The role of genes and their encoded products in aging.
Normal aging begins after 60 years of age. According to Harman, the accumulation of free radicals, which results from weakening of repair and protective mechanisms, takes place in the aging brain. It is believed that especially in the population of the most elderly there is increased incidence of both dementia and depression. The causes of these central nervous system disorders in the aging human body are changes at the molecular level, such as changes in the biochemical parameters, the accumulation of mutations in nuclear and mitochondrial DNA, and epigenetic changes. Biomarkers associated with aging of the brain include accumulated deposits of β-amyloid (Aβ), disturbed cholesterol homeostasis, altered neuroimaging parameters, and impaired glucose metabolism. Genetic factors are also responsible for normal aging, for example, SIRT1, AKT1, and CDKN1A, and among them the longevity genes, such as FOXO3A and CETP. Dementia as well as cognitive decline may be modified by poly-T variants of TOMM40 and APOE alleles via influencing the level of apolipoprotein E (apoE) in the brain and in the plasma as well as by its ability of Aβ clearance.
- molecular factors
- normal aging
Currently, average life expectancy in the world is over 60 years. The world’s longest life expectancy is in Japan, at 82.2 years, and in Australia, at 80.6 years. In Europe, the longest-lived people are the French, at 80.6 years, the Swedes, at 80.6 years, the Italians, at 79.9 years, the Greeks, at 79.3 years, the Dutch, at 79.1 years, and the Germans, at 78.9 years. It is predicted that in Europe from 2005 to 2050, the number of people following into their 80th year of life will increase by 43 million .
In psychological studies on the elderly, three subperiods were stratified among people over 60 years old; these included
Progressive aging of the population is one of the factors determining the increasingly frequent occurrence of cognitive impairment and dementia syndromes. Dementia, due to its prevalence in the population (it occurs in approximately 10% of those 65 years of age and in approximately 30–40% of those 90 years of age), requires great concern and clinical care. It is estimated that by 2040, the number of elderly people with dementia in the world will exceed 80 million .
According to the classification of mental disorders in the American Psychiatric Association’s DSM – IV (Diagnostic and Statistical Manual of Mental Disorders) , there is no isolated, separate diagnostic category for “dementia,” but the criteria for this diagnosis are contained in the various types of dementia, for example, Alzheimer’s disease (AD), vascular dementia (VD), or in other diseases. According to these criteria, a diagnosis of dementia is necessary to determine the presence of multiple cognitive deficits that cause significant disturbances in the functioning of social exclusion and mental illness (depression) and delirium.
It is believed that the functional and cognition changes observed in older persons are associated with disturbances at the molecular level in the aging body. Molecular changes in the aging process may relate to genomic instability as a result of accumulation of mutations, telomere attrition and epigenetic alterations, and alteration in the level of brain biomarkers .
To select significant studies for this review, the authors conducted multiple searches through public databases, including PubMed and Scopus, by using the following search strategy: (“normal aging” or “aging”) and (“dementia” or “cognitive decline”) and (“biomarker” or “SNP” or “genetic polymorphism” or “mutation”). The last search was performed in February 2016. A subsequent data mining through review articles and references facilitated finding additional eligible studies.
2. Brain biomarkers and cognitive function in normal aging
Central changes leading to impaired brain and cognition functions have been reported in normal brain aging, but data are inconclusive [5, 6]. A study  using functional magnetic resonance imaging (MRI) with gadolinium contrast confirmed changes in the hippocampus associated with impairment of cognitive function in elderly people. Also, a study conducted on 564 cognitively normal individuals (average age was 78 years) using MRI and fluorodeoxyglucose positron emission tomography (FDG-PET) and Pittsburgh Compound B (PiB) PET indicated impairment of cognition and imaging biomarkers. The causes of these central changes in the brain of the aged subjects seem to have been increased β-amyloid (Aβ) levels . In the senescent brain, accumulation of Aβ deposits is eminent, in the form of senile plaques as well as fibrillary tangles in the neurons. The lesions may develop in the human brain as late as in one’s 80s (frequently with no signs of dementia). The slower the accumulation of lesions is, the longer the time period required to develop dementia [7, 8]. Cerebral amyloidosis has been associated not only with Aβ deposition but also with higher pulse pressure in the presence of neurodegeneration, which may lead to more rapid progression of dementia . However, more recent data indicate that Aβ deposition may in time exceed brain structural changes, such as grey matter volume, as measured by MRI, and neuronal hypometabolism assessed using PET with 18F-fluorodeoxyglucose (FDG) . Moreover, the cognitive decline in elderly patients is associated with brain infarcts . Also dietary factors, such as ω-3 polyunsaturated fatty acids (PUFAs), were shown to be associated with normal brain function; the PUFA concentration remains in reverse correlation with brain atrophy in cognitively normal elders .
3. Molecular changes in normal aging and dementia
In the aging process, the epigenetic changes lead to expression alteration of genes associated with vital functions of cells, such as mitochondrial function, as well as protective and repair mechanisms, as shown in Table 1.
One of genetic hallmarks of aging is genomic instability which includes accumulation of genetic damage both in nuclear DNA (nDNA) and mitochondrial DNA (mtDNA). mtDNA is more susceptible to DNA damaging factors than nDNA due to its oxidative environment. Also DNA repair mechanisms are less efficient in mitochondria than in nucleus , although mitochondria have most of the DNA repair pathways existing in the nucleus . The mitochondrial reactive oxygen species (ROS) theory of aging (mitochondrial free radical theory of aging [MFRTA]) proposed by Harman assumes that free radicals generated in normal metabolism cause mtDNA mutations and ageing is a result of oxidative damage accumulation. According to MFRTA, maximum life span can be decreased by mtDNA damage caused by oxidative stress . Mitochondrial reactive oxygen species (mtROS) may play signaling role in mitochondrial stress during ageing . It is also suggested that mtDNA mutation accumulation and mitochondrial dysfunction during aging are a result of decreased activity of autophagy and mitophagy . Moreover, a study performed on 18 three-generation families of women shows decline in mtDNA copy number, mitochondrial protein expression, and oxidative function with age . Oxidative DNA damage and mitochondrial dysfunction lead to neuronal loss and may play a role in the development of dementia. The decreased level of antioxidants was observed among dementia individuals [29, 30]. It was suggested that high levels of ROS and decline in neuronal DNA damage response may be associated with neuronal dysfunction and cognitive impairment characterized by lower Mini-Mental State Examination (MMSE) score . Additionally, it is known that oxidative damage leads to frontal-executive dysfunction .
|Gene/encoded product||Locus||Role in aging||References|
|10q21.3||Age-related decreased level of SIRT1 is associated|
with impaired oxidative stress response and changes in glucose metabolism. Indirectly may be involved in age-related diseases, for example, retinal degeneration, hypertension, and cardiovascular diseases.
protein Kinase B
|14q32.32||Decreased level of AKT1 with age alters|
regulation of glucose metabolism, apoptosis,
cell proliferation and cell migration, and PI3K/AKT/mTOR pathway.
Cyclin-dependent kinase inhibitor 1 (p21)
|6p21.2||Possible promoter of aging due to pro-aging activity of p53.Oxidative stress increases expression of CDKN1A and overexpression of p21 may be involved in age-related diseases such as atherosclerosis, amyloidosis, AD, and arthritis.|||
Cholesterol ester transfer protein
|16q21||Responsible for cholesterol homeostasis in central nervous system. Decreased level of CETP|
results in healthier aging, slower memory decline, less frequency of dementia, and lower AD risk.
Transcriptional factor FOXO3A
|6q21||Involved in insulin metabolism and insulin/IGF1 signaling pathway.|
Protection from oxidative stress and reduction of age-related diseases.
|12q23.2||Decreased level of IGF-1 with age leads to cell senescence.|||
|7q21.3||Decreased level of PON1 with age impairs oxidative stress response and is a risk factor for cardiovascular diseases due to LDL oxidation.|||
Another mechanism involved in aging is epigenetic alterations. Epigenetics is defined as molecular traits that are “stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence.” The epigenetic pathways include DNA methylation, histone modifications, and noncoding RNA .
The analyses of CpGs methylation changes show that genome-wide global levels of DNA methylation decrease during aging. Despite this, many promoters of genes which are unmethylated in young gain methylation in old individuals [34, 35]. Other authors  suggest that DNA methylation changes may result in age-associated immune deficiency. It is possible that hypermethylation is caused by programmed changes, while hypomethylation may be the result of environmental and stochastic processes. Multiple studies have identified genes undergoing hyper- and hypomethylation with age. The first group includes genes involved in process such as cell adhesion, cell-cell signaling, ion transport, neuron differentiation, and other genes associated with development. The hypermethylated genes are regulated by a common group of transcription factors, whereas hypomethylated genes are involved in metabolic process, RNA splicing, regulation of ligase activity and protein ubiquitination, transmission of nerve impulse, and many others. The hypomethylation in neurons may cause age-related memory deficits [36, 37]. The abnormal profile of methylation may contribute to dementia. It is shown that mutation in DNA methyltransferase 1 (
Histone modification includes acetylation, methylation, phosphorylation, citrullination, ubiquitination, SUMOylation, adenosine diphosphate (ADP) ribosylation, deimination, and proline isomerization, in which the first three are the most-studied topics. Modifications can change the chromatin structure by histone-histone or histone-DNA interactions. The chromatin packaging affects many processes such as transcription, repair, replication, and chromosome condensation. Acetylation is associated with activation of transcription, while the result of lysine methylation can be either activation or repression . Chromatin packaging changes during aging . It is shown that higher level of histone acetylation facilitates memory and learning processes; therefore, acetylation decrement may lead to cognitive impairments and is associated with aging . Also histone methylation may affect the life span; loss of H3K9 trimethylation which results in reduction of heterochromatin is the hallmark of aging . The acetylation of H4K16 is necessary for maintaining the structure of chromatin and when impaired, the process of double-strand break repair is less efficient . Subsequently, histone tail proteolytic cleavage, especially H3, may be involved in aging, but the exact mechanism remains unclear . Additionally, the decline in histone chaperon levels is observed during aging and may be the answer for defective DNA repair mechanisms .
Both age-related changes in DNA methylation and histone modification alter the experience-dependent synaptic plasticity in hippocampus by changing the chromatin structure. Thus, it may be involved in memory loss and learning difficulties. Epigenetic changes may be possible brain biomarkers of cognitive aging .
4. Genes associated with age-dependent dementia
Several genes are involved in age-dependent dementia. Most of them, such as
APP, PSEN1, and PSEN2
Aβ is formed in a process called an amyloid cascade which involves the amyloid precursor protein (APP), encoded by the
TOMM40, translocase of the outer mitochondrial membrane 40 homolog (TOM40)
It has been suggested that Aβ may exert intracellular toxicity mediated by TOM40, for example, by affecting the function of cellular power plants—the mitochondria. According to , the mechanism underlying TOM40’s role in dementia involves its ability to uptake Aβ to the mitochondrion, as Aβ has been found to be co-localized with cristae proteins in the mitochondria. Subsequently, after absorption the intracellular Aβ would cause increased production of ROS, thus leading to DNA damage and premature neuronal death.
Several studies have shown that poly-T
These studies suggest that the
5. Cholesterol, lipoproteins, and dementia
Of the many lipids, cholesterol is believed to play a major role in brain function and development, as the brain contains as much as 23% of the total cholesterol deposits . One of the most pronounced groups of genes described as dementia risk factors are involved in the transport of cholesterol and may be accounted for as apolipoproteins . A misbalanced lipid metabolism may be associated with memory loss . According to another paper , patients with higher levels of high-density lipoproteins (HDLs) had a decreased risk of developing dementia at the time of the study and in the future. For patients from the upper quartile (with a plasma HDL concentration higher than 55 mg/dL), the dementia hazard was decreased by 60%. Studies on the subjects were continued by several other teams; however, the obtained results seem to be rather inconsistent [61–67].
Generally, the HDL level is believed to negatively correlate with the prevalence of dementia in elderly people; however, many studies have implied that HDL influence may be characteristic of the VD development [58, 68].
CLU, apolipoprotein J (apoJ)
Apolipoprotein J (apoJ, also known as clusterin), encoded by the
APOE, apolipoprotein E (apoE)
ApoE is encoded by the
ApoE in physiological conditions is a major cholesterol carrier and one of the most vital proteins responsible for maintaining cholesterol homeostasis in the brain. ApoE is mostly synthesized by astrocytes and probably does not cross the blood-brain barrier .
A recent study  on transgenic rabbit
A study  analyzing the association of plasma and CSF apoE concentrations showed that the CSF/serum ratios of apoE levels were associated with progression of dementia. Schmidt et al. observed that “the lower the ratio, the faster the deterioration,” as measured by the MMSE, instrumental activities of daily living (iADL), or Geriatric Depression Scale (GDS). Subsequently, another study  showed that plasma apoE may be a biomarker of dementia, as patients suffering from memory decline had lowered concentrations of plasma apoE.
The first reports indicating that
According to , the most significant
It is also interesting that multiple studies have confirmed that the
APOEin a healthy Polish population under 60 years of age
Despite the many years of research, AD remains a disease that is difficult to predict and diagnose, with few blood-derived biomarkers possible for use in routine clinical setting. As was stated before,
6.1. Aim of the study
In this study, we tried to assess the influence of the
A total of 83 healthy adults (70 females, mean age: 51.9 ± 7.2; 13 males, mean age: 44.9 ± 11.7) under 60 years of age with no signs of dementia or other neurological disorders were enrolled in the study. All participants provided signed, written consent. The research project was approved by the Bioethical Committee at the Poznan University of Medical Sciences, decision no. 1031/13, dated May 5, 2013.
Each volunteer’s blood was collected on an anticoagulant—K3EDTA (Monovette™ vacuum system, Sarstedt, USA). A total of 3 ml of blood was immediately aliquoted, then frozen and stored at −80°C upon nucleic acid isolation. Subsequently, the remaining blood was centrifuged (1400 relative centrifugal force [RCF], 10 min) and the collected plasma was aliquoted and stored at −80°C.
6.4.1. APOE genotyping
First, a subject’s DNA was extracted from frozen blood using gravity flow microcolumns (Genomic Micro AX Blood Gravity, A&A Biotechnology, Poland). The DNA concentration was measured by a microplate spectrophotometer (Take3, Epoch, BioTek, USA) and adjusted to 20 ng/μL with Milli-Q® water. Subsequently, genotyping was performed according to a modified mismatch primer method . Briefly, three quantitative polymerase chain reaction (qPCR) specific to each
6.4.2. ApoE quantification
Determination of the plasma apoE concentration was performed by the enzyme-linked immunosorbent assay (ELISA) method. The analysis was performed according to the manufacturer’s protocol (Human apoE ELISA Kit, Mabtech, Sweden) using 10,000× diluted plasma samples. Absorbance was measured by an EPOCH microplate reader (BioTek, USA). The concentrations were calculated from a four-parametric standard curve (
Our study on Polish subjects showed that the observed genotype frequencies of
Our results indicate that the apoE plasma concentration depends on the
|Female||1.98 ± 0.67||2.91||2.35 ± 0.78||2.02 ± 0.52||0.69|
|Male||–||–||2.91 ± 0.63||1.72 ± 0.22||–|
|Combined||1.98 ± 0.67||2.91||2.43 ± 0.79||1.95 ± 0.49||0.69|
As was stated before, the
According to our results, the
Our study shows that
The plasma concentration of apoE may be a valuable dementia biomarker because it is easily available and, according to literature data, decreased apoE may be a risk factor for developing dementia. The above-mentioned Australian follow-up cohort study, comprising mostly Caucasian subjects, showed that the reduced apoE plasma level may be a predictor of a transition from MCI to AD. Moreover, the plasma apoE concentration correlates positively with cognitive function, and patients with a lower apoE level tend to perform worse in neuropsychological tests assessing spatial memory and language abilities .
Hence, the assessment of the plasma apoE concentration and the
The appearance of dementia in old age is influenced by both biochemical and genetic factors leading to structural disorders in the brain of elderly persons. The level of Aβ is mentioned among the other biochemical factors associated with dementia. The deposition of Aβ in the brain is controlled by
Finding a way to control the genetic factors and their protein products may contribute to the prevention of diseases of old age, including depression and dementia, and to improve the quality of life of elderly people.
This study was supported by the Poznan University of Medical Sciences grant no. 502-14-01111677-10342.