Re: Can Germline Gene Therapy Cause mutation

6.1.99

Hi Rafe,

Germline gene therapy would alter the offspring's characteristics, not the 
parents.  As it is usually imagined, cells would be removed from the 
parent, altered, fertilized in vitro, then reimplanted in the mother.  
Therefore, the parents would never actually have the genetic change 
themselves.  

Germline gene therapy in humans is still science fiction but the science 
is progressing rapidly in other animals.  We'll see this technology used 
increasingly to create animals expressing traits we want.  For instance, 
cows that produce human factor VIII in their milk that can be purified and 
used for humans with factor VIII deficiency.  

The state of the art transgenic mammalian system right now is mice.  A 
transgenic mouse is made as follows:
1) embryonic stem cells (ES) cells extracted from a mouse (the "parent") 
are grown in culture.
2) ES cells are transfected with genetic construct expressing gene of 
interest
3) transfected ES cells are added into a embryo (from different parents) 
at the blastula stage, embryo is implanted in a host mother
4) genetic mosaic baby mouse is born made up of cells from original embryo 
and transfected ES cells
5) test multiple such mosaic mice to find one where ES cells make up 
germline
6) mice where ES cells make up germline are breed to create mouse carrying 
transgenic change in all cells

Obviously, the strategy used for making transgenic mice is incompatible 
with human germline gene therapy.  We can't have a mosaic generation, 
etc.  So, change will need to be made so that it is complete in the first 
generation (F1).

Hope this answers your question and gives you a sense of the current 
technical challenges of applying germline gene modification to animals.

For information on how this is being commercialized see:
www.genzyme.com/transgenics

Best wishes,

Jim McCarter MD,PhD
Genome Sequencing Center

mccarter@genetics.wustl.edu