Wilson's Disease–Molecular Basis of the Disease State

Wilson’s Disease is a copper transport disorder. Although the signs and symptoms of the disease were characterized by Dr. Kinnier Wilson in 1912, it wasn’t until several decades later that the metabolic cause of the disease was fully elucidated.

Familial or Inherited?

When Wilson first characterized Wilson’s Disease in 1912 he asserted that the disease was familial, but not hereditary. (Wilson, 1912) Despite his insistence, there was suspicion in the scientific community that Wilson’s Disease was an inherited disorder. In 1921 Hall reviewed Wilson’s patient cases and an additional seven cases and stated that the disease was likely inherited in a recessive pattern. Later studies also lent support to the idea that Wilson’s Disease was an inherited, recessive disorder, but up until this point all genetic analysis had been done using old patient case reports. This methodology left a lot of room for error if everything hadn’t been properly recorded in the initial case report. Finally in 1953 A.G. Bearn used genetic ratio analysis calculations, instead of patient case reports, to confirm that Wilson’s Disease is inherited in an autosomal recessive pattern. (Bearn 1953) Now that it was clear that Wilson’s Disease was hereditary, the scientific community could move on to determine what was actually causing the disease.

The Wilson’s Disease Gene– Genetics and Epigenetics:

In 1993 a suite of Nature Genetics papers were published which mapped the Wilson’s Disease gene to chromosome 13q14.3 and found that the gene codes for ATP7B, a copper-transporting ATPase that binds copper to ceruloplasmin, a copper carrying protein,  and then transports the bound copper out of the cell and into the circulatory system. (Bull et. al. 1993) (Petrukhin et. al. 1993) (Tanzi et. al. 1993) Case studies have uncovered a wide variety of mutation types in the Wilson’s Disease gene that can disrupt the coding of ATP7B enough to cause Wilson’s Disease. (Bull et. al. 1993) (Petrukhin et. al. 1993) (Tanzi et. al. 1993) These mutations often vary by family and currently the information is limited to small genetic case studies. So far the mutations that have been determined to cause Wilson’s Disease include truncating mutations, splice mutations, missense mutations, splicing defects, frameshift mutations, deletions, and nonsense mutations all of different base pair lengths in different parts of the gene. Although scientists have not yet determined if there is a critical residue or region of the gene that when mutated causes Wilson’s Disease, it has been observed that the majority of these mutations disrupt the stability or activity of ATP7B in such a way that is it unable to function.

Many scientists believed that all mutations that caused Wilson’s Disease disrupted ATP7B in this way, but a new study altered this paradigm when the authors discovered a mutation that impairs ATP7B’s ability to exit the Golgi. (Braiterman et. al. 2014) The disease causing mutation induces local distortions at ATP7B’s transmembrane 1 domain which alters the interactions between transmembrane domains 1 and 2 (TM1 and TM2), thus preventing ATP7B from exiting the trans-golgi network. (Braiterman et. al. 2014) The author’s characterization of this new type of mutation and the proposed mechanism for how it functions also sheds light on the broader role of TM1 and TM2 as regulators of copper trafficking in ATP7B. (Braiterman et. al. 2014)

Recent research has looked beyond mutation of the Wilson’s Disease gene to the role of epigenetics in the regulation of the Wilson’s Disease gene. A new study characterized hepatic copper accumulation, methionine metabolism, global DNA methylation and gene expression in a mouse model for Wilson’s Disease throughout its lifespan from fetus through adulthood. (Le et. al. 2014) The authors found that there is a close relationship between copper accumulation and the decreased expression and increased methylation of several of the enzymes necessary for methionine metabolism. (Le et. al. 2014) In healthy people, S-adenosylhomocysteinase regenerates S-adenosyl homocstiene (SAH) which is the enzyme that inhibits DNA methylation.(Le et. al. 2014) The ratio of SAH to SAM (S-adenosylmethionine–the enzyme that methylates DNA) is imperative for maintaining normal DNA methylation regulation.(Le et. al. 2014) In Wilson’s Disease ATP7B is not functional due to a mutation in the WD gene. Therefore, copper accumulates in the liver. This copper accumulation inhibits S-adenosylhomocysteinase, which means that SAH is not regenerated and so it increases in concentration.(Le et. al. 2014) This throws off the SAH to SAM ratio and therefore there is more DNA methylation than normal.(Le et. al. 2014) The authors hypothesize this indicates a correlation between genetic and epigenetic regulation of the onset and later progression of Wilson’s Disease. (Le et. al. 2014) The role that epigenetics plays in Wilson’s Disease will be a new direction for research in this field in the coming years.

Copper-transporting ATPase ATP7B:

The Wilson’s Disease gene codes for the copper-transporting ATPase ATP7B. ATP7B is responsible for managing copper in the cell. THe highest concentration of ATP7B is found in hepatocytes, but it is also found in smaller concentrations in brain and kidney cells. Proteins in the ATPases family are transport proteins that use ATP to power the transport of heavy metals. ATP7B uses ATP to power the transport of copper. ATP7B facilitates the binding of copper to ceruloplasmin, the copper carrying protein, in the liver so that copper can be carried through the blood stream to other tissues that require it for metabolic processes. When copper is in excess, ATP7B packages excess copper into vesicles that are sent to the bile so that the copper can later be excreted in feces. (Turnlund 1998)

There have been homology models proposed for the structure of ATP7B. However, because ATP7B is a membrane bound protein, a crystal structure of the entire protein has not yet been accomplished. The homology model for the structure of ATP7B contains an ATPase consensus sequence, a hinge domain, a phosphorylation site, and at least two putative copper-binding sites. Figure 1 below show a homoloy model for ATP7B.  (USCN)

The above figure shows a homology model for ATP7B.
Figure 1: The above figure shows a homology model for ATP7B which contains a consensus sequence, a hinge domain, and at least two putative copper binding sites. (Image from USCN)

In Wilson’s Disease, the gene coding for ATP7B is mutated and therefore ATP7B’s stability and activity are disrupted, making copper transport difficult, if not impossible.

It was possible to elucidate the role of ATP7B in Wilson’s Disease because of the similarity of ATP7B to ATP7A, another copper-transporting ATPase which is linked with Menkes Disease, another copper transport disorder. (Tanzi et. al. 1993) ATP7A is found in almost all tissues of the body except the liver, brain, and kidneys where ATP7B is found. Both ATP7A and ATP7B contain 6 heavy metal binding domains and 8 membrane spanning helices. Copper is delivered to ATP7A and ATP7B via chaperones like ATOX1. (Gourdon et. al. 2012) In 2012 a study proposed a homology model for ATP7B and ATP7A based on the crystal structures available for the individual, soluble domains by comparing them to a recently solved protein, LpCopA, from a bacterial model. (Gourdon et. al. 2012) The authors also determined that the heavy metal binding domains 5 and 6 (HMBD5 and HMBD6) are crucial for function by mapping disease causing missense mutations. (Gourdon et. al. 2012) Finally, further analysis of the HMBDs allowed the authors to propose a structural model and mechanism where the 6 HMBDs associate in three pairs and communicate via a connecting loop domain between HMBD5 and HMBD6 so that when copper is bound, the autoinhibitory HMBD6 is displaced from the ATPase protein core. (Gourdon et. al. 2012)

This figure shows the
Figure 2: This figure shows a basic diagram of the architecture of ATP7A and ATP7B. Both proteins have 6 heavy metal binding domains and 8 transmembrane domains. Chaperons like ATOX1 are thought to deliver copper to both ATP7A and ATP7B. In Wilson’s Disease the gene coding for ATP7B is mutated so the stability of this ATP7B structure is disrupted. (Gourdon et. al. 2012)

The Impact of Wilson’s Disease on Global Copper Metabolism:

Humans consume copper as part of our diet. The copper is then absorbed by the intestinal mucosa and transported via the portal blood to the liver. (Turnlund 1998)  When copper arrives at the liver in a patient with Wilson’s Disease, ATP7B does not function properly. (Gourdon et. al. 2012) Therefore, ATP7B is unable to use copper to bind and modify the ceruloplamsin protein, which is made in the liver. Ceruloplasmin is the protein responsible for carrying copper through systemic circulation to the other tissues of the body that require copper for their enzymes. Because of this, the copper is released as free copper, meaning that it is only loosely bound to albumin or other small molecules. Free copper is not capable of being picked up by the surrounding tissues requiring copper, so free copper accumulates in the blood stream. The excess copper acts as a heavy metal to donate electrons to oxygen species resulting in reactive oxygen species such as superoxide. These reactive oxygen species can reek havoc on the patient’s system. (Kalita et. al.)

Not only does ATP7B mutation cause problems with free copper in the blood stream, but it also causes issues with accumulation of copper in vital organs such as the liver and brain. When there is excess copper in the liver of a healthy patient, ATP7B leaves the Golgi and instead works to package copper into vesicles to remove the excess copper before it damages the liver. Those vesicles are then sent out of the liver via bile. (Turnlund 1998) In a person with Wilson’s disease, ATP7B is unable to package the copper into vesicles for export from the liver via bile and subsequent excretion in feces. Therefore, the excess copper accumulates in the liver, and other vital organs, of the Wilson’s Disease patient. This copper accumulation causes an inflammatory response in the patient’s liver and brain. This inflammation leads to the symptoms we associate with Wilson’s Disease such as cirrhosis. (Turnlund 1998)

Other Pages on Wilson’s Disease:

Wilson’s Disease–Title Page

Wilson’s Disease–History and Metabolic Context

Wilson’s Disease–Treatments and Disease Management

Wilson’s Disease–Conclusions and Proposals for Future Work

6 Replies to “Wilson's Disease–Molecular Basis of the Disease State”

  1. Re: Treatments and disease management

    1. Nice job on the article, the most difficult thing about communicating your science is being able to call your parents up and explain to them what you’re working on and have them understand it. You did a very nice job communicating.

    2. I am a liver immunologist; and it’s important to keep in mind that a complication of wilson’s disease (copper buildup as a chronic inflammatory insult to the liver) can result in cirrhosis, rather than because you have wilson’s disease you will have liver cirrhosis. Liver transplant would address this complication in the short term, but at the heart of the issue is still the mutation in copper transport protein. This would also carry the risk that the transplanted liver could fail if the patient doesn’t manage their disease appropriately. Do you think that replacement of mutated ATP7B protein with functional protein could be a reasonable treatment in the future?

    3. Once treatment is initiated, are the other symptoms (ring around the eye, psychiatric, etc) reversed?

    1. Hi Dana! Thank you so much for taking the time to read ad comment! I completely agree that the issue with Wilson’s Disease is indeed chronic inflammation as an insult to the liver. The Wilson’s Disease mutation itself does not in fact cause the inflammation and cirrhosis–that is caused by copper accumulation. I will do my best to make that clearer on my website. Gene therapy and hepatocyte transplantation are the new hope for future treatment. This would involve transplanting hepatocytes with normal ATP7B protein into the diseased liver to try to regenerate the liver. There is promising research happening here (especially by Gupta et al in their 2014 paper) and hopefully this will be widely available for patients in the future. (See the conclusions and proposals for future work webpage for a cool figure and more info!).
      After treatment, many of the Wilson’s Disease symptoms can be reversed. Penicillamine focuses on decoppering the vital organs, so many of the symptoms related to the vital organs subside. Zinc therapy focuses instead on decreasing the amount of copper getting into the body and free copper levels. so this may reverse symptoms caused by elevated free copper levels and simply manage the symptoms associated with vital organs. Patients with Wilson’s Disease are really treating the symptoms of their disease in either case though since there is currently no way to reverse the genetic mutation. Thanks!

  2. Great job on the website! You present the material in a very thorough yet accessible manner.

    I am interested in the connection between copper accumulation and increased methylation of genes required for methionine metabolism. Has methionine or homocysteine been demonstrated to accumulate in patients with Wilson’s disease as a result of these epigenetic changes? Are there any clinical features of the disease that could be attributed to methionine metabolism dysregulation such as increased coronary artery disease?

    1. Hi Rebecca! Thanks for taking the time to read and comment! So far, the 2014 study I cited above was the first to address the role of genetics and global DNA methylation with respect to Wilson’s Disease and the study was rather preliminary. I have not yet seen any studies specifically looking at accumulation of methionine or homocycteine in Wilson’s Disease patients. This study was done in mouse models and looked at the ratio of SAM to SAH, but did not actually look specifically at methionine or homocystiene levels (although if SAM is decreased and SAH is increased, I would expect the methionine levels and homocystiene levels to be altered as well–we just have no experimental evidence for this yet). However, at this stage it is likely that the majority of physicians and researchers are not yet looking too closely at methionine and homocystiene levels, since this study is so new. I have not seen any evidence for coronary artery disease either. Again, it is possible that this is a correlation that is occurring and people just haven’t thought to look for it or take note of it. It is also possible that WD is not progressing to an extent where coronary artery disease would be evident since we are able to control WD fairly well with the currently available treatments. I hope that in the future there will be further research into this area and we will have more information to share! I responded to Tommy’s comment below with the biochemical explanation for how the SAM:SAH ratio they looked at is impacted by amino acid metabolism in case you are interested! Thanks!

  3. Hey Kelly,

    My question is also concerned with coppers role in methionine metabolism. You say that the symptoms of Wilson’s disease could be associated with global corruption of DNA methylation, however, the enzyme involved in the disease is clearly localized to the liver. Is there a mechanism behind which the liver siphons and transports metabolic intermediates from the methionine pathway to other parts of the body to be used as sources of methylation? I guess my main question is, how does problems with methylation in the liver correlate to global porlblems with DNA methylation?

    On another note, if the problem with the disease is due to the fact that copper concentrations don’t allow for the proper metabolism of methionine, do you think identification and supplementation of the relevant metabolites involved could be a useful in conjunction with some of the other treatments?

    1. Hi Tommy! Thanks for reading and commenting! Alright, here’s the pathway explanation I think you are looking for that explains the link between global DNA methylation and copper accumulation, according to Le et. al. In healthy people, S-adenosylhomocysteinase regenerates S-adenosyl homocstiene (SAH) which is the enzyme that inhibits DNA methylation. The ratio of SAH to SAM (S-adenosylmethionine–the enzyme that methylates DNA) is imperative for maintaining normal DNA methylation regulation. In Wilson’s Disease ATP7B is not functional due to a mutation in the WD gene. Therefore, copper accumulates in the liver. This copper accumulation inhibits S-adenosylhomocysteinase, which means that SAH is not regenerated and so it increases in concentration. This throws off the SAH to SAM ratio and therefore there is more DNA mehtylation than normal. It seems that supplementation with metabolites wouldn’t necessarily help the situation here. It seems that a decreases in copper accumulation is the best bet, so decoppering therapies such as chelating agents likely address this. It would be possible to try enzyme therapy to supplement S-adenosylhomocysteinase, but it seems you wouldn’t want an excess of that in your system either because it would likely skew the SAM:SAH ratio in the other direction and you would not have enough methylation of DNA. Hope this helps! Thanks!

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