A Tale of Monozygotic Twins With Down Syndrome: Divergent Clinical Paths to Dementia

Objectives: Individuals with Down syndrome (DS) have a very high risk for developing Alzheimer's disease (AD) due to the triplication of the amyloid precursor protein gene on chromosome 21. We describe a unique set of female monozygotic twins with Trisomy 21 and mild intellectual disability with significantly discordant rates of cognitive decline. Methods: The twins were followed longitudinally, starting at age 42, using cognitive assessments (Down Syndrome Mental Status Examination and Modified Cued Recall). Caregiver assessments included the Dementia Questionnaire for People with Learning Disabilities, National Task Group - Early Detection Screen for Dementia and the Neuropsychiatric Inventory. Blood was collected for AD biomarkers. Results: Performance on baseline cognitive assessments was similar; however, by Timepoint 1, Twin 2 met criteria for mild cognitive impairment (MCI) and by Timepoint 2 met criteria for dementia due to AD. In contrast, Twin 1 remained cognitively stable. Caregiver assessment at baseline showed concerns about Twin 2's social skills, which remained stable across timepoints. AD protein biomarker levels were similar. Discussion: Discordant cognitive decline could not be explained by clinical co-morbidities, medications, or environment, suggesting that other factors, such as epigenetics, could underlie phenotypic variability and variable risk for AD in DS and deserves further study.