Alexander the Great. Genghis Khan. George Washington. Three figures that once held great influence over the masses and have held pivotal roles in defining much of human history. Yet this triumvirate of consolidated and insurmountable power succumbed to nature’s most derisory and puny of parasites- plasmodium, also referred to as sporozoites. It is responsible for inflicting 3% of the world’s population with the debilitating disease Malaria, which has been at the forefront of global attention, and shows minimal signs of slowing down, mainly due to extensive population of its host, the mosquito. This account focuses on the nature of this exploitative parasite’s life cycle, explores its excoriating impact upon the human body and expands upon the innovative utilisation of CRISPR, in order to initiate a gene drive, with the hope of eradicating this ‘plague’.
From bite to infection.
It is the Anopheles female mosquito that is responsible for the transmission of the disease-inducing parasite. Sporozoites are present in the insects’ saliva, which is discharged into a victim to prevent clotting of the blood, thus allowing for the direct transfer of the parasite into the blood stream. The sporozoite seeks out the liver, which is the body’s central detoxification unit, where it gains entry into the liver tissue via the Kupffer cell, often killing cells upon entry and leaving a trail of necrotic cells. Upon infecting a portion of liver cells, the plasmodia undergo nuclear division as they asexually reproduce to procreate thousands of new parasites, or an army of merozoites. This lasts an incubation period of 2 weeks, after which the liver cells eventually burst, unleashing a wave of merozoites modified to actively seek out red blood cells. Once contact is made with a red blood cell, the parasite garners refuge from the body’s immune system, within the cell itself, from where it can continue division and replication. The cell’s contents are devoured and it too bursts, thus releasing a consequential wave. However, certain merozoites experience a distortion in their shape after invading red blood cells, and change form to produce immature, male and female gametocytes. When the infected organism is bitten again, it is highly likely that the mosquito gorges on blood containing these male and female gametocytes, which results in another host. A phenomenon then occurs. It is only in the gut of mosquitoes that these gametocytes are capable of developing and hold the ability to fuse in order to form a zygote, which in turn develops into a newly formed sporozoite. This completes the cycle of transmission.
CRISPR AND THE GENE DRIVE-
The prophesied theories come to life…
After years of speculation and the publishing of several papers, scientists have proposed routes via which it may very well be possible to eliminate the threat of malaria. This can be achieved through the application of CRISPR/CAS 9, which is a recent tool which allows for precise edits to be made to the genes of organisms, along with a gene drive, which allows us to modify the genetic makeup of a species by editing the DNA of select members, and thus spreading the change to the entire populace through reproduction, with the hope of severely diminishing mosquito populations, through rendering female members of the species as infertile, or through the editing of genes to introduce an immunity to plasmodia, and turning the insects into non-carriers. The term CRISPR is used as reference to a sequence of naturally occurring bacterial DNA which acts as a potent genetic editing tool when used in accordance with the Cas9 enzyme. The tool is credited to the biochemists Jennifer Doudna and Emmanuelle Charpentier. It successfully edits a single organism which may or may not be successful in passing on the favourable genes to its offspring, but has a limited potency as it cannot change an entire species when used on somatic cells (all cells excluding those involved in reproduction). Gene drives incorporate “selfish” genes, which are able to manipulate the reproductive process to give themselves a better chance of being spread. The genes eliminate sperm that do not contain them or alter the DNA replication of the cells to reduce the frequency of their replication. When the two concepts are applied to mosquitoes, the gene which allows for plasmodia-immunity or induces infertility in females, uses CRSIPR to insert itself into all reproductive cells. This allows for this gene to be passed on to almost 100% of the offspring, rather than previous rates of 50%.
A team of scientists at UC San Diego and UC Irvine announced that they had made a CRISPR gene drive for the Anopheles Stephensi mosquito, and tested it upon 2 designated, edited males (10.1 and 10.2). Two generations of cross breeding layer, out of the 3,894 third-generation mosquitoes, 3,869 had the resistance gene. This meant that 99.5% of the offspring had successfully received the gene, which prevented them from becoming carriers of plasmodia. This is a significant breakthrough, which will allow for a significant reduction in carrier-hosts of the malarial parasite, and drastically reduce the number of infant deaths. Each mosquito births approximately 300 nymphs, which means that a swift transfer in this resistant gene will be made possible across the Anopheles gambiae, Anopheles coluzzii, and Anopheles arabiensis species, which hold the greatest responsibility in transmitting the disease.
In addition, scientists are also looking towards cutting down the populations of mosquitoes by producing infertile female mosquitoes. This will prevent the rapid rates of population consolidation, and lower the threats posed bye malaria. However, significant backlash is being produced from biotech watchdogs, such as Friends of the Earth, due to the potential harm that may be unleashed. It is necessary for rigorous testing to take place, so as to avoid disaster, yet speed and efficiency are essential in removing this pandemic from the face of the Earth.