Proceed with care: direct predictive testing for Huntington disease

The cloning of the Huntington disease (HD) gene allows highly accurate predictive testing using direct analysis of the CAG repeat. This new test provides results with almost complete certainty but poses unique counseling issues related to direct testing for an adult-onset disease. These include testing individuals who are at 25% risk, without the need for blood from a 50% at risk relative; the assessment of symptomatic individuals; the need for ongoing counseling despite simplification of laboratory procedures; and counseling of persons from families who represent a new mutation for HD. This paper describes protocols for direct predictive testing for adult and prenatal assessment, on the basis of the experience of the Canadian Collaborative Study on Predictive Testing (CCSPT). Over the past 8 years, we have provided > 400 results by using linked markers and, more recently, 416 results by using direct assessment of CAG expansion in the HD gene. The vast majority (86%) of requests for direct predictive testing have been from persons who have not previously received results by using linked markers. Despite the ability to now directly assess for the mutation associated with HD, we still recommend assessment of DNA from an affected relative, as this may significantly enhance the accuracy of information to be provided to the at-risk individual. Distance from a genetics center has previously limited the availability of testing, and therefore we have developed approaches to providing predictive testing in the patient's own community.