Objectives How can gene therapy be used in the treatment of disease? What are the differences between somatic and germ line therapies? What are the advantages and disadvantages of gene therapy?
What is gene therapy? Treatment of a genetic disease by providing the sufferer with a corrected copy of their defective gene. Transfection is the technical term for inserting a corrected gene in to a cell.
There are two approaches to gene therapy: 1. Somatic cell therapy Copies of the corrected gene are inserted directly into the somatic (body) cells of the sufferer. Doesn’t prevent the disease from occurring in the next generation because it doesn’t affect the gametes. Effects only last for the lifetime of the cells, so treatment has to be repeated.
*There are two approaches to gene therapy: 2. Germ line therapy The corrected gene is inserted into a fertilised egg produced via IVF. If successful, all cells of the embryo will contain the corrected gene. Germ cell therapy is permanent and also ensures offspring inherit corrected gene. Currently illegal.
Cystic fibrosis Caused by mutant recessive allele. Caused by a deletion mutation of 3 bases (AAA) in CFTR gene which encodes the CFTR protein. Role of CFTR is to transport Cl - ions across epithelial cell membranes. Water follows by osmosis so membranes are kept moist and mucus runny. In sufferers of cystic fibrosis, CFTR is non-functional so water is retained, membranes are dry and mucus is very sticky.
Sticky mucus causes the symptoms of cystic fibrosis Thick, sticky mucus accumulates in lungs causing breathing difficulties and infections. Only treatment is antibiotics and physiotherapy. Mucus blocks pancreatic duct so release of digestive enzymes affected (patients given enzymes instead) Sperm duct blockage in males – infertility.
Gene therapy is targeted at the lungs A vector is needed to introduce the corrected gene into cells of the lung. Two main vectors you need to know about: 1. Harmless virus (retrovirus or adenovirus) 2. Liposomes
Viruses as vectors 1.Adenovirus grown in lab to produce harmless strain. 2.Add recombinant plasmids that contain functional CFTR gene. 3.Gene becomes part of virus DNA. 4.Virus sprayed into the nostrils of patients via aerosol. 5.DNA (including normal CFTR gene) injected (by virus) into epithelial cells of patients lungs. Retroviruses can be used in a similar way, particularly when stem cells from bone marrow are removed, treated, then re-implanted into the patient. This is a treatment for leukaemia.
Liposomes as vectors 1.CFTR gene inserted into plasmids. 2.Plasmids wrapped in lipid molecules (forming a liposome). 3.Liposomes sprayed into patients airways via a nasal aerosol. 4.Liposomes fuse with the plasma membrane of cells and the plasmids enter the nucleus.
Gene therapy - problems Lots of potential but not many successful trials because: Liposomes sometimes not small enough to pass into the lung cells though the bronchioles. Poor expression of the CFTR genes. Adenoviruses may cause infection. Adenoviruses may trigger immune response or patients may develop an immunity to them (useless for future treatments). Patients have to undergo multiple rounds of gene therapy as effects short lived, depending on cell lifespan.
Ethical considerations - Gene Therapy Are disabilities diseases? Should they be cured or prevented? Does searching for the cure demean the lives of those who are presently affected by disabilities? It is very expensive – so who’ll have access to these? And who’ll pay?