Brona McVittie reports :: June 2006

“At times in their lives they have definitely striven to be perceived as a unit and at other times seem to want people to acknowledge their differences and respect them as individuals,” says Jimmy of his friends Gavin and Jason. Gavin and Jason are identical twins. One is a literally a clone of the other. Their similarities cannot be disputed, but when I first met them, I found them to be quite different people. Had I met them a few years ago, I doubt I would have been able to tell them apart.

Monozygotic twins arise with an incidence of 1 in every 250 births worldwide. For reasons yet unknown, a fertilized egg cell can clone itself and give rise to separate embryos. Each will begin and end life with the same genetic make-up, but as they grow and develop they will experience differences in their environment, some of which might alter their appearance and behaviour.

Gavin and Jason have exactly the same DNA. If one committed a crime and unwittingly left samples for forensic analysis, it would be impossible to determine the baddie from DNA fingerprint analysis. However, closer inspection of their molecules may reveal significant differences. Although the lads share the same genes, recent evidence suggests that some genes might be active in one twin and not the other. They might be identical genetically but not epigenetically.

Such differences are discernable at the molecular level in the way that their chromosomes are arranged within the nucleus of each cell. Twisted around tiny protein balls, the same DNA can have different consequences for a cell. Both balls and string assume complex 3D structures depending on their biochemical flavour. A variety of small molecules can affect the nuclear infrastructure by adhering to both DNA and associated histone proteins. Such flavours are influenced by the environment, most notably our diet.

Biochemical fine-tuning of the genome determines which genes get switched on, so twins are not necessarily destined to share the same fate. Recent research on monozygotic twins has revealed that their DNA is marked in different ways by a tiny molecule called methyl. So it’s not really true to say that they are identical. What’s more, these differences were much more pronounced in older than younger twins.

This is new fodder for the age-old debate over how much the environment influences our fate relative to our genes. Although the similarities between identical twins are more striking than the differences, their inequalities could offer new avenues for disease research. For example, although Gavin and Jason are both at risk of type II diabetes, Jason was recently admitted to hospital with serious pancreatic problems and had to inject himself with insulin for some time afterwards. The doctors were unable to make a specific diagnosis. Had they been equipped with better diagnostic tools, such as are offered by new advances in epigenetic research, they might have been able to profile the twins to see whether DNA methylation patterns were involved.

Many diseases have a known genetic component, but may be modified by epigenetics. Epigenetic features like DNA methylation are much more viable targets for treatment because it’s much easier to change the way DNA is methylated than to change the underlying DNA sequence.