Ben Trumble inside ASU's lab at the School of Human
Evolution and Social Change. Credit: Charlie Leight/ASU News
Previous
research has revealed that the Apolipoprotein-ε4 (APOE-ε4) allele increases the risk for
a variety of diseases in aging populations, specifically Alzheimer's and
cardiovascular disease. Yet despite its negative effects, this allele remains
prevalent in approximately 20% of the human population. In a quest to determine
how this negative allele is surviving natural selection, a group of researchers
have discovered that the APOE-ε4 allele is associated
with increased fertility in women.
In a new paper titled "Apolipoprotein-ε4 is associated with higher fecundity in a natural fertility
population" released today in Science
Advances, the authors worked alongside the Tsimane (Chi-mahn-eh) community
in Bolivia, a forager-horticultural society, to look at the effects of the
allele through an evolutionary anthropological lens.
To do so, lead author Benjamin Trumble, who is an associate professor of evolutionary anthropology at Arizona State University's School of Human Evolution and Social Change as well as the university's Center for Evolution and Medicine, headed to the Bolivian Amazon lowlands where the Tsimane live. Trumble, along with the Tsimane Health and Life History Project (co-directed by Trumble, Hillard Kaplan, Michael Gurven, and Jonathan Stieglitz), has a longstanding relationship with the Tsimane to collect demographic and biomedical data and help provide medical care—a relationship that has been in effect for over 20 years. With over 17,000 people living across 90 villages, their hunter-farmer lifestyle is more similar to human life prior to the Industrial Revolution than it is to life in modern cities, offering a unique glimpse into health and aging without modern day influences.
New research shows genetic mutation known for
Alzhemeir's disease also is associated with increased fertility in women.
Credit: Arizona State University School of Human Evolution and Social Change
"What I do is I try to understand what human health was like prior to
industrialization," Trumble explained.
"For 99% of human history, we were hunter-gatherers. The world we're
living in today is really weird. It's this built environment that we've created
that's entirely different from what it was throughout most of human evolution. We are now
essentially operating outside of the manufacturer's recommended warranty,"
he added.
For this particular study, Trumble and the other researchers collected data
from 795 Tsimane women, ranging in age from 13 to 90 years old. The team of
researchers not only retrieved genetic data to determine which alleles were
prevalent for each individual, but also information about their fertility,
including age of first birth, how long between births, total number of live
births, and more.
When looking at the data, the researchers found that Tsimane women with the presence of one APOE-ε4 allele had an increase of .5 births, compared to those without the APOE-ε4 allele. The number of live births increased even more when there was the presence of two copies of the APOE-ε4 allele, where these women had an average increase of two live births compared to those without this specific allele.
Trumble (center) with Tsimane men in Bolivia.
Credit: Ben Trumble/ASU
"What we
found in this population was that women began reproducing almost a year earlier
if they had the APOE-ε4 allele and they had shorter
interbirth intervals. Those two things combined allow them to have about half
an additional kid if they have one copy or two additional kids if they have two
copies," Trumble explains.
This advantageous effect on fertility could help to
explain how an allele that has such a negative impact in one's later life
through the increased chances of developing Alzheimer's or cardiovascular
disease has not been weeded out by natural selection—the advantages take place in one's early to midlife years, and
therefore are passed along to their offspring.
"Genes that are associated with diseases that occur
after the age of reproduction, or after reproduction has already started, are
in selections shadow. There have been a lot of arguments about the APOE-ε4 allele, that it may be an example of 'selection's shadow,' that you
don't develop Alzheimer's until after you've already had all your kids,"
Trumble said.
Other, smaller studies have shown other advantageous
aspects of the APOE-ε4 allele, including that children
in Brazil are able to handle environmental pathogens and parasites, such as
Giardia, better than those without the allele, which resulted in better
cognitive functions and higher growth rates.
"If you spend a calorie on growth, you can't
spend it on immune function and vice versa, so if you're sick all the time, you
have to invest a lot of energy in immune function, and you can't grow as fast.
Children with the APOE-ε4 allele seem to have better
immune function, so they spend less time sick and can grow faster,"
Trumble explained.
Even with these benefits of the APOE-ε4 allele that have been discovered, there is still the major deleterious
effect of the increased risk of Alzheimer's and cardiovascular disease in the
later stages of life.
Interestingly, this negative effect of the allele is
mostly seen in Westernized nations. The Tsimane, for instance, have the lowest
rates of dementia and Alzheimer's in the world, as discovered in research by
the large interdisciplinary team Trumble is a part of last year. This is
despite having the same 20% prevalence of the APOE-ε4 allele in their population.
What can this mean for understanding and treating the
negative effects of the allele in areas affected the most?
"We need to better understand global variation,
and in particular, we need to kind of think outside the box and move beyond our
focus on 'this allele causes X disease' and that's just how it is. Instead, we need to
take a step back and say, what about in different environments? What about in
the environments in which humans evolved? Because that actually opens up a
whole other possibility for prevention or treatment by being able to mimic some
of the aspects of that lifestyle. If we aren't seeing the same associations
between APOE-ε4 and some outcome, then maybe if
we kind of take a step back and we say, okay, what are the differences?"
The Tsimane Health and Life History Project is a large interdisciplinary team effort, combining anthropologists, cardiologists, neurologists, and gerontologists, as well Bolivian physicians and Tsimane anthropologists.
No comments:
Post a Comment