3.8.2.3 Gene expression and cancer

• Explain how a mutation in the DNA sequence of a proto-oncogene could lead to cancer. (4 marks)

  • A mutation in a proto-oncogene can cause it to become an oncogene. (1)
  • This mutation may lead to the overproduction of proteins that stimulate excessive cell division. (1)
  • As a result, the cell cycle is disrupted, and cells divide uncontrollably. (1)
  • This uncontrolled cell division can lead to the formation of a tumour. (1)
• Discuss the potential effects of epigenetics on the development of diseases such as cancer. (6 marks)

  • Epigenetic changes can activate oncogenes or silence tumour suppressor genes. (1)
  • Hypermethylation of a tumour suppressor gene prevents its transcription. (1)
  • This leads to uncontrolled cell division and tumour formation. (1)
  • Hypomethylation of oncogenes increases their expression. (1)
  • This leads to overproduction of growth factors, promoting cancer. (1)
  • Epigenetic changes can be inherited or influenced by environmental factors such as diet and stress. (1)
• Explain the difference between benign and malignant tumours. (4 marks)

  • Benign tumours are non-cancerous and do not spread. (1)
  • Malignant tumours invade surrounding tissues and can metastasize. (1)
  • Benign tumours grow slowly and remain localized. (1)
  • Malignant tumours grow rapidly and disrupt normal tissue function. (1)
• Describe how a mutation in a tumour suppressor gene can lead to the formation of a tumour. (5 marks)

  • Tumour suppressor genes regulate the cell cycle and prevent uncontrolled division. (1)
  • A mutation can inactivate the tumour suppressor gene. (1)
  • This removes its ability to stop cell division. (1)
  • As a result, cells divide uncontrollably. (1)
  • This can lead to tumour formation. (1)
• Cancer is caused by uncontrolled cell division, leading to the formation of tumours. Tumours can be malignant or benign.

  • Malignant tumours grow rapidly and can spread to neighbouring cells via metastasis (via the blood stream or lymph system).
  • Benign tumours grow slowly and do not spread.
• Cancer happens due to mutations in genes that regulate the cell cycle:

  • Tumour suppressor genes (TSG): control cell division, causing the cell cycle to stop when damage is detected, triggering apoptosis
  • Proto-oncogenes: stimulate cell division when needed.
• Apoptosis is programmed cell death.
• Mutation in Tumour suppressor gene (TSG):

  • A mutation in a tumour suppressor gene means the gene is inactivated.
  • The cell no longer stops dividing even if there is DNA damage.
  • The result is uncontrolled cell division, leading to tumour formation.
• Mutation in Proto-oncogenes:

  • A proto-oncogene mutates into an oncogene.
  • The oncogene produces too many growth factors or causes proteins that stimulate cell growth and division to be constantly activated.
  • Cells divide uncontrollably, forming a tumour.
• Epigenetic changes can activate oncogenes or silence tumour suppressor genes, increasing the risk of cancer.
• Increased DNA Methylation of TSGs

  • Methyl groups (-CH₃) are added to tumour suppressor genes.
  • This switches them off, preventing them from stopping cell division as the necessary regulatory proteins coded for by TSG are not produced.
  • Leads to uncontrolled cell division → Cancer.
• Decreased Acetylation of Histones

  • DNA is tightly wrapped around histones, preventing transcription.
  • This can silence tumour suppressor genes, leading to cancer.
• Hypomethylation of Proto-Oncogenes

  • Reduced methylation keeps proto-oncogenes permanently switched on.
  • This leads to excessive cell division → Tumour formation.
• Increased oestrogen concentrations can be linked to breast cancer development.

  • Oestrogen binds to the transcription factor, which activates the genes promoting cell division, leading to tumour formation.
• Explain how the methylation of tumour suppressor genes can lead to cancer. (3)

  • Methylation of DNA prevents transcription of gene
  • Protein not produced prevents cell division
  • No control in mitosis
• Describe how alterations to tumour suppressor genes can lead to the development of tumours. (3)

  1. Increased methylation (hypermethylation)
  2. Causes a mutation in the tumour-suppressor genes.
  3. Tumour suppressor genes are not transcribed so are not expressed.
  4. This results in uncontrolled cell division
• A mutation of a tumour suppressor gene can result in the formation of a tumour, explain why (2)

  • gene inactivated / not able to control cell division
  • rate of cell division too fast / out of control