SUMMARY In chapter 2, we investigated the ATN classification system. Six hundred ninety-three individuals were classified into ATN profiles, as determined by amyloid PET or CSF amyloid-β (A), CSF p-tau (T), and MRI-based medial temporal lobe atrophy (MTA; N). Compared to A-T-N-, individuals in A+ profiles had a higher risk of dementia with a dose-response pattern for number of biomarkers affected. Individuals in A+ profiles also showed a steeper decline on tests addressing memory, attention, language, and executive functions. These findings implicate that the ATN classification can help identify which SCD individuals are at risk of dementia. In chapter 3, we determined thresholds for amyloid pathology and investigated the clinical relevance of grey zone amyloid burden, using dynamic [18F]florbetapir PET. Scans were visually assessed as amyloid positive/negative. Additionally, we calculated the mean binding potential (BPND) and standardized uptake value ratio (SUVr50-70). We determined six amyloid positivity thresholds using various data-driven methods (resulting thresholds: BPND 0.19/0.23/0.29; SUVr 1.28/1.34/1.43). Overall, concordance between definitions for amyloid positivity were comparable. Memory slopes gradually became steeper with higher amyloid load. We found that not only high, but also grey zone amyloid burden was already associated with suboptimal memory function. Multiple biomarkers have been suggested to measure neurodegeneration (N) in the ATN framework. In chapter 4, we compared five N biomarkers (CSF total (t)-tau, MTA visual rating on MRI, hippocampal volume (HV), serum neurofilament light (NfL) and glial fibrillary acidic protein (GFAP)). N biomarkers were only modestly to moderately correlated, indicating differint N biomarkers reflect different aspects of neurodegeneration and should therefore not be used interchangeably. T-tau was strongly correlated with p-tau. All N biomarkers individually predicted clinical progression, but only HV, NfL and GFAP added predictive value beyond abeta and p-tau. Our results illustrate the added prognostic value of N beyond A and T. In chapter 5, we investigated the relationships between genetic determinants of AD and ATN biomarkers. A polygenic risk score (PRS) was calculated based on 39 genetic variants. We used amyloid PET or CSF abeta for A, CSF p-tau for T, and MTA on MRI for N. The PRS and APOE ε4 carriership were associated with A+ ATN profiles, and APOE ε4 additionally with isolated increased tau (A-T+N-). A high PRS and APOE ε4 separately predicted AD dementia. Combined, a high PRS increased while a low PRS attenuated the risk associated with ε4 carriers. These results imply genetic variants beyond APOE are clinically relevant and contribute to the pathophysiology of AD. In chapter 6, we investigated the associations between relative cerebral blood flow (rCBF), amyloid burden and cognition. Individuals underwent dynamic [18F]florbetapir PET, and we calculated mean binding potential (BPND) and R1 (proxy for relative (r)CBF). Both amyloid burden and rCBF independently contributed to cognitive decline over time. Despite absence of cross-sectional associations, high baseline amyloid burden was associated with a subsequent decrease in rCBF, while inversely low baseline rCBF was associated with subsequent increase in amyloid burden. These results suggest that amyloid accumulation and decrease in rCBF are two disease processes which enhance each other longitudinally. In chapter 7, we studied longitudinal change in ATN biomarkers. We used [18F]florbetapir PET for ‘A’, [18F]flortaucipir PET for ‘T’ and medial temporal atrophy score on MRI for ‘N’. Seventeen individuals (44%) changed to a different ATN profile over time. Only 6/17 followed the hypothetical sequence of A→T→N, while 11/17 followed a different order, showing the large variability in the order of ATN biomarkers becoming abnormal. APOE ε4 carriership inferred an increased risk of changing from A- to A+. Individuals who became A+ at follow-up showed subtle decline on tests for attention and executive functioning.