I read an article today which called for the NHS to ready itself for a ‘genetic revolution’ in healthcare. Leaving aside the perpetually optimistic claims of the genetic world, and the cost implications for a health service that is being asked to cut back on some basic surgical procedures, there was a throwaway line that made me cringe:
“Cancer can also be described as a genetic disease and some genes are known to increase the risk of cancer.”
Cancer is indeed a ‘genetic’ disease in that the behaviour that makes cells cancerous is influenced by genetically encoded processes. The trouble is that a lot of people read the word ‘genetic’ and think ‘heritable’ or ‘hereditary’, which is not the same thing. Though some people are at increased risk of cancer having inherited a ‘bad gene’, these people actually make up a small proportion of all cancer cases compared to the numbers of people whose cancer is triggered by external factors such as smoking, excess alcohol consumption or exposure to certain environmental triggers. But the underlying process of cancer development is still genetic in these cases. I’ll explain.
You started life as a fertilised egg, a single cell combining genetic material from each of your parents. In due course this divided into two cells, then four, then eight. But your cells did not continue to double indefinitely, as otherwise you would quickly become enormous, á la the wheat and chessboard problem.
What happened instead is that various mechanisms for regulating how and when your cells divide started to kick in. Some of these mechanisms suppress the growth and division of certain cell types at certain times. Others actively encourage it – for example, those which promote the on-going renewal of skin cells or red blood cells. The signalling pathways that control these processes are encoded in the genes.
In a normal cell, there are multiple pathways that control growth and division, so if something goes wrong in one pathway, others may compensate and things carry on just fine. But if a few mutations accumulate in the genes controlling these pathways, the effects might start to be seen via abnormal cell behaviour. Cells may start to divide too rapidly, or in the wrong place, or lose their identity. If this renegade behaviour isn’t kept in check then what we refer to as ‘cancer’ develops.
Now, in some people a mutation may have been inherited from one or other of their parents that puts them at elevated risk of cancer from day one. The BRCA mutations associated with breast and ovarian cancers are a good example. But overall the number of breast cancer cases that can be attributed to those mutations is small. In reality, the causes of most breast cancer cases lie in genetic mutations accumulated over the course of a lifetime, with the ageing process itself being a risk factor. In fact most cancer cases in general are not hereditary though the cancer-generating processes are rooted in genetic errors.
There will be applications for genetics in clinical medicine, though I don’t see widespread implementation happening any time soon, and the limitations go beyond those mentioned in the article. Even full-scale sequencing of the genome of a patient’s mutated cancer cells won’t yield a clinical advantage if targeted treatments don’t exist. But my main issue here is that describing cancer as a ‘genetic disease’ without clarifying the difference between ‘genetic’ and ‘hereditary’ is misleading, and may well lead to a rather fatalistic view of the extent to which we can manage our own personal risk.