Associations between homocysteine, tau, and neurodegenerative markers in apolipoprotein ε4 non‐carriers with clinical Alzheimer’s disease

Abstract

AbstractBackgroundHomocysteine (Hcy) is an amino acid generated as a byproduct of methionine metabolism. It is a risk factor for vascular and neurodegenerative diseases. This study investigated relationships between plasma Hcy concentrations and neurodegenerative biomarkers in people clinically diagnosed with Alzheimer’s disease (AD) or mild cognitive impairment (MCI) due to AD.MethodParticipants were identified from the Ontario Neurodegenerative Disease Research Initiative. Brain parenchymal fraction (BPF) was quantified using T1‐ and T2‐weighted structural magnetic resonance imaging with an in‐house Semi‐Automated Brain Region Extraction and Lesion Explorer package. Plasma amyloid beta (Aβ) 42, Aβ40, phosphorylated tau 181 (p‐tau181), and neurofilament light chain (NfL) were measured with Single molecule array (Simoa) assays. Linear regression analyses were used to test associations between Hcy and neuroimaging or plasma biomarkers, controlling for age, sex, hypertension, hyperlipidemia, diabetes, cholesterol, and vitamin B12. Analyses were stratified by apolipoprotein E (APOE) ε4 carrier status (non‐carriers: n = 64, mean age = 70.9 years, 41% female; carriers: n = 62, mean age = 71.2 years, 50% female).ResultIn APOE ε4 non‐carriers, Hcy was associated negatively with BPF (β = ‐0.233, p = 0.047, 95% CI [‐0.460, ‐0.003]), notably in temporal lobe regions in exploratory regional analyses. Hcy was also associated positively with plasma concentrations of NfL (β = 0.403, p = 0.007, 95% CI [0.117, 0.688]) and p‐tau181 (β = 0.357, p = 0.023, 95% CI [0.051, 0.663]) in APOE ε4 non‐carriers. Also, in APOE ε4 non‐carriers, Hcy was associated positively with Aβ40 (β = 0.367, p = 0.015, 95% CI [0.074, 0.660]) and Aβ42 (β = 0.305, p = 0.041, 95% CI [0.012, 0.598]), but not with the Aβ42/40 ratio (β = 0.155, p = 0.339, 95% CI [‐0.166, 0.476]). None of these associations were seen in APOE ε4 carriers.ConclusionIn non‐carriers of the APOE ε4 allele clinically diagnosed with AD or MCI, homocysteine was associated with Amyloid, tau and neurodegeneration. The specificity of these relationships to ε4 non‐carriers identifies a unique risk factor profile for AD that is independent of APOE ε4 status.