Transcript Genetics & Gene Therapy

Genetics & Gene Therapy
Chapter 30
‫وسام عبد الرؤوف العايدي‬
220060666
 The
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study of viral genetics :
(1) mutations and their effect on replication and
patho- genesis
(2) the interaction of two genetically dis- tinct
viruses that infect the same cell. In addition,
(3) viruses as vectors in gene therapy and in
recombi- nant vaccines,
viral mutations
Mutations in viral DNA and RNA occur by:
 (1) base substitution,
 (2)deletion,
 (3) frameshift
viral mutations
(1)The most important practical use of
mutations is in the production of vaccines
containing live, attenuated virus.
These attenuated mutants have lost their pathogenicity but have retained their
antigenicitymthey therefore induce immunity
without causing disease.
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(2) Antigenic variants such as those that occur
frequently with influenza viruses, which have an
altered surface protein and no longer inhibited
by host preexisting antibody. The variant can
thus cause disease, whereas the original strain
cannot.
(3)Drug-resistant mutants, which are
insensitive to an antiviral drug because the target
of the drug, usu- ally a viral enzyme, has been
modified
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Conditional-lethal mutations
These mutations function normally under
permissive condi- tions but fail to replicate or to
express the mutant gene under restrictive
conditions
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For example, temperature- sensitive conditionallethal mutants express their phe- notype
normally at a low (permissive) temperature, but
at a higher (restrictive) temperature the mutant
gene product is inactive..
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To give a specific example, tempera- ture- 
sensitive mutants of Rous sarcoma virus can
trans- form cells to malignancy at the permissive
temperature of 37°C.
When the transformed cells are grown at the 
restrictive temperature of 41°C, their phenotype
reverts to normal appearance and behavior. The
malignant phenotype is regained when the
permissive temperature is restored
Note that temperature-sensitive mutants have 
now entered clinical practice.
Temperature-sensitive mutants of influenza 
virus are now being used to make a vac- cine,
Defective interfering partides
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Some deletion mutants have the unusual property
of being defective interfering partides. They are
defective because they cannot replicate unless the
deleted func- tion is supplied by a "helper" virus.
They also interfere with the growth of normal
virus if they infect first and preempt the required
cellular functions. Defective in- terfering particles
may play a role in recovery from viral infection;
they interfere with the production of progeny
virus, thereby limiting the spread of the virus to
other cells.
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INTERACTION
(1)Recombination 
(2)Complementation
(3)Phenotypic mixing
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Recombination
(1) Recombination is the exchange of
genes between two chromosomes that is based
on crossing over within regions of significant
base sequence homology.
DNA 
RNA 
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Reassortment
Reassortment is the term used when viruses 
with segmented genomes, such as influenza
virus, exchange segments. This usually results in
a much higher fre- quency ofgene exchange than
does recombination. Reas- sortment of
influenza virus RNA segments is involved in the
major antigenic changes in the virus that are the
basis for recurrent influenza epidemics.
Complementation
(2) Complementation can occur when either one
or both of the two viruses that infect the cell
have a muta- tion that results in a nonfunctional
protein
The nonmutated virus "complements" the 
mutated one by making a functional protein that
serves for both viruses. Complementation is an
important method by which a helper virus
permits replication of a defective virus
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One clinically important example of
complementation is hepatitis B virus providing
its sur- face antigen to hepatitis delta virus,
which is defective in its ability to produce its
own outer protein.
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This phenomenon is the basis for the
complementa- tion test, which can be used to
determine how many genes exist in a viral
genome.
Phenotypic mixing
(3) In phenotypic mixing, the genome of virus 
type A can be coated with the surface proteins
of virus type B (Figure 30-2).
This phenotypically mixed virus can infect cells 
as determined by its type B protein coat.
However, the progeny virus from this infection
has a type A coat; ir is encoded solely by its type
A generic material.
An interesting example of phenotypic mixing is
that of pseudotypes, which consist of the
nucleocapsid of one virus and the envelope of
another.
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Pseudotype composed of the nucleocapsid of 
vesicular stomatitis virus (a rhabdovirus) and the
envelope of human immunodeficiency virus
(HIV, a retrovirus) are currently being used to
study the immune response to HIV.
Gene Therapy
Retroviruses are currently being used 
as vectors of the gene encoding
adenine deaminase (ADA) in patients
with immunodeficiencies resulting
from a defective ADA gene.
Retroviruses are excellent vectors
because a DNA copy of their RNA
genome is stably integrated into the
host cell DNA and the integrated genes
are ex- pressed efficiently.
Recombinant Vaccines 
Recombinant viral vaccines contain viruses that
have been genetically engineered to carry the
genes of other viruses. Viruses with large
genomes, eg, vaccinia virus, am excellent
candidates for this purpose..
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To construct the recombinant virus, any vaccinia
virus gene that is not essential for viral
replication is deleted, and the gene from the
other virus that encodes the antigen that dicits
neutralizing antibody is introduced.
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For example, the gene for the surface antigen of
hepatitis B virus has been introduced into
vaccinia virus and is expressed in in- fected cells.
Recombinant vaccines are not yet dinically
available, but vaccines of this type promise to
greatly improve the efficiency of our
immunization programs
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