Each year, tens of thousands of babies are born in the United States as the result of IVF (in vitro fertilization) – where several eggs are fertilized outside the mother's body and then an embryo is selected for transfer into her womb. For a number of years now, it has been possible for prospective parents to screen their embryos for rare genetic defects such as cystic fibrosis or Tay-Sachs disease. However, new technical advances now make it possible for parents to have their embryos screened for genetic risk factors for diseases such as cancers, diabetes, heart disease, or psychiatric disorders such as schizophrenia.
With this new method, genetic testing provides each embryo with a risk score – an estimate of the chances of the given embryo to develop a particular disease. After perusing the scores, parents can then select the embryo they want implanted. This selection process clearly has considerable ethical implications. However, there was one key question that needed addressing: Can screening actually reduce the probability that an embryo will develop one of these diseases? This was the focus of a statistical analysis carried out by a team led by Professors Shai Carmi and Or Zuk at the Hebrew University of Jerusalem (HU) and Professor Todd Lencz at the Feinstein Institutes for Medical Research in New York. Their findings were published today in eLife.
For decades, couples whose offspring were at risk of developing a devastating childhood disease have had the option to screen the genetic makeup of their embryos. In these cases, only a single mutation on a specific gene is responsible for the disease, and consequently, these diseases are known as monogenic. However, most adult diseases are not monogenic, but are polygenic – affected by a large number, sometimes thousands, of different genes and variants, each having a small effect on the risk of developing a particular disease. Polygenic risk scores combine genetic variants in these numerous genes to estimate the risk of an individual. By now, already one company is taking advantage of polygenic risk scores to offer screening of embryos for polygenic diseases.
In their present work, the researchers analyzed scenarios where parents are provided with the risk scores of their embryos for one given disease. These parents would then be faced with two selection strategies: To eliminate embryos with a particularly high score for an undesirable disease and then randomly choose one of the remaining embryos for implantation or to select the embryo with the lowest predicted risk score.
