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  • Subject area(s): Engineering
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  • Published on: 7th September 2019
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Genetic engineering starts with DNA (deoxyribonucleic acid), DNA molecules are very long chains or units composed of simple sugars and a phosphate group. Attached along this chain at regular points are nitrogen bases. Nitrogen bases are chemical compounds in which carbon, hydrogen, oxygen, and nitrogen atoms are arranged in rings. Four nitrogen bases occur in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). The way these bases are arranged along a DNA molecule form a type of genetic code for the cell in which the molecule occurs. For example, the sequence of nitrogen bases T-T-C tells a cell that it should make the amino acid known as lysine. The sequence C-C-G, on the other hand, instructs the cell to make the amino acid glycine. A very long chain (tens of thousands of atoms long) of nitrogen bases tells a cell, therefore, what amino acids to make and in what sequence to arrange those amino acids. A very long chain of amino acids arranged in a particular sequence, however, is what we know of as a protein. This specific arrangement of nitrogen bases, tells a cell what kind of protein it should be making. Also, the instructions stored in a DNA molecule can easily be passed on from generation to generation. during cell division (reproduction), the DNA within it also divides. Each DNA molecule separates into two identical parts. Each of the two parts then makes a copy of itself. At the beginning of the process one DNA molecule existed, now two identical copies of the molecule exist. That process is repeated over and over again, every time a cell divides. The nitrogen bases forms a code or set of instructions when they are arranged in a specific way this instructs a cell to make a specific protein. The protein might be the protein needed to make red hair or blue eyes (to simplify the possibilities). Therefore the sequence of bases holds the code for some genetic trait.

Diagram of DNA

  Genetic engineering, often called genetic modification is the process of altering the DNA in an organism’s genome. This manipulation is used to change an organism’s traits in a certain way. For this to happen it could mean changing a base pair, deleting a whole region of DNA or inserting an extra copy of a gene. It could also mean the extraction and combination of DNA from another organism’s genome. Genetic engineering can be applied to any organism from bacteria and viruses to plants and even humans (Your Genome, 2016). Genetic engineering requires three elements: the gene that is to be transferred, a host cell into which the gene is inserted, and a vector to facilitate the transfer. If for example someone wanted to insert the gene for making insulin into a bacterial cell. Insulin is a naturally occurring protein made by cells in the pancreas in humans and other mammals. It controls the breakdown of complex carbohydrates in the blood to glucose. People whose bodies can no longer produce insulin become diabetic. The first step in the genetic engineering procedure is to obtain a copy of the insulin gene. This copy can be obtained from a natural source (from the DNA in a pancreas, for example), or it can be manufactured in a laboratory. The second step in the process is to insert the insulin gene into the vector. The term vector means any organism that will carry the gene from one place to another. The most common vector used in genetic engineering is a circular form of DNA known as a plasmid. Endonucleases are used to cut the plasmid molecule open at almost any point chosen by the scientist. Once the plasmid has been cut open, it is mixed with the insulin gene and a ligase enzyme. The goal is to make sure that the insulin gene attaches itself to the plasmid before the plasmid is reclosed. The hybrid plasmid now contains the gene whose product (insulin) is desired. It can be inserted into the host cell, where it begins to function just like all the other genes that make up the cell. In this case, however, in addition to normal bacterial functions, the host cell also is producing insulin, as directed by the inserted gene. Notice that the process described here involves nothing more in concept than taking DNA molecules apart and recombining them in a different arrangement. For that reason, the process also is referred to as recombinant DNA (rDNA) research (Genetic Engineering 2002).

Diagrams of

  One way in which genetic engineering is used is in treating x-linked SCID (x-linked Severe Combined Immunodeficiency) which is an inherited disorder of the immune system that occurs almost exclusively in males. Boys with X-linked SCID are prone to recurrent and persistent infections because they lack the necessary immune cells to fight off certain bacteria, viruses, and fungi. Many infants with X-linked SCID develop chronic diarrhoea, a fungal infection called thrush, and skin rashes. Affected individuals also grow more slowly than other children. Without treatment, males with X-linked SCID usually do not live beyond infancy (2016).

  Just under half of the volume of blood consists of blood cells which are tiny and can only be seen through a microscope. The rest of the blood volume is plasma, a watery liquid which contains dissolved proteins, sugars, fats, salts and minerals. There are three main types of cells in the blood: red blood cells, white blood cells and platelets. It is the white blood cells that play an important role in defending the body against infection. All blood cells are derived from immature cells known as stem cells, sometimes referred to as "mother cells". Some stem cells can be found in the blood, but the richest supply is found in the bone marrow. SCID is caused by a defect or mutation in a child's genetic make-up. It is an inherited condition - passed on in families in the same way as physical characteristics are passed from parent to child. In the centre of every cell in the body there are 46 string-like structures known as chromosomes. The chromosomes are arranged into 23 matching pairs. For ease of identification the chromosome pairs are numbered according to size from one to 23. The last pair ‘number 23’ are the sex chromosomes which determine whether a child is male or female. Each pair of chromosomes is like a double string of thousands of beads. Each bead is a gene. Each gene is responsible for a certain characteristic (hair colour for instance). Scientists know that each gene is responsible for producing a particular protein many of which are necessary for the development of a normal immune system. A defect in one of these genes results in absence of the protein that is necessary for a functioning immune system. There are a number of different genes that can be affected, each causing a different type of SCID. These important findings now enable doctors to make a much more specific diagnosis. The names given to the different types of SCID are based on the particular protein or gene that is deficient, e.g. gamma chain deficiency, JAK 3 kinase deficiency, purine nucleoside phosphorylase (PNP) deficiency, adenosine deaminase (ADA) deficiency, MHC class II deficiency or recombinase activating gene (RAG) deficiency (GOSH NHS 2015).

  Traditionally bone marrow transplantation is a treatment option for SCID. In a transplant, healthy haematopoietic stem cells are given to the patient from a donor. Haematopoietic stem cells are very early blood cells which can differentiate and divide into all possible types of mature immune cells, including the B and T lymphocyte cells. Being able to make working B and T lymphocytes provides transplant patients with some level of protection against infection. Bone marrow transplantation is most successful if a fully-matched family donor is available. It is also possible from matched unrelated donors and mismatched donors, although long-term survival is reduced. It’s estimated that only one in five children finds a fully-matched bone marrow donor. Specific therapies for some of the different types of SCID are also available. For example, individuals with SCID caused by mutations in the adenosine deaminase enzyme can be treated with enzyme replacement therapy. However, immune function recovery is variable with this treatment.

  Gene therapy for SCID works by correcting the genetic mutation in the hematopoietic stem cells (required for all immune cells) of the affected individual. Cells are removed from the patient’s bone marrow and, using special viral material, scientists introduce a functioning copy of the faulty gene that causes SCID. The corrected cells are then re-transplanted into the patient, and can use this functioning copy of the gene as a blueprint for making working immune system cells. Chemotherapy is also performed in some cases. Chemotherapy may provide an initial advantage to the corrected cells and create space in the bone marrow, therefore improving incorporation of the corrected cells. As this technique uses only cells taken from the affected individual, it does not carry the risk of illness caused by the body reacting to donor material. There’s also a reduced risk of the graft itself being rejected.

  In two longer-term follow-up studies, the underlying genetic defect was repaired in four out of six patients with Adenosine Demaninase-Deficient SCID, and 10 out of 10 patients with X-linked SCID. Immune cell production was restored, and the effects persisted up to nine years after therapy (the most recent point of measurement). The procedure produced minimal side effects, and patients could attend typical schools. Combining the results with the results of other studies shows that 30 patients with Adenosine Demaninase-Deficient SCID have been treated with gene therapy to date. All patients have survived (follow-up of 1-10 years) and 21 (67%) have been able to stop enzyme replacement therapy.

   The major danger is that gene therapy may activate an oncogene. These are genes (often a mutated form of a normal gene) that cause cancer. In the London trial, one of the 10 children treated for X-linked SCID developed leukaemia. He was treated with chemotherapy and is now in remission. Leukaemia also developed in four patients in the French trial. However, no cases of leukaemia have been observed in any of the 30 patients treated with gene therapy for Adenosine Demaninase-Deficient SCID. It is unclear whether this occurrence is due to the nature of the DNA inserted to correct the mutation, the nature of the condition itself or some other factor. ‘Next generation’ retroviral and lentiviral vectors (carriers for introducing new genes) are being developed to reduce the risk of leukaemia. At present, clinical trials using these vectors are commencing in Europe and the United States.

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