After any dementia diagnosis, it’s only natural to think about risks for your family. The good news is most forms of dementia are not hereditary. But some are. Understanding some basic things about the heritability of dementia may help you determine if you should seek genetic testing.
Understanding Genetic Risk
Sometimes, a single gene is all it takes to develop a disease. For example, if you receive one copy of the HTT gene from one parent, then you have a 50% chance of developing Huntington’s disease. And if you receive a copy of that gene from both parents, you’re guaranteed to develop the disease.
There are thousands of dangerous single-gene mutations like Huntington’s. Affecting a single gene makes them easy to test for, but most genetic disorders aren’t so simple. Most dangerous mutations are multifactorial, meaning the result of large groups of genes interacting with each other and with their environment.
Even though there’s no such thing as a tall gene, as much as 80% of a person’s height is determined by genetics. Likewise, the genetics of dementia can work the same way, which makes testing for these types of genes more complicated and less predictive.
Can You Inherit Dementia?
Is dementia hereditary? It depends on the type of dementia. There are rare forms of highly heritable dementia, and there are more common genes that may increase your risks. The majority of all cases are not hereditary.
Is there genetic testing for dementia? Once again, it depends on the type of dementia. Testing will only show the presence or absence of a gene. You can test for a single-gene disease like Huntington’s and end up with practical information about your chances. But most of the time, the mere presence of a gene will not give you any clear idea.
Types of Dementia
Four of the most common types of dementia are vascular dementia, Lewy body dementia, frontotemporal dementia (FTD), and Alzheimer’s disease. With respect to Lewy body dementia, about 10% of cases are believed to be hereditary. Hereditary forms of vascular dementia are rarer still, though an extremely small number of cases are caused by mutations to the NOTCH3 gene.
One of the most heritable forms of dementia is frontotemporal dementia. Between 30%-50% of all cases can be traced to genes called MAPT, GRN, and C9ORF72. If you possess these genes and live to old age, you will develop dementia. One in ten people suffering from FTD have an extensive family history of the disease.
It’s estimated more than 99% of people with Alzheimer’s did not inherit it genetically. There are a number of genes that can make small contributions to your risk of Alzheimer’s. The strongest is called ApoE. People with two copies of the ApoE4 gene, one from each parent, have been found to be at greater risk to develop late-onset Alzheimer’s.
In other words, having someone in the family who has suffered from Alzheimer’s does slightly increase the risks faced by later generations. But it’s far from guaranteed. The one exception is young-onset Alzheimer’s disease, which is highly heritable. With symptoms starting in as early as your 20s or 30s, young-onset Alzheimer’s can be traced to mutations in genes called PSEN1, PSEN2, and APP.
Having a family member suffering from dementia could mean you’re at greater risk. But that information alone can’t tell you anything about your chances. You can get tested for heritable forms of dementia, but unless someone in your family has been diagnosed with one of the rarer forms of dementia, genetic testing is unnecessary and will tell you little.
With most forms of dementia, genetics are not the leading risk factor. The leading risk factor for Alzheimer’s is age. Although we can’t control risk factors like age, there are many we can control. Quitting smoking, eating a healthier diet, and doing a better job of managing your health conditions can have far greater impact than your genes alone.