The effects of mosaicism on biological and clinical markers of Alzheimer's disease in adults with Down syndrome.

Background: Individuals with Down syndrome (DS) are at high risk of early-onset Alzheimer's disease (AD); yet, some 20 percent do not develop any signs of dementia until after 65 years or in their lifetime. Mosaicism could contribute to this phenotypic variation, where some disomic cells could lead to lower levels of gene products from chromosome 21.

Methods: We examined longitudinal neuropsychological and biomarker data from two large studies of DS: the Alzheimer Biomarker Consortium-Down syndrome study (ABC-DS) (n = 357); and a legacy study (n = 468). We assessed mosaicism using karyotyping or GWAS data. Participants had data on plasma AD biomarkers (Aβ₄₀, Aβ₄₂, tau, and NfL) and longitudinal cognitive measures. A subset had cerebrospinal fluid biomarkers (Aβ₄₀, Aβ₄₂, tau, ptau181, and NfL) and amyloid and tau PET data.

Findings: For both cohorts, the prevalence of mosaicism was <10% (ABC-DS: 7.3%; Legacy: 9.6%), and those with mosaicism had lower plasma Aβ₄₀ and Aβ₄₂ concentrations. For the older legacy cohort, when compared to those with full trisomy, those with mosaicism had significantly smaller decline in total and annualized neurocognitive scores, and lower incidence and prevalence of dementia.

Interpretation: Mosaicism in DS was associated with lower concentrations of plasma Aβ peptides, possibly leading to lower AD risk. However, its clinical impact was less clear in the younger ABC-DC cohort, and a follow-up study is warranted.

Funding: National Institutes of Health (R01AG014673, P01HD035897, R56AG061837), NIA (U01AG051412, U19AG068054), NICHD, ADRC programs, the Eunice Kennedy Shriver Intellectual and Developmental Disabilities Research Centers Program, and NCATS (UL1TR001873).