Viral Vector-Based Gene Transfer

The term ‘vector’ comes from the word ‘vehere’ which means “to carry” in Latin. Hence, in biology, vectors refer to biological agents or carriers that play a crucial role in the transmission of genetic material, such as genes, from one organism to another. In gene therapy, it is a key step to successfully transfer a specific gene to the target cells or tissues, and this is where the vectors come in. They are usually categorized into two types: viral vectors and non-viral vectors. Viral Vector-Based Gene Transfer has proven to be a good approach. Let’s dive into more details. 

Definition of a Viral Vector

A viral vector is derived from viruses already known for their ability to deliver genetic information into cells. They are used as tools by molecular biologists for several applications including gene therapy. This is done through modifications that allow the viral vector to retain only some of the features of the virus, for example, the genes responsible for finding and entering the target cells are retained. However, certain functions are reduced such as the pathogenicity of the viral particle. The use of viral vectors for gene transfer is far and wide, including gene therapy, genetic engineering, and research.

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Types of Viral Vectors for Viral Vector-Based Gene Transfer

Viral vectors come in a variety of forms, some of which are frequently employed in medicine. They include:

Adeno-Associated Viral Vectors (AAV):

These vectors, which are derived from adeno-associated viruses, can also carry genetic material. They, however, need a helper adenovirus or herpes virus because of their reduced replication ability. In vivo strategies use AAV vectors to treat genetic diseases since they have a natural ability to transfer genetic material. They can be directly administered to the patient. The AAV-Helper-free system is primarily employed in clinical research at the moment. Since they haven’t been conclusively connected to any of the recognized human illnesses, AAVs are now considered to be non-pathogenic on their own.

Advantages: Adeno-associated virus (AAV) vectors are commonly preferred for in vivo gene therapy, due to several advantages that contribute to their popularity in the field. Firstly, AAV vectors can transduce both dividing and quiescent (non-dividing) cells. Secondly, AAV vectors exhibit robust transduction efficiency in vivo, meaning they are effective at delivering genetic material into target cells within a living organism. This efficiency is crucial for achieving therapeutic effects in the context of gene therapy.

Examples: AAV serotypes, such as AAV2 and AAV9

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Adenoviral Vectors (Ads):

Source: Lundstrom, K. (2018). Viral Vectors in Gene Therapy. Diseases, 6(2), 42. https://doi.org/10.3390/diseases6020042

These vectors, derived from adenoviruses, usually cause temporary gene expression. Similar to AAVs, they do not integrate into the host genome. Adenovirus is characterized as a large and complex virus with a non-enveloped, icosahedral structure. It has a double-stranded DNA (dsDNA) genome and measures 70 to 90 nanometers in size. Adenovirus is known for its structural stability and the ability to protect its genetic material. Genes can be effectively delivered to various dividing and non-dividing cells using them. It is known to be the most efficient gene delivery system for a broad range of cells and tissues. The reason is the presence of adenovirus receptors in most human cells.

Advantages: Importantly, adenoviruses have proven to be highly efficient tools in the field of gene therapy. Their ability to efficiently deliver genes into target cells has resulted in the approval of over 450 protocols for clinical trials. In gene therapy applications, adenoviral vectors are often used to carry therapeutic genes into host cells, offering a promising approach for the treatment of various genetic disorders and diseases. Their ability to suit both dividing as well as non-dividing cells is also of advantage.

Examples: Human adenoviruses, including Adenovirus type 5, are often used as vectors.

Lentiviruses (LVs) :

These vectors, which belong to a subfamily of genus retroviruses, incorporate their genetic material into the genome of the host cell. Unlike AAVs and Ads, LVs are less likely to elicit the production of neutralizing antibodies. Neutralizing antibodies can hinder the effectiveness of gene therapy by recognizing and neutralizing the vector before it reaches its target cells.

Advantages: In addition to reduced neutralizing antibody production, LV vectors offer other advantages over traditional gene delivery systems. One of the most significant advantages of LV vectors is their capability to provide long-term and stable gene expression. This feature is particularly crucial for gene therapy applications in adolescents or pediatric patients where sustained therapeutic effects are desired. Unlike some other vectors that may lead to transient expression, LVs allow for enduring gene expression.

Examples: HIV-derived lentiviral vectors, equine infectious anemia virus (EIAV), and feline immunodeficiency virus (FIV)

Moreover, the types of vectors are also categorized into RNA or DNA virus vectors having single-stranded or double-stranded genomes. Examples of a few viral vectors are given in the table given above. 

Mechanism of Action

The process of viral vectors for gene transfer involves the delivery of specific genetic material into target cells via modified viruses. Here’s a basic overview of the steps involved in viral vector gene transfer:

a. Viral Genome Modification-

Scientists change a virus’s genetic material to remove or replace its normal genes with the therapeutic genes or gene-editing tools they want to convey. This change is critical to ensuring that the virus may transmit the desired genetic material without infecting the host organism.

b. Insertion of Therapeutic Genes-

Image source: https://www.beckman.com/resources/applied-science/immunotherapy/about-viral-vectors

Therapeutic genes are inserted into the viral genome, including genes for protein production, genetic defect correction, or other therapeutic objectives. The virus then carries these genes.

c. Modified Virus Replication-

The modified virus, which now contains therapeutic genes, is replicated in the laboratory. Thus producing a vast number of viral vectors that are ready to be used for gene transfer.

d. Viral Vector Delivery to Target Cells-

Viral vectors are administered into the target organism via injection into specified tissues or intravenous (IV) administration. The viral vectors are subsequently exposed to the target cells.

e. Cell Recognition and Attachment-

The modified viral vectors’ surfaces contain specialized proteins that allow them to recognize and attach to receptors on the surface of target cells. This recognition is often restricted to specific cell types. Soon after attachment, viral vectors reach target cells by direct fusion with the cell membrane or receptor-mediated endocytosis, in which the cell engulfs the vector in a membrane-bound vesicle.

f. Viral Genome Release into the Cell-

Once within the cell, the viral vector releases its changed genetic material (therapeutic genes) into the cytoplasm of the host cell. Viruses can release their genetic material in a variety of ways, each with its own unique set of characteristics.

g. Incorporation into Host Genome (For Some Viruses)-

The viral genome, coupled with therapeutic genes, can integrate into the host cell’s genome in the case of retroviruses and lentiviruses. Because of this integration, the therapeutic genes become a permanent part of the cell’s genetic material. The host cell’s machinery begins transcribing and translating the inserted therapeutic genes, resulting in the synthesis of functional proteins or the correction of genetic abnormalities.

h. Producing Therapeutic Benefits-

The expressed proteins or corrected genetic functions contribute to the intended therapeutic outcomes, such as genetic condition treatment or therapeutic protein manufacturing.

Challenges of using Viral Vectors for Gene Transfer

While viral vectors come with great advantages, they can also be associated with risks. It is important to address these risks and challenges to overcome problems related to safety, efficiency, and broader applicability of viral vector-based approaches.

1. Immune Response

Viral vectors can elicit immune responses in the host. Eventually, the immune system may recognize the viral components as foreign and mount an immune response against the vector, potentially leading to the clearance of the vector and reduced effectiveness of the gene therapy.

2. Vector Toxicity

Some viral vectors can be toxic to host cells, either due to the expression of viral genes or other factors associated with the vector. Thus, toxic effects can impact the viability and function of the cells.

3. Insertional Mutagenesis

Some viral vectors, particularly integrating vectors such as retroviruses, carry the danger of insertional mutagenesis. Integrating the viral genome into the host cell’s DNA has the potential to alter normal gene activity, resulting in unexpected effects or even oncogenesis.

4. Limited Packaging Capacity:

Viral vectors have limited load capacity, which means they can only carry so much genetic material. This constraint can be difficult to overcome when attempting to deliver big or several therapeutic genes at the same time.

Consequently, viral vectors play a pivotal role in gene transfer, offering a versatile platform for a range of applications in gene therapy and genetic engineering. In gene therapy, viral vectors are employed to deliver therapeutic genes into target cells, addressing genetic disorders, inherited diseases, and certain cancers. Furthermore, viral vectors serve as indispensable tools in genetic engineering, enabling the introduction of desired genes or gene-editing tools into cells for research purposes, fostering advancements in biotechnology, and understanding cellular mechanisms. Continuous advancements in vector design and engineering are expanding the scope of applications, undoubtedly reinforcing the significance of viral vectors in the rapidly evolving landscape of gene transfer technologies.

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Team MBD