Mosaicism for trisomy 21 in a patient with young-onset dementia: a case report and brief literature review.

OBJECTIVE To describe a case of young-onset Alzheimer disease (AD) due to mosaicism for trisomy 21. DESIGN Case report of a single patient. SETTING Tertiary referral dementia clinic. PATIENT A 55-year-old man with a mild degree of developmental delay but no previous diagnosis of Down syndrome and only minimal physical manifestations of Down syndrome presented with gradually progressive cognitive impairment consistent with probable AD. RESULTS Fluorescent in situ hybridization analysis of interphase chromosomes revealed trisomy 21 in 10% of peripheral lymphocytes. CONCLUSIONS As mosaicism for trisomy 21 can present with no or minimal manifestations of Down syndrome, it may be underdiagnosed as a cause of early-onset AD. Occult mosaicism for trisomy 21 may explain in part the previously described association between family history of Down syndrome and risk of AD. Screening for mosaicism with fluorescent in situ hybridization is indicated in selected patients with mild developmental delay and those with AD of young onset.