Genetic Disease and Gene Therapy Molly Niska

Gene therapy is the treatment of a genetic disorder by altering a person's genotype.

Genetic Disease Treatment

The process consists of a normal gene being inserted into a person's cells and making them work properly. This is done by the use of vectors (viruses, liposomes, or naked DNA).

SCID (severe combined immunodeficiency)

  • causes immune system to fail, often SCID patients die in infancy
  • inability to make enzyme ADA (adenosine deaminase), used in immune system

SUCCESSFUL GENE THERAPY FOR SCID

1990

  • removed T-lymphocytes
  • introduced normal alleles of ADA with a virus vector
  • replaced cells
  • temporary cure due to short life of cells, continued transfusions every 3-5 months

1992

  • using retrovirus transferred harvested bone marrow stem cells
  • 2000 four children with gene therapy using retrovirus got leukemia
  • leukemia came from retrovirus randomly inserting the new alleles into genome (when inserted at random location can change regulatory gene sequence resulting in activation of cancer genes)
  • now use lentivirus as vector (insert genes randomly but have GM to inactivate replication)
  • AAV (adeno-associated virus) used as vector, doesn't put gene into host genome and not passed on in cell division (only valuable in long living cells such as liver or neurones)

Recent Success with Gene Therapy

  • improved eyesight of young men with hereditary blindness (Leber congenital amaurosis)
  • normal allele of β-globin into blood stem cells fixing β-thalassaemia
  • reduced symptoms for those with haemophilia B
  • 2013 five children cured from SCID
  • naked DNA inserted into tissue without vector in trial for skin, muscular, and heart disorder (advantageous because no vector)

Cystic Fibrosis

Genetic disorder in which abnormally thick mucus is produced in the lungs and other parts of the body.

  • prone to bacterial infections because mucus is not properly removed resulting in breeding of bacteria
  • caused by recessive allele of gene coding for transported protein CFTR (allows chloride ions to pass out of cells pulling water out of cells to mix with mucus to make thin for removal)
  • CFTR protein missing one amino acid (deletion of three base pairs)
  • ideally gene therapy could insert normal dominant allele into cells in the lung and the correct CFTR would be produced
  • 1993 trials to insert normal allele with liposome (tiny lipid balls) by nasal spray, worked for a week because nasal cells have short lifespan
  • USA trial with inserted allele into harmless virus to carry through gas exchange system (worked until volunteers experienced side-effects of infection from "harmless" virus)
  • to be successful allele needs to get into many cells throughout respitory system that will divide
  • new technique replaces mutated base to make 'stop' codon, ribosomes stops at codon producing short CFTR proteins
  • drug PTC124 allows translation across stop codon, making whole protein with 1 missing amino acid (hoping good enough)

Social and Ethical Implications of Genetic Technology

  • 2004 UK law- allow embryo without allele for genetic disease or with tissue to transplant into sick siblings, but not addition of allele to egg, sperm, or zygote
  • some say laws overstep boundaries, other say not far enough
  • fetus screened for genetic disease, may result in parents terminating pregnancy (even if the 'defect' was minor)
  • many consider the use of genetic technology to select sex of child to be unethical
  • genetic screening allows for potential parents to find if they carry mutant alleles before conception
  • couples may receive medical advice to terminate pregnancy if mutant allele (known as therapeutic abortion)
  • Huntington's disease has no cure and is often not diagnosed until after childbearing age, causes deterioration of brain. Debate on if parents should be tested because negative new could seem like a death sentence
Ethics are sets of standards by which people agree to regulate behavior...each group must decide whether research into gene technology is acceptable, and then whether or not it is acceptable to adopt the successful technologies"

Credits:

Created with images by eLife - the journal - "Yeast helicases unwinding DNA" • nadya_il - "bacteria black health" • M Pinarci - "DNA strand" • quapan - "DNA-Double-Helix through an electron microscope" • M Pinarci - "DNA Strand"

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