Is CRISPR/Cas9 capable of erasing the third copy of chromosome 21?

by Zafeiroula Katsara, MSc Student in Bioeconomy: Biotechnology and Law, International Hellenic University

CRISPR/Cas 9 technology gives scientists the ability to add, remove or alter specific sequences on the genome in an easier, economically efficient but also more accurate way. This means that CRISPR technology could be a valuable application on an entire new field of studies on gene therapy and may be able to cure disorders and diseases like cancer, HIV, cystic fibrosis etc. However, when it comes to Down’s Syndrome, is it applicable for any possible cure?

Is it possible to eliminate an entire chromosome using this technology? And even more generally speaking, is it a potential tool to be applied to prevent and cure any type of aneuploidy?

CRISPR/Cas9 has a role that could be described as a molecular scissor, which can specifically cut designated positions in a DNA sequence through the guide of sgRNA. The chromosome is a series of very long, repetitive DNA sequences and the repetition of those sequences could be targeted by designed sgRNAs. CRISPR/ Cas9 identifies targeted sgRNAs and cuts the repeats in the chromosome and breaks them. That results the destruction of the chromosome, in the point beyond of cell repair, which means that it is no longer capable on participating in the replication process and in the end will not be expressed in the offspring. ¹

Of course, that method is not as simple to apply as it seems. The main obstacle is not only how to induce multiple DNA cleavages efficiently but also to predict whether these repetitive sequences “work well”. Also, how would it be possible to target only one out of three chromosomes, without causing any damage to the other two. ²

There is an emerging, promising approach of the use of CRISPR/Cas9 technique either via silencing the entire chromosome by using XIST transgene ³ or via deletion of the overexpressed genes HSA21. ⁴ It is possible that this technique could stand in the future as a potential therapeutic approach to cure aneuploidy diseases; however, it is well highlighted that off-target effects should be examined in silico and in vivo before passing into the clinical trial to eliminate risks. ⁵   

The main obstacles that method is due to face are

• The moderate expression of genes that is a result with the interaction of the Cas9 and guide RNA.⁶ However, even those modest expressions could have significant phenotypic variations ⁷ and that is an indicator that there might not be necessary to silence the entire chromosome. ⁸
• The off-target effects ⁹
• In vivo delivery of CRISPR/Cas9 is challenging since the method is applied ¹⁰
• Epigenetic suppression of the epigenetic targets may lead to unwanted phenotypes ¹¹

For the method to be applicable to prevent cognitive damage, the main way to be applied is before birth, as the brain is significantly advanced after the second trimester. ¹² By applying a prenatal therapy to the foetus through the mother is not only risky due to the possibility of various complication in the integrity of the foetus itself, it is also unethical.¹³

The prenatal application of CRISPR/Cas9 with the possibility of the most effective results leads us to germline genome editing, which is unethical since the changes will be inherited.  Generally and theoretically, the silence or elimination of an entire chromosome seems to be possible, however in practise it is rather inefficient. Also, it is difficult to predict in advance the Down Syndrome in an embryo, as most of the cases is not heritable from parents. Might have been economically inefficient, but in order to locate the trisomy, there is the possibility for checking it through preimplantation genetic testing. Instead of moving to genome editing, there is also the opportunity to choose to transfer the embryo without the trisomy to the mother’s womb. That does not withdraw the potential of this CRISPR/Cas9 method on a possible therapy of Down’s Syndrome, however there is a long road ahead before moving forward to it.

Acknowledgements
We are grateful to Ms Katsara for kindly providing the original article.

References

^{1 Hui, Y. (2017). CRISPR/Cas9 gene editing can be used to eliminate entire chromosomes: a Q&A – On Medicine. [online] On Medicine. Available at:}

CRISPR/Cas9 gene editing can be used to eliminate entire chromosomes: a Q&A

[Accessed 19 Aug. 2019].

^{2 Id}

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^{4 Mentis, A. (2016). Epigenomic engineering for Down syndrome. Neuroscience & Biobehavioral Reviews, 71, pp.323-327. 10.1016/j.neubiorev.2016.09.012}

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^{9 Id}

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^{11 Mentis, A. (2016). Epigenomic engineering for Down syndrome. Neuroscience & Biobehavioral Reviews, 71, pp.323-327. 10.1016/j.neubiorev.2016.09.012}

^{12 Keiichi Ishihara, Kenji Amano, Eiichi Takaki, Atsushi Shimohata, Haruhiko Sago, Charles J. Epstein, Kazuhiro Yamakawa, Enlarged Brain Ventricles and Impaired Neurogenesis in the Ts1Cje and Ts2Cje Mouse Models of Down Syndrome, Cerebral Cortex, Volume 20, Issue 5, May 2010, Pages 1131–1143, https://doi.org/10.1093/cercor/bhp176}

^{13 Mentis, A. (2016). Epigenomic engineering for Down syndrome. Neuroscience & Biobehavioral Reviews, 71, pp.323-327. 10.1016/j.neubiorev.2016.09.012}

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