Something that is becoming increasingly clear is that early development is a key period when this control of transcriptional noise first becomes established. After all, very little of the variation in body weight in the original inbred strains could be attributed to the post-natal environment (just 20–30 per cent). Interest is increasing all the time in the role of a phenomenon called developmental programming, whereby events during foetal development can impact on the whole of adult life, and it is increasingly recognised that epigenetic mechanisms are what underlie a major proportion of this programming.
Such a model is entirely consistent with Emma Whitelaw’s work on the effects of decreased levels of Dnmt3a or Trim28 in her mouse studies. The body weight effects were apparent when the mice were just three weeks old. This model is also consistent with the fact that decreased levels of Dnmt3a resulted in the increased variability in body weight, but decreased levels of the related enzyme Dnmt1 had no effect in Emma Whitelaw’s experiments. Dnmt3a can add methyl groups to totally unmethylated DNA regions, which means it is responsible for establishing the correct DNA methylation patterns in cells. Dnmt1 is the protein that maintains pre-established methylation patterns on DNA. It seems that the most important feature for dampening down gene expression variability (at least as far as body weight is concerned) is establishing the correct DNA methylation patterns in the first place.
Scientists and policy-makers have recognised for many years the importance of good maternal health and nutrition during pregnancy, to increase the chances that babies will be born at a healthy weight and so be more likely to thrive physically. In more recent years, it’s become increasingly clear that if a mother is malnourished during pregnancy, her child may be at increased risk of ill-health, not just during the immediate post-birth infancy, but for decades. We’ve only recently begun to realise that this is at least in part due to molecular epigenetic effects, which result in impaired developmental programming and life-long defects in gene expression and cellular function.
As already highlighted, there are extremely powerful ethical and logistical reasons why humans are a difficult species to use experimentally. Tragically, historical events, terrible at the time, conspire to create human scientific study groups by accident. One of the most famous examples of this is the Dutch Hunger Winter, which was mentioned in the Introduction.
This was a period of terrible hardship and near-starvation during the Nazi fuel and food blockade of the Netherlands in the last winter of the Second World War. Twenty-two thousand people died and the desperate population ate anything they could find, from tulip bulbs to animal blood. The dreadful privations of the population created a remarkable scientific study population. The Dutch survivors were a well-defined group of individuals all of whom suffered just one period of malnutrition, all of them at exactly the same time.
One of the first aspects to be studied was the effect of the famine on the birthweights of children who had been in the womb during the famine. If a mother was well-fed around the time of conception and malnourished only for the last few months of the pregnancy, her baby was likely to be born small. If, on the other hand, the mother suffered malnutrition for the first three months of the pregnancy only (because the baby was conceived towards the end of this terrible episode), but then was well-fed, she was likely to have a baby with normal body weight. The foetus ‘caught up’ in body weight, because foetuses do most of their growing in the last few months of pregnancy.
But here’s the thing – epidemiologists were able to study these groups of babies for decades and what they found was really surprising. The babies who were born small stayed small all their lives, with lower obesity rates than the general population. Even more unexpectedly, the adults whose mothers had been malnourished only early in their pregnancy had higher obesity rates than normal. Recent reports have shown a greater incidence of other health problems as well, including certain aspects of mental health. If mothers suffered severe malnutrition during the early stages of pregnancy, their children were more likely than usual to develop schizophrenia. This has been found not just in the Dutch Hunger Winter cohort but also in the survivors of the monstrous Great Chinese Famine of 1958 to 1961, in which millions starved to death as a result of Mao Tse Tung’s policies.
Even though these individuals had seemed perfectly healthy at birth, something that had happened during their development in the womb affected them for decades afterwards. And it wasn’t just the fact that something had happened that mattered, it was when it happened. Events that take place in the first three months of development, a stage when the foetus is really very small, can affect an individual for the rest of their life.
This is completely consistent with the model of developmental programming, and the epigenetic basis to this. In the early stages of pregnancy, where different cell types are developing, epigenetic proteins are probably vital for stabilising gene expression patterns. But remember that our cells contain thousands of genes, spread over billions of base-pairs, and we have hundreds of epigenetic proteins. Even in normal development there are likely to be slight variations in the expression of some of these proteins, and the precise effects that they have at specific chromosomal regions. A little bit more DNA methylation here, a little bit less there.
The epigenetic machinery reinforces and then maintains particular patterns of modifications, thus creating the levels of gene expression. Consequently, these initial small fluctuations in histone and DNA modifications may eventually become ‘set’ and get transmitted to daughter cells, or be maintained in long-lived cells such as neurons, that can last for decades. Because the epigenome gets ‘stuck’, so too may the patterns of gene expression in certain chromosomal regions. In the short term the consequences of this may be relatively minor. But over decades all these mild abnormalities in gene expression, resulting from a slightly inappropriate set of chromatin modifications, may lead to a gradually increasing functional impairment. Clinically, we don’t recognise this until it passes some invisible threshold and the patient begins to show symptoms.
The epigenetic variation that occurs in developmental programming is at heart a predominantly random process, normally referred to as ‘stochastic’. This stochastic process may account for a significant amount of the variability that develops between the MZ twins who opened this chapter. Random fluctuations in epigenetic modifications during early development lead to non-identical patterns of gene expression. These become epigenetically set and exaggerated over the years, until eventually the genetically identical twins become phenotypically different, sometimes in the most dramatic of ways. Such a random process, caused by individually minor fluctuations in the expression of epigenetic genes during early development also provides a very good model for understanding how genetically identical Avy/a mice can end up with different coat colours. This can be caused by randomly varying levels of DNA methylation of the Avy retrotransposon.
Such stochastic changes in the epigenome are the likely reason why even in a totally inbred mouse strain, kept under completely standardised conditions, there is variation in body weight. But once a big environmental stimulus is introduced in addition to this stochastic variation, the variability can become even more pronounced.