Gene Therapy Flashcards
What are non-viral delivery (NVD)?
These are artificial means of delivery to target cells. They include mechanically delivered ‘naked’ DNA and ‘facilitated’ DNA
What modifications have been produced in non-viral DNA?
Some hybrid vectors have been produced: DNA + lipids + viral proteins for targeting
What limit does viruses have but not NVD?
Unlike viruses, NVD usually has no DNA size capacity limit
What are most NVD based around?
cationic agents (positively charged) which complex (and sometimes cause compaction) of negativelycharged nucleic acids
What are the two main categories for NVD?
cationic liposome or cationic polymer
What are the names Lioplex and polyplex given to?
complexes formed between cationic agents and DNA/RNA
What are some delivery methods used to deliver naked DNA to cell targets?
microinjection, electroporation & biolistics
What has electroporation been shown to induce?
transient gene expression in tissue (e.g. muscle), before DNA is degraded
What has In vivo electroporation been used to deliver?
NAs to skin, muscle, brain and liver in situ
How does electroporation work?
- Clusters of electrodes are inserted into target tissue, which emit controlled electrical pulses
- Creates trans-membrane potentials sufficient to allow naked DNA into cells, via transient pores
What is a disadvantage of electroporation?
Method causes temporary tissue damage then regeneration
What does Biolistics entail?
Biolistics entails projection of nucleic acid molecules into cells or tissue
How does Biolistics work?
The ‘gene gun’ uses a high velocity gas jet (e.g. helium) to accelerate NA-coated metal microparticles (e.g. gold, tungsten)
What is Biolistics particularly suitable for?
Epithelial GT
What do you need to assess when using Biolistics?
The potential damage vs recovery
What was the first gene therapy trial mailing to treat?
adenosine deaminase (ADA) deficiency
What is adenosine deaminase (ADA) deficiency?
It is a rare, monogenic, autosomal (not sex-linked) recessive disorder which leads to severe combined immunodeficiency or SCID
What is ADA?
ADA is an enzyme that plays an important role in the purine salvage pathway
What does a lack of ADA lead to?
Lack leads to high levels of the metabolite dATP which is toxic to immature T-lymphocytes, blocking maturation and inducing apoptosis in thymus → impaired immune system and likely infection
What is the usually therapy of ADA deficiency?
Usual therapy is bone marrow transplant (matched donor) or bovine ADA supplements (helps but not a solution)
What is deoxyadenosine?
an intermediate in the synthesis and degradation of DNA.
What does ADA do?
Adenosine deaminase (ADA) enzyme converts deoxyadenosine to nontoxic deoxyinosine
What happens within a cell during ADA deficiency?
deoxyadenosine and its toxic product deoxyadenosine triphosphate builds up to toxic levels in T (and B) cells, causing immunodeficiency → frequent infections
How was the ADA gene used in gene therapy?
the ADA gene was isolated from chromosome 20 and cloned as a complementary DNA (cDNA) 1.1kb fragment into a retrovirus vector, under control of a strong gene promoter
In ADA-SCID gene therapy, what is used as a vector?
MuLV retrovirus
What was the % efficiency of this retrovirus?
10%
What was the correlation when using this retrovirus?
Correlation between patient response and % transduction
What is cytokines like interleukin 2 used to do?
stimulate T-cell proliferation in vitro, before transduction and re-infusion
How was a patient with ADA-SCID treated?
- Bone marrow sample removed, CD34+ cells isolated treated, rest of bone marrow ablated with chemotherapy.
- CD34+ (cell surface marker for pluripotent haemopoetic stem cells) cells retrovirus/ADAtransduced, then re-infused.
How many causes of SCID are X-linked?
~50%
What have recent rail of ADA-SCID gene therapy used instead of mature T cells?
‘pluripotential’ stem cells from bone marrow
What gene was there a mutation in in the most famous cases of X-linked SCID?
il2rg
What are the strategies gene therapy van take form of?
replacement of defective or missing genes
augmentation of existing genes
sensitising target cells
interference with replication of infectious agents
What are the two main types of gene therapy?
Germ-line gene therapy
Somatic gene therapy
What does germ-line gene therapy involve?
This would involve the insertion of therapeutic genes into the reproductive or ‘germ’ cells (e.g. unfertilised eggs) or into embryos very early in development
What could germ-line gene therapy lead to?
be to treat diseases that could be passed on to subsequent generations and could lead to eradication of some hereditary diseases
What is the problem with germ-line gene therapy?
ethically controversial and no clinical applications of germline GT have been carried out
What is somatic gene therapy?
This involves the introduction of therapeutic genes into specific body (not germ) cells or tissues of a patient. Only this individual is affected
What are the main categories of somatic gene therapy?
Ex vivo gene therapy
In vivo gene therapy
What is somatic GT Ex vivo?
Cells are removed from the patient, genetically modified outside the body then retransplanted (e.g. genetic immunodeficiency/ bone marrow stem cells)
What is are the two forms of somatic GT In vivo?
In situ delivery
In vivo delivery
What is In situ delivery?
where genetic material is directly administered to the target tissue (e.g. cystic fibrosis/ respiratory tract)
What is In vivo delivery?
involves systemic administration of the genetic material (e.g. metastatic cancer)
What may viruses show for different tissues/cells?
some ‘tropism’ for different tissues/ cells (e.g. dividing cells, specific cell type)
What is the most commonly used virus for clinical GT?
adenoviruses
What is done to some adenovirus before GT use?
Ad genes are often deleted to prevent replication (replication-deficient) – can infect cells, but no progeny produced
Others Ad vectors can replicate (replication-competent), but only under specific conditions
What can adenovirus do regardless if replicating or not?
infect a wide range of human cell
What tropism does adenovirus have for?
Respiratory tract (cystic fibrosis GT)
Where is Ad genome maintained?
in the host cell nucleus but outside the host genome (episomal) and is lost over time by nuclease degradation or as cells divide
What will gradual loss of Ad genome from infected cells lead to be done?
Repeated administration
What is a problem for replacement GT but sometimes useful e.g. for cancer GT?
Ads are common viruses, this can lead to deleterious immune reactions
What capacity do most adenoviruses have for inserted DNA?
~7kb
What are retroviruses?
Retroviruses are also commonly used for clinical GT. Most retros are naturally nonpathogenic (HIV is one of exceptions). Murine leukaemia virus (MuLV) most widely used
What do retroviruses infect?
Replicating/dividing cells
What are Lentitviruses?
Lentiviruses (e.g. HIV) are members of the retrovirus family
They can infect nondividing cells
Can be engineered to be nonpathogenic
What do retroviruses have a insert capacity of?
~8kb
What do retroviruses usual have deleted and what does this mean?
There main 3 genes
This lack of genes means that they are replicationdefective (infect cells → no progeny)
What genome do retrovirus have?
ssRNA that is converted to dsDNA (by reverse transcriptase enzyme) and is integrated into the host genome (as a ‘provirus’) by viral integrase Advantage
What is an advantage of retrovirus?
transgene not lost - potential for longterm gene expression
What are the problems of artificial propagation of retroviruses?
the titre of virus is less than that for adenoviruses, thus less virus particles are delivered per volume
Second, RNA viruses are more genetically unstable than DNA ones, and undergo more recombinations and rearrangements.
What do the problems of retroviruses mean?
that during propagation viruses can incorporate retrovirus genes from the packaging cell line and become replication-competent (a particular problem if resultant virus is pathogenic like HIV).
What is the problem with the virus randomly integrating into the host genome?
host genes can be altered by this integration. Most significantly, there is the potential for insertional activation of genes (e.g oncogenes)
What are Adeno-associated viruses (AVV)?
Adeno-associated viruses