Designer Humans: A Response to the First Report of Human Germ Cell Line Editing with CRISPR-Cas9 / by James Wade

The word is out. Scientists have successfully rewritten the genes of a human embryo. Gattaca, here we come! Or do we?

Last month, researchers out of Sun Yat-sen University in Guangzhou, China, published the first report on editing the genes of a human zygote. They used the gene editing technology that is exploding in popularity called CRISPR-Cas9. The paper, published in the low-impact journal Protein & Cell (impact factor 2.851), came right on the heels of a comment in Nature calling for an outright ban on editing human germ line cells. 

Before we call on the angry mobs to storm the lab performing the germ line alteration studies, we should first look at what the researchers actually did. Quoting directly from the controversial study:

“In this report, we used tripronuclear (3PN) zygotes to further investigate CRISPR/Cas9-mediated gene editing in human cells. We found that CRISPR/Cas9 could effectively cleave the endogenous β-globin gene (HBB)."

The researchers used tripronuclear (3PN) zygotes. These cells are a byproduct of in vitro fertilization (IVF), and they contain DNA from an egg along with the DNA from two sperm cells. Prior to fertilization, germ line cells (egg and sperm cells) are haploid, which means that they only contain a single copy of each chromosome. A normal fertilized egg has two copies of each chromosome, termed diploid. The 3PN zygotes have an additional copy of each chromosome (from the extra sperm), making it a triploid cell. These embryos are known to produce poor quality embryos and are viewed as a waster product of the IVF process. Since they are typically discarded, the researchers sought to use these cells to study the effects of the CRISPR-Cas9 system on germ line cells. As a proof-of-concept study, they attempted to remove the β-globin gene (HBB), a gene encoding a sub-unit of adult hemoglobin protein.

Quoting, again, from the abstract of the paper:

“We found that CRISPR/Cas9 could effectively cleave the endogenous β-globin gene (HBB). However, the efficiency of homologous recombination directed repair (HDR) of HBB was low and the edited embryos were mosaic. Off-target cleavage was also apparent in these 3PN zygotes as revealed by the T7E1 assay and whole-exome sequencing."

The CRISPR-Cas9 system has been shown to edit many different cell types across a number of genes. Feng Zhang and Jennifer Doudna, pioneers of this technology, have published extensively on the development of the CRISPR-Cas9 gene editing method. Unsurprisingly, when the researchers in this study attempted to target the HBB gene, they successfully removed it from the genome. The CRISPR-Cas9 gene editing method works by forming a double strand break (DSB) in the DNA. This can be repaired in two ways: non-homologous end joining (NHEJ) and homologous recombination directed repair (HDR). When the researchers attempted to repair the DSB created by removing the HBB gene, they were unable to perform the repair with perfect control. This imperfect repair creates what are called mosaic embryos, making predicting the outcomes of gene editing impossible.  Here are some videos to help explain the process:

Not only were the researchers unable to control the repair process after removal of the HBB gene, they also observed cleavage of DNA at off-target locations in the genome. In other words, the method to remove the HBB gene removed additional pieces of the genetic code.  From these results, the researchers claim (correctly in my opinion) that the CRISPR-Cas9 gene editing system is not ready for clinical application. Clearly, much work needs to be done to improve the accuracy of gene removal and the reliability of putting proper genes back in place.

So, what are we left with? Scientists used cells that were never intended to develop fully into humans to demonstrate that gene editing technologies are dangerous and much more research should take place before we consider using these methods to improve the next generation. This study initiated a lot of discussion, some of it quite vitriolic,  but it seems that it was the small spark that triggered a burst of excitement and concern.

In a perspective article (paywall, sorryin Science in early April of this year, top scientists expressed their recommendations for the future of human genetic engineering. Though they do not cite the 3PN zygote study directly, they were most certainly aware of its imminent publication, motivating their release of the perspective piece. In it, they list four recommendations:

1) Strongly discourage, even in those countries with lax jurisdictions where it might be permitted, any attempts at germline genome modification for clinical application in humans, while societal, environmental, and ethical implications of such activity are discussed among scientific and governmental organizations. (In countries with a highly developed bioscience capacity, germline genome modification in humans is currently illegal or tightly regulated.) This will enable pathways to responsible uses of this technology, if any, to be identified.
2) Create forums in which experts from the scientific and bioethics communities can provide information and education about this new era of human biology, the issues accompanying the risks and rewards of using such powerful technology for a wide variety of applications including the potential to treat or cure human genetic disease, and the attendant ethical, social, and legal implications of genome modification.
3) Encourage and support transparent research to evaluate the efficacy and specificity of CRISPR-Cas9 genome engineering technology in human and nonhuman model systems relevant to its potential applications for germline gene therapy. Such research is essential to inform deliberations about what clinical applications, if any, might in the future be deemed permissible.
4) Convene a globally representative group of developers and users of genome engineering technology and experts in genetics, law, and bioethics, as well as members of the scientific community, the public, and relevant government agencies and interest groups, to further consider these important issues, and where appropriate, recommend policies.

Each of their recommendations are well founded and sensible. I'm really not seeing what all of the fuss was about. Are people in some hidden lab attempting to genetically engineer embryos to be brought to term? It is a strong possibility. But, we at the societal level cannot effectively block these sorts of efforts. Genetic engineering is coming, and I believe the approach that the Chinese scientists took for the most recent study is a good example for others to follow.  Let's all hope others are as responsible in the future.