When I think of the word Latency
in medicine, I think of just one thing: Cancer. Although
latency can also refer to viral infections such as HIV, Cancer Latency is the
time between the initial exposure of a carcinogen (such as radiation), and
eventual development of cancer. When I think of latency, I often times think of
inevitability. It gives the impression that you know what the outcome will be,
but there’s nothing you can do to change it. Or for those who did not know the
origins of their cancer, the reality is they were already harboring the
cancerous cells and would eventually develop the disease, and unfortunately the
available interventions to prevent the outcome would have been insufficient.
With our current knowledge of cancer biology, latency makes
sense. Take radiation for example. Ionizing radiation is very good at causing
trouble for DNA in your cells. Not only can it directly damage DNA, it can
actually ionize water within your cells, generating highly reactive hydroxyl
anion radicals. So this means our lovely H2O that fills our cells
can become HO·-
(that's Hydrogen attached to Oxygen, and it is negatively charged with an extra
electron, or a free radical as we call them). And if there’s one thing HO·- knows how to do,
it’s damage DNA and anything else it can get its hands on. In fact, HO·- is the most
powerful free radical oxidant that ever appears in our cells. So, with all this
bad stuff going on after radiation exposure, we get mutations in our DNA. In
cancer biology, this is called initiation, this is when the timer starts for
our latency period.
Normally, our cells can detect these mutations, and
gratefully, these cells decide to kill themselves before anything bad happens.
Next time you get sunburned (UV light is a form of ionizing radiation) and a
few days later you pull off the dead skin, give a little thanks to their
wonderful sacrifice. But, every once in a while, a damaged cell will escape the
programmed death. Now, without the control to know when to die, these cells can
gather further mutations, continue to divide and proliferate, and with a
certain amount of time, a cancer will appear. The time between the initiation
from the first radiation exposure, to the time when the cancer appeared is the
latency, and typically we think of these times in the forms of years to
decades. This is not to say that everyone who is exposed to a carcinogen like
this will develop cancer, not at all, but for those who do develop cancer,
there has been effort to understand what the latency periods are. Arsenic, for
example, is good at causing skin cancers, and this typically takes 10-15 years.
Latency period for cigarette smoke and lung cancer, is somewhere around 30
years. The latency period for radiation exposure when looking at medically
exposed radiation therapies has been observed to be between 4 and 44 years.
This concept that an initiating step can predetermine one’s
risk of disease decades later makes sense in cancer, and we accept it. But, can
we think this way about other diseases? Can our risk of a heart attack at age
60 be predetermined from an exposure that happened decades earlier? Can our
risk of developing Parkinson’s Disease already be set in childhood? Or can our
child’s risk of developing autism already be determined at birth? These
questions fall into the field of “fetal origins of adult disease,” where there
is growing evidence that our risk of disease in adulthood may be strongly
influenced by what exposures occurred during fetal and early life development.
Fetal origins of
adult disease
Interestingly, the story of this field begins with World War
II. In the winter of 1944, the Nazi occupied region of the Netherlands had
grown scarce with food supplies. With active attempts from the Allied Forces to
liberate the country, liberation efforts stalled upon failure to gain control
of a crucial bridge, which would give much needed access and food supply. In
response, the former Dutch government influenced the national railways to go on
strike to aid the Allied Forces. In September of 1944, as a retaliation effort
to the railway strike, the German government placed an embargo on the transport
of all food into the western Netherlands. An unusually cold winter had come
early to the Netherlands, and although the embargo was partially lifted in
November of that year, canals had already frozen over, preventing the transport
of food by barge.
As a result, food stocks began to run out, and daily rations
for adults dropped to below 1,000 Cal in large cities such as Amsterdam in
November of 1944, and reached as low as 580 Cal in February of 1945 (compare
that to our recommended caloric intake of somewhere around 2,500). The famine
ended in May of 1945 upon the liberation of the western Netherlands. During
this winter, millions of Dutch people were exposed to famine conditions, and
roughly 22,000 people died. This terrible winter is called the Hongerwinter, or the Dutch Hunger
Famine.
Out of nearly 4.5 million people who suffered from the
rations, you can imagine how roughly half of these victims are women. Out of
over 2 million women affected, you can expect than many of these women would be
pregnant. This is exactly what happened. In an unexpected positive outcome of
this historical event, these children became the perfect group to study the
effects of starvation during pregnancy, low birthweight, and adult disease.
When those children reached 19, they registered for national service, which
included a visit with a physician and a clinical exam. At this point, there was
the creation of a large cohort of Dutch men and women who were either exposed
or not exposed to the famine conditions during their in utero (in the womb)
development. In addition, with nationalized medicine, there was routine
follow-up with these individuals that allowed for a lifetime of clinical data
to be collected.
The first publications examining this cohort began to emerge
in the mid 1970’s, where it was reported in the New England Journal of Medicine that young men exposed to famine
conditions during the first trimester of pregnancy were more likely to be obese.
These initial observations were subsequently followed up
with many more extensive analyses, predominantly by the physician and clinical
epidemiologist Dr. David Barker. In examining these individuals exposed to
prenatal famine conditions, Dr. Barker discovered that malnutrition during
pregnancy increases adult risk of obesity, diabetes, and other cardiovascular
disorders including hypertension (high blood pressure) and coronary artery
disease (plaque build up within arteries). These discoveries led to what is
widely known as the “Barker Hypothesis”, which is that maternal malnutrition
during pregnancy ‘programs’ the developing fetus to be more susceptible to
metabolic disorders. Dr. Barker just passed away September, 2013 at age 75.
In his obituary in The Guardian, they gave this quote that
I think is fantastic and quite correct:
"Across the
world there is now general agreement that human beings are like motor cars.
They break down either because they are being driven on rough roads or because
they were badly made in the first place. Rolls-Royce cars do not break down no
matter where they are being driven. How do we build stronger people? By
improving the nutrition of babies in the womb. The greatest gift we could give
the next generation is to improve the nutrition and growth of girls and young
women."
So how do we build
stronger people? I think before we can answer that question, we need to
understand what is leading these children to be more susceptible to disease.
When we think about
disease, we often times think of genetics. We will hear ourselves saying, “oh
it’s probably due to genetics…” And there are a lot of reasons why this makes
perfect sense, as we know that genetics and mutations to our DNA code can be
very important in determining disease risk. But, there is another aspect of
genetics that might be more important to our health than just the order in
which our A’s, T’s, C’s, and G’s line-up. This field is called “Epigenetics,”
or literally, “above” the genome. Our DNA code is used to make proteins, and
these proteins do their specific jobs. But, what proteins should be made? How
much protein should be made? Under what conditions? Determining what proteins
to make and how much is what truly influences disease, and it is called
transcriptional regulation. Epigenetics is now known to be one of the principle
factors of transcriptional regulation. Since our DNA is tightly wrapped up
around proteins (called histones, the complex DNA and protein package is called
chromatin) the vast majority of it is unavailable to be turned into protein. So
our cells have evolved amazingly complex machinery to regulate the access to
the DNA. In particular, we have two dueling epigenetic mechanisms working with
each other, 1) chromatin modification, and 2) DNA methylation. With chromatin
modification, the protein can be modified to either open up the DNA or close
the DNA. When it is opened, proteins that turn on the protein making machinery
get in there and get to work. With DNA methylation, the C’s of our DNA can be
modified by the addition of a small methyl group (-CH3). As a
result, the DNA methylation typically prevents that DNA from being turned into
protein.
Below is an image
that diagrams chromatin modification and DNA methylation.
Gluckman, NEJM 2008
Now all this
epigenetics stuff may sound pretty technical and obscure, but, one thing that
we’re realizing is that much of these epigenetic markings, in particular DNA
methylation, is passed down each time a cell divides. In addition, we’re finding
that certain environmental stressors, such as malnutrition, inflammation,
injury, and toxic exposures, can cause changes in DNA methylation. So think
about that, if you are pregnant and exposed to famine conditions, as the heart,
lungs, brain of your child develop in the womb, the epigenetic make-up might be
altered. And what we know from Barker is that these children are more
susceptible to disease.
The general theory
is that if you are exposed to famine conditions during pregnancy, the
developing fetus adapts by changing the epigenetic profile to ensure immediate
survival. But these adaptations in the womb may have adverse consequences later
on. In particular, the developing infant in some ways “expects” a low nutrient
environment, this is called the “predictive adaptive response.” When the infant
is born, it is expecting a low nutrient environment, but when it actually is
living in a nutrient rich environment, it is almost too good at using all the
available energy and this is why we see increased risk of obesity, diabetes,
cardiovascular disease, etc.
Below is a figure
that shows how the adult risk of metabolic disease (obesity, diabetes,
cardiovascular disease), is a product of the genetic and epigenetic
inheritance, followed by subsequent prenatal and postnatal cues. The belief is that if there is a predicted sparse environment, but the child is met with a nutritionally rich environment, metabolic disease risk is increased. Alternatively, if there is a predicted rich environment, and is met with a nutritionally rich environment, metabolic disease risk is not increased.
Gluckman, NEJM 2008
In utero toxic exposure as the origin of adult disease
Barker’s work gave us the understanding that a malnourished
environment during fetal development can predispose to cardiovascular disease,
and his quote above gives his prescription for fixing this; improve the
nutrition of babies in the womb. But what about those mothers who are
perfectly well nourished? Can other toxic exposures cause the same or similar long
lasting effect on cardiovascular disease?
In my postdoctoral fellowship at UW, I have focused on
trying to answer this question using the exposure model of air pollution. Exposure
to air pollution during pregnancy is known to promote a reduction in
birthweight, and with the Barker Hypothesis, it stands to reason that those
with a reduced birthweight would be at a higher risk of cardiovascular disease.
What we’re trying to test here is not an easy thing to test.
We are asking the question of whether or not an in utero exposure to something
can promote cardiovascular disease in adulthood. To date, there aren’t any
epidemiological studies with human populations and air pollution to try and
answer this, so we began this study with a bit of an acceptance that it’s a high-risk
project. By using mouse models, we have the ability to test the effect of an in
utero exposure on adult susceptibility to cardiovascular disease in roughly 4-5
months, this gives us a way to address this in a reasonable amount of time
(FYI, a mouse lives roughly 2 years, and we generally consider a mouse that is
2 months of age to be an adult).
In our first experiment, we exposed mice to diesel exhaust
air pollution, at a level roughly the same of what you would find in Beijing on
a modestly bad air day, during their in utero development. We let the offspring
be raised in normal filtered air conditions, then when the offspring reached
adulthood (3 months of age), we measured how well their hearts functioned by
echocardiography at 1) baseline, and 2) after we performed a surgery that simulated
a high blood pressure. Our hypothesis is that the in utero exposure to diesel
exhaust would produce a long lasting susceptibility to heart failure, so when
we performed the surgery to simulate high blood pressure, we predicted the hearts wouldn’t do
as well and would fail.
Below is a video of a normal mouse heart under
echocardiography.
The view in this video is called the parasternal short axis
view, where we hold the probe directly on the chest and look at a slice of the
heart perpendicular to the orientation of the heart. This allows us to see the
left ventricle, measure how well it contracts, as well as determine the size of
the heart and the ventricle walls. In cardiology, when the heart gets big, the
walls of the ventricles get thick, and when the heart doesn’t contract as well,
it means there’s trouble. When the heart fails to pump well enough, and there
is not enough blood flow making it to the organs, it is heart failure, and this
is one of the major causes of death worldwide. The heart in the video above is
normal, since the mouse heart beats around 500-600 bpm, this is slowed down so
we can get a good look. You can see that the walls of the ventricle contracts
so there is almost no blood left inside the lumen of the ventricle, and this is
what we want, good pumping means good blood flow around the body.
So after our baseline measurements, we performed a surgery
on the mice to cause high pressure, allowing us to ask how well the heart
responds to a stress. What we found was that the in utero exposure to diesel
exhaust caused the hearts to respond in a much worse way. Below are two videos
that show how a typical control mouse, as well as an in utero diesel exhaust
mouse would respond to the increased pressure.
What you can see in the second video is that the heart
begins to fail. Pumping doesn’t work as well, and the heart has enlarged to a
much greater extent. What this says is that the in utero diesel exhaust
exposure appears to cause a long lasting effect on how well the mouse hearts
can respond to stress.
In my view, this is important. This means that the in utero
diesel exhaust exposure is programming the adult mouse to be more susceptible
to heart failure. In follow-up studies, we also found that the in utero diesel exhaust exposure
causes direct effects to the placenta, that crucial organ that regulates blood
and nutrient flow to the fetus, and that in addition to increased
susceptibility to heart failure, the mice gain more weight and have altered
blood pressure.
Overall, we believe that an in utero and early life exposure
to air pollution can promote a long lasting susceptibility to heart failure in
mice, and it is very likely that a similar effect happens in humans, but this
has yet to be determined.
What can we do about
this?
Our findings with air pollution are not completely unique.
Similar findings have been made with in utero caffeine exposure, cocaine
exposure, as well as exposure to anti-retroviral medications used on patients
with HIV.
We believe all of this comes down to epigenetics. As I
discussed above, epigenetics, those changes on DNA without changing the DNA
sequence, can alter how well genes are turned into proteins. When the heart is
under stress, it needs to be able to turn those genes on in very regulated
ways, but if the epigenetics in the heart is altered as a fetus, it’s very
likely the same changes will be present in adulthood, and it could prevent how
well the heart responds to those stressors. We are currently trying to figure
this out, but I believe there are two paths forward that we should start to
think about.
1) Let’s build stronger people
If we expose developing infants to
such things as air pollution, we are likely setting a trajectory of their
health that won’t reach their potential. If we want to improve public health,
we need to begin to realize that health begins before birth, and our regulatory
policies around environmental exposures need to take this into account.
2) New clinical therapies should push hard on epigenetics
and cardiovascular disease
Epigenetics
and pharmaceutical development has largely focused on cancer so far. Cancer
cells have very messy epigenetics, and it may be a fantastic way to treat
cancer patients, but we now understand that epigenetic factors are likely at
play in cardiovascular disease, and we need to push this. But, as of now, we
don’t have enough basic science research to fully understand what’s going on.
The path to new therapies that will address the epigenetic regulation of
cardiovascular disease needs simultaneous basic science research funding as
well as pharmaceutical company interest.
With latency in cancer, there is a clear line that can be
traced between the initiating event and the subsequent disease. With fetal
origins of adult cardiovascular disease, there isn’t a clear line, but in many
ways I feel that we can think of it as a latency. With the Barker Hypothesis, the
initiating event is the epigenetic or morphologic change that occurred during
fetal development, which inevitably will lead to a higher disease risk. With
our growing understanding of how developmental factors can contribute to adult
disease, we need to start changing how we look at disease progression. If
disease progression has a developmental origin, we need to work to understand
what programming occurs and work to prevent and treat these patients.
Chad Weldy
P.S. If you are interested in reading more about our work, we recently published these reports describing our findings. They are available open access, meaning they are free for anyone to access.
- Weldy CS, Liu Y, Chang YC, Medvedev IO, Fox JR, Larson TV, Chien WM, Chin MT. In utero and early life exposure to diesel exhaust air pollution increases adult susceptibility to heart failure in mice. Particle and fibre toxicology. 2013;10(1):59. Epub 2013/11/28. doi: 10.1186/1743-8977-10-59. PubMed PMID: 24279743.
- Weldy CS, Liu Y, Liggitt HD, Chin MT (2014) In Utero Exposure to Diesel Exhaust Air Pollution Promotes Adverse Intrauterine Conditions, Resulting in Weight Gain, Altered Blood Pressure, and Increased Susceptibility to Heart Failure in Adult Mice. PLoS ONE 9(2): e88582. doi:10.1371/journal.pone.0088582.
