#161 Gene therapy

Gene therapy: treatment of a genetic disorder by altering a person's genotype (insert functional alleles of genes into cells)







Common vectors:

• virus: retrovirus, lentivirus, HIV, adeno-associated virus (AAV)
• liposomes - small spheres of phospholipids
• naked DNA

- retrovirus: inserts genes randomly into host's genome. If the gene is inserted into another gene or regulatory sequence of a gene, it could activate a nearby gene and cause cancer

- lentivirus: inserts genes randomly into host's genome, but this virus can be modified to inactivate replication e.g.: HIV

- adeno-associated virus (AAV): does not insert genes into host genome --> gene is not passed to daughter cells when a cell divides --> the virus can be used successfully with long-lived cells

Severe combined immunodeficiency (SCID) 

• a crippled immune system
• sufferers may die at infancy due to normal infections
• inability to make adenosine deaminase (ADA)

--> T-lymphocytes of sufferers are removed and normal alleles of the ADA gene are introduced into them using a virus vector. This is not a permanent cure.

They call him bubble boy.

Cystic fibrosis: genetic disorder where abnormally thick mucus is produced in the lungs and other body parts. It is caused by a recessive mutated allele of the gene for a functional transporter protein for chloride ions.

• deletion mutation of 3 bases (AAA) in CFTR gene
• role of CFTR: transport chloride ions across epithelial cell membranes of the pancreasm alveoli in the lungs,...  ---> water follows via osmosis across membranes as water potential decreased due to the entry of chloride ions ---> membranes remain moist and runny
• sufferers of cystic fibrosis: CFTR non-functional --> water is retained inside cell --> results in dry membranes and sticky mucus

Symptoms:

• thick mucus accumulates in lungs --> breathing difficulties and higher risks of infections
• thick mucus blocks pancreatic duct and therefore blocks off oral enzyems that help digestion
• male infertility: thick mucus blocks sperm ducts

Somatic and germ cell therapy

- somatic cells = body cells

- germ cells = cells involved in sexual reproduction (alleles in the germ line are passed on through generations and generations)

Applications

• electrophoresis of DNA is used in genetic profiling (fingerprinting) in forensic science
• PCR is also used in forensic science to solve crimes: used to amplify DNA from small tissue samples

Syllabus 2016-2018 19.2 Genetic technology applied to medicine a) define the term bioinformatics  b) outline the role of bioinformatics following the sequencing of genomes, such as those of humans and parasites, e.g. Plasmodium (details of methods of DNA sequencing are not required)  c) explain the advantages of producing human proteins by recombinant DNA techniques (reference should be made to some suitable examples, such as insulin, factor VIII for the treatment of haemophilia and adenosine deaminase for treating severe combined immunodeficiency (SCID))  d) outline the advantages of screening for genetic conditions (reference may be made to tests for specific genes such as those for breast cancer, BRCA1 and BRCA2, and genes for haemophilia, sickle cell anaemia, Huntington’s disease and cystic fibrosis)  e) outline how genetic diseases can be treated with gene therapy and discuss the challenges in choosing appropriate vectors, such as viruses, liposomes and naked DNA (reference may be made to SCID, inherited eye diseases and cystic fibrosis)  f) discuss the social and ethical considerations of using gene testing and gene therapy in medicine (reference should be made to genetic conditions for which treatments exist and where none exist, also to IVF, embryo biopsy and preselection and to therapeutic abortions)  g) outline the use of PCR and DNA testing in forensic medicine and criminal investigations