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What is Gene Therapy?

Gene therapy aims to treat or cure genetic diseases through the introduction of foreign, normally-functioning genes into target cells to correct or compensate for defective or abnormal genes. For a long time, traditional large/small molecule drugs mainly act on protein targets, which has limited its indications and ability to effectively exert the therapeutic effect. This is especially true for genetic diseases, where genetic factors occupy the main cause of disease. The number of people suffering from such gene-related diseases is actually quite large, even though most genetic disorders are considered rare diseases. For example, in our traditional understanding, the number of patients with each rare disease should be very small, but the collective number of patients worldwide has reached 350 million, of which about 80% are directly caused by genetic mutations.



R&D difficulty

Success rate

Target range



Gene therapy






Small molecule drugs






Large molecule drugs






Table 1. Comparison of several treatment methods


The current treatment methods are limited for these rare genetic diseases, they are all treated from the perspective of improving symptoms and cannot be cured. The patient's illness will continue to worsen and seriously affect the quality of life, and even eventually leading to death. In this context, gene therapy came into being, which can treat diseases from the root cause. Although gene therapy has related clinical application cases as far back as 20 years ago, over the past five years it has been widely studied and many clinical trials have started. According to comprehensive analysis data from many authoritative organizations, the global gene therapy market has grown from US$440 million in 2018 to US$2.07 billion in 2020, and is predicted to grow rapidly to US$30.53 billion in 2025. More than a million people could be eligible for gene therapy in the next 15 years. The conversations researchers have today, both with the general population and with their patients, may ultimately determine how the field evolves. With the right support, gene therapy could revolutionize medicine and public health.


Vectors Used in Gene Therapy

Gene therapy can be divided into in vivo gene therapy and ex vivo/in vitro gene therapy. Among them, in vivo therapy is mainly to deliver non-viral vectors with therapeutic genes (such as the adeno-associated virus, AAV) directly into the patient's body; while ex vivo therapy introduces therapeutic genes into the patient's cells in vitro ( Lentivirus is often used), which is amplified and then injected back into the patient's body. Compared with ex vivo therapy, in vivo therapy has more stringent requirements for gene delivery tools (i.e., vectors).


Figure 1. In vivo gene therapy and ex vivo gene therapy [1]


The selection of gene delivery tools is a key part of gene therapy, as the final effect of drugs in the human body depends to a large extent on a stable and efficient delivery system. At present, the commonly used vectors for gene therapy include viral vectors and non-viral vectors. Due to its wide host range and long action time, viral vectors are more and more widely used in gene therapy research. Viral vectors can perform operations such as overexpression, interference, knockout, and endogenous activation of coding or non-coding genes. Commonly used virus tools include adeno-associated virus (AAV), adenovirus (ADV), and lentivirus (LV), among which, adeno-associated virus (AAV) is currently the most widely used delivery vector for in vivo gene therapy. This is primarily due to AAV-mediated gene editing presenting good tissue specificity, low immunogenicity, and high safety for in vivo applications.






Particle diameter





4.7kb single-stranded DNA or 2.4kb double-stranded DNA

36 kb double-stranded DNA

9.3 kb double-stranded RNA






Phased expression at medium to high level

High-level transient expression

Moderately stable expression

Expression Duration

Begin to express within 7-14 days, generally 6 months to 24 months

Begin to express in 1-2 days, lasts ≤ (2-4) weeks

Begin to express in 2-4 days, lasts ≥6 months

Diffusion capacity in vivo





Small amount of targeted integration

No integration

High frequency random integration


High yield, simple composition, good safety

High yield and easy purification

Low yield, stable expression


Animal experiments

Cell/animal larger gene overexpression

Cell experiments

Table 2. Comparison of virus characteristics of commonly used tools (Source: Cyagen)


How does AAV achieve the expression of the target gene? First, the capsid protein of AAV can recognize each other with receptors on the host cell surface, and then the internalization of the virus is triggered by clathrin-mediated endocytosis. The AAV virus entering the cell causes a conformational change in the VP1/VP2 region due to the change of the intracellular pH value, and is then drawn into the nucleus. The viral inverted terminal repeat (ITR) present in the AAV genome can drive intermolecular or intramolecular recombination to form a circular episomal genome that can persist in the nucleus and be expressed for a long time in the cell.


Figure 2. The process of AAV infection of host cells [2]


Gene Therapy Strategies

Choosing corresponding treatment strategies for different diseases is a crucial step in the development of gene therapy.

At present, there are three main treatment strategies in gene therapy:

1. Gene overexpression strategy refers to the introduction of normal genes or genes with therapeutic effects into patients through viral vectors. This strategy does not change the patient's inherent gene sequence, and the effect is better. Gene therapy products currently on the market or in the clinical stage mostly adopt overexpression strategies.

2. Gene interference strategy is often used to treat diseases caused by abnormal gene activation.

3. Gene editing strategies involve manipulation of the native gene sequence, and aim to treat the numerous genetic diseases caused by gene mutations. Simple overexpression or interference cannot achieve therapeutic effects, but requires the use of gene editing technology to intervene in disease-causing genes. However, due to the problems of editing efficiency and off-target potential, there are still many issues to be solved in this strategy.


Figure 3. Three strategies for gene therapy[4]


Difficulties in basic research on gene therapy

Although every gene therapy drug must go through preliminary research and development, IND application, expansion of production, clinical trials and other processes, it remains very different from traditional large/small molecule drugs. From the initial design of the treatment strategy to the final product approval, the various research stages are very different between traditional drugs and gene therapies. For example, the particularity of gene therapy delivery methods has led to significant changes in its development process compared to traditional drugs. In the preclinical research phase of gene therapy, every researcher must consider the following core issues:

  • For a certain genetic disease with a clear pathogenesis, how to screen out sequence fragments with therapeutic effects?  

  • How to design an effective delivery vector (represented by AAV) to deliver the therapeutic gene sequence to the target tissue and ensure its expression?

  • Is there a disease model for the disease I am studying?

  • Which injection method should be adopted to meet the convenience of operation and the effectiveness of delivery?

  • How to evaluate and determine the end point of gene therapy at the animal level?

  • How to reduce the immune response in the body of the developed gene therapy and avoid being eliminated by neutralizing antibodies in the body?

These issues involve the identification of therapeutic targets, the selection and design of therapeutic vectors, the construction of disease models, and the evaluation of therapeutic effects. As a comprehensive solution provider, Cyagen has established a multi-in-one gene therapy innovative CRO platform service network. In the past ten years, Cyagen has accumulated a large amount of genetic information. The continuous deep cultivation of model animals has also allowed us to be at the forefront of the industry in terms of gene editing technology. Combined with our in-depth exploration in the field of artificial intelligence, we can provide a one-stop overall solution for efficient gene function analysis and gene therapy.


Cyagen gene therapy one-stop solution

Cyagen can provide more efficient gene function analysis and one-stop gene therapy solutions for researchers worldwide. Our gene therapy platform includes target screening and functional research, construction of viral vectors (such as AAV, LV, ADV) and other virus design and packaging, development of animal models, and phenotypic analysis - providing end-to-end services for your study.

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