Genetic Diseases and Expression of the Genome


Mechanism of Cystic Fibrosis

  • A malfunction or absence of the CFTR protein means that chloride ions cannot exit the cell and instead build up inside the cell.
  • Since there is no inhibition from the CFTR protein, many sodium ions move into the cell
  • The build up of NaCl (salt) in the cell means water moves into the cell from surrounding mucus by osmosis to restore equilibrium.
  • This makes mucus very thick and sticky
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Genetics of Cystic Fibrosis

  • The CFTR protein is an enormous channel protein that contains 1480 amino acids.
  • The gene that codes for this protein is also large, and is located on chromosome 7.
  • Any mutation to this gene can affect the CFTR and therefore cause cystic fibrosis.
  • Cystic fibrosis is seen phenotypically if the genotype is a homozygous ressesive allele.
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Effects of Cystic Fibrosis: Digestive System

  • Cystic fibrosis causes mucus lining pancreas cells to be sticky and thick.
  • Thick mucus forms fibrous cysts which block the pancreatic duct.
  • This prevents digestive enzymes from reaching the small intestine, so food is not digested properly.
  • The digestive enzymes trapped in the pancreatic duct can damage the pancreatic cells, so there is an increased risk of pancreatic diseases. eg damage to pancreas can effect release of insulin, causing diabetes.
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Effects of Cystic Fibrosis: Digestive System (cont

  • Thick mucus can form a barrier between gut and small intestine wall, so there is less surface area for absobtion.
  • This can lead to malnutrition, leading to poor growth, weakness and delayed puberty.
  • Also an increase risk of bone diseases eg rickets. 
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Effects of Cystic Fibrosis: Reproductive System

  • Thick mucus can block the cervix, meaning sperm cannot get to the egg.
  • Mucus can also block oviducts, so egg is not released.
  • Men with cystic fibrosis are often infertile
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Effects of Cystic Fibrosis: Respiratory System

  • Thick, sticky mucus builds up in the lungs, because it is difficult to clear. 
  • The mucus is a breeding site for bacteria, increasing risk of lung infection/damage.
  • Mucus is too thick for antibodies to kill pathogens.
  • Smaller bronchioles can be blocked by mucus, leading to less surface area for gas exchange.
  • This makes a patient feel more tired, breathless and lacking energy. 
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Effects of Cystic Fibrosis: Sweat Glands

  • Sweat is more concentrated and salty due to excess Na and Cl ions.
  • Too much salt loss (from extretion of sweat) can lead to heart disease.
  • Concentration of the body's fluids are difficult to control
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Treatments- Physiotherapy

  • Involves chest precussions and vibrations. These vibrations create mini bursts of air to dislodge mucus.
  • Also involves breathing techniques.
  • Removes mucus, so it is easier to breathe.
  • Reduces risk of infection
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Treatments- Exercise

  • Exercise increases lung capacity, strength, endurance, energy and life expectancy.
  • It improves airway clearance, which helps prevent respiratory infections.
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Treatments- Diet and Enzymes

  • Because mucus blocks pancreatic duct, supliment enzymes are taken to replace lost enzymes.
  • People with cystic fibrosis need a high calorie, balanced diet with lots of fat, carbohydrate and protein (to counter lack of digestion).
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Treatments- Drug Therapy

  • Antibodies destroy built up bacteria in lungs that can cause infection.
  • Insulin is taken to treat diabetes.
  • Asthma drugs are taken to reduce lung inflammation.
  • Mucolites are taken to make mucus runnier.
  • Inhaled drugs are reach airwaves quickly.
  • Liquid medicine can be injected directly into blood
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Treatments- Transplant Surgery

  • New organs can replace those that have been damaged by cystic fibrosis.
  • New organ will not be affected by cystic fibrosis directly. 
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Treatments- Gene Therapy

Gene therapy is the process in which a healthy gene replaces a faulty gene in a host cell, so that the correct proteins may be synthesised.

  • Isolate the required gene.
  • Cut gene out of DNA molecule using restriction enzymes.
  • Take a vector, e.g a plasmid, and cut out a space in it using the same restriction enzymes.
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Treatments- Gene Therapy (cont)

  • Insert gene into vector, and join the two using DNA ligase to produce recombinant DNA.
  • Introduce vector into correct location in body eg by nasal spray or injection.
  • This will allow correct CFTR protein to be synthesised.
  • Intracellular problems: getting replacement DNA to be transcribed is difficult and unlikely.
  • Extracellular problems: Vectors can be attacked by immune system, mucus makes vector delivery difficult, changes may not be perminant
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Treatments- Germ Line Engineering

  • Germ line engineering is gene therapy preformed on germ cells instead of somatic cells, so that the faulty gene is not passed onto offspring.
  • Advantages: Perminant cure, not passed onto offspring
  • Disadvantages: Fears of what else could be engineered in offspring, unethical creation, against nature
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Genetic Screening

  • Genetic screening is the testing of human cells to determine whether they have genetic mutations or diseases.
  • Done by using cells from inside the mouth and testing them for certain alleles
  • If a parent is found to be a carrier of a disease (eg cystic fibrosis), they are offered medical and financial advice, and offered prenatal screening
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Prenatal Screening

  • Prenatal screening is genetic screening carried out on an embryo to test for genetic diseases/mutation.
  • There are 2 methods: amniocentisis and chorionic villus sampling.
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Prenatal Screening- Amniocentisis

  • Remove 20cm3 of amniotic fluid (using a syringe) during 16th week of pregnancy.
  • Centrifuge fluid and retrieve foetal epithilial cells and blood cells.
  • Culture cells for 2-3 weeks and test chromosomes for mutations.
  • Disadvantages: 1% risk of spontaneous abortion, choice to abort because of results can be traumatic because tests occur so late in pregnancy.  
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Prenatal Screening- Chorionic Villus Sampling

  • Small sample of embryonic tissue taken from developing placenta.
  • Cells are tested for range of genetic abnormalities.
  • Carried out much earlier in pregnancy that amniocentisis, so choice of abortion is less traumatic.
  • Results are available more quickly than for ammniocentisis.
  • Disadvantages: 5% chance of spontaneous abortion, X chromosomes are inactivated in foetal placental cells so mutation not detected on X. 
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Preimplantation Genetic Screening

  • This is used for in vitro fertilisation (egg and sperm fertilised outside of body).
  • Genetic screening is preformed on an embryo before it is implanted in the uterus.
  • A single cell is removed from an embryo and its genetic makeup is tested.
  • Embryos free of disease are implanted into the uterus. 
  • This technique ensures no faulty genes enter the gene pool
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  • Epistasis is when one gene affects or alters the expression of another gene (the genes occupy different chromosomal loci).
  • The alleles that mask other alleles are called epistatic alleles.
  • The alleles who's effect is being masked are called hypostatis alleles
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Genes and the Envirnonment

  • Environmental factors can affect the way certain genes are expressed phenotypically.
  • The effect of genetic influence vs environmental influence in humans is studied using identical twins.
  • Identical twins that have been brought up in different environments can show what genetics are altered by different environments. 
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Monoamine Oxidase A

  • Monoamine oxidase A is an enzyme found in the nervous system that breaks down monoamines, which make neurotransmitters that diffuse across synapses.
  • Levels of MAOA are determined genetically, and many different mutations can cause raised or lowered levels of this enzyme.
  • Too much MAOA means too little neurotransmitter, linked to Parkinson's and depresssion.
  • Too little MAOA means too much neurotransmitter, linked to hyperactivity and ADHD.  
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