Baseline levels of soluble amyloid precursor proteins predict conversion to subcortical small-vessel disease

Abstract

Introduction: The Gothenburg Mild Cognitive Impairment (MCI) study is a longitudinal mono-center study of patients seeking help for cognitive complaints at the memory clinic in Gothenburg, Sweden. The study includes both pre-clinical diagnoses, e.g. subjective cognitive impairment (SCI) and mild cognitive impairment (MCI), as well as patients with manifest dementia, including Alzheimer's disease (AD), subcortical small-vessel disease (SSVD) and mixed AD/SSVD. The International Working Group (IWG) for New Research Criteria for the Diagnosis of AD has proposed a conceptual framework for guiding diagnosis by biomarker analysis. In short, patients with typical AD and mixed AD/SSVD should display decreased amyloid-β (Aβ) 1-42 together with increased total tau or phosphorylated tau in cerebrospinal fluid (CSF). Methods: In this project, we used the IWG-2 criteria to help the exploration of preclinical CSF biomarkers for SSVD. To this end, we investigated the odds of SCI and MCI patients with and without AD pathology converting to SSVD. Moreover, we investigated how CSF biomarker levels at baseline affected risk of converting to SSVD. The applied cut-offs for AD pathology were Aβ-42 350 ng/L and p-tau181>59 ng/L. The study cohort consisted of 29 SCI with AD pathology and 173 SCI without AD pathology, and 98 MCI with AD pathology and 203 MCI without AD pathology. CSF biomarkers Aβ-38, Aβ-40 and Aβ-42, and soluble amyloid precursor protein (sAPP) alpha and beta were analyzed at baseline. Results: While SCI and MCI patients with pathological AD biomarkers were 30 times more likely to convert to AD and 15 times more likely to convert to mixed AD/SSVD, pathological AD biomarkers did not predict conversion to SSVD. Using Cox regression analysis, we found that higher levels of sAPP-α and sAPP-β at baseline were associated with lower risk of converting to SSVD. Moreover, Kaplan- Meier estimations showed that when preclinical subjects were stratified according to baseline levels of sAPP-α or Aβ-40, those with the lower levels of the proteins had increased probability of converting to SSVD. No other biomarkers were associated with conversion to SSVD. Discussion: sAPP-α/-β and Aβ-40 could be used as potential biomarkers to evaluate progression to SSVD.