Comparison of CSF neurofilament light chain and total tau as neurodegeneration markers: Associations with synaptic markers and cognitive outcomes.

Clinically, Alzheimer's disease (AD) is characterized by progressive cognitive decline due to neuronal and synaptic degeneration. Neurofilament light chain (NfL) and total tau (T-tau) reflect neurodegeneration, NfL putatively more related to white and T-tau to grey matter. This study examines how cerebrospinal fluid (CSF) neurodegeneration markers (T-tau, NfL or both) are correlated with synaptic markers and clinical progression. We included 331 individuals with (n = 212) and without (n = 119) pathological CSF Aβ42/40 ratios. Associations between CSF NfL, T-tau, and the synaptic biomarkers neurogranin and BACE1 were assessed using Pearson's correlation. Group differences in synaptic marker levels were evaluated using linear regression comparing individuals with isolated pathological T-tau, NfL, or both, versus biomarker-negative individuals. Clinical progression to MCI or dementia was assessed using a Cox proportional hazards model (n = 257; mean follow-up = 3.75 years). Linear regression and Cox proportional hazards models included age, sex, and dichotomized APOE-ε4 carriership as covariates. T-tau had a stronger correlation with neurogranin(r = 0.84) and BACE1(r = 0.73) than NfL(r = 0.51 and 0.48; p < 0.001). Group-wise comparisons confirmed this, showing that only individuals with pathological T-tau-alone or with NfL-had significantly higher synaptic marker levels (p < 0.001). Only the combination of pathological T-tau and NfL was associated with a significantly increased risk of clinical progression(HR=6.79; p < 0.001). These findings suggest that T-tau is more closely related to early synaptic dysfunction in AD than NfL. The combined elevation of both biomarkers, linked to greater clinical decline, supports a dual contribution of grey- and white matter degeneration to disease progression.