Wilson Disease Robert Baumgartner, Kira Melamud and Amelia Wnorowski

Transcription

1 Wilson Disease Robert Baumgartner, Kira Melamud and Amelia Wnorowski Basic Genetics Wilson Disease is an autosomal recessive genetic disorder of impaired copper metabolism. The Wilson Disease gene is composed of 21 exons 1 and is located on chromosome 13q It is responsible for creating ATP7B as its protein product. This gene product is present in the trans-golgi network of hepatocytes and in the central nervous system, although much less is known about its role there 3. Therefore, patients with Wilson Disease present with both neurological symptoms and impaired liver function. Over 200 defects have been identified within the gene, which manifest themselves as nonsense, frameshift, and splice-site mutations. Missense mutations, which are responsible for half of the occurring disease-causing mutations, have been localized mostly to the regions of the gene encoding the ATP-binding domain and the membrane-spanning segments 4. The two most commonly found mutations of the ATP7B gene are H1069Q, which is common in Northern, Central, and Eastern Europeans with an allele frequency of 43.5%, and R778L in Asian populations 5. Overall, the disease frequency is estimated to be around 1 in 30,000 and the carrier frequency at 1 in 90 worldwide 1. Molecular Biology and Biochemistry Normal Copper Metabolism Copper facilitates electron transfer reactions and is essential for the function of cuproproteins, which are required for a diverse array of processes in all living organisms. These processes include: mitochondrial oxidative phosphorylation, free radical detoxification, neurotransmitter synthesis and maturation, and iron metabolism 6. However, the intracellular concentration of copper must be carefully balanced, as excessive copper will lead to increased free radical formation and a variety of subsequent detrimental effects 7. Consequently, it is imperative that the cell maintains proper control over the importation of copper into the cell as well as intracellular trafficking, compartmentalization, and cuproprotein synthesis. The elimination of copper, which is only possible through biliary excretion, has drawn particular interest from researchers as defects in the pathway have been implicated in copper related diseases in humans such as Wilson and Menkes Diseases. The most important membrane bound copper transport protein identified to date is copper transporter 1 (Ctr1). Human Ctr1 (hctr1) has been found to transport copper metal-specifically with high affinity and in a time-dependent and saturable manner 6. While hctr1 has been found to be expressed in multiple cell types, it is most likely only one member of an entire family of clinically important copper transporters. Once copper has entered the cell, it is important that it be efficiently bound so that the copper ion (CuI) is not oxidized and so that vulnerable intracellular proteins are protected from copper toxicity. Therefore, copper is bound by cytosolic metallothioneins, low molecular weight, cystein-rich proteins that are able to tightly bind a variety of metals including copper, zinc, cadmium, and mercury. Both isoforms of metallothioneins, MT I and MT II are ubiquitously expressed 8. A whole family of cytosolic copper chaperone proteins is responsible for transporting the intracellular copper ions from metallothioneins to the various cuproproteins. These chaperone proteins are able to move copper ions from one intracellular location to another and cross

2 membrane boundaries, while also specifying the target proteins through particular structural features 9. Copper chaperone proteins also appear to lower the activation barrier for metal transfer into specific protein-binding sites through a mechanism that has yet to be elucidated 10. Diseased Metbolism Elimination of Copper from Cells Wilson Disease is a disorder that results from a mutation in the ATP7B gene, which encodes a copper-transporting P-type ATPase, essential for trafficking of copper in and out of hepatocytes. The P-type ATPase encoded by ATP7B is essential in transport of copper into Golgi apparatus, in combining copper and ceruloplasmin, and in excretion of copper from hepatocytes to the biliary tract 11. Mutations in ATP7B can disrupt intracellular transport of copper at various points during the trafficking pathways. For instance, certain mutations lead to reduction in ceruloplasmin, a six-copper binding protein which is normally released into blood to act as a source of copper for peripheral organs like the brain and kidney. Ceruloplasmin under normal circumstances is formed by the ATP7B mediated transfer of copper to apoceruloplasmin. ATP7B also regulates the excretion of copper into biliary canaliculi. This is crucial, since copper can only be excreted from liver through bile. Failure to do so results in toxic build-up of copper within hepatocytes. Accumulation of copper damages mitochondria and leads to oxidative damage of cells 5. Consequently copper is released from the liver into blood, and overloads organs like kidneys, brain, and red blood cells, resulting in their damage. In the brain, the main regions affected are lenticular nuclei. Progression of the disease is manifested into necrosis and gliosis with lesions found in the brainstem, thalamus, cerebellum, and cerebral cortex 1. Proliferation of Alzheimer cells takes place during early stages of the disease. The kidneys also become affected in Wilson Disease, and the vacuolar degeneration in proximal tubular cells presents itself in patients as Fanconi Syndrome and as the appearance of the golden-brown Kayser-Fleischer ring in the cornea 1. Excess of copper in red blood cells induces hemolysis. Symptomology Though no two patients with Wilson Disease have identical symptoms, most patients can be classified as having either hepatic or neurological presentation of the disease. Those affected by hepatic Wilson Disease develop symptoms in childhood or adolescence and present with acute hepatitis, hepatic failure, or progressive chronic liver disease such as hepatitis or cirrhosis. 2

3 However, the degree of liver damage is variable ranging from asymptomatic hepatosplenomegaly and elevated liver enzymes, to complete failure. Also, there is a direct relationship between age of onset and severity of the disease; the earlier the age of onset, the more severe liver damage. Neurological Wilson Disease can present itself as early as the second decade of life, with the latest age of onset reported being 72 years of age. However, the majority of patients develop symptoms by the age of It has been suggested that the later onset of neurological presentation of Wilson disease as compared to the onset of the disease with hepatic presentation is due to the fact that copper is now readily taken up and retained by neural tissues 11. In its early progression the most common manifestations of the disease include difficulties in speech and swallowing, and drooling. Some patients present with cerebellar abnormalities such as unsteadiness of gait and incoordination. A third of the patients present with psychiatric disturbances, such as depression and reduced performance in school or work. In rare instances patients exhibit chorea and tremors of head, neck, and limbs. Dystonia of the facial and jaw muscles is also a common feature of Wilson Disease, and can produce a stiff face with a gaping mouth known as vacuous smile 1. Also, 95% of patients with neurologic symptoms present with the hallmark sign of Wilson disease the Kayser-Fleischer ring. However, only % of the patients with non-neurological presentation demonstrate this feature of the disease 5. Diagnostic Criteria Using clinical presentation alone, two of the typical symptoms presented together can be a definitive diagnosis for Wilson Disease without any additional tests. Therefore, to definitively diagnose, a patient must present with two of the following: Kayser-Fleischer Rings; typical neurological symptoms; and low serum ceruloplasmin levels 3. However, due to the large variation of symptoms in many patients, it is often not possible to definitively diagnose Wilson Disease based on symptomology alone. When a patient presents with a mixture of symptoms that could indicate Wilson Disease, it is therefore necessary to confirm a diagnosis through abnormal test results. The most common tests for Wilson Disease are low serum ceruplasmin, increased urinary copper excretion, and increased copper in the liver 2. A test for serum free copper greater than 10 micrograms/deciliter, 24 hour urinary copper showing greater than 100 micrograms/deciliter, and hepatic copper greater than 250 micrograms/gram of dry weight are all indicative of Wilson Disease 3. Even though these tests are available, there are a high number of false positives for each of them, as each of the results can be caused by other conditions 3. Therefore, it is necessary to use these tests with presenting symptoms as well as genetic information in order to make a diagnosis. A DNA analysis is not always practical given the length and number of known mutations in the Wilson Disease gene. In addition, most affected individuals are compound 3

4 heterozygotes, making identifying the gene mutation responsible even harder 5. Therefore, population frequencies must be used to narrow the number of mutations being searched for using PCR. Prognosis and Treatment Wilson Disease is a chronic disease and patients must be treated for their entire lives. The disease is fatal if not treated and symptomatic patients who stop treatment usually only live another 9 months to 3 years 1. However, as long as the patient is diagnosed early enough in the progression of the disease, the short and long-term prognosis for him or her is very good. Many of the symptoms are preventable and completely reversible with appropriate treatment. If diagnosed much later in the progression of the disease, the prognosis is much worse. He or she may have irreversible neurological symptoms and may have liver symptoms so severe that he or she may need a liver transplant 3. Initially, a chelating agent, such as penicillamine or trientine, is used. After symptoms are under control or in presymptomatic patients, these chelating agents or zinc can be given in reduced doses 3. The zinc works by interfering with copper uptake in the intestine by blocking the copper transporter and inducing a ligand to bind copper inside cells 3. The progression of liver symptoms can be stopped through the use of chelation therapy. However, while some MRI findings are completely reversible, the neurological symptoms are much more difficult to treat 3. Therefore, it is important to treat Wilson Disease early because if an asymptomatic individual or patient with liver symptoms alone is left undiagnosed or untreated, the disease may progress to include neurological symptoms years later 3. An individual with multiple advanced symptoms should be treated using therapies specific to each symptom. For example, anticholinergics may be used to control parkinsonian symptoms and antiepileptic drugs may be used to control convulsions 1. Disease Risks for Other Family Members: Genetic Counseling Though it is rare to see any clinical manifestations of Wilson Disease before age five, the age of onset of many of the different symptoms varies 2. However, it is helpful to diagnose those with Wilson Disease before they show symptoms because many of them can be prevented and reversed. Therefore, it is important to identify affected family members early on so that they can make educated decisions about treatment options. Since Wilson Disease is autosomal recessive, the risk of family members in question can be calculated depending on the degree of relationship to the original patient. The status of each family member can be determined definitively using PCR analysis if the specific mutation of the original patient is known 2. If the allele or alleles in question are unknown, haplotype analysis can be done using highly polymorphic microsatellite markers that flank the Wilson Disease gene with both the family member s and original patient s DNA 5. Depression is a common symptom seen in about one third of patients affected by Wilson Disease 3. In addition, each individual will react differently after learning his or her status as a homozygous affected or carrier of the Wilson Disease gene. Therefore, it is important for the genetic counselor to watch for such symptoms. 4

5 References 1. Das S, Ray K. Wilson s Disease: An Update. Nature Clinical Practice, Neurology 2006;2: Langner C, Denk H. Wilson Disease. Virchows Arch 2004;445: Kitzberger R, Madl C, Ferenci P. Wilson Disease. Metabolic Brain Disease 2005;20: Forbes JR, Cox DW. Functional Characterization of Missense Mutations in APT7B: Wilson Disease Mutation or Normal Variant? American Journal of Human Genetics 1998;63: Ferenci P. Pathophysiology and Clinical Features of Wilson Disease. Metabolic Brain Disease 2003;19: Lee J, Pena MM, Nose Y, Thiele DJ. Biochemical characterization of the human copper transporter Ctr1. J Biol Chem 2001;277: Vulpe CD, Packma S. Cellular copper transport. Annu Rev Natr. 1995;15: Kelley EJ, Palmiter RJ. A murine model of Menkes disease reveals a physiological function of metallothionein. Nat Genet 1996;13: Harris ED. Cellular copper transport and metabolism. Annu Rev Nutr. 2000;20: Huffman DL, O Halloran TV. Function, structure, and mechanism of intracellular copper trafficking proteins. Annu Rev Biochem 2000;70: Hayashi M, Fuse S, Endoh D, Horigushi N, Nakayama K, Kon, Y, and Okui T. Exp Anim 2006;55: