Prenatal Diagnosis of Alagille Syndrome

Abstract

Alagille syndrome is an autosomal dominant disorder characterized by cholestasis, paucity of interlobular bile ducts, and anomalies of the cardiovascular system, eyes, skeleton, and facies (1,2). The phenotype of Alagille syndrome can range from an apparently healthy individual to severe morbidity from liver failure and complex cardiac malformations (3). The disease is caused by defects in the human JAG1 gene (4,5), which encodes a ligand for the Notch signaling pathway which has been shown to be important for cell differentiation (6,7). We report a DNA-based prenatal diagnosis for Alagille syndrome. CASE REPORT Alagille syndrome was diagnosed in a Turkish male infant with clinical and biochemical cholestasis and the following features: liver biopsy with severe paucity of interlobular bile ducts and fibrosis, posterior embryotoxon, tetralogy of Fallot, and characteristic face with prominent forehead and hyperteloric deep set eyes. The clinical course was characterized by a progressive deterioration of liver function and the patient died of cardiac failure at 8 months. The parents were consanguineous. The child's mother had had a spontaneous abortion 2 years before presentation in our department. Evaluation of the parents indicated that the disease was inherited by the mildly affected mother, who had minor facial anomalies, posterior embryotoxon, elevated serum gamma-glutamyltransferase, alkaline phosphatase, and total bile acids. DNA analysis of the deceased patient and his mother revealed a heterozygous T to G transversion in exon 6 leading to a nonsense mutation by exchange of a tyrosine at position 255. The abnormality resulted in a truncated protein (TAT>TAG, Y255X). The chain termination in the epidermal growth factor (EGF)-like repeats domain abolishes the membrane anchoring of JAG1. Additionally, both parents exhibited a heterozygous silent polymorphism at the same codon (TAT>TAC, Y255Y) (Fig. 1).FIG. 1.: Pedigree of the described family with the JAG1 sequence of codon 254 to 256 in exon 6. Filled symbols denote affected family members. The termination mutation is marked bold.The mother requested genetic counseling and prenatal diagnosis during three subsequent pregnancies. The described nonsense mutation was not detected in the chorionic villous sampling obtained at 13 weeks of gestation during the first subsequent pregnancy. At 39 weeks of gestation, a healthy male child was born. Despite a contraceptive coil the mother became pregnant again. Since the fetus aborted spontaneously during the 10th gestational week, prenatal diagnostics were not performed. In the third pregnancy, DNA analysis of chorionic villous sampling obtained in the 12th week of gestation showed the nonsense mutation described above. The mother decided to terminate the pregnancy by induced abortion. DISCUSSION Clinical manifestations of Alagille syndrome are highly varied, making clinical diagnosis of children with an incomplete syndrome difficult. The differentiation of Alagille syndrome from other forms of cholestatic diseases, such as extrahepatic biliary atresia, is thought to be important in early infancy to avoid unnecessary surgery. In 132 patients with Alagille syndrome presenting with neonatal cholestasis nine underwent a Kasai procedure (8). To diagnose Alagille syndrome is usually easy in the presence of the fully expressed syndrome showing all five major features: chronic cholestasis, characteristic facies, cardiac malformations, vertebral abnormalities, and posterior embryotoxon. Alagille syndrome can be ascertained if at least three of the five main features - including cholestasis - are present (3). Diagnosis is more difficult in children with cholestasis and minor symptoms: in these cases, molecular analysis of JAG1 might be feasible. However, in a recent study of 102 cases of extrahepatic biliary atresia approximately 10% of patients were found to have a JAG1 mutation (9). None of these patients displayed features of Alagille syndrome. Moreover, in an extended pedigree of a family with tetralogy of Fallot, a JAG1 missense mutation, G274D, segregated with the disease (10). It is important to note that the mutation spectrum seems to differ between the above mentioned entities. In Alagille syndrome, nonsense or frameshift mutations leading to a truncated protein predominate (11,12). The identification of the underlying disease-causing gene carries with it the hope that DNA analysis may facilitate diagnosis. However, the finding of JAG1 variations in different diseases such as Alagille syndrome, biliary atresia and tetralogy of Fallot points out that a correct diagnosis cannot be based on a DNA analysis alone. The autosomal dominant inheritance of Alagille syndrome includes a high recurrence risk of 50% for subsequent pregnancies if one parent is affected. Since parental mosaicism has been reported to occur in the range of 8% of cases (13), genetic testing is indicated in familial cases as well as in sporadic cases. Identifying the underlying disease-producing mutation will enable rapid prenatal diagnosis in a subsequent pregnancy. To our knowledge, this report represents the first DNA-based prenatal diagnosis of Alagille syndrome by chorionic villous sampling. However, we are concerned that the ability to diagnose JAG1 mutations prenatally carries with it some critical ethical issues. The phenotype of Alagille syndrome is highly variable even within the same family and only minor symptoms may develop with a normal quality of life and life expectancy in patients with the same mutation as a more seriously affected sibling. This unpredictablilty of clinical presentation is illustrated by a report of monozygotic twins with an identical de novo splice site mutation. Both twins displayed a severe form of Alagille syndrome, however, one twin had severe pulmonary atresia with mild liver involvement while the other had tetralogy of Fallot with severe hepatic disease requiring liver transplantation (14). Another approach to determine the severity of the disease prenatally using repeated fetal echocardiography has been proposed (15). This procedure may give information about the degree of cardiac anomalies, but not about the course of the liver involvement. Environmental triggers or modifying genes are presumed to contribute to the phenotype. For a long time, Alagille syndrome was considered to be a “benign syndrome of intrahepatic cholestasis” having a good long term prognosis (16). Liver complications were suspected to be responsible for death in about 5% of the patients (3). In a recent study of 163 patients with Alagille syndrome, however, ten year survival with a native liver was 45% and overall survival 65% in patients presenting with neonatal cholestasis (8). Forty four of 132 patients with neonatal cholestasis (33%) underwent liver transplantation at a median age of six years and nine months (8). Unfortunately, no valid data are available concerning the recurrence of a severe phenotype in familial Alagille syndrome. Since the phenotype cannot be predicted from the genotype, it is impossible to give families prenatal information about the clinical manifestations and outcome of an affected fetus. The ultimate decision to use prenatal molecular diagnostics has to be made by the parents. As physicians it is our responsibility to provide appropriate medical information and counseling to support the parents in their difficult decision.