Association between Cerebrospinal Fluid Biomarkers for Alzheimer’s Disease,APOE Genotypes and Auditory Verbal Learning Task in Subjective Cognitive Decline,Mild Cognitive Impairment,and Alzheimer’s Disease

Abstract

In the course of Alzheimer’s disease (AD), early pathological changes in the brain start decades before any clinical manifestation. The concentration levels of AD cerebrospinal fluid (CSF) biomarkers, such as amyloid-β_1-42 (Aβ_1-42), total tau (T-tau), and phosphorylated tau (P-tau), may reflect a cerebral pathology facilitating an early diagnosis of the disease and predicting a cognitive deterioration. The aim of this study was to estimate the prevalence of AD CSF biomarkers in those individuals with a subjective cognitive decline (SCD), a mild cognitive impairment (MCI), and Alzheimer’s dementia (AD-D), together with the relationships between the biomarkers, an APOE ɛ4 presence, and a verbal episodic memory performance. We included 252 patients from the memory clinic with a diagnosis of SCD (n = 85), MCI (n = 87), and AD-D (n = 80). A verbal episodic memory performance level was assessed and was based on a delayed recall trial from the 10-word list of an auditory verbal learning task (AVLT). We found that the patients with more severe cognitive impairments had significantly lower levels of Aβ_1-42 and higher levels of T-tau and P-tau. This pattern was also typical for the APOE ɛ4 carriers, who had lower levels of Aβ_1-42 than the noncarriers in the AD-D and MCI groups. The levels of T-tau and P-tau were significantly higher in the APOE ɛ4 carriers than in the noncarriers, but only in the MCI patients. The AVLT performance in the whole study samples was predicted by age, Aβ_1-42, and the T-tau CSF biomarkers, but not by the APOE genotyping.

Keywords

Alzheimer’s disease APOE genotype auditory verbal learning task cerebrospinal fluid biomarkers diagnosis mild cognitive impairment subjective cognitive decline

INTRODUCTION

Alzheimer’s disease (AD) is the most common type of dementia, where prevalence increases along with the aging of the world’s population. According to the World Health Organization’s (WHO) report, there were 35.6 million people living with dementia in 2010, with a growing number of new cases of 7.7 million each year [1]. AD pathology accounts for an estimated 60% – 80% of the demented population and may even occur 20 years before the clinical manifestations of demented disorders [2].

There is a growing interest in improving an early diagnosis and a detection of those patients who represent the stage in which only a mild neuronal damage is present. The most promising cerebrospinal fluid (CSF) biomarkers of AD when used to assess an in vivo pathology are the total tau protein (T-tau) levels, the tau phosphorylated at threonine 181 (P-tau) levels, and the amyloid-β (Aβ) peptides. The Aβ_1-42 peptide is especially the one most prone to an aggregation resulting in amyloid plaques [3], which are a neuropathological hallmark of the disease, along with the formation of neurofibrillary tangles (NFTs). The combination of these three markers increases the diagnostic accuracy for determining AD. Specificity, even as high as 80% –90%, and sensitivity at a level of 90% –95%, have been reported [4]. An increased concentration of the tau peptides and a decreased concentration of Aβ reflect the abnormalities that are consistent with AD.

Data also suggest that an apolipoprotein E ɛ4 allele (APOE ɛ4) presence appears to make the brain more prone to age-related pathological mechanisms, and carriers are more likely to develop AD at an earlier age than noncarriers [5, 6]. Moreover, normal elderly APOE ɛ4 carriers present decreased levels of Aβ_1-42 in the CSF biomarkers [6, 7].

From a neuropsychological point of view, the deficits in episodic memory are the hallmark of AD [8]. The brain regions known to be crucial for an episodic memory – such as the hippocampus and the surrounding entorhinal, parahippocampal, and perirhinal cortex [8, 9], are also the first ones where amyloid plaques are noticed, even in the preclinical stages of AD [10]. There is a growing number of studies suggesting that abnormalities in the CSF biomarkers are related to episodic memory deficits in AD and in the mild cognitive impairment (MCI) populations [11, 12].

Those patients with complaints concerning cognitive impairments, which have not been objectively confirmed, are often seen in memory clinics. However, studies have shown that some of them can later develop AD. Based upon meta-analysis data, researchers have estimated that 24.4% of patients with subjective memory complaints will probably develop cognitive disorders consistent with MCI within four years, while 10.9% of them will convert to dementia [13]. There is also some evidence that biomarker abnormalities consistent with an AD pathology are associated with a subjectively experienced cognitive decline, but are not detectable in a neuropsychological assessment [14, 15]. These findings lead to the conclusion that healthy individuals reporting a self-perceived impairment in cognition are at a greater risk of developing dementia. Therefore, a subjective cognitive decline (SCD) has been recently recognized as a precursor stage of preclinical AD occurring before an MCI and the dementia phase [14 , 17].

In this study, we investigated how the CSF biomarkers and an APOE ɛ4 presence contribute to the heightened risk of developing dementia in the SCD and MCI groups. There are many findings confirming that pathological levels of AD CSF biomarkers can be found, even in MCI patients, but there are fewer studies concerning the SCD patients.

Our second aim was to estimate the relationships between the biomarkers and a verbal episodic memory performance. We used delayed recall trial from the 10-word list of an auditory verbal learning task (AVLT). AVLT as a free recall memory task is commonly used in clinical practice for diagnostic purposes in dementia studies [12 , 18–21], as it is a highly sensitive measure of episodic memory decline in early AD and correlates with hippocampal volumes [22]. The question was whether the AVLT performance could be predicted on the basis of CSF biomarkers pathology, an APOE ɛ4 presence, and by the demographic characteristics in the groups at various stages of cognitive impairment. Since there has been evidence that pathological CSF profiles, even in MCI patients, were related to a reduced episodic memory [11], we wanted to explore the pattern of AD-related biomarkers among patients, who were without a confirmed cognitive decline, yet were complaining of a subjectively experienced worsening of intellectual skills (SCD). A patient’s performance on a verbal episodic memory test and its associations with the risk factors, such as those with an APOE ɛ4 presence, together with the AD-related CSF biomarker results, are all essential for a better understanding of the disease and to facilitate the design of AD prevention studies.

METHODS AND MATERIALS

Participants

252 participants were recruited from the outpatient memory clinic. All of the assessments were made during a two-day hospitalization in the Neurology Clinic of the Central Clinical Hospital in Warsaw, Poland. Due to the invasive nature of a lumbar puncture and the need for hospitalization, a patient’s willingness to undergo the procedures was crucial in the recruitment process. Therefore, the final sample is convenience sample, but no systematic demographic differences between those patients and the rest of memory clinic patients were observed. The participants underwent a neurological and psychiatric examination. The screening test included: a Mini-Mental State Examination (MMSE) [23] and a Clock Drawing Test [24]. The functional assessments were performed independently by using a Global Deterioration Scale (GDS) [25] and a Clinical Dementia Rating (CDR) [26]. All of the patients underwent neuropsychological testing, neuroimaging (CT or MRI), and a blood analysis (including APOE genotyping). This protocol was previously described elsewhere [27]. There was also a CSF biomarker analysis, both for general and biomarker testing. The diagnosis was established by neurologists, who also performed a lumbar puncture. All of the clinicians were blinded to the results of the CSF analyses and the APOE genotyping, which were obtained from the laboratory approximately three months later. Thus, the diagnosis was established blind to the values that are the subject of this study, and the incorporation bias was avoided [28]. The following inclusion criteria have been followed: an age above 50 y and fulfilling the criteria for SCD, MCI, and mild AD – D (CDR 1). Those patients with an MCI and AD were diagnosed according to the National Institute on Aging-Alzheimer’s Association (NIA-AA) criteria [29, 30]. The patients with SCD were classified on the basis of their performance on the neuropsychological tests using the guidelines of Jessen and colleagues [17]. The following exclusion criteria were implemented: an age below 50 y, fulfilling the criteria for severe depressive symptoms, a previous diagnosis of a major psychiatric disorder (i.e., bipolar disorder, psychosis), and a relevant history of alcohol or drug abuse, together with other serious medical conditions which might influence one’s cognition. This study was approved by the Ethical Committee for Medical Research of the Central Clinical Hospital in Warsaw, Poland. A written informed consent was obtained from all of the patients.

Cerebrospinal fluid

The CSF assays were performed blind to any clinical data. The samples were collected by a lumbar puncture using a non-traumatic spinal anesthesia needle through the L3/L4 or L4/L5 interspace, during the first day of a two-day hospitalization in standardized conditions, usually between 1 p.m. and 2 p.m. The AD biomarker concentrations were examined by using a sandwich enzyme-linked immunosorbent assay kit (ELISA) (Innogenetics, Gent, Belgium) in the hospital laboratory. The samples were stored at –80°C using polypropylene tubes. Raw scores of the Aβ_1-42, the T-tau, and the P-tau variables, were used in all of the analyses. However, the hospital laboratory estimated internal cut-off values that were pathological for AD. The CSF biomarkers level in 50 patients with AD-D were compared to 29 healthy subjects. To define optimal cut-off values, receiver operating characteristic (ROC) curve analyses were used to obtain the area under the curve (AUC) values, which were determined using the maximal sum of sensitivity and specificity (Youden index). The results are presented in Table 1.

APOE

The APOE evaluation was performed blind before the diagnosis. The DNA was obtained from peripheral blood lymphocytes by using a standard procedure. Two polymorphic sites, rs.429358 and rs.7412, consisted of an APOE genotype, and that was determined by using a TaqMan assay (Step One Plus Detection System, Applied Biosystems, Foster City, CA, USA) according to the manufacturer’s instructions. All of the patients were divided into two groups: noncarriers – without an APOE ɛ4 allele (APOE ɛ4–), and carriers – with one or more APOE ɛ4 alleles (APOE ɛ4+).

Neuropsychological assessments

A neuropsychological examination (a detailed description has been described previously) [27] was performed during the first day of the two-day hospitalization, before the lumbar puncture, and this was executed by experienced neuropsychologists. An evaluation of the specific cognitive domains served to establish the diagnosis in the described population. The clinicians were blind to the AD biomarkers in the CSF analysis and the APOE genotyping. In order to assess a verbal episodic memory, the 10–word list of an auditory verbal learning task (AVLT) [31] was used. For an episodic memory variable, we used the results (raw scores) in a delayed recall from the AVLT. Since episodic memory has been known as a neuropsychological marker for AD, it was decided not to take into account other neuropsychological results, but to focus only on the AVLT scores.

Statistical analysis

The statistical analyses were performed by using IBM SPSS Statistics 20.0.0. Before the hypothesis testing, we conducted a two-way ANOVA for the demographic variables (age, education, gender), and with the groups defined by a diagnosis as the first factor, and by the APOE status as the second factor. This was done in order to assess if all of the subgroup groups were comparable with respect to the demographic variables. Gender was coded as a binomial variable and was also included in the ANOVA model in order to maintain clarity of analysis. To access the differences in the AD biomarkers in the three groups of patients, while including the APOE ɛ4 status in the analysis, we conducted a series of two-way ANOVAS with a diagnosis and an APOE ɛ4 status, as the two independent factors. In the first step, we also controlled the influence of age and education (the years of education as a continuous variable) as a covariant on the analysis, but all age-related and education related effects in the model failed to reach any significance. As a consequence, age and education were excluded from the analyses. To evaluate the differences in an AVLT performance, while simultaneously taking into account the presence of an APOE ɛ4 genotype, we conducted two factors of ANOVA with a diagnosis status and the APOE ɛ4 presence, as independent variables. The Pearson correlation coefficient was used to test for associations between an AVLT delayed recall and the CSF biomarkers in the whole samples of the patients under investigation. Finally, in order to assess factors that could predict an AVLT performance in the entire study samples, we entered age, the three CSF biomarkers, and the presence of an APOE ɛ4 genotype, into a regression model, using a stepwise method. P-tau was excluded from this particular analysis, as it correlated too strongly with the T-tau variable. The results were considered significant at a level of p < 0.05.

RESULTS

Participant characteristics: Age, gender, education

Among the 252 subjects recruited for the study, 63% were women. The mean age for the whole sample was 65.49 (SD = 9.5), and the age range was 50–87 y. 17 people had a primary school education, 125 had a high school diploma, 95 had a university degree, and 15 had more than one faculty, or a Ph.D. The detailed descriptive statistics for age, gender, and education, are shown in Table 2.

Participant characteristics in the studied groups with regard to the APOE ɛ4 status and the AD biomarker concentrations in the CSF

The mean values of the AD biomarkers in the CSF and any ɛ4 APOE allele presence are shown in Table 3. In order to identify those patients who were at a greater risk of developing AD, we assessed how many patients had decreased levels of Aβ_1-42 and increased levels of T-tau and P-tau, simultaneously. We dichotomized Aβ_1-42, T-tau, and P-tau, based on the cutoff values that were estimated in the hospital’s laboratory, in order to construct an AD profile in the CSF. The frequencies of the pathological levels of the biomarkers are shown in Table 3.

Demographic differences between the study groups

Because we have compared the three groups, which were diverse in terms of age, gender, and education levels, we have controlled for the differences in those factors, as shown in Table 4.

The two-way ANOVA models showed that all of the groups were statistically comparable for gender. The age and education differences in the APOE ɛ4 status did not contribute to any differences. However, two significant differences regarding the diagnosis were found, and accordingly, were additionally post hoc tested. The participants in the AD-D group were, as was expected, significantly older than those in the MCI group (LSD test, MD = 5.81, p < 0.001) and those in the SCD group (LSD test, MD = 9.37, p < 0.01), together with those in the MCI group that were older than the SCD group (LSD test, MD = 3.56, p = 0.008). The AD-D group had slightly fewer years of education than those in the MCI group (LSD test, MD = –1.14, p = 0.033) and in the SCD group (LSD test, MD = –2.50, p < 0.01), and those in the MCI group had less years of education than those in the SCD group (LSD test, MD = –1.36, p = 0.010).

Differences in the AD biomarker concentrations in the CSF and in the APOE ɛ4 status in the studied groups

Differences in the AD biomarkers in all of the groups of patients, including the APOE ɛ4 status in the analysis, are shown in Table 5.

We observed a significant main effect of diagnosis on all of the three biomarkers. Those patients with a more severe cognitive impairment had significantly lower levels of Aβ_1-42 and significantly higher levels of the proteins T-tau and P-tau. There was also a significant main effect of the APOE ɛ4 presence in all of the study groups. The APOE ɛ4+ group had the lowest level of Aβ_1-42 and the highest levels of the proteins T-tau and P-tau. However, a statistically significant effect of interaction was observed only in the Aβ_1-42 peptide group. Figures 1, 2, and 3 show the described differences in the biomarker levels in details.

The AD-D group had, as was expected, lower levels of amyloid Aβ_1-42 than those in the MCI group (LSD test, MD = –244.51, p < 0.001) and in the SCD group (LSD test, MD = –374,62, p < 0.01), and those in the MCI group had lower levels than did the SCD group (LSD test, MD = –130.10, p = 0.002). The presence of APOE ɛ4 differentiated the participants in the AD-D group (t_df = 78 = 3.58, p = 0.001), and as shown, the interaction effect was even more pronounced in the MCI group (t_{df = 75.4} = 5.12, p < 0.001). The presence of APOE ɛ4 had no significant effect in the SCD group.

The participants in the AD-D group had, as was expected, higher levels of T-tau than those in the MCI group (LSD test, MD = 250.66, p < 0.001) and those in the SCD group (LSD test, MD = 329.41, p < 0.001), and those in the MCI group had higher levels than did the SCD group (LSD test, MD = 78.75, p = 0.023). The presence of APOE ɛ4 differentiated participants only in the MCI group (t_df = 32.6 = –2.8, p = 0.008).

The AD-D group had, as was expected, higher levels of P-tau than those in the MCI group (LSD test, MD = 23.52, p < 0.001) and those in the SCD group (LSD test, MD = 34.39, p < 0.001), and those in the MCI group had higher P-tau levels than did the SCD group (LSD test, MD = 10.86, p = 0.014). The presence of APOE ɛ4 differentiated participants only in the MCI group (t_df = 33,8 = –2.22, p = 0.033).

Verbal episodic memory

AVLT and an APOE ɛ4 presence

To assess the differences in the AVLT scores, while taking into account an APOE ɛ4 presence, we conducted a two-way ANOVA. The results are presented in Table 6. As expected, we observed the main effect being the diagnosis status. The patients with a more severe impairment had significantly lower scores in delayed recall from the AVLT. However, we did not observe the main effect of an APOE ɛ4 presence or any interaction effect.

AVLT and the AD biomarker concentrations in the CSF

The correlations between the AVLT scores and the levels of the AD biomarkers in the whole sample are shown in Table 7. As expected, decreased levels in the AVLT performance correlated with a decreased level of the Aβ_1-42 peptide, together with increased levels of the T-tau and P-tau proteins. Those correlations are computed for the whole study sample. We did not analyze the groups separately, as in this case the variance of memory test would be too low for the most severely impaired patients (due to a floor effect).

AVLT: Regression analysis

In order to assess which factors influenced the AVLT performance, we entered age, Aβ_1-42, T-tau, P-tau, and an APOE ɛ4 presence, into a regression model using the stepwise method. The protein P-tau was excluded from this analysis, as it correlated too strongly with the T-tau variable. The model was matched to data F (3,248) = 52.2; p < 0.001; adjusted to r² = 0.38. The APOE ɛ4+ variable did not predict with any statistical significance scores in the AVLT performance and it was not added to the model. However, the age variable (β= –0.214, p < 0.001), Aβ_1-42 (β= 0.289, p < 0.001), and T-tau (β= –0.328, p < 0.001) were important predictors of the AVLT scores.

DISCUSSION

This study has shed light on identifying factors that might be helpful in improving the early detection of patients at a risk of developing AD. We investigated the associations between the presence of cognitive complaints and verbal episodic memory deficits in the SCD group and the MCI group when compared to the AD-D group, and we focused on the AD-related biomarkers in the CSF and the APOE genotypes.

An APOE ɛ4 presence, along with the CSF biomarkers, identified patients at a greater risk of developing dementia due to AD. The most valued finding of this study was the abnormal concentration of biomarkers, suggesting an underlying AD pathology that was present, not only in a significant part of the MCI group but also in a number of the SCD patients. In detail, 23.5% of the SCD group already had abnormal levels of Aβ_1-42 and 31.8% had the T-tau protein. Although Aβ_1-42 is known for being a biomarker of AD, the T-tau protein is not specific, and since this group was younger, the abnormality of this biomarker in the SCD group needs to have a different origin. It may possibly be caused by other diseases affecting the neuronal tissue.

Accordingly, nearly half of the MCI patients had pathological levels of Aβ_1-42 (44.8%) and the T-tau protein (43.7%). It suggested that the presence of an objective cognitive decline, especially in the AVLT performance, was caused rather by AD pathology. Therefore, it is not surprising that the full CSF AD profile (which is defined as an Aβ level below norm and T-tau and P-tau proteins above norm) was present in 6 of the SCD patients (7.1%) and 24% of the MCI patients, respectively. Visser and colleagues [15] have also reported that an AD CSF profile is common in SCD patients. However, their percentage was much higher than in our sample (about 52%). These differences may be due to the fact that the authors defined the CSF AD profile differently – as the ratio of Aβ_1-42: tau. Nevertheless, the described findings concordantly point to the possibility that subjectively experienced cognitive decline may reflect an early pathological manifestation of the underlying disease.

As expected, the AD biomarker concentrations in the CSF differentiated those patients due to a diagnosis, which had been established independently, and before any CSF assays. Those subjects with a more severe cognitive impairment had significantly lower levels of Aβ_1-42 and significantly higher levels of the proteins T-tau and P-tau. The AD-D group had higher levels of T-tau and P-tau and lower levels of Aβ_1-42 than the nondemented groups, and the MCI group had higher levels of the tau proteins and lower levels of Aβ than did the SCD group, respectively. Risacher and colleagues in their study found that diagnosis was associated with the tau proteins, but not with Aβ, in groups of healthy controls and MCI patients [6], which is partially consistent with our findings. As regards to the differences between the MCI and the AD-D participants, the study reports were inconsistent. De Riva and colleagues found that only Aβ_1-42 differentiated those groups of patients [32], but Struyfs reported that they could not find any differences between the MCI group and the AD group when using these three biomarkers [33]. They expected to find no difference in those particular two groups, because the underlying pathology is the same, and that the levels of the biomarkers are changed minimally in the course of a disease’s evolution.

With regards to an APOE ɛ4 presence, we found that the APOE ɛ4 carriers had a lower level of Aβ_1-42 and a higher level of the proteins T-tau and P-tau than did the noncarriers. However, a statistically significant effect of an interaction was only observed in the case of amyloid Aβ_1 - 42 .

By analyzing each group of patients separately, we found that the APOE ɛ4 carriers in the MCI group and in the AD-D group had statistically lower levels of Aβ_1-42 than did the noncarriers. Although carriers in the SCD group had a lower level of Aβ_1-42 than did the noncarriers, the effect did not reach any statistical significance. Struyfs and colleagues found that the noncarriers had higher levels of Aβ_1-42 when compared to the carriers, but this difference was not significant when the AD group and the MCI group were analyzed separately [33]. This can be explained by the fact that an APOE ɛ4 presence is a risk factor for AD, but it is also for AD co-pathology in non-AD dementias [34].

The levels of the tau proteins were significantly higher in the APOE ɛ4 carriers than in the noncarriers, but only in the MCI patients. These findings are not consistent with previous studies [35], which have shown that even in healthy control APOE ɛ4 carriers, they had significantly higher levels of the T-tau and P-tau proteins when compared with the noncarriers. However, the results of Mosconi and colleagues are limited by relatively small sample sizes.

The neuropsychological assessment also revealed some interesting results. We found a positive correlation between the AVLT performance levels and Aβ_1-42 and a negative correlation with the T-tau and P-tau proteins, respectively. These findings are similar to other study findings that reported the presence of a positive correlation between the verbal episodic memory performance and the amyloid levels in the MCI patients, but not in the AD-D group [12]. Haldenwanger and colleagues did not find a correlation between the tau proteins and the cognitive performance in their studied groups [12], which is the opposite of our findings. In contrast, Pettigrew and colleagues found that verbal episodic memory was not correlated either with the Aβ_1-42 peptide or the T-tau protein [36].

Consequently, we investigated whether an AVLT performance can be predicted by CSF pathology and APOE genotyping, while taking the demographic information into account. We found that of all the studied factors, only the patient’s age, the Aβ_1-42 peptide, and the T-tau CSF biomarkers, were important predictors of the AVLT performance. Grambaite and colleagues reported that the CSF biomarkers were significant predictors of a cognitive performance [37]. They showed in their study that Aβ_1-42 was a predictor for verbal episodic memory and visuospatial memory in a group with SCD and T-tau in a MCI group.

APOE genotyping did not predict the AVLT performance, and also, did not differentiate the investigated groups of patients. The APOE ɛ4 carriers had lower scores in the delayed recall trials in all of the groups, but these differences did not reach any statistical significance. These results are similar to previous findings. Pettigrew and colleagues found that an APOE ɛ4 presence was not associated with a cognitive performance and that there were no significant differences between the carriers and the noncarriers [36].

When interpreting those results, some theoretical issues regarding episodic memory should be taken into consideration. Some studies prove that free recall tests may not be good predictors of real-world cognitive problems [19 , 38]. The original theoretical concept of episodic memory is strongly related to recollecting events in their spatiotemporal context and it is explicitly tied in with self-awareness of this process [39]. Tulving suggests that in verbal learning tests those two important features of episodic memory are missing [39]. In this sense AVLT based on free recall has less ecological validity than What-Where-When (WWW) memory tasks. WWW tests are commonly used in animal studies on episodic-like memory [40, 41]. In a human subject, experimental research on memory is often conducted in the form of a computer game, virtual reality, or real-world tasks [19 , 42]. Still, the AVLT is more user-friendly in the memory clinic settings due to the less time-consuming procedure and the fact that many older people may struggle with the virtual reality version of the task. Also, researchers prove that free recall tests reflects hippocampal impairment [18, 22] and are referred as standard episodic memory tests in specialized clinical settings [19, 43].

Some additional demographic factors must be taken into account when interpreting obtained results. Since age and education related effects were controlled in the analysis, the overall influence of those demographic variables is worthy of a consideration. The severity of the impairments differentiated the groups in accordance with age. The AD-D group was, as expected, significantly older than the nondemented ones, and the MCI group was older than the SCD group, respectively. This can be explained by the fact that the SCD patients came earlier to the physicians in order to obtain a diagnosis and receive treatment, before their symptoms became noticeable, than did those in the MCI group. Some of the MCI and AD-D patients might have been less critical of their difficulties, and they would only report any cognitive problems to their physicians, when someone from their families had concerns about their memory.

With regards to education, the AD-D group was slightly less educated than the nondemented ones, and accordingly, the MCI group had fewer years of schooling than did the SCD patients. These differences correspond to general age-related educational cohort differences in Poland [44]. The older populations are generally less educated than the younger generations. The more educated participants have more demanding occupations, more challenging social activities, and any minor cognitive problem could really affect their normal functioning. The better educated people are more aware of their symptoms and the risk factors of developing dementia, or have an AD history in their families, so they report their cognitive impairments earlier.

Rolstad and colleagues found that a higher level of education possibly had a modifying effect on the CSF concentrations [45]. They reported that the higher educated MCI patients had lower levels of T-tau at the baseline, when compared to those with a lower educational level. It was also observed that age was significantly associated with the T-tau concentrations. In another study that considered MCI patients who had converted to dementia, those that were highly educated had lower levels of the Aβ_1-42 peptide when compared to those with a lower education [46]. This was probably due to the cognitive reserve (CR) effect.

This study has had some limitations, as expected. One of them was a cross-sectional design, which does not allow for causal interpretations and restricts the predictability of the CSF values to those of a current cognitive performance. Also, the subjects were recruited from the memory clinic, which makes a generalization of the whole population less reliable. Our sample of patients differs from the general population in terms of education. There are only 17 subjects with a primary school education. Some studies suggest that APOE ɛ4 genotype frequency is lower in population samples than in clinical-based samples [47]. Also, there could be more subjects with a family history of AD, which may have been an additional motivation to report to the memory clinic. Another potential difference is that surprisingly men are overrepresented in the group of MCI subjects in our sample. In general, women are disproportionally affected with AD because they live longer, on average, than men [48, 49]. However, some studies show that there is a higher prevalence of MCI in men [48, 50]. Mielke and colleagues suggest that men may die earlier in life due to competing causes of death, and only the most resilient may survive to older ages. The higher incidence of MCI in men may also suggest that women convert to dementia at a later age but more abruptly [50]. The following additional drawback was the lack of a healthy control group – in other words, those without any cognitive complaints. Assessing healthy patients would be difficult for organizational and ethical reasons, as the procedure of a lumbar puncture is invasive.

In conclusion, the SCD patients appear to be a group of particular interest for the early detection of a neurodegenerative disease. The fact that these patients reported a cognitive decline, when there were no significant deficits in the neuropsychological evaluation, is important and points to the existence of a cognitive reserve. Those subjects with a higher CR may have a more severe pathology, especially when deciding to contact a clinician, than those with a lower CR. This can be explained by the fact that a more pronounced neuronal damage is needed to expose any signs of cognitive impairment, because of the more effective connections and the neuronal pathway usages in those particular cases [45]. That is why a more and more efficient criterion is needed to select those subjects who can see and display a subjectively worsening cognitive performance, and in particular, when that condition was failed to be observed by the clinicians. Intensive studies are needed so that more efficient prevention strategies can be planned. Some studies imply that future AD prevention treatment is the application of anti–amyloid therapies, to be taken as soon as possible, preferably in the pre-symptomatic stages [7 , 52].

To sum up, a CSF analysis is crucial for a very early identification of those patients who are at a risk of developing AD. In the SCD group, its value is not to be underestimated. The AD CSF biomarkers correlate with one’s AVLT scores, which is an established marker of AD. However, the obtained results of an APOE genotyping analysis failed to support its utility in predicting an AVLT performance. The results of our cross-sectional study are preliminary and they need further confirmation in longitudinal studies on a larger sample of patients.

Footnotes

ACKNOWLEDGMENTS

This study is part of the BIOMARKAPD project within the EU Joint Program for Neurodegenerative Disease Research and is supported through The National Center for Research and Development (3/BIOMARKAPD/JPND/2012).

Authors’ disclosures available online ().

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