DNA (deoxyribonucleic acid) , the long, spiraling, double helix structure that is found on the 23 pairs of chromosomes human beings normally possess, is a blueprint for human characteristics. 

New gene diagnosis methods make it possible to find the chromosomal location of genes that are responsible for neurological and psychiatric conditions and to identify structural changes in these genes that are responsible for causing disease. This information can be quite useful for identifying individuals that carry ‘faulty’ genes. Through understanding the precise cause of a diseases, methods of prevention and treatment can be improved. Furthermore, the susceptibility and malignancy of certain tumors can be evaluated.

Current knowledge

So far, defective genes for more than 50 neurological disorders have already been identified and the chromosomal location of the defect for up to 100. For many of these illnesses, prenatal and carrier tests have been developed.

• Huntington’s Disease

For example, the gene that goes ‘rogue’ in Huntington’s disease has been located on chromosome 4. The defect is an expansion of a CAG repeat. CAG is the genetic code for the amino acid glutamate and the expanded repeat results in a long string of glutamines in the protein. This alters the function of the protein.

• Alzheimer’s Disease and Down Syndrome

The gene coding for the beta amyloid precursor that is abnormally cut to form the smaller peptide beta amyloid, has been located on chromosome 21. It is this peptide that accumulates in the plaques that clog the brain of patients with Alzheimer’s disease. This discovery has also shed light on why individuals with Down syndrome invariably accumulate amyloid deposits. They carry three instead of two copies of this gene and thus produce too much amyloid. Mutations in this gene have been shown to underlie Alzheimer’s disease in a distinct subset of these patients.

Several other genetic factors have been identified in Alzheimer’s disease, including two genes coding for the proteins presenilin 1 and 2, located on chromosome 14 and 1, respectively. A risk factor for late-onset Alzheimer’s is the gene for the apolipoprotein E protein located on chromosome 19.

Future Developments

• The characterizations of the structure and function of individual genes causing diseases of the brain and nervous system are still in the early stages. Factors that determine the variations in the genetic expression of a single-gene abnormality are still largely unknown.
• In addition to further research in that area, scientists are also studying the DNA in mitochondria, structures outside of the cell nucleus that carry their own DNA and are responsible for the energy production in the cell. Recently, several mutations in mitochondrial genes are found to cause several rare neurological disorders. It has been speculated that inheritable variation in mitochondrial genes may play a role in diseases such as Alzheimer’s, Parkinson’s and some childhood diseases of the nervous system.