Thursday, April 30, 2015

Blogger's Desk #6: Spark of Ethical Debate on editing the embryonic genome


There is a lot of pages right now talking about, the sparked ethical debate of editing a human embryo. Though I see a very responsible reporting of what actually the whole study is about in some of standard reference journals and science websites, I see a lot of pages and even some news articles hyping the actual work or there is a clear misinterpretation of the whole work. In this blog space, I almost never touch upon topics such as ethics, human genetics etc. But in this case I would like to make an exception. Well that's what Blogger's desk series is about...

With increasing genetic tools to edit the gene for a variety of research purposes in laboratory, there is an increase in fidelity and ability to edit genomes of both small (such as viruses) and large genomes (such as mouse genomes). Lab created animals via genetic techniques (such as mouse models) are a common place. Studies also have been conducted to rectify human genes especially in an attempt to correct genetic disorders. For example there is a reasonable expected success in treating SCID (Severe combined immunodeficiency syndrome), caused by a single gene mutation using gene therapy.

Fig 1: Method for mitochondrial gene therapy.
Mitochondrial genetic diseases is another case. In a recent historical decision (Link), mitochondrial gene editing tool has been approved to be used so as to avoid passing the mutations to progeny. The procedure is controversial since the genetic makeup of the resulting progeny is derived from 3 people instead of natures standard. In a recent paper published in cell, mitochondria-targeted nucleases selectively reduced mtDNA haplotypes in germline. This was successfully used to reduce human mutated mtDNA levels responsible for Leber’s hereditary optic neuropathy (LHOND), and neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP), in mammalian oocytes under lab conditions. Bruce Whitelaw speaks, "Conceptually this is an alternative to the ‘three person embryo’ strategy. Society needs to grapple with this. You could imagine every IVF clinic in the country being able to do this. But is the genome editor technology robust enough yet? I think that’s an open question. I genuinely believe it will be in the near future, so we have to have the debate now: what applications are beneficial and which ones does society has concerns about?”

Editing genomes has never been easier. CRISPR/CAS technology (See my earlier post) has exponentially improved in the last 3 years. The technique has allowed editing of genome in an unprecedented scale in the laboratory. By using some tricks in design of the guide RNA, Gantz and Bier showed that they could create homologous stable genetic changes in fruit fly models. The method was called as Mutagenic Chain Reaction.

I'm trying to impress on you the fact that the gene editing is a technology that has not suddenly emerged. Genetic modification is of two types- Somatic and Germ Line. Most of the genetic editing technology has been focused on Somatic cell line changes. In case of genetically modified mice or other laboratory animals, the mutations are in the germline. In other words changes are made in embryonic system therefore they continue to breed changes. In contrast human embryo is a special case.  One of the first large scale genetic editing technology was the Zinc finger Nuclease technique (ZFN method; Link). With advent of CRISPR the technology has been powerful enough in cellular models to warrant further application. There has been a general unscripted agreement that heritable genetic changes will not be made in humans till we have the sufficient technology to try to do so. The question is how do we know we are ready or not?

Photo 1: 8-cell embryo, at 3 days.
The debate on embryonic gene editing was raised with a series of articles by multiple people sounding an alarm over gene editing in human embryo. It was cited that people have already sought permission to do so, from discarded embryo's. The paper's further urged scientists for temporary voluntary moratorium to discuss and debate the issue before proceeding further. In the meantime as the stir of debate was heating up, publication in Protein & Cell, of having done the procedure accelerated the stem of debate.

A little bit about of the study. Chinese scientists have used CRISPR gene-editing technique to modify non viable human tripronuclear zygotes. Of course the use of used non-viable embryos obtained from fertility clinics was a deliberate attempt to avoid ethical problems. These are embryos that will die and will never mature to complete human though it will divide to produce a few cells. The team injected 86 embryos with CRISPR/Cas9 custom designed to add in new DNA. After 48 hours, by which time the embryos would have grown to about eight cells each. Of the 71 embryos that survived, 54 were genetically tested by exome sequencing revealing that just 28 were successfully spliced endogenous β-globin gene, and that only 4 of those contained the genetic material designed to repair the cuts. The efficiency of homologous recombination directed repair was low and the edited embryos were mosaic. The relative inefficiency was evident and the research was halted at this point.

I gathered from reliable sources, the paper was peer reviewed in Nature and Science, but was rejected because of questions raised on ethical concerns. However, John Harris a bioethicist says, “It’s no worse than what happens in IVF all the time, which is that non-viable embryos are discarded. I don’t see any justification for a moratorium on research”.

Fig 2: Off-target cleavage in human embryos. Source
In my view the paper actually gives the data to talk about if research on human embryo needs to go further and is it time to make it to clinics. The idea of authors in this case was to answer the question "Are we ready for embryonic gene therapy?". The paper also noted that there were serious off target effects. For example there was 8 bp mismatch between the G1 gRNA and C1QC gene. Yet the C1QC locus was targeted. The follow up sequencing was done only for Exome and not whole genome. As the authors point out, there will be likely a lot more they have missed in the non exome region and is a concern.

The data from the paper basically says "We are not yet ready for germline level genetic editing in humans". But the debate has gone further and NIH is seriously considering not encouraging these type of experiments. However, David Baltimore remarks "I am not in favor of the NIH policy and I believe that the Chinese paper shows a responsible way to move forward. But it is the will of Congress that there be no work with human embryos and I assume that means even ones that are structurally defective".

The basic ethical debate in the current study doesn't question if the study was useful. There is almost an universal agreement, indeed we are not yet there and the paper has provided the data for it. But the debate is about the use of human embryo's. Supporter's of the study pointed that the there was no other ethical way to know and the study was ethically and morally right.
Thrasher AJ etal (2014). A modified γ-retrovirus vector for X-linked severe combined immunodeficiency. The New England journal of medicine, 371 (15), 1407-17 PMID: 5295500

Ewen Callaway. Scientists cheer vote to allow three-person embryos. Nature. Link

Gantz VM, & Bier E (2015). Genome editing. The mutagenic chain reaction: a method for converting heterozygous to homozygous mutations. Science (New York, N.Y.), 348 (6233), 442-4 PMID: 25908821

Reddy P, Ocampo A, Suzuki K, Luo J, Bacman SR, Williams SL, Sugawara A, Okamura D, Tsunekawa Y, Wu J, Lam D, Xiong X, Montserrat N, Esteban CR, Liu GH, Sancho-Martinez I, Manau D, Civico S, Cardellach F, Del Mar O'Callaghan M, Campistol J, Zhao H, Campistol JM, Moraes CT, & Izpisua Belmonte JC (2015). Selective elimination of mitochondrial mutations in the germline by genome editing. Cell, 161 (3), 459-69 PMID: 25910206

Lanphier E, Urnov F, Haecker SE, Werner M, & Smolenski J (2015). Don't edit the human germ line. Nature, 519 (7544), 410-1 PMID: 25810189

Cyranoski, D. (2015). Scientists sound alarm over DNA editing of human embryos Nature DOI: 10.1038/nature.2015.17110

Cyranoski D (2015). Ethics of embryo editing divides scientists. Nature, 519 (7543) PMID: 25788074

Liang P, Xu Y, Zhang X, Ding C, Huang R, Zhang Z, Lv J, Xie X, Chen Y, Li Y, Sun Y, Bai Y, Songyang Z, Ma W, Zhou C, & Huang J (2015). CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes. Protein & cell PMID: 25894090

Reardon, S. (2015). NIH reiterates ban on editing human embryo DNA Nature DOI: 10.1038/nature.2015.17452

Baltimore D, Berg P, Botchan M, Carroll D, Charo RA, Church G, Corn JE, Daley GQ, Doudna JA, Fenner M, Greely HT, Jinek M, Martin GS, Penhoet E, Puck J, Sternberg SH, Weissman JS, & Yamamoto KR (2015). Biotechnology. A prudent path forward for genomic engineering and germline gene modification. Science (New York, N.Y.), 348 (6230), 36-8 PMID: 25791083

No comments:

Post a Comment