13.1 Essential ideas
13.1.4 Medicine (HL)
Biotechnological tools can be used in the diagnosis and treatment of different types of disease.
Detecting enzyme deficiencies
- A mutation in one gene results in a deficiency in one enzyme – either the enzyme is absent, or it is malformed and doesn’t function.
- Enzyme deficiencies disrupt metabolic pathways and cause the accumulation of metabolites in the blood or urine.
- For example, phenylketonuria (PKU) can be detected by accumulation of phenylpyruvic acid in the blood, as illustrated below:
- When a person is infected by a pathogen, the pathogen can be detected by the presence of its antigens, or by the presence of its genetic material.
Antigens can be rapidly detected in the blood by using an Enzyme-Linked Immunsorbent Assay (ELISA).
- In an ELISA, immobilised antibodies on a testing surface are used to capture antigens from infected blood.
- The testing surface is bathed with blood, rinsed, then bathed with more antibodies that have been linked to an enzyme. If antigen is present, it is at this step “sandwiched” between the two antibodies.
- Finally, a solution containing substrate is added. The substrate reacts with the linked enzyme, so that if there are antigens present, the testing well changes colour.
Application: PCR to diagnose different strains of flu
- Some pathogens, namely viruses such as HIV and influenza, use RNA as their genetic material. These can be detected using a special type of PCR called reverse-transcriptase polymerase chain reaction (RT-PCR)
- Reverse transcriptase is an enzyme that is capable of synthesising DNA from RNA.
- Once the genetic material is amplified by RT-PCR, the specific strain of influenza virus can be determined by comparison using gel electrophoresis. (see 3.1.5)
- Fast diagnosis of different flu strains is important to prevent epidemic, and to protect vulnerable groups (the elderly, pregnant women, etc).
What are genetic markers?
- Geneticists have identified many alleles that predispose people to acquiring genetic disease. These are called genetic markers.
- Genetic markers are found all over the genome – in coding and non-coding regions, as single alleles or as a combination of multiple genes.
- Genetic markers do not necessarily cause disease – instead statistical probabilities indicate the level of risk associated with the alleles.
Detecting genetic markers using DNA microarray
- DNA microarray uses the principle of complementary base pairing to compare many DNA sequences to a database of known genetic markers.
- The process begins when mRNA from a target cell is isolated and put in an environment with reverse transcriptase. Complementary DNA (cDNA) is made from mRNA.
- The cDNA is then labelled with fluorescence, and exposed to a DNA microarray chip. The chip contains millions of DNA strands of known sequences, held in place at specific locations.
- When the chip is scanned, the computer produces an image showing the locations where base pairing has occurred.
Protein tracking experiments
- Information about the location and interactions of proteins can be gained using tracking experiments.
Application: Tracking transferrin with luminescent probes
- Transferrin is a plasma protein that binds to iron and is taken into cells by receptor-mediated endocytosis.
- When transferrin is labelled with a luminescent probe, it can be tracked as it moves through the blood plasma and into different tissues in vivo.
Figure 13.1.4e – Luminescence helps to identify and track tumour cells
- This is significant because the concentration of transferrins is higher in cells with a high proliferation rate, including tumor cells.
- Tracking experiments have been used to identify and monitor tumor cells in the body.
Biopharming therapeutic proteins
- Biopharming refers to the use of genetically modified animals and plants to produce proteins for therapeutic use.
- We have seen how microorganisms can be used to produce insulin for therapeutic use 3.1.5. Microorganisms are not able to perform post-transcriptional modifications necessary to produce complex proteins for therapy.
Application: Biopharming of antithrombin
- Herds of transgenic goats are bred to produce antithrombin, ATryn, a human blood-clotting factor.
- There are a number of ways to transfer the gene construct containing human ambithrombin gene into a goat’s genome.
- The gene construct may be microinjected into a fertilised zygote, or it may be transferred along with a complete genome into an enucleated oocyte.
- In both cases, the construct contains other regulatory sequences to ensure that the protein is produced in the mammary glands rather than in other parts of the body.
- When the surrogate female gives birth, the transgenic offspring is induced to produce milk that contains ambithrombin.
- Transgenic goats that produce milk are then bred normally to produce a herd.
Viral vectors in gene therapy
- Gene therapy may be used to treat inherited diseases, such as cystic fibrosis, or inborn errors of metabolism.
- Gene therapy works by infecting cells with viruses that have been genetically modified to contain working copies of defective genes.
- In this way, the virus is a vector, or a delivery system, for working genes.
- Gene therapy is not a cure – working genes are inserted into somatic cells. The therapeutic benefit is not passed on to the next generation of cells.
Treatment of Severe Combined immunodeficiency (SCID)
Figure 13.1.4g – Gene therapy
- SCID is a deficiency of the enzyme adenosine deaminase (ADA) that leads to toxic accumulation of metabolites in lymphocytes.
- With low counts of both T and B lymphocytes, a person with SCID is unable to neutralise weak infections.
- SCID was the first disease successfully treated using gene therapy and a retrovirus vector.
Biotechnology can be used in the diagnosis and treatment of disease
Figure 13.1.4i – BRCA 1 and BRCA2 are genetic markers associated with breast and ovarian cancer in women.
Figure 13.1.4j – Real size of a DNA microarray chip