Medical Biotechnology

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Medical Biotechnology

• Viruses

• Virology The study of viruses
• The structure of viruses
• How viruses infect host cells
• How viruses are used in gene therapy

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Viruses The stuff of nightmares

• Bring diseases such as small pox, polio, AIDS,
  some cancers, influenzas, the bird flu.
• Evolved from plasmids-the small circular sections
  of DNA from bacteria
• Are so simple, but can control so much
• BUT Are now the messengers of hope for those in
  need of gene therapy
• Read Measles Structure article now

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Structure

• Classification depends upon
• Whether or not it has RNA or DNA
• Whether or not it has an envelope
• The structure of its capsid (protein coat)
• It is 1/10 of the size of a bacterium
• Carries only enough nucleic acid for a dozen or
  so genes

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Are they alive?

• No!!
• Viruses do not meet all the criteria for the
  definition of a living organism.
• They lack a metabolism, and can only reproduce by
  invading a living cell.
• Once inside that cell they literally override the
  genetic instructions of that cell and make new
  viruses.

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The retrovirus

• HIV is an example
• HIV has as an envelope for protection
• Has reverse transcriptase makes DNA from RNA
• Has overlapping genes-a precursor RNA will
  duplicate, split and allow for 2 proteins to be
  made.
• Also polio, smallpox, some cancers

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The adenovirus

• Contains double- stranded DNA
• Terminal proteins add stability to DNA
• Protein filaments on outside helps virus get
  inside.
• Cause common cold and upper respiratory tract
  infections.

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Viral life cycle

• Regardless of virus type, the overall procedure
  is the same Viral nucleic acid will code for
  the production of new capsids and new viral
  nucleic acid. How they enter and leave the cell
  is dependent upon the type of virus.

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Retrovirus reproduction

• The reverse transcriptase means that an
  additional transcription must take place. The
  envelope protecting the new viruses comes from
  the host cell

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Gene therapy

• Viruses are excellent at entering cells. Given
  that they inject their nucleic acid into the
  host, they make the perfect vector for
  delivering desirable DNA.
• However
• Ability to replicate their own DNA must be
  blocked
• The desired gene must be inserted into the genome
  so that it will function, but not interfere with
  the viruses ability to make its capsid.
• These challenges are able to be met.

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Gene therapy, cont

• Adenovirus AD-2 and retrovirus MuLV have been
  used in over 90 of all trials.
• Big challenge is to get the quantity of viruses
  correct Too few will not let enough cells have
  the correct gene. Too many can cause a spill
  over and cause a hyperimmune response.

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Drawbacks

• After about 2 weeks the expression of the
  therapeutic gene begins to decline, because it
  has not been integrated into the host cell
  chromosome.
• Retroviruses have an increased ability to invade
  immune cells. The therapeutic gene is
  incorporated into the host cell, but the
  expression of the desired gene is frequently too
  low to cure the patient.

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Drawbacks, cont

• If there is a mistake using an integrating virus,
  there is no way to get that gene out.
• Having a virus inside cells could lead to
  recombination if any other virus is present, and
  therefore a replication competent virus that has
  become pathogenic/deadly.
• An Ebola/HIV hybrid has worked at increasing the
  therapeutic protein production, but the scare
  factor with this is high.

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Ashi DeSilva

• 1st gene therapy trial
• SCID (Severe combined immunodeficiency)
• Combined both B T cells
• Received abnormal ADA gene from each parent
• Codes for an enzyme used to recycle nucleotides
• Her lymphocytes (T B cells) were grown in
  culture
• Infused with a good copy of the gene via
  retrovirus
• Now stem cells being used

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Jesse Gelsinger

• OTC (Ornithine Transcarbamylase) Deficiency
• Removes ammonia from body
• Why do we have ammonia?
• Used adenovirus as a vector to bring in good gene
• Resulted in a viral infection
• Jesse passed away
• Gene therapy came to screeching halt!