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 40 or more such repeats. Some
individuals have a gene that has between 27 and 39 repeats, which is considered an
intermediate result and may be 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 has an
intermediate repeat size between 27 and 39.
A geneticist will analyse the number of repeats in a very small sample of DNA, which
can be obtained from blood samples and from most tissues.
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.