We invite you to participate in this "offline" Forum Discussion led by Brian Balin (Philadelphia College of Osteopathic Medicine). This discussion will extend our previously held live discussion on the Pathogen Hypothesis of Alzheimer Disease. Take advantage of this more leisurely format to express your ideas in depth. Please contact us with questions, comments, critiques, and kudos. We will post your commentaries and forward them to other participants for their responses.
By Brian Balin
Arguments put forth for the importance of pathogens, as opposed to genetics, in chronic diseases of aging, including neurological ones, are reminiscent of the perennial debate over nature versus nurture in human development. Which has the greatest influence: something inherent or something extraneous? The answer, obviously, lies in both. Yet a disproportionate amount of research effort and funding on the causes of Alzheimer disease is directed towards finding genes for late-onset AD. Even epidemiological studies focus on factors such as nutrition, medications, and lifestyle, and largely overlook the possibility of infection. However, research by my laboratory and a handful of others suggests that pathogens may play a more important role—more so than genetics, perhaps—than some may be willing to believe.
The environmental influences of the microbial world in chronic diseases of aging cannot be dismissed. Time and again, science and medicine have been rocked by the discovery of infectious processes where none was expected. In just the last quarter of the twentieth century we have seen dramatic instances of new infectious diseases, including AIDS, and have come to realize that maladies such as gastric ulcers, once thought to result from bad diet, bad lifestyle, or simply bad genes, can actually be caused by microorganisms.
With the sequencing of the human genome, the stage is set for enormous breakthroughs in our understanding of genetics and disease. However, what is often missed, or given little consideration, is that we are not pure organisms. The microbial world insinuates into our existence, often, as retroviruses can, by influencing our genetic makeup. And just as we cannot discount the importance of that makeup—because it can predispose individuals to either resist or succumb to disease—likewise, we cannot discount the fact that acute and chronic infectious diseases are today the major contributors to the morbidity and mortality of the world's human and animal populations.
While acute infectious disease is often readily diagnosed, chronic infection is a different matter. Paul Ewald points out in his book Plague Time that in the absence of an acute phase of disease, infectious causation is either dismissed or not even considered. He goes on to examine how the scientific and medical communities have considered infectious disease in the last two centuries. During the last quarter of the nineteenth century, all diseases of infectious origin were primarily acute, that is, the sufferers had obvious symptoms shortly after infection. In contrast, in the last quarter of the twentieth century, most newly identified human infectious diseases have been of a chronic nature. Ewald terms these the "Stealth Infections." Among the chronic diseases that Ewald proposes may be microbial in origin are cancer, atherosclerosis, and Alzheimer disease. The key arguments in support of the "pathogen hypothesis" have been detailed on the Alzforum [see "The Pathogen Hypothesis" and "Viruses and Dementia, and the Role of Apolipoprotein E (ApoE)"]. In this discussion, I would like to invite colleagues to debate specific points raised against the pathogen hypothesis. Specifically, I encourage colleagues to think creatively about how to reconcile the role of microbial pathogens with genetics and known molecular pathways involved in Alzheimer disease.
Objection: ApoE proves that genetics is critically important in late-onset AD.
Balin: Paul Ewald presents a very intriguing argument with regard to chronic disease. He states that "chronic diseases, if they are common and damaging, must be powerful eliminators of any genetic instruction that may cause them." Thus, he goes on to argue that the only factor capable of staying ahead of natural selection is pathogens, which can mutate their way rapidly around any genetic adaptations. In essence, we are in a genetic arms race against microbes that we cannot win. Mathematical models based on this concept imply that nongenetic factors are almost entirely responsible for chronic degenerative diseases.
What, then, about ApoE? Ewald argues that genetic causes of chronic disease will persist only if a genetic instruction provides a compensating benefit. For example, sickle cell anemia is caused by a genetic mutation that, in heterozygotes, protects against malaria, which kills millions worldwide each year. The relatively few individuals who inherit two copies of the sickle cell gene suffer a painful and crippling disease, but the gene has persisted because it contributes significantly to the survival of the more numerous heterozygotes. It has been hypothesized that the ApoE4 allele evolved late in human evolution and may have conferred a benefit in younger individuals, perhaps by modifying lipid metabolism as the amount of animal fats in the diet increased. The e4 allele might be one that is beneficial during reproductive years but becomes a detriment in old age.
ApoE4 may confer increased susceptibility to infection in Alzheimer disease. Studies by Dobson and Itzhaki, 1999, demonstrated that AD patients with the e4 allele were more likely to also harbor HSV1. Furthermore, in a recent study by Gerard et al., 2005, the bacterial load of Chlamydia pneumoniae in the Alzheimer brain was significantly higher in the ApoE4-bearing AD brain tissues, as compared to the AD brain tissues not expressing the e4 allele. Is it possible that HSV1, Chlamydia pneumoniae, and possibly other infectious microbes could be hijacking molecules for cholesterol delivery to enter the brain? This scenario offers a novel interpretation for how a genetic risk factor and infectious agent might interact to cause disease.
Objection: The risk for late-onset AD is clearly heritable, even in individuals with the ApoE4 allele, so there must be other risk-factor genes waiting to be discovered.
Balin: No doubt, with further analysis, other risk-factor genes will be discovered. However, studies of monozygotic twin populations clearly demonstrate that genetic factors account only partially for an increased risk of AD. Genes predispose individuals to AD, but environmental influences must come into play to ultimately cause disease. Just as previously mentioned, there are a multitude of examples that suggest that the combination of a pathogen and genetic susceptibility can result in disease.
Objection: Most of us harbor pathogens in the CNS with no ill effect.
Balin: There is a presumption that the presence of infectious agents in tissues is not harmful if there is no obvious acute or reactive process. Any microbial invasion of the human central nervous system that results in acute, chronic, and/or permanent residence cannot be good. Of all the tissues in the body, one would expect the central nervous system, which controls our entire being, to be most "sterile." Ironically, the central nervous system is the one site in the body where infectious agents can persist over a long time because in this part of the body, they can evade immune surveillance by the body's defense system.
However, this begs the question of the brain's "internal immune surveillance" that is accomplished principally by microglia and astroglia. Chronic and/or persistent "turn-on" of these cells due to the presence of infection or products of infection would not be healthy for nerve cells. One could envision a tissue response by the brain that would result in chronic degenerative damage. A perfect analogy is tuberculosis in lung tissue in which chronic inflammation due to a persistent stimulus (i.e., mycobacterium or products of mycobacterium in the macrophages) results in granuloma formation, a focus of inflammation around damaged or dead, necrotic lung tissue. Could the well-circumscribed dense-core amyloid plaque be a comparable "granuloma" of the brain?
Objection: Many neurologists and neuropathologists will object that "since most people have viruses in their nervous systems, the viruses must not be contributing to disease."
Balin: This argument is flawed. Consider Helicobacter pylori infection in the stomachs of human populations. This organism has been proven in the last two decades of the twentieth century to cause gastric and peptic ulcers, gastric cancer, and mucosal-associated lymphoid tissue tumors in the stomach. Approximately 3.5 billion people are infected with this organism, but only 10 percent show obvious disease. Are we to believe that because such a large population is infected with this organism, but only a small percentage actually fall ill, that the presence of the organism in the gut is of little significance? Obviously, this is not the case.
And, there are parallels in the central nervous system! Herpes viruses that infect neurons are known to cause annoying chronic problems such as cold sores and shingles. Other pathogens are known to cause or contribute to more significant problems such as Bell's palsy and Guillain-Barre syndrome. But there may be still others that may trigger even more severe disease pathogenesis. For example, the autoimmune responses that lead to multiple sclerosis might be triggered by infection, and AIDS dementia results from long-term infection with the HIV-1 virus. And, it just so happens, that HIV-dementia appears to be worsened in those patients who express the ApoE4 allele (Corder et al., 1998)!
Objection: If pathogens cause AD, why don't we see AD brains full of viruses or other microbes?
Balin: Microbes have many tricks for hiding out in tissues. Viruses, for example, can insinuate themselves into host cells' genes and become invisible. Chlamydia pneumoniae is an extremely interesting organism and candidate for numerous diseases. This bacterium is endocytosed into vacuoles within a variety of cell types and may be retained in certain cells indefinitely. Our own studies have determined that infection by this organism into neuroblastoma cells in culture confers an antiapoptotic effect on the cells, thus ensuring infection in a chronic to persistent state. Difficulties in recognizing this type of infection are a major challenge. Changes in gene and antigen expression with persistency have led, at times, to the undetectability of the organism in tissues and culture samples. Furthermore, homology of some bacterial proteins with eukaryotic proteins may result in antigenic mimicry that can incite an autoimmune response without a clearly identified or identifiable infectious component. These are just a few reasons why associating infection with chronic disease or proving that infection is causing chronic disease is so difficult.
Objection: By what mechanism can chronic infection result in a chronic degenerative disease?
Balin: Chronic diseases can result from direct impact of infection on the genetics of the system. For example, cervical cancer, a chronic disease, is caused by the human papilloma virus (HPV). Different strains of this virus code for proteins that directly impact two proteins, the retinoblastoma tumor suppressor protein, and the p53 protein, which is important in DNA repair. The viral proteins block the ability of the eukaryotic proteins to apply breaks to the cell cycle, thus leading to cellular proliferation and cancer.
Another possible mechanism is autoimmunity such as mentioned above. A microbe may express genes that are molecular mimics of a human protein (say, Aß). The body mounts an immune attack on the microbe, which is cleared from the scene. Rheumatic heart disease is a familiar example of how this could occur. Years later, some trigger (a new infection, or overexpression of Aß?) results in an immune attack against the endogenous protein and causes severe tissue damage.
Objection: People have searched in good faith, but the data are not convincing. Ergo, there's nothing to this hypothesis.
Balin: Several studies, including our own, have reported a remarkably strong association between a microbe and AD, but other studies have not shown such an association. There are numerous reasons why there are discrepancies in the different studies. These include tissue sampling, use of different polymerase chain reaction (PCR) primers and probes, and different antibody probes to different antigenic determinants of the organism at different stages of infection. We must work on standardizing approaches to resolve some of these issues. Numerous laboratories have applied real-time PCR approaches in this effort. However, even in these instances, newer findings on gene expression and gene modifications found in microorganisms are clouding these approaches. Thus, numerous techniques and collaborative approaches are required to objectively address these issues prior to ruling out infectious risk and/or causation of chronic diseases. The solution is still unfolding in the laboratory, where increasingly sophisticated techniques are being used to discover if infection is involved in the pathogenic processes. Evolution of newer techniques and development of animal models of infection are some of the key reasons we can now more efficiently investigate pathogen causation in chronic diseases.
Objection: Why mess around with microbes when there are other, more compelling ideas, such as the amyloid cascade hypothesis, with so much more supporting data, including persuasive genetic findings on the cause of early-onset familial AD and the increased risk of LOAD associated with ApoE4?
Balin: Until we can clearly determine the cause of AD and other neurodegenerative diseases, as well as many other chronic diseases, infection must be included as a hypothesis. In the history of medicine and science, this has proven to be and continues to be the greatest determinant of disease. Researchers must ask themselves the following questions: 1) Has all previous work on a given chronic disease of aging demonstrated that infection is not involved in the disease? 2) Have we put enough resources behind the investigators and investigations studying infection in chronic diseases of aging to rule out infection as causing the problems? And last, but not least, 3) do we currently have enough knowledge of infectious diseases in chronic conditions to proclaim that chronic infection does not cause chronic disease? If the answers to any of these questions is no, then pathogen involvement in chronic diseases of aging has to be given high priority for consideration.
The possibility that treatable common microbial infections are contributing to the global crisis of Alzheimer disease has implications for public health that are too important to ignore. A thorough review of many of the issues discussed here can be found through the American Academy of Microbiology. A report entitled "Microbial Triggers of Chronic Human Illnesses" was compiled from a colloquium held in June 2004. This report highlights many features relevant to this discussion. For example, two sections report on host factors and microbial factors that contribute to illness. Host factors that are considered are: genetics, concomitant infections, age, dose, gender, hormonal factors, immune status, nutritional status, behavioral factors, and exposure to non-infectious agents. Microbial factors considered are: viral or infectious genetic integration into host genome, latency factors, ability to bind to mucosal surfaces or other tissues, characteristics of the target organ, high mutation rate, and immune evasion. In addition, the report highlights currently available techniques for the detection of pathogens with an analysis of the technical strength and weakness.
In summary, this report provides excellent insight into current strategies and the required needs for addressing microbial triggers of chronic illnesses such as Alzheimer disease.
Book page Citations
- Dobson CB, Itzhaki RF. Herpes simplex virus type 1 and Alzheimer's disease. Neurobiol Aging. 1999 Jul-Aug;20(4):457-65. PubMed.
- Gérard HC, Wildt KL, Whittum-Hudson JA, Lai Z, Ager J, Hudson AP. The load of Chlamydia pneumoniae in the Alzheimer's brain varies with APOE genotype. Microb Pathog. 2005 Jul-Aug;39(1-2):19-26. PubMed.
- Corder EH, Robertson K, Lannfelt L, Bogdanovic N, Eggertsen G, Wilkins J, Hall C. HIV-infected subjects with the E4 allele for APOE have excess dementia and peripheral neuropathy. Nat Med. 1998 Oct;4(10):1182-4. PubMed.
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