Over the past several decades, it has become increasingly clear that inheritance plays a major role in Alzheimer’s disease.The roughly 25,000 genes in the human genome are composed of DNA packaged into 24 different chromosomes. A gene’s job is either to make proteins or control the activity of other genes. Over many generations, the DNA of a gene can mutate to create a “variant.” A very rare DNA variant is called a "mutation," while a variant that is common in the population is called a "polymorphism."

DNA variants allow for all of us to be a little different from each other. There are about 3 million variants that differ between any two individuals. Variants in certain genes can directly cause a disease like Alzheimer’s, can increase susceptibility to disease or even can confer protection against disease. One’s risk for most age-related diseases such as cancer, diabetes, heart disease, stroke and Alzheimer’s is strongly influenced by our genes. For all of these age-related diseases, we know of mutations that guarantee onset of these diseases with no need for input from any other genes or environmental factors. And we know of polymorphisms that can increase (or decrease) one’s susceptibility to the disease, but without guaranteeing onset of the disease. In this latter case, other genes and environmental factors usually work together to determine when and whether one will get disease. Any gene that can contain a variant(s) that significantly influences one’s susceptibility to Alzheimer’s, whether it be to guarantee the disease or serve to increase (or decrease) risk, is called an “Alzheimer’s gene.” It is important to remember that all genes are “good”; it is only the variants in the DNA of these genes that can influence one’s lifetime risk for a disorder such as Alzheimer’s disease.

In the 1980s and ’90s, my laboratory co-discovered the three known genes that can carry mutations causing early-onset (younger than 60 years of age) familial Alzheimer’s disease.  These mutations are rare, accounting for only 1 percent to 2 percent of Alzheimer’s cases. Inheritance of one of these mutations from just one parent virtually guarantees onset of Alzheimer’s, usually by age 60. If a parent carries such a mutation, each child has a 50 percent chance of inheriting the same mutation and getting early-onset Alzheimer’s disease with virtual certainty before age 60. Genetic testing is available for the early-onset Alzheimer’s gene mutations, but is usually reserved for those who have a family history of early-onset Alzheimer’s.

In the early 1990s, investigators at Duke University found that a common gene variant (polymorphism) of APOE, a fourth affected gene, can increase risk for late-onset (older than 60 years of age) Alzheimer’s disease. This variant is present in about 20 percent of the general population, but in greater than 50 percent of Alzheimer’s patients. Unlike the early-onset AD gene mutations, this variant does not guarantee Alzheimer’s, but only serves to increase risk. Inheriting one copy of the variant (from one parent) increases risk fourfold (vs. the general population), and two copies (from both parents) increases risk tenfold. Importantly, a person can inherit this gene variant from one or both parents and never get Alzheimer’s in the span of a normal lifetime.

With regard to genetic testing for the common late-onset form of Alzheimer’s, we are not yet able to do so reliably. This is because the APOE gene variant is not sufficient on its own to predict one’s risk for Alzheimer’s reliably. Other genes and environmental factors need to combine with the APOE gene variant to cause Alzheimer’s. We do not yet know the full set of gene variants that can increase or decrease risk for Alzheimer’s when inherited together with the APOE gene variant. Thus, APOE gene testing is not recommended as a sole means for predicting Alzheimer’s risk. The other late-onset Alzheimer’s genes must first be identified in order to reliably test for risk for late-onset Alzheimer’s disease.

So how many other Alzheimer’s genes are there?

We know that the four known Alzheimer’s genes account for roughly 30 percent of the inheritance of Alzheimer’s. Thus, 70 percent of the genetics of Alzheimer’s remains undefined. Cure Alzheimer’s Fund (CAF) researchers and others have been engaged in comprehensive projects to find the other Alzheimer’s genes. Once we have all of the Alzheimer’s genes in hand, we will be able to more reliably predict one’s lifetime risk for the common late-onset form of Alzheimer’s disease. However, one might ask, “Why bother to test if there is nothing we can currently do to prevent, stop, or reverse it?” This is certainly a fair question, since we still do not have drugs that stop the disease process in Alzheimer’s. We only have drugs like Aricept and Namenda that modestly and temporarily alleviate the symptoms of cognitive decline, but without affecting the progress of the disease.

We need to find better, more effective therapies for Alzheimer’s, but how do we get there?

First we need to identify all of the genes and variants involved in influencing risk for Alzheimer’s disease. Studies of the known Alzheimer’s disease genes have provided the vast majority of information being used to guide novel drug discovery aimed at preventing, stopping and maybe even reversing Alzheimer’s disease. Every new Alzheimer’s gene defect we find provides new clues regarding the cause of the disease and what we need to do to stop the disease. Thus far, all four genes have pointed to a small protein called “Abeta” as the cause. Abeta is normally made in the brain, but is found in excessive amounts in the brains of Alzheimer’s patients, e.g., in senile plaques that litter the Alzheimer’s brain around nerve cells. Small clumps of Abeta can gum up the connections between nerve cells known as synapses. Billions of nerve cells in the brain form trillions of synapses, making up our neural network. The neural network, in all its complexity, is needed for all brain function, including memory and learning. Excessive Abeta disrupts synaptic communication between nerve cells, leading to loss of memory and learning and eventually dementia. Dementia is defined as global and catastrophic cognitive failure; Alzheimer’s disease is the most common form of dementia in the elderly.

Beyond the original four Alzheimer’s genes.

Most drug discovery for Alzheimer’s today is based on studies of the four original Alzheimer’s genes. But we know there are many more Alzheimer’s genes yet to be identified. Since 2005, CAF has supported a project called the Alzheimer’s Genome Project™, carried out in my laboratory at Massachusetts General Hospital. The goal of this project is to study 5,000 families with multiple members who are affected with the common late-onset form of Alzheimer’s disease in an effort to identify all of the other Alzheimer’s genes. In addition to the Alzheimer’s Genome Project™, the International Genomics of Alzheimer’s Project (IGAP), in which we are members, uses tens of thousands of individual Alzheimer’s cases from the general population in the United States and Europe to find common DNA variants that influence risk for Alzheimer’s. The family-based method of our Alzheimer’s Genome Project™ and the population-based method of IGAP have identified some of the same Alzheimer’s genes, but also find different ones with different effects on risk.

In the family-based studies of the Alzheimer’s Genome Project™, we are able to find not only common DNA variants that influence one’s risk for Alzheimer’s, but also rare mutations that profoundly affect risk for the disease or directly cause it. The Alzheimer’s Genome Project™ places a high priority on finding these rare but very potent gene mutations because, historically, among the four known Alzheimer’s genes, it has been the rare early-onset familial Alzheimer’s gene mutations that have been most effectively guiding drug discovery efforts. This is mainly because hard-hitting mutations have clear-cut adverse effects on biological systems, which can be elegantly recapitulated in animal models. This then allows for more effective drug discovery and development.

One of the first new genes to be identified in the Alzheimer’s Genome Project™ was ADAM10.

This gene was specifically chosen for testing as a potential Alzheimer’s gene because, like the four original Alzheimer’s genes, it affects the production of Abeta in the brain. We identified two rare mutations in this gene that strongly predispose adults to Alzheimer’s disease at around age 70. These two mutations were found in only seven of 1,000 AD families tested. Thus, they are very rare. We recently have demonstrated these two mutations dramatically impair the activity of ADAM10. ADAM10 normally blocks the production of Abeta. Accordingly, we have found these two rare mutations greatly enhance Alzheimer’s amyloid pathology in animal-based models of the disease. With the validation of these mutations to be “pathogenic,” or disease causing, the Alzheimer’s Genome Project™ considers ADAM10 to be the fifth Alzheimer’s gene.

Also beginning in 2005, with CAF support and as part of the Alzheimer’s Genome Project™, we carried out the first family-based “genome-wide association study” for new Alzheimer’s genes. This entailed a screen of the entire human genome in patients and their relatives in thousands of Alzheimer’s families. The first phase of this study was completed in 2008 and led to the identification of more than 100 new Alzheimer’s candidate genes. We reported the top four Alzheimer’s candidate genes from this study in 2008; it was named by TIME/CNN to be one of the Top 10 Medical Research Breakthroughs of 2008.

The CAF Alzheimer’s Genome Project™.

This was the first large-scale study of the human genome performed on the world’s largest collection of families affected by Alzheimer’s disease. It was also the first genome-wide study for Alzheimer’s in the world to discover novel Alzheimer’s gene candidates with statistically significant results and confirmation in thousands of subjects from families with a high incidence of Alzheimer’s disease. The four new Alzheimer’s genes reported by the Alzheimer’s Genome Project™ in 2008 were ATXN1, CD33, GWA14Q34 and DLGAP1. ATXN1 is known to carry mutations that cause another neurodegenerative disease called spinal cerebellar ataxia, a movement disorder. We found that when this gene is inactive, Abeta levels increase dramatically, leading to cognitive decline in mouse models. Another gene, CD33, is perhaps the most interesting, since it controls the brain’s innate immune system and inflammation in the brain. In a related study funded by Cure Alzheimer’s Fund, Abeta was found to play a role in the brain’s innate immunity system. CD33 regulates the brain’s immune system and, concurrently, levels of Abeta. We now are developing CD33 as a drug target for Alzheimer’s based on the genetic findings of the Alzheimer’s Genome Project™. It should be emphasized that without the Cure Alzheimer’s Fund Alzheimer’s Genome Project™, we likely never would have guessed that genes like ATXN1 and CD33 might be involved with Alzheimer’s.

As part of the current activities of the Alzheimer’s Genome Project™, we now are testing these genes, as well as more than 100 other new Alzheimer’s candidate genes coming out of our genome screen, to identify all of the DNA variants and mutations that influence risk for Alzheimer’s in the 5,000 Alzheimer’s families under study in the Alzheimer’s Genome Project™. We specifically are searching for DNA mutations and variants in these genes that very strongly affect risk for onset of Alzheimer’s. As new defects are found in these genes, we not only will increase our ability to reliably predict risk for Alzheimer’s but, more importantly, we will garner new clues regarding the causes of Alzheimer’s and, in doing so, gather new ideas and biological targets for novel drug discovery aimed at preventing, stopping and reversing the disease.

Finally, it should be noted that even if a DNA mutation in an Alzheimer’s gene is rare or restricted to a small subset of families, most researchers think new drugs or therapies for Alzheimer’s based on what is learned from that mutation will be useful in preventing and treating all cases of Alzheimer’s. As noted above, in the Alzheimer’s Genome Project™, we place a high priority on family-based gene studies of Alzheimer’s, since there we have the highest odds of finding DNA mutations with very strong effects on risk for Alzheimer’s, akin to those of the early-onset familial Alzheimer’s disease gene mutations discussed above. These mutations are most useful for driving successful drug discovery, since their biological effects on the disease process are of much greater impact and more clear-cut in terms of mechanism by which they cause disease. They also lend themselves to more useful animal models for drug testing. Ultimately, the full list of Alzheimer’s genes emerging from the family-based genetic studies of the Alzheimer’s Genome Project™ are getting us closer and closer to someday being able to eradicate Alzheimer’s disease using a strategy of early prediction and early intervention. n

Links:

• 2nd Quarter Report - Cure Alzheimer's Fund

One of the most important, outstanding genetic questions about Alzheimer's disease is the relationship between the APOE genotype and the risk associated with the disease.  Following a CAF-sponsored meeting with members of our Research Consortium and several other noted researchers, CAF awarded a grant to David Holtzman, MD, in his lab at Washington University in St. Louis to further study this question.

Understanding the APOE linkage to AD. Although the genetic link has been known for 18 years, the critical links remain a puzzle. Other than age, the strongest risk factors for Alzheimer’s disease (AD) are genetic. The most common form of AD is late-onset AD, which accounts for more than 95 percent of Alzheimer’s cases and typically is defined by cognitive decline and dementia beginning after age 60. In this form of the disease, by far the strongest genetic risk factor for AD is one’s APOE genotype. The APOE gene has several subtle differences, or “alleles,” numbered APOE2 through APOE4. Although the APOE4 genetic variant is found in about a quarter of the population, not everyone with the variant will develop AD. Inheritance of the APOE4 form of APOE is associated with increased risk (three times the normal risk for those inheriting one copy of the variant; about 12 times the risk for those people inheriting two copies of the gene), and the APOE2 form is associated with decreased risk. How the APOE genotype is linked with altered risk to develop AD is the big question.

There is mounting evidence that one of the major reasons APOE is linked with AD is the ability of the APOE protein to influence when the amyloid-beta peptide (Abeta) begins to accumulate in the brain to instigate damage.  

Understanding the APOE linkage to AD. Although the genetic link has been known for 18 years, the critical links remain a puzzle. Other than age, the strongest risk factors for Alzheimer’s disease (AD) are genetic. The most common form of AD is late-onset AD, which accounts for more than 95 percent of Alzheimer’s cases and typically is defined by cognitive decline and dementia beginning after age 60. In this form of the disease, by far the strongest genetic risk factor for AD is one’s APOE genotype. The APOE gene has several subtle differences, or “alleles,” numbered APOE2 through APOE4. Although the APOE4 genetic variant is found in about a quarter of the population, not everyone with the variant will develop AD. Inheritance of the APOE4 form of APOE is associated with increased risk (three times the normal risk for those inheriting one copy of the variant; about 12 times the risk for those people inheriting two copies of the gene), and the APOE2 form is associated with decreased risk. How the APOE genotype is linked with altered risk to develop AD is the big question.

There is mounting evidence that one of the major reasons APOE is linked with AD is the ability of the APOE protein to influence when the amyloid-beta peptide (Abeta) begins to accumulate in the brain to instigate damage.  The specific hypothesis for Dr. Holtzman’s project, inspired by the APOE meeting, is that anti-APOE antibodies that specifically target APOE in amyloid-beta containing plaques in the brain will result in less Abeta accumulation in the brain and decrease Abeta-related pathology, and that this treatment will have fewer side effects than the use of anti-Abeta antibodies.

Dr. Holtzman writes: APOE is the most important genetic risk factor for Alzheimer’s disease. A major reason it appears to act as an AD risk factor is via its effects on Abeta (Aβ) metabolism. In vivo (cell) and in vitro (animal) data strongly suggest that APOE influences both soluble Abeta clearance as well as the probability of Abeta aggregation into clusters of the protein called “oligomers” and “fibrils,” which have been determined to be toxic to neural synapses. These effects of APOE on Abeta buildup in the brain are APOE isoform-dependent. In addition, in the absence of APOE, Abeta can still aggregate in the brain; however, the conversion of Abeta into fibrils and probably oligomers is markedly inhibited. In the brain of a mouse genetically engineered to demonstrate Alzheimer’s pathology (APP Tg mouse), and in humans, APOE strongly co-localizes with both amyloid plaques and the buildup of Abeta in certain blood vessels in the brain, a condition known as cerebral amyloid angiopathy (CAA). All of these facts argue that a treatment that could modify APOE/Aβ interaction may provide a novel treatment strategy for AD.

One way to modify the APOE/Abeta interaction once Abeta aggregates in the brain, or in CAA, is to use antibodies, which are proteins produced by the immune system to protect the organism. Numerous studies have injected anti-Abeta antibodies either systemically as well as intracerebrally into various types of APP Tg mice that develop Abeta accumulation in the brain. Via a variety of mechanisms, many of these antibodies are able to decrease Abeta accumulation when given in a prevention mode, and some of them are able to reduce existing Abeta oligomers and fibrils when given in a treatment mode (after plaques have begun to form). While this is potentially good news for treatment development, systemic treatment with certain anti-Abeta antibodies can also result in vasogenic edema (brain swelling) as well as cerebral hemorrhages in both animal models and in humans. Recent studies have attempted to utilize antibodies to reduce a minor component of Abeta plaques in the brain, such as pyro-glutamate-Abeta. These studies have shown a robust plaque-clearing effect, suggesting that if antibodies can bind to Abeta deposits, even if the component is only a fraction of the total material deposited, this can have a strong effect. To date, no one has published whether antibodies to other components of Abeta plaques, such as APOE, can have useful effects on the total amount of Abeta-deposited, Abeta-associated neuritic damage and inflammation, behavior or potentially any side effects.

Stay tuned for updates and progress on this critical work.

Links:

• Dr. David Holtzman Bio