Gene replacement and DNA repair [was Re: The fountain of youth.]

Robert Bradbury bradbury at sftwks.UUCP
Thu May 14 08:50:33 EST 1992


In article <9205111852.AA24470 at rust.zso.dec.com> french at RUST.ZSO.DEC.COM writes:
>
>> Once gene therapy is an off the shelf process you will be able 
>> to replace any of your alleles with the "optimal" one for your 
>> lifestyle ...
>
>How can broken genes be replaced without also messing up the gene
>expression state of somatic cells?    For example, if my insullin 
>producing gene is broken, I want to replace it and have the 
>new gene turned on in my pancreas but turned off elsewhere.
>
The current model on which much gene therapy is based is to take the
gene and stick into a retrovirus vector which contains the retrovial
packaging sequences but not the retroviral genes themselves.  This
is usually done in a specially designed cell line.  The result is
a virus package which will transfer the gene to only those cell types
which have certain receptors.  The reverse transcriptase from the virus
converts the viral RNA to DNA which integrates somewhere in the host DNA.
People have in the past been worried about this integration transforming
the cells but that doesn't seem to happen at a very high rate.

This is great for something like diabetes, you only need a "working"
insulin gene with its normal promoters, you don't need to replace
the defective insulin gene.  It is not good for some tumors where
you need to replace a gene which is promoting cell division or
Alzheimer's where you may need a substitute for a defective protein.

There are several approaches to this.  One would be to develop DNA
viral vectors packaged with proteins which mediate homologous
recombination between the DNA in the viral package and your defective
gene.  Another would be to add a gene generating anti-sense mRNA to
disable the defective gene and then add the defective gene seperately.
You avoid much of the regulatory problem by using viral vectors which
target only the cell types which are defective and you try as much
as one can given our limited knowledge to include promoters/repressors
which are only active in those cell types.

>In any event, you would probably have to replace all of your genes
>periodically (assuming that every cell in the human body contains
>a broken gene by age 80).   I find it difficult to see how it would
>be possible to replace your entire genome without also disrupting
>the gene expression state of your somatic cells.
>
You don't need to replace the entire genome, you only need to replace
the specific "active" genes in the cells in which they are important.
I envision the point where we will develop "pseudo-artificial mini
chromosomes" which contain complete biochemical pathways under common
regulatory control which are packaged in such a way so as to be
delivered to specific cell types.  You don't have to mess with
a person's normal DNA, you simply need to add those genes which
do the proper thing when detecting specific signals in specific cells.
Now, if you favor the disdifferentiation theory of aging (that aging
is caused in part or whole because cells lose their differentiation)
we are going to have to come up with a way to keep genes which are
normally off off.  But the anti-sense RNA I mentioned above give
you one way to do this, I'm sure that others will be developed
as we understand the normal means by which cells turn off genes
during development.

>Furthermore, I suspect that the easier route to immortality will be 
>to prevent DNA errors from accumulating by building more sophisticated 
>error correction machinery into our cells.  
>
Yes, of course, but this doesn't help the people who are alive now
and may be in need of "renewed" DNA when they are elderly and the
techniques become available to provide this.

>Meiosis seems to provide the right model for how we might engineer
>such a DNA repair mechanism.  The following excerpts on
>recombinatorial repair and meiosis from the  text "Aging, Sex, and 
>DNA Repair" are interesting:
>  ... book excerpts from pgs 255, 259

Yes, there are certainly very active repair process in mitosis during
S and G2, during meiosis in gamete production and during early
development when metabolic rates are very high.

The key thing about mitosis and meiosis is not so much that they have
higher DNA repair activity but that they make all of the DNA available
for DNA repair to occur which is something that doesn't happen in
non-dividing cells where much of the DNA is turned off by being
packed into protective structures.  There is a distinct difference
in the speed with which different species repair "inactive" DNA.
Longer lived species seem to be faster at making inactive DNA
available to repair proteins.  This makes sense if you think
of aging as a process whereby DNA damage is occuring to genes
which are "normally" off but are sometimes required by a cell.
(In aging one thing that consistantly happens is you lose your
ability to respond to stress and you take longer to return to
homeostasis).  It would appear that to have a long lifespan
you want to do a better job repairing inactive genes for those
times when they are called into action.  It doesn't do you much
good to turn on a burned out lightbulb!
-- 
Robert Bradbury			uunet!sftwks!bradbury




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