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A team led by Dr Jeffrey Craig in 2010 at the Royal Children’s Hospital in Melbourne also examined DNA methylation in identical and fraternal twin pairs[32]. They investigated a few relatively small regions of the genome in greater detail than in Manel Esteller’s earlier paper. Using samples just from newborn twin pairs, they showed that there was a substantial amount of difference between the DNA methylation patterns of fraternal twins. This isn’t unexpected, since fraternal twins are genetically non-identical and we expect different individuals to have different epigenomes. Interestingly, though, they also found that even the MZ twins differed in their DNA methylation patterns, suggesting identical twins begin to diverge epigenetically during development in the uterus. Combining the information from the two papers, and from additional studies, we can conclude that even genetically identical individuals are epigenetically distinct by the time of birth, and these epigenetic differences become more pronounced with age and exposure to different environments.

Of mice and men (and women)

These data are consistent with a model where epigenetic changes could account for at least some of the reasons why MZ twins aren’t phenotypically identical, but there’s still a lot of supposition involved. That’s because for many purposes humans are a quite hopeless experimental system. If we want to be able to assess the role of epigenetics in the problem of why genetically identical individuals are phenotypically different from one another, we would like to be able to do the following:

Analyse hundreds of identical individuals, not just pairs of them;

Manipulate their environments, in completely controlled ways;

Transfer embryos or babies from one mother to another, to investigate the effects of early nurture;

Take all sorts of samples from the different tissues of the body, at lots of different time points;

Control who mates with whom;

Carry out studies on four or five generations of genetically identical individuals.

Needless to say, this isn’t feasible for humans.

This is why experimental animals have been so useful in epigenetics. They allow scientists to address really complex questions, whilst controlling the environment as much as possible. The data that are generated in these animal studies produce insights from which we can then try to infer things about humans.

The match may not be perfect, but we can unravel a surprising amount of fundamental biology this way. Various comparative studies have shown that many systems have stayed broadly the same in different organisms over almost inconceivably long periods. The epigenetic machinery of yeast and humans, for example, share more similarities than differences and yet the common ancestor for the two species lies about one billion years in the past[33]. So, epigenetic processes are clearly fairly fundamental things, and using model systems can at least point us in a helpful direction for understanding the human condition.

In terms of the specific question we’ve been looking at in this chapter – why genetically identical twins often don’t seem to be identical – the animal that has been most useful is our close mammalian relative, the mouse. The mouse and human lineages separated a mere 75 million or so years ago[34]. 99 per cent of the genes found in mice can also be detected in humans, although they aren’t generally absolutely identical between the two species.

Scientists have been able to create strains of mice in which all the individuals are genetically identical to each other. These have been incredibly useful for investigating the roles of non-genetic factors in creating variation between individuals. Instead of just two genetically identical individuals, it’s possible to create hundreds, or thousands. The way this is done would have made even the Ptolemy dynasty of ancient Egypt blush. Scientists mate a pair of mice who are brother and sister. Then they mate a brother and sister from the resulting litter. They then mate a brother and sister from their litter and so on. When this is repeated for over twenty generations of brother-sister matings, all the genetic variation gets bred out, throughout the genome. All mice of the same sex from the strain are genetically identical. In a refinement of this, scientists can take these genetically identical mice and introduce just one change into their DNA. They may use such genetic engineering to create mice which are identical except for just one region of DNA that the experimenters are most interested in.

A mouse of a different colour