18 September, 2021

CRISPR gene editing: In search of perfection

In 2012, Biochemists Jennifer Doudna and Emmanuelle Charpentier made one of the most important discoveries in the history of biology, the implications of which could revolutionize the human species for generations to come. This is the story of CRISPR.

It all began in 2000. A small German research group tried to figure out how bacteria fought viral infections utilizing a programmable system that allows RNA molecules to guide proteins into virus DNAs or RNAs in infected cells and destroy them.

Bacteria have to deal with viruses in their environment. These viral infections are like a ticking timebomb to the bacterium, who only have a few minutes to defuse the ‘bomb’ before it destroys them. Many bacteria have an adaptive immune system in their cells called CRISPR, allowing them to detect viral DNA and destroy it. Part of the CRISPR system is a protein called Cas9. Cas9 can seek out, cut and eventually degrade viral DNA in a specific way.

Piggybacking off the German research team’s discoveries, Doudna and Charpentier realized they could harness the function of Cas9 as a genetic engineering technology. A way for scientists to delete or insert specific bits of DNA into cells with incredible precision that would offer opportunities that have never been done before in science. CRISPR can change the DNA in the cells of mice, monkeys and other organisms. In 2014, Chinese scientists showed that they could use CRISPR to change genes in human embryos, while scientists in Philadelphia showed they could use CRISPR to remove the DNA of an integrated HIV from infected human cells.

What is CRISPR?

CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. It is a mechanism that allows cells to record, over time, the viruses they were exposed to. These bits of DNA are then passed on to the cells’ progeny, thus cells are protected from viruses over many generations of cells. This allows the cells to keep a record of infection. The CRISPR locus is effectively a genetic vaccination card in cells. Once those bits of DNA are inserted into the bacterial chromosome, the cell then makes a little copy of a molecule called RNA (orange), an exact replicate of the viral DNA. RNA is a chemical cousin of DNA, and it allows interaction with DNA molecules that have a matching sequence. The RNA from the CRISPR locus above binds to a protein called Cas9 (white) and forms a complex that functions as a sentinel in the cell. It searches through all DNA in the cell to find sites that match the sequence in the bound RNAs. When those sites are found, this complex will associate with that DNA (blue) and allow the Cas9 cleaver to cut the viral DNA. In simple terms, the point where the DNA helix is cut acts as an edit point, where cells can make changes precisely at the break.

Implications of CRISPR

Genome engineering is not new. It has been in development since the 1970s. We have had technologies for sequencing, copying, and even manipulating DNA. Though these technologies were promising, they were either inefficient or too difficult to use, hence the low adoption uptake in laboratories. CRISPR however seems to have emerged at the right moment.

As DNA sequencing gets cheaper, more and more organizations, companies and countries are amassing computer server-busting amounts of human DNA data, typically for studies of the links between genes, lifestyle factors, and disease risk.

Most importantly with CRISPR, the complex is programmable, it can be programmed to recognize particular DNA sequences, and make a break in the DNA at that site. An activity that could be harnessed for genome engineering, to allow cells to make a very precise change to the DNA, at the site where the break was introduced.

Doudna and Charpentier envisaged using the CRISPR system for genome engineering, but how best to do so? Cells can detect broken DNA and repair it. When a plant or animal cell detects a double-stranded break in its DNA, it can fix that break. It can paste the ends of the broken DNA together with a small change in the sequence of that position, or it can repair the break by integrating a new piece of DNA at the site of the cut. If we can introduce double-stranded breaks into DNA at precise places, we can trigger cells to repair those breaks by either disruption or incorporation of new genetic information. If we were able to program the CRISPR technology to make a break in DNA at the position at or near a mutation causing cystic fibrosis, for example, we could trigger cells to repair that mutation.

Once a double-stranded break is made in the DNA, we can induce repair and thereby potentially achieve astounding things such as correcting mutations that cause sickle cell anemia, or Huntington’s disease, etc.

CRISPR is best applied through the blood, due to the ease of delivery to the cell, compared to solid tissues.

In 2020, Victoria Gray (pictured below) was the first patient to get gene editing for sickle cell disease. 

Doctors removed bone marrow cells from Gray’s body, edited a gene inside them with CRISPR, and infused the modified cells back into her system. Scientists discovered the cell produced a protein that alleviated the worst complications of her sickle cell, restoring the production of fetal hemoglobin that can compensate for the defective adult-hemoglobin sickle cells that patients produce.

A study in Philadelphia, utilizing CRISPR-engineered t-cells in patients with refractory cancer, is underway, though the process to edit these cells is laborious, with costs that will only increase over time. Therefore, a more streamlined option is needed for mass rollout. Similarly, CAR-T therapy is in its early stages, and the ideal integration of CRISPR into this technology is one to watch.

The possibilities to reshape the world using CRISPR are inviting. Scientists could make fundamental discoveries in cell function and development. In medicine, new therapies, antibiotics, targeted drugs and new diagnostics could increase life expectancy to inconceivable ages. In agriculture, nutritious, disease-resistant, climate tolerant crops could be engineered, eliminating hunger.

The opportunity to do this kind of genome editing raises various ethical issues that must be considered as this technology can be deployed not only in adult cells but also in the embryos of organisms, including in human beings. CRISPR technology can be used for human enhancement. Venture capitalists and start-up companies are undergoing private research on human upgrading: Stronger bones, low risk to any cancers, 20/20 vision, removal of Alzheimer’s/breast cancer/stroke genes, impervious to diseases like COVID, high IQ, different eye color, blonde hair, height, baldness, the list is endless. The genetic designs that make up these traits are as yet unknown, but CRISPR can nonetheless make such changes once that knowledge becomes available. The Bionic Man/Six Million Dollar Man, the advent of the designer human is not too far from us.

The last six years have involved a multitude of ethical and societal debates with heads of science as to the implications of this technology. In 1970, there was a moratorium on the safety of gene editing, and over the last five years, there’s a call for a global pause to allow more time to understand this technology. However, the powers that be always find a way, with money behind them, with itchy trigger fingers, who knows how irresponsible decision-makers can be? Prudence is the best step forward in the future understanding of CRISPR and how to harness its power optimally and safely.

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