Results obtained from Notch signaling inhibition and Notch reporter assays showed that this chord-like phenotype, exhibited by mutant JAGGED1 transfectants, may result from an inhibitory effect on Notch signaling. Altogether, our results favor a dominant-negative mechanism of some JAGGED1 mutations in AGS.

Expression of mutant JAGGED1 alleles in patients with Alagille syndrome.

Heterozygous mutations in JAGGED1 (JAG1), encoding a ligand for Notch receptors, have been identified in patients with Alagille syndrome (AGS). These mutations map to the extracellular and transmembrane domains of JAG1, giving rise in 70% cases to a premature termination codon (PTC). Although haploinsufficiency has been hypothesised as the main mechanism of AGS, a dominant negative effect of truncated forms of Serrate/Jagged has been suggested. Only few studies of the mutant mRNAs and proteins from AGS patients have been performed to elucidate the molecular mechanisms of the disease. To gain insight into the stability of mutant mRNAs, we studied transcripts from five livers and 24 lymphoblastoid cell lines (LCLs) of AGS patients. Mutant JAG1 transcripts were recovered (albeit in different relative amounts) from RNAs with missense mutations (five) or in-frame deletions (two), and from all but two of the 21 with PTCs. In addition, results from LCL RNAs correlated well with results from liver RNAs. Mutant transcripts were also recovered from tissues of a 23-week-old AGS foetus with a PTC mutation. This suggests that most mutant transcripts with PTCs escape nonsense-mediated mRNA decay (NMD) and could lead to the synthesis of soluble forms of JAG1. Production of a truncated protein was indeed observed after transfection of COS cells with a mutant JAG1 cDNA. In conclusion, mutant JAG1 transcripts are present in LCLs, livers and tissues of AGS patients, whatever the mutation type, and mutant proteins can be produced, suggesting a dominant negative effect of some mutant proteins as another molecular mechanism of AGS.

Bleeding tendency in children with Alagille syndrome.

OBJECTIVE: Spontaneous intracranial bleeding is now a widely recognized complication and cause of mortality in patients with Alagille syndrome. The pathogenesis of intracranial bleeding in these patients remains unclear. The aim of the study was to look for other sites of bleeding in these patients that could suggest a factor of multiorgan morbidity. METHODS: The records of 174 patients with Alagille syndrome were reviewed, and 38 (22%) patients without liver failure who experienced hemorrhage that led to a drop in hemoglobin level of at least 3 g/dL or to blood transfusion were identified. RESULTS: In 38 patients, 49 bleeding episodes occurred at a median age of 3.75 years (range: 1 month-27 years). Seventeen patients had 23 episodes of spontaneous bleeding; 21 patients bled during surgery or other medical procedures, and 5 among these 21 patients also had a spontaneous bleeding episode. Nine patients bled at least twice. Median platelets count and prothrombin time were normal. Severe cholestasis existed in 33 patients. One patient has a deletion of the 20p12 region, and 13 of 17 patients studied have a JAGGED1 mutation. Blood transfusion was necessary in 23 patients. Eight patients died secondary to bleeding (4 after surgery, 2 after gastrointestinal bleeding, 1 after needle liver biopsy, and 1 after intracranial bleeding). CONCLUSION: These results suggest that patients with Alagille syndrome are at special risk for bleeding; this should be taken into account before deciding on an invasive procedure. The mechanism of the bleeding is still unclear; the role of hypercholesterolemia cannot be excluded, but it may be speculated that JAGGED1 signaling abnormalities may impair the hemostatic function.