Gene editing and CRISPR Cas9

IN NEWS:

• Scientists Jennifer Doudna and Emmanuelle Charpentier bagged the Nobel Prize for Chemistry for the development of one of gene technology’s sharpest tools: the CRISPR/Cas9 genetic scissors.

WHAT IS GENE?

• A gene is a small section of DNA that contains the instructions for a specific molecule, usually a protein.
• A chromosome contains a single, long DNA molecule, only a portion of which corresponds to a single gene. Humans have approximately 20,000 genes arranged on their chromosomes.
• The gene is the basic physical unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify traits, like eye and hair colour.
• Genes come in different forms, called alleles.
  • An individual inherits two alleles for each gene, one from each parent. An individual’s phenotype is determined by the combination of alleles they have.
  • If the two alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous.

GENE, DNA AND GENOME:

GENE EDITING:

• Gene editing is a type of genetic engineering in which DNA is inserted, deleted, modified or replaced in the genome of a living organism.
• Gene editing is performed using enzymes, particularly nucleases that have been engineered to target a specific DNA sequence.
• These enzymes introduce cuts into the DNA strands, enabling the removal of existing DNA and the insertion of replacement DNA.
• The most significant genome editing technologies are the Transcription Activator-Like Effector Nucleases (TALENs), and Zinc-Finger nucleases (ZFNs) and CRISPR-Cas9.

CRISPR & Cas9:

• CRISPR technology was adapted from the natural defence mechanisms of prokaryotic organisms like bacteria and archaea.
• Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) is a family of DNA sequences found in the genomes of the prokaryotic organisms. These pieces of DNA are snipped off by the bacteria from viruses (bacteriophages) that once attacked them.
• These organisms use CRISPR-derived RNA and various Cas proteins, including Cas9, to foil attacks by viruses and other foreign bodies.

CRISPR SCISSORS:

• In their natural form, the scissors recognise DNA from viruses, but Charpentier and Doudna proved that they could be controlled so that they can cut any DNA molecule at a predetermined site.
• They discovered that they could “fool” the Cas9 protein by feeding it artificial RNA. Then, the enzyme would search for anything with that same code, not just viruses, and cut them.

SIGNIFICANCE OF CRISPR TECHNOLOGY:

• Promote gene therapeutics: The tool has enabled scientists to edit human DNA in a lab and early-stage clinical trials are being attempted to use the tool to treat a few diseases, including inherited disorders/diseases and some types of cancer. In the future, it could make it easy to “rewrite the code of life”.
• Expand gene editing: The technology is not species-specific and hence can be used on organisms previously resistant to genetic engineering. The technique is already being explored for a wide number of applications in fields ranging from agriculture through to human health.
• Precise editing: Unlike TALENs and Zinc-Finger Nucleases, CRISPR has a higher degree of flexibility and accuracy in cutting and pasting DNA. Using the tool, researchers can change the DNA of animals, plants and microorganisms with greater precision.
• Cheaper and faster: CRISPR makes it possible to carry out genetic engineering on an unprecedented scale at a very low cost. Also, it allows for the introduction or removal of more than one gene at a time. This makes it possible to manipulate many different genes very quickly, reducing the process from taking a number of years to a matter of weeks.

SOME APPLICATIONS:

• Food preservation: The CRISPR/Cas 9 system was first exploited by a food processing company to improve the immunity of bacterial cultures against viruses. Today, many food manufacturers now use the technology to produce cheese and yoghurt.
• Treating genetic disorders: Scientists have reported the successful use of CRISPR/Cas 9 in laboratories in treating muscular dystrophy, thalassemia, sickle cell disease and in making human cells immune to HIV.
• Organ cultivation: CRISPR is being investigated in conjunction with pluripotent stem cells for the creation of transgenic animals to produce organs for transplants into human patients.
• Vector control: The technology is also being investigated as a means to genetically engineer insects so as to wipe out insect-borne diseases such as malaria, transmitted by mosquitoes, and lyme disease, transmitted by ticks.
• COVID-19 control: The CSIR-Institute of Genomics and Integrative Biology (CSIR-IGIB) in Delhi developed a COVID-19 paper strip testing kit, nicknamed ‘Feluda’, based on the CRISPR/Cas9 system.

CONCERNS:

• Grey areas: The technology is still in its infancy and knowledge about the genome remains very limited. Many scientists caution that the technology still needs a lot of work to increase its accuracy and prevent side effects.
• Unforeseen outcomes: Gene editing can inadvertently wipe out and rearrange large swaths of DNA in ways that may imperil human health. This was supported by recent studies showing that CRISPR-edited cells have inadvertently triggered cancer in some cases.
• Ethical concerns: Its questionable use has been a matter of controversy. The most controversial application was in 2018, when a Chinese researcher used it to create ‘gene-edited twins’ via in-vitro fertilisation. The desired mutations were not achieved, and there were a host of other unintended mutations too.
• Philosophical dilemma: Several religious philosophies and beliefs are against researches in germline manipulations. Also, while it will take many more years before the technology will be viable to use to create ‘designer babies’, a public debate has already begun on this issue.
• Unclear regulatory regime: Policy-makers are still debating about what limitations to put on the technology. The US National Institutes of Health has stated that it will not fund any research that uses genome editing tools such as CRISPR in human embryos. Meanwhile, the UK's Authority has allowed its use on human-animal hybrid embryos under 14 days old.
• Threat to ecology: Once an organism, such as a plant or insect, is modified, they are difficult to distinguish from the wild-type and once released into the environment could endanger biodiversity.

CONCLUSION:

The system purportedly holds promise for treating more complex diseases, such as cancer, heart diseases, mental illnesses, and HIV infection and researches are already underway in this direction. However, it is yet to reach the level of precision required to be applicable in a massive scale. Hence, the technology needs a dynamic and effective regulatory regime that encourages research and investment, but at the same time maintains ethical and legal boundaries.

PRACTICE QUESTION:

Q. What is CRISPR-Cas9 technology. Enumerate its applications. What are the challenges associated with it?