Day-to-day, my PhD research involves investigating the molecular basis of inherited heart muscle disorders or scientifically – cardiomyopathies. Unsurprisingly then, one morning after being alerted by my father that cardiomyopathy was mentioned on the news I frantically googled: “nature crispr cardiomyopathy korea” and hey presto! Google [1] swiftly suggested the article in question: Correction of a pathogenic gene mutation in human embryos [2] published this month in the world-leading scientific journal Nature. In this article, scientists used the relatively recent technique of genome editing to “fix” an inherited cardiomyopathy mutation in a human embryo! The mutation was carried by the father whilst the mother was “healthy” meaning that she carried two wild-type (WT) alleles of MYBPC3. This gene encodes the sarcomeric protein myosin-binding protein C.

Cardiomyopathy, as the name defines, means heart-muscle disorder. The father in the study suffered from hypertrophic cardiomyopathy (HCM). Typically, the left side of the myocardium is enlarged or thickened, resulting in a stiff heart with impaired pumping capabilities. Particularly during strenuous exercise, HCM can lead to a person collapsing such as in the case of the footballer Fabrica Muamba [3]. In severe cases it can even cause sudden cardiac death. Currently there aren’t any cures for this condition, hence, there is a requirement for in-depth research into this field.

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Figure 1. Normal heart (left) vs HCM heart (right). At times hypertrophy can be so severe almost blocking outflow of the blood through the aorta. Image from [4].

This[2] was one of the few studies in which the genome editing technique called CRISPR was applied to a human embryo. The schematic below (Figure 2) gives an overview of the technique. In essence, a DNA cutting enzyme (Cas9) is combined with a guide RNA sequence that targets it to the specific location in the genome. Cas9 then cuts the DNA, generating a double stranded break. Subsequently, this break can be “fixed” by a mechanism called homology directed repair. Whilst the exact ins-and-outs of this are still unknown (oh the joys of biology!) a homologous DNA template already present in the nucleus or one that is supplied by injection is used to repair the break. In this case, this was the WT sequence from the healthy mother.

 

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Figure 2. A schematic showing how CRISPR works. Edited from [5].

However, as with any technique there are risks involved: homology directed repair can lead to a phenomenon known as mosaicity, meaning a mixture of WT and mutant cells in the embryo. Furthermore, there is a possibility of the guide RNA binding to a highly homologous but not identical sequence, leading to off-target editing of the genome. This could be very dangerous and lead to disastrous effects if not identified. You wouldn’t want your genome being unspecifically chopped up!

In this article they demonstrated that by injecting the CRISPR/Cas9 complex directly into the sperm before fertilisation, 72% of embryos were “fixed” (now WT) and they did not find any “evidence of off-target mutations”. This technique could in potentially increase the number of healthy embryos available for IVF implantation, hence, preventing the pathogenic genetic mutation being passed onto offspring.

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Figure 3  Cas9 (yellow) and guide RNA are injected into the sperm cell which then fertilizes the egg. Image adapted from [6].

 

There are a number of other very important uses where the CRISPR technique is being put to use. Scientists are researching ways to modify the genome of mosquitos in order to stop them transmitting the P. falciparum parasite which causes malaria in humans [7] Whilst there is still much speculation and concern about how this technique could, in theory, lead to a future of “designer babies” and “genetically superior” humans [8], let’s face it, even with IVF there’s so much regulation it’s highly unlikely to be allowed, at least not in our lifetimes. Nevertheless, attempting to preventing transmission of inherited conditions has just got one step closer. Watch this space!

 

Article by Roksana N.  A final year PhD student at King’s College London

 

[1] Google is so amazing nowadays!
[2] Ma, H. et al., (2017) Correction of a pathogenic gene mutation in human embryos, Nature, advance online publication doi:10.1038/nature23305
[3] https://www.theguardian.com/football/2012/mar/18/heart-disorder-hocum-fabrice-muamba [Accessed Aug 2017]
[4]www.mayoclinic.org/diseases-conditions/hypertrophic-cardiomyopathy/home/ovc-20122102 [Accessed Aug 2017]
[5] Royal Society of Biology (2017)
[6] Winblad, N. and Lanner, F., (2017) Biotechnology: At the heart of gene edits in human embryos, Nature, advance online publication
[7] Gantz et al., (2014) Highly efficient Cas9-mediated gene drive for population modification of the malaria vector mosquito Anopheles stephensi, PNAS,  doi: 10.1073/pnas.1521077112
[8] Gattaca (Film), 1997