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.

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

1. any three from:
2. [1] donor not needed ;
3. [1] immediate effect ;
4. [1] idea that easy to administer treatment ;
5. [1] no immune response / no rejection ;
6. [1] less invasive ;
7. [1] AVP; e.g. lower risk to health / cheaper/quicker
8. any four from:
9. [1] given a drug to increase number of stem cells (in bone marrow) ;
10. [1] ref. to virus / vector, containing, normal / healthy, allele ;
11. [1] remove, bone marrow / stem cells ;
12. [1] mix stem cells with, viral / vector (to allow transfer of normal allele) ;
13. [1] radiotherapy / drug, to make space in bone marrow / to kill stem cells (in bone marrow) ;
14. [1] (transduced stem) cells, infused / injected, into blood ;
15. [1] (lymphocytes) produce functioning ADA ;
16. [1] 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
17. any three from:
18. [1] expensive ;
19. [1] cure / long term treatment / no longer chronically ill / better quality of life ;
20. [1] no need for regular, injections / treatments or only a single treatment ;
21. [1] cultural/religious, objections ;
22. [1] no donor needed ;
23. [1] ref. more money available to health system in the long term;
24. [1] may cause cancer ;
25. [1] stressful ;