BY Dr. Sanjiva Wijesinha

The fact that cigarette smokers place themselves at serious risk of ending up with lung cancer is widely known. What’s not known as much is that smoking tobacco is associated not only with lung cancer but other types of cancer as well.

Smokers – as well as passive smokers who are regularly exposed to environmental tobacco smoke (a.k.a secondhand smoke) – run an increased risk of cancer because the smoke from burning tobacco contains several chemicals that can damage our DNA.

In addition to lung cancer, smokers are prone to develop certain forms of leukaemia, as well as cancers of organs such as the throat and larynx, mouth, oesophagus, bladder and kidney.

Although we yet do not know precisely how tobacco causes cancer, the simple explanation is that the noxious chemicals in cigarette smoke – termed carcinogens (cancer causers) – induce mutations in the DNA that make up the genes in our cells.

DNA, which is the acronym for deoxyribonucleic acid, is the hereditary material that we have in all our cells – a sort of manual that instructs the body to perform certain activities that build, repair and maintain it.

Recently, the complicated process by which each cell in our body reads its specific instructions from its DNA code has been better understood.

The basic instructions in the DNA are interpreted differently by the cells in our muscles, the cells that line our gut or airways etc. Essentially, it is the way the DNA in different cells is packaged that makes some sections of the instruction manual unreadable at times.

One method by which the activity of a DNA sequence can be changed without the DNA itself changing is through a process called methylation whereby chemical clusters called methyl groups are added to the DNA molecule.

Professor Martin Widschwendter of University College London (UCL) is a well-known medical scientist working on the question of how cancers take root. While studying the process of DNA methylation, his UCL team discovered that important changes take place in the DNA of cells in an organ before they became cancerous.

They found that tiny changes to the DNA methylation of certain genes greatly increased the risk of those cells turning into cancer cells.

Smoking was shown to be a significant cause of aberrant DNA methylation. Assistant Professor Ludmil Alexandrov of the University of California in San Diego, the lead researcher of a 2016 study published in Science magazine, says: “Tobacco smoking damages DNA in organs directly exposed to smoke, and also speeds up a mutational cellular clock in organs that are both directly and indirectly exposed to smoke.”

Teams of researchers like his have begun elucidating the distinct types of mutations caused by cigarette smoke to better understand the biological pathways that lead to tobacco induced cancer. Organs directly exposed to tobacco smoke such as the airways suffer particular damage.

Alexandrov’s team discovered that within a year of exposure to cigarette smoke, 150 mutations were found to occur in lung cells, 97 in the larynx and 39 in the oral cavity.

Significantly, this DNA methylation has been shown to be reversible. In other words, if you stop smoking, the DNA methylation in your body reverts in time to its safer form – which explains why the risk of cancer falls in those who have quit smoking.