Gene Therapy

Because certain mutations can change gene function and cause disease, geneticists have been working hard to use healthy, cloned genes to compensate for faults in mutant genes.

Genetic disorders

Over 7,000 genetic disorders have been identified in humans that involve abnormalities in a single gene. Examples include cystic fibrosis, sickle cell anaemia, and haemophilia. These conditions can be very good candidates for gene therapy.

Furthermore, many research efforts in gene therapy have been invested into treating conditions such as cancer and cardiovascular diseases that may occur at later stages of life. There is also a significant amount of research with regards to utilising gene therapy for combating infectious diseases such as AIDS.

There are six main steps involved in a gene therapy plan:

1. Making a functional gene.
2. Constructing a therapeutic vector that contains the functional gene.
3. Conducting clinical trials to determine the safety and efficacy of the novel therapy.
4. Determining the eligibility status of the patient.
5. Delivering the functioning gene.
6. Keep an eye on the long-term safety and efficacy of the treatment.

Types of gene therapy

Since all cells in the human body possess genetic material, they can all be potential candidates for gene therapy. Cells can be broadly classified into two categories of somatic and germline cells.

This type of classification also provides categorization for gene therapy. Somatic gene therapy refers to interventions targeting somatic cells. Meanwhile, germline gene therapy targets reproductive cells or embryos before implantation.

It is possible to carry out gene therapy in humans exclusively in somatic cells since germline gene therapy raises ethical implications, and many countries have banned it.

Despite the ethical and moral arguments, a Chinese scientist claimed in 2018 to have performed germline editing on two human embryos that had already been implanted and born¹. The scientific community was taken aback by this bold and, some would say, questionable move!

Strategies of gene therapy

There are a few gene therapy techniques that scientists use. These include gene augmentation, gene inhibition, and suicide gene therapy.

Gene augmentation therapy

For genetic recessive illnesses caused by a dysfunctional gene, a single normal copy of the gene may be enough to prevent or reverse the disease phenotype. Hence, the only thing needed to transfer would be a functional gene to replace the mutated, dysfunctional one.

This method is called gene augmentation therapy, and it is best for treating disorders caused by a loss-of-function mutation in a gene that has resulted in a faulty or absent protein. This type of gene therapy could effectively treat conditions such as severe combined immunodeficiency and cystic fibrosis.

Nevertheless, from a practical standpoint, the success of gene augmentation therapy is contingent on at least two factors:

1. The amounts of the functional protein generated by the augmented gene must be adequate.
2. The disease's effects and consequences must still be reversible.

Fig. 1 - A schematic diagram of gene augmentation therapy

Gene inhibition therapy

For many disorders, restoring normal protein function is not sufficient to reverse the disease phenotype. They require the mutant gene's expression to be repressed.

Gene inhibition therapy (also known as gene silencing therapy) may be appropriate for dominant genetic disorders, some forms of infectious diseases, or certain forms of cancer.

For dominant genetic disorders, the logical setup of this technique would be to introduce a gene that can block the mutant gene's expression or interfere with the mutant protein's action.

Fig. 2 - A schematic diagram of gene inhibtion therapy

With the discovery of the RNAi pathway by Andrew Fire and Craig Mello in 1998, this strategy became highly viable². RNA interference (RNAi) is an endogenous biological process that regulates gene expression in the cells via short RNA sequences that are complementary to mRNA. RNAi is a form of post-transcriptional modification. You can learn more about it in our “Transcriptional Regulation” article.

The RNAi pathway provided an option to employ endogenous cellular machinery to modulate the expression of defective genes. Inserting a gene that encodes small interfering RNAs – short single-stranded RNA molecules complementary to the mRNA of the defective gene – can lead to degradation of the mutant gene's mRNA via RNAi and prevent it from being translated. By inhibiting the expression of the mutant gene, the cell would be able to function normally.

The development of gene-editing tools such as CRISPR has provided further options for gene therapy. Gene editing techniques can be used to alter the genome by eliminating the faulty gene or accurately repairing it.

Suicide gene therapy

The two techniques mentioned above aim to restore cellular physiology and function and reverse the pathological phenotype. However, in certain types of conditions such as cancer, we are more interested in eliminating the defective cell rather than reversing it back to physiological states.

Suicide gene therapy can be used in this context, by using a transgene that works in one of two ways:

1. The inserted transgene codes for a highly toxic protein that kills the defective cell. This transgene is called a 'suicide' gene.
2. The inserted transgene encodes a protein that designates the defective cell as a target that the body's immune system can attack.

Fig. 3 - A schematic diagram of suicide gene therapy

Gene delivery systems

Transferring foreign genetic material into a cell or tissue is not easy because organisms have developed numerous defensive mechanisms to avoid it. Consequently, one of the most important concerns to address in a gene therapy plan is finding out which delivery method is most suited to maximise the treatment's effectiveness. There are currently two major types of delivery systems used for gene therapy: viral and non-viral systems.

The viral systems use modified viruses and their intrinsic capacity to infect cells with their genetic material. The big advantage of viral systems is their remarkable efficiency compared to other systems. Meanwhile, their main disadvantage is the safety problems associated with employing modified viruses.

Non-viral systems, on the other hand, incorporate a variety of chemical or physical approaches that have the benefit of being safe but the main disadvantage of being somewhat inefficient.

The optimum delivery mechanism for a specific gene therapy strategy is determined by a number of factors, including the size of the gene, the predicted outcome, and the side effect profile.

Ethical concerns about gene therapy

Gene therapy is prone to causing major ethical concerns, controversies, and disputes. Despite the numerous concerns, gene therapy has immense therapeutic potential and benefits. Like any other medical intervention involving human subjects, gene therapy must meet important requirements and considerations, such as ensuring informed consent and having a promising risk-benefit balance.

Nevertheless, the possibility of altering one's personal genetic information in germline cells raises the question of whether it should be allowed or not. The reason for this is the difficulty in drawing a line between gene enhancement and gene therapy since people may abuse this technology to alter the characteristics of their children to their own taste.

2. Fire A, Xu S, Montgomery MK, Kostas SA, Driver SE, Mello CC, Potent and specific genetic interference by double-stranded RNA, Nature, 1998