52 // DISCOVERY | RESEARCH
Breakthroughs in Biotechnology: Curing Disease and Crossing Lines with CRISPR AUTHOR // RUTH PURCELL The field of biotechnology has flourished in the last 10 years and an incredible number of advances have been made in our ability to exploit natural biological systems for desirable artificial outcomes. One such biotechnological advance is the development of CRISPR gene editing which is based on part of the bacterial immune system and has allowed for incredible strides to towards curing genetic disorders to be made. Recently, however, a favourite argument made by those opposed to genome-editing has again acquired substantial traction: the idea that making artificial alterations to the human genetic code is morally dubious because it puts one in the position of ‘playing God’. Personally, I see this logic as beyond absurd as I am not sure how anyone can start citing ‘morals’ as the reason for being in favour of denying fellow humans a way out of the life-sentence that is a incurable genetic illness. Nevertheless, I do support the move that brought about this sudden rise in the notion that editing human genomes is unethical: the firing of He Jiankui, a Chinese scientist, from his university in late January 2019 for producing the first genetically-modified human babies. Although I fully-support the science of human genome modification, I acknowledge that this field was not ready for such a bold move to be made in an ethical fashion— we have no idea what off-target effects may have occurred in these ‘test-subject babies’ as CRISPR is not yet a foolproof system. So here I present to you the facts of CRISPR gene editing; you can decide for yourself whether you think giving human genes an artificial nudge in the direction of health is truly more reprehensible than sitting back and letting people suffer from diseases we could be curing. Bacterial genomes(complete sets of genetic material present in a single organism), unlike animal genomes, tend to be very compact with few ‘unnecessary’ regions. In animal genomes, seemingly useless repeated DNA segments are commonplace. Consequently, in 1987 when Japanese scientist Yoshizumi Ishino observed strange repeating patterns of DNA while investigating genes in Escherichia coli bacteria he was baffled because “so far, no sequence homologous [corresponding] to these has been found elsewhere in prokaryotes [the taxonomic domain to which bacteria belong]”, and Ishino deemed “the biological significance of these sequences [to be] not known.”. Fast forward 30 years and Ishino’s discovery has lead to CRISPR/Cas9—the recently developed technology currently revolutionising the way scientists edit genes. Genes are distinct units of DNA sequence that control how we look, how we behave, and how our bodies function. These ‘DNA units’ are passed
on from parents to their offspring through generations and are, therefore, responsible for why you might have your mother’s laugh or your grandfather’s eyes. CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats and is a guide RNA (gRNA), meaning that it serves as a template directing relevant protein to the particular part of a DNA molecule with the sequence corresponding to the gRNA. Casis aprotein—more specifically, an enzyme (a protein which facilitates biochemical reactions)— which is directed to the DNA sequence of interest and acts as a pair of ‘molecular scissors’. Together, CRISPR and Cas form a complex capable of recognising very specific sequences within an entire genome of a cell and then cutting that cell’s DNA at the recognised site so as to create a double-strand break in the DNA double-helix. Being able to make such double-strand DNA breaks is important to bacteria when they come under the attack of viruses. Viruses that attack bacteria (bacteriophages) do so by injecting their DNA into their victim (that is, the host cell). This viral DNA is then merged with the host genome, allowing replication of viral particles. However, sometimes invading genetic material can be chopped up and incorporated into a surveillance system instead (i.e. CRISPR) and used to protect the cell against future attacks by the same virus. Consequently, should the cell (or any of its descendants) encounter that virus again, the CRISPR/Cas system will ensure that the virus is unable to take over and kill the cell. This remarkable feat is achieved by the initially-infected cell recording the sequence of the viral DNA with which it was injected and storing it within its own genome, interspersed by short characteristic repeated, palindromic sequences. In the case of bacteria, CRISPR is used to detect a sequence of viral DNA which has been incorporated into the cell’s genome and then Cas is used to make an incision in the viral DNA, effectively destroying the viral DNA. This serves to inhibit the virus’s ability to replicate and, therefore, prevents the virus from killing the cell. In animals, however, the CRISPR/Cas system can be harnessed for a different purpose. Once DNA has been subjected to a double-strand break (as is inflicted by Cas enzymes), the cell essentially goes into a state of ‘panic’ and recruits its DNA repair mechanisms. Although this damaged DNA can be repaired by joining the severed strands back together, the process by which this occurs is error-prone. Consequently, mutations tend to be incorporated into the repaired DNA as some nucleotide bases (the building blocks of DNA) may
