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19.2.319 · Genetic Technology

Gene Therapy — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 16 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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19.2.3 Gene Therapy — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe gene therapy, its delivery systems, examples for SCID and Leber congenital amaurosis, and its limits.Gene therapy — altering a person's genetic material to treat or cure disease.Possibiliti…

  • Somatic gene therapy
  • Germline therapy
  • Recombinant
  • Germline gene therapy
  • Lentivirus
  • Adeno-associated virus

Part 1 of 8What gene therapy is

Gene therapy
  • Gene therapy altering a person's genetic material to treat or cure disease.
Possibilities as the human genome is understood: replace a faulty gene, inactivate a faulty gene or insert a new gene.
It is used experimentally for SCID, Leber congenital amaurosis, β-thalassaemia and haemophilia B.
Most gene therapies are still in clinical trials because delivery and correct expression are difficult.

Part 2 of 8Vectors for gene therapy

Vectors for gene therapy
Vectors are the delivery system.
  • Viruses (retroviruses, lentiviruses) are most common because they recognise cells and deliver genetic material.
  • Non-viral vectors are being researchedliposomes and "naked" DNA.
Retroviruses insert randomly and may disrupt genes; adeno-associated viruses do not integrate, so genes are lost when cells divide.
Viral vectors are disabled so they cannot replicate or cause disease.

Part 3 of 8Somatic and germline therapy

Somatic gene therapy
  • Somatic gene therapy targets specific body cells, so changes are not inherited and are often short-lived.
Ex vivo
  • Ex vivo blood or bone marrow cells are removed, a virus inserts the gene, the cells are grown and returned by injection into a vein.
In vivo
  • In vivo the vector inserts the gene into cells inside the body.
Germline therapy
  • Germline therapy altering germ cells would be permanent and inherited, so it is illegal in humans.
    • (gametes or an early embryo)

Part 4 of 8Severe combined immunodeficiency (SCID)

Severe combined immunodeficiency (SCID)
Caused by inability to make adenosine deaminase (ADA), which is key to immune function.
  • Children can die from common infections and may be kept in plastic bubbles.
Treated by ex vivo somatic gene therapy.
  • A virus transfers a normal ADA allele into T-lymphocytes taken from the patient, which are returned by injection.
  • Not a permanent cure, since T-lymphocytes are replaced, so treatment is repeated every three to five months.
Retroviruses inserted genes randomly, possibly into regulatory sequences, and caused leukaemia, so lentiviruses and adeno-associated viruses are used.

Part 5 of 8Inherited eye diseases

1Leber congenital amaurosis
  • Leber congenital amaurosis inherited blindness from damage to retinal light receptors.
  • It affects children from birth; by their 20s or 30s the person is blind; there is no cure.
  • At least 18 mutated genes cause the group of diseases.
2
In vivo somatic gene therapy injects adeno-associated viruses carrying the normal allele into the retina.
3
All treated patients showed improved eyesight.
4
Photoreceptor cells are long-lived, so the effect lasts.

Part 6 of 8Cystic fibrosis

Cystic fibrosis
The normal CFTR allele is delivered to airway epithelium by liposomes in an aerosol or by viruses.
The effect is short-lived because the epithelial cells are replaced and the immune system responds.
Treatment must be repeated.

Part 7 of 8Gene editing and limits

Gene editing and limits
CRISPR-Cas9 uses a guide RNA to direct the Cas9 nuclease to cut DNA at a chosen site, so the faulty allele can be repaired.
Limits of gene therapy:
  • The allele may not be expressed highly or for long enough.
  • Random insertion can disrupt a gene or activate an oncogene.
  • The body may react to the vector.
  • Diseases involving several genes cannot be cured by adding one allele.
  • Treatments are expensive.

Part 8 of 8More from the specification

Cystic fibrosis
  • A recessive condition causing thick sticky mucus.
Genetically modified
  • Having had genes from another organism inserted.
Recombinant
  • Offspring with a new combination of characteristics formed by crossing over.
Screening
  • Testing blood or people to find infection.
Protein
  • A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
Insulin
  • A pancreatic hormone that lowers blood glucose.
Medicine
  • A drug used to treat illness.

Key terms

TermMeaning
Gene therapyAltering genetic material to treat or cure disease.
Somatic gene therapyGene therapy in body cells, not inherited.
Germline gene therapyAlteration of gametes or embryos, inherited.
Ex vivoCells treated outside the body.
In vivoGene delivered to cells inside the body.
LentivirusVirus vector that inserts randomly but can be modified not to replicate.
Adeno-associated virusVirus vector that does not insert genes into the host genome.

Avoid these mistakes

1
Saying somatic gene therapy is permanent — it is short-lived because cells are replaced.
2
Saying germline therapy is legal — it is illegal in humans.
3
Forgetting that early retroviral vectors caused leukaemia.
4
Saying gene therapy removes the faulty allele — it adds a functional allele.

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