Cancer is one of the leading causes of mortality across the globe, with a substantial social and economic burden. Management of cancer using conventional therapeutics involve the use of anticancer drugs, chemotherapy, and radiation. These methods have limited success in treating cancer and have their own side effects. Current technological advancements have considerably contributed to our understanding of cancer at the molecular level, and efforts need to be directed towards the development of new cancer therapeutic strategies.
Cancer is known to be associated with multiple genetic and epigenetic mutations that lead to tumorigenesis, proliferation and metastasis. These genetic alterations also confer resistance to anticancer drugs. Thus, the key to developing successful cancer therapy lies in the ability to rectify the aberrant gene mutations.
Since the discovery of the CRISPR/Cas system, it has been one of the most popularly used genome editing tools among researchers and has revolutionised genome engineering. The simple approach, high specificity, efficiency, affordability, and scalability of CRISPR/Cas gene editing technique has contributed to its popularity over other genome editing technologies such as TALENs and ZFNs.
In addition to being a robust research tool, CRISPR/Cas9 has immense potential in cancer therapeutics. For example, catalytically inactive dCas9 is capable of regulating endogenous gene expression by binding to transcription activation or inhibition domains of the target DNA. Furthermore, targeted epigenome editing can be performed by tagging dCas9 to histone modifiers or fusion proteins i.e., dCas9-DNMT3A, or dCas9–DNMT3A-DNMT3L that can alter DNA methylation of the CpG motif in the target regions. Since epigenetic factors are known to play a critical role in different types of cancers such as lymphoblastic leukaemia or Ewing sarcoma, CRISPR/dCas9 system can be useful in dysregulation of such cancers.
CRISPR/Cas9 screens can help to identify tumour biomarkers and the same gene editing technique offers the possibility of rectifying genetic alterations that lead to tumour proliferation and metastasis. However, the challenge lies in the accurate identification of the driver genetic mutations within the mutational landscape of a tumour.
Another breakthrough application of CRISPR technology in cancer therapeutics is related to cancer immunotherapy development. Over the last few years, cancer immunotherapy has emerged as a promising therapeutic solution for cancer. Compared to conventional cancer therapy drugs, cancer immunotherapy has low risks, is capable of treating cancers that were previously untreatable, and generates a durable immune response within the body.
Adoptive cell therapy (ACT) is an immunotherapy approach that involves the extraction of T cells from a cancer patient, followed by genetic modification of the T cells, in vitro expansion of the modified cells, and finally re-fusion of the T cells into the patient. The ACT includes chimeric antigen receptor (CAR)-T therapy. CRISPR can help to engineer immune cells such as T cells harvested from the cancer patient and genetically edited to produce CARs for use in immunotherapy. The engineered CAR-T cells are thereafter re-infused into the patient. These CAR-T cells are capable of eliciting a stronger immune response against tumour cells than unmodified T cells. CRISPR technique is also employed in developing universal ‘off-the-shelf’ allogeneic CAR-T cells that find application in cancer immunotherapy. Unlike CAR-T therapy that is patient-specific and involves time-consuming process of development, allogeneic CAR-T therapy is already prepared from donor samples and is ‘ready-to-use’.
Cancer cells like normal cells often express programmed cell death protein ligand 1 (PD-L1) or other receptors that are recognised by cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and PD-1 immune checkpoint receptors expressed on the surface of T cells. This helps the cancer cells to evade T cell attack. Adoptive cell therapy also employs CRISPR/Cas9 to delete the PD-1 gene in extracted T cells that are later expanded and re-infused into the patient. The PD-1 gene-deficient T cells can now recognise and eliminate tumour cells.
In addition to these immunotherapies, oncolytic viruses have emerged as potential cancer therapeutic agents. These genetically engineered oncolytic viruses lack virulence against normal cells in the body but are capable of attacking and killing cancer cells. CRISPR/Cas9 is used to manipulate the genome of the oncolytic viruses to improve their tumour selectivity, remove their virulence against normal cells, and increase their immune stimulation.
Thus, CRISPR/Cas9 technology finds diverse application in the development of cancer therapeutics and immunotherapy.