reconstitution. Consequently, we may not see significant changes in the 
patient's immune functions prior to one year after therapy. 
One factor that may increase the speed of immunological reconstitution in 
these patients is the fact that the transplant is autologous. Since no 
transplantation mismatches are present, the patient's treated cells should be 
able to grow unaffected by any residual immune functions the patient has. This 
may mean that the totipotential stem cells would be able to contribute to the 
Immune reconstitution through all cell lineages and not just the T cell 
lineage. As has been shown (and forms the basis for replacement therapies) the 
presence of ADA(+) cells can rescue ADA(-) cells to some extent. 
Although Immune reconstitution will probably take a long period of time, 
we would expect that tests for the presence or absence of the transferred ADA 
gene would be positive within weeks of a successful gene transplant. These 
tests are much more sensitive than the Immune function tests and do not require 
the complex Intercellular Interactions required of a normal Immune response. 
Some of the tests are exquisitely sensitive. The polymerase catalyzed reaction 
(PCR) Is able to detect gene transfer In as few as one cell in a million. In 
situ hybridization (ISH) can be used to detect as few as 50 mRNA molecules per 
cell. Since this is essentially an assay of individual cells, as few as 1 
positive cell in a thousand may readily be demonstrated. Both PCR and ISH are 
especially useful tests because few cells are needed to perform the assays, an 
Important consideration since the first clinical trials might involve Infants. 
If we were unable to demonstate any gene transfer by four months after the gene 
therapy procedure, we would assume that the procedure was not successful and 
would then consider alternatives which would include a repeat attempt at gene 
therapy, alternative forms of therapy, or no therapy (except for continued 
careful monitoring and symptomatic care of the patient). 
Recombinant DNA Research, Volume 12 
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