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tudies into the heritability of insomnia go back over half a
century. Recent research points the finger at hundreds of genes, yet they can
only explain part of the risk.
Share on PinterestHow much
do our genes contribute to our risk of developing insomnia?
If insomnia affects you,
there is a good chance that members of your extended family will have
experienced it as well.
In fact, the chance is at least 30 percent, according to
a recent article in the journal Nature
Reviews Disease Primers.
"Insomnia tends to aggregate in families," explain
the authors.
A 2015 study by
Mackenzie J. Lind, from the Virginia Institute for Psychiatric and Behavioral
Genetics at Virginia Commonwealth University School of Medicine in Richmond,
and colleagues estimated the heritability of insomnia to be around 59 percent
in women and 38 percent in men when comparing insomnia symptoms in identical
and nonidentical twins.
Speaking about her work to Medical News Today, Lind explained: "The amount of interest
and research into the genetics of insomnia has definitely increased in recent
years. However, [...] twin and family studies examining the heritability of
insomnia and related phenotypes have been published since the 1960s."
So, which genes are to blame?
Finding
insomnia genes
It makes sense to start with genes known to play a role in
sleep, since insomnia is characterized by difficulties falling or staying
asleep.
There is some evidence to suggest that the obvious culprits
— namely, circadian rhythm genes and neurotransmitters involved in the
regulation of sleep — play a role in insomnia. Yet Lind and others point out
that there is a lack of
robust and reproducible evidence for many of these candidate
genes.
Over the past 2 years, MNT have reported on a couple of studies that used a
different approach to identify candidate genes or genetic regions. Genome-wide
association studies (GWAS)
scan vast amounts of genetic code to find locations with ties to particular
traits.
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These studies identified a
number of novel candidate genes and genetic regions, including some specific to
women and some that were specific to men.
The story got a lot more complex last week, when two new
studies appeared in the journal Nature
Genetics. Both dug deep into the human genome to identify additional
regions of the genetic code with links to insomnia.
From genes
to brain regions
In the largest GWAS
of insomnia to date, the researchers analyzed data from 1,331,010 people and
found 956 genes across 202 locations in the genome with a link to insomnia.
Some of these genes were enriched
in cortical areas and the striatum in the brain, particularly in a type of cell
called medium spiny neurons, which are implicated in reward processing. There
was also a link to pyramidal neurons in the claustrum, which is important for
the processing of incoming stimuli.
The second study identified
57 genetic locations with links to insomnia.
The results point to increased expression of 135 genes in
the cerebellum, frontal cortex, anterior cingulate cortex, hypothalamus, basal
ganglia, amygdala, and hippocampus.
The researchers also found an association with a type of
cellular process called ubiquitin-mediated proteolysis. This is a mechanism
that targets proteins for destruction within cells, and scientists have
previously linked it to insomnia.
Research is clearly making headway in identifying those
genes that contribute to the genetic risk of developing insomnia — but the
heritability is not 100 percent. What else is involved?
Stress
also affects our genes
When asked which other factors might contribute to a
person's risk of insomnia, Lind explained that environmental factors —
particularly stress — are to
blame.
While we generally accept the connection between stress and
insomnia, what actually happens in the brain is less clear.
There is plenty of evidence to suggest that our environment
— particularly stressful life events — can change which genes are expressed in
our cells without directly altering our genetic code. This phenomenon is called
epigenetics.
Importantly, epigenetic changes can be passed from parents
to children, but they are also thought to be reversible. Researchers have begun
to uncover epigenetic influences on how our bodies regulate
sleep and respond to stress.
The bottom line is that insomnia
has a significant genetic component. Scientists have now identified hundreds of
genetic locations that may each contribute a little bit to a person's overall
risk of developing insomnia. Environmental influences can further shape how
these genes are expressed, linking life events to insomnia before and after we
are born.
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