Read The Epigenetics Revolution Online
Authors: Nessa Carey
Tags: #Science/Life Sciences/Genetics and Genomics
The Epigenetics Revolution (16 page)
This seems like a good example of transgenerational (Lamarckian) inheritance, but has it has been caused by an epigenetic mechanism? Did an epigenetic change (altered DNA methylation and/or variations in histone modifications) that had occurred in Basje as a result of malnutrition during her first twelve weeks of development in the womb get passed on via the nucleus of her egg to her own child? Maybe, but we shouldn’t ignore that there are other potential explanations.
For example, there could be an unidentified effect of the early malnutrition which means that when pregnant, Basje will pass more nutrients than normal across the placenta to her foetus. This would still create a transgenerational effect – Camilla’s increased size – but it wouldn’t be caused by Basje passing on an epigenetic modification to Camilla. It would be caused by the conditions in the womb when Camilla was developing and growing (the intrauterine environment).
It’s also important to remember that a human egg is large. It contains a nucleus which is relatively small in volume compared to the surrounding cytoplasm. Imagine a grape inside a satsuma to gain some idea of relative sizes. The cytoplasm carries out a lot of functions when an egg gets fertilised. Perhaps something occurred during early developmental programming in Basje that ultimately resulted in the cytoplasm of her eggs containing something unusual. That might sound unlikely but egg production in female mammals is actually initiated early in their own embryonic development. The earliest stages of zygote development rely to a large extent on the cytoplasm from the egg. An abnormality in the cytoplasm could stimulate an unusual growth pattern in the foetus. This again would result in transgenerational inheritance but not through the direct transmission of an epigenetic modification.
So we can see that there are various mechanisms that could explain the inheritance patterns seen through the maternal line in the Dutch Hunger Winter survivors. It would help us to understand if epigenetics plays a role in acquired inheritance if we could study a less complicated human situation. Ideally, this would be a scenario where we don’t have to worry about the effects of the intra-uterine environment, or the cytoplasm of the egg.
Let’s hear it for fathers. Because men don’t get pregnant, they can’t contribute to the developmental environment of the foetus. Males also don’t contribute much cytoplasm to the zygote. Sperm are very small and are almost all nucleus – they look like little bullets with tails attached. So if we see transgenerational inheritance from father to child, it isn’t likely to be caused by intra-uterine or cytoplasmic effects. Under these circumstances, an epigenetic mechanism would be an attractive candidate for explaining transgenerational inheritance of an acquired characteristic.
Greedy fellows in Sweden
Some data suggesting that male transgenerational inheritance can occur in humans comes from another historical study. There is a geographically isolated region in Northern Sweden called Överkalix. In the late 19th and early 20th centuries there were periods of terrible food shortages (caused by failed harvests, military actions and transport inadequacies), interspersed with periods of great plenty. Scientists have studied the mortality patterns for descendants of people who were alive during these periods. In particular, they analysed food intake during a stage in childhood known as the slow growth period (SGP). All other factors being equal, children grow slowest in the years leading up to puberty. This is a completely normal phenomenon, seen in most populations.
Using historical records, the researchers deduced that if food was scarce during a father’s SGP, his son was at decreased risk of dying through cardiovascular disease (such as stroke, high blood pressure or coronary artery disease). If, on the other hand, a man had access to a surfeit of food during the SGP, his grandsons were at increased risk of dying as a consequence of diabetic illnesses
4
. Just like Camilla in the Dutch Hunger Winter example, the sons and grandsons had an altered phenotype (a change in the risk of death through cardiovascular disease or diabetes) in response to an environmental challenge they themselves had never experienced.
4
. Just like Camilla in the Dutch Hunger Winter example, the sons and grandsons had an altered phenotype (a change in the risk of death through cardiovascular disease or diabetes) in response to an environmental challenge they themselves had never experienced.
These data can’t be a result of the intra-uterine environment nor of cytoplasmic effects, for the reasons outlined earlier. Therefore, it seems reasonable to hypothesise that the transgenerational consequences of food availability in the grandparental generation were mediated via epigenetics. These data are particularly striking when you consider that the original nutritional effect happened when the boys were pre-pubescent and so had not even begun to produce sperm. Even so, they were able to pass an effect on to their sons and grandsons.
However, there are some caveats around this work on transgenerational inheritance through the male line. In particular, there are risks involved in relying on old death records, and extrapolating backwards through historical data. Additionally, some of the effects that were observed were not terribly large. This is frequently a problem when working with human populations, along with all the other issues we have already discussed, such as our genetic variability and the impossibility of controlling environment in any major way. There is always the risk that we draw inappropriate conclusions from our data, rather as we believe Lamarck did with his studies on the families of blacksmiths.
The heretical mouse
Is there an alternative way of investigating transgenerational inheritance? If this phenomenon also occurs in other species, it would give us a lot more confidence that these effects are real. This is because experiments in model systems can be designed to test specific hypotheses, rather than just using the datasets that nature (or history) provides.
This is where we come back to the
agouti
mouse. Emma Whitelaw’s work showed that the variable coat colour in the
agouti
mouse was due to an epigenetic mechanism, specifically DNA methylation of a retrotransposon in the
agouti
gene. Mice of different colour all had the same DNA sequence, but a different degree of epigenetic modification at the retrotransposon.
agouti
mouse. Emma Whitelaw’s work showed that the variable coat colour in the
agouti
mouse was due to an epigenetic mechanism, specifically DNA methylation of a retrotransposon in the
agouti
gene. Mice of different colour all had the same DNA sequence, but a different degree of epigenetic modification at the retrotransposon.
Professor Whitelaw decided to investigate if the coat colour could be inherited. If it could, it would show that it’s not only DNA that gets transmitted from parent to offspring, but also epigenetic modifications to the genome. This would provide a potential mechanism for the transgenerational inheritance of acquired characteristics.
When Emma Whitelaw allowed female
agouti
mice to breed, she found the effect that is shown in
Figure 6.2
. For convenience, the picture only shows the offspring who inherited the
A
vy
retrotransposon from their mother, as this is the effect we are interested in.
agouti
mice to breed, she found the effect that is shown in
Figure 6.2
. For convenience, the picture only shows the offspring who inherited the
A
vy
retrotransposon from their mother, as this is the effect we are interested in.
If the mother had an unmethylated
A
vy
gene, and hence had yellow fur, all her offspring also had either yellow fur, or slightly mottled fur. She never had offspring who developed the very dark fur associated with the methylation of the retrotransposon.
A
vy
gene, and hence had yellow fur, all her offspring also had either yellow fur, or slightly mottled fur. She never had offspring who developed the very dark fur associated with the methylation of the retrotransposon.
By contrast, if the mother’s
A
vy
gene was heavily methylated, resulting in her having dark fur, some of her offspring also had dark fur. If both grandmother and mother had dark fur, then the effect was even more pronounced. About a third of the final offspring had dark fur, compared with the one in five shown in
Figure 6.2
.
A
vy
gene was heavily methylated, resulting in her having dark fur, some of her offspring also had dark fur. If both grandmother and mother had dark fur, then the effect was even more pronounced. About a third of the final offspring had dark fur, compared with the one in five shown in
Figure 6.2
.
Figure 6.2
The coat colour of genetically identical female mice influences the coat colour of their offspring. Yellow female mice, in whom the agouti gene is expressed continuously, due to low levels of DNA methylation of the regulatory retrotransposon, never give birth to dark pups. The epigenetically – rather than genetically – determined characteristics of the mother influence her offspring.
The coat colour of genetically identical female mice influences the coat colour of their offspring. Yellow female mice, in whom the agouti gene is expressed continuously, due to low levels of DNA methylation of the regulatory retrotransposon, never give birth to dark pups. The epigenetically – rather than genetically – determined characteristics of the mother influence her offspring.
Because Emma Whitelaw was working on inbred mice, she was able to perform this experiment multiple times and generate hundreds of genetically identical offspring. This was important, as the more data points we have in an experiment, the more we can rely on the findings. Statistical tests showed that the phenotypic differences between the genetically identical groups were highly significant. In other words, it was very unlikely that the effects occurred by chance
5
.
5
.
The results from these experiments showed that an epigenetically-mediated effect (the DNA methylation-dependent coat pattern) in an animal was transmitted to its offspring. But did the mice actually inherit directly an epigenetic modification from their mother?
There was a possibility that the effects seen were not directly caused by inheritance of the epigenetic modification at the
A
vy
retrotransposon, but through some other mechanism. When the
agouti
gene is switched on too much, it doesn’t just cause yellow fur.
Agouti
also mis-regulates the expression of other genes, which ultimately results in the yellow mice being fat and diabetic. So it’s likely that the intra-uterine environment would be different between yellow and dark pregnant females, with different nutrient availability for their embryos. The nutrient availability could itself change how particular epigenetic marks get deposited at the
A
vy
retrotransposon in the offspring. This would look like epigenetic inheritance, but actually the pups wouldn’t have directly inherited the DNA methylation pattern from their mother. Instead, they’d just have gone through a similar developmental programming process in response to nutrient availability in the uterus.
A
vy
retrotransposon, but through some other mechanism. When the
agouti
gene is switched on too much, it doesn’t just cause yellow fur.
Agouti
also mis-regulates the expression of other genes, which ultimately results in the yellow mice being fat and diabetic. So it’s likely that the intra-uterine environment would be different between yellow and dark pregnant females, with different nutrient availability for their embryos. The nutrient availability could itself change how particular epigenetic marks get deposited at the
A
vy
retrotransposon in the offspring. This would look like epigenetic inheritance, but actually the pups wouldn’t have directly inherited the DNA methylation pattern from their mother. Instead, they’d just have gone through a similar developmental programming process in response to nutrient availability in the uterus.
Indeed, at the time of Emma Whitelaw’s work, scientists already knew that diet could influence coat colour in
agouti
mice. When pregnant
agouti
mice are fed a diet rich in the chemicals that can supply methyl groups to the cells (methyl donors), the ratios of the differently coloured pups changes
6
. This is presumably because the cells are able to use more methyl groups, and deposit more methylation on their DNA, hence shutting down the abnormal expression of
agouti
. This meant that the Whitelaw group had to be really careful to control for the effect of intrauterine nutrition in their experiments.
agouti
mice. When pregnant
agouti
mice are fed a diet rich in the chemicals that can supply methyl groups to the cells (methyl donors), the ratios of the differently coloured pups changes
6
. This is presumably because the cells are able to use more methyl groups, and deposit more methylation on their DNA, hence shutting down the abnormal expression of
agouti
. This meant that the Whitelaw group had to be really careful to control for the effect of intrauterine nutrition in their experiments.
In one of those experiments that simply aren’t possible in humans, they transferred fertilised eggs obtained from yellow mothers and implanted them into dark females, and vice versa. In every case, the distribution of coat patterns in the offspring was the same as was to be expected from the egg donor, i.e. the biological mother, rather than the surrogate. This showed unequivocally that it wasn’t the intra-uterine environment that controlled the coat patterning. By using complex breeding schemes, they also demonstrated that the inheritance of the coat pattern was not due to the cytoplasm in the egg. Taken together, the most straightforward interpretation of these data is that epigenetic inheritance has taken place. In other words, an epigenetic modification (probably DNA methylation) was transferred along with the genetic code.
This transfer of the phenotype from one generation to the next wasn’t perfect – not all the offspring looked exactly the same as their mother. This implies that the DNA methylation that controls the expression of the
agouti
phenotype wasn’t entirely stable down the generations. This is quite analogous to the effects we see in suspected cases of human transgenerational inheritance, such as the Dutch Hunger Winter. If we look at a large enough number of people in our study group we can detect differences in birth weight between various groups, but we can’t make absolute predictions about a single individual.
agouti
phenotype wasn’t entirely stable down the generations. This is quite analogous to the effects we see in suspected cases of human transgenerational inheritance, such as the Dutch Hunger Winter. If we look at a large enough number of people in our study group we can detect differences in birth weight between various groups, but we can’t make absolute predictions about a single individual.
There is also an unusual gender-specific phenomenon in the
agouti
strain. Although coat pattern showed a clear transgenerational effect when it was passed on from mother to pup, no such effect was seen when a male mouse passed on the
A
vy
retrotransposon to his offspring. It didn’t matter if a male mouse was yellow, lightly mottled or dark. When he fathered a litter, there were likely to be all the different patterns of colour in his offspring.
agouti
strain. Although coat pattern showed a clear transgenerational effect when it was passed on from mother to pup, no such effect was seen when a male mouse passed on the
A
vy
retrotransposon to his offspring. It didn’t matter if a male mouse was yellow, lightly mottled or dark. When he fathered a litter, there were likely to be all the different patterns of colour in his offspring.
But there are other examples of epigenetic inheritance transmitted from both males and females. The kinked tail phenotype in mice, which is caused by variable methylation of a retrotransposon in the
Axin
Fu
(Axin fused) gene, can be transmitted by either the mother or the father
7
. This makes it unlikely that transgenerational inheritance of this characteristic is due to intra-uterine or cytoplasmic influences, because fathers don’t really contribute much to these. It’s far more likely that there is the transmission of an epigenetic modification at the
Axin
Fu
gene from either parent to offspring.
Axin
Fu
(Axin fused) gene, can be transmitted by either the mother or the father
7
. This makes it unlikely that transgenerational inheritance of this characteristic is due to intra-uterine or cytoplasmic influences, because fathers don’t really contribute much to these. It’s far more likely that there is the transmission of an epigenetic modification at the
Axin
Fu
gene from either parent to offspring.
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