Predictive Testing

(taken from the Canadian HD  Web Site)

Predictive testing for Huntington disease is a complicated process, both techncially and personally. It is only available to individuals 18 years of age or over. Detailed information is provided in an HSC booklet.

Technical Aspects

In most instances, a blood sample from the at risk individual and an affected family member will be required to perform the predictive test to determine whether or not the HD gene has been inherited. In some cases, it may be possible to test only the at risk person. All predictive testing is based on recognizing patterns within genes.

Each cell of the human body contains 46 chromosomes, arranged in 23 pairs. In each pair, one chromosome comes from the mother and the other from the father. Chromosomes are made up of small units of genetic material called genes. The gene for HD is located on chromosome number 4.

Genes are made up of DNA (deoxyribonucleic acid.) DNA-molecules consist of chains of four small elements called bases. There are four bases: A (adenine), T (thymine), G (guanine) and C (cytosine.) Their order constitutes a code which determines the type of protein that the particular gene produces. Any change in the sequence of bases may cause a problem in the functioning of that protein.

The gene causing HD has been shown to have a region in which three of the bases, specifically C, A and G, are repeated many times. In the normal gene, this region contains approximately 10 to 30 CAG repeats, (that is, a repeat of C-A-G, 10 to 30 times.) In the gene causing HD, this region contains approximately 40 or more such repeats. Some individuals have a gene that has between 30 and 40 repeats, which is more difficult to interpret.

To complicate matters further, because genes come in pairs, each person will have a certain number of repeats in one gene and another number of repeats in the other. The repeat sizes can be the same (such as 18 in both genes) or different (such as 18 repeats in one gene and 42 in the other.)

In a small number of families, predictive testing may not even be possible. An example: the clinical diagnosis in the family is HD, but none of the affected individuals in the family demonstrates the increased number of repeats found in the HD gene. The family could have been misdiagnosed with HD or there could be a change in this gene that we do not yet know how to detect or the at risk person may inherit a gene that has a difficult-to-interpret repeat size between 30 and 40.

A geneticist will analyze the number of repeats in a very small sample of DNA, which can be obtained from blood samples and from most tissues

Personal Considerations

The decision about whether or not to take the predictive test is a deeply personal one. The at risk person must balance the advantages and disadvantages of knowing whether s/he is likely to develop Huntington disease.

At the present time, there is neither a cure nor adequate treatment for HD. For that reason, the vast majority of at risk people choose not to be tested. However, some individuals see testing as an opportunity, even though it is frightening.

Generally speaking, it would appear that the people who cope best are those who have known about HD from childhood and have discussed it freely and openly with relatives and close friends. They often have a remarkable appreciation for happiness and quality in life and a deep understanding of certain aspects of life which many others take for granted. Out of possible adversity comes an enriched perspective and a whole new range of more satisfying values and relationships.

Counselling is a vital part of predictive testing, both before and after the actual test. Before testing, a genetic counsellor helps the individual explore reasons for taking the test, and prepare for receiving results. After testing, counselling is geared to offering support and assistance in understanding the impact of the either negative or positive test results on the individual's life and that of the family.