Gene Therapy and Genome Projects

Gene therapy is the treatment of genetic disorders by introducing functional copies of genes into the cells of affected individuals. This lesson also covers the Human Genome Project, pharmacogenomics, CRISPR gene editing, and the ethical issues surrounding these technologies — all of which feature in the AQA A-Level Biology specification.

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What Is Gene Therapy?

Key Definition: Gene therapy is the introduction of a normal (functional) allele of a gene into the cells of an individual who carries a defective allele, with the aim of treating or curing a genetic disorder.

Key points:

• Gene therapy targets genetic disorders caused by a defective allele — the aim is to supplement the faulty gene with a functional copy so that the correct protein can be produced.
• Gene therapy does not alter the defective gene itself (unless CRISPR is used) — it adds a working copy alongside it.
• There are two categories of gene therapy: somatic cell gene therapy and germ line gene therapy.

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Somatic Cell Gene Therapy vs Germ Line Gene Therapy

Feature	Somatic Cell Gene Therapy	Germ Line Gene Therapy
Target cells	Body (somatic) cells of the patient	Gametes (eggs or sperm) or early embryo cells
Effect on offspring	Not inherited — only the treated individual is affected	Changes are inherited by all future generations
Current status	Permitted and used in clinical trials and treatments	Illegal in most countries, including the UK (under the Human Fertilisation and Embryology Act)
Ethical concerns	Fewer ethical objections — affects only the individual patient	Major ethical objections — altering the human germline raises concerns about "designer babies," unforeseen effects on future generations, and consent issues
Duration of effect	May be temporary (if the treated cells are replaced or the gene is not permanently integrated); may require repeated treatments	Permanent — the change is incorporated into the germline and passed to all descendants

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Methods of Delivering Genes (Gene Delivery Vectors)

The functional gene must be delivered into the target cells. Several methods exist:

Viral Vectors

• Modified viruses (with their pathogenic genes removed) are used to deliver the therapeutic gene into host cells.
• Advantages: Viruses are naturally efficient at entering cells and delivering genetic material.
• Disadvantages: Risk of immune response against the viral proteins; potential for insertional mutagenesis (the viral DNA may insert into a tumour suppressor gene or near an oncogene, potentially triggering cancer).

Types of viral vector:

Viral Vector	Features
Adenovirus	Does not integrate into the host genome; gene expression is temporary; low risk of insertional mutagenesis but requires repeated treatment
Retrovirus	Integrates into the host genome using reverse transcriptase and integrase; provides permanent gene expression but risk of insertional mutagenesis; only infects dividing cells
Lentivirus	A type of retrovirus that can infect both dividing and non-dividing cells; used in some modern gene therapies
Adeno-associated virus (AAV)	Small virus with low immunogenicity; can integrate or remain episomal; widely used in current clinical trials

Non-Viral Methods

• Liposomes — the gene is enclosed in a lipid vesicle that fuses with the cell membrane, delivering the DNA into the cell. Less efficient than viral vectors but lower risk of immune response.
• Naked DNA / plasmid injection — direct injection of DNA into target tissue. Simple but very low efficiency.
• Electroporation — electrical pulses create temporary pores in cell membranes for DNA entry.

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Cystic Fibrosis — A Gene Therapy Case Study

Cystic fibrosis (CF) is one of the most studied candidates for gene therapy.

The Disease

• CF is caused by a mutation in the CFTR gene (cystic fibrosis transmembrane conductance regulator) on chromosome 7.
• The most common mutation is ΔF508 (deletion of three bases coding for phenylalanine at position 508), which results in a misfolded CFTR protein that is degraded before reaching the cell membrane.
• The CFTR protein functions as a chloride ion channel in epithelial cell membranes.
• Without functional CFTR, chloride ions cannot be transported out of cells. Water follows by osmosis, and the mucus produced by epithelial cells becomes thick and sticky.
• This thick mucus blocks airways (leading to lung infections), pancreatic ducts (leading to malabsorption), and reproductive tracts (leading to infertility).
• CF is an autosomal recessive condition — an individual must be homozygous (ff) to be affected.

Gene Therapy Approaches for CF

• The aim is to deliver a functional copy of the CFTR gene into the epithelial cells of the lungs.
• Adenoviral vectors: The functional CFTR gene is inserted into a modified adenovirus, which is then delivered to the lungs via an inhaler or nasal spray. The virus infects the epithelial cells and delivers the gene.
• Liposome delivery: The CFTR gene is encapsulated in liposomes and delivered to the lungs by inhalation.

Challenges

• Lung epithelial cells are regularly replaced (turnover ~every 2–3 months), so the therapy must be repeated regularly as the treated cells die and are replaced by untreated cells.
• Immune responses to the viral vector can occur, reducing effectiveness with repeated treatments.
• Efficiency of gene delivery is low — not all target cells receive the functional gene.
• Ensuring adequate expression levels of the CFTR protein is difficult.
• Progress has been slow, but newer approaches (including CRISPR-based strategies) are being investigated.

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The Human Genome Project