| Abstract|| |
Wilson's disease, first described in 1912, is a familial disease involving copper metabolism. This article reviews recent developments in the genetics of the disease and their implications for the diagnosis and treatment.
|How to cite this article:|
Roberts EA. Wilson's disease: A new direction
. Saudi J Gastroenterol 1995;1:163-8
| History|| |
In many ways, Wilson's disease is a paradigm for charting the progress of contemporary medicine. Wilson's disease was first described in 1912 by the American neurologist, Kinnear Wilson, who was working in England at the time. This was the era of Garrod, and genetic diseases inborn errors of metabolism were only just being identified. Kinnear Wilson described Wilson's disease as a familial disease characterized by progressive, lethal neurological disease along with chronic liver disease and corneal changes, known as the Kayser-Fleischer ring More Details, - a phenomenon previously-described in 1902-3. He also observed that some younger sibs of patients with this constellation of findings, died of severe liver disease without developing neurological abnormalities.
Although numerous observations pointed to copper overload, the etiological role of copper was not firmly established until 1948. The importance of low concentrations of plasma ceruloplasmin, the major carrier protein for copper, was recognized only in 1952. However, by the mid 1950's, the main diagnostic criteria for Wilson's disease were clear, and in 1956 chelation therapy with penicillamine, a metabolite of penicillin, was first used. This treatment changed the outlook for the disease, radically. Patients could be restored to good health in most cases. However, our knowledge of the pathobiology of Wilson's disease was still imperfect. An autosomal recessive pattern of inheritance was determined only in 1960. A defect in biliary excretion of copper appeared the most likely mechanism of the disease, but this remained unproved. In 1985 the gene was localized to chromosome 13. In 1993 the gene was identified: the gene, technically named ATP7B (but conventionally called WND, codes for an enzyme which is a "P-type ATPase"), has six copper-binding sites and acts as a copper transporter. The LongEvans cinnamon rat, which has a liver disease somewhat similar to Wilson's disease, has an analogous genetic abnormality. Studies of the pathophysiology of Wilson's disease are now a realistic possibility. Moreover, given these advances in our knowledge of Wilson's disease, we can develop genetic methods for diagnosis and possibly, genetic treatment.
| Clinical features|| |
Wilson's disease can be defined as an autosomal recessive disorder of the hepatic metabolism of copper ,,,,. Copper is not incorporated into its carrier protein ceruloplasmin within the liver, and it is not excreted adequately into bile. Consequently, copper accumulates in the liver. Over time, copper is released into the blood and is deposited in various other organs, notably the brain, kidneys and the cornea of the eyes. Wilson's disease occurs world-wide. Its incidence appears to be on the order of 15-30 affected individuals per million population.
The clinical presentation of Wilson's disease is extremely variable. It may present as chronic liver disease in childhood; as a progressive neurological disorder without clinically-prominent hepatic dysfunction; or as a psychiatric disease. Some patients have a liver disease which appears virtually identical to that of autoimmune hepatitis. Occasionally Wilson's disease presents with intravascular hemolysis and acute liver failure. This clinical variability makes confirming the diagnosis of Wilson's disease less facile than it is sometimes portrayed. It invites the question of whether there is a genetic basis for such variability.
Conventional wisdom holds that Wilson's disease is not symptomatic before 5 years of age . Some asymptomatic affected sibs are to the identified before this age by biochemical or genetic testing. However, patients have presented with severe liver disease at age 4 years, or exceptionally, at age 3 years ,. This latter patient was found to have a severe mutation. It is possible that some poorly-defined liver disease in toddlers is actually a non- classic presentation of Wilson's disease. Likewise, although most patients are diagnosed in their teens or 20's, recent reports indicate that much older patients, in their 50's may be found to have Wilson's disease. Although such patients often have more prominent neurological or psychiatric disease than liver disease, not all patients reflect a missed diagnosis.
The younger the patient, the more likely the presentation of Wilson's disease will be as liver disease. Wilson's disease should be considered as a possible diagnosis in any child with hepatomegaly. Most patients have some evidence of abnormal liver function. Symptoms may be vague and non-specific, such as fatigue and anorexia. Mood disturbance (mainly, a depressed effect), changes in school performance and/or handwriting, and clumsiness should be sought by careful direct questioning. There may be a history of unexplained hemolysis. Some patients have impressive, isolated splenomegaly, but no hepatomegaly. Physical examination rarely discloses Kayser-Fleisher rings; a slit-lamp examination is usually required. A small percentage of patients do not have Kayser-Fleischer rings, and these are less common in very young patients. Although Kayser-Fleischer rings may develop in chronic cholestatic liver disease and also with certain systemic disease, finding Kayser-Fleischer rings in a child without chronic cholestasis or previously-known liver disease, is virtually diagnostic of Wilson's disease.
Laboratory investigations frequently suffice for diagnosis of Wilson's disease. However, multiple studies are needed, and in some patients, the diagnosis still remains elusive. Routine liver function tests are usually abnormal with mild to moderate elevations of aminotransferase. Serum ceruloplasmin concentration is typically below normal range and is usually very low; normal concentration is found in approximately 5% of patients . Serum copper concentration is also low, but the free (non-ceruplasmin- bound)copper concentration is elevated. Serum uric acid concentration may be low: this reflects the renal tubular dysfunction of untreated Wilson's disease. Other renal manifestations include microscopic hematuria, proteinuria, aminoaciduria, phosphaturia, and defective acidification of the urine.
Studies of urinary excretion of copper prove useful for the diagnosis of Wilson's disease. Basal urinary copper excretion is elevated. This should be assessed with several carefully-obtained 24hour urine collections. After . these baseline studies, a provocative test of urinary copper excretion may be performed. Penicillamine (500 mg by mouth every 12 hours) is given while 24hour urinary collections are obtained. It is prudent to obtain three such collections. Although a normal individual may put out 20 times baseline excretion after penicillamine administration, an individual with Wilson's disease will put out considerably more. Mowat and colleagues  have shown that urinary excretion of 25 µmoles of copper per 24 hours (or more) is diagnostic of Wilson's disease and they argue that this test is more reliable than the measurement of hepatic tissue content of copper.
Hepatic tissue copper concentration is an important diagnostic test for Wilson's disease. This can be measured by neutron activation analysis. Hepatic copper content greater than 250 µg per gram of dry weight is taken as diagnostic of Wilson's disease. However, there are problems associated with this test. Copper distribution in the liver parenchyma may be inhomogeneous, as commonly found by histochemical staining of liver biopsies. Some patients with Wilson's disease have a hepatic tissue copper concentration intermediate between normal and definitely elevated (between 100-250 µg per gram of dry weight). Some heteroxygotes have similar moderate elevations of liver tissue copper. The finding is not specific; patients with chronic cholestasis or diseases such as Indian childhood cirrhosis may have elevated hepatic tissue copper. Finally, liver biopsy may not be safe in some patients with Wilson's disease because of coagulopathy or ascites, and therefore this diagnosis parameter is not available.
Apart from diagnostic information, the liver biopsy in Wilson's disease may also be useful for determining the degree of damage. In the presymptomatic patient, parenchymal architecture is usually normal and abnormalities are generally mild and even non-specific: large-droplet fat and glycogenated nuclei. The stain for copper may be positive. However, electron microscopic findings may be more informative; abnormalities in the mitochondria, namely, dilatation of the tips of the cristae, abnormal mitochondrial morphology, and condensation of dense bodies, may be identified even at an early stage of the disease . In more advanced cases, cirrhosis may be present with nodules showing variable amounts of stainable copper.
Two specific clinical presentations of Wilson's disease deserve special comment. Wilson's disease may present in children and young adults with clinical features indistinguishable from autoimmune hepatitis: elevated aminotransferase, greatly increased serum IgG concentration, and positive non-specific autoantibodies such as anti-nuclear antibody and anti-smooth muscle (anti-actin) antibody ,. Wilson's disease must always be excluded in such patients. The minimum assessment is measurement of serum copper and ceruloplasmin, and slit-lamp examination for Kayser-Fleischer rings. Wilson's disease may also present as a fulminant hepatic failure, with severe coagulopathy and encephalopathy. Acute intravascular hemolysis is usually present. Since the patient typically has not been suspected to have underlying liver disease, acute viral hepatitis is the working diagnosis. Unlike acute viral hepatitis-producing fulminant liver failure, the fulminant hepatic failure presentation of Wilson's disease is characterized by disproportionately-low aminotransferase (usually < 500 U/L) from the onset of clinically apparent disease. The serum alkaline phosphatase is low or in normal range . Slit-lamp examination may be impractical in such patients; lunulae ceruleae, bluish discoloration at the base of the fingernails indicating copper deposition, may be discerned. Urinary copper excretion is elevated significantly in such patients.
| The gene for Wilson's disease|| |
The search for the gene for Wilson's disease began in the mid-1980's. When linkage with the esterase D gene was established; the gene for Wilson's disease was localized to chromosome 13, also the site of the retinoblastoma protein gene. Further extensive linkage studies narrowed the location to 13g14. In the meantime, the gene for Menkes disease, another human disorder of copper metabolism in which copper is not transported out of intestinal epithelial cells, leading to generalized copper deficiency, was cloned. The encoded protein was identified as a P-type ATPase for transporting copper with six copperbinding domains. The gene for Menkes disease is technically named ATP7A but conventionally called MNK. The gene for Wilson's disease, WND, was finally cloned by looking for homology with copper-binding domains of MNK . Nearly simultaneously, it was also identified from a brain cDNA library in studies relating to amyloid-like genes ,. There is a 57% homology overall between the Wilson's disease gene and the gene for Menkes disease, and greater homology in certain critical functional regions.
The protein encoded by WND has features which are typical of P-type ATPases, which are bacterial cation-transporting enzymes. Basically, the protein appears to consist of six copper-binding domains, identified by the presence of heavy metal-binding sequences, near the amino-terminal. Toward the middle of the protein, a region can be identified which supports cation transport. In the same general area are transmembrane regions which may form a type of pore or channel. Finally, toward the carboxy-terminal of the protein, is a region which can be identified as an ATPbinding domain. Thus, this protein has the necessary apparatus to take energy from breaking down ATP and use it to move copper within or out of a cell. Details of the transmembrane region of the protein are still not available, although it is evident that at least the first two of these regions exist in the liver .
The initial reports of the cloning and identification of WND also described mutations in the gene in patients with Wilson's disease. At the present time 25 mutations have been identified . Most of the mutations are rather subtle, not large deletions as found in Menkes disease. There are 13 small insertions and deletions, 7 missense, 2 nonsense, and 3 splice-site mutations, described thus far. Most patients are compound heterozygotes; they have one each of two different mutations. This may account for some of the broad clinical diversity of the disease. Nevertheless, patients with mutations which make it impossible to produce functional gene product, appear to have earlier onset of disease. The average age of clinicallyidentifiable disease in such patients is approximately 7 years old, but as mentioned previously, one patient presented with severe hemolysis and decompensated cirrhosis at the age of 3 years. Whether patients presenting later in life with mainly neurological disease have characteristic mutations, remains to be determined.
| Animal model for Wilson's disease|| |
It is now evident that there is a suitable model for Wilson's disease. The Long-Evans cinnamon (LEC) rat is a highly-inbred strain of rat which develops hepatic disease due to copper accumulation in the liver. A prominent characteristic of the liver disease is chronic inflammation, and an important later complication is the development of hepatocellular carcinoma, which is rarely encountered in patients with Wilson's disease. Moreover, LEC rats do not develop neurological abnormalities. In normal rats, a gene with 82% identity to WND has been identified (Atp7b); in LEC rats there is a large deletion of the 3'-end of this gene ,. Thus, the LEC rat is an appropriate animal model for Wilson's disease. It is not clear whether the clinical differences between humans and rats are related to the specific LEC mutation or to host-specific differences in hepatic physiology.
| Genetic diagnosis of Wilson's disease|| |
A genetic disease should be diagnosed by genetic means. Although Wilson's disease can usually be diagnosed by a battery of biochemical tests with or without liver biopsy, the diagnosis is not invariably straightforward. Mutation analysis is a future prospect for diagnosis. At the present time, DNA linkage studies have been shown to aid in identifying affected sibs of index cases or for prenatal diagnosis ,. Haplotype analysis offers an alternative approach. With three DNA markers, haplotype patterns can be defined, which distinguish affected individuals from normals, and in many instances, have a strong correspondence to specific mutations. Haplotype studies may also enhance identification of currently unrecognized mutations in Wilson's disease.
| Treatment of Wilson's disease|| |
There are three generally-recognized treatments for Wilson's disease. With effective medical treatment most patients live normal, healthy lives. Early treatment is critical, and the best outlook is reserved for patients who are diagnosed and begun on treatment in the presymptomatic phase of the disease. Thus, screening of sibs is vitally important. Liver transplantation is reserved for patients who present with severe, decompensated liver disease, unresponsive to medical therapy or for those who present with fulminant liver failure . The role of liver transplantation for improving severe neurological disease in the absence of decompensated liver disease remains disputed.
Penicillamine was introduced in 1956 by Walshe. Penicillamine, a metabolite of penicillin, is the sulfhydryl-bearing amino acid cysteine, doubly-substituted with methyl groups. Walshe found that penicillamine was excreted in urine with the sulfhydryl group intact and reasoned that it could function as a chelator because it had this free sulfhydrl group. Penicillamine has revolutionized the outlook for Wilson's disease and remains the firstline therapy. It promotes urinary excretion of copper and may induce production of metallothioneins. It has other actions as well: it interferes with collagen cross-linking and acts as an immunosuppressant. Penicillamine is effective in most patients with Wilson's disease. It is not effective in those who present with acute liver failure; these patients require urgent liver transplantation. The neurological status of patients who present with mainly neurological symptoms may worsen initially after penicillamine treatment is started. Some patients develop a febrile reaction with rash and proteinuria within 7-10 days of beginning treatment .
Penicillamine can have serious side-effects. Adverse reactions involving the skin are prominent: various types of rashes, pemphigus, and elastosis perforans serpiginosa. Other side-effects include: thrombocytopenia, proteinuria, gastrointestinal upset, arthralgias, and loss of the sense of taste. Severe global bone marrow depression may rarely occur. Nephrotic syndrome, systemic disease resembling lupus erythematosus and Goodpasture's syndrome have all been reported, as well as a myasthenia syndrome. Clearly, these severe side-effects require stopping penicillamine treatment. A different chelation strategy must then be used. Approximately 30% of patients with Wilson's disease develop some side-effect of penicillamine, requiring a change of treatment.
The usual alternative for people with severe toxicity from pecillamine is triethylene tetramine dihydrochloride (2, 2, 2- tetramine), known by its official short name "trien" or conventionally as "trientine" ,. Trien is one of a chemical family of chelators and differs chemically from penicillamine by not having sulfhydryl groups. Copper is chelated by forming a stable complex with the four constituent nitrogens in a planar ring. Trien increases urinary copper excretion and may interfere with intestinal absorption of copper. It appears slightly less potent as a chelator than penicillamine. It has little clinically important toxicity in Wilson's disease apart from inducing iron deficiency; apparently by chelating dietary iron. More importantly, adverse effects of penicillamine resolve and do not recur during treatment with trien .
Zinc is the latest addition to treatments for Wilson's disease . Its mechanism of action is entirely different from that of chelators. Zinc in pharmacological doses, interferes with absorption of copper from the gastrointestinal tract. The postulated mechanism is that excess zinc induces metallothionein in enterocytes. As this metalothionein has greater affinity for copper than for zinc, it preferentially binds copper present in the intestinal contents. Once bound, the copper is not absorbed, but is lost in the feces as enterocytes are shed in normal turnover. Since copper enters the gastrointestinal tract from the diet and from internal secretions, it is likely that zinc treatment mobilizes endogenous copper. Problems with zinc therapy include gastritis as a common acute side effect, and uncertainty about dosing. The long-term effectiveness and adverse side-effects of zinc require further investigation, and thus zinc therapy is not yet widely accepted as a conventional treatment for Wilson's disease .
| Summary|| |
We are now in a new era with Wilson's disease. With the identification and cloning of the gene responsible for Wilson's disease, we can begin to understand the mechanism of the disease and the reasons for its clinical diversity. Conclusive diagnosis is now possible. Consequently, recategorization of some patients previously-thought to be either "homozygote-affected" or "heterozygote" may prove necessary. Moreover, the spectrum of clinical disease in Wilson's disease may require some redefinition, especially with respect to severe disease in early childhood. The possibility of gene transfer therapy now exists; although current pharmacological treatment is generally satisfactory.
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Eve A Roberts
Associate Professor of Pediatrics, Medicine and Pharmacology Room 8415, University Wing, Division of Gastroenterology & Nutrition, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8
Source of Support: None, Conflict of Interest: None