Which process is used to insert normal genes into human cells to correct disorders quizlet?

Gene therapy works by altering the genetic code to recover the functions of critical proteins. Proteins are the workhorses of the cell and the structural basis of the body’s tissues. The instructions for making proteins are carried in a person’s genetic code, and variants (or mutations) in this code can impact the production or function of proteins that may be critical to how the body works. Fixing or compensating for disease-causing genetic changes may recover the role of these important proteins and allow the body to function as expected.

Gene therapy can compensate for genetic alterations in a couple different ways.

• Gene transfer therapy introduces new genetic material into cells. If an altered gene causes a necessary protein to be faulty or missing, gene transfer therapy can introduce a normal copy of the gene to recover the function of the protein. Alternatively, the therapy can introduce a different gene that provides instructions for a protein that helps the cell function normally, despite the genetic alteration.
• Genome editing is a newer technique that may potentially be used for gene therapy. Instead of adding new genetic material, genome editing introduces gene-editing tools that can change the existing DNA in the cell. Genome editing technologies allow genetic material to be added, removed, or altered at precise locations in the genome. CRISPR-Cas9 is a well-known type of genome editing.

Genetic material or gene-editing tools that are inserted directly into a cell usually do not function. Instead, a carrier called a vector is genetically engineered to carry and deliver the material. Certain viruses are used as vectors because they can deliver the material by infecting the cell. The viruses are modified so they can't cause disease when used in people. Some types of virus, such as retroviruses, integrate their genetic material (including the new gene) into a chromosome in the human cell. Other viruses, such as adenoviruses, introduce their DNA into the nucleus of the cell, but the DNA is not integrated into a chromosome. Viruses can also deliver the gene-editing tools to the nucleus of the cell.

The vector can be injected or given intravenously (by IV) directly into a specific tissue in the body, where it is taken up by individual cells. Alternately, a sample of the patient's cells can be removed and exposed to the vector in a laboratory setting. The cells containing the vector are then returned to the patient. If the treatment is successful, the new gene delivered by the vector will make a functioning protein or the editing molecules will correct a DNA error and restore protein function.

Gene therapy with viral vectors has been successful, but it does carry some risk. Sometimes the virus triggers a dangerous immune response. In addition, vectors that integrate the genetic material into a chromosome can cause errors that lead to cancer. Researchers are developing newer technologies that can deliver genetic material or gene-editing tools without using viruses. One such technique uses special structures called nanoparticles as vectors to deliver the genetic material or gene-editing components into cells. Nanoparticles are incredibly small structures that have been developed for many uses. For gene therapy, these tiny particles are designed with specific characteristics to target them to particular cell types. Nanoparticles are less likely to cause immune reactions than viral vectors, and they are easier to design and modify for specific purposes.

Researchers continue to work to overcome the many technical challenges of gene therapy. For example, scientists are finding better ways to deliver genes or gene-editing tools and target them to particular cells. They are also working to more precisely control when the treatment is functional in the body.

Scientific journal articles for further reading

Bulcha JT, Wang Y, Ma H, Tai PWL, Gao G. Viral vector platforms within the gene therapy landscape. Signal Transduct Target Ther. 2021 Feb 8;6(1):53. doi: 10.1038/s41392-021-00487-6. PMID: 33558455. Free full-text article from PubMed Central: PMC7868676.

Duan L, Ouyang K, Xu X, Xu L, Wen C, Zhou X, Qin Z, Xu Z, Sun W, Liang Y. Nanoparticle Delivery of CRISPR/Cas9 for Genome Editing. Front Genet. 2021 May 12;12:673286. doi: 10.3389/fgene.2021.673286. PubMed: 34054927. Free full-text article from PubMed Central: PMC8149999.

• Gene therapy uses sections of DNA (usually genes) to treat or prevent disease.
• The DNA is carefully selected to correct the effect of a mutated gene that is causing disease.
• The technique was first developed in 1972 but has, so far, had limited success in treating human diseases.
• Gene therapy may be a promising treatment option for some genetic diseases, including muscular dystrophy and cystic fibrosis.
• There are two different types of gene therapy depending on which types of cells are treated:
  • Somatic gene therapy: transfer of a section of DNA to any cell of the body that doesn’t produce sperm or eggs. Effects of gene therapy will not be passed onto the patient’s children.
  • Germline gene therapy: transfer of a section of DNA to cells that produce eggs or sperm. Effects of gene therapy will be passed onto the patient’s children and subsequent generations.

Gene therapy techniques

There are several techniques for carrying out gene therapy. These include:

Gene augmentation therapy

• This is used to treat diseases caused by a mutation that stops a gene from producing a functioning product, such as a protein.
• This therapy adds DNA containing a functional version of the lost gene back into the cell.
• The new gene produces a functioning product at sufficient levels to replace the protein that was originally missing.
• This is only successful if the effects of the disease are reversible or have not resulted in lasting damage to the body.
• For example, this can be used to treat loss of function disorders such as cystic fibrosis by introducing a functional copy of the gene to correct the disease (see illustration below).

Gene inhibition therapy

• Suitable for the treatment of infectious diseases, cancer and inherited disease caused by inappropriate gene activity.
• The aim is to introduce a gene whose product either:
  • inhibits the expression of another gene
  • interferes with the activity of the product of another gene.
• The basis of this therapy is to eliminate the activity of a gene that encourages the growth of disease-related cells.
• For example, cancer is sometimes the result of the over-activation of an oncogene (gene which stimulates cell growth). So, by eliminating the activity of that oncogene through gene inhibition therapy, it is possible to prevent further cell growth and stop the cancer in its tracks.

Killing of specific cells

• Suitable for diseases such as cancer that can be treated by destroying certain groups of cells.
• The aim is to insert DNA into a diseased cell that causes that cell to die.
• This can be achieved in one of two ways:
  • the inserted DNA contains a “suicide” gene that produces a highly toxic product which kills the diseased cell
  • the inserted DNA causes expression of a protein that marks the cells so that the diseased cells are attacked by the body’s natural immune system.
• It is essential with this method that the inserted DNA is targeted appropriately to avoid the death of cells that are functioning normally.

How is DNA transfer done?

• A section of DNA/gene containing instructions for making a useful protein is packaged within a vector, usually a virus, bacterium or plasmid.
• The vector acts as a vehicle to carry the new DNA into the cells of a patient with a genetic disease.
• Once inside the cells of the patient, the DNA/gene is expressed by the cell’s normal machinery leading to production of the therapeutic protein and treatment of the patient’s disease.

An illustration to show the transfer of a new gene into the nucleus of a cell via a viral vector. Image credit: Genome Research Limited

Challenges of gene therapy

• Delivering the gene to the right place and switching it on:
  • it is crucial that the new gene reaches the right cell
  • delivering a gene into the wrong cell would be inefficient and could also cause health problems for the patient
  • even once the right cell has been targeted the gene has to be turned on
  • cells sometimes obstruct this process by shutting down genes that are showing unusual activity.
• Avoiding the immune response:
  • The role of the immune system is to fight off intruders.
  • Sometimes new genes introduced by gene therapy are considered potentially-harmful intruders.
  • This can spark an immune response in the patient, that could be harmful to them.
  • Scientists therefore have the challenge of finding a way to deliver genes without the immune system ‘noticing’.
  • This is usually by using vectors that are less likely to trigger an immune response.
• Making sure the new gene doesn’t disrupt the function of other genes:
  • Ideally, a new gene introduced by gene therapy will integrate itself into the genome of the patient and continue working for the rest of their lives.
  • There is a risk that the new gene will insert itself into the path of another gene, disrupting its activity.
  • This could have damaging effects, for example, if it interferes with an important gene involved in regulating cell division, it could result in cancer.
• The cost of gene therapy:
  • Many genetic disorders that can be targeted with gene therapy are extremely rare.
  • Gene therapy therefore often requires an individual, case-by-case approach. This may be effective, but may also be very expensive.

This page was last updated on 2021-07-21

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