Memory and Genetics: The Role of Genes in the Physiology of the Brain
and Resistance to Environmental Factors on Mental Abilities
Rachel LeMalefant
BIOL 303, Section 501
November 1, 2012
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The human ability to remember past experiences and information, and then to build upon
that knowledge with new information, is essential to the human way of life. Without memory, an
individual would not be able to learn and incorporate new information or form relationships with
other people. One specific type of memory, episodic memory, allows us to remember events
from the past during a specific time and place. Memory, like many cognitive abilities, is
susceptible to age-related changes, such as weakening of neurological pathways, as well as
environmental factors, like nutrition or drugs. An increasing number of studies are showing that
there are genetic effects on memory in addition to these other factors and, in fact, that humans
with different genotypes differ in the extent to which these other factors affect their memory.
Many studies have explored dopaminergic pathways in the brain and how varying genotypes are
affected in memory performance by age and environmental factors.
The key to understanding these experiments is to understand how dopamine and regions
in the brain work together to influence memory. Dopamine, a neurotransmitter that regulates
motor behavior, motivation, pleasure, and emotional arousal, plays an active role in episodic
memory. One study, performed by Li et al. (2012), specifically looked at the dopamine
transporter gene (DAT) and the D2 dopamine receptor gene. The dopamine transporter gene
acted in terminating the action of dopamine by reuptake into the presynaptic neuron (the neuron
that fired the dopaminergic signal in the first place), while the D2 receptor gene acted in
encouraging the action of dopamine by receiving the neurotransmitter and initiating it in the
postsynaptic neuron (the neuron receiving the dopaminergic signal in order to pass it on to the
next neuron). This study considered DAT polymorphisms with variable number of tandem
repeats (VNTR). A VNTR is a stretch of DNA on which there are repeated sequences, usually
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10-100 base pairs long, that are clustered together and repeat a different number of times in
different individuals (see Figure 1).
Figure 1:
http://www.nlm.nih.gov/visibleproofs/education/dna/vntr.pdf
For example, the genotypes in this study were the 9/9 homozygotes, the 9/10 heterozygotes, and
the 10/10 homozygotes. The 9/9 homozygotes had a sequence that was 40 base pairs long and
was repeated 9 times in both of their alleles, while the 10/10 homozygotes had that same
sequence repeated 10 times in both of their alleles. Li et al. (2012) also focused on the D2
dopamine receptor gene at the C957T single nucleotide polymorphism (SNP). A SNP is a DNA
sequence variation occurring when a single nucleotide in the genome differs between members
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of a biological species; in this case the differing nucleotides are Cytosine and Thymine at
position 957 (957th base pair on the DRD2 gene).
Li et al. (2012) hypothesized that the 9/9 homozygotes for the DAT gene and
homozygotes for the 957C allele for the D2 receptor gene would perform better in backward
serial recall, a specific type of episodic memory that requires an individual to recall a series of
items in their reverse presentation order. The DAT 10/10 homozygotes expressed higher density
of the dopamine transporter (Cheon et al. 2005; Heinz et al. 2000), meaning that there is more
terminating action on the effects of dopamine, so they would not perform as well on memory
performance tasks as the 9/9 homozygotes for the DAT gene. Also, the T allele for the C957T
SNP for the D2 gene has been found to reduce messenger RNA (mRNA) stability and as such to
reduce the density of D2 receptors (Duan et al. 2003) that act to encourage the action of
dopamine, so homozygotes for the 957T allele would not recall as much information from
episodic memory as homozygotes for the 957C allele.
Li et al. (2012) performed their experiment with 479 younger adults (mean age about 26)
and 809 older adults (mean age about 65) and compared how they performed on an episodic
memory task while also taking into account their genotype. Participants were genotyped using
standard methods, and amplification of the specific region around the DAT VNTR region was
achieved by Polymerase Chain Reaction (PCR). The episodic memory task consisted of 3
different lists of words with 12 words on each list, and participants were asked to repeat the
words presented on each list in backward order from memory. Participants were scored based on
correct identification of the words and their serial positions (where they were on the list). The
DAT gene and the D2 receptor gene were found to be associated with omission errors (the words
were not recalled). Li et al. (2012) obtained results showing that the DAT and D2 receptor
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genotype was more influential on episodic memory performance in the older adult group than in
the younger adult group (see Figure 2). Results also showed that the DAT and D2 receptor genes
modulated each other’s effects on backward serial recall, meaning that the benefits of having the
DAT homozygous 9-repeat genotype were enhanced if the participant was also homozygous for
the 957C allele in the D2 receptor gene.
Figure 2:
(Li et al. 2012)
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This experiment showed that genes can affect behavioral phenotypes and that age can reveal the
effects of certain genes on phenotypes. It was suggested that in older participants, the other
factors in the brain that affect memory performance in younger adults, such as prefrontal or
hippocampal memory systems, are significantly weakened, and therefore the effects of having
one type of allele for DAT or D2 are greatly heightened and more apparent. The purpose of this
study was to show that genes are capable of alterations in behavioral phenotypes, but how the
genes affect the brain to accomplish these alterations was not fully explained.
Another study, performed by Schott et al. (2006), was able to show more insight as to
what exactly is going on in the brain that allows for better memory. The previous experiment
attributed differences in performance to genes and how they affected the density of dopamine
transporters and receptors in the brain, while this study focused more on the activation of
midbrain regions and regions in the prefrontal cortex (PFC) and their effects on encoding
memory. Encoding is the process by which our brains transform what we perceive into memory.
Schott et al. (2006) looked at the DAT dopamine transporter gene previously discussed to see if
there was a difference in midbrain activation between individuals homozygous for the 9-repeat
allele of the DAT VNTR and individuals homozygous for the 10-repeat allele of the DAT
VNTR. They also considered the role of the catechol-O-methyl transferase (COMT)
Val108/158Met polymorphism in the activation of encoding regions in the right prefrontal
cortex. The COMT gene is a gene that encodes one of several enzymes that degrade
catecholamines. Examples of catecholamines are dopamine, epinephrine, and norepinephrine. As
discussed, dopamine is essential for episodic memory performance, so with its degradation it was
expected that memory performance would be weakened. The Val108/158Met SNP leads to a
valine to methionine substitution at position 158 (in membrane-bound form) in the COMT
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protein. In another study, it was found that the methionine variant leads to threefold to fourfold
lower enzymatic activity (Lotta et al. 1995). Other studies also show that DAT is abundantly
expressed in the midbrain, but sparsely in the prefrontal cortex (Sesack et al. 1998). Thus, Schott
et al. (2006) hypothesized that DAT and COMT polymorphisms might influence midbrain and
prefrontal activations (respectively) related to episodic encoding.
Participants in the Schott et al. (2006) study were genotyped using standard PCR methods
and were given tasks meant to study two levels of processing; deep vs. shallow. The deep-study
task required participants to judge whether a word was pleasant or unpleasant, while the shallowstudy task required participants to judge whether or not a word had exactly two syllables. Study
word lists were followed by a brief distracter task and then participants were asked to recall all of
the words that they could in 90 seconds. Results were modeled based on number of deep hits
(correct recall of words in the deep study), deep misses (incorrect recall of words in the deep
study), shallow hits (correct recall of words in the shallow study), and shallow misses (incorrect
recall of words in the shallow study). Results were compared for levels of processing (deep vs.
shallow) and overall recall (hits vs. misses). Also, participants were tested with an fMRI to see
which areas of their brains were activated while performing the tasks. fMRI stands for functional
resonance imaging; this is a test that measures brain activity by detecting associated changes in
blood flow. fMRI uses the change in magnetization between oxygen-rich and oxygen-poor blood
as its basic measure to show which regions of the brain become activated. This fMRI study
allowed Schott et al. (2006) to assess the influences of each genotype on brain region activations
(see Figure 3 and Figure 4).
Faster recall of words in shallowly studied items as opposed to deeply studied items was
noted, but overall there was no DAT or COMT effect on memory performance or level of
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processing. The main effects of the DAT and COMT genes were apparent in the fMRI brain
activation scans. After comparison of fMRI contrasts of DAT VNTR 10-repeat homozygotes and
DAT VNTR 9-repeat homozygotes, it was found that 9-repeat homozygotes showed higher
activation of the dopaminergic midbrain during correct recall of words in both shallow and deep
processing (Schott et al. 2006). The COMT Val108/158Met genotype was not associated with
midbrain activity, but cortical activation patterns showed differences between Val/Val and
Met/Met subjects. Val/Val subjects showed increased memory related activations of the right
prefrontal cortex. This study also demonstrated that functional connectivity of brain regions can
be affected by genetic polymorphisms. DAT VNTR 9-repeat carriers showed higher functional
coupling of the hippocampus with portions of the cortex.
Figure 3:
(Schott et al. 2006)
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Figure 4:
(Schott et al. 2006)
These findings support the hypothesis of Schott et al. (2006), that dopaminergic
transmission through activation of brain regions is involved in episodic memory encoding. In
both studies previously discussed, DAT VNTR 9-repeat carriers showed lower density of the
dopamine transporter, so the neural pathways in the brain that work to transfer dopamine work
harder to compensate, and the fMRI shows an increase in midbrain activation. Since the
methionine variant shows lower enzymatic activity, the Val/Val homozygotes would naturally
show increased activation of the prefrontal cortex. Both cases prove that this activation was
effective in allowing the subjects to remember words correctly. It is also important to mention
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that this second study by Schott et al. (2006) may not have found an association between the
DAT gene and memory performance, but their younger sample population (18-31 years) would
have had many mechanisms in the brain to aid in memory that the older population in the first
study by Li et al. (2012) would not have had (such as prefrontal or hippocampal memory
systems). Therefore, the effects of the DAT gene on memory performance were not apparent in
the Schott et al. (2006) study. Up to this point, only gene-related effects on memory have been
evaluated. However, environmental effects on memory cannot be overlooked.
External influences, such as nutrition or drugs, can affect memory, but it is interesting to
note that these effects can be strengthened or weakened based on an individual’s genotype. In a
study performed by Jacobsen et al. (2006), scientists attempted to determine whether the C957T
SNP of the D2 receptor gene previously discussed can modify the effects of nicotine on verbal
working memory. Working memory is the active maintenance of information in short-term
storage. To understand how different genes alter the effects of nicotine on a person’s memory
ability, one must understand the normal effects of nicotine on the brain. Nicotine is a highly
addictive drug (well known for its role in addiction to cigarettes) that leads to a burst of
unregulated receptor activity in the brain. Addiction results when the stimulation of cholinergic
neurons promotes the release of the neurotransmitter dopamine in reward pathways of the brain,
thus bringing on pleasant and happy feelings that encourage the use of nicotine. Previous studies
have shown that smokers homozygous for the 957T allele are more likely to stop smoking after
treatment with a nicotine patch than are carriers of the 957C allele (Lerman et al. 2006). There is
also evidence that the 957T allele increases dopamine binding availability (Hirvonen et al. 2004).
Therefore, since nicotine releases dopamine, Jacobsen et al. (2006) hypothesized that nicotine
administration would be associated with impaired working memory performance in carriers of
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the 957T allele. Dopamine signaling and performance are related by an inverse u-shaped
function that shows that both deficient and excessive modulation are suboptimal for cognitive
functions (Cools and D’Esposito, 2011; Li and Sikstrom, 2002; Mattay et al. 2003;
Vijayraghavan et al. 2007). They also hypothesized that impaired working memory performance
in the 957T allele carriers would be accompanied by reduced efficiency (increased activation) of
working memory regions in the brain.
Participants in the Jacobsen et al. (2006) study were categorized into smokers (people
who smoked 25±10 cigarettes per day for 21±9 years) and nonsmokers (have never had more
than one cigarette in their lifetime). Subjects were genotyped and scanned in fMRI twice, once
after placement of a placebo patch and once after placement of a nicotine patch. Participants
were required to abstain from tobacco products for at least 15 hours before scanning, and 6 hours
before scanning the patch was placed on the participant’s upper thigh. The memory task was an
auditory n-back task. There were two levels of working memory load, one-back, meaning the
subjects were listening for a back-to-back repeat such as “bip bip,” and two-back, meaning the
subjects were listening for a repeat separated by a word such as “bip gex bip”. There were also
two levels of selective attention load, binaural, meaning the stimuli would be the same in both
ears, and dichotic, meaning the stimuli would be different in each ear. Subjects were tested twice
for each of the four tasks. It is important to note that due to small sample size (36 adult subjects),
smokers and nonsmokers were combined in all analyses.
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Figure 5:
(Jacobsen et al. 2006)
Comparing carriers of the 957T allele to homozygotes for the 957C allele in the D2
receptor gene showed that the genotype and patch condition (placebo or nicotine) interaction
effects on n-back task performance accuracy were only significant for the dichotic two-back task
condition. These results showed that the performance of this task was significantly worse during
nicotine administration among carriers of the 957T allele (see Figure 5). So, results supported the
hypothesis that nicotine worsened accuracy of verbal working memory in people who carried the
957T allele, but not in individuals who were 957C homozygotes. fMRI data showed that
activation of many memory-related regions of the brain increased during nicotine administration
in the 957T carriers, while memory-related regions of the brain decreased during nicotine
administration in the 957C homozygotes. This data showed that people with 957T allele work
harder to efficiently remember information. It is important to note that the Jacobsen et al. (2006)
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study did show that the D2 receptor 957T allele homozygotes performed better than 957C allele
homozygotes in a working memory task, which is not consistent with the data found in the Li et
al. (2012) study. Li et al. (2012) attributed this inconsistency to difference in sample sizes and
specific cognitive functions assessed. The Jacobsen et al. (2006) study is effective both in
demonstrating how different genes react differently to the environment, and in setting the
groundwork for possibly being able to predict responses to certain therapies to stop smoking in
the near future.
These studies indicated that genes play an active role in behavioral phenotypes,
specifically memory performance, and that both aging and environmental factors affect
individuals with different genes differently. In one study, the homozygotes for the DAT 9-repeat
allele and the 957C allele for the D2 receptor gene showed better backward serial recall at a
more advanced age (Li et al. 2012). In another study, the DAT 9-repeat allele homozygotes were
shown to have higher midbrain activation. The individuals with the genotype Val/Val for the
COMT gene also were shown to have higher brain region activation, except in the right
prefrontal cortex as opposed to the midbrain (Schott et al. 2006). The last study showed that
environmental factors such as the drug nicotine had a stronger effect on carriers of the 957T
allele of the D2 receptor gene because of its role in modulating dopamine levels in the brain
(Jacobsen et al. 2006). The results from these studies suggest that genes coding for production
and regulation of neurotransmitters in the brain greatly affect behavior and memory performance
in humans. Further, these genes are affected by the environment in different ways. Using this
information, researchers can estimate memory aptitude in old age and predict patient responses
to new drugs, and relate brain activation to genotype and cognitive performance, all with a DNA
sample. From identifying genes that determine eye color, to discovering which genes determine
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our memory and thus many essential aspects of our life, genetics has been making huge
advancements with many more to come.
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