By Paul Whaley and Dr John Newby, PhD; cross-posted from Health & Environment
To understand the importance of the new science of epigenetics for health, we have to visit cell development and the cellular processes which, if they go wrong, lead to cancer. Understanding these processes could help us better anticipate and prevent possible health hazards from environmental chemicals, develop better models for risk assessment, and even lead to novel treatments for cancer.
Epigenetics and development
One single fertilised cell, in order to become a human, has to differentiate itself into about 200 cell types. Every single cell, however, contains the same complete set of around 25,000 genes. This means different genes have to be turned on and off at certain times in order for a cell to develop into and function as, for example, a skin cell rather than a liver cell.
This regulation of when genes are turned on and off is governed by epigenetic processes. Rather than mutations, which are changes to the genetic code, epigenetic changes affect genes themselves, like software in relation to DNA hardware.
During development, epigenetic regulation is one factor responsible for determining the course of development of a cell, setting it on the path to becoming a skin cell rather than a liver cell, or a brain cell instead of a muscle cell.
Sometimes, however, external influences can result in genes being silenced or activated at the wrong times. In effect, this can confuse the developmental instructions being acted on by a cell, subtly taking it away from its natural developmental pathway and down an altered route, with a range of potential knock-on effects.
The effects of these alterations have been studied in humans, in relation to parental dietary changes and effect on adult health, finding that the children of starving mothers are more likely to give birth to underweight babies and parental diet can affect longevity of grandchildren, such as in this Dutch and this Swedish study.
In animals, revealing research has been done into the effects which exposure to chemicals can have on epigenetic processes. One particularly famous example of the effects of chemically-induced epigenetic change is the feeding of pregnant mice on a diet rich in substances known to change gene expression by adding methyl groups to DNA.
In this case, the mouse offspring were not expressing the genes which would have made them yellow (agouti) rather than brown: the diet had switched the agouti genes off.
Mice which were not fed the DNA-methylating diet ended up with fully-activated agouti genes, which meant they were yellow in colour and they even became obese; one of the effects of leaving active the genes which make the mice yellow is to also prevent them from being able to tell when they are full, so the yellow mice eat their way to obesity.
The relationship between epigenetic changes and cancer
In general, cancer is understood by scientists to be a genetic disease, where mutated genes are thought to initiate transformation of normal cells into malignant cells. These cells show uncontrolled growth and the ability to invade other tissues and spread to other locations in the body.
The dominant theory is that a mutation initiates cancer by affecting a gene involved in the regulation of cell division, cell survival and/or DNA repair processes. This disables the safeguards preventing uncontrolled growth, allowing the cells to endlessly multiply and become destructive and invasive.
The genes involved in regulation of cell division and cell survival are known as tumour suppressor genes and cell proliferation regulator genes (also known as “protooncogenes”). Damage to suppressor genes is well-recorded in early-onset breast cancers, while mutations in the protooncogenes turns them into cancer-causing genes.
Epigenetic research, however, is starting to reveal another layer of complexity to the process: while cancer cells show mutations in protooncogenes, in addition these genes almost always display epigenetic changes in addition to the mutations.
The concern is that epigenetic changes are switching off tumour suppression genes, thereby initiating or causing progression in malignant transformation of the cell. Malignant growth could also be initiated by the epigenetic switching-on of oncogenes.
A particularly infamous example of cancer-causing interference in epigenetic processes is the drug diethylstilbestrol (DES), a synthetic oestrogen often prescribed to pregnant women until around 1971. DES was prohibited for this use when it was discovered that it elevated the risk of rare vaginal cancers in the daughters of women who took the drug. The theory is that in utero exposure to DES turns on persistent over-expression of certain protooncogenes.
Should we treat more chemicals as potentially cancer-causing?
In an attempt to limit the number of cases of cancer caused by chemicals, risk assessments are carried out to determine a chemical’s ability to cause mutations, and its use is then restricted accordingly.
The underlying assumption in carrying out risk assessments is that cancer is caused by genotoxins and mutagens altering the DNA sequence, resulting in uncontrolled cell replication.
Epigenetics, however, is making this assumption look false: the evidence coming from new studies is strongly suggesting that some substances are toxic because of their epigenetic effects, not because they directly damage the DNA sequence itself.
For example, the environmental contaminants cadmium, nickel, chromium and arsenic are known carcinogens that induce epigenetic alterations. Chemicals such as vinclozan (a fungicide), methoxychlor (an insecticide), bisphenol A (found in food containers), benzene (in air pollution), diethylstilbestrol (a synthetic oestrogen) and persistent organic pollutants (particularly dioxin) have been shown to cause epigenetic alterations by increasing or decreasing DNA methylation.
This knowledge is not yet making its way into assessing potential for harm in risk assessment, which still assumes chemicals cause cancer by mutating genes, even though exposure to chemicals which have epigenetic effects is common and could be influencing incidence rates of cancer.
Suspected carcinogens are currently only risk-assessed and categorised by their capacity to alter the DNA sequence, and not whether or not a gene is expressed.
What does all this mean for cancer prevention?
Although epigenetics, as a new field, shows us there is a lot we don’t really know about carcinogenesis, it has already opened up several important avenues for implementing a conservative approach to preventing harm to health from the environment.
For one thing, there is good reliable evidence that epigenetic changes occur early in the cancer initiation and progression stages. The fact epigenetic alteration is reversible gives us targets for novel cancer treatments and possibly, as the example of the agouti mice shows, chemoprevention.
We also know that chemical exposures can cause epigenetic alterations, and that epigenetic changes during the development stages have serious implications later in life. These changes could initiate a cancer in utero, while exposure to something later in life could cause the progression of malignant transformation. Elimination of these chemical exposures could help reduce incidence rates of a number of cancers.
Given that cancer is easier to prevent than to treat, epigenetics also gives us helpful targets for primary prevention. This is where improvements in risk assessment and chemical regulation aimed at limiting exposure to environmental contaminants with known epigenetic effects (one candidate being bisphenol-A) is of critical importance.
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Paul Whaley is editor of the website and monthly e-publication Health & Environment, which emphasises to medical practitioners and public health policy-makers the importance of the environment as a determinant of public health. He also serves as curator of the Environmental Chemicals Stream for Sustainability for Health: an Evidence Base for Action (currently still in beta) and is Communications Manager for the Cancer Prevention and Education Society. He can be reached at firstname.lastname@example.org.
Dr John Newby, PhD, is Medical Information Officer for the Cancer Prevention and Education Society. His publications include The Cancer Incidence Temporality Index and Environmental Influences in Cancer Aetiology.