Background HD is relatively rare. Thus we need advanced, multi-centre, multi-national research frameworks to study simultaneously multiple complementary aspects of HD.

Methods/techniques First cross-sectional data cut of the first 1766 participants from 13 European countries of REGISTRY, the European Huntington's Disease Network's (EHDN), multi-lingual, multi-national prospective observational study of HD in Europe.

Results/outcome The phenotype, and the HD genotype, of manifest HD participants across different European regions was similar. Motor onset was most common (48%); however, a non-motor onset was observed in more than a third of participants. Motor signs increased, and cognitive abilities and functional capacity declined, as the disease burden ((CAG_n-35.5) x age) increased. A life-time history of behavioural symptoms was common, but the behavioural score was not related to disease burden. Severe psychiatric problems were also common including suicidal ideation and attempts, and/or irritability/aggression, in one fifth of participants with psychosis being less common. Participants on anti-dyskinetic medication were more likely to have a higher motor score, were older, more prone to trauma and had a lower cognitive score. Motor score and cognitive score were the best predictors for HD disease stage, with a higher motor and a lower cognitive score predicting more advanced disease.

Conclusions The hypotheses generated here can be addressed using the unparalleled collection of clinical data and biomaterials within EHDN's REGISTRY. This initiative will also expedite the search for disease modifiers (genetic and environmental) of age at onset and disease progression that could be harnessed for the development of novel treatments.