Chapter 19

Gene Therapy

Gene therapy:

   Is the treatment of genetic disease by delivering replacement genes to correct the genetic deficiency.


Gene therapy: Three Examples:
1. adenosine deaminase deficiency

Severe combined immune deficiency can be caused by adenosine deaminase (ADA) deficiency. 


Lack of ADA blocks the breakdown of metabolic toxins to uric acid. 


Presence of the toxins destroy T cells and causes susceptibility to infections and cancer.


Replacement of ADA in individuals genetically deficient was attempted.



Injections of PEG-ADA (enzyme from cow stabilized with polyethylene glycol) given to first child in 1986:

Increased ADA levels

Increased T cell survival

Improved immune function


The ADA white blood cells from an ADA deficient patient is replaced with a functional copy and the cells are returned.

Increased T cells present with functioning ADA gene.


Umbilical cord blood is used to isolate stem cells.  Stem cells are treated to replace mutated ADA allele with normal allele and returned. 

T cells with normal allele accumulate in patient.

Gene therapy:

2. ornithine transcarbamylase

Deficiency of ornithine transcarbamylase  (OTC) is inherited as an X-linked recessive mutation.  

OTC normally breaks down amino acids present in dietary protein.

Lack of OTC allows build up of ammonia which damages brain function.

Low protein diets and ammonia-binding drugs are used to treat OTC deficiency.

Clinical trials to treat OTC deficiency were established using adenovirus as a vector for the normal OTC gene.

Adenovirus has been used in over 330 gene therapy trials in 4,000 patients.

Jesse Gelsinger had a mild OTC deficiency.  When he turned 18, Jesse volunteered for the OTC gene therapy trial and was accepted as the 17th volunteer. 


Four days after gene therapy Jesse died of massive immune reaction and associated complications.

OTC and Jesse Gelsinger

Jesse’s story led to immediate halt of this trial and review of all gene therapy trials to determine what happened and how to prevent future tragedies.


Autopsy results suggest that Jesse had had an infection with parvovirus which may have sensitized his immune system so it attacked the adenovirus used to treat the OTC.


The adenovirus did not target liver cells as intended but macrophages, immune cells that set off an immune response.


Reform of gene therapy trials continues. 

Weekly reports of adverse effects have been instituted.


 3.  Canavan disease is caused by aspartoacylase enzyme deficiency


Neurons release N-acetylaspartate (NAA).

NAA is normally broken down by the enzyme aspartoacylase to harmless components. 

Enzyme deficiency creates NAA buildup which destroys oligodendrocytes.

Lack of oligodendrocytes prevents myelin formation.

Without myelin, neurons cease functioning.

Causes brain degeneration in children.

First observed as developmental delay, inability to stand or sit, poor muscle control and vision and lack of reaction to surroundings.


It is a good gene therapy candidate:


Gene and protein are well known.

Window of time exists for treatment.

Only the brain is affected.

Brain scans can be used to monitor treatment.

There is no existing treatment.



Gene therapy trials

There are three types of experiments:


Ex vivo gene therapy 

cells are altered outside of patient’s body and returned.


In situ gene therapy 

healthy gene and vector DNA is injected at isolated site on body that is easily accessible (e.g. skin tumor).


In vivo gene therapy

 vector and gene is introduced into the body systemically.

Requirements for approval of a clinical trial

Knowledge of defect and how it causes symptoms


An animal model


Success in growing human cells in vitro


No alternative therapies or group of patients for whom therapies are not possible


Safe experiments


Bioethical concerns about gene therapy

Does the participant of the trial truly understand the risks?


If the gene therapy is effective, how will recipients be chosen assuming it is expensive?


Should rare or common disorders be the focus of gene therapy trials?


What effect should deaths among volunteers have on research efforts?

Scientific concerns about gene therapy

Which cells should be treated?


What proportion of target cells must be corrected to alleviate or halt disease progression?


Is overexpression of the therapeutic gene dangerous?


Is it harmful if the altered gene enters other types of cells?


How long will the treated cells function? (How long lasting is the treatment?)


Will the immune system attack the altered cells?


Germline versus somatic gene therapy

Somatic gene therapy

Involves alteration of the DNA of somatic cells implicated in the disease.

Changes are not heritable.


Germline gene therapy

Involves alteration of the DNA of a gamete or fertilized egg.

Changes are heritable; passed from treated individual to offspring.

Currently there is no germline gene therapy done in humans.

Sites of somatic gene therapy

Gene therapy for cancer treatment

   Two strategies have been developed using recombinant methods for the treatment of cancer.


Suicide gene therapy involves expressing a protein in cancer cell that will kill them.


Cancer vaccines allow tumor cells to produce proteins activating the immune response.

Gene therapy for cancer treatment

Gene therapy for treating skin cancer

Delivery systems for gene therapy

Adeno-associated virus (AAV) integrates into specific chromosomal sites infecting both dividing and nondividing cells.  Expression is long-term, nontoxic.  Can carry small genes. e.g. cystic fibrosis, thalassemias


Adenovirus (AV) is a large virus that can carry large genes.  It infects dividing and nondividing cells.   Expression is transient.  Invokes an immune system reaction. e.g. cystic fibrosis, hereditary emphysema


Herpes virus can infect nerve cells and has long term expression. e.g brain tumors


Retroviruses provide a stable but imprecise integration method.  Infects predominantly dividing cells.  Expression is long term and nontoxic.  This method has the longest history. e.g. Gaucher disease, HIV infection, ADA deficiency.


Drug treatment can modify genetic disease