Evaluate how gene therapy delivers or expresses a functional allele to restore a missing protein or pathway, while considering treatment limits in Cambridge A Level Biology questions.
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.
Outline the procedure used for gene therapy treatment of a person with SCID.
any four from:
1 given a drug to increase number of stem cells (in bone marrow) ;
2 ref. to virus / vector, containing, normal / healthy, allele ;
3 remove, bone marrow / stem cells ;
4 mix stem cells with, viral / vector (to allow transfer of normal allele) ;
5 radiotherapy / drug, to make space in bone marrow / to kill stem cells (in bone marrow) ;
6 (transduced stem) cells, infused / injected, into blood ;
7 (lymphocytes) produce functioning ADA ;
8 AVP ;
e.g. (gamma) retrovirus / adeno-associated virus
e.g. tissue from bone marrow, purified / sorted, to obtain stem cells
e.g. cells are grown in culture to check the ADA gene is active