Molecular Genetics of Wilson Disease

Abstract

Wilson disease is an autosomal‐recessive genetic disorder of hepatocellular copper disposition. It is rare but has a worldwide distribution. It is clinically diverse. Disease patterns include various kinds of liver disease; neurological movement disorders and psychiatric disease including psychosis, and less commonly recurrent haemolytic anaemia, osseomuscular abnormalities and cardiac dysrhythmias. The classic eye finding, the Kayser–Fleischer ring, has no functional impact. Thus far, only one gene has been identified whose mutations are causal for Wilson disease, ATP7B on chromosome 13q14.3, cloned in 1993. More than 500 mutations have been identified. The encoded protein is a metal‐transporting P‐type adenosine triphosphatase (ATPase), the Wilson ATPase, similar to copper transporters across the phyla. Simple genotypic explanation for the phenotypic diversity does not exist. The important genotype–phenotype correlation is that if the Wilson ATPase is absent or nonfunctional, severe disease (usually hepatic) commonly develops in the first decade of life. Clinically evident Wilson disease is fatal if not treated. The broad age range of diagnoses (3–80+ years) raises the question of incomplete penetrance, an issue along with gene modifiers currently under investigation. The possibility that mutations in other genes may result in Wilson disease has not been entirely eliminated. The contribution of ATP7B to non‐Wilsonian diseases is being determined. Key Concepts: Wilson disease is a rare (30 per million population, on average) autosomal‐recessive disorder of hepatic copper disposition. Wilson disease can present as almost any form of hepatic disease, or as neurological movement disorders, or as psychiatric disease. Genetic diagnosis is definitive; clinical application of genetic findings requires skilled interpretation; success rate for identifying one or both mutations can be highly variable. Most affected individuals are compound heterozygotes. Genetic testing is most efficient for investigating first‐degree relatives, who must be assessed for Wilson disease once a single family member has been diagnosed with Wilson disease. Wilson disease is an example of ‘endogenous hepatotoxicity’ where a normal component of the functioning organism is mishandled and becomes toxic. The mechanism of liver (and likely brain) damage involves oxidative stress with damage to mitochondria; apoptosis is typical. Concepts relating to drug‐induced hepatotoxicity are highly relevant to understanding the mechanism of damage and devising treatments. Metalloproteomics is a research strategy for examining copper disposition in normal and abnormal models/organisms. More than 500 mutations in ATP7B have been identified. These are mainly missense mutations, and less frequently small deletions or insertions, nonsense or splice‐site mutations. In contrast, the homologous gene ATP7A , abnormal in the X‐linked recessive disorder Menkes disease (with copper insufficiency), displays mainly large deletions. Predictably, the phenotypic variation depends on complex factors besides the genotype. The only important genotype–phenotype correlation is that severe disease (usually hepatic) typically develops very early in life if the Wilson ATPase is absent or nonfunctional. A convincing example of this correlation is provided by comparison of the Atp7b ‐knockout mouse and the toxic milk (tx‐j) mouse, which has a point mutation. Few modifying genes have been identified. Candidates include apolipoprotein E, human prion gene and BIRC4/XIAP. Females seem to have more severe Wilson disease. Other disorders of copper disposition exist. These include defects in ceruloplasmin production (aceruloplasminaemia and AT‐1 defect), various congenital glycosylation disorders, Indian childhood cirrhosis and its variants. In the Bedlington terrier, hepatic copper toxicosis is usually related to mutations in the gene for COMMD1. ATP7B may play a secondary role in some forms of Alzheimer disease. The Wilson ATPase plays a direct role in the hepatocellular handling of platinum compounds such as cisplatin.