Genome Editing to Protect Humanity from Infectious Diseases – The Battle of CRISPR/Cas9 and Malaria

Threat of Infectious Diseases

Since the confirmation of a cluster infection in Wuhan, China in 2019, the novel coronavirus infection, known as COVID-19, has caused a global pandemic. COVID-19 can be considered one of the most notorious infectious diseases in human history, significantly altering our lives in ways unparalleled before.

However, infectious diseases posing a threat to humanity are not a novel occurrence. Throughout history, diseases like smallpox, which caused numerous deaths since ancient times, the Black Death that claimed one-third of Europe’s population in the 14th century, and the Spanish flu that spread after World War I. Human history has always been a battle against infectious diseases. Among them, “Malaria infection” is known as an infectious disease that has continued to claim numerous lives as a zoonotic infection from prehistoric times, affecting both humans and animals. Even in modern times, people continue to lose their lives due to malaria infections. So this time, the discussion will approach attempts at treating malaria from the standpoint of genome editing. About Malaria

Before delving into the discussion of malaria treatment using genome editing, let’s first organize information about malaria. Malaria is a bloodborne infectious disease transmitted by mosquitoes of the Anopheles genus, carrying parasites known as Plasmodium spp[1]. There are five types of Plasmodium parasites that infect humans: P. falciparum,　P. vivax, P. malariae, P. ovale, and P. simian. Plasmodium falciparum, causing tropical malaria, is particularly known as “malignant malaria” and poses a higher risk of severe complications.

The malaria parasite primarily infects red blood cells. Infected red blood cells are destroyed due to the activity of the malaria parasite. As a result, there is an increased burden on the spleen, which serves as the waste disposal site for red blood cells, causing it to swell significantly. Additionally, blood clots, referred to as “thrombi,” can form, blocking blood vessels in various organs and impairing their function. Among these complications, cerebral malaria, which affects the brain, is particularly lethal. Patients who develop cerebral malaria often fall into a coma and ultimately succumb to death.

Therapeutic drugs for malaria infection have been developed and put into practical use. For example, quinine is a medication derived from substances found in the bark of the cinchona tree and is known to be the precursor to major antimalarial drugs such as chloroquine and primaquine. Additionally, artemisinin is a medication derived from the sweet wormwood plant (Artemisia annua), and the Chinese researcher Tu You-you, who discovered it, received the Nobel Prize in Physiology or Medicine in 2015 [2].

Humanity has been developing various therapeutic drugs to eradicate malaria infections. However, considering the current situation where over 600,000 people still lose their lives to malaria infections worldwide every year, it is believed that an approach solely based on conventional drug development may find it challenging to eliminate malaria infections. Therefore, this time will introduce some research efforts utilizing CRISPR/Cas9 to address malaria infections.

Genome Editing and Malaria

Mainly now CRISPR/Cas9, a genome editing technology that is currently primarily utilized, was first applied to malaria parasites in the world through the research conducted by Mehdi Ghorbal and colleagues, published in Nature Biotechnology in 2014 [3]. Mehdi and his team demonstrated that CRISPR/Cas9 could induce double-strand breaks in malaria parasites with ease, given the appropriate design of guide RNA (gRNA). Furthermore, they confirmed the ability to introduce foreign genes into malaria parasites by utilizing homologous recombination with the introduction of donor DNA.

In this study, Targeting the egfp region of the malaria parasite’s chromosome, double-strand breaks were induced. As a result, resistance genes against folate antagonist metabolism inhibitors in humans were introduced. Successful reproduction of malaria parasites lacking egfp and possessing resistance to folate antagonist metabolism inhibitors was achieved within approximately three weeks. In all of these experiments, no significant off-target effects were observed. (Off-target effects refer to unintended genetic mutations that may occur in the context of CRISPR/Cas9.) With the insights gained from these studies, it is anticipated that research on the pressing issue of drug resistance in malaria parasites will continue to advance by through the judicious use of CRISPR/Cas9.

Andrea and his team aimed to create mosquitoes that have lost reproductive function by knocking down the intron 4–exon 5 regions of the doublesex gene, involved in mosquito reproduction, using CRISPR/Cas9. Consequently, the genome-edited mosquitoes in the cage areas, confined in cages, lost reproductive function in nearly all individuals by the 7th generation, ultimately leading to the extinction of the mosquito population.

While this experiment was confined to cages, if these genome-edited mosquitoes were released into the actual natural environment, it could result in the loss of reproductive function in mosquitoes worldwide, potentially leading to the extinction of the mosquito population. The extinction of the vector for malaria parasites would effectively mean the eradication of malaria infections.

From the perspective of eradicating malaria, the use of CRISPR/Cas9 for gene drives can be considered beneficial for humanity. However, further research is needed to understand how the extinction of mosquitoes, such as Anopheles, through CRISPR/Cas9-driven gene drives, would ultimately impact humanity. Ethical considerations surrounding the deliberate extinction of populations through artificial gene drives also require careful discussion. Interference with ecosystems through casual genome editing should be approached with caution.

Furthermore, not only for malaria but also for pandemics that occur approximately every ten years, such as SARS, new influenza, and COVID-19, The use of CRISPR/Cas9 technology may be effective. Even as COVID-19, which has spread at the fastest rate in recorded history, begins to subside, it is not difficult to imagine the emergence of different infectious diseases in the future. Taking the lessons from this pandemic, it is hoped that humanity, leveraging various technologies, including the wisdom of CRISPR/Cas9, will carefully consider the balance between eradicating infectious diseases and their impact on ecosystems. The optimal use of CRISPR/Cas9 for the benefit of humanity and the Earth is desired.

References

[1] World Health Organization. “Malaria”.
[2] The Nobel Foundation. “Press release: The Nobel Prize in Physiology or Medicine 2015.” The Nobel Prize. 15 October, 2020.
[3] Ghorbal, M., Gorman, M., Macpherson, C. et al. Genome editing in the human malaria parasite Plasmodium falciparum using the CRISPR-Cas9 system. Nat Biotechnol 32, 819–821 (2014).
[4] Ariey, F., Witkowski, B., Amaratunga, C. et al. A molecular marker of artemisinin-resistant Plasmodium falciparum malaria. Nature 505, 50–55 (2014).
[5] Kyrou, K., Hammond, A., Galizi, R. et al. A CRISPR–Cas9 gene drive targeting doublesexcauses complete population suppression in caged Anopheles gambiae mosquitoes. Nat Biotechnol 36, 1062–1066 (2018).