Clinical implications of loss of interruption variants for diagnosis, genetic counselling, and clinical trials in Huntington's disease.

Age of onset in Huntington disease (HD) is influenced by cis-acting genetic variants, particularly the loss of interrupting codons in the HTT CAG and CCG repeats (CAG-CCG LOI variant). The CAG-CCG LOI variant is not detectable by current diagnostic assays, leading to underestimation of CAG repeat length, misdiagnosis, and inaccurate prediction of risk of symptom onset in the reduced penetrance range. In clinical trials, unidentified CAG-CCG LOI variants may affect interpretation of results, particularly for small trials. Accurate ascertainment and reporting of the CAG-CCG LOI genotype therefore has important implications for HD diagnosis, genetic counselling, and clinical trial design.