Can molecular biology cure cancer?

Figure 1 Signal transduction. How a cell receives a mitogenic signal and translates this into cell proliferation. (A) Signalling OFF. The absence of mitogens means that the receptor has low enzymatic activity. Its substrates are not being modified and therefore the signalling pathway is turned OFF. The transcription factors do not activate genes to drive cell division. (B) Signalling ON. Mitogens outside the cell bind and activate the cell surface receptor. This turns on its enzyme activity inside the cell, which then modifies its substrates (such as by phosphorylation — Figure 2). These modifications alter the function of the next protein, which can then activate the transcription factor that turns on cell division genes. In this way a chain of events is started that causes the cell to divide. (C) Cancer. If an enzyme in the pathway is mutated, it may be turned on in the absence of mitogens. In this case, the cell will always be getting the signal to divide.

Cancerous tumours form when cells start to grow in an uncontrolled way. Tumours disrupt the function of the organs they grow in, ultimately leading to death. Cancer is a genetic disease caused by mutations in specific genes (see Box 1 and BIOLOGICAL SCIENCES REVIEW, Vol. 20, No. 3, pp. 6–9). Mutations happen when DNA gets damaged and is not repaired properly by the cell. DNA can be damaged by a number of things, such as the carcinogens in cigarettes (see pp. 2–4) and irradiation (for example, by staying out in the sun too long, which is associated with skin cancer). Mistakes can also happen when cells copy their DNA before dividing (see pp. 5–8). Although cells are good at checking and repairing damaged DNA, sometimes errors are copied into the next generation of cells.

When damage occurs in a section of a gene coding for a protein, it can change the amino acid sequence of that protein. This alteration inproteinsequencecanresult in a change in protein function, and this altered function can lead to disease. There are many examples of diseases caused by mutations that alter proteins — muscular dystrophy, cystic fibrosis and sickle-cell anaemia are examples. In cancer, mutations alter the function of genes that have important roles in the control of cell growth.