Without twins, these five mysteries might never have been solved

Twin research has led to all kinds incredible insights into an important mystery: nature vs. nurture or how the environment and our genes affect our health.

“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 John Hopper, director of Twins Research Australia at the University of Melbourne.

“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 in its database and is the largest volunteer twin registry in the world.

“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,” Hopper says. “Twin studies are very important for the translation of this new genomic knowledge into public health.”

Here are some of the significant discoveries to come from the facility’s more than 230 twin studies:

1. Breast cancer risk

“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,” Hopper says.

Researchers discovered that identical (monozygotic) twins are highly similar for mammographic density, which predicts future risk of breast cancer. Non-identical (dizygotic) twins are 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,” Hopper says. The next step, 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. 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 in the genes

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

But scientists found a genetic basis to specific forms of epilepsy, changing how its definition and treatment.

“Twin research was really fundamental in these discoveries,” Hopper says. “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.”

Researchers also studied instances in which one twin 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. Life in the womb

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.

A comparison of epigenetic differences in identical and non-identical twins showed that experiences in the womb help determine the epigenetic profile we are born with, which 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,” Hopper says.

4. Aging brains

The longitudinal Older Australian Twins Study is following twins over the age of 65 to investigate healthy brain aging.

Comparing identical and non-identical twins offers insight into 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,” Hopper says.

“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. Math and reading skills

Researchers analyzed test scores of about 3,000 sets of twins in grades 3, 5, 7 and 9 and discovered that up to 75 percent of differences in children’s abilities in math, reading, and spelling, and up to 50 percent 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,” Hopper says. “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 maximize their talents.”