Gene Therapy 2 (MJC)
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Describe the inheritance of cystic fibrosis.
- -It is a genetic disorder caused by a mutation of the autosomal recessive CFTR (cystic fibrosis conductance regulator) allele on chromosome 7.
- -Homozygous recessive people displays the disease while heterozygotes are carriers of this disease.
- - Two carrier/heterozygous parents have 1 in 4 chances at each conception of producing offsprings with cystic fibrosis.
Explain how a deletion of 3 base pairs in the CFTR gene results in a person having cystic fibrosis.
- -The deletion of 3 base pairs in the CFTR gene on chromosome 7 will result in an inframe-mutation.
- -This will cause the mRNA sequence transcribed from the mutated CFTR gene to be missing a codon at position 508
- -As a result, the amino acid phenylalanine is missing from the protein found in the CFTR protein.
- -This thus affects the secondary, tertiary and quaternary structure of the protein thus changing its 3D configuration and the protein is no longer able to transport Cl- out of the cell
- -This leads to an accumulation of Cl- in the cell causing Na+ to enter the cell to neutralise the negative charge, causing the cell concentration to increase, thus preventing water from leaving the cell
- -As a result, the mucus secreted becomes thicker due to the lack of water causing cystic fibrosis.
Explain how a mutation in the sex chromosomes can result in diseases such as SCID to arise.
- -This is known as X-linked SCID where interleukin 2 receptor gamma (IL2RG) gene on the X chromosome is mutated
- -The mutated gene hence results in the synthesis of the mutated IL2RG protein, which is a gamma chain in the receptors on lymphocytes.
- -This prevents the signal molecules from binding to the IL2RG which is no longer complementary which is needed to induce a signal pathway needed for the development of T lymphocytes.
Define the term autosomal recessive inheritance.
- -It means that the person with the disease is homozygous
- -and inherits two copies of the defective autosomal recessive allele
- -where the defective allele is due to a mutation of genes.
Explain why the use of bone marrow transplant to treat SCID is not effective and has a low success rate.
- -there may be tissue rejection where the T cells from the transplanted bone marrow can attack the cells of the host, causing a graft-versus-host disease and finding a suitable donor can be very difficult for some people
- -the donor cells may also be infected with a virus which could overwhelm the recipient before the host's immune system is restored, causing the death of the host.
Define the following term.
- - Gene therapy is a technique for correcting defective genes responsible for disease development by replacing the non-functional or absent gene with the introduction of a functional normal gene
- -The new gene allows the cell to generate functional protein product.
- -This restores the cell back to its normal function.
State the limitations of using retrovirus as a vector in gene therapy for SCID.
- 1) The treatment is not permanent and is a short-term solution and hence the injections have to be repeated as the T cells die off after 6 to 12months.
- 2) The integration of the ADA gene into the host genome is a random process which may result in the disruption of important genes such as tumor-suppressor genes which may result in the development of cancer.
Why is retrovirus a suitable vector to treat SCID?
- -Retrovirus infects only dividing cells such as the bone marrow cells
- - The retrovirus has an envelope with envelope proteins which can be modified, enabling the virus to recognise cell surface receptors and infect specific cells
Explain the advantages and disadvantages of using adenovirus as a vector to treat cystic fibrosis
- -The DNA is not integrated into the host genome and thus do not disrupt host genes.
- - It can trigger immune response in body due to the expression of viral genes
- -The viral DNA can be hydrolysed and gene expression will be lost and the therapy have to be repeated and hence making this an expensive treatment.
- -Adenovirus can cause the host cell to undergo apoptosis.
Explain why adeno-associated viruses are used more often as a vector compared to adenovirus.
- - Adeno-associated viruses do not trigger an immune response in the host cells as they do not normally infect human cells while adenovirus will trigger an immune response due to the expression of viral genome.
- - Adeno-associated viruses reduces the chances of disrupting other genes and the normal gene can permanently become part of the cell thus enabling long-term treatment as the DNA is inserted into a specific region on chromosome 19 unlike the adenovirus which do not cause the integration of the gene into the host gene genome and hence the viral DNA can be hydrolysed, making it a short-term treatment.
State the advantages of using viral gene delivery system over non-viral gene delivery system.
- -Specific receptors on virus allows genes to be transduced into the correct target cells
- - There is greater efficiency in transduction of gene into target cells compared to non-viral vectors
- -It also allows for the incorporation of alleles into the target cell's genome.
State the 4 methods in which a non-viral vector carrying the normal gene is delivered into the target cell.
- 1) Electroporation
- 2) Gene gun
- 3) Cationic liposome
- 4) Cationic polymer
Explain how a gene gun is used to introduce non-viral vectors into the target cells.
- -The plasmid DNA is coated onto microscopic gold or tungsten particles.
- -The particles are accelerated towards the target cells using a gene gun.
- -The particles penetrate the plasma membrane of the target cells and deliver the DNA into the nucleus.
- -The target cells are then transferred into the body of the
Explain how protein replacement can be used as a therapy for cystic fibrosis and describe the immediate effect of this therapy.
- -The normal function CFTR protein can be introduced into the cell membrane.
- -The normal transportation of chloride ions across the cell membrane out of the cell can take place.
Explain the ethical and social objections of therapy in general.
- -The decision of what is normal and what is a disability should not rest on just a small group of individuals or medical professionals. The question of whether disabilities are considered disease and whether these need to be cured or prevented requires thorough consideration.
- -There is a concern that the presence of a cure might demean the lives of individuals presently affected by disabilities. There could be a new definition of what contributes to a ‘normal’ being.
- -There can be a formation of class distinctions, because expensive gene therapy is easily available to individuals with financial means, but not to those financially less well off.
- -Changing the genetic makeup of individuals goes against many religious and cultural beliefs.
- -There may be incidental elimination of genes that are important but with unknown functions and this may lead to decreased species fitness
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