By contrast, if the mother’s Avy 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.
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[43].
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 Avy 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 Avy 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[44]. 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 intra-uterine 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.
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 Avy 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 AxinFu (Axin fused) gene, can be transmitted by either the mother or the father[45]. 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 AxinFu gene from either parent to offspring.
These model systems have been really useful in demonstrating that transgenerational inheritance of a non-genetic phenotype does actually occur, and that this takes place via epigenetic modifications. This is truly revolutionary. It confirms that for some very specific situations Lamarckian inheritance is taking place, and we have a handle on the molecular mechanism behind it. But the agouti and kinked tail phenotypes in mice both rely on the presence of specific retrotransposons in the genome. Are these special cases, or is there a more general effect in play? Once again, we return to something that has a bit more immediate relevance for us all. Food.
As we all know, an obesity epidemic is developing. It’s spreading worldwide, although it’s advancing at a particularly fast rate in the more industrialised societies. The frankly terrifying graph in Figure 6.3 displays the UK figures for 2007[46], showing that about two out of every three adults is overweight (body mass index of 25 or over) or obese (body mass index of 30 or over). The situation is even worse in the USA. Obesity is associated with a wide range of health problems including cardiovascular disease and type 2 diabetes. Obese individuals over the age of 40 will die, on average, 6 to 7 years earlier than non-obese people[47].
Figure 6.3 The percentage of the UK population that was overweight or obese in 2007.
The data from the Dutch Hunger Winter and other famines support the idea that poor nutrition during pregnancy has effects on offspring, and that these consequences can be transmitted to subsequent generations as well. In other words, poor nutrition can have epigenetic effects on later generations. The data from the Överkalix cohort, although more difficult to interpret, suggested that excess consumption at key points in a boy’s life can have adverse consequences for later generations. Is it possible that the obesity epidemic in the human population will have knock-on effects for children and grandchildren? As we don’t really want to wait 40 years to work this out, scientists are again turning to animal models to try to gain some useful insights.