Metals dyshomeostasis in Alzheimer’s Disease: A Systematic Review.

Citation: Lucaroni F , Ambrosone C, Paradiso F, Messinese M, Di Domenicantonio R, Alessandroni C, Cerone G, Cerutti F, Di Gaspare F, Morciano L, Tran L, Pietroiusti A and Palombi L - Metals dyshomeostasis in Alzheimer’s Disease: A Systematic Review. - Biomedicine & Prevention (2017) - vol. 2 - (112) - DOI:10.19252/000000070

Lucaroni F,1* Ambrosone C,1 Paradiso F,1 Messinese M,1 Di Domenicantonio R,2 Alessandroni C,1 Cerone G,1 Cerutti F,1 Di Gaspare F,1 Morciano L,1 Tran L,3 Pietroiusti A,1 Palombi L.1
1 Department of Biomedicine and Prevention, University of Rome Tor Vergata – Rome, Italy
2 School of General Practice – Lazio Region, Italy
3 Institute of Occupational Medicine – Edinburgh, United Kingdom

Introduction

Dementia represents a major healthcare problem, involving 46.8 million people worldwide, with over 9.9 million new cases every year, one every 3.2 seconds.1 Dementia is a leading cause of disability and dependency in the elderly, leading to high physical, psychological, social and economic impact on caregivers and families. It is also responsible for higher direct and indirect costs to society. The estimated worldwide cost of dementia is US $818 billion and is expected to reach one trillion dollars by 2018.1

Alzheimer’s disease (AD) is the most frequent cause of dementia, involving 50 to 75 per cent of the global burden of disease,2 with almost 46 billion people affected worldwide3 and 1.54 million deaths globally.4 Indeed, according to WHO,4 Alzheimer’s disease is the seventh cause of death among high-income countries. Therefore, preventing AD is a main issue: unfortunately, the detailed knowledge of its pathogenesis that would be necessary in order to implement rational preventive measures, is not yet available. 

AD is a neurodegenerative disorder, characterized by progressive brain deposition of the amyloid β peptide (Aβ), which is generated by proteolytic cleavage of amyloid precursor protein (APP) by β- and γ-secretases. The abnormal aggregation and accumulation of neurotoxic Aβ has been proposed as the primary driving force for AD in the amyloid hypothesis.5

Amyloid-β is also a major constituent of senile plaques. According to recent studies, the cerebral accumulation of metal ions might contribute to Aβ aggregation, through the production of a reactive oxygen species (ROS).6 Although there is a plethora of studies concerning pathogenesis of AD, it has not been completely clarified yet because of the complex interaction between endogenous (age, family history, genetic factors, gender) and exogenous (environment) factors that could contribute to the development of this illness.7 Genetic predisposition seems to be the most important risk factor of Alzheimer’s disease, contributing 70% of the overall risk of AD, with the remaining 30% likely to be caused by lifestyle and chronic exposure to environmental factors.8 The “brain trace metal dyshomeostasis” is a fascinating, although controversial, hypothesis to explain the residual part of ethiopathogenesis of Alzheimer’s disease. Although the role of metals as hazardous substances is largely accepted, with even the Agency for Toxic Substances and Disease Registry (ATSDR) including arsenic, lead, mercury, and cadmium in the first ten places on the annual list of hazardous substances,9 much has yet to be clarified as to their impact on neurodegenerative disorders. Some evidence of an association with AD seems to be available only for aluminium. According to a recent metanalysis undertaken on 34 studies involving 1,208 participants, aluminium levels were significantly higher in serum, brain, and the CSF of patients with Alzheimer’s disease.10

The aim of this systematic review is to evaluate the currently available studies concerning metal concentration, except for aluminium, in biological human matrices (blood, serum, hair, brain tissue, nails, cerebrospinal fluid) of AD and non-AD subjects.

Methods

This systematic review was performed according to a-priori protocol, designed following the PRISMA guidelines.11

Pubmed/Medline, Scopus and Cochrane databases were examined on 21st November 2016, through a combination of keywords and relevant medical subject heading (MeSH) terms, as extensively shown in Table 1. Search inclusion criteria were restricted to studies on humans, published in the last 10 years, written in English and with an abstract available.

The flow diagram of the complete process selection is shown in Figure 1.

Only original studies measuring metal concentrations – except for Aluminium – in biological matrices (blood, urine, hair, nails, cerebrospinal fluid, brain tissue) in AD subjects were considered suitable for inclusion. Cases followed Alzheimer’s disease international diagnostic criteria: NINCDS—ADRDA, DSM-IV, or BRAAK criteria for post-mortem studies.

Four authors (CA, RdD, MM, FP) independently extracted the articles that met the eligibility criteria. Disagreements on the potential relevance of the selected studies were resolved by consensus or by discussion with a fifth author (FL). Furthermore, overlapping articles were excluded.

Additionally, five further works coming from the examination of references of the previews articles were included.16,47,48,52,53

The mean difference in metal concentrations with p value <0.05 was considered as statistically significant result.

A quality assessment of the included studies was performed using the NIH specific tool12 for observational studies evaluation.

Results

Forty-four articles meeting the eligibility criteria were identified: 39 from search engines and 5 additional studies checked from reference lists of the included works. (Table 1; Figure 1).

All the studies were of low to moderate quality, according to the NIH tool score:12 the median score of the selected studies being 5.5 out of 12.

The main characteristics of the included studies are summarized in Table 2-16.

 

Arsenic (As)

As shown in table 2, an overall number of four studies evaluating arsenic concentrations in biological matrices were considered eligible. Three of the included studies14,15,16 were achieved on serum samples. Park et al. and Paglia et al. found no statistically significant difference (p=0.309 and p=0.312, respectively) in metalloid concentration between AD subjects and healthy controls in two large populations of 207 (89 cases and 118 controls) and 118 subjects (34 AD, 20 MCI, 24 SMC, and 40 non-AD) respectively. A similar result occurred in Baum research: among the 85 enrolled subjects, 44 with a diagnosis of Alzheimer’s disease and 41 healthy controls, there were no relevant differences in arsenic blood concentration between the two groups: a mean level of 35.7 nmol/L in AD subjects compared with 38.7 nmol/L in healthy controls. A post mortem case control study13 carried out by Szabo et al. on 31 brain samples showed significantly lower concentration both in frontal cortex and in ventricular fluid brain regions (p=0.033 and p=0.044, respectively) of AD patients when compared with non-AD subjects (frontal cortex: 145 ng/g vs 159 ng/g; ventricular fluid: 18 ng/ml vs 13 ng/ml, respectively).

 

Cadmium (Cd)

Among the five studies assessing cadmium concentration in blood samples of patients with Alzheimer’s disease compared with healthy controls, no significant differences emerged between the two groups (p>0.05). Only one paper21 reported different results, showing a statistically significant (p=0.001) increase in cadmium levels among AD patients. Of note, this study included only 39 subjects, having the smallest size among the considered studies. Only one study19 investigated variations in cadmium cerebrospinal fluid level, showing no statistically significant difference between AD and non AD subjects.                                                          

Concerning nails and hair, Koseoglu et al. carried out a case control study on 122 subjects with and without dementia, showing a statistically significant decrease in metal concentration in patients with Alzheimer’s disease compared with healthy controls. Two post mortem studies analysing cadmium levels in brain tissue gave no univocal results. Szabo et al. found a statistically significant (p=0.031) decrease in Cd frontal cortex levels in AD patients, whereas. Akatsu et al. reported a significant (p<0.05) increase in amygdala region of AD compared with non-AD subjects.

 

Chrome (Cr)

As shown in Table 4, five articles evaluating a difference in chrome concentration between cases and controls met the eligibility criteria. In 2014 Gonzalez-Dominguez et al. carried out a case-control study to assess serum levels of the metal in 65 subjects: 25 with diagnosis of Alzheimer’s disease, 15 with mild cognitive impairment and 25 healthy patients without dementia. No statistically significant difference in chrome blood concentration was found among the three groups. Similar results were reported by Baum et al., 2010 and by Paglia et al., 2016. Finally, post mortem case control studies by Szabo et al. and Akatsu et al. on brain tissue, found no substantial difference in chrome content, in particular in frontal cortex, ventricular fluid, hippocampus and amygdala.

 

Cobalt (Co)

As shown in Table 5, three studies16,19,22 focused on blood samples (plasma and serum). Alimonti et al. found a significant decrease in serum cobalt among AD subjects in a population of 177 (53 AD and 124 controls), while the studies carried out by Gerhardsson et al. in 2008 and by Paglia et al. in 2016 showed no substantial changes in metal concentration between the two groups. A recent case control study (2017) by Koseoglu et al. focused on Co levels in other biological matrices, hair and nails. The research was undertaken on 122 subjects (62 AD and 60 non-AD) and showed a statistically significant decrease in cobalt concentration (p < 0.001) among AD patients in hair. On the contrary, no real variations emerged by comparison of nail samples between the two investigated groups.

Finally, two studies19,23 assessing metal levels in cerebrospinal fluid on large population samples [N=314 (cases+controls) and N=318 (cases+controls), respectively] did not detect statistically significant differences between cases and controls.

 

Copper (Cu)

Copper is the most widely investigated metal, with 36 studies assessing its concentrations in different biological matrices (blood, cerebrospinal fluid, hair, nails and brain tissue). As shown in Table 6, most studies assessed copper levels in blood, serum or plasma. The results were extremely variable with 42% of the studies (N=11) showing a significant increase in total and free copper in AD subjects compared with non-AD, 42% (N=11) showing no statistically significant difference between the two groups, and 16% (N=6) showing a significant decrease in AD patients. Also the most robust study in terms of sample population19,33,40,43 gave no univocal results. It must be pointed out that the number of subjects included in studies showing decreased concentrations was generally much lower than in studies showing an increase or no difference. Concerning cerebrospinal fluid, three case control studies19,23,35 found no substantial difference in CSF copper concentration between AD subjects and healthy controls   whereas only one study31 showed significantly higher copper levels amongst cases compared with the control group.

Few studies have been made on hair or nail matrices. Koseoglu et al.20 evaluated copper levels both in hair and nails on a population of 122 people, reporting a significant decrease in patients with Alzheimer’s disease. On the contrary, Koc et al.,38 assessing metal concentration in hair on a smaller sample (N=78) found no significant difference between AD and non-AD subjects. Finally, a number of post mortem case control studies (N=7), were carried out on brain tissue. More than 70% of eligible articles (5 out of 7)6,18,36,39,41 found a statistically significant decrease in copper brain concentration among AD subjects when compared with healthy controls. Only two studies13,29 showed no relevant variations in metal levels between cases and controls.

 

Iron (Fe)

As reported in Table 7, an overall number of 25 articles on iron were considered eligible. Most of the included studies (18 out of 25) were carried out on blood as a biological matrix. One study,50 published in 2014 by Faux et al., was carried out on a large population (N=1,112: 211 with diagnosis of Alzheimer’s disease, 133 with mild cognitive impairment, and 768 healthy controls). In this study there was no evidence of any significant difference in blood iron concentration between AD and non-AD subjects. Non univocal results were reported in the remaining studies. Indeed, ten studies17,19,24,27,37,43,45,50,52,53 didn’t show substantial differences between cases and controls. Six studies15,16,22,25,32,51 showed a significant decrease in blood iron levels of AD patients when compared with healthy controls. Conversely, only one study21 showed a significant increase of iron in AD subjects compared with control group (p=0.001).

Concerning hair and nails, a recent study (2017) by Koseoglu et al. gave evidence of a significant decrease of iron levels (p < 0.001 for nails and p=0.001 for hair) among cases when compared to non-AD subjects.

Three further articles investigated cerebrospinal fluid metal concentrations, but with no statistically significant variation between the two groups.

Finally, four post mortem case control studies were performed on different regions of brain tissue and reported discordant results. Szabo et al., Graham et al. and Akatsu et al. found significantly higher iron levels in patients with a diagnosis of Alzheimer’s disease when compared with subjects without dementia. Graham et al. found higher iron levels only in severe AD patients. In Szabo et al.’s work there was an increase in iron concentration in the frontal cortex (p=0.018), but not in the ventricular fluid. The study of Akatsu et al. showed a similar increase in metal but only in the amygdala. The article by Yu et al. showed no statistically significant difference in the temporal cortex region.

Lead (Pb)

Eight articles concerning lead concentrations in AD and non-AD subjects met the eligibility criteria, as shown in Table 8. Six case control studies, carried out on serum or plasma samples, showed discordant results. Giacoppo et al., in their research on 89 AD and non-AD subjects, found a statistically significant decrease in blood lead levels (p<0.001) in patients with Alzheimer’s disease. The study by Lee et al., carried out in 210 subjects: 80 with a diagnosis of Alzheimer’s disease and 130 without, identified a significant reduction in blood lead concentrations between AD and non-AD subjects (p=0.035). Conversely, the same study also reported an increase in lead levels of patients with Alzheimer’s disease (0.20µg/dL compared with 0.17 µg/dL in the control group) when the evaluation of metal concentration was determined on serum samples.

Furthermore, five articles13,14,16,22,23 didn’t report any statistically significant variations in Pb levels between the two groups.                                                           

Concerning studies evaluating cerebrospinal fluid samples, similar discordant results were obtained, although on a smaller number of studies.19,23 In their article published in 2009, Gerhardsson et al, evaluating CSF samples in a population of 318 subjects (264 with AD and 54 controls) found no statistically significant difference in metal concentration between AD and non-AD patients. Conversely, the year before another case-control study,18 testing cerebrospinal fluid lead levels in 227 patients, reported a significant reduction (p<0.010) of metal in patients with Alzheimer’s disease compared with healthy controls.                                                           

Finally, a recent post mortem case control study, carried out in 2016 by Szabo et al. on brain tissue samples (frontal cortex and ventricular fluid) of 31 subjects, didn’t show any statistically significant differences in Pb levels among AD (N=16) and non-AD subjects (N=15).

 

Manganese (Mn)

As shown in Table 9, twelve articles concerning manganese concentration in biological matrices met the eligibility criteria. Six case-controls studies tested differences in blood metal levels, reporting discordant results. Baum et al. and Alimonti et al. didn’t report any significant variations in serum between AD subjects and healthy controls.

Gonzalez-Dominguez et al., compared 25 AD, 15 MCI and 25 non-AD, found significantly lower manganese serum levels (p<0.05) in AD patients when compared with healthy controls. Similar findings were reported by Arslan et al., in a study performed on 39 subjects (24 AD and 15 non-AD), (p=0.001), and by Paglia et al. on 118 subjects (34 AD, 20 MCI, 24 SMC and 40 non-AD) (p <0.001).

By contrast, one study19 showed a significant (p≤0.001) increase in mean plasma concentrations in 173 patients with AD, compared with 54 healthy controls.

Koseoglu et al., in their study on hair and nails samples of 122 subjects, found a significant decrease in metal concentration in both biological matrices among patients with diagnosis of Alzheimer’s disease when compared with non-AD subjects.

Concerning studies on cerebrospinal fluid matrices, Gerhardsson et al. in two articles, published in 2008 and 2009 respectively, reported significantly lower concentrations of manganese in subjects with AD (p ≤0.010 and p=0.004, respectively) than in controls.

Finally, three post-mortem case control study reported no statistically significant difference in Mn brain levels between AD and non-AD subjects, irrespectively from which brain region was investigated (frontal cortex, temporal cortex, hippocampus and amygdala).

 

Mercury (Hg)

Eight studies evaluated the difference in mercury concentration between subjects diagnosed with Alzheimer’s disease and healthy patients, as shown in Table 10. Among the five articles assessing blood levels,14,16,19,26,52 only one19 showed statistically significant variations (an increase) in metal levels, while the others found no substantial difference between cases and controls.

Concerning cerebrospinal fluid, Gerhardsson et al. described similar concentrations in cases and controls <0.21 µg/L whereas, a significant decrease in metal concentration in 62 AD patients was reported by Koseoglu et al in comparison to 60 controls.

Finally, a post mortem case control study showed no statistically significant difference in metal concentration of frontal cortex and ventricular fluid between 16 AD and 15 non-AD subjects.

 

Molybdenum (Mo)

Among the three evaluated articles, only one16 reported an increase (p=0.008) in molybdenum blood concentration in patients with diagnosis of Alzheimer’s disease compared to healthy controls. No statistically significant changes in metal levels were found between cases and controls in the other two reports, irrespectively from the biological matrix investigated (blood, cerebrospinal fluid) (Table 11).

 

Nickel (Ni)

As shown in Table 12, six studies assessing differences in nickel concentration between AD and non-AD subjects met the eligibility criteria.13,16,19,22,23,29 Independently from the biological matrix evaluated (plasma, serum, cerebrospinal fluid, brain tissue) no significant variation between patients with diagnosis of Alzheimer’s disease and healthy controls was found.

 

Selenium (Se)

Six articles evaluated selenium levels in patients with diagnosis of Alzheimer’s disease and healthy controls (Table13). Differences in metal concentrations were assessed in two biological matrices: blood and cerebrospinal fluid.                                                                                                

The five studies on blood samples15,16,17,26,32 showed non univocal results: three studies showed a decrease in selenium, whereas no substantial differences were reported in two studies. In detail, Giacoppo et al. reported a significant (p<0.001) decrease in selenium blood levels of 15 AD patients compared with the healthy control group (N=33). A study by Vural et al. found similar results, with blood Se levels that were significantly lower (p<0.001) in patients with Alzheimer’s disease compared to non-AD subjects ([Se]=58.15µg/dl in male and 58.43 µg/dl in female with AD; [Se]=67.84 µg/dl in male and 68.70 µg/dl in female without AD). A study by Paglia et al. found a significant decrease (p= 0.026) of selenium blood levels in AD subjects.

Another two case control studies on selenium blood concentration15,17 showed no substantial variation in metal levels between case and control.

The only available study performed on different biological matrices (Gerhardsson L et al., 2009) found no statistically significant difference between cases and controls in the cerebrospinal fluid samples of 318 subjects (264 AD and 54 healthy controls).

 

Tin (Sn)

As shown in Table 14, among the four articles that met eligibility criteria, no statistically significant variation in metal levels were found between case and control, irrespective of the biological matrix investigated (blood, brain tissue).

 

Vanadium (V)

As shown in Table 15, four articles met the eligibility criteria.13,16,17,19 The study by Paglia et al., carried out on 118 subjects (34 AD, 20 MCI, 24 SMC and 40 non-AD), showed a significant decrease (p< 0.001) in vanadium blood levels in AD subjects.

By contrast, Gonzalez-Dominguez et al. (studied population: 25 AD, 15 MCI and 25 Healthy Control), and Gerhardsson L et al. (studied population: 173 AD and 54 non-AD patients) found no significant differences among different groups. Conversely from plasma concentrations, Gerhardsson L et al. reported significantly decreased V concentrations in AD patients when compared with healthy controls ([V] =2.9 µg/L in AD versus [V] =3.2 µg/L in non-AD). Furthermore, Szabo ST et al. in their study carried out on brain tissue of 31 subjects (16 AD and 15 non-AD) detected no significant changes in frontal cortex and ventricular fluid vanadium levels.

 

Zinc (Zn)

As shown in Table 16, an overall number of 13 studies assessing zinc concentration in different biological matrices (blood, cerebrospinal fluid, hair, and brain tissue) were eligible. Many of the studies (N=9) evaluated metals levels in serum samples, with not univocal results: 5 studies out of 916,17,26,44,53 showed a significant decrease in zinc blood levels in patients with Alzheimer’s disease; no statistically significant variation in metal concentration emerged from a comparable number of articles.25,38,45 Only the research carried out by Alimonti et al. on 177 subjects resulted in increased levels of zinc among AD subjects when compared with healthy controls. Three further studies were post mortem case controls, undertaken on brain tissue samples, and all showed no significant variations in zinc concentrations between cases and healthy controls. One study19 was carried out on 318 subjects (264 AD and 54 healthy controls) and showed almost identical levels of CSF zinc in cases and controls.

Finally, Koç et al., 2015, in their case control study on a population of 78 subjects found a statistically significant (p=0,02) increase in hair zinc concentration among AD patients compared with healthy controls.

 

Discussion

Strenghts and limits of the study

Worldwide high incidence and prevalence of Alzheimer’s disease make its prevention and early diagnosis one of the most promising Public Health challenges. Nevertheless, to the best of our knowledge, this is the first systematic study focused on metals as environmental risk factors for Alzheimer’s disease.

Albeit the plethora of researches achieved, demonstrating the clear interest in the topic, mainly poor quality studies are available at the moment. Small sample sizes, no clear choice of exposed and unexposed subjects, high variability of results, lack of adjustment techniques for the most important confounding factors make it often difficult to draw conclusions. Indeed, as reported in Tables 2-16, overall quality of the included studies is quite poor, neither reaching the sufficiency threshold: according to the NIH tool score,12 the median score of the selected studies is 5.5 out of 12.                  

Furthermore, it is surprising not to find researches investigating interactions among metals, although a tight intercommunication among some of them is well known to influence their absorption, their bond to circulating transporters and regulatory proteins.56

 

Main findings

The most part of the 44 eligible studies investigating difference in metals concentration reported no univocal outcomes.57 Only some elements gave a clear tendency of results. Chrome, molybdenum, nickel and tin showed no real difference between AD and non-AD subjects, irrespectively to the biological matrix evaluated.

Conversely, a defined tendency to significant variation is available in blood and post-mortem studies investigating copper: blood levels were significantly increased in AD subjects while a decrease in brain tissue concentrations was observed in 70% of the studies, substantially agreeing with results of the metanalysis by Schrag et al.,58 that underlined heterogeneity of the study results on iron, copper and zinc except for brain copper levels, that were significantly depleted in patients with AD. Less robust results were recorded for nail copper levels, that were significantly decreased in patients with Alzheimer’s disease, although this data comes from only one study.

Differently from copper, higher levels of iron were found in the brain tissue of AD subjects, in particular in the Brodmann area, frontal cortex, and amygdala, while there was no significant difference in metal concentration in other biological matrices.

Conclusions

Although much is known about pathogenesis of Alzheimer’s disease, available evidence doesn’t explain all the factors involved in the pathologic process and further models, as “the brain trace metal dyshomeostasis” hypothesis, take place in order to explain those residual part of disease risk. Lifetime absorption of low-moderate metal concentrations, as those recorded in environmental exposure, make it difficult to clearly distinguish between exposed and unexposed subjects.

Besides aluminium, widely recognized as a risk factor for Alzheimer’s disease onset, it has to be pointed out that there is also some evidence of different metal concentrations between AD patients and healthy people, in particular for copper and iron. Concerning the first metal, blood levels are significantly increased in AD subjects, as there is a higher transport towards blood circulation to the detriment of noble organs, such as brain tissue, where copper is decreased, in particular in the hippocampus, amygdala, frontal cortex, cerebellum, motor and sensory cortex, cingulate gyrus, temporal gyrus, and entorhinal cortex. Conversely, in patients with Alzheimer’s disease, higher levels of iron were found in the brain tissue of AD subjects, in particular in the Brodmann area, frontal cortex, and amygdala, while there was no significant difference in metal concentration in other biological matrices. Moreover, because of the complex interaction among iron, copper and zinc, it could be effective to focus on studies investigating Cu/Zn, Cu/Fe, and Zn/Fe ratios as possible biomarkers of effect, instead of concentrating on measuring single metal levels in biological matrices.

Limited evidence was also found on manganese and vanadium: significantly lower blood and cerebrospinal fluid levels were recorded respectively.

Although these hypotheses are fascinating, with the aim to identify groups at risk and to begin to understand Alzheimer’s disease in terms of prevention, interpretation of this study should proceed with caution, in order to avoid speculation.

Indeed, in the environmental field, statistically significant associations do not provide direct evidence of a causal relationship between exposure to metals and Alzheimer’s disease. They trace a direction for future research, reinforcing support to the hypothesis that a dyshomeostasis of metals could be behind the pathogenesis of this disease, also without being its main causal factor.

Therefore, focusing further studies on a limited group of metals, excluding those with a clear tendency to non significant variations in AD subjects (e.g. chrome, molybdenum, nickel and tin), would be a useful approach.


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