Alzheimers

Can Gene Editing Eliminate Alzheimer’s Disease?

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The development of the CRISPR/Cas9 system has made gene editing a relatively simple task.  While CRISPR and other gene editing technologies stand to revolutionize biomedical research and offers many promising therapeutic avenues (such as in the treatment of HIV), a great deal of debate exists over whether CRISPR should be used to modify human embryos. As I discussed in my previous Insight article, we lack enough fundamental biological knowledge to enhance many traits like height or intelligence, so we are not near a future with genetically-enhanced super babies. However, scientists have identified a few rare genetic variants that protect against disease.  One such protective variant is a mutation in the APP gene that protects against Alzheimer’s disease and cognitive decline in old age. If we can perfect gene editing technologies, is this mutation one that we should be regularly introducing into embryos? In this article, I explore the potential for using gene editing as a way to prevent Alzheimer’s disease in future generations.

Alzheimer’s Disease: Medicine’s Greatest Challenge in the 21st Century

Can gene editing be the missing piece in the battle against Alzheimer's? (Source: bostonbiotech.org)

Can gene editing be the missing piece in the battle against Alzheimer’s? (Source: bostonbiotech.org)

I chose to assess the benefit of germline gene editing in the context of Alzheimer’s disease because this disease is one of the biggest challenges medicine faces in the 21st century. Alzheimer’s disease is a chronic neurodegenerative disease responsible for the majority of the cases of dementia in the elderly. The disease symptoms begins with short term memory loss and causes more severe symptoms – problems with language, disorientation, mood swings, behavioral issues – as it progresses, eventually leading to the loss of bodily functions and death. Because of the dementia the disease causes, Alzheimer’s patients require a great deal of care, and the world spends ~1% of its total GDP on caring for those with Alzheimer’s and related disorders. Because the prevalence of the disease increases with age, the situation will worsen as life expectancies around the globe increase: worldwide cases of Alzheimer’s are expected to grow from 35 million today to over 115 million by 2050.

Despite much research, the exact causes of Alzheimer’s disease remains poorly understood. The disease seems to be related to the accumulation of plaques made of amyloid-β peptides that form on the outside of neurons, as well as the formation of tangles of the protein tau inside of neurons. Although many efforts have been made to target amyloid-β or the enzymes involved in its formation, we have so far been unsuccessful at finding any treatment that stops the disease or reverses its progress. Some researchers believe that most attempts at treating Alzheimer’s have failed because, by the time a patient shows symptoms, the disease has already progressed past the point of no return.

While research towards a cure continues, researchers have sought effective ways to prevent Alzheimer’s disease. Although some studies show that mental and physical exercise may lower ones risk of Alzheimer’s disease, approximately 60-80% of the risk for Alzheimer’s disease appears to be genetic. Thus, if we’re serious about prevention, we may have to act at the genetic level. And because the brain is difficult to access surgically for gene therapy in adults, this means using gene editing on embryos.

Preventing Alzheimer’s through Gene Editing

Luckily, research has identified a plausible means to address Alzheimer’s disease at the genetic level.  A study of people in Iceland found that a particular mutation in the APP gene seems to protect against Alzheimer’s disease. The researchers saw that this gene variant was present in 0.45 percent of the general population but only 0.13 percent of those with Alzheimer’s, suggesting that it lowers one’s risk of Alzheimer’s by a factor of about 3.5. Despite the fact that this study was performed only in Scandinavian individuals and shows only a correlation between the presence of the mutation and protection against Alzheimer’s, there are many reasons to believe that this particular mutation would be protective if introduced into the broader population. As mentioned above, the formation of amyloid-β aggregates is thought to be the root cause of Alzheimer’s disease. Amyloid-β forms when a cellular enzyme cuts the amyloid-β fragment from the amyloid precursor protein, the product of the APP gene. The particular protective mutation in APP sits near the site where the precursor protein gets cleaved, and experiments in cultured human cells confirm that the mutation interferes with the formation of amyloid-β peptides.  Therefore, it is likely that the mutation directly protects against Alzheimer’s by interfering with the production of amyloid-β.

Given these findings, it’s tempting to consider what might happen if we were to use gene editing to insert this protective allele into embryos. This year, there will be ~ 3.9 million children born in the US. Using current estimates of life expectancy and the lifetime risk of developing Alzheimer’s, we would expect 420,000 of these children to eventually develop Alzheimer’s disease. If we were to successfully engineer the APP mutation into every child born this year in the US, we’d estimate that only 120,000 of these children would develop Alzheimer’s in their lifetime. Thus, gene editing carries the potential of preventing a large fraction of Alzheimer’s cases.

Is Gene Editing a Viable Solution to Alzheimer’s?

An important number to consider when evaluating the potential risks and benefits of a medical intervention is the number needed to treat (NNT). This figure tells you how many patients you would need to treat in order to prevent one case of the disease. A treatment that causes serious side effects in 1 in 20 patients may be tolerable if the NNT is below 20, but not if the NNT is higher. Based on the figure above, treating 3.9 million children prevents 0.3 million cases of Alzheimer’s, giving a NNT of 13. Therefore, the benefit of preventing one case of Alzheimer’s disease must outweigh the combined cost and side effects of treating 13 individuals in order for the treatment to be practical. Given that there are healthy individuals living today that carry this mutation, it does not seem likely that it causes severe problems, especially not at the 1 in 13 level. However, an important worry with gene editing technologies is the introduction of “off-target” mutations, inadvertent changes to the genome that result as a side-effect of the gene editing process. These are concerning given that off-target mutations could cause significant problems like cancer. If we trust that the NNT is ~ 10-20, an off-target mutation rate of < 0.5% would ensure that the benefits of gene editing would outweigh the costs, even if every off-target mutation caused a problem ten times worse than Alzheimer’s.

What technological advances must be made for this therapy to be practical?  First, the NNT calculation assumes that every embryo we edit contains the correct mutation.  We are not close to that goal yet, and more work is required to reach that level of efficiency.  However, because the protection requires only one copy of the mutation to be present, it should be possible to edit eggs cells, fertilize them, screen the resulting embryos for the mutation, and implant only those containing the correct mutation.  Second, we need to improve the off-target mutation rate.  Although the recent embryo modification study from Sun Yat-sen University reported off-target mutation rates much higher than 0.5%, they did not use newly developed CRISPR tools that decrease off-target mutations 50-1000 fold.  The level of off-target mutation required for the proposed APP therapy is likely to be feasible soon. As the scientific and medical arguments for initiating such gene editing trials grow, society should begin considering the real social and ethical considerations of gene editing.  How would society accept individuals with edited genomes?  Would there be discrimination against them?  If gene editing became prevalent, would there be discrimination against non-edited individuals?  If this therapy is available only to the wealthy due to cost issues, would such a situation worsen problems with social inequality?  Determining the answers to these questions is just as important as addressing the scientific questions in determining the path forward.

Prospects for the Future

Despite our incredible advances in gene editing technologies, we still do not understand enough about gene-gene and gene-environment interactions to know with absolute certainty the effect of this mutation in every individual, so testing will certainly be required before this gene editing of this allele can done regularly. The requirement for testing turns out to be the Achilles’ heel of this approach. Determining the safety and efficacy of this treatment would require tracking a large number of individuals born from edited embryos over their entire lives, especially because Alzheimer’s rarely occurs before the age of 65. Thus, if we were to initiate a clinical trial today, we would not know the result of the trial until ~2100.

Of course, even if we knew today that introducing this mutation were safe and effective, we would not see the benefits of this treatment pay off until near the turn of the century when the cohort of edited children enter old age. While gene editing could potentially eliminating 70% of Alzheimer’s cases in the next generation of people, the difficulty of testing the effects of these mutations means that the gene-edited embryos are unlikely to have any immediate effect in the reduction of diseases like Alzheimer’s.  So despite gene editing’s enormous promise, we must still seek other therapies to if we wish to address Alzheimer’s in our lifetimes.  I would hope we would not have to wait until the 22nd century to develop an effective means of taming this scourge of the 21st century.

 

 

Postdoctoral researcher in biophysics. My areas of expertise include single molecule spectroscopy, structural biology, biochemistry, and evolutionary biology.
13 replies
  1. Greg Bernhardt
    Greg Bernhardt says:

    With most disease, early detection is critical. I believe I watched a TEDtalk where a scientist claimed there were clear physical signs in the brain of early Alzheimer’s disease long before any external signs.

  2. Ygggdrasil
    Ygggdrasil says:

    I agree, early detection is crucial for Alzheimer’s, especially because some of the failed clinical trial suggest that the disease is too far advanced to be reversed once patients are showing symptoms. There is research into trying to [url=http://www.nature.com/nm/journal/v20/n4/full/nm.3466.html]identify biomarkers that predict Alzheimer’s risk[/url], but although there has been progress, the test is [url=http://languagelog.ldc.upenn.edu/nll/?p=11015]not yet accurate enough[/url] to be [url=http://www.medpagetoday.com/Neurology/Dementia/44688]useful in guiding treatment[/url]. The proposal to prescribe drugs to treat Alzheimer’s at an early pre-clinical stage or to attempt to prevent Alzheimer’s disease, however, has been met with some [url=http://www.nytimes.com/2013/03/18/opinion/drugs-for-early-stage-alzheimers.html?hp&_r=4&]controversy[/url].

  3. Jimster41
    Jimster41 says:

    Amazing. Clearly a lot of challenges, given embryos are already legally complex, and controversial. However, I would also think that families with high risk of alzheimer’s would have the kind of perspective to judge. I was reading sort of fast. Is there currently a means of detecting the genetic markers for alzheimer’s risk? Is the only leverage point for intervention at the embryo?

    Now I have read the other posts…

  4. gravenewworld
    gravenewworld says:

    I keep hearing people in the media talk about fixing errors in the genes for Huntington’s disease or cystic fibrosis in human embryos, but why wouldn’t you instead just screen for embryos without the disease-causing mutations? It is easier and probably far cheaper.

    I do see therapeutic potential for genome editing of somatic cells for people with some diseases, especially ones affecting blood cells or other easily transplanted cells. And of course, CRISPR and other genome editing technologies will continue to be very, very useful in the lab to create model organisms and genetically modified cell lines. But for human embryos, I just don’t see a lot of upside to the technology.

  5. Ygggdrasil
    Ygggdrasil says:

    Amazing. Clearly a lot of challenges, given embryos are already legally complex, and controversial. However, I would also think that families with high risk of alzheimer’s would have the kind of perspective to judge. I was reading sort of fast. Is there currently a means of detecting the genetic markers for alzheimer’s risk? Is the only leverage point for intervention at the embryo?

    There are a few genetic markers that have been found to affect one’s risk of developing Alzheimer’s disease. Probably the best known one is the APOE gene. There are three forms of the gene, ε2, ε3, and ε4. ε3 is the most common form, but it seems that the ε2 form may lowers one’s risk and the ε4 form raises one’s risk of developing Alzheimer’s.

    Since any genetic intervention would have to target brain cells, intervention at the embryo stage may be the most practical. Performing brain surgery to do gene therapy to insert a gene that will lower one’s risk of Alzheimer’s does not seem like a feasible solution. Researchers, however, are taking clues from genetic studies to try and design drugs that may mimick the effects of protective mutations or block the effects of deleterious mutations in order to provide another means to tackling Alzheimer’s.

    I keep hearing people in the media talk about fixing errors in the genes for Huntington’s disease or cystic fibrosis in human embryos, but why wouldn’t you instead just screen for embryos without the disease-causing mutations? It is easier and probably far cheaper.

    I do see therapeutic potential for genome editing of somatic cells for people with some diseases, especially ones affecting blood cells or other easily transplanted cells. And of course, CRISPR and other genome editing technologies will continue to be very, very useful in the lab to create model organisms and genetically modified cell lines. But for human embryos, I just don’t see a lot of upside to the technology.

    I agree that, for most genetic disorders, PGD is much easier, safer and cheaper way to ensure an embryo is free from genetic disease (a point which I made in my previous article on [url=https://www.physicsforums.com/insights/dont-fear-crispr-new-gene-editing-technologies-wont-lead-designer-babies/]CRISPR gene editing technologies[/url]). Where gene editing has an application is in the introduction of rare protective alleles into embryos to significantly lower disease risk. For example, in this article, I look at a rare mutation (present in ~ 0.5% of Scandinavian people and virtually absent in other populations) that seems to lower one’s risk of developing Alzheimer’s disease by ~3.5 fold. There’s certainly a lot more work that can be done before this rare allele could be introduced into embryos safely and efficiently, but such a therapy could potentially eliminate a large fraction of Alzheimer’s cases in the next generation of people.

  6. mfb
    mfb says:

    My position is that Alzheimer’s could be entirely eliminated by genetic editing.
    We know this is true.
    There are super agers who live past a century without developing Alzheimer-like pathology.

    Gene editing, properly done would eliminate 100% of Alzheimer’s dementia.

    There are also persons who don’t die in a traffic accident, that does not mean gene editing will allow to eliminate traffic deaths.
    There is no evidence that the Alzheimer risk could be zero with any genetic code.

    Given that there are healthy individuals living today that carry this mutation, it does not seem likely that it causes severe problems, especially not at the 1 in 13 level.

    Are we sure about that? Did someone screen the population with this mutation for all potentially problematic health issues? Would a 7% higher risk of getting disease X have been spotted, for all X? What about a 1% higher risk to get diseases A, B, C, D, E, F and G?

  7. mfb
    mfb says:

    Genetic markers have been found that help prevent the onset of Alzheimer`s disease.

    They just reduce the risk.

    The APOE 3b variant that I mentioned had profound effects on lowering Alzheimer risk, up to 90%.

    While I would be interested in a source for the claimed 90%, it does not help: your original claim was 100%.

    It cannot be assured, though I strongly suspect that CRISPering a set of genes could have a profound effect in reducing risk.

    That is a much weaker statement.

  8. Laroxe
    Laroxe says:

    While we are getting increasing evidence that the risk of dementia is influenced by specific genes, the discussion here seems to be based on the idea that we have evolved genes to help us develop dementia and I suspect this isn’t the case. We know that certain genes in certain combinations influence risk of all sorts of diseases but presumably these genes have survived selective pressures for a reason. The question really is do we know enough about the processes underpinning dementia and do we know enough about how the products of our genes, often in complex combinations influence our physiology.
    The genetics of early onset Alzheimer’s is pretty well understood but this is rare and isn’t really the same disease as late onset at all. The genetics of late onset is far more muddy with a number of genes identified that influence risk but in an unpredictable way. The APOE gene in general controls the production of lipoprotein’s, a major physiological process and the various subtypes have been linked to risks of a variety of diseases including cardiovascular, the fact that there is variation in the level of risk doesn’t really help when all the subtypes are common and even in those with 2 type e3 variants that shouldn’t have any increased risk, 60% will develop the condition by 80. Some people argue that the only effect of the APOE gene is in when people develop the condition.
    I think gene editing has potential but only when we actually understand what we are doing. It may have been possible to eliminate the sickle cell trait, which has a clear single gene origin, carried by up to 50% of the population of some areas in Africa and can cause a lot of problems, doing so, would have probably killed millions, the discovery of its effects on malaria only being discovered later than its identification as a cause of disease.

  9. dlgoff
    dlgoff says:

    Evidence is also emerging for a role of copper-2 found in copper plumbing as perhaps the causative agent of
    the current Alzheimer pandemic.

    [URL]http://www.mdpi.com/2072-6643/7/12/5513/htm[/URL]

    This is shocking to know, yet could be valuable knowledge for the general public IMO.

  10. rootone
    rootone says:

    Copper piping has been around for quite a while though.
    If it does have some detrimental effect for health that may be so, but it can’t explain a sudden pandemic outbreak if that is the case.

  11. Ygggdrasil
    Ygggdrasil says:

    Copper piping has been around for quite a while though.
    If it does have some detrimental effect for health that may be so, but it can’t explain a sudden pandemic outbreak if that is the case.

    I don’t know if it’s accurate to say that there has been a sudden pandemic outbreak of Alzheimer’s disease. First, the main risk factor for Alzheimer’s is age, so the prevalence and incidence of the disease depends on the size of the otherwise healthy, elderly population in a society in which doctors could notice symptoms of the disease. Second, diagnostic criteria for many neurological diseases change over time, so increases in incidence of a disease could simply be due to changes in how doctors diagnose the disease. For example, diagnoses of Alzheimer’s disease used to be reserved for individuals between the ages of 45 and 65, and it wasn’t until 1977 that the term was applied to dementia in individuals over the age of 65 ([URL]https://en.wikipedia.org/wiki/Alzheimer’s_disease#History[/URL]).

    That said, whether copper contributes to Alzheimer’s disease is still controversial. While the review that @Alzoom cites makes a compelling case, there are conflicting reports, for example, [URL=’http://www.nature.com/articles/srep01256′]this study[/URL] suggesting that copper could actually prevent amyloid formation. More research is likely required to come to a more solid conclusion. However, the recommendations that Brewer makes in the [URL=’http://www.mdpi.com/2072-6643/7/12/5513/htm’]Nutrients paper[/URL] (avoid supplement pills containing copper, filter water to remove trace copper, wash fruits and vegetables, and reduce dietary fat and meat intake) are very reasonable suggestions for a healthy lifestyle.

  12. Laroxe
    Laroxe says:

    I think the work on metal toxicity is interesting but really quite unconvincing, it seems to change to another candidate metal every couple of years and in fact many of these metals are very common in the natural environment. Aluminium is I think the 3rd commonest mineral on the planet, is widely present in both plants and animals and appears pretty inert, any discussion on poisonings that does not in some way relate symptoms to dosages is rarely useful and most poisons have well described and specific effects. The vast majority of potent neuro-toxins our neurones are exposed to are actually produced in the brain as products of metabolism. There are just to many theories to make much sense out of but again as someone said the clearest risk factor is age, with virtually everyone over 80 having significant accumulations of amyloid proteins in their brain whether diagnosed or not. I recognise the life expectancy figures don’t help much, but it is a fact that populations are ageing, there are more elderly people with risk very clearly associated with age. Interestingly there appears to be a new spanner in the works, most people have perhaps heard that despite al the panics about obesity the actual incidence of heart disease is falling and no one is quite sure why. Well it also seems to be the case that the incidence of Alzhiemer’s is developed countries is also falling & again its a bit of a mystery. [URL]http://www.cam.ac.uk/research/news/dementia-prevalence-figures-in-the-uk-show-decline-over-past-20-years[/URL]

  13. jakob.worleycun
    jakob.worleycun says:

    While alzheimers and some ofher forms of dementia can be prevented by the altering of genes, once the damage has been done changing the genes may stop the detrimental expression of said gene from further expression, it will not however reverse damage that has been done.

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