Cancer Misconceptions

There are a number of misconceptions I believe some people have about cancer, which I thought I would attempt to clear up.

  1. All cancers are alike
  2. We should find a cure for cancer soon
  3. Getting cancer is unlucky
  4. Cancer is a man-made disease

 

1. All cancers are alike

By using the term cancer we tend to think of it as a single illness, with a range of risk factors, such as smoking, eating unhealthily, drinking alcohol to excess and being exposed to radiation. But from a biological standpoint, cancers vary hugely, with over 200 types of cancer, and subtypes that vary drastically. All cancers are the result of abnormal cell growth. Normally, growth of our cells is controlled by oncogenes, promoting it, and tumour suppressor genes, inhibiting it. But occasionally a mutation will arise in a critical gene, which various safeguard mechanisms attempt to identify – so they can correct the mistake or else instruct the cell to self-destruct. However, very occasionally a mutation is missed by the safeguards (or the mutation is in a gene for the safeguards themselves!) and the abnormal cell can start growing uncontrollably; replicating, spreading, mutating and evolving as cancerous tissue1.

Almost all cancers are caused in this way, but the precise genetic fault that causes the cell to start growing abnormally and avoid self-destruction can be one of many thousand, or even million! As with any existing complicated system, the vast majority of changes will be detrimental to its function. Even in the same tissue type, there can be numerous different kinds of cancers, with very different causes. But cancers can start to grow in any tissue type in the body, and these usually differ substantially from one another. This brings us on to misconception number 2.

 

2. We should find a cure for cancer soon

Cancers vary widely in their causes and this means that there will never be a ‘cure for cancer’. Compared to most diseases cancers are incredibly difficult to treat because the cause is the body’s own cells, growing uncontrollably. Once it has begun there is almost no stopping it, as the body is unable to recognise the cancerous tissue as being a threat, as it has the “self” marker that foreign pathogens causing other diseases lack. Attempts at fighting the cancer by doctors are similarly thwarted – it is difficult to target the abnormal cells alone, hence why chemotherapy typically causes harmful side effects.

On top of this, a single cancer tumour has a whole succession of mutations, varying by location in the tumour, plus a tendency to mutate rapidly. Treatments can be aimed at the genetics of a sample unrepresentative of the entire tumour, and drug resistance can quickly emerge if a drug fails to destroy every cell quickly enough. As with other diseases that can evolve resistance to antibiotics, cancers can quickly mutate and overcome our most effective treatments2. Doctors compensate by administering a huge dose and combination of different drugs during chemotherapy, reducing the odds that the tumour will become resistant to the treatment. Unfortunately, the large dosage also contributes to the severe side effects that many cancer patients experience.

It is likely that many cancers will prove to be incurable, but by concentrating our efforts on the most common killers, we may hopefully be able to cure some of them in future. However, I think it is unlikely that cancers will ever stop being a leading cause of death. If you look at causes of death over the past couple of hundred years you can see that cancers have increased hugely. This rise in cancers can be attributed to people living longer due to improved sanitation, nutrition and medical care. No longer are infants lucky to make it to their teenage years, and no longer are adults lucky to make their 60th birthday. But unfortunately cancers become much more likely in old age, and this leads us on to misconception number 3.

 

3. Getting cancer is unlucky

The average life expectancy in the UK today is 81, which by the standards of any of our ancestors is incredibly long! While we are no longer likely to die from infectious diseases like diarrhoea, pneumonia and tuberculosis, which killed many of our ancestors, we are now more prone to developing a cancer. The reason for this is simple; cancers are caused by genetic mutations and these accumulate over time from mistakes in DNA copying (in cell cycles) and exposure to environmental risk factors. Now that we are lucky to live longer lives, we are much more likely to get and die from cancer.

According to Cancer Research UK, 1 in 2 people in the UK born after 1960 will develop cancer in their lifetime2. So getting cancer is just as unlucky as losing a coin toss. However, it should be recognised that lifestyle and environmental risk factors play a vital role too. Behaviours such as smoking, drinking, failing to exercise, eating a poor diet and getting sun burnt can vastly increase your chances of developing a cancer. So you obviously shouldn’t think of cancer as inevitable and throw caution to the wind!

While cancer is terrible as it takes many of us to our graves, it is actually a sign that we are living long lives in the first place. It should be noted that some cancers do strike people while they are young, but these are quite rare cases (less than 2% of all UK cancer cases are in under 25’s2). Why are cancers more common when we are older? Because there was a strong selective pressure for people to live to rear offspring, and to care for those offspring until they reached reproductive age in turn. So there was a strong pressure for people not to get cancer and die at a young age, before passing on their genes – those that failed left no offspring and those that succeeded were our ancestors. In contrast, the selective pressure on our ancestors to stay alive and be free of disease into old age was very relaxed. If a genetic disease doesn’t strike until we are 50, then it won’t be rapidly selected out of the population, as our children will already be passing on our genes!

All organisms face a trade-off in allocating energy to different aspects of their lives – growth, reproduction and maintenance (immune system and repair). Natural selection has led to strategies that produce optimal allocations of their resources, on average, resulting in a maximum possible number of strong, healthy offspring (biological fitness). The body spends energy defending itself against environmental, as well as genetic, factors that can trigger cancer, such as sunlight, viruses, bacteria and carcinogens. It pays to invest large amounts of energy supressing mutations that could lead to cancer in early and reproductive years, but the payoff to this maintenance reduces dramatically post-reproduction3.

There are so many different possible forms of cancer that we could get, and our chances increase rapidly as we age. Unfortunately, it is not bad luck if you get cancer and die from it in old age; that is how many of us can expect to die. Improved nutrition and medicine are allowing us to live longer and longer lives, and it is estimated that a third of all babies born today in the UK will live to be over 1004. However, even with great advances in genetic therapies, cancers will always be a huge difficulty to overcome; one that we may never be able to beat. So it is possible that many of us will live for over 100 years, but it is not very likely that we’ll be able to stay free of cancer forever.

 

4. Cancer is a man-made disease

This is absolute nonsense. The entirety of multi-cellular life has the ability to get cancer, and many other animals do get cancer; if they are lucky enough not to get injured, diseased or eaten before they get old.

A more accurate statement is that cancer is more common in modern times. Some environmental risk factors for cancer such as smoking, drinking alcohol, eating unhealthily and air pollution are no doubt more common in modern society. But the main reason cancer is more common is that we live longer lives; our bodies deteriorate with exposure to carcinogens and our cellular defences against cancer can’t protect us forever.

 

Special thanks to Jonathan Lockett, my cancer expert, for his technical knowledge and tips.

 

  1. Casás-Selves, M., & DeGregori, J. (2011). How cancer shapes evolution, and how evolution shapes cancer. Evolution4(4), 624–634. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3660034/
  2. Cancer Research UK http://www.cancerresearchuk.org/cancer-info/cancerstats/keyfacts/Allcancerscombined/
  3. Aktipis, C., & Nesse, R. M. (2013). Evolutionary foundations for cancer biology. Evolutionary applications6(1), 144-159. http://www.athenaaktipis.com/Home_files/AktipisNesse2013.pdf
  4. Office for National Statistics http://www.ons.gov.uk/ons/rel/lifetables/historic-and-projected-data-from-the-period-and-cohort-life-tables/2012-based/sty-babies-living-to-100.html
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The Problem With Incest

Humans find the idea of incest disgusting and thus avoid mating with close relatives. Most people have a vague understanding that children born to closely related parents are likely to have physical or mental abnormalities, like the royal family of years gone by and some of that dodgy village down the road. This is known as inbreeding depression. But why does it occur? Surely it is beneficial to give children not only half of your set of genes, but also some that your sibling or parent shares with you, thus passing more than 50% of your genes to the next generation. Inbreeding ought to allow you to maximise the proportion of your genes in the next generation, the ‘goal’ of reproduction. Unfortunately, for most animals on this planet there are severe problems with inbred offspring that outweigh any benefits in terms of propagating their genes.

To understand inbreeding depression you must first have some basic knowledge of errors that occur when genes are copied or repaired incorrectly, called mutations. Every time cells divide they must copy all of their genetic material using microscopic ‘machinery’, and these are subject to the occasional error, which can result in a new version of a gene. Additionally, mutations can arise due to environmental damage through radiation, heat or chemical agents. These mutations become permanent features in the genetic lineage of a given cell, hence those that appear in the germ line (egg and sperm cells) are passed from parent to offspring eternally down the generations.

Within a single cell a mutation happens at a completely random point in the genome, effecting any of the ~25,000 genes. In most cases the mutation is not expressed, as there are two copies, called alleles, of each gene. And mutations usually result in a recessive allele, meaning it is submissive and masked by the ‘normal’, dominant allele, which works as usual. Therefore, any person with a random recessive mutation is unaffected and a ‘carrier’ for the genetic disorder associated with faults in that particular gene.

Most individuals inherit between 3-5 random recessive mutations and when they mate with an unrelated person in the population there is a very low probability that they are both carriers of the same mutation. This means that their children are pretty much guaranteed to inherit at least one healthy, working version of each gene. However, when closely related people mate they are likely to share mutations they’ve inherited from a common ancestor, meaning each of their children would have a 1 in 4 chance of receiving both faulty versions of a particular gene, giving them a genetic disorder. As most people carry multiple mutations (and these are likely to be shared by related parents) the chance that their child will have genetic abnormalities becomes seriously high.

So that is inbreeding depression: the accumulation of faulty versions of genes in descendants of closely related sexual partners, where they are shared through common ancestry.

Note: Not all mutations are harmful; most are in fact neutral, producing no effect on the protein and function of the gene. And while some mutations are harmful, a small but important subset result in improved function of a gene, and this is a crucial way in which new adaptations can arise.