In 2017, a team of researchers led by Shoukhrat Mitalipov, a geneticist at Oregon Health and Science University in Portland, reported that human embryos carrying a mutation could be coaxed into this process without a synthetic template. The researchers generated embryos from a union between two cells: a sperm carrying a mutation that can make it harder for the heart to pump blood, and an egg with a healthy version of the gene. Dr. Mitalipov and his team used Crispr-Cas9 to cut the broken copy of the gene to see if the intact version would guide its repair. They reported the experiment a success and published it in the journal Nature.
“In principle, this could be a way to correct a mutation in a human embryo” that has only one broken copy of a gene, Dr. Egli said.
But the new findings could cast some doubt on the 2017 work, Dr. Egli added.
The researchers of the Cell study focused on a different mutation — one that causes hereditary blindness and affects a different part of the genome — but adopted a similar setup. Using donated sperm containing a mutation in a gene called EYS, they fertilized eggs that had normal copies of EYS, then sent in Crispr-Cas9 to snip the mutation.
Several of the cells managed to sew the Crispr-cut pieces of DNA back together with a few minor changes, Dr. Egli said.
But about half the embryos seemed unable to cope with the trauma of the break. The genetic damage failed to heal, eventually forcing cells to tear off and toss aside large chunks of the chromosome that harbored the mutated EYS. In some cells, the entire chromosome was lost.
“That is not a correction,” Dr. Egli said. “That is a vastly different outcome.”
Instead of gently goading the cell into editing the genetic “text” at which it was targeted, the Crispr machinery gouged irreparable gaps in cells’ DNA, said Maria Jasin, a geneticist at Memorial Sloan Kettering Cancer Center and another author of the study. The negative consequences of this, she added, were disproportionately disastrous. “They were talking about trying to repair one gene, and you have a substantial fraction of the genome being changed,” Dr. Jasin said.
Dr. Egli and Dr. Jasin said that this probably happened in Dr. Mitalipov’s 2017 paper as well, but it went unnoticed. After Dr. Mitalipov’s team carried out their Crispr-Cas9 treatment, they could no longer detect the mutation in embryos. But Dr. Egli and Dr. Jasin noted that, technically, dumping or destroying a huge segment of a chromosome would have wiped out evidence of the mutation as well. Dr. Mitalipov and his team, they said, might have mistaken a deletion for an edit.