The Genetics of Alzheimer’s: Be Careful About Inferring Too Much
Many individuals often refer to the underlying genetic basis of Alzheimer's disease. The inference that many individuals make is genes are destiny. In other words, once a person holds the given genetic makeup they ultimately assume or infer the underlying quality that the gene has coded for will undoubtedly manifest itself in the phenotypic quality, which in this case is Alzheimer's disease. However, one has to be cautious about inferring an inevitable cause and effect between genes and disease.
In understanding the underlying genetic basis of Alzheimer's disease, we do know that genes play an important role. During the last couple of decades genes discovered on chromosomes 14, 19, and 21 have been implicated in this disease. However it appears that the genetic influence on chromosome 19 appear to be the most significant. On chromosome 19 there are a family of genes called the APO genes, which is short for the apolipoprotein genes. The subfamily of genes called the epsilon genes, often referred to as E2, E3, and E4, are the ones that appear to be the most significant for memory. In fact, these genes are also related to controlling triglycerides and cholesterol in the body and are therefore pleiotropic, meaning that a single gene can have an affect on many different body areas.
Of the above epsilon genes, the E4 type appears to be the most insidious, especially toward Alzheimer's disease. Genes come in pairs called alleles. Having one E4 allele is bad but having two matching E4 genes is highly problematic, or as many have come to infer, a death sentence. But one has to remember that genes do not automatically dictate cause and effect qualities for diseases. Genes, like switches, have to be turned on. If they are not turned on they cannot express themselves. Most individuals do not carry two E4 alleles. For those that do the probability for getting Alzheimer's increases dramatically, but, and this is an important but, some individuals do have two E4 alleles in old age yet are fully competent and have acute intellectual ability. Therefore, the first thing that this demonstrates is there is not an automatic, 100 percent likelihood of obtaining Alzheimer's disease even if you have two E4 alleles.
Also, people of Asian descent often have the least likelihood of having two E4 alleles, followed by the white population and the African American population. Subsequently, one would therefore expect then that African Americans would have a greater likelihood of obtaining Alzheimer's disease, especially the early-onset type, given their greater predisposition for holding two E4 alleles. But here again, things do not fit nicely into the larger scheme of the disease. Those who are most likely to acquire Alzheimer's disease are white individuals. Furthermore, just as some individuals who have both the E4 alleles do not acquire Alzheimer's disease, some individuals that do not even have one E4 end up developing Alzheimer's disease quite early in their lives. What this appears to be demonstrating is that Alzheimer's disease is polygenetic, with often more than just one type of gene involved in the process.
What I hope that I have demonstrated is that many individuals that think their genetic predisposition is ultimately their destiny are wrong. Yes, we do know that certain genetic profiles, such as being homozygous for the E4 allele (having both the alleles), make a person more likely to develop Alzheimer's disease and possibly earlier in life than others. However, one's underlying genetics are not a mandate that ultimately provides 100 percent certainty that a person will acquire the disease. There are too many more elements that appear to be involved in developing Alzheimer's disease, and although genetics is important, it by know means provides the total picture.
An ethical question that often has come up with our new knowledge of genetics, especially gathered in tracing the entire genome, is if we are able to know where the genes are located for this type of disease, why not test the person for them to provide individuals with information about their potential genetic predisposition for the disease. However, one question that needs to be posed is what potential benefit would be gained if someone knew his or her genetic makeup? Given that there currently is no cure for the disease, could harboring the knowledge that one carries the genetic predisposition actually cause more harm? Harboring the knowledge of having the genetic predisposition could lead to psychological turmoil for a person, often impairing the quality of their life as they continue to live out their days waiting for the first evidence of the disease.
Also, as was mentioned, even if the genetic testing revealed that the person had the most insidious genetic makeup, there is no guarantee that the person will ultimately get the disease. The genetic makeup often creates a predisposition, not a guarantee, yet we often view the predisposition of the genetic makeup as an inevitable end result of certainty. Therefore the person that obtains the test results and spends his or her days waiting for the first impending signs of the disease may be doing so for no reason, since the test cannot predict with certainty that you will acquire the disease.
In an age when there are many tests that are available and are very important and well used for preventative purposes, our current ability to test for a genetic basis of Alzheimer's disease may not be one of them. Given that there are currently no cures for the disease, that a positive test is not a guarantee of acquiring the disease, and that there are probably numerous other factors that work in concert with a genetic predisposition, the ethical implications for harming the individual psychologically could be great. Although our current knowledge to pinpoint important genes is great, and often this scientific knowledge can be quite beneficial, we also have to be aware that sometimes attempting to interject too much science in our lives can do more harm than good.