Alzheimers And Down Syndrome
Down syndrome may give researchers hints at how the 21st chromosome affects the development of Alzheimers disease.
In Down syndrome, an individual has an extra copy of chromosome 21. This 43rd chromosome overall leads to symptoms, such as a flattened facial appearance, slant eyes, short neck, small ears, small hands and feet, and intellectual and/or developmental problems.
Moreover, individuals with Down syndrome develop Alzheimers at a much higher rate than the rest of the population, and at a much younger age.
Since Down syndrome seems to increase risk of Alzheimers disease, scientists believe the APP gene, which is present on chromosome 21, playsa major role in Alzheimers development.
Genetic Testing And Counselling
If someone has a relative with Alzheimers disease and is concerned about whether it may be familial, they should gather as much information as they can about their relatives family history so that they can discuss it with the doctor caring for their relative. The doctor will also be able to refer them to a specialist genetic counselling service if appropriate to discuss their concerns and arrange genetic testing if needed.
The results of any genetic test have important implications not only for the person being tested but also for the rest of the family. It is important to discuss these carefully with a specialist who has expertise in genetic counselling, before any genetic tests are performed.
A genetic test for FAD can be performed on a blood sample and is usually done because several other family members have Alzheimers disease. However, it may also be performed when symptoms or brain scans suggest familial Alzheimers. For more information on the different types of genetic testing and counselling for FAD, please visit the I think I am at risk of FAD. What next? page.
You can find out more about genes and typical Alzheimers disease by visiting the Alzheimers Research UK website. One of our members has shared their journey undergoing genetic counselling and testing for FAD. You can read Jess story.
You may also find visiting our FAQs section useful.
Living with FAD: A short film
I think I am at risk of FAD
Living with FAD
What Is The Difference Between Alzheimer’s And Dementia
First, some explanation of dementia vs. Alzheimer’s. According to the Alzheimer’s Association, dementia is a general term for a decline in mental ability that interferes with daily life. Not a normal part of aging, most dementias are typically caused by damaged brain cells.
Of all the dementias, Alzheimer’s disease is the most common, accounting for 60-80% of the cases. In other words, it is a specific disease while “dementia” is a general term for a life-altering decline in brain function .
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How Can Genes Cause Dementia
Most often, dementia is caused by a complex disease in which genes are only one factor. When this happens, the dementia develops as a result of many different factors. In these cases, genes do not directly cause it to develop.
It is not possible to directly inherit dementia when it is caused by a complex disease.
Blessing From A Curse: This Family’s High Rate Of Alzheimer’s Might Help Researchers Find A Cure
Alzheimer’s disease often runs in families, but the risk that runs through the Chastain family, which has been working with Emory, is higher than researchers have encountered before. Do family members hold clues encoded in their DNA that can help ward off this looming epidemic?
More than a decade ago, a woman in her early 70s came to see neurologist Allan Levey for an evaluation. She was experiencing progressive memory decline and was there with her children.
Part of the evaluation involved taking a family history. One of the womans sisters had died with dementia and an autopsy had confirmed Alzheimers disease. But it was the previous generation that captured Leveys attention. Her father was one of nine children, and all had dementia. And three family members had had autopsies where Alzheimers was confirmed, says Levey, director of Emorys Alzheimers Disease Research Center.
Every one of the fathers generation was affected, he says. This defied what we knew about even the strongest genes.
The Lowman family recalls touching moments from more than a decade of caring for their mother, Mildred “Lily” Chastain Lowman , who died of Alzheimer’s disease at age 78. Her daughters say she loved the outdoors, gardening, and family, and carefully tended to her prize rosebush that her daughter now maintains.
Additionally, family members tend to develop not early-onset Alzheimers but the much more common late-onset type, with symptoms emerging in their 60s and 70s.
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Areas Of Future Research
According to the National Institute on Aging, here are some questions to answer in future research into the association between Alzheimers and genetics:
- How genetics interact with lifestyle factors to affect Alzheimers risk
- How the disease process begins
- Why Alzheimers is the most common cause of dementia symptoms
- What the difference in Alzheimers between various racial groups and sexes
- How to prevent Alzheimers
- What treatments are possible and effective
- Who is most at risk for Alzheimers, and why
Genes And Dementia With Lewy Bodies
Dementia with Lewy bodies is caused by a build-up of abnormal proteins in the brain and may have symptoms similar to those seen in Parkinsons disease. Age is currently the biggest known risk factor for dementia with Lewy bodies, although research is underway to find out whether genes may also play a role.
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Genes Which May Influence Alzheimers Disease
Having a close relative with the Alzheimers disease is not evidence of a genetic link. People who are influenced by risk factor genes are only at a slightly increased risk in developing the disease than the average population.
The most important gene discovered to date is the Apolipoprotein E gene, which is found in chromosome 19. This gene occurs in three forms in humans: types 2, 3 and 4. Every person in the world carries two Apolipoprotein genes: they can be the same type , or a mixture of two types . What has been found is that people with at least one type 4 and especially those with two, such as 4,4, are at an increased risk of developing Alzheimers disease earlier in life than those with the other types of Apolipoprotein E. Nevertheless half of the people aged 85 who have 2 copies of apolipoprotein E 4 do not have symptoms of Alzheimers disease at that age.
People with type 2, especially 2,2, appear to be protected against developing Alzheimers disease, until much later in life. Researchers do not understand why this is so, and there is much research underway to find out why.
The type of Apolipoprotein does not mean definitely that Alzheimers disease will or will not occur. Indeed it is known that some people can reach 90 with type 4 and not develop dementia, whereas others with type 2 can develop dementia much earlier in life. What this means is that the type of Apolipoprotein a person has, is not enough on its own to cause Alzheimers disease.
Genetic Causes Of Dementia
One rare form of Alzheimers disease is passed from generation to generation. This is called Familial Alzheimers disease . If a parent has a mutated gene that causes FAD, each child has a 50% chance of inheriting it. The presence of the gene means that the person will eventually develop Alzheimers disease, usually in their 40s or 50s. This form of Alzheimers disease affects an extremely small number of people probably no more than 100 at any given time among the whole population of Australia.
Three genes have been identified which, if mutated in certain ways, will cause FAD. These are called presenilin 1 , presenilin 2 and the amyloid precursor protein gene on chromosome 21.
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What Is A Gene Variant
The same gene can differ between individuals these are known as gene variants. These can help to explain why people are different to each other for instance, why one person has blue eyes and another person has brown eyes.
If a gene variant increases a person’s risk of developing a disease, it is known as a ‘risk variant’ for that disease.
Age At Onset As A Highly Heritable Factor In Ad
AAO is highly heritable in AD families. In early-onset cases, mutations in three genes are known to account for around half of familial cases : Amyloid Precursor Protein , Presenilin-1 and Presenilin-2 . However, the genes involved in AAO variance mostly remain to be identified linkage and candidate gene studies have established several possible associations with different loci, but the only consistently replicated modulator of AD AAO, in both familial and sporadic cases, is also the strongest genetic risk locus for the development of the disease . More recently, Naj et al. performed a large genome-wide association study in over 9000 patients to detect effects of known AD risk loci in modifying AAO. They confirmed the association of APOE4 allele with earlier onset and identified associations with CR1, BIN1 and PICALM. Burden analyses showed that APOE contributed 3.7 % of AAO variation the other nine loci studied contributed 2.2 % when considered together .
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People Affected By Dementia Are Often Concerned About Whether The Condition Can Be Passed Along In Families
Here we discuss the role of heredity in Alzheimers disease and other forms of dementia.
Dementia is the term used to describe the symptoms of a large group of illnesses which cause a progressive decline in a persons functioning. It is a broad term to describe a loss of memory, intellect, rationality, social skills and what would be considered normal emotional reactions. Alzheimers disease is the most common form of dementia, accounting for 50% to 70% of all cases of dementia. It occurs relatively frequently in older people, regardless of family history. For females aged 65 to 69 years dementia affects 1 person in 80 compared to 1 person in 60 for males. For both males and females aged 85 and over the rate is approximately 1 person in 4.
Genes: A Blueprint For Health And Disease
The genes encoded in our DNA are profoundly involved in many aspects of our health. They act as a sort of blueprint for the construction, operation, and repair of our bodies throughout life. Genes provide instructions for the creation and regulation of our bodys building blocks. We inherit one copy of each gene from each of our parents, which is one reason why every person has a unique appearance and metabolism.
The effects of a gene can be dramatically changed by mutation of even one pair of its molecules. Specific versions of genes, called alleles, are passed down through a familys lineage, potentially creating an entire population of people who share a healthy characteristic such as resistance to cancer a more neutral characteristic such as eye color or a heightened risk for a specific disease such as Alzheimers disease .
Many of us have learned about Mendel, who was an Austrian monk, and the pea plants he bred as he discovered basic principles of genetic inheritance. Just as with Mendels plants, some physical characteristics can be transmitted to our offspring through inheritance of even one version of a gene. Many diseases are inherited this way, too.
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Genetic Testing For Alzheimer’s Disease
A blood test can identify which APOE alleles a person has, but results cannot predict who will or will not develop Alzheimer’s disease. Currently, APOE testing is used primarily in research settings to identify study participants who may have an increased risk of developing Alzheimer’s. This knowledge helps scientists look for early brain changes in participants and compare the effectiveness of possible treatments for people with different APOE profiles.
Genetic testing is also used by physicians to help diagnose early-onset Alzheimers disease and to test people with a strong family history of Alzheimers or a related brain disease.
Genetic testing for APOE or other genetic variants cannot determine an individuals likelihood of developing Alzheimers diseasejust which risk factor genes a person has. It is unlikely that genetic testing will ever be able to predict the disease with 100 percent accuracy, researchers believe, because too many other factors may influence its development and progression.
Some people learn their APOE status through consumer genetic testing or think about getting this kind of test. They may wish to consult a doctor or genetic counselor to better understand this type of test and their test results. General information about genetic testing can be found at:
How Can Genes Cause Disease
Genes often play a role in the development of diseases. This can happen in two main ways.
Single-gene diseases
Sometimes, a gene changes and this causes a disease. This is known as a single-gene disease because it is caused by a change in just one gene. Single-gene diseases are often serious, but they are rare.
It is possible to directly inherit a single-gene disease. This means that if a child inherits the same changed gene that a parent has, they are very likely in some cases almost certain to develop the same disease.
Complex or multiple-gene diseases
Complex diseases are less straightforward. They develop through the interaction between several different factors. These are known as risk factors for that disease. They include:
- the persons environment or lifestyle for example, their diet and whether they smoke or not
- multiple gene variants known as risk variants because they increase the persons risk of developing the disease.
It isnt possible to inherit a complex disease in the same way as a single-gene disease. This is because complex diseases are caused by multiple factors acting together.
However, a person with a family history of a complex disease will often have an increased chance of developing it. This is because they are likely to share risk factors with other birth relatives. Despite this increased chance, the person still might never develop the disease.
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Family History By The Numbers
Studies of family history say that if you have a close relative who has been diagnosed with Alzheimer’s diseasethe most common form of dementia in older adultsyour risk increases by about 30%. This is a relative risk increase, meaning a 30% hike in your existing risk.
If you are age 65, the risk of being diagnosed with Alzheimer’s is 2% per year, although this also means a 98% chance per year of not developing Alzheimer’s. In absolute numbers, a 2% annual risk means that two out of 100 65-year-olds will develop dementia every year.
Family history raises the 2% annual risk by about 30%, to 2.6% per year. That means going from 20 cases in a group of 1,000 to 26 in 1,000, or six additional cases in 1,000. “So the absolute increase is relatively small,” Dr. Marshall says.
Age raises the chance of Alzheimer’s more than family history. People in their 70s have a 5% chance of being diagnosedmore than twice that of people in their 60s. Family history raises this by 30%, from 5% to 6.5%. Again, the absolute change is relatively small.
Definition Of Associated Loci
A region of ±500kb was defined around each variant with a stage I P value below 1×105. These regions were then merged to define nonoverlapping regions. The region corresponding to the APOE locus was excluded. We then used the PLINK clumping procedure to define independent hits in each region. An iterative clumping procedure was applied to all variants with a stage I P value below 1×105, starting with the variant with the lowest P value . Variants with a stage I P value below 1×105, located within 500kb of this index variant and in LD with the index variant were assigned to the index variants clump. The clumping procedure was then applied until all the variants had been clumped. LD in the EADB-TOPMed dataset was computed using high-quality imputed genotypes.
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What Is Alzheimer’s Disease
Alzheimer’s disease is an irreversible brain disorder. It is a progressive condition that causes issues with memory, cognition, and behavior.
It was first discovered in 1906 by Dr. Alois Alzheimer, a German psychiatrist and neuropathologist. He examined the brain of a woman who had died following symptoms including memory loss, language problems, and altered behavior. He found unusual clumps in her brain, known as amyloid plaques, as well as tangles in her brain fibers that are some of the main features of Alzheimer’s disease.
Since this initial discovery, many other complex changes in the brain have been discovered among people suffering from disease, including the loss of connection between neurons.
Is Alzheimers Genetic Or Hereditary Heres What We Know
Alzheimers disease is not completely genetic or hereditary. In very rare cases, a specific gene can directly lead to Alzheimers. In more common cases, having a first-degree family member with Alzheimers increases your risk but does not guarantee you will develop Alzheimers.
Being older than 65 is the most common non-genetic risk factor for developing Alzheimers disease.
Simply put, having a family history of dementia makes it more likely that you will experience dementia. However, people with no family history still develop Alzheimers and people with a robust family history of dementia may not get it.
Alzheimers disease is a neurodegenerative disease and the most common form of dementia, which results in memory impairment and general cognitive decline. About 6 million people have Alzheimers disease in the United States alone, including those who developed it at a younger age.
It is caused by the buildup of beta-amyloid plaque and tau tangles in the brain, destroying brain cells . These buildups may be influenced by many factors:
- Environmental factors
- Other unknown factors
C2N Diagnostics released a simple blood test in 2020 that can measure levels of amyloid plaque particles in your blood plasma, as well as determine if an individual possesses the APOE gene that increases your risk of developing Alzheimers.
Lets dive into everything you need to know about the genetics behind Alzheimers disease.
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