Effect of CAG repeat length on psychiatric disorders in Huntington’s disease

There is strong evidence that the length of CAG repeats, in patients with Huntington’s disease (HD), govern the age of onset and the rate of clinical progression of neurological symptoms. However, psychiatric manifestations of the disease have not been examined as comprehensively. Seventy two Greek patients with Huntington’s disease had DNA testing and were clinically assessed by means of a semi-structured interview (SCID) and four self-rated questionnaires. Genotype–phenotype correlations were examined. The CAG repeat length had a significant negative association with the age of onset of psychiatric disorders, the total level of functioning and the MMSE. However, the probability of developing a psychiatric disorder and the severity of psychiatric symptoms were not determined by the trinucleotide expansion, after controlling for the duration of illness, sex, and age of the subjects. The factors that determine the development of psychiatric symptoms in HD patients seem not to be limited to a dose related toxicity of the expanded Huntington. It is hypothesized that alternative genetic or environmental factors underlie the pathogenesis of the psychiatric phenotype.