Deamidation of asparagine (Asn) residue is a spontaneous, non-enzymatic protein degradation process resulting in the formation of damaging isoaspartate (isoAsp). As isoAsp residues accumulate in proteins, the latter lose their native structure, function and solubility [1]]. A body of research has demonstrated that the accumulation of isoAsp triggers protein aggregation, which ultimately leads to the onset and progress of multiple ailments, including neurodegenerative diseases (NDDs) [2–7]. NDDs, such as Alzheimer’s disease (AD), vascular dementia (VaD), frontotemporal dementia (FTD), and Parkinson’s disease (PD), are usually defined as the progressive loss of structure and function in neurons and axons of the central nervous system. The NDD diagnostics is usually based on clinical features, such as cognitive decline and movement disorders, as well as on neuroimaging techniques positron emission tomography (PET) and magnetic resonance imaging (MRI). Recently, molecular biomarkers reflecting protein pathology and aggregation has gained increased interest of NDD researchers. Apart from the cerebrospinal fluid (CSF) biomarkers (e.g., amyloid β (Aβ), tau, α-synuclein, etc.) obtaining of which is sometimes viewed as complicated, invasive and/or expensive, several blood biomarkers have emerged in recent years, such as neurofilament light chain (NfL) [8–10], phosphorylated tau (pTau) [11–13] and glial fibrillary acidic protein (GFAP) [14–16]. However, the sensitivity and specificity of these blood biomarkers is not yet fully established, especially for early disease stages, and thus new reliable, specific, low-cost blood biomarkers are highly desirable. Besides, the majority of blood biomarker characterization studies concerned separating diagnosed AD from healthy controls, while much less results have been published on the more challenging task of diagnosing early neurodegeneration stages, such as mild cognitive impairment (MCI) ahead of diagnosed dementia.
Our recent findings have strengthened the link between isoAsp and AD pathology [17]. Using a novel enzyme-linked immunosorbent assay (ELISA) [18], we discovered that the accumulation of isoAsp in human serum albumin (HSA) leads to a diminished HSA capacity to carry Aß and pTau, and thus to clear these damaging molecules in the blood circulatory system, which has been recognized as a major contributor to their accumulation in brain [19]. By studying an Amsterdam cohort, we have found that isoAsp levels in HSA strongly correlate with AD [17]. Furthermore, native immunoglobulins (IgGs) specific to deamidated HSA that could protect the human organism from the damaging effect of deamidation have been found significantly reduced in AD [17]. However, questions remained related to the performance of these biomarkers in non-Caucasian patients, specificity to AD of these biomarkers, and their ability to detect early disease stages.
To address these issues, we recruited a Shanghai cohort with several kinds of NDDs as well as subjects with MCI, and compared the performance of the deamidation biomarkers with other plasma NDD markers. The central hypothesis under verification was that accumulation of deamidation products due to breakdown of the isoAsp repair/removal mechanisms precedes most processes in AD initiation, and thus deamidation biomarkers might exhibit high sensitivity in detecting early stages of neurodegeneration.
1.2 Study conclusions and implications
In this follow-up study we first confirmed on Shanghai cohort our earlier findings on Amsterdam cohort that isoAsp in plasma HSA is significantly increased (P < 0.0001) while the anti-aHSA IgG levels are lower (P < 0.0001) in AD patients compared to healthy controls. Then we measured the levels of these two biomarkers in other NDDs, such as VaD, FTD and PD, and found behavior similar to AD in the first two diseases (P < 0.01). However, in PD the results didn’t reach statistical significance, possibly due to a large data spread within the PD group. Then we tested the performance of the deamidation biomarkers for MCI ahead of dementia diagnosis. This performance was found to be superior compared to other tested blood biomarkers, reaching the impressive accuracy of 92%. Then we tested the association of deamidation biomarkers with other analyzed parameters and discovered strong correlation with cognitive scores of the patients and controls. Taken together, these results strongly support the role of deamidation in NDD etiology, and open prospects of using deamidation biomarkers in early disease diagnostics.