Genetic testing for Huntington's disease: Past, present and future. How could genetic data be used to improve clinical practice?

The identification of the repeat expansion which causes Huntington's disease in 1993 soon led to a clinical genetic test for the condition, enabling people at risk to have a test to determine whether they will get the disease. The primary determinant of age at onset in Huntington's disease is CAG repeat length, but in recent years there have been advances in identifying and characterising genetic modifiers which influence age at onset. This has led to the question of whether these data may be applied clinically to improve clinical practice. Here, on behalf of the European Huntington's Disease Network (EHDN) Genetic Testing and EHDN Genetic Modifiers Working Groups, we review the current state of genetic testing for Huntington's disease and consider the personal impact that pre-symptomatic genetic testing has on those that undertake it. We then discuss how genetic information could be used to improve onset prediction clinically, and whether it could be applied in clinical trials stratification. We conclude by proposing short, medium and long-term recommendations to improve the use of genetic data to in clinical practice and clinical trials.