Gene Therapy — Revision Notes

⚡ 30-Second Revision

Gene Therapy: — Modifying genes to treat disease.
Goal: — Replace faulty genes, inactivate problematic genes, introduce new therapeutic genes.
Vectors: — Delivery vehicles for genes. Primarily modified viruses (Retrovirus, Adenovirus, AAV).
Retrovirus: — Integrates, long-term, dividing cells only, risk of insertional mutagenesis.
Adenovirus: — Episomal, transient, dividing & non-dividing, strong immune response.
AAV: — Mostly episomal, long-term, dividing & non-dividing, low immunogenicity (popular).
Ex vivo: — Cells modified outside body, then reinfused (e.g., ADA-SCID).
In vivo: — Vector directly administered into body.
Somatic Cell GT: — Non-heritable, clinically accepted.
Germline GT: — Heritable, ethically prohibited.
ADA-SCID: — Classic example. Deficiency of Adenosine Deaminase enzyme. Treated by ex vivo gene therapy using retroviral vector to introduce functional ADA gene into lymphocytes.

2-Minute Revision

Gene therapy is a revolutionary technique to treat diseases by correcting or modifying genetic defects. Its core principle involves delivering a functional gene into a patient's cells to replace a faulty one, inactivate a harmful gene, or introduce a new therapeutic gene.

This delivery is achieved using 'vectors,' most commonly modified viruses like retroviruses, adenoviruses, and adeno-associated viruses (AAVs). Retroviruses integrate their DNA for long-term expression but only in dividing cells, with a risk of insertional mutagenesis.

Adenoviruses offer transient expression in both dividing and non-dividing cells but can elicit strong immune responses. AAVs are favored for their low immunogenicity and ability to infect both cell types, often remaining episomal.

Gene therapy can be 'ex vivo' (cells modified outside the body, then reinfused, as in ADA-SCID) or 'in vivo' (vector directly administered). Crucially, 'somatic cell gene therapy' (non-heritable changes) is clinically accepted, while 'germline gene therapy' (heritable changes) is ethically prohibited due to concerns about future generations.

The classic example for NEET is Severe Combined Immunodeficiency (SCID) due to Adenosine Deaminase (ADA) deficiency, treated by ex vivo gene therapy using a retroviral vector to introduce the functional ADA gene into lymphocytes.

5-Minute Revision

Gene therapy represents a cutting-edge biotechnological application focused on treating or preventing diseases by altering a patient's genetic makeup. The fundamental idea is to address the root cause of genetic disorders, certain cancers, and viral infections.

This is achieved through three main strategies: gene addition (introducing a functional gene to compensate for a defective one), gene silencing (turning off an overactive or harmful gene), or gene correction (precisely editing a faulty gene).

The delivery of these therapeutic genes into target cells is critical and relies on 'vectors.' Modified viruses are the most efficient vectors. Key viral vectors include:

1
Retroviruses: — Integrate their genetic material into the host genome, leading to stable, long-term gene expression. However, they can only infect actively dividing cells and carry a risk of insertional mutagenesis (random integration potentially disrupting host genes).
2
Adenoviruses: — Can infect both dividing and non-dividing cells but typically remain episomal (do not integrate), resulting in transient gene expression. They can also trigger a strong immune response.
3
Adeno-Associated Viruses (AAVs): — Small, non-pathogenic, and can infect both dividing and non-dividing cells. They are known for their low immunogenicity and generally remain episomal, though they can integrate at a specific site at low frequency. AAVs are increasingly popular in clinical trials due to their favorable safety profile.

Gene therapy approaches are broadly categorized as:

Ex vivo gene therapy: — Cells are removed from the patient, genetically modified in the laboratory (e.g., using a viral vector), and then reinfused into the patient. This allows for precise control over the modification process. A prime example is the treatment of Severe Combined Immunodeficiency (SCID) caused by adenosine deaminase (ADA) deficiency.
In vivo gene therapy: — The gene-carrying vector is directly administered into the patient's body, targeting the affected cells or tissues. This is simpler but requires highly specific targeting and careful management of potential immune responses.

An important distinction is between somatic cell gene therapy and germline gene therapy. Somatic cell gene therapy targets non-reproductive cells, meaning the genetic changes are not heritable and only affect the treated individual.

This is the only type currently approved for clinical use. Germline gene therapy, conversely, targets reproductive cells or early embryos, leading to heritable genetic changes that would be passed to future generations.

This raises significant ethical and safety concerns and is not currently practiced in humans.

Example: ADA-SCID Gene Therapy

Children with ADA-SCID lack the enzyme adenosine deaminase, crucial for immune cell function. The gene therapy involves:

1
Isolation of lymphocytes (or hematopoietic stem cells) from the patient.
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Introduction of a functional ADA gene into these cells using a retroviral vector in the lab.
3
Reinfusion of the genetically corrected cells back into the patient, restoring immune function. This is a classic example of successful ex vivo gene therapy.

Prelims Revision Notes

Gene therapy is a biotechnological application focused on treating diseases by modifying an individual's genetic material. The core idea is to correct genetic defects at their source. This involves introducing a functional gene, inactivating a harmful gene, or adding a new gene for therapeutic benefit.

Key Components:

1
Therapeutic Gene: — The DNA or RNA sequence to be delivered.
2
Vector: — A delivery vehicle, most commonly modified viruses, to transport the gene into target cells.
3
Target Cells: — The specific cells in the patient that need genetic modification.

Types of Viral Vectors and their Characteristics:

Retroviruses (e.g., HIV-based):

* Integration: Integrates genetic material into host genome. * Expression: Long-term. * Cell Tropism: Infects only actively dividing cells. * Risk: Insertional mutagenesis (potential for cancer).

Adenoviruses:

* Integration: Remains episomal (does not integrate). * Expression: Transient. * Cell Tropism: Infects both dividing and non-dividing cells. * Risk: Strong immune response.

Adeno-Associated Viruses (AAVs):

* Integration: Mostly episomal, low frequency site-specific integration. * Expression: Long-term. * Cell Tropism: Infects both dividing and non-dividing cells. * Advantage: Low immunogenicity, widely used in clinical trials.

Gene Therapy Approaches:

Ex vivo: — Cells are removed from the patient, modified in vitro, and then reinfused. Offers precise control. Example: ADA-SCID.
In vivo: — Vector is directly administered into the patient's body to target cells.

Types of Gene Therapy (Ethical Distinction):

Somatic Cell Gene Therapy:

* Target: Non-reproductive cells. * Heritability: Non-heritable (changes not passed to offspring). * Status: Clinically accepted and practiced.

Germline Gene Therapy:

* Target: Reproductive cells (sperm/egg) or early embryos. * Heritability: Heritable (changes passed to future generations). * Status: Ethically prohibited and not practiced in humans due to profound concerns.

Classic Example: Severe Combined Immunodeficiency (SCID) due to ADA Deficiency

Disease: — Genetic disorder causing severe immune system impairment due to lack of Adenosine Deaminase (ADA) enzyme.
Treatment: — Ex vivo gene therapy.
Mechanism: — Lymphocytes (or hematopoietic stem cells) are isolated from the patient. A functional ADA gene is introduced using a retroviral vector. Modified cells are reinfused, restoring immune function.

Risks/Challenges: Immune response to vector, insertional mutagenesis, off-target effects, achieving specific and sustained gene expression.

Vyyuha Quick Recall

VECTORS: Viruses Enter Cells To Offer Remedial Solutions.

ADA-SCID: All Deficient Antibodies Stop Combating Infections Due to Defective ADA gene. (Gene therapy is the fix!)