Molecular pathology of Crigler-Najjar type I and II and Gilbert's syndromes

Crigler-Najjar syndromes type I and II and Gilbert's syndrome are familial unconjugated hyperbilirubinemias caused by genetic lesions involving a single complex locus encoding for bilirubin-UDP-glucuronosyltransferase which is involved in the detoxification of bilirubin by conjugation with glucuronic acid. Over the last few years a number of different mutations affecting this gene have been characterized. The aim of this work is to review the molecular pathology of Crigler-Najjar and Gilbert's syndromes, to discuss its impact on the clinical and genetic classification of these conditions, and on the diagnostic evaluation of clinical pictures associated with unconjugated hyperbilirubinemia.

The authors of the present review are involved in the clinical management of patients with familial unconjugated hyperbilirubinemia as well as in the characterization of its molecular bases. Evidence from journal articles covered by the Science Citation Index and Medline has been reviewed and collated with personal data and experience.

It has been known for many years that mild to severe deficiency of bilirubin UDP-glucuronosyltransferase in the liver is the cause of two types of familial unconjugated hyperbilirubinemia, Crigler-Najjar syndromes I and II, and Gilbert's syndrome. Since the characterization of the gene encoding for bilirubin UDP-glucuronosyltransferase, a number of mutations affecting the expression of this gene have been identified. These mutations can be classified into three groups: mutations which result in a reduced production of a normal enzyme; mutations which give rise to the synthesis of a structurally abnormal and dysfunctional enzyme; mutations which completely abolish the expression of the affected allele. The combination of mutations affecting the coding region of the gene and of promoter mutations which reduce the expression of the gene accounts for the wide clinical spectrum of familial unconjugated hyperbilirubinemias, ranging from the clinically negligible Gilbert's syndrome to the severe and often fatal Crigler-Najjar type I syndrome. A better understanding of the genetics of these conditions and the availability of molecular diagnosis will improve the diagnostic efficiency and afford better informed genetic counseling, not only for Crigler-Najjar and Gilbert's syndromes, but also for several acquired conditions characterized by unconjugated hyperbilirubinemia.