Gene Therapy and Genome Projects
Gene therapy is the treatment of genetic disorders by introducing functional copies of genes — or, increasingly, by directly editing the defective allele — in the cells of affected individuals. The field has had a turbulent history: the 1999 death of Jesse Gelsinger in an early adenoviral trial and the leukaemia cases in the early X-SCID retroviral trials produced a profound caution that took two decades to recover from. The modern era — built on AAV vectors, lentiviruses, ex vivo manipulation of haematopoietic stem cells, and CRISPR-Cas9 — has produced licensed gene therapies for inherited retinal dystrophy, spinal muscular atrophy, sickle cell disease, and several other conditions. This lesson covers the molecular logic, the case-study landscape, the Human Genome Project that made the field possible, pharmacogenomics, CRISPR editing, and the persistent ethical issues — all of which feature in the AQA A-Level Biology specification.
Spec mapping: This lesson sits in AQA 7402 Section 3.8.4 — Gene technologies allow the study and alteration of gene function. The specification expects candidates to describe the principles of gene therapy (somatic vs germline), explain the role of vectors, evaluate ethical implications, and to understand the Human Genome Project and its applications including personalised medicine. (Refer to the official AQA specification document for exact wording.)
What Is Gene Therapy?
Key Definition: Gene therapy is the introduction of a normal (functional) allele of a gene — or the precise editing of a defective allele — in the cells of an individual carrying a genetic disorder, with the aim of treating or curing the disease.
Key points:
- Traditionally, gene therapy means adding a functional copy of the gene alongside the defective copy, allowing the missing protein to be produced. The defective allele itself remains.
- Gene editing approaches (CRISPR-Cas9, base editing, prime editing) directly correct the defective sequence, leaving a normal allele in place.
- Targets are typically monogenic disorders — diseases caused by mutation in a single gene (cystic fibrosis, sickle cell anaemia, severe combined immunodeficiency, haemophilia, retinal dystrophies, spinal muscular atrophy). Polygenic diseases (diabetes, hypertension, schizophrenia) are not currently amenable to gene therapy.
- The two principal categories are somatic cell gene therapy and germline gene therapy, distinguished by whether the genetic change can be inherited by the patient's offspring.
Somatic Cell Gene Therapy vs Germline Gene Therapy
| Feature | Somatic Cell Gene Therapy | Germline Gene Therapy |
|---|
| Target cells | Body (somatic) cells of the patient (lung epithelium, haematopoietic stem cells, retinal cells, hepatocytes) | Gametes (eggs or sperm) or early embryo cells |
| Effect on offspring | Not inherited — only the treated individual is affected | Inherited by all future generations from the modified germline |
| Current status | Permitted; multiple licensed therapies; many active clinical trials | Illegal in the UK and most jurisdictions for clinical use (research on embryos is permitted in some countries up to 14 days under licence) |
| Ethical concerns | Fewer — affects only the consenting patient | Major — altering the human germline raises concerns about consent (the future offspring cannot consent), unforeseen effects on future generations, and a "slippery slope" to genetic enhancement |
| Duration of effect | May be temporary (if treated cells turn over and the gene is not integrated) or permanent (if integrated into a long-lived stem cell lineage) | Permanent and heritable |
| Examples | Luxturna (RPE65 retinal dystrophy), Zolgensma (SMN1 spinal muscular atrophy), CAR-T cell therapies, ex vivo β-globin editing in sickle cell disease | He Jiankui's 2018 announcement of CRISPR-edited human embryos (CCR5 ΔΔ32) — widely condemned and resulted in criminal conviction in China |
Exam Tip: The somatic vs germline distinction is the single most commonly examined point on this topic. Always state explicitly: somatic = patient only, not inherited, permitted; germline = heritable, currently banned. The 14-day rule in the UK applies to research only, not to clinical use.
Methods of Delivering Genes (Gene Delivery Vectors)
The therapeutic gene must be delivered into the target cells and expressed at appropriate levels. The choice of vector is the central engineering problem in gene therapy.
Viral Vectors
- Modified viruses (with their pathogenic and replication genes removed) deliver the therapeutic gene into host cells, exploiting the virus's evolved ability to penetrate the cell.
- Advantages: viruses are naturally efficient at entering cells and delivering genetic material; some integrate into the host genome for permanent expression.
- Disadvantages: immune response against viral proteins (limiting repeated dosing); potential for insertional mutagenesis (the viral DNA may integrate into or near an oncogene or tumour suppressor, potentially triggering cancer — this was the cause of the X-SCID leukaemia cases).
Types of viral vector:
| Viral Vector | Features |
|---|
| Adenovirus | Does not integrate into the host genome; gene expression is transient; low risk of insertional mutagenesis but requires repeated treatment; strong immunogenicity is the principal limitation (it was an adenoviral vector that caused Jesse Gelsinger's death in 1999) |
| Retrovirus | Integrates into the host genome via reverse transcriptase and integrase; provides permanent gene expression but carries insertional mutagenesis risk; only infects dividing cells |
| Lentivirus | A type of retrovirus (e.g. modified HIV-1) that can infect both dividing and non-dividing cells; integrates with somewhat reduced oncogenic risk compared with classical retroviruses; used in modern ex vivo therapies including some CAR-T cells |
| Adeno-associated virus (AAV) | Small, low-immunogenicity virus; predominantly episomal (does not integrate), reducing insertional mutagenesis risk; multiple serotypes target different tissues (AAV2 retina, AAV8 liver, AAV9 CNS); the workhorse of modern in vivo gene therapy |
Non-Viral Methods
- Liposomes — the gene is enclosed in a lipid vesicle (cationic or neutral lipid) that fuses with the cell membrane, delivering DNA into the cell. Less efficient than viral vectors but lower risk of immune response. The lipid nanoparticle (LNP) systems used in mRNA COVID-19 vaccines are descendants of this technology.
- Naked DNA / plasmid injection — direct injection of DNA into target tissue. Very low efficiency; used mainly for some DNA vaccines and proof-of-concept studies.
- Electroporation — electrical pulses create temporary pores in cell membranes for DNA entry. Used for ex vivo modification of cells.
Ex vivo vs in vivo gene therapy
- Ex vivo gene therapy — target cells (typically haematopoietic stem cells) are removed from the patient, modified in the laboratory with the vector, expanded, and reinfused. Allows tight quality control. Used for haematological diseases (sickle cell disease, β-thalassaemia, X-SCID, ADA-SCID).
- In vivo gene therapy — the vector is delivered directly to the patient (intravenous, subretinal, intrathecal, inhaled). Used when target cells cannot easily be extracted (retina, CNS, liver).
Cystic Fibrosis — A Gene Therapy Case Study
Cystic fibrosis (CF) is one of the most studied candidates for gene therapy and illustrates both the promise and the persistent challenges.
The Disease
- CF is caused by mutations 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), present in ~70% of affected alleles in northern European populations. This produces a misfolded CFTR protein that is degraded by the endoplasmic reticulum quality-control machinery 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.
- The thick mucus blocks airways (leading to recurrent lung infections, particularly Pseudomonas aeruginosa), pancreatic ducts (causing malabsorption and pancreatic insufficiency), and reproductive tracts (causing infertility in most male patients).
- CF is autosomal recessive — an individual must be homozygous (or compound heterozygous) for CFTR mutations to be affected. Carrier frequency in northern Europeans is ~1 in 25.
Gene Therapy Approaches for CF
- The aim is to deliver a functional copy of the CFTR gene into the epithelial cells of the lungs, where the most serious clinical consequences arise.
- Adenoviral vectors: the functional CFTR gene is packaged in a modified adenovirus, delivered to the lungs via aerosol or nasal spray. Adenoviruses naturally infect airway epithelium but provoke strong immune responses that limit repeated dosing.
- Liposome delivery: the CFTR gene is encapsulated in cationic liposomes and delivered by inhalation. Less immunogenic but less efficient than viral delivery.
- AAV vectors: more recent trials use AAV serotypes that target airway epithelium.
Challenges
- Lung epithelial cells are continually replaced (turnover several weeks), so the therapy must be repeated regularly as treated cells die and are replaced by untreated cells.
- Immune responses to the viral vector or to the CFTR protein itself reduce effectiveness with repeated treatments.
- Efficiency of gene delivery is low — only a fraction of target cells take up the gene and express functional CFTR.
- Ensuring adequate expression levels of CFTR in the right cell types is non-trivial.
- The disease affects multiple organs (lung, pancreas, gut, reproductive tract); systemic correction would require a delivery strategy reaching all of them.
Modern alternatives
- CFTR modulators (small-molecule drugs) such as ivacaftor, lumacaftor and the triple-combination Trikafta target the protein directly and improve folding/function of mutant CFTR. They have substantially altered CF management for patients with eligible genotypes, somewhat reducing the immediate clinical urgency of gene therapy for the major mutation classes (though gene therapy remains highly relevant for patients with mutations that do not respond to modulators).
Severe Combined Immunodeficiency (SCID) — Gene Therapy Successes and Setbacks
X-linked SCID and ADA-SCID are the gene therapy success stories of the early 2000s — and also the warning cases.
- Children with SCID lack a functional immune system from birth.
- ADA-SCID is caused by deficiency of adenosine deaminase; X-linked SCID by mutations in the IL2RG gene (the common γ chain of several cytokine receptors).
- Both conditions have been treated with retroviral gene therapy: bone marrow cells are removed, the corrected gene is introduced ex vivo, and the cells are reinfused.
- Successes: most treated children developed functional immune systems and have lived disease-free for years.
- Setbacks: in the early X-SCID trials in France (early 2000s), several children developed T-cell leukaemia because the retroviral vector integrated near the LMO2 oncogene, activating it and driving leukaemic transformation. This insertional mutagenesis was a defining cautionary event for the field.
- Modern X-SCID trials use lentiviral vectors with self-inactivating long terminal repeats, designed to reduce the risk of activating nearby oncogenes.
The Human Genome Project
Key Definition: The Human Genome Project (HGP) was an international research programme (1990–2003) that determined the complete nucleotide sequence of the human genome — approximately 3.2 billion base pairs across 22 autosomes plus the X and Y chromosomes.
Key Findings
- Humans have approximately 20,000 protein-coding genes — substantially fewer than the 100,000+ that some pre-HGP estimates had predicted.
- Only about 1.5% of the genome codes for proteins; the rest is regulatory DNA, repetitive elements (LINEs, SINEs, satellite DNA), transposon remnants, and non-coding RNAs.
- The genome contains large amounts of repetitive DNA, including the STRs and VNTRs used in genetic fingerprinting (Lesson 6).
- Many genes are shared across species (high conservation between humans and chimpanzees, mice, fish, fruit flies, yeast), confirming evolutionary relationships and enabling functional inference from model organisms.
- Over 99.9% of the DNA sequence is identical between all humans; the remaining ~0.1% (millions of single-nucleotide polymorphisms — SNPs — plus structural variation) underlies individual differences in physiology, drug response and disease risk.
Major contributions to biology
- Medical diagnosis: identification of genes associated with genetic disorders (e.g., BRCA1/2 for breast cancer risk; CFTR for cystic fibrosis; HTT for Huntington's disease).
- Drug development: understanding disease mechanisms at the molecular level enables the development of targeted therapies (Lesson 3).
- Pharmacogenomics: tailoring drug treatments to an individual's genetic makeup (see below).
- Comparative genomics: comparing the human genome with other species clarifies evolutionary relationships, dates of divergence, and the genetic basis of species-specific traits.
- Forensics: identification of STR loci and other variable regions for DNA profiling.
Successor projects
- 1000 Genomes Project (2008–2015) — catalogued common human genetic variation across multiple populations.
- UK Biobank — half-million participants with genome-wide genotyping and detailed health records.
- GWAS (genome-wide association studies) — link common variants to common diseases.
- Genome England 100,000 Genomes Project — whole-genome sequencing in the NHS for rare disease and cancer patients.
Pharmacogenomics
Key Definition: Pharmacogenomics is the study of how an individual's genetic makeup affects their response to drugs. The clinical promise is personalised medicine — using a patient's genetic profile to select the most effective drug and the right dose.
Examples
- Warfarin dosing. Variations in CYP2C9 (a cytochrome P450 enzyme that metabolises warfarin) and VKORC1 (the drug's target) affect both warfarin clearance and sensitivity. Genetic testing can guide initial dose selection, reducing the trial-and-error period that traditionally accompanies warfarin initiation.
- Cancer treatment. Tumour genotyping identifies driver mutations that determine which targeted therapies will work — HER2 amplification → trastuzumab; BCR-ABL → imatinib; BRAF V600E → vemurafenib; EGFR mutations → gefitinib/osimertinib. (Lesson 3 covers these in detail.)
- Thiopurine drugs. TPMT polymorphisms determine how quickly mercaptopurine and azathioprine are metabolised; pre-treatment TPMT testing is now standard before starting these drugs to avoid severe myelosuppression in poor metabolisers.
- Abacavir hypersensitivity. Patients carrying HLA-B*57:01 have a high risk of severe hypersensitivity reaction to abacavir (an HIV drug). Pre-treatment screening is now mandatory and has essentially eliminated this adverse reaction.
Limitations
- Genetic information is only one input; environment, age, comorbidity and drug-drug interactions also affect drug response.
- Pharmacogenomic testing is unevenly available; integration into routine clinical pathways is gradual.
- Many polymorphisms have effects too small to be clinically actionable.
CRISPR-Cas9 Gene Editing
Key Definition: CRISPR-Cas9 is a gene-editing technology that allows precise changes to the DNA sequence of a living organism. It uses a guide RNA (gRNA) to direct the Cas9 nuclease to a specific location in the genome, where it makes a double-strand break. The natural system is the adaptive immune system of bacteria, which use CRISPR arrays to recognise and cleave invading phage DNA.
The technology was developed by Emmanuelle Charpentier and Jennifer Doudna (Nobel Prize in Chemistry 2020, awarded jointly) and rapidly adapted for genome editing in mammalian cells. (Paraphrased throughout — no verbatim quotation from their 2012 Science paper is reproduced.)
How CRISPR-Cas9 Works