Recombinant DNA technology can be used in the diagnosis of several diseases. Most of the tools and techniques are involved in the construction of probes, i.e., short segments of single stranded DNA attached to a radioactive or fluorescent marker. Such probes are now used for identification of infectious agents, such as food poisoning Salmonella, pus forming Staphylococcus, hepatitis B virus, HIV, etc.
By testing DNA genetic disorder carrier parents, their genotype can be determined and predictions can be made about their afflicted children.
In humans, gene therapy is used to replace a faulty gene by a normal healthy functional gene.
The intensive investigation has been done in several human diseases, such as sickle cell anaemia caused by single mutations and on the other hand Severe Combined Immuno-Deficiency (SCID)
Example of SCID is mentioned here:
SCID is caused by a defect in the gene for the enzyme adenosine deaminase (ADA) in 25 per cent of the cases.
The patients having SCID do not have functioning T-lymphocytes and therefore, they cannot mount immune responses against invading pathogens.
The ideal approach for SCID treatment is to give the patient a functioning ADA that breaks down toxic biological products.
For this purpose, lymphocytes, a kind of white blood cells are extracted from the bone marrow of a SCID patient.
Now, a good copy of human gene encoding this enzyme (ADA) is introduced into these cells. This is done by exploiting the properties of special viruses called retroviruses, as vector.
This vector (retrovirus) is engineered to incorporate the ADA gene which has the packing sequence but no viral genes.
Now, co-infection of the bone marrow stem cells, removed from a SCID patient, with this vector and helper virus produces new virus particles with ADA gene inserted inside, but lacks the packing sequence.
This equips these cells with a good copy of the gene, in addition to the existing faulty copy of the gene.
Now, the treated cells are re-injected to the patient’s bone marrow.
The lymphocytes subsequently produced by these bone marrow stem cells have a fully functional ADA gene. These cells can play their normal role in patient’s immune system.
Such molecular surgical approach, is confronted by many practical challenges but even then this provides a ray of hope to the medical world.