Recombinant DNA Advisory Committee - 2/10-11/92 
all of the cells express the HSV-TK gene, the researchers could selectively ablate 
problem-causing cells by administering ganciclovir. Using the TK function as an ablatable 
marker, introduces a new concept in safety regarding gene transfer into human somatic 
cells. 
Dr. Mclvor explained that HSV-TK is not just a standard TK gene which is being inserted 
into the cells, but a fusion protein between TK of HSV and hygromycin 
phosphotransferase (hph), which is similar functionally to the neomycin resistance gene. 
HyTK is a fusion protein that has two separate functions. The retrovirus is a standard 
LTR-based construct similar to LNL6 and GIN. The investigators have established that 
this fusion protein will be effective. 
Another concern relates to cells expressing TK; there will be a low but measurable 
mutation frequency to TK-deficiency. In somatic cell genetic studies, this measurement 
turns out to be on the order of 1 in 10 6 cells. If there was a mutation frequency to 
ganciclovir resistance of 1 in 10 6 cells among those T-cells that would be reinfused, that 
frequency would probably not cause problems in these patients. 
As far as the retroviral vector is concerned, it is a standard retroviral vector, similar to 
LNL6. There are some additional sequences which have been removed in comparison to 
LNL6 that probably make the vector safer in terms of recombinational activities that could 
lead to the establishment of replication-competent helper virus in the producer cell line or 
in the transduced cell clones. 
T-cell clones represent a new target cell population other than heterogeneous lymphocytes 
which have been reviewed previously. Since extensive in vitro manipulation could change 
the character of these cells and pose a risk to the patient, the investigators have inserted 
an ablatable gene. Therefore, there should not be any increased risk to the patient. The 
original protocol did not include in vitro data which substantiated the ability to generate 
and transduce T-cell clones. However, the investigators did present an in vivo experiment 
in mice demonstrating that infused congenic lymphocytes were transduced with HyTK 
construct and subsequently ablated with ganciclovir administration. 
Another issue relates to the administration of ganciclovir to these patients. BMT 
recipients frequently receive ganciclovir for the treatment of cytomegalovirus (CMV) 
infections. Following a discussion at the HGTS meeting, these patients will not be given 
ganciclovir prophylactically, as is the practice for some BMT patients. However, if the 
patients develop a CMV infection, these patients will be given ganciclovir immediately, 
although this treatment would obviously preempt continuation of the research protocol. In 
conclusion, the protocol introduces a new concept by inserting a gene with an ablatable 
function into a cell population that could have some therapeutic benefit to the patient 
Recombinant DNA Research, Volume 15 
[355] 
