Atypical course of Parkinson's disease with clinical manifestations of Huntington's disease in a patient with an allele of 27 CAG repeats in the HTT gene

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disease. Its molecular cause is a cytosine-adenine-guanine (CAG) trinucleotide repeat dynamic expansion in the huntingtin (HTT) gene. Alleles with 36-39 CAG-repeats are incompletely penetrant, as individuals might develop symptoms but typically with a later age of onset. When repeats are equal or greater than 40, the symptoms of the disease occur. It is considered that CAG-repeats in the "intermediate" alleles (27-35 repeats) also cause the symptoms of the HD. We present here the case of a patient who has clinical phenotype and family history of Parkinson's disease (PD), but 27 CAG-repeats. The feature of this patient is early development of non-motor manifestations such as cognitive impairment, psychotic disorders, early dystonia in a hand, camptocormia and poor response to levodopa. It is believed that the intermediate allele of HTT gene might modify the clinical phenotype of PD in this patient.