The Genetics of Risk

In my ongoing series “The Causes of Cancer“, we examine the environmental causes of that increase your risk of being diagnosed with cancer. However, environmental causes are only one side of the coin. The more we understand about the way cancer works, the more we appreciate how genetics is at the core of why some people get cancer and some people don’t, irrespective of whether they have engaged in risky behavior.

Most of the genetics that regulates cancer risk is unbelievably complex. It helps to think of the human DNA as a blueprint for how each cell in the body must behave in order for the whole organism to function correctly. The blueprint is made up of different parts, each of which regulates a specific part of the living cell. Each one of these parts is known as a “gene” and contains instructions to manufacture a specific protein – a microscopic molecular machine that carries out a specific purpose within the cell. The ensemble of all the genes in the human DNA is known as the human genome. Each gene can be present in several varieties, which are known as alleles. To continue the blueprint metaphor, while all house blueprints have a part describing how the front door is going to be built, some houses have wooden doors, some have metal doors, some have red doors, some have green doors and so on. While all human beings have the same basic genes – that is, the same parts of the blueprint, they all differ in what alleles of these genes they carry. Since there are hundreds of thousands of possible variations throughout the human genome, the combination of all the alleles that we carry is, in a way, what makes us unique. Alleles are transmitted from parent to child, which is how you inherit your parents’ characteristics – like the color of your eyes or the shape of your nose. However, alleles can also carry an increased risk for some diseases, which is also unfortunately transmitted from parent to child. That is why having a family history of any non-infectious disease is the first red-flag to any general practitioner to look out for symptoms of that particular disease.

In the majority of cases, the risk of cancer is distributed through several genes and is therefore dependent on a combination of several alleles. These risk-carrying alleles usually occur through genes that detail instructions on how cells repair damage to their DNA, reproduce and control their own reproduction (for more details on how genetics drives cancer, check out my Understanding Cancer series here). When risk is distributed over hundreds of alleles, each allele carries a very small part of the overall risk. This means that it is quite difficult to understand exactly which alleles carry cancer risk and which ones don’t. A good analogy for this is looking at a group of friends lifting a table. If there are 20 people all helping to lift, each one is only lifting a small fraction of the wight. This means that if any of them drops out, the table is not going to fall down. However, if there are only one or two people lifting, they are each carrying much more of the weight. This means that if one of them drops out, the table is going to smash into the ground.

In some cases, however, there are only one or two alleles involved in carrying the cancer risk (in other words, only one or two people lifting the table). In other words, sometimes a specific allele is essentially directly responsible for a tumor forming. These cases are both relatively rare and very much at the forefront of scientific research. While testing everyone for a specific combination of several hundreds or even thousands of alleles is unfeasible, testing for one specific allele is not only doable, but a great tool to help prevent cancer as well as treat it.

This new series of posts is going to focus on examples of genes that are directly linked to certain types of cancer.

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