Epigenetics & Disease (AQA A Level Biology)

Inheritance of epigenetic modifications

• Epigenetic imprinting is the DNA methylation of certain genes and it occurs during the formation of oocytes and sperm cells
• A child, therefore, inherits two sets of DNA, one from the mother and one from the father, each with its own epigenetic imprint
• Imprinting is reversible, therefore the maternal epigenetic imprints that are inherited by a male will become paternal imprints when his sperm are produced so that he passes on paternally imprinted DNA
• Prader-Willi syndrome is an example of a condition with epigenetic links
  • The syndrome is caused by the silencing of an allele on chromosome 15
  • The severity of the syndrome depends on whether an individual receives the affected DNA from their mother or their father
  • If the mother is a carrier for the defective chromosome, individuals that inherit the chromosome do not develop the syndrome
  • However, if the defective chromosome is inherited from the father, the individual will develop the syndrome

A family tree showing the inheritance of Prader-Willi Syndrome. An individual is only fully affected by the syndrome if they inherit the faulty chromosome from their father.

Epigenetic cancer treatment

• DNA in human tumour cells have changes in DNA methylation and histone acetylation which causes tumour suppressor genes to be silenced and oncogenes to be activated
  • This leads to deregulation of the cell cycle and the formation of tumours

• Some cancer treatments involve drugs that reverse the epigenetic changes through the removal of acetyl and methyl tags
  • Removal of methyl groups from the DNA of tumour suppressor genes will enable the genes to be expressed
  • The proteins produced can then regulate the cell cycle and stop tumours forming from faulty or cancerous cells
  • Removal of acetyl groups from histone proteins attached to oncogenes causes the DNA to wrap more tightly, silencing these genes
  • Reducing the expression of oncogenes stops cancer as faulty cells are able to die through programmed cell death (apoptosis) rather than continuing to replicate, causing cancer