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This lesson covers gene therapy — the treatment of genetic disorders by introducing functional genes — and genetic screening — testing individuals for specific genetic conditions. Both topics are required by the Edexcel A-Level Biology specification (9BI0, Topic 7).
Gene therapy is the introduction of a functional copy of a gene into the cells of a patient to treat or prevent a genetic disorder. The aim is to compensate for a faulty or missing gene by providing a working version.
Gene therapy targets disorders caused by single gene defects (monogenic disorders), such as:
Exam Tip: Be clear about the difference between somatic and germ line gene therapy. Examiners often ask about this distinction and the ethical implications of each type.
The functional gene must be delivered into the patient's cells using a vector. The main approaches are:
| Vector type | Mechanism | Advantages | Disadvantages |
|---|---|---|---|
| Adenovirus | Infects cells and delivers gene; DNA remains separate from host genome | Efficient delivery; can infect non-dividing cells | Gene expression is temporary; immune response |
| Retrovirus | Integrates gene into host chromosome using reverse transcriptase | Long-term expression; gene is permanently integrated | Risk of insertional mutagenesis (gene may insert into an oncogene or tumour suppressor gene, potentially causing cancer) |
| Adeno-associated virus (AAV) | Small virus; integrates at a specific site or remains as an episome | Low immune response; can infect non-dividing cells | Small capacity for inserted DNA |
| Lentivirus | Modified HIV; integrates into host chromosome | Can infect non-dividing cells; long-term expression | Safety concerns due to HIV origin |
| Method | Description |
|---|---|
| Liposomes | DNA is enclosed in lipid vesicles that fuse with the cell membrane; used for CF gene therapy (inhaled) |
| Electroporation | Electrical pulses create temporary pores in cell membranes |
| Microinjection | Direct injection of DNA into cells |
| Naked DNA | Injection of plasmid DNA directly into tissue |
Exam Tip: The most significant risk of retroviral gene therapy is insertional mutagenesis. This occurred in a clinical trial for X-linked SCID in 2002, where several patients developed leukaemia because the retrovirus inserted near an oncogene.
Severe Combined Immunodeficiency (SCID) is caused by a mutation in the gene coding for the enzyme adenosine deaminase (ADA). Without ADA, toxic metabolites accumulate and destroy T lymphocytes, leaving the patient with no functional immune system.
Cystic fibrosis (CF) is caused by mutations in the CFTR gene, which codes for a chloride ion channel protein. The most common mutation is ΔF508 (deletion of phenylalanine at position 508).
| Approach | Method |
|---|---|
| Viral vector | Adenovirus delivers CFTR gene to lung epithelial cells via aerosol inhalation |
| Liposomes | CFTR gene enclosed in lipid vesicles, delivered by nebuliser |
A revolutionary new approach to gene therapy uses CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats):
| Advantage | Detail |
|---|---|
| Precise | Targets a specific sequence with high accuracy |
| Versatile | Can knockout, correct or insert genes |
| Relatively cheap | Much less expensive than previous gene editing technologies |
| Efficient | Higher success rate than older methods |
Exam Tip: CRISPR-Cas9 is an increasingly common exam topic. Be able to explain how it works (guide RNA + Cas9 nuclease), how it differs from traditional gene therapy (it edits the existing gene rather than adding a new copy), and its advantages and ethical concerns.
Genetic screening is testing an individual's DNA (or gene products) to determine whether they carry a specific allele or are at risk of developing a genetic condition.
| Type | When/who | Purpose |
|---|---|---|
| Pre-natal screening | During pregnancy (amniocentesis or chorionic villus sampling) | Detect chromosomal abnormalities (e.g. Down syndrome) or genetic diseases |
| Neonatal screening | Newborn babies (heel prick test) | Detect conditions such as phenylketonuria (PKU) and sickle cell disease for early treatment |
| Carrier testing | Individuals with a family history | Determine if a person carries one copy of a recessive allele (e.g. CF carrier testing) |
| Pre-implantation genetic diagnosis (PGD) | During IVF | Test embryos for genetic conditions before implantation |
| Predictive testing | Individuals with family history of late-onset conditions | Assess risk of developing conditions like Huntington's disease or BRCA-related cancers |
| Pharmacogenomics | Before drug treatment | Identify genetic variants that affect drug response |
In the UK, the Genetic Information Nondiscrimination Act equivalent provisions and the Equality Act 2010 provide some protection against genetic discrimination, though insurance companies may request genetic test results for policies above certain thresholds.
| Topic | Key Points |
|---|---|
| Gene therapy | Introduce functional gene to treat genetic disorder |
| Somatic vs germ line | Somatic = not inherited; germ line = inherited (illegal in UK) |
| Vectors | Viral (retrovirus, adenovirus, AAV) or non-viral (liposomes) |
| CRISPR-Cas9 | Guide RNA + Cas9 nuclease = precise gene editing |
| Genetic screening | Testing for genetic conditions at various life stages |
| Ethics | Balance between medical benefit and concerns about discrimination, consent, and selection |
Exam Tip: Gene therapy and genetic screening questions often have a discussion/evaluation component. Make sure you can present arguments for and against, supported by specific examples and case studies.
This material sits in Edexcel 9BI0 Topic 8 (Grey Matter — Coordination, Response and Gene Technology), which expects candidates to describe gene therapy as the introduction of a functional gene (or the editing of a defective gene) into a patient's cells to treat a monogenic disorder, distinguishing somatic therapy (treats only the patient — changes are not transmitted to offspring) from germline therapy (alters egg, sperm or early-embryo DNA — changes are heritable across all descendant cells), and to describe genetic screening in its principal clinical contexts: prenatal screening (during pregnancy), newborn screening (at birth), carrier screening (in adulthood, often with a family history) and pre-implantation genetic diagnosis (PGD) during IVF. Synoptic links run backwards to lesson 7 on genetic engineering (gene therapy reuses the recombinant-DNA toolkit — restriction enzymes, ligase and viral / plasmid vectors — but the host cell is a human somatic cell rather than E. coli); to lesson 6 on PCR (PCR amplifies the diagnostic target sequence in carrier and prenatal screening, and the corrective insert in a gene-therapy construct); and to Topic 1 (DNA structure) for the molecular basis of how a single base substitution (sickle-cell HBB Glu6Val) or a three-base deletion (CFTR ΔF508) can disable a protein and thus a clinical phenotype. Synoptic links run forwards to lesson 9 on genomics and bioinformatics (population-scale screening relies on reference-genome resources and bioinformatic variant-calling pipelines), and sideways to Topic 6 (immunity, vaccines and infection) — both because vaccine development draws on the same molecular techniques and because the CRISPR-Cas system repurposed in modern gene editing originated as a bacterial adaptive immune defence against bacteriophages. Topic 4 (population genetics and the Hardy-Weinberg principle) underpins the carrier-frequency reasoning that justifies which heterozygous-carrier conditions warrant screening at a population level (Tay-Sachs in Ashkenazi Jewish populations; sickle-cell in West African and Mediterranean populations; CF in Northern European populations). Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
(a) Distinguish between somatic and germline gene therapy, and explain why germline gene therapy is currently prohibited in the UK while somatic therapy is clinically available. (6)
(b) Explain why an AAV (adeno-associated virus) vector is the preferred delivery system for the retinal gene therapy Luxturna, whereas an ex vivo CRISPR-Cas9 edit of haematopoietic stem cells is the strategy used by Casgevy for sickle-cell disease. (2)
Solution with mark scheme:
(a) M1 (AO1) — definition of somatic. Somatic gene therapy introduces (or edits) a functional gene in non-reproductive body cells of the patient — for example haematopoietic stem cells, lung epithelial cells, retinal cells or T lymphocytes. Because the gametes are not modified, the change cannot be transmitted to the patient's offspring; the therapy benefits only the treated individual.
A1 (AO1) — definition of germline. Germline gene therapy alters the DNA of gametes (egg, sperm) or of an early embryo before differentiation, so the modification is propagated to every cell of the resulting individual, including their gametes, and is therefore heritable across all descendant generations.
A1 (AO2) — clinical examples. Approved or clinically administered somatic therapies include CAR-T cell therapy (autologous T cells edited ex vivo to express a chimeric antigen receptor against B-cell leukaemia), beti-cel / Casgevy (autologous haematopoietic stem cells edited or augmented for β-thalassaemia and sickle-cell disease), and Luxturna (AAV-delivered RPE65 to the retina for inherited retinal dystrophy). No germline gene therapy is approved in any jurisdiction.
A1 (AO3) — ethical / regulatory contrast. Germline editing raises issues that somatic editing does not: (i) future generations cannot consent to a heritable change made before they exist; (ii) off-target edits are propagated forever rather than dying with the patient; (iii) the technology opens a route to non-therapeutic enhancement ("designer babies"); (iv) the 2018 case in which a researcher in China used CRISPR-Cas9 to edit human embryos for CCR5 and produce live births was internationally condemned by professional societies and led to a criminal conviction for the lead researcher.
A1 (AO3) — UK regulatory position. Under UK law, heritable genome editing of human embryos for clinical implantation is prohibited; somatic gene therapies are regulated as advanced-therapy medicinal products by the MHRA, with prior approval by NICE for NHS reimbursement on a per-product basis.
A1 (AO3) — risk asymmetry. A failed somatic therapy harms only the consenting patient and is reversible (in the sense that it does not propagate). A failed germline edit affects an entire descendant lineage and is, in practical terms, irreversible. This asymmetry is the central reason a global regulatory consensus tolerates somatic but not germline therapy.
(b) M1 (AO3). Luxturna treats inherited retinal dystrophy caused by RPE65 mutations. The retina is a small, immune-privileged compartment accessible by a single subretinal injection of AAV carrying a functional RPE65 cDNA. AAV is the vector of choice because it provokes a low immune response, infects non-dividing post-mitotic retinal cells, and gives long-term episomal expression without integrating into the patient's chromosomes — minimising insertional-mutagenesis risk in a non-renewing tissue.
A1 (AO3). Casgevy treats sickle-cell disease (an HBB single-base substitution that distorts haemoglobin under low O₂). A viral vector delivering a corrective HBB copy is poor strategy here because the disease originates in the patient's own renewing haematopoietic stem cell pool and the dominant-negative sickle mutation must be addressed at the DNA level. Instead, haematopoietic stem cells are harvested, CRISPR-Cas9 is delivered ex vivo to disrupt the BCL11A erythroid enhancer (reactivating fetal haemoglobin), and the edited stem cells are reinfused after myeloablative conditioning. The therapeutic problem dictates the technology — small, immune-privileged, non-dividing target → AAV in vivo; renewing, accessible-by-bone-marrow, gene-correction-required → CRISPR ex vivo.
Total: 8 marks (M2 A6).
Question (6 marks): A clinical-genetics service is planning a screening programme for cystic fibrosis (CF) and considering a gene therapy trial for severe CF. (i) Describe how carrier screening for CF is performed and the role of Hardy-Weinberg-derived carrier-frequency estimates in deciding whom to offer the screen. (ii) Explain why CFTR gene therapy by liposome inhalation has historically delivered modest clinical benefit, and describe one molecular reason CRISPR-Cas9 ex vivo editing of airway basal stem cells is now a more promising approach.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Naming PCR (or restriction-fragment length polymorphism / sequencing) of a buccal- or blood-derived DNA sample to test for the CFTR ΔF508 allele and other common pathogenic variants |
| 2 | AO1.2 | Stating that carrier screening identifies heterozygous (Ff) individuals — phenotypically unaffected but at risk of an affected (ff) child if their partner is also a carrier |
| 3 | AO2.1 | Using the Hardy-Weinberg equation 2pq to estimate carrier frequency in the population (≈1 in 25 in Northern European populations for CF) — justifying universal vs targeted screening on the basis of the resulting cost-benefit calculation |
| 4 | AO2.7 | Explaining that liposome / aerosol CFTR gene therapy delivers a temporary, episomal copy of CFTR to terminally differentiated airway epithelial cells, which turn over rapidly; effects fade as cells are shed, requiring repeated dosing, and the mucus layer plus immune response further reduce delivery efficiency |
| 5 | AO3.1 | Explaining that CRISPR-Cas9 can edit the CFTR locus in long-lived airway basal stem cells ex vivo; reintroduction of edited stem cells gives a renewing source of corrected differentiated epithelium, in principle sustaining benefit beyond a single dose |
| 6 | AO3.2 | Synoptic — connecting screening to lesson 6 (PCR amplifies the diagnostic locus), lesson 7 (gene therapy reuses the recombinant-DNA toolkit), lesson 9 (population-scale variant data come from genomics resources), and Topic 4 (Hardy-Weinberg gives the carrier-frequency model) |
Total: 6 marks (AO1 = 2, AO2 = 2, AO3 = 2). Edexcel reliably tests gene-therapy / screening through scenario prompts ("a clinical service is planning…") that demand a paired molecular + ethical / population-level answer. Candidates who answer only the molecular half (PCR, CFTR, liposome) without the carrier-frequency / Hardy-Weinberg reasoning lose the AO2 marks; candidates who answer only the population half without naming CFTR ΔF508, PCR-based variant detection, AAV / liposome / CRISPR lose the AO1 marks. A* candidates integrate both halves and explicitly link the molecular target to the choice of vector or editing strategy.
Lesson 7 (genetic engineering and recombinant DNA). Gene therapy is recombinant-DNA technology applied inside a living patient. The corrective gene is obtained as cDNA by reverse transcription, ligated into a viral vector backbone (AAV, lentivirus) by DNA ligase with restriction-enzyme-cut sticky ends, and the resulting recombinant viral genome is packaged into capsids that infect the patient's somatic cells. Same molecular toolkit; clinical-grade context, immunogenicity and safety constraints replace the fermenter.
Lesson 6 (PCR and gel electrophoresis). PCR is the workhorse of every clinical-genetics screen. Carrier and prenatal screening for known pathogenic alleles (CFTR ΔF508, HBB sickle, HEXA Tay-Sachs) typically use allele-specific PCR or PCR followed by restriction digest of the amplicon. Pre-implantation genetic diagnosis must amplify a target locus from a single biopsied blastomere — the most demanding application of PCR amplification fidelity in routine clinical practice.
Lesson 9 (genomics and bioinformatics). Population-scale screening is no longer one variant at a time. Expanded carrier screening panels test hundreds of genes in parallel by next-generation sequencing; non-invasive prenatal testing (NIPT) sequences cell-free fetal DNA from maternal plasma. Both depend on the reference-genome assembly, variant databases (ClinVar, gnomAD) and bioinformatic variant-calling pipelines that lesson 9 introduces.
Topic 6 (immunity, vaccines and the bacterial origin of CRISPR). CRISPR-Cas9 — the editing tool now central to gene therapy — was discovered as a bacterial adaptive immune system against bacteriophage DNA. Bacteria capture short fragments of phage DNA into CRISPR arrays, transcribe them as guide RNAs, and direct Cas nucleases to cleave matching invading sequences. Biotechnology repurposed this defence: replace the natural guide RNA with a synthetic 20-nt guide and Cas9 will cut any complementary genomic locus. Vaccine development draws on the same recombinant-DNA workflow.
Topic 1 (DNA structure and the molecular basis of disease). A single base substitution (HBB Glu6Val → sickle-cell), a three-base in-frame deletion (CFTR ΔF508), or a CAG-trinucleotide expansion (HTT → Huntington's disease) is sufficient to disable a protein and produce a clinical phenotype. The chemistry of the DNA double helix — antiparallel strands, complementary base pairing — is what allows a 20-nt guide RNA to find a single matching genomic locus among ~3 × 10⁹ base pairs.
Topic 4 (Hardy-Weinberg and carrier frequency). The decision to screen a population for a recessive disease allele rests on Hardy-Weinberg-derived carrier frequencies. CF carrier frequency ≈ 1 in 25 in Northern European populations (so ≈ 1 in 2,500 affected); sickle-cell carrier frequency reaches ≈ 1 in 4 in some West African populations. Hardy-Weinberg is the statistical framework that converts an affected-individual rate into a carrier-screening cost model.
Healthcare-policy and ethics dimensions. Equitable access (Casgevy lists at ~2.2millionperdose;Zolgensmaat 2.1 million); informed consent (especially for predictive testing of late-onset conditions like Huntington's); insurance non-discrimination (UK has a moratorium covering most policies, the US has GINA); reproductive autonomy and disability-rights perspectives on PGD. None of these is "outside biology"; they are the social system the molecular work lives inside.
CRISPR-Cas9 vs traditional gene-replacement vectors. Traditional gene therapy adds a functional copy of the gene; CRISPR edits the existing gene in situ. The two strategies are complementary — recessive loss-of-function disorders can in principle be rescued by either, but dominant-negative disorders (where the mutant protein actively interferes) generally require editing or knockdown rather than supplementation. The therapeutic question dictates the molecular tool.
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