20.5 Gene expression and cancer

Types of tumour:

• Not all tumours are cancerous.

• Malignant tumours (cancerous) and benign tumours (non-cancerous)

Comparison between malignant and benign tumours:

Cancer and the genetic control of cell division:

Oncogenes

• Most oncogenes are mutations of proto-oncogenes.

What do proto-oncogenes do?

• Stimulate a cell to divide when growth factors attach to a protein receptor on its cell-surface membrane. This activates genes that cause DNA to replicate and the cell to divide.

If a proto-oncogene mutates into an oncogene it can become activates for two reasons:

Tumour suppressor genes:

• Tumour suppressor genes slow down cell division, repair mistakes in DNA, and ‘tell’ cells when to die - a process called apoptosis

• They have the opposite roles of proto-oncogenes

• Normal tumour suppressor gene maintains normal rates of cell division and so prevents the formation of tumours.

• If a tumour is suppressed it becomes mutated it is inactivated

• Stops inhibiting cell division and cells can grow out of control.

• These mutated cells that are formed are usually structurally and functionally different from normal cells.

• While most of these die, those that survive can make clones of themselves and form tumours

Types of tumour suppressor genes:

• TP53, BRCAI, BRCA2

Inherited cancer genes

The process by which hypermethylation may lead to cancer is as follows: