Gene editing in human embryos: Your questions answered

Research published in the scientific journal Nature has, for the first time, shown how editing genes in human embryos can repair a disease-causing mutation and produce apparently healthy embryos. Although it is a long way from clinical use, it raises the possibility that gene editing, in the future, may protect babies from hereditary conditions.

What is a gene mutation?

A gene mutation is a permanent alteration in the DNA sequence that makes up a gene. Mutations range in size; they can affect anything from a single DNA building block to a large segment of a chromosome that includes multiple genes.

There are two types of gene mutation: hereditary mutations, which are inherited from a parent and are present throughout a person’s life in virtually every cell in the body; and acquired mutations, which occur at some time during a person’s life and are present only in certain cells. These changes can be caused by environmental factors or can occur if a mistake is made as DNA copies itself during cell division.

What disease did the researchers focus on?

Hypertrophic cardiomyopathy , which affects the muscles in the heart, occurs in about one in 500 people. It leads to heart failure and has been implicated in some cases of sudden adult death. It is caused by a mutation in a gene called MYBPC3.

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What did they do?

Using sperm from a man with hypertrophic cardiomyopathy and eggs from 12 healthy women, the researchers created fertilised eggs. They injected Crispr-Cas9, which works as a genetic scissors, to cut out the mutated DNA sequence on the male MYBPC3 gene. Next they injected a synthetic healthy DNA sequence into the fertilised egg. The male gene then copied the healthy sequence from the female gene, thereby eliminating the mutation that would otherwise have caused the heart-muscle problem to develop.

Will the new treatment work for other diseases?

Yes, but it will not be of use in all diseases. Most are caused by multiple factors – often a combination of genetic factors, the environment, lifestyle and infection. The new technique may work in disorders caused by mutations in a single gene, for example sickle cell disease and cystic fibrosis.

But common medical problems such as heart disease and diabetes do not have a single genetic cause: they are associated with the effects of multiple genes in combination with lifestyle and environmental factors. And although complex disorders often cluster in families, they do not have a clear-cut pattern of inheritance. This means the latest gene-editing breakthrough cannot be used to treat many common diseases.

Are there ethical or legal issues surrounding this treatment?

Critics of the Crispr technology have argued that gene editing could lead to eugenics and to the production of embryos with certain features, in order to develop so-called designer babies.

Gene editing has yet to be shown to be completely safe in people; there are concerns it may affect future generations in unexpected ways. The technique already faces substantial regulatory hurdles. The United States Congress has barred the US Food and Drug Administration from even considering human trials with edited embryos. Earlier this year a US National Academy of Sciences committee endorsed modifying embryos, but only to correct mutations that cause "a serious disease or condition" and when no "reasonable alternatives" exist. In the UK it is illegal to implant genetically modified embryos in women.