Evaluation of Plasma Amyloid Peptides Aβ_1-40 and Aβ_1-42 as Diagnostic Biomarker of Alzheimer's Disease, its Association with Different Grades of Clinical Severity and 18F-Fluorodeoxyglucose Positron Emission Tomography Z score in the Indian Population: A Case-Control Study

Introduction

Alzheimer's disease (AD) is the most common cause of dementia in older patients (>60–65 years) and accounts for 4.9% of deaths among elderly people in the USA.[1] Global prevalence was about 25 million in 2010 which is anticipated to be doubled by 2030 because of increased life expectancy. AD is predicted to affect one in 85 people globally by 2050.[23] For populations above 65 years, the prevalence of AD in Asian countries varies from 6.44% in South India, 4.86% in Shanghai (China), and 3.92% in Sri Lanka.[4]

Despite such a significant effect of AD on the human race and decades of research devoted to finding a cure for this dementing illness, little has been achieved in terms of cure or reduction in the rate of its progression. This is partly related to an inherent problem in that the pathogenic process in AD starts years before clinical onset and drugs are likely to be most effective if started in preclinical phase or in the early stage of mild cognitive impairment (MCI) or AD. To know the effects of the intervention, one should be able to diagnose MCI with certainty and to determine which MCI patients are going to progress to Alzheimer's disease and also the rate of disease progression.

There are various imaging and laboratory biomarkers (decreased cerebrospinal fluid [CSF] Aβ_1-42, increased CSF tau, decreased 18F-Fluorodeoxyglucose [18F-FDG] uptake on cerebral cortices positron emission tomography [PET], amyloid PET imaging, and measures of brain atrophy on magnetic resonance [MR]), which can assist in the diagnosis of AD. However, these are either invasive (CSF), expensive, and not readily available.[56]

Recently, significant attention had been given to the role of plasma biomarkers in the early diagnosis of AD as well as in its differentiation from other forms of dementia. The most commonly used plasma biomarkers include serum amyloid peptides. Because plasma sampling is simpler and less invasive than lumbar puncture, it is well suited to use in old age patients or when multiple measures are required, such as in clinical trials. However, the published data on plasma Aβ levels in AD is conflicting. One study indicated that low or decreasing plasma Aβ42 levels and Aβ42/Aβ40 ratio were related to cognitive decline during the follow-up.[7] A high variation in the prevalence and progress of AD among different geographic regions is noted, which can be an indicator of the difference in the pathogenesis of AD among different geographic regions (e.g., variation in the incidence of different AD causing mutations in different population, variation in cultural and dietary factors, and prevalence of different forms of inherited patterns). Again, amyloid-beta negative Alzheimer's disease is also a known entity. However, till now no study has evaluated the association between plasma amyloid-beta level, clinical dementia stages, and 18F-FDG PET Z score in the Indian population. This is the first study to evaluate the same in the Indian population. Thus, we planned the current study to determine the role of plasma Aβ_1-40 and Aβ_1-42 levels in the diagnosis of Alzheimer's disease.

Patients and Methods

The current cross-sectional study was conducted in the Department of Pharmacology and Neurology at apex care and teaching hospital in northern India. The study was started after getting approval from the institutional ethics committee (Histo/15/IEMEC/37) and written informed consent from all participants. The patients were recruited from 2014 to 2015. During this period, the patients with dementia were screened for inclusion in the study. The diagnosis of dementia was made based on the diagnostic and statistical manual (DSM)-IV criteria.[8] Patients with dementia were then evaluated in detail to determine the exact etiology of dementia. All these patients underwent detailed hematological (complete hemogram including erythrocyte sedimentation rate and c reactive protein) and biochemical (blood sugars, renal and liver function tests, thyroid function tests, serum electrolytes, calcium, and phosphorus) investigations. All these patients also underwent electrocardiogram and echocardiogram, serum venereal disease research laboratory, and testing for human immunodeficiency and hepatitis viruses. Neuroimaging (MR imaging) and 18F-FDG PET imaging were done in a few of these patients. Other investigations including chest X-ray, ultrasonography of abdomen, vasculitis profile, thyroid peroxidase antibodies, toxicology profile, serum Vitamin B12 levels, electroencephalography, and CSF analysis were performed wherever indicated. The patients who were diagnosed to be suffering from AD based on Dubos criteria[9] and MCI[10] were included in the study. The procedure for the selection of cases is depicted in Figure 1.

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Figure 1

Study flow chart and patient enrollment

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The inclusion and exclusion criteria for the study groups are given below:

Results

The current study included 47 cases of cognitive impairment (AD-29; MCI-18) and 33 controls after the screening of 191 participants. The mean (±SD) age was 69.8 (±9.9) years in the AD group, 68.7 (±7.07) years in the MCI group, and 60.9 (±9.05) years in the control group. Men constituted 14 (48.3%) of participants in the AD group, 15 (83.3%) in the patients of the MCI group, and 23 (70%) in the control group [Table 1]. Among AD patients, 7 (24.1%) patients had mild, 15 (15.8%) had moderate, and 7 (24.1%) patients had severe dementia. The mean age was significantly lower in controls to AD and MCI patients. Regarding associated medical diseases, hypertension was seen in 10 patients in the AD group with a mean duration of 10 years, 11 patients in the MCI group with a mean duration of 14.7 years, and seven patients in the control group with a mean duration of 8.1 years. Seven patients in the AD group had diabetes mellitus with a mean duration of 8.1 years, 5 in the MCI group had diabetes mellitus with a mean duration of 15 years, and 4 in the control group had diabetes with a mean duration of 6.75 years. These, as well as other demographic data, are reported in Table 1. All the patients underwent detailed laboratory investigations as mentioned in the patients and methods section. In comparison, all the investigations were comparable between AD and MCI patients.

Discussion

The treatment of AD continues to be far from satisfactory. This is partially related to the fact that by the time AD is diagnosed clinically, the pathological process is already in the advanced stage. Furthermore, to test the efficacy of the new intervention, it is imperative that it is applied at a stage when the pathological process has just begun. In other words, to test the efficacy of a new intervention, one needs to diagnose presymptomatic AD with reasonable certainty. Current investigational modalities (radiological imaging, nuclear imaging, and various CSF biomarkers [Aβ peptides, p-tau, t-tau, and mmp-9]) which are being used for this purpose, are either too costly or invasive and difficult to applied widely mostly in peripheral hospitals. Thus, in the present study, we tried to assess the role of two amyloid peptides of plasma in the diagnosis of AD. Currently, the CSF level of these biomarkers has been included in the research diagnostic criteria offered by the National Institute on Aging and Alzheimer's Association, and the international working group. Recently, a “Biological definition” of AD has been suggested with the A/T/N classification which used biomarker of β-amyloid pathology (A), tau (T), and neurodegenerative markers (N).[22]

The published data on the role of plasma amyloid peptides level in AD are conflicting. Previous studies suggested that during early-stage AD, there is a gradual rise in plasma levels of amyloid peptides but as the disease process progress, their level gradually decreases and finally becomes normal so much so that once AD is clinical evidence, plasma levels of Aβ_1-42 levels are comparable to healthy controls.[23] A study concluded that decreasing levels of Aβ_1-42 in serial measurements may be associated more with cognitive decline than the plasma amyloid-beta peptides and indicate the development of AD[7] while numerous large studies have consistently reported that a lower Aβ_1-42/Aβ_1-40 ratio in plasma is associated with a higher risk of dementia.[24]

In the present study, plasma Aβ_1-42 levels were found significantly higher in AD patients as compared to controls. However, other measures such as plasma Aβ_1-40 and Aβ_1-40/Aβ_1-42 ratio did not show any significant differences between the three groups. Plasma Aβ_1-42 levels of 5.7 ng/mL had a sensitivity of 27.6% and specificity of 97% in differentiating AD from control with insignificant P value which could be due to large variability in patients and control group's age and study with larger sample size is recommended.

18F-FDG PET is a common molecular imaging technique which used as a biomarker. Basically, it measures the intracellular glucose metabolism and used in various applications in neuroscience including in the study of dementia where it has been used from the past three decades. 18F-FDG PET has become the most sensitive and specific imaging modality for the diagnosis of AD and nowadays it is considered an imaging biomarker for AD before the onset of dementia and in clinical trials.[25] The quantitative analysis of brain hypometabolism shown in 18F-FDG is done by Z score. A positive Z score >2 represents a significant reduction in metabolic activity comparable to the normal reference data. AD patients show hypometabolism in bilateral temporal lobes (middle and inferior temporal gyri), bilateral limbic system (parahippocampal gyrus and posterior cingulate gyrus), bilateral parietal lobe, and bilateral lateral parietal cortex.[26] Womack et al., have found temporoparietal hypometabolism was more sensitive (sensitivity, 93.6% P = 0.003), but posterior cingulate hypometabolism was more specific (specificity, 71.4% P = 0.01) for diagnosing AD.[2728]

In the present study, we found a moderate positive correlation of amyloid peptide ratio (Aβ_1-40/Aβ_1-42) to a Z score of PET in commonly affected brain areas indicating higher Aβ_1-40/Aβ_1-42 ratios in patients with hypometabolism on PET scan. This promising finding needs to be evaluated in larger studies.

Our studies have several limitations. The main limitation is the smaller sample size. Other is the control groups were not fully matched to cases with respect to age and gender distribution. Furthermore, we could not do serial measurements of plasma amyloid peptides in patients with dementia.

Conclusion

The results of our study reveal relatively low sensitivity of plasma amyloid-beta peptides for differentiating AD from healthy controls. Future studies involving larger sample size and longitudinal measurement of plasma levels of various amyloid peptides will help in better characterization of the role of various biomarkers in differentiating AD from healthy controls. The identification of AD disease in the early phase is still a major challenge, so the combined plasma amyloid and FDG PET approach might be helpful in the early detection of pathological changes in older age individuals.