CRISPR cannot correct genetic mutations in human embryos

Tech Science 19. feb 2021 4 min Professor in Molecular Genetics, NNF Young Investigator Eva Ran Hoffmann Written by Kristian Sjøgren

A new study on which a Danish researcher has commented shows that trying to repair the genes of embryos with CRISPR is a very, very bad idea. The cells simply do not have the tools to repair the damage CRISPR causes.

For many years, researchers have enthused about the possibility of correcting genetic mutations at the earliest stages of human embryo development, also known as heritable genome editing.

These mutations may cause breast cancer or deafness, and the idea has been that CRISPR, a technology equivalent to molecular scissors, could be used to cut out the dysfunctional genes and replace them with healthy copies.

Although researchers have already experimented with CRISPR on human embryos, this is an extremely bad idea.

A new study reports that CRISPR does more harm than good in embryos. An accompanying comment by two researchers, including one from Denmark, further elucidates why CRISPR should not applied to the biological material destined to become a person.

“Researchers have been divided into two camps. Some think that editing errors out of human embryos is a good idea, whereas others think that CRISPR itself is still plagued by too many errors. The study we reviewed shows very clearly that we should definitely avoid using CRISPR. The study also questions whether human embryos can be gene edited at all, at least with current molecular tools,” explains Eva Hoffmann, Professor at the Department of Cellular and Molecular Medicine, University of Copenhagen.

Both the original study and comments by Eva Hoffmann and a colleague have been published in Cell.

Great expectations for CRISPR

The background for the new study and Eva Hoffmann’s accompanying comments are previous studies published in Nature and elsewhere in which researchers showed that they could use CRISPR to repair genetic mutations in fertilized eggs, which later developed into viable fetuses and children.

However, according to Eva Hoffmann, these studies had serious errors.

The researchers behind these studies concluded that, since they could not find the mutated gene in the embryos after gene editing, it must have worked. However, this is a fallacy.

“The researchers behind these very famous studies neglected to verify that the disease genes were actually repaired and not just deleted. Many of us therefore had serious concerns that the studies were flawed. In the new study, the researchers show that our fears were well founded,” says Eva Hoffmann.

Disease genes were not edited – the whole chromosome just disappeared

In the new study, the researchers very thoroughly investigated what happened to the chromosomes when they tried to edit mutated genes using CRISPR.

The researchers used eggs donated by healthy women, which they fertilized with sperm from a man who was blind because of a mutation in a specific gene. The aim was to edit this mutated gene in the fertilized eggs.

The researchers used CRISPR to cut out the defective piece of the chromosome so that the body could repair the gene by inserting a good copy of the gene for vision from the part of the genome that came from the woman who donated the eggs.

The researchers found that, in half the cases, the gene appeared to have been repaired because they could no longer find the defective gene, only the healthy one from the mother.

Closer examination, however, showed the alarming situation.

“The researchers found that chromosome 6, which had been cleaved, was missing in half the embryos. The missing chromosome would otherwise have come from the father. Something had gone completely wrong in the repair process, because the gene was not repaired and the entire chromosome had been deleted,” explains Eva Hoffmann.

In a further 43% of the cases, the cleaved DNA was incorrectly repaired. Instead of using the healthy gene as a copy for repair, the father’s mutated gene was pieced back together after a few bases were cut away – resulting in small deletions in the gene and leading to mutations. Gene repair was detected in only 7% of embryos; however, this only occurred in a subset of cells in the embryo, meaning that not all cells had been edited and the embryos were mosaic.

CRISPR arbitrarily cleaves the DNA

Further studies showed that CRISPR had also arbitrarily cleaved the DNA on a different chromosome (off target) and that the cells’ own repair mechanisms could not repair the damage in the vast majority of cases.

When the damage is not repaired, the cell discards the chromosome completely, and the fetus will subsequently lack a chromosome.

When the researchers in the original Nature studies could not find the genetic mutation, they concluded that it had been repaired. However, they have yet to demonstrate that this is due to gene editing and not loss of the chromosome.

“The idea behind CRISPR is basically sound, but the stumbling block is that the cells in the embryo are not very good at repairing the DNA that CRISPR has cleaved. Evolution appears to have chosen a strategy to only allow embryos that are viable to develop and otherwise discard the defective ones instead of trying to repair these cells,” says Eva Hoffmann.

The end for CRISPR in heritable genome editing?

Eva Hoffmann says that the discovery may well be the end for CRISPR for the purpose of editing the human genome to repair genetic mutations.

There are better ways to get viable fetuses than to count on a small probability of success, which was as low as 7%.

Assuming that CRISPR can be used to edit mutations in embryos and that the chance of succeeding is so small, extensive genetic screening would be required to ensure that no new dangerous damage to the genome had been introduced in the process.

“We have other ways to avoid the transmission of disease genes to children: by thoroughly screening fertilized eggs before implanting them in a woman. In this situation, half the embryos we examine are do not contain the disease gene. CRISPR has only a 7% success rate of re pair, and then only in some cells. This is very problematic,” explains Eva Hoffmann.

CRISPR can still advance researchers’ knowledge on chromosome defects

Eva Hoffmann thinks, however, that we can learn a lot about repairing the genes in human cells from the experiments performed with CRISPR.

The researchers have learned a great deal about what repair mechanisms are involved, and this may advance our understanding of why 50% of embryos naturally suffer chromosome breaks and loss without CRISPR.

Some repair mechanisms are active and others are not, and these occur at different times in life.

Researchers can also use the technology to advance their knowledge about chromosome defects that are already present when an egg is fertilized.

“But embryos should not be edited using the technology we have today because it is just not good enough,” says Eva Hoffmann.

Allele-specific chromosome removal after Cas9 cleavage in human embryos” and “Cas9 in human embryos: on target but no repair” has been published in Cell. In 2015, the Novo Nordisk Foundation awarded a grant to Eva Hoffmann for the project Mapping the Genomic Landscape in the Human Germline. Eva Hoffmann was elected as a member of the European Molecular Biology Organization, a lifetime honour in recognition of her extraordinary results within the biological sciences.

Chromosomes are rearranged and organized into new sets to create diversity as they are passed from parent to offspring through the germline. The genet...

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