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The UK Huntington's Disease Consortium has reported the combined results, from all UK centres, of predictive testing for Huntington's Disease over the 10 years from 1987 to 1997 [Harper, P.S. et al (2000) J Med Genet 37, 567-571 (Abstract)]. Before 1993, testing had to be done indirectly using markers linked to the gene, and required samples to be taken from several family members. Since the identification of the Huntington's Disease gene in 1993, and the discovery that there is virtually no genetic heterogeneity, that is, all those with the disease have the same type of mutation in the same gene, testing has been both simpler and much more accurate. The Consortium reports that 2937 presymptomatic tests were carried out over the 10-year period, 85% of them based on direct analysis of the mutation. It is difficult to know exactly what percentage of people at risk of Huntington's Disease have taken up the offer of testing, but the Consortium estimates that about 18% of those at 50% prior risk (those with an affected parent) have done so. About 40% of tests results showed that the individual tested carried the Huntington's mutation. The analysis of testing over the 10 year period shows some interesting features, including a significant excess of women among those who elect to be tested (almost 60% were women) and a surprisingly high percentage of mutation-positive results (almost 30%) among people over 60 years old, indicating that the age of onset of the disease can be very variable. The Consortium suggests that experience with predictive testing for Huntington's will prove useful as testing becomes possible for more late-onset neurodegenerative diseases.

Comment: The results of this survey confirm the general experience that uptake of genetic testing for late-onset conditions, particularly untreatable ones, is quite low. The protocol for predictive testing for Huntington's, which includes extensive counselling, provides a good model for predictive testing for other rare, late-onset single-gene diseases. It is less easy to extrapolate to a possible future where testing may be available for predisposition to more-common late-onset diseases, where results will almost always be much less clear-cut. Here, the Huntington's counselling protocol may be less useful as a model: on the one hand, such rigorous psychological counselling might not be necessary when the result is likely to be much less dreadful, but on the other hand the complexity of the underlying genetics of common disease and the strong role of environmental factors mean that most people will need careful explanation of exactly what the result means for them.