“Chemical surgery” on embryos to free them of a gene causing thalassemia

By Dr. Ananya Mandal, MDOct 2 2017

A team of Chinese researchers have for the first time “mended” defective embryos using chemical surgery to free them of a faulty gene that leads to beta thalassemia. Until now this defect was found to be incorrigible caused due to a single misspelling in the DNA code. Beta thalassemia is a dreaded blood disorder wherein the child is unable to make healthy blood needing blood transfusions on a regular basis. The study was published this week in the journal Protein and Cell.

As of now making babies out of the embryos that have been modified is illegal in several countries including the United Kingdom and so it was purely an experiment on an academic basis. However the scientists are thrilled to have found a method by which they could prevent genetically inherited disease conditions by using a method called “base editing”. This new term was called “chemical surgery” by David Liu at Harvard University who invented it.

Crispr-Cas9 till date is one of the most powerful tools to edit genetic codes and has been making waves in the field of genetic engineering for the past few years. Crispr is a method by which enzymes are used to make precise cuts within the faulty genetic sequences to correct them and replace them with a sequence of correct codes.

Junjiu Huang at Sun Yat-sen University in Guangzhou, China and team in 2015 had tried using Crispr to make the alterations and corrections in the abnormal beta thalassemia genes in the human embryos that carried the mistakes. This experiment did not show much success and this time the team tried base editing on the cloned embryos. As a first step Huang and his team had to create a batch of cloned embryos. For this they took skin cells off patients with beta thalassemia after obtaining their consent. They used these cells and removed its DNA-containing nuclei. These cells devoid of the nuclei were them combined with donor eggs without nuclei. These eggs now developed into early stage of human embryos that had the beta thalassemia mutation.As a next step the team used base editing. Unlike Crispr, this technique does not cut through the double strands of the DNA helix but the enzyme is used to change single letters or bases. The DNA is made up of a combination of four bases G, T, A and C. In most cases of beta thalassemia, there is a switch of an A into a G in the HBB gene. This mutation at the 28th position is one of the three most common mutations seen among the beta thalassemia patients in China and Southeast Asia.

Now every person has a pair of genes or alleles. In some cases having one normal allele would mean no disease (a recessive trait) and in others both alleles have to be normal to avoid the disease – a condition known as a dominant trait. This experiment was capable of correcting only one faulty allele. That could be a problem for diseases inherited as a dominant trait. Further base editing was not always successful in the experiment and sometimes created its own mutations. Thus it is far from ready to be actually used on human embryos explain the team.