A recent gene editing therapy, having seen great success in infants, has provided scientists with a breakthrough in cancer research. British doctors claim that the infants were cured via “designer cells,1” citing the first instance of genetically engineered donor immune cells inserted into the host body without high levels of rejection. CRISPR, clustered regularly interspaced short palindromic repeats, are short repetitive DNA fragments, which are attached to the Cas system, resulting in the Cas9 recognizing and cutting exogenous DNA.2 2012 marked the year when the CRISPR-Cas9 pathway was first recognized for its genome editing ability.2 It was found that a select RNA sequence combined with Cas9 could recognize and cleave a specific DNA sequence, allowing nucleotide manipulation and efficient targeting of tumor cells. CRISPR is also equipped with modified transcription factors, allowing scientists to silence and activate selective genes.3

The path to this groundbreaking gene editing therapy has followed a succession of gene editing therapy, each being more selective than the last. The first gene editing therapy was introduced in the late 1990s into the early 2000s, by the name of zinc finger nucleases (ZFNs). These proteins were designed to recognize a specific DNA sequence and cut the desired DNA fragment. The hypothesis was that host cells would recognize the cut and repair the genome segment from the injected foreign DNA, introducing a new DNA segment into the existing genome. Even though this technology provided an intellectual breakthrough in using new gene editing therapies, it was difficult to use. Each modification required a new protein sequence that was tailored to a specific host DNA sequence, making the task tedious and time consuming.3 TALENs, transcription activator like effector nucleases, another gene editing technique, were introduced in 2010. TALENs were similarly difficult to modify because they required a specific protein sequence. Finally, in 2012, CRISPR was introduced as being readily able to modify host DNA sequences with only a 20 base pair RNA sequence. Now, researchers are testing its ability to remove viral DNA from tumor cells.

The method of gene editing via CRISPR is significantly easier than prior mechanisms.  It requires the CRISPR-Cas9 complex, containing a Cas9 enzyme, which act as “molecular scissors” and gRNA(guide RNA), which guides the Cas9 enzyme to the right place in the genome, ensuring a correct cleavage. As the host cell recognizes DNA damage, it uses the complementary RNA base pairs, through reverse transcription, to insert a new DNA sequence into the genome.

However, the recent discovery, resulting in the elimination of cancer in two infants, was not due to the CRISPR-Cas9 system, but rather to a TALEN mechanism. The TALEN mechanism edits genomes by inducing double stranded breaks, inducing the cells to begin their repair mechanism. Once the target mechanism is identified, TALENs are engineered and inserted into a plasmid, which is then inserted into the target cell. When combined with nucleases, TALENs cleave the DNA at the target sequence. The treatment entailed collecting blood from donors, isolating healthy immune T-cells, and then using TALENs to deactivate certain T-cell genes. While TALENs are targeting the gene, the protein encoded by the gene causes rejection once transplanted into a leukemia patient. The T cells were also engineered to attack cancer cells directly.1  A study was done by scientists at Memorial Sloan Kettering Cancer Center, in which CRISPR T cell engineering was compared to conventional T cell engineering. Here, scientists synthesized CRISPR engineered CAR T cells, and tested their potency in a mouse model of acute lymphoblastic leukemia. CAR stands for chimeric antigen receptor, and it allows T cells to recognize a specific antigen on tumor cells. As CRISPR engineered T cells into CAR T cells, the CAR T cells arrive at the target loci genome, and the modified T cells worked more efficiency, as they were able to kill tumor cells immediately.  As a result, tumor rejection was enhanced.4

The primary function of T cells is to patrol the body for non-self foreign cells, identify the marked antigens, and attack the foreign cells. However, in cancer, tumor cells develop using host machinery and thus are marked as “self-cells” which T cells do not attack, as this would be homologous to attacking one’s own cells. Typically, in cancers, the B cells are infected with foreign tumors, and so CAR T-cell therapy introduces T cells to target and and attack the marked tumor B cells, a job that the internal T cells cannot accomplish, as they are not able to distinguish between one’s cells and invading tumor cells.5 However, a side effect of CAR T cell therapy has been that the modified T cells destroy both cancerous and normal B cells, reducing the B lymphocyte count, resulting in an overall condition of B-cell aplasia.

An exciting new breakthrough is in using CRISPR to create “killer” T cells which specifically target antigens displayed by a particular patient’s cancer. Such a study was conducted using gene editing to simultaneously introduce the CAR and disrupt TCR and CD52 in T cells to generate functional CAR T cells that could evade host immunity. These CAR T cells were then used to treat two infants with relapsed acute lymphoblastic leukemia via sessions of chemotherapy.6 This bridged the gap between these gene editing therapies and the resulting ease of transplants, making gene editing a more viable option for treatment.

The procedure for CAR T-cell therapy begins with apheresis, in which blood is extracted, T-cells are separated, and the remaining blood is inserted back into the body. The T-cells are then engineered to have chimeric antigen receptors on their surface, and allowed to proliferate. When the CAR T-cells are returned to the patient’s bloodstream, they identify and attack cancerous cells with the antigen marker. CAR T-cell therapy has recently emerged as an option for ALL, Acute Lymphoblastic Leukemia, as studied in the previous study, but also has had positive results in other blood cancers, such as Chronic Lymphoblastic Leukemia, and non-Hodgkin Lymphoma. Although CAR T-cell therapy is available only to patients undergoing clinical trials, it provides great hope for a future in gene editing in battling cancers and an amazing breakthrough for the field of medicine.


REFERENCES

  1. Brown, Kristen. A Groundbreaking Gene-Editing Therapy Eliminated Cancer in Two Infants. January 2017.
  2.   Jinek M, Chylinski K, Fonfara I, Hauer M, Doudna JA, Charpentier E (August 2012). “A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity”. Science. 337 (6096): 816–21.Bibcode:2012Sci…337..816J. doi:10.1126/science.1225829.PMID 22745249.
  3.   Young S (11 February 2014). “CRISPR and Other Genome Editing Tools Boost Medical Research and Gene Therapy’s Reach”. MIT Technology Review. Cambridge, Massachusetts: Massachusetts Institute of Technology. Retrieved 2014-04-13.
  4. CRISPR Turbocharges CAR T Cells, Boosts Cancer Immunotherapy. Genetic Engineering and Biotech News. February 2017
  5.   Porozinski, Roxanne. Gene Editing Breakthrough In Leukemia Treatment. January 2017.
  6.   Qasim et al., Molecular remission of infant B-ALL after infusion of universal TALEN gene-edited CAR T cells, Science, Jan 25, 2017. Vol. 9, Issue 374
  7.  Nicola Dagg, Marc Döring, Dr Joachim Feldges, Daniel Lim. Editing the future: A brief introduction to CRISPR

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