A gene provides the code, or blueprint, for the type and order of amino acids needed to build a specific protein. Everything in the body is made up of proteins. Bones and teeth, muscles and blood, for example, are formed from different proteins. Genes direct almost every aspect of the construction, operation, and repair of all living things. For example, genes carry information that determines eye and hair color and other traits inherited from our parents. Genes also ensure that we have two hands and can use them to do things, like play the piano.
A healthy body depends on the proper interaction of thousands of proteins (as well as other molecules such as lipids and carbohydrates), in just the right place, and in just the right amount. Malfunctions in proteins are sometimes caused by an alteration, or mutation, in the gene that directs the creation of the protein. As many as 5,000 diseases, such as early-onset Alzheimer's disease, cystic fibrosis, Huntington disease, and sickle cell anemia, are believed to result from mutated genes inherited from either biological parent.
Human cells usually contain 46 chromosomes, which are individual strands of deoxyribonucleic acid (DNA), 23 from each parent. A chromosome is a threadlike structure which can carry hundreds, sometimes thousands, of genes. The number of genes in human beings is still not known, but is probably between 30,000 and 100,000.
DNA consists of chemical bases called nucleotides. Each nucleotide contains one chemical base, one phosphate molecule, and the sugar molecule deoxyribose. Each strand of DNA is made up of millions of sequences of nucleotides.
The bases in DNA nucleotides are adenine, thymine, guanine, and cytosine, hence the letter sequence A, T, G, C. The order - combinations and repeats - in which these bases occur determines the genetic code that directs the construction of a specific protein. A single misspelling in a sequence can cause a hereditary disease. Changes in a person's DNA can also occur during his/her lifetime, through mistakes in cell division, or influences of environmental toxins or radiation, as well as by inheriting the change from one or both parents.
The search to identify the genes that may be linked to late-onset AD will present many challenges. Long stretches of DNA strands appear to have no function, at least as far as scientists can tell presently. Some genes only have a particular function at a particular time of a person's life. The only known risk factor gene for late-onset AD is a form (allele) of the apolipoproteinE (apoE) gene. Scientists are studying several regions in chromosomes that have shown potential for links to late-onset AD, but they will need many more DNA samples to achieve positive identification.
The potential benefits of finding these other AD risk factor genes are numerous. They will aid in diagnosis, prognosis, prevention, and targeted drug treatments.
Until more is known about late-onset AD, both the NIA and the Alzheimer's Association support apoE testing for diagnostic purposes only in conjunction with other tests to evaluate people who have symptoms of AD. People who learn that they have an increased risk of AD may experience emotional distress and depression about the future because there is not yet an effective way to prevent or cure the disease.