Re: How would an alteration of a cell's DNA affect the cell?

Question: How would an alteration of a cell's DNA affect the cell?
From: James L. Gouldsmith Email: AdrienVeidt@yahoo.com
Grade: nonaligned
City: Savannah, State/Prov.: Georgia Country: USA
Area: Genetics Message ID Number: 929171605.Ge

In detail, say a new color-determining gene were to replace the
previous gene in a person's right eye by how I understand gene
therapy to operate.  It is my understanding that every cell in
the human body is replaced every seven years.  After seven years,
would the right eye now have a different color?  If so, what
(other than the actual knowlege of how to effect the mechanics)
would prevent a person from becoming an entirely different person
through an enormous, entire-DNA code replacement?  Will it be
possible to improve the current height of an adult human through
such a process?  Grow out of diabetes?  Change gender?  Change
species?

Dear James:

Greetings!  Wow!  Your question is insightful and you have touched on 
several complicated issues regarding gene therapy.  I will try to address 
most of them here, but only in broad terms.  (I'll warn you now that I have 
written a lot here!!)  If you are interested in more "nitty-gritty" 
details, I shall refer you to some sources where you can find more in-depth 
information.

Generally speaking, gene therapy is a process whereby exogenous DNA is 
delivered to human cells as a treatment for a particular disease.  In some 
cases, the new DNA contains a "good" copy of a certain gene because the 
patient has a defective copy contained in his or her cells.  This is the 
theory behind the use of gene therapy to treat metabolic diseases, many of 
which are due to a genetic mutation in a single enzyme required for normal 
metabolism.  In other cases, DNA is being delivered not because there are 
specific mutations in the patient's own gene, but rather because the 
patient's copy isn't actively making protein in a large enough quantity, at 
the appropriate time and location.  This is the rationale for treating 
patients with blocked coronary arteries in their heart, often causing 
severe chest pain ('angina'), with a gene for the growth factor VEGF 
(Vascular Endothelial Growth Factor).  Ideally, this growth factor will 
stimulate the growth of new blood vessels in the heart and relieve 
patients' chest pain.  In this situation, there isn't anything specifically 
wrong with an individual's VEGF gene-there just simply isn't enough of the 
protein in the right place to get the job done.

There are several factors which influence the efficacy of gene therapy, 
only a few of which we are beginning to understand.  Two of the major 
issues are (1) effective gene transfer and (2) stable gene expression.  The 
main hurdle regarding the first point is how the gene of interest will be 
introduced into a cell, so it can travel to the nucleus and ultimately 
incorporate into the patient's DNA.  "Naked DNA", meaning just the gene 
alone, does not find its way into cells very efficiently.  For the most 
part, genes need to be targeted to cells using "carrier DNA" known as a 
vector.  Many methods of gene delivery are currently focused on using 
viruses as vectors for gene therapy.  Normally, during the process of viral 
infection, a virus must get its DNA into host cells and "hijack" the cells' 
metabolic machinery in order to propagate itself.  Thus, since viruses 
already have these capabilities, they are a logical choice for use in gene 
therapy.  By inserting the gene of interest into the genome of a particular 
virus, one can create a package for gene delivery.  While simple in theory, 
this approach is not without its drawbacks.  The second point, achieving 
stable gene expression, simply means that the exogenous gene needs to 
insert itself into the patient's own DNA in order to be continually 
converted into protein.  This is not a trivial concern, because only a 
small percentage of the cells that receive gene therapy will incorporate 
the piece of DNA into their genome.  Another issue to consider is whether 
the gene will be delivered to a patient's cells while they are still in the 
body ('in vivo'), or alternatively, whether a small number of cells will be 
removed from the body, propagated in a petri dish ('in vitro'), and then 
treated with the gene of interest.   Then, these cells can be reintroduced 
into the patient.  

To address some of the specifics you mention in your question, I would 
first like to dispel a common myth, the origin of which I am uncertain.  
Every cell in the human body is not replaced every seven years.  There are 
many cell types in the human body, each with different capabilities for 
renewal.  For example, the muscle cells in your heart cannot replace 
themselves-this is one reason why a heart attack can inflict such severe 
damage to the heart.  If the heart muscle cells are deprived of oxygen for 
too long (due to blocked coronary arteries, for instance), they die and 
cannot regenerate, leaving the heart weakened and structurally compromised. 
 The neurons in your brain and spinal cord are another example of a cell 
population that can rarely, if ever, regenerate.  In contrast, the cells 
that line the gastrointestinal tract proliferate rapidly-the lining of the 
gut is turned over about once every three days.  With regard to the 
pigmented cells in the iris which give eyes their color, it is likely that 
they are also a non-renewing cell population.

What would happen if you replaced a color-determining gene in the eye with 
another color-determining gene? As far as I understand, we do not yet have 
the technical capabilities for "gene replacement" in humans.  There is a 
subtle, but important distinction between gene therapy and gene 
replacement.  With gene therapy, a gene is delivered which will ultimately 
integrate randomly into the genome of a particular cell.  Exactly where it 
inserts itself isn't a major concern, at least in principle.  Currently, we 
do not have the ability to target where a gene integrates-for instance, 
whether it jumps into the site occupied by a color-determining gene or some 
other gene. 

Essentially, what prevents a person from becoming an entirely different 
person through an "enormous, entire DNA code replacement" is the 
limitations of gene therapy as a technique.  The odds of getting stable 
gene expression in a cell population are rather low.  However, if one can 
achieve a decent level of gene expression, it is then important to consider 
the types of cells that are modified.  If, for example, you hit a cell that 
is rapidly replaced, such as the cells that line the gut, this can be a 
problem.  The cells may be genetically modified, but since they are 
sloughed from the gut lining every three days, they will be lost.  One way 
to remedy this is to try and select a special subset of these cells, often 
referred to as stem cells, which have special properties.  Stem cells are 
like cellular Xerox machines-they are constantly dividing and renewing the 
cells that die.  So, if one could effectively target a particular stem cell 
with gene X, then this cell will continue to make copies of itself and 
replace the dying cells with new cells, all of which will contain gene X.  
However, this is much easier said than done because all tissues in the 
human body do not have stem cells, or at least, we haven't identified them 
yet! 

I don't think gene therapy will be a feasible technique for altering the 
current height or gender of an individual.  These are qualities that are 
most certainly under the control of many genes and are acquired during a 
long process of growth and development.  Along those lines, I do not 
foresee gene therapy as a means for changing species in the near or distant 
future.  It will be a significant triumph if we can successfully implement 
techniques for correcting single gene disorders.  These other things are 
complicated entities under genetic as well as environmental control.  
However, gene therapy may hold promise as a treatment for diabetes.  While 
this may not be a single gene disorder, it is essentially due to the lack 
of control over blood glucose levels.  In some forms of diabetes, this is 
due to the lack of insulin-producing cells in the pancreas.  Therefore, it 
may be possible to deliver the insulin gene to some other cells in the body 
to restore the proper regulation of blood glucose.  

Whew!  I hope this information helps (and there certainly is a lot of it!). 
Please feel free to email me with further questions!  I have listed a 
couple references below.

Nikki
nmdavis@fas.harvard.edu

Introduction 
to Gene Therapy, a website for a course at Vanderbilt University.

An article on gene therapy in Scientific 
American in October 1996-perhaps a bit out-of-date, but still worth 
reading.

The American Society for Gene Therapy

Towards gene therapy of diabetes mellitus.  Molecular Medicine Today.  1999 
5(4):165-171.
This article describes some of the approaches to gene therapy for diabetes. 
If you are interested and can't find it at your library, contact me and I 
will send you a copy.