What is gene therapy?

Gene therapy is a medical field which focuses on the utilization of the therapeutic delivery of nucleic acid into a patient's cells as a drug to treat disease. It provides a unique approach to treat both inherited and acquired diseases including hematological diseases, cancer, AIDS, diabetes, heart failure, and neurodegenerative diseases. 

Researchers are testing several approaches of gene therapy, which include: (1) Replacing a mutated gene that causes disease with a healthy copy of the gene; (2)Inactivating, or “knocking out”, a mutated gene that is functioning improperly; (3) Introducing a new gene into the body to help fight a disease.

In most of the situations, gene therapy always starts with the identification of mutant gene related with the disease. The next step is cloning the correct gene and loading it with a vector. Then the vector will deliver the therapeutic gene to the patient’s target cells, and at last the genetic material gets integrated into DNA and corrects the defective or mutated gene. Choosing the vector and making it effectively deliver therapeutic gene into the target cell is the most critical step in performing gene therapy.

Schematic illustration of key steps in gene therapy (Murali Ramamoorth and Aparna Narvekar, 2015)

A variety of non-viral and viral vectors have been developed for gene therapy. 

Viral Vectors

Viral vectors are engineered viruses to deliver genetic material into cells and have been used for gene therapy. Several types of viruses, including retrovirus, adenovirus, adeno-associated virus (AAV), and herpes simplex virus, have been modified in the laboratory to deliver genetic materials into cells and can’t cause disease when used in people. The two current vector systems which are most promising for the treatment of genetic diseases are AAV for in vivo gene transfer to postmitotic cells, and lentiviral vectors for ex vivo gene transfer to stem cells and hematopoietic cells.
AAV cause a very mild immune response in humans, and several additional features make it an attractive candidate for creating viral vectors for gene therapy. AAV vectors can infect both dividing and quiescent cells and persist in an extrachromosomal state without integrating into the genome of the host cell, although in the native virus integration of virally carried genes into the host genome does occur. Integration can be important for certain applications, but can also have unwanted consequences. AAV is believed to be the safest viral vectors for gene therapy, and have been used in a few successful gene therapies, for example, the U.S. FDA approved for spinal muscular atrophy last year, and a treatment for the blood-clotting disorder hemophilia B that’s expected to receive FDA approval this year. But recently a dog study hints that AAV-based gene therapy may pose cancer risk which raises the concern of viral vector’s long-term safety again. The researchers found that in the experimental dogs, AAV-ferried DNA had integrated in many spots across the genome in the dogs’ liver cells, sometimes near genes affecting cell growth. Some of those cells had divided more than other cells, forming pockets of multiple cells or “clones” in some animals.

Non-viral Vectors

The non-viral vectors are naked DNA, particle based and chemical based. They are administered by direct administration (plasmid DNA/naked DNA), chemical or physical. The main drawbacks of using virus vectors are its immunogenicity, cytotoxicity, and insertional mutagenesis. Non-viral vectors have important safety advantage over viral approaches. However, their poor efficiency of delivery and low transient expression of transgenes have limited their application for a long time. In the recent years, with great advances in efficiency, specificity, gene expression duration and safety, there are an increased number of non-viral vector products entering clinical trials. (Murali Ramamoorth and Aparna Narvekar, 2015)

CRISPR for Gene Therapy

CRISPR technology as the most widely used gene-editing technology is revolutionizing many research areas. One of the most promising application for gene therapy. In March 2020, Nature reported that a person with a rare genetic condition called Leber’s congenital amaurosis 10 (LCA10) that causes blindness has become the first to receive a CRISPR-Cas9 gene therapy.  The CRISPR-Cas9 gene editing system are injected directly into the eye to remove mutations in the gene CEP290 that causes LCA10. This experiment is a significant jump from treating cells in the lab to real clinical use in humans. (Heidi Ledford, Nature, 2020)

Although the recent advances represent a significant step toward the eventual application of CRISPR-Cas9 in the clinic, there are still many hurdles to overcome, such as the off-target effects, efficacy of homology-directed repair, fitness of edited cells, immunogenicity of therapeutic CRISPR-Cas9 components, as well as efficiency, specificity, and translatability of in vivo delivery methods. (Weijing Dai et al, 2016) 

Conclusion

Gene therapy holds the promise to transform medicine and treat a wide range of diseases. The FDA, EMA and China are all beginning to approve gene therapy products. In 2018, a landmark FDA approval for a gene therapy product called Luxturna. This product is developed by Spark Therapeutics using an AAV virus to deliver a gene that is missing in patients suffering from an inherited eye disease. In 2019, Novartis received a historic FDA approval for spinal muscular atrophy (SMA). This method called Zolgensma also uses an AAV viral vector to deliver SMN protein into the motor neurons of afflicted patients. Now in 2020, we will see more gene therapy products been approved.