Gene probes and Medical diagnosis

• to produce a DNA probe, you first need to sequence the allele that you want to screen for. You then use PCR to produce multiple complementary copies of part of the allele - these are the probes
• DNA probes can be used to locate specific alleles of genes (e.g. on chromosomes) or to see if a person's DNA contains a mutated allele that causes a genetic disorder
• DNA probes are short strands of DNA:

  • they have a specific base sequence
  • complementary to the base sequence of part of a target allele (allele that you are looking for)
  • e.g. an allele that causes a genetic disorder
  • means that DNA will bind (hybridise) to the target allele if it is present in a sample of DNA
• • a DNA probe also has a label attached
  • so that it can be detected
  • the most common labels are a radioactive label (detected using X-ray film)
  • or a fluorescent label (detected using UV light)
• how fluorescently labelled probes are used:

  1. a sample of DNA is digested into fragments using restriction enzymes and separated using electrophoresis
  2. the separated DNA fragments are then transferred to a nylon membrane and incubated with a fluorescently labelled DNA probe. if the allele is present, the DNA probe will bind (hybridise) to it
  3. the membrane is then exposed to UV light and if the gene is present there will be a fluorescent band
• if a radioactively labelled probe is used, the fragments are transferred to X-ray film

  if the gene and probe are present, a shadow will form on the film
• Screening for multiple genes:

  • the probe can be used a part of a DNA microarray, which can screen for lots of different genes at the same time
  • a DNA microarray is a glass slide with microscopic spots of different DNA probes attached to it on rows
• 1. a sample of fluorescently labelled human DNA is washed over the array
  2. if the labelled human DNA contains any DNA sequences that match any of the probes, it will stick to the array
  3. means you can screen the DNA for lots of different mutated genes at the same time
• 1. array is washed to remove any fluorescently labelled DNA that hasn't stuck and then visualised under UV light
  2. any labelled DNA attached to a probe will show up (fluoresce)
  3. any spot that fluoresces means that the person's DNA contains that specific allele e.g. if the probe is for a mutated allele that causes a genetic disorder, the person has the allele
• microarrays aren't just used to diagnose genetic diseases, researchers can use them to analyse other DNA samples or even to analyse mRNA samples
• uses of screening with DNA probes:

  • can be used to help identify inherited conditions
  • can be used to help determine how a patient will respond to specific drugs
  • can be used to help identify health risks
• e.g. Huntington's disease is an inherited condition:

  • affects the nervous system
  • does not usually start to display symptoms until aged between 30-50
  • people with a family history may choose to be screened for the mutated allele - see if they have inherited it
• • the NHS offers to screen all newborns for CF
  • can cause breathing and digestive difficulties
  • so treatment can begin as soon as possible
• • breast cancer can be caused by a mutation in the HER2 proto-oncogene and treated with the drug Herceptin
  • Herceptin only effective against this type of breast cancer bc it targets a specific receptor
  • screening for this particular mutation helps determine whether Herceptin will be a useful treatment or not
• • inheriting particular mutated alleles increases risk of developing certain types of cancer (although doesn't make it certain)
  • if a person knows they have these alleles - might help them make choice that could reduce the risk of the disease developing
• • some people concerned that genetic screening may lead to discrimination by insurance companies and employers if people are known to have a high risk of developing a condition
• genetic counselling:

  • the results of screening can sometimes be used for genetic counselling
  • GC is advising patients and their relatives about the risks of genetic disorders
  • involves advising people about screening (e.g. looking for mutated alleles if there is a family history of cancer) and explaining the results of a screening
• • screening can help to identify if someone is the carrier of a mutated allele
  • the type of mutated allele they are carrying (indicating the type of genetic disorder or cancer)
  • and the most effective treatment
  • if the results of a screening are positive (indiv. has the mutation) then genetic counselling is used to advise patients on the options of prevention or treatment available
• e.g. a woman with a family history of breast cancer may have genetic counselling to help her decide whether or not to be screened for known mutations that can lead to breast cancer e.g. a mutation in the BRCA1 tumour suppressor gene

  • if screened and result positive - genetic counsellors might explain to the woman what lifetime chance of developing breast cancer is (with mutated BRCA1 gene - 50-85% chance)
  • counselling could also help the woman to decide if, e.g. she wants to take surgical steps to reduce risk of developing - mastectomy
• e.g. sickle-cell anaemia is a recessive genetic disorder caused by a mutation in the haemoglobin gene

  • a couple who are both carriers may want kids - may undergo genetic counselling to help them understand their chances of having a child with sickle-cell anaemia ( 1 in 4)
  • GC also provides unbiased advice on possibility of having IVF and screening embryos for the allele, so embryos without the mutation are implanted in the womb
  • could also provide info on the help and drugs available if they have a child with sickle-cell anaemia
• a carrier is a person with an allele that is not expressed in their phenotype but that can be passed on to offspring
• personalised medicine:

  • results of screening can be used in personalised medicine
  • your genes determine how your body responds to certain drugs
  • different people respond to the same drugs in different ways - makes certain drugs more effective for some people than others
  • where personalised medicines come in
• • personalised medicines are medicines that are tailored to an individual's DNA
  • theory is that if doctors have your genetic info they can use it to predict how you will respond to different drugs
  • only prescribe the ones that will be most effective for you