A single blood test for all cancers – too far-fetched?

Imagine it is the year 2084. You turn up for your annual medical check and all you have to do is: present your index finger to an austere-looking astronaut-like figure, dressed in a white one-piece space suit, who takes a drop of blood from your finger and puts it into an analyser. A minute later, you have an all-clear on a hundred different types of cancer. Even better, a drop of urine, saliva or sweat so you don’t have to feel the sharp prick of the blood draw. Science fiction? No, just science…but it still has some way to go.

The more we find out about cancer,  the more we understand that it leaves a tracer of itself in many different ways in the body. At the heart of this, is understanding that a cancer is a foreign entity that occupies the host, in a parasitic manner, and is distinct from the host in many ways. Host versus tumour – a war between two genetically distinct species!

Although this may sound alarming, it is the very differences of the cancer  from the host that are its Achilles’ heel. Because a cancer is so unlike the host, we can induce the immune system to attack it, as if it were an infecting bacteria or virus. We can give drugs   that turn off the defective genes, in      the cancer, that are causing it to grow, without harming the host’s tissues significantly. Moreover, we can detect these differences in many ways – to pick up cancers using blood tests or tests of other secretions like urine.

So, how close are we to getting this right for all cancers in a single test? The tracers of a cancer that can be detected include free-floating DNA in the blood, chemicals given off by the abnormal metabolism of the cancer, and even whole cancer cells floating around the bloodstream away from the main tumour mass.

The problem with many current tumour markers that are detected in the blood is that they are present in the normal healthy state as well, just at very low levels. As they increase in amount, there is a strong correlation with the presence of a cancer (depending on the tumour marker used), but finding some cut-off level that is useful is difficult, and we try to compromise between not mistakenly labelling healthy people as having cancer and not missing people who actually have the disease.

It follows, that a much better approach to the problem of developing a universal cancer test would be: to detect changes in the cancer that are not shared by the normal tissue of the host; mutations in DNA and abnormal products of metabolism have great potential for this reason, and also because we have great technology to detect them.

The small obstacle in the path of realising the Holy Grail of a single cancer test is that cancer, even from one site in the body, is not a single disease. For example, take breast cancers that respond to hormones – those that are HER2 positive and those that are described as triple-negative; all of these types of breast cancer have a very different biology. If we look closer at breast cancers by studying the DNA of the cancers, we see an even greater level of differences in cancers from one individual to the next, let alone differences between breast cancer, colon cancer and lung cancer! Therefore, how do we rationalise this into a single test?

Until we find the smoking gun that is common to all tumours, if ever, this would be an expensive process to cover all mutations, all changes that are unique to the cancer, and also identify all cancers from different sites of the body. In bringing such technology to everyday use, we need to diligently validate each and every cancer among thousands of cases to ensure that such testing does what we  need it to do, which is detect cancer early.

And finally, a very important factor, in what we would like such a test to do, is all of the unique changes of a cancer described above appear to test for detection when the cancer is already invading. We would actually like to detect pre-cancers and cure them before they have invaded the body and have a risk of spreading and causing death. Such pre-cancers, like ductal carcinoma in situ (DCIS) in the breast, do not have access to the blood system or secretions, and they are hidden from detection methods like blood tests. Oi! No one said conquering cancer would be easy, but we certainly aren’t giving up here…

MEET THE EXPERT

Dr Christopher Maske is a molecular pathologist at Lancet Laboratories in Johannesburg and head of the Molecular Pathology Laboratory. He qualified in medicine from the University of Cape Town and did his PhD at Oxford University in Molecular Cell Biology, and worked as a research scientist in the university before moving into diagnostic pathology. His main interest is in molecular biomarkers in solid tumours for treatment, diagnosis and therapy decisions.

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