Among ten second-generation epigenetic clocks tested across two independent cohorts—AIBL and ADNI—the Principal Components PhenoAge (PCPhenoAge) clock emerged as the sole measure showing a replicated, statistically significant association between biological age acceleration and reduced hippocampal volume. Additional cross-sectional signals appeared between hippocampal volume and GrimAge, PCGrimAge, PCHorvathS2013, PCHorvathS2018, and PCHannumG2013, and a DNA methylation-derived telomere length estimate correlated with change in brain amyloid-β burden—but none of these secondary findings replicated in ADNI.
The hippocampus is among the first regions to atrophy in Alzheimer's disease, making any reliable biological-age marker tied to its volume clinically meaningful. PCPhenoAge is trained on blood biomarkers linked to morbidity and mortality rather than chronological age alone, theoretically capturing dysregulated physiology more sensitively than first-generation clocks like Horvath's or Hannum's. That this clock's signal replicated across two well-characterized, multicenter cohorts elevates it above the noise typical in epigenetic-clock research, where single-cohort findings frequently fail to generalize.
Critical caveats apply: the design is cross-sectional for the primary hippocampal outcome, precluding causal inference—accelerated epigenetic aging could be a consequence, not a driver, of neurodegeneration. Cohort participants skew older and clinically selected, limiting generalizability. Still, this is confirmatory-to-incrementally-advancing work: it narrows the field from ten candidate clocks to one robust predictor and motivates prospective trials testing whether interventions that reduce PCPhenoAge acceleration demonstrably preserve hippocampal integrity.