estrogen receptor alpha - polymorphism, genetic - aging - cognition
receptor alfa de estrogênio - polimorfismo genético - envelhecimento - cognição
Global aging is a challenging reality of this century. In 2010, there were 576 million people aged 65 years and over, worldwide. The expectation for 2050 is that this number will triple, reaching 1.5 billion people[1]. It is important to note that the highest population aging rates occur in developing countries. It is expected that between 2010 and 2050 the number of older people in developing countries will grow around 250%, while in developed countries the rate will be 71%[1]. Notably, the change in aging is rising: a child born in Brazil in 2015 has a life expectancy 20 years longer than Brazilians who were born just 50 years ago[2].
The maintenance of cognitive skills is fundamental for preservation of autonomy and quality of life among elderly individuals. Age-associated cognitive decline is characterized by changes in attention regulation, processing speed and memory capacity[3]. Cognitive decline is one of the most feared aspects of old age. It is also the costliest, in terms of financial, personal and social burdens[3].
Normal aging is characterized by some impairment in cognitive performance, which is insufficient to cause dependence. Mechanisms have been proposed as causes of this decline, such as a decrease of brain size and reduction in neurons and synapse counts[4]. Other candidates include oxidative stress, telomere attrition, hormonal dysregulation and immunosenescence[5]. However, a lack of understanding about the physiopathology of cognitive aging remains. Since cognition is a core feature associated with functional performance in senescence, knowledge about mechanisms and strategies for its preservation is crucial.
The estrogen receptor-alpha (ESR-1) belongs to the nuclear hormone receptor superfamily and acts as a ligand-activated transcription factor. The human ESR-1 gene is located on the long arm of chromosome 6 (6q25.1) and contains eight exons separated by seven intronic regions[6]. The ESR-1 is widely expressed in the female genital tract, however, it is also found in the kidneys, liver, brain, heart and most immunologic system cells[7]. Estrogen receptors are located in the brain, especially in regions involved in learning and memory, such as the hippocampus and amygdala[8]. The main polymorphism in the ESR-1 gene is located in intron 1 and consists of the -351A/G substitution (rs9340799, XbaI A > G), which creates a restriction site for the XbaI enzyme[9],[10]. A possible functional mechanism attributed to this polymorphism of the ESR1 gene includes a change in the mRNA processing, producing different variants or isoforms of the protein with impairment of its function. Moreover, estrogens are important in maintaining brain function in regions typically affected by Alzheimer’s disease (AD), thus variations in estrogen exposure over a lifetime may affect cognitive decline associated with AD[11].
The aim of this study was to evaluate whether genotypes and alleles of XbaI polymorphism in the ESR1 gene are associated with short-term cognitive decline observed in healthy oldest-old individuals from a previous community-based study.
METHODS
Study design
This prospective investigation was conducted with a subgroup of participants from the Pietà study, a population-based investigation on brain aging in subjects aged 75+ years from Caeté city, Brazil. Baseline characteristics of the sample, diagnostic criteria and methods are described elsewhere[12].
Subjects
A total of 28 cognitively healthy participants (14 men and 14 women) from the Pietà study were included in the present analysis. Absence in cognitive impairment or dementia was ascertained by standard criteria and consensus discussions among clinical investigators. All cognitively healthy individuals participated of cognitive evaluations in 2008 and 2009 and donated a peripheral blood sample for genetic analysis. All participants or their legal representatives signed the written informed consent and the study protocol was approved by the Ethics Committee of the Federal University of Minas Gerais.
Cognitive evaluation
In this study, 52 cognitively healthy individuals were genotyped for the XbaI A > G polymorphism, but only 28 were evaluated in cognitive follow-up assessments. The cognitive evaluation of the 28 elderly individuals [median age (IQ) = 78 (5) years] was performed through brief cognitive tests, including the Mini-Mental State Examination (MMSE), animal category fluency test, and picture drawings memory test, with learning and delayed recall phases. The association of the MMSE and these brief cognitive tests is useful for discriminating healthy individuals, and those with dementia in Brazilian epidemiological studies[13]. All cognitive tests were administered twice, within a one-year interval, and were normalized by the participant’s schooling level. The Z scores derived from the cognitive tests were used to calculate a global cognitive score. The individuals were then categorized in two distinct groups according to the difference between the global cognitive scores obtained in 2008 and in 2009: decliners, those who showed short-term cognitive decline; and non-decliners, those who demonstrated short-term stability or even improvement in cognitive abilities on follow-up evaluation.
Molecular analyses
Polymorphism genotyping was performed in DNA extracted from peripheral blood, collected in tubes with EDTA (VacuetteTM) using Biopur (BiometrixTM). Genotyping of the XbaI A > G was performed through PCR-RFLP using the restriction enzyme XbaI (FermentasTM) and electrophoresis in polyacrylamide gel 6%.
RESULTS
The main results for cognitive evaluations are shown in the [Table]. We found that individuals with XbaI AA genotype were more frequent in the cognitive decliner group (p = 0.037). Additionally, XbaI G allele carriers more frequently showed cognitive stability or improvement (OR = 1.667, CI95% = 1.103-2.519; p = 0.018) compared to carriers of the A allele. Logistic regression analysis confirmed that these findings were independent from age, sex and the presence of the ApoEε4 allele (all p > 0.05).
Table
Short-term cognitive outcome in cognitively healthy participants according to XbaI A > G genotype. Variables expressed in n (%).
|
SNP XbaI A>G
|
Decliners (n = 13)
|
Non-decliners (n = 15)
|
p-value*
|
|
GG
|
0 (0.0)
|
2 (26.7)
|
|
|
GA
|
0 (0.0)+
|
4 (13.3) ++
|
0.037*
|
|
AA
|
13 (100.0) ++
|
9 (60.0) +
|
|
|
GG
|
0 (0.0)
|
2 (13.3)
|
0.484
|
|
A carriers
|
13 (100.0)
|
13 (86.7)
|
|
|
AA
|
13 (100.0) ++
|
9 (66.7) +
|
|
|
G carriers
|
0 (0.0) +
|
6(33.3) ++
|
0.018*
|
*significant, χ2 test. + less frequent ++ more frequent. A carriers: genotype AA + GA; G carriers: genotype GG + GA; SNP: Single Nucleotide Polymorphism.
DISCUSSION
Our findings indicated that the genetic variant XbaI A > G may have an impact on cognitive performance in cognitively normal aging. Some hypotheses for the functional significance of this polymorphism shall be discussed. Cheng et al.[14] reported that ERS-1 polymorphisms might influence the gene expression and affect estrogen function. Given its location, 351 bp upstream from the start of exon 2, a mechanism including changes on ESR-1 expression via altered binding of transcription factors and influence on alternative splicing of the ESR-1 gene has been suggested[15]. Such activity appears to be influenced by the genotype at the time of enhancer activity of the G allele when compared to the A allele. In the present study, our results corroborate this hypothesis, as the elderly participants carrying the G allele remained stable or showed improvement on cognitive evaluations. As well, several studies have already found an association between the A allele and increased risk of developing AD[16],[17],[18]. Usui et al.[19] found a marginal association between the A allele of XbaI and an increased risk for AD in elderly Japanese men and women. Similarly, male Italian old individuals with XbaI AA genotype showed an increased risk for AD[20].
Amantea et al.[11] demonstrated that estrogen reduces cell death induced by exocitotoxic stimulation in neuronal cultures. Additionally, Xu et al.[21] proposed that ERS-1 acts as a transcriptional factor that regulates the gene expression and function by interacting with regulatory regions of target genes. Moreover, some neuroprotective actions such as growth and survival of cholinergic neurons, increased cholinergic activity and antioxidant properties have been attributed to estrogen in pathological contexts[22]. It was also verified that physiological concentrations of estrogen reduce in vitro β-amyloid peptide[11]. Therefore, estrogen decreases the risk for the development of AD. Here, we suggest that the estrogen pathway may be involved in neuroprotection, even in healthy brain aging.
Finally, we suggest that the genetic variant XbaI A > G may exert a positive impact on cognitive performance in cognitively healthy aging. Methodological limitations need to be considered, such as the absence of estrogen levels data and the small sample size. However, we were unable to find another similar study that has investigated the influence of XbaI A > G on cognitive decline in the healthy oldest-old. Therefore, our preliminary data may motivate more robust studies designed for this purpose.
