Alzheimer's disease is one of the most common forms of dementia in the world. Some of the most prominent risk factors for this disease would be age and gender. The older you get, the more likely you are to have damaged proteins that can predispose you to Alzheimer's, and being a female also makes you more vulnerable.
The genetics of early-onset Alzheimer's disease
Although age is considered to be the most significant risk factor for Alzheimer's, genetics do play a role in the likelihood of developing the disease. The best way to explain the heritability of this disease is to look at the two most common forms of Alzheimer's and analyze the genetic link between them.
When looking at the genetics of this disease, numerous studies show that about 60 percent of families with cases of early-onset Alzheimer's will have multiple members of their family eventually diagnosed with the disease. If looking at the inheritance pattern of the disease, approximately 13 percent of families have an autosomal gene for Alzheimer's where at least three generations of the family can expect to have the disease.
This may be a good time for a brief explanation of the idea of genes and inheritance. It can get quite confusing quite quickly, so I will do my best to keep it understandable. Chances are you remember some of your high-school biology classes, and know that there are genes in your DNA that determine your appearance, diseases you may eventually develop, and aspects of your personality. These genes can either be considered dominant, where the majority of people in your family will have similar characteristics, or recessive, where only one family member may have it over a few generations.
Each person will get one of these genes from their mother and one from their father. The gene can be expressed or not, something that depends on whether it's dominant or recessive. A simple formula would be that 75 percent of the genes would be considered dominant and only 25 percent would be recessive if each parent had a copy of one dominant gene and one recessive gene. If a person were to inherit a copy of one dominant gene and one recessive gene, the dominant gene is able to shut off the expression of the recessive gene, so you will not see it in the final product.
If you were able to follow that all the way through, well done. Now it will become a little more complicated. On the surface, this may seem to be a very straightforward process but numerous other factors in our DNA can influence how much these genes can be affected, so it typically will not be these nice three to one ratio of expression.
The genetics of late-onset Alzheimer's disease
The genetics of the more common late-onset Alzheimer's disease are much less obvious. There are much lower prevalence rates in families where a patient develops Alzheimer's after the age of 65. Numerous studies have concluded that the lifetime risk of developing late-onset Alzheimer's about doubles in a person with a first-degree relative diagnosed with Alzheimer's compared to the general population. Due to the complex mechanism of how Alzheimer's occurs, however, numerous genetic and environmental factors are believed to affect gene expression. This makes it almost impossible to guess the genetic pattern of inheritance in this type of the disease.
At this stage of the investigation, all we know about the late-onset form of Alzheimer's disease is that these families tend to have a higher concentration of a specific gene called Apo lipoprotein E. From studies, we know that this gene has some influence on the age of onset and potentially the rate of deterioration, but high levels of the gene do not perfectly correlate with the disease so it is hard to explain the full impact of this gene.
We have seen that in both cases, the early-onset and late-onset form of Alzheimer's, families will have a higher risk of having the disease but there is no guarantee that you will ever develop symptoms. It is a far better plan for you to try to limit other modifiable risk factors that can also increase your risk of Alzheimer's. Controlling long-term conditions like hypertension and diabetes, routine exercise and making sure you keep your brain sharp by continuously trying to learn new things have the biggest protective impact.