PSYC23H3 Lecture Notes - Lecture 3: Epigenome, Fetus, Asthma

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28 Jun 2015

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Week 3C: Early Experiences Can Alter Gene Expression and Affect Long-Term
The Issue:
Scientists liken the structural genome to the hardware of a computer— both determine the boundaries of
what’s possible, but neither works without an operating system to tell it what to do. In the genome, that
operating system is called the epigenome.
Like the software in an operating system, the epigenome determines which functions the genetic
“hardware” does and does not perform.
This system is built over time as positive experiences, such as exposure to rich learning opportunities, or
negative influences, such as environmental toxins or stressful life circumstances, leave a
chemical“signature” on the genes.
These signatures can be temporary or permanent, and both types affect how easily the genes are switched
on or off.
-For example, even though identical twins have the same structural genomes, their different experiences
result in different epigenomes.
-These differing experiences leave signatures on the epigenome that cause some genes to be expressed
*This explains why genetically identical twins, though similar in many ways, can exhibit different
behaviors, skills, health, and achievement in both school and, later, in the workplace.
To date, scientists have found that temporary epigenetic chemical modifications control when and where
most of our genes are turned on and off.
This, however, is not the entire story.
Certain experiences can also cause enduring epigenetic modifications in hundreds of genes that have
already been identified, and the list is growing.
Increasing evidence shows that experience-driven, chemical modifications of these latter genes appear to
play particularly key roles in brain and behavioral development.
This new knowledge has motivated scientists to look more closely at the factors that shape the epigenome
and to study whether interventions can reverse these modifications when negative changes occur.
Nutritional status, exposure to toxins and drugs, and the experiences of interacting with varied
environments can all modify an individual’s epigenome.
Epigenetic instructions that change how and when certain genes are turned on or off can cause temporary
or enduring health problems.
Moreover, research in both animals and humans shows that some epigenetic changes that occur in the fetus
during pregnancy can be passed on to later generations, affecting the health and welfare of children,
grandchildren, and their descendents.
-For example, turning on genes that increase cell growth, while at the same time switching off genes that
suppress cell growth, has been shown to cause cancer.
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Repetitive, highly stressful experiences can cause epigenetic changes that damage the systems that manage
one’s response to adversity later in life.
On the other hand, supportive environments and rich learning experiences generate positive epigenetic
signatures that activate genetic potential.
In this second case, the stimulation that occurs in the brain through active use of learning and memory
circuits can result in epigenetic changes that establish a foundation for more effective learning capacities in
the future.
As we get older, new experiences can continue to change our epigenome.
However, science tells us that the chemical signatures imprinted on our genes during fetal and infant
development can have significant influences on brain architecture that last a lifetime.
*the discovery of the epigenome provides an explanation, at the molecular level, for why and how
early positive and negative experiences can have lifelong impacts.
Policymakers can use this knowledge to inform decisions about the allocation of resources for interventions
that affect the life circumstances of young children—knowing that effective interventions can literally alter
how children’s genes work and, thereby, have long-lasting effects on their mental and physical health,
learning, and behavior.
In this respect, the epigenome is the crucial link between the external environments that shape our
experiences and the genes that guide our development.
What Science Tells Us:
Early prenatal or postnatal experiences and exposures influence long-term outcomes by chemically
altering the structure of genes.
Known as epigenetic modification (from the Greek root epi, meaning upon or over), these chemical
signatures are written on top of the gene without actually altering the genetic code itself.
-Instead, the signatures attract or repel other chemicals that help the genes produce the proteins that are the
building blocks our brains and bodies need to develop.
Research tells us that some genes can only be modified epigenetically during certain periods of
development, defined as critical periods of modification, while other genes are open to alterations
throughout life.
Epigenetic modification typically occurs in cells that comprise organ systems, thereby influencing how
these structures develop and function.
Experiences that change the epigenome early in life, when the specialized cells of organs such as the brain,
heart, or kidneys are first developing, can have a powerful impact on physical and mental health for a
We are also learning from new scientific discoveries in both animals and humans that environmental
factors, such as certain drugs or the nutritional status of the mother, have the potential to cause epigenetic
changes to genes in egg or sperm cells in the fetus.
When such changes occur, this new chemical signature of the DNA is enduring and can be inherited by
future generations.
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