Cassandra Smith (seated) and her student researchers, (clockwise) visiting scientist Giang Nguyen, research associate Joe Bouchard (CAS'99), molecular biology major Karen Pimpis (CAS'00), summer student Dana Willner, molecular biologist Gregg Surdi (ENG'97, GRS'04), and high school student Mitch Maloof.

Identical twins are an obvious source of data for studying nature/nurture issues; when one twin develops, say, a disease and the other does not, the cause appears obviously environmental.

But appearance is not necessarily reality. In the first place, says Biomedical Engineering Professor Cassandra Smith, scientists have sometimes been remarkably unscientific in determining if twins are identical. "They've used questionnaires: Are you identical? What did your parents say? Your pediatrician? They've looked at photographs and made measurements of heads or ears."

Just looking alike isn't proof that twins are monozygotic, that is, produced by a single ovum fertilized by a single sperm. Further, says Smith, although monozygotic twins are identical at the moment that egg divides, genetic changes begin in utero, influenced by differences in position, nutrition, and blood and oxygen supply. Throughout life, disease and simple aging bring other changes.

Although geneticists now generally acknowledge genetic differences between so-called identical twins, these differences haven't been studied in much detail, Smith says, partially because researchers just haven't thought about it and partially because methods of genetic analysis have been cumbersome. Locating the areas of genetic variation between twins when only one has developed a disease could be a significant step toward determining a genetic cause. "Identifying the cystic fibrosis gene took more than ten years and $120 million," she says. Sequencing only those areas of the genome that differ could simplify the process by more than 90 percent.

Testing for Twinship

Now Smith -- who is deputy director of the Center for Advanced Biotechnology and a professor at the School of Medicine as well as at Engineering -- has developed testing methods that are more accurate and also cheaper and quicker, making it feasible to look at hundreds or even thousands of data points. Further, traditional methods of determining genetic similarity between twins have used only blood, which Smith finds unreliable because blood cells can migrate between twins in utero. She uses skin cells and hair.

Her methods may have a profound effect on disease studies. Twins who have appeared to be monozygotic may in fact be fraternal, increasing the possibility that a disease afflicting only one is genetic. And since all pairs of twins probably have some genetic differences, "one extreme possibility is that there is no such thing as environmental causes," says Smith.

Such a major upset in medical theory is unlikely, but improved genetic testing may well challenge other accepted theories. "Progress in science is always a matter of reevaluation," Smith says. For instance, "we don't understand heritability. People always assume that all parts of the genome are equally heritable, so that first-degree relatives [parents and children, or siblings] have, on average, genome similarity levels of 50 percent, and second-degree relatives [aunts or uncles and their nieces or nephews] have average levels of 25 percent. Those are simple statistical assumptions, and our experiments are now questioning those numbers."

Multiple Applications

Beyond medicine, her work has potential applications in many areas. Take forensic testing: DNA matches placing a suspect at a crime scene have been based on as few as four markers, and while the chances of another match being found in the population as a whole are infinitesimal, the chances of a match in the suspect's immediate family are considerable, a significant fact in, for example, domestic murder cases, when probable cause might be applicable to several family members. "The excuse for the small sample has been that testing has been so expensive," Smith says. "But our methodology eliminates that excuse. There is no excuse, especially when a person's life is at stake."

The increase in multiple births expands another area for improved genetic studies. "Multiple pregnancies are always problematic, so understanding them is important," Smith says. And what is true of monozygotic siblings is true of clones: genes change, and determining where and how will improve cloning in agriculture, for both plants and large animals, the fishing industry, and pharmaceuticals. "Dolly [the cloned sheep] is aging more quickly than they'd expected, but I'm not surprised," Smith says. Understanding genetic change is a vital first step in controlling such deterioration.

With that much potential, the testing method has become the foundation of a company, virtual at the moment but headed toward reality. Among Smith's anticipated customers are twins interested in learning what they were once certain of: whether or not they are "identical." But the primary market will be related to food production and medicine; Smith cites as one example a company now cloning skin tissue for burn victims.

Her own research interests center on the genetic causes of hypertension, schizophrenia, and homosexuality, as well as how DNA changes in different tissues. "But there are so many applications," she says. "It's really blossoming. This is the kind of science you get very excited about."