Objective: We report a case of probable symptomatic Huntington’s Disease (HD) in a patient with intermediate range trinucleotide repeats.

Background: Huntington’s Disease is an autosomal dominant condition caused by more than 40 CAG trinucleotide expansions on the short arm of chromosome 4p16.3, in the Huntingtin gene. The disease has reduced penetrance between 36 and 39 repeat and typically does not manifest in those with less than 27 repeats. There is an intermediate range between 27 and 35 repeats with case reports of patients with the clinical phenotype of HD, but none with asymmetric signs and symptoms.

Method: This patient’s clinical course and neurologic exam were monitored over a period of three years. Serial MRIs were obtained. Stepwise genetic testing was planned to evaluate for diagnoses that may cause her presenting neurologic signs and symptoms.

Results: Her initial neurologic exam was most notable for dystonia, ataxia, and chorea that were more pronounced on the right side of her body. These findings corresponded with asymmetric contralateral caudate atrophy. Genetic testing for HD revealed that she had an expanded CAG repeat length of 29 on one allele and a normal number of copies of 17 on the other allele. Genetic testing for spinocerebellar ataxia type 17 (SCA17) via the TBP CAG/CAA repeat expansion was also sent. The patient was heterozygous for two normal TBP alleles, one with 36 CA(G/A) repeats and one with 37 CA(G/A) repeats. Further genetic testing for conditions that may cause similar neurologic findings was deferred at the patient’s request.

Conclusion: Her asymmetric contralateral caudate atrophy suggests mosaicism and somatic trinucleotide expansion. A better understanding of the factors resulting in clinical manifestations of HD in patients with intermediate range CAG repeats will enable clinicians to provide proper diagnosis and more meaningful counseling to patients and their families.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/asymmetric-symptomatic-huntingtons-disease-in-a-patient-with-intermediate-range-trinucleotide-repeats/