outline the advantages of genetic screening, using the examples of breast cancer (BRCA1 and BRCA2), Huntington’s disease and cystic fibrosis

Genetic Technology – Advantages of Genetic Screening (Cambridge AS/A‑Level Topic 19)

Learning outcomes (what you should be able to do)

• Explain the molecular basis of inheritance – DNA structure, replication fidelity, transcription, translation and the central dogma (AO1).
• Describe the main DNA‑based techniques used in genetic screening – PCR, gel electrophoresis, RFLP/SNP analysis, DNA sequencing, recombinant DNA, cloning and CRISPR/Cas9 (AO2).
• Analyse the advantages and limitations of genetic screening for human disease (AO2).
• Evaluate the ethical, legal and social issues (ELSI) that arise from screening programmes (AO4).
• Design a simple screening assay, record data, and evaluate its reliability (AO3 – experimental skills).

1. Core concepts of genetic technology

1.1 DNA – the molecule of heredity

• Structure: double‑helix of deoxyribonucleic acid; nucleotides (A, T, C, G) pair A–T and C–G.
• Replication fidelity:

  • DNA polymerase δ/ε synthesises the new strand with 3′‑5′ exonuclease proofreading.
  • Mismatch‑repair (MMR) system corrects mis‑paired bases, reducing the spontaneous mutation rate to ≈10⁻⁹ per base per cell division.

• Transcription → translation – central dogma (DNA → mRNA → protein).

1.2 Mutation and its consequences

• Types of mutation: point mutations, insertions, deletions, repeat expansions, chromosomal rearrangements.
• Mutation rates: spontaneous (≈10⁻⁸–10⁻⁹ per base per generation) vs. induced (by mutagens).
• Induced mutagens:

  • Physical: UV radiation (pyrimidine dimers), ionising radiation (double‑strand breaks).
  • Chemical: alkylating agents, base analogues, tobacco smoke.

• Phenotypic effect: benign, advantageous or deleterious; penetrance and expressivity determine how a genotype manifests.

1.3 DNA‑based biotechnological tools (AO2)

1.4 Quality of a screening test (AO2)

• Sensitivity – proportion of true positives correctly identified.
• Specificity – proportion of true negatives correctly identified.
• Positive predictive value (PPV) – likelihood that a positive result truly indicates disease (depends on prevalence).
• Negative predictive value (NPV) – likelihood that a negative result truly indicates absence of disease.
• False‑positive / false‑negative results – clinical and psychological implications; importance of confirmatory testing.

2. Why genetic screening is important – general advantages (AO1/AO2)

• Early detection & preventive action – disease can be managed before symptoms appear.
• Targeted surveillance – high‑risk individuals receive intensified monitoring (e.g., MRI for BRCA carriers).
• Personalised / precision medicine – choice of drug or therapy based on genotype (e.g., CFTR modulators).
• Informed reproductive decisions – carrier screening, pre‑implantation genetic diagnosis (PGD), prenatal testing.
• Cascade testing of families – identification of other at‑risk relatives.
• Research participation – recruitment of genotype‑confirmed participants for clinical trials.
• Psychological preparedness – individuals can plan finances, career and lifestyle.
• Public‑health impact – population‑level programmes reduce disease burden (e.g., newborn screening for sickle‑cell disease).

3. Illustrative examples of screening programmes

3.1 Breast cancer – BRCA1 & BRCA2 (hereditary)

• Gene function – tumour‑suppressor proteins involved in DNA double‑strand‑break repair.
• Risk – pathogenic variants confer up to 70 % lifetime risk of breast cancer and 40 % risk of ovarian cancer.
• Screening methods – targeted PCR & NGS panels; multiplex ligation‑dependent probe amplification (MLPA) for large deletions.
• Advantages

  • Identification of high‑risk women before disease onset.
  • Preventive options: prophylactic mastectomy/oophorectomy, chemoprevention (tamoxifen), intensified imaging (annual MRI + mammography).
  • Family cascade testing → early detection in relatives.

• Limitations – incomplete penetrance, possible anxiety, cost of long‑term surveillance.

3.2 Huntington’s disease (HD)

• Gene & mutation – CAG repeat expansion in the HTT gene; >36 repeats = disease.
• Inheritance – autosomal dominant; each child of a carrier has a 50 % chance of inheriting the mutation.
• Screening method – PCR followed by capillary electrophoresis to count repeat length.
• Advantages

  • Predictive testing for at‑risk adults (before symptom onset, usually 30–50 y).
  • Allows life‑planning (career, finances, family).
  • Enables enrolment in clinical‑trial cohorts for emerging gene‑silencing therapies (ASOs, CRISPR‑based approaches).

• Ethical issues – right to not know, impact on insurance/employment, need for pre‑ and post‑test counselling.

3.3 Cystic fibrosis (CF)

• Gene & mutation – >2 000 CFTR variants; ΔF508 (deletion of phenylalanine at position 508) accounts for ~70 % of alleles in Caucasian populations.
• Screening strategies

  • Newborn screening – immunoreactive trypsinogen (IRT) test followed by DNA analysis for common CFTR mutations.
  • Carrier screening – PCR‑RFLP, SNP arrays or targeted NGS for couples planning pregnancy.

• Advantages

  • Early diagnosis → immediate physiotherapy, nutritional support, antibiotics, and mutation‑specific drugs (ivacaftor, lumacaftor, tezacaftor).
  • Improved survival (median life expectancy >50 y in screened cohorts).
  • Reproductive choices – PGD or prenatal diagnosis to avoid affected births.

• Limitations – false‑positive IRT results, variable response to CFTR modulators depending on genotype.

3.4 Population‑level example – Sickle‑cell disease (SCD) / β‑Thalassaemia

• Gene & mutation – point mutations in the β‑globin (HBB) gene; e.g., Glu6Val (HbS) for SCD, various deletions/point mutations for β‑thalassaemia.
• Screening programme – national newborn screening (heel‑prick blood spot) using high‑performance liquid chromatography (HPLC) or isoelectric focusing, followed by confirmatory DNA analysis (PCR‑RFLP or NGS).
• Advantages

  • Early identification allows prophylactic penicillin, vaccination and parental education, dramatically reducing mortality.
  • Data inform public‑health policies (carrier‑frequency mapping, premarital counselling).
  • Cascade testing of families reduces incidence in subsequent generations.

• Limitations – need for robust follow‑up services; potential stigma in communities with high carrier rates.

4. Practical/experimental skills (AO3)

Designing a simple screening assay – example: BRCA1 exon 11 deletion

1. Collect peripheral blood or buccal swab; extract genomic DNA (spin‑column or Chelex method).
2. Set up PCR with primers flanking exon 11 (include a positive control DNA and a no‑template control).
3. Run PCR products on a 2 % agarose gel stained with ethidium bromide or SYBR Safe.
4. Interpret band pattern:

   • Band of expected size → wild‑type allele.
   • Smaller band or additional band → deletion/duplication.

5. Confirm ambiguous results by Sanger sequencing or MLPA.
6. Evaluate assay performance (sensitivity, specificity, repeatability).

Data‑recording table (template)

Sample evaluation checklist (AO3)

• Were appropriate positive, negative and no‑template controls included?
• Was DNA quality assessed (A260/280 ratio) before PCR?
• Did any lane show contamination or primer‑dimer artefacts?
• Is the observed band size within ±5 % of the expected size?
• Was the assay repeated on an independent extract to confirm reproducibility?
• How do the calculated sensitivity and specificity compare with published values?
• Identify any systematic errors and propose improvements (e.g., redesign primers, use hot‑start polymerase).

5. Ethical, legal and social implications (ELSI) (AO4)

• Informed consent – clear explanation of purpose, possible outcomes, limits of confidentiality, and right to withdraw.
• Right to not know – individuals may decline predictive information, especially for untreatable conditions.
• Data privacy – secure storage of genetic data; compliance with GDPR (EU) or the UK Data Protection Act.
• Legislation – examples to name:

  • Genetic Information Nondiscrimination Act (GINA, USA).
  • UK Human Tissue Act 2004 and the forthcoming UK Genetic Information Act.
  • European Convention on Human Rights (Article 8 – right to privacy).

• Discrimination – potential misuse by insurers or employers; importance of protective laws.
• Psychological impact – anxiety, stigma, family dynamics; provision of pre‑ and post‑test genetic counselling.
• Equity of access – screening should be available regardless of socioeconomic status; consideration of cultural beliefs.

6. Summary table – Advantages of screening by disease

7. Key take‑aways (revision checklist)

1. Genetic screening relies on a solid understanding of DNA structure, replication fidelity, transcription, translation and mutation mechanisms.
2. Core laboratory techniques (PCR, electrophoresis, RFLP/SNP, sequencing, recombinant DNA, CRISPR) enable detection of disease‑causing variants.
3. Advantages include early intervention, personalised treatment, informed reproductive choices, family‑wide risk reduction and public‑health benefits.
4. Every programme must balance benefits against false results, psychological impact and ethical/legal considerations.
5. AO3 skill: design an assay, record data in a structured table, evaluate reliability (controls, sensitivity, specificity) and suggest improvements.