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YOU ARE WHAT YOUR
GENES EXPERIENCE
T
he nature/nurture dilemma has
long framed debates about the relative
importance of genes and environment on
human development. For CIFAR member
Clyde Hertzman, who is a professor of
population health at the University of
British Columbia, the question is far more
nuanced. Ongoing research illustrates how,
rather than genes being responsible for one
job and environmental factors for another,
the real story centres around how nature
and nurture interact with one another.
Events in the external world can leave
biochemical fingerprints on a child’s DNA,
changing the course of everything from
brain complexity to motor skills – some
of the fundamental characteristics of
individual identity.
“We’re coming to understand now that early
experience gets under the skin,” he says.
That thesis holds enormous implications
for early childhood education, parenting
techniques and health policy. The
phenomenon also suggests that
an individual’s genetic make-up is
programmed to adapt during the formative
early years to both positive and negative
external stimuli.
The mechanics of this process involve
subtle changes in brain chemistry.
When very young children experience
things – anything from the soothing
sound of a parent’s voice to the screams
emanating from a domestic assault – those
occurrences are carried into the brain
in the form of electrical signals. Stressinducing
experiences are associated with
heightened cortisol levels, Hertzman says.
These signals create a kind of biochemical
“cascade” that can trigger structural and
chemical changes to cytosine, one of the
four building-block components of DNA.
The cascade leaves behind distinctive
patterns of a methyl compound, which
in turn affects the way these genes will
express themselves.
This is “the outside world and DNA talking
to one another,” he says.
Animal and early human studies on blood
and saliva cells suggest that methylation
patterns differ noticeably with exposure to
positive and negative stimuli.
Events in the external world can leave biochemical fingerprints
on a child’s DNA, changing the course of everything from
brain complexity to motor skills – some of the fundamental
characteristics of individual identity.
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“Right now, we’re at the statistical
association point,” Hertzman says,
noting that researchers thus far have
had to infer methylation patterns in
brain tissue.
How gene-environment interactions
affect children’s development and
help shape their identities may be
“way upstream,” affecting not only
the brain circuits that regulate stress
hormones, but also the evolution of
the organ systems that manufacture
these hormones.
“Preliminary evidence suggests that
the capacity of early experience to leave
epigenetic marks is greater than with
later experiences,” Hertzman says,
(although the phenomenon has also
been observed among people who
have endured profoundly traumatic
experiences later in life, including living
through the Holocaust).
It may also be the case that these
changes can become intergenerational,
explaining how the effects of trauma can
indeed be passed from parent to child.