A level Biology 19.3 Genetically modified organisms in agriculture- Exam style question

Question

One cause of the genetic disease severe combined immunodeficiency (SCID) is a mutation in the
ADA gene. This mutation results in a deficiency of the enzyme adenosine deaminase (ADA).
Although ADA is found throughout the body, it is especially active in lymphocytes. The absence of
functional ADA causes the build-up of toxic metabolites that kill lymphocytes and damage organs.
Babies are often diagnosed with SCID by six months old. Treatment can greatly improve the life
expectancy of children with SCID.
Some treatment options are available.
• Enzyme replacement therapy with recombinant human ADA made by genetically modified
(GM) Escherichia coli. Weekly intra-muscular injections are given.
• Bone marrow transplant if a well-matched donor, such as a close relative, can be found.
• Gene therapy.

(a) Suggest and explain why it may be more appropriate to use enzyme replacement therapy to
treat SCID instead of a bone marrow transplant.

(b) Outline the procedure used for gene therapy treatment of a person with SCID.

(c) Suggest the social and ethical implications of gene therapy for SCID that need to be
considered before treatment is carried out.

▶️ Answer/Explanation

Solution

(a) Enzyme replacement therapy is more appropriate than a bone marrow transplant because:

1. No donor needed: Unlike bone marrow transplants, enzyme replacement therapy does not require a matching donor, making it more accessible.
2. Immediate effect: The therapy provides quick relief by directly supplying the missing ADA enzyme.
3. Less invasive: Weekly injections are simpler and safer than the surgical procedure required for a bone marrow transplant.
4. No immune rejection: Since no foreign cells are introduced, there is no risk of graft-versus-host disease (GVHD).

Explanation: Enzyme replacement therapy avoids the complications of finding a donor and reduces risks associated with transplants, offering a safer and faster treatment option.

(b) Gene therapy procedure for SCID:

1. Stem cell collection: Harvest stem cells from the patient’s bone marrow or blood.
2. Gene insertion: Use a viral vector to introduce the functional ADA gene into the stem cells.
3. Conditioning: Use chemotherapy or radiation to clear space in the bone marrow for the modified cells.
4. Reinfusion: Return the genetically corrected stem cells to the patient’s bloodstream, where they migrate to the bone marrow and produce healthy lymphocytes.

Explanation: This method aims to provide a permanent cure by enabling the patient’s cells to produce ADA naturally.

(c) Social and ethical implications of gene therapy:

1. Cost: Gene therapy is expensive, limiting accessibility for some patients.
2. Long-term effects: Potential risks, such as unintended mutations or cancer, must be carefully monitored.
3. Ethical concerns: Some religious or cultural groups may oppose genetic modification.
4. Psychological impact: The process can be stressful for families, despite its potential to cure SCID.

Explanation: While gene therapy offers a transformative cure, its high cost and ethical dilemmas require careful consideration before widespread adoption.

A level Biology 19.3 Genetically modified organisms in agriculture- Exam style question – Paper 4

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