LIFE-DSR-Biomarker Sub-study of Biomarkers in Down Syndrome Related Alzheimer's Disease (DS-AD)

This is an optional sub-study that will enroll participants from the LIFE-DSR parent protocol. Participants will undergo assessments at two timepoints, including: additional blood samples for PBMC and RNA extraction, as well as a lumbar puncture for collection of CSF, and/or MRI and tau PET imaging. Sub-study visits will be scheduled around a similar calendar-day as the parent protocol study visits (e.g., Month (M) 0 (M0) and M16, or M16 and M32). The aim is to increase knowledge of biomarkers of interest in DS-AD clinical progression and perform deep immunophenotyping of PBMC samples. Intervention studies in people with DS depend upon determining the optimal age for treatments to be given, by identifying or developing reliable outcome measures that are most sensitive to decline and discovering biomarkers most closely related to disease progression. For tau deposition, the aim of this study is to ascertain the degree and distribution of tau pathology in DS as a function of age using tau PET, and to determine the longitudinal progression of that pathology. As such, enrichment of the study population included in the tau PET sub-study for presence of tau PET positive scans, and in particular tau PET scans in the earliest stages of positivity, is desirable. Accomplishing this may require flexibility in recruitment, including adjustments of age eligibility as a response to emerging new information in the sub-study population as the study proceeds. PET images will be reviewed in near real-time to enable adaption of the age strata based on the presence or absence of tau PET signal. All the above endpoints will be acquired with informed consent and assent. All data is intended for research purposes only and associated clinical data will be stored securely. Participants will have the option to participate in: * LP to collect CSF and venipuncture to collect a blood sample OR * Imaging (MRI and tau PET) and venipuncture to collect a blood sample OR * Both * LP to collect CSF and venipuncture to collect a blood sample AND * Imaging (MRI and tau PET).