Twins have long held a fascination in our popular imagination.

And while stories about telepathy are intriguing, says Director of Twins Research Australia John Hopper, twins hold the key to a more important mystery: nature versus nurture.

"Twins allow us to control perfectly for one of the main variables in our health – genetics. Then we can drill down on the role that environmental factors are playing," says Professor Hopper, who has been leading the University of Melbourne based research facility since 1990.

"For example, studying twin pairs where one smoked and the other didn't revealed tobacco use is a major cause of osteoporosis."

Launched in 1982, the facility has more than 35,000 pairs of twins on its database and is the largest volunteer twin registry in the world.

And while the field has evolved considerably since, the facility remains an increasingly important tool for understanding how genetics and the environment interact to impact our health.

"We have seen a massive leap forward in DNA technology since the early 80s, notably the human genome project and the new field of epigenetics," says Professor Hopper.

"Twin studies are very important for the translation of this new genomic knowledge into public health."

To mark their 35th anniversary, we asked Professor Hopper and his team to nominate five of the most significant discoveries to come from its more than 230 twin studies.

1. Identifying the genetic and epigenetic risk of breast cancer

"Through studying twin pairs we identified the first gene that influences both mammographic density (the amount of white areas on a mammogram) and risk of breast cancer, called LSP1," says Professor Hopper.

His team found monozygotic (identical) twins were highly similar for mammographic density, which predicts future risk of breast cancer. Dizygotic (non-identical) twins were half as similar, suggesting a major genetic role underlying this risk factor.

"Incredibly, we also found a new epigenetic risk factor for breast cancer that is determined in the uterus – a finding we could only have discovered by studying twin pairs," says Professor Hopper. The next step, he says, is to find out why the environment in the womb influences the risk of breast cancer.

Ultimately, this work could change breast screening and breast cancer prevention across the world. Professor Hopper and his colleagues are working with Breast Screen Victoria to develop plans for tailored screening, where the timing of screening is based on personal risk, rather than age alone.

2. Epilepsy can be inherited

Twenty years ago the medical profession thought epilepsy was an acquired disease, caused by head injuries or a difficult birth.

But Professors Sam Berkovic and Ingrid Scheffer from the University of Melbourne found a genetic basis to specific forms of epilepsy, transforming how epilepsy is defined and treated.

"Twin research was really fundamental in these discoveries," says Professor Hopper. "It revealed which of the many types of epilepsy had a genetic basis, and which didn't, so as to better target their gene discovery studies."

The team also studied twin pairs in which one had epilepsy and the other didn't, and looked at data on any injuries or complications at birth. "There was no evidence to suggest obstetric complications caused epilepsy – a huge relief for so many parents."

3. The environment in the womb impacts our future health

The relatively new field of epigenetics studies the effect of the environment on how our genes work. An important area is methylation, the process by which genes are switched on and off.

The field is helping researchers better understand the impacts of lifestyle factors like diet and stress on our health, and how these are possibly passed down through generations.

By comparing the level of epigenetic differences in identical and non-identical twins, Associate Professor Jeff Craig from the Murdoch Children's Research Institute and his team showed that experiences in the womb help determine the epigenetic profile we are born with.

This may explain why identical twins, despite sharing the same DNA, often experience different health conditions.

"Again, twin research is showing our health is shaped by the environment right from the time of conception, and especially in early life," says Professor Hopper.

4. Healthy brain ageing

The longitudinal Older Australian Twins Study, led by neuro-psychiatrist Professor Perminder Sachdev from the University of New South Wales, is following twins over the age of 65 to investigate healthy brain ageing.

By comparing identical and non-identical twins, researchers are learning what genes and lifestyle factors are causing cognitive decline, or cognitive resilience, in later life.

"By studying older twins, the researchers are uncovering novel findings about what causes brains to age faster, or slower, by comparing their lifetime of experiences in exercise, diet, smoking and so on," says Professor Hopper.

"By controlling perfectly for genetic factors, the results are much more compelling than if the researchers had studied a random sample of the general population, given how much underlying genetic risk of cognitive decline differs across people."

5. Maths and reading skills are largely genetic –

Professor Brian Byrne from the University of New England analysed the NAPLAN scores of about 3000 sets of twins in Years 3, 5, 7 and 9.

His team discovered that up to 75 per cent of differences in children's abilities in maths, reading and spelling, and up to 50 per cent of their differences in writing skills, could be due to genetic differences.

"There tends to be an argument that if genes are important, then the environment doesn't matter," says Professor Hopper.

"In fact the opposite is true. Environmental and genetic factors multiply on one another. Whether children are genetically inclined to be academic or not, they need an environment that supports them to maximise their talents."

If you are an identical or non-identical twin, please consider joining Twins Research Australia at www.twins.org.au or freecall 1800-037-021.

Researchers interested in conducting twin studies can contact Professor Hopper at info@twins.org.au.

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