Monozygotic twins suffering from Huntington's disease show different cognitive and behavioural symptoms.

Monozygotic male twins, carrying the same number of trinucleotide repeats in the IT 15 Huntington disease (HD) gene, showed a different clinical course. Patient 1 presented with anxiety and chorea at the age of 40. Patient 2 showed persecution paranoia and motor impersistence at the age of 42. Both patients were monitored for 30 months using currently recommended motor and behaviour scales. No differences were observed in motor scoring besides small interevaluation fluctuations. However, on the cognitive and behaviour scales, patient 1 showed a significant worsening when compared with patient 2. Our cases support the belief that the motor symptoms and signs in HD are highly dependent on the trinucleotide expansion. However, the differences in the evolution of mental status in our patients suggest that other still unknown environmental factors are important in the phenotypic expression of Huntington’s disease.