Can Your Genes Predict Your Diseases?
DNA testing can be incredibly useful — or highly misleading
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When the phenomenal Human Genome Project was completed in 2003, two years ahead of schedule and fifty years after the discovery of the structure of DNA, it looked like we had all diseases nailed. A simple DNA test would reveal which disease you were most likely to fall victim to, so you could pre-empt your nemesis. Forewarned is forearmed.
This project, described as “..one of the great feats of exploration in history”, is a complete mapping of all the genes that form the blueprint for a human being. These genes are collectively known as the genome.
Know yourself
Your individual genome can now be dissected, analysed, and the results plotted on charts. Depending on the type of test you choose, your genome can reveal what diseases you might inherit, who you are related to, or where your ancestors came from.
No wonder that a craze for genetic testing has swept across much of the world, especially the US and UK, creating a global boom in the industry. A quick Internet search reveals that the competition among laboratories for your DNA is fierce.
The procedure is simplicity itself. For a (usually) reasonable fee, you order a test kit from your chosen laboratory, take a swab, post the swab back to the lab and wait for the results to arrive.
That’s exactly what over 26 million people have already done, since testing became available. Genetic sleuthing allows you to trace interconnected lines of evidence that form a spiderweb of intrigue, and unearth the stories of notoriety or fame that form your life’s back catalogue. Who doesn’t want to indulge their inner narcissist, just a teeny bit?
Know your nemesis
When it comes to identifying potential disease, genetic testing represents a huge advance in the management and treatment of inherited conditions. Genetic mutations reveal susceptibility to these conditions. Although most types of genetic mutation diseases are rare, awareness can be invaluable. A well-known example of this are the BRCA1 and BRCA2 genes: mutation of these genes is an indication of high risk of developing breast and ovarian cancer.
Knowing that you have this mutation means you can work with your doctor to manage your risk, make lifestyle changes or have treatment.
DNA analysis can be a double-edged sword. One unfortunate corollary of the genome-centric view of disease has been the widespread but mistaken belief that our genes predict our future diseases, like stars determining our fate, leaving us powerless to control its trajectory. Finding out that you possess gene variants that are linked to common chronic diseases can cause unnecessary stress and anxiety.
The chronic diseases that are now so prevalent across the globe include cardiovascular disease, type 2 diabetes, obesity, Alzheimer’s disease and most cancers.
When it comes to these chronic conditions, genetic variants have surprisingly little influence. Take Alzheimer’s disease. Cases of this neurodegenerative disease are rising exponentially and are expected to double every 20 years, something that already suggests that ‘bad’ genes are not the cause. Variants of the APOE (apolipoprotein E) gene are associated with Alzheimer’s, but the association is weak — because variants are quite common and also occur among people who don’t go on to develop the disease.
As the UK Alzheimer’s Society website states:
“Many people fear that Alzheimer’s disease in the family may be passed on to children and grandchildren. In the vast majority (99 per cent) of cases, this is not so.”
What this means is that although genes may suggest susceptibility to common chronic conditions, they are not ‘necessary determinants’ of disease.
Researchers at the University of Alberta, Canada, analysed the data from almost 600 studies that examined the relationship between genetic variants and over 200 common conditions. According to senior study author David Wishart, “Most common, chronic diseases have little to do with genetics or to do with your parents or the genes you inherited from them.”
Common variations in DNA sequence are called single-nucleotide polymorphisms (SNPs). The researchers of the study found that more than 95% of common chronic diseases could not be predicted from SNPs. According to Wishart, the risk for contracting most diseases arises from a number of factors, including environment and diet.
That wasn’t the first research of its kind. More than 2,000 studies into the association between genetic predisposition and disease outcome have ‘rarely’ reported more than the smallest of risks. The minor associations found from studies of identical twins “cast further doubt on the notion that our inherited genomes are the primary causes of chronic diseases.”
Human genes evolve very, very slowly. You couldn’t even call it a glacial pace. And, bearing in mind that rates of common chronic diseases are snowballing (keeping with the Arctic theme), it doesn’t take a genius to work out that the association is probably not genetic.
Our ancient genome has an average mutation rate of 0.5% per million years, meaning that it “..still resides for the greater part in the Paleolithic era.” That’s right — you have a Stone Age body. You have passed down the evolutionary chute from your cave-dwelling ancestors and landed in the modern world, virtually unchanged.
“In fact, our genes today are very similar to the genes of our ancestors during the Paleolithic period 40,000 years ago, at which time our genetic profile was established”.
Clash of civilisations
In 1985, anthropologists Stanley Boyd Eaton and Melvin J Konner proposed their ‘discordance hypothesis’, which states that the human genome was determined by the conditions of the Palaeolithic era, and that changes to our lifestyles have occurred too rapidly for our bodies to adapt, resulting in a mismatch that leads to chronic disease.
The Palaeolithic lasted 2.6 million years and ended just 10,000 years ago. Since then, our genes have changed by just 0.005%. The processed, junk food industry took off less than a hundred years ago, meaning that in a flash our diets and lifestyles have been transformed beyond all recognition, but our genes have not.
However — and this is a big however — the fact that our genes haven’t changed doesn’t mean that they are not affected by external factors. The study of environmental influences on genetic expression that occur without altering the DNA sequence is known as epigenetics. Epigenetic influences include diet, stress, exposure to sunlight, chemicals, and infection. These influences are able to switch genes on and off, creating changes in gene expression within just a few generations. These changes can be passed on to offspring.
A DNA test may reveal that you have a faulty gene that makes you susceptible to a certain disease, such as cardiovascular disease, but that gene has to be ‘switched on’ by an external factor, such as diet, or a toxic chemical.
“In recent years, several studies have demonstrated that disruption of epigenetic mechanisms can alter immune function and that epimutations not only contribute to certain cancers but also to lifestyle diseases such as type 2 diabetes, allergies, cardiovascular disease, and rheumatoid arthritis, as well as unhealthy aging.”
The risk of acquiring a chronic disease such as heart disease, type 2 diabetes and certain cancers is influenced by the interactions between genes and environmental factors. This risk applies to mental as well as physical health.
“The rapidly increasing incidence, and perhaps severity, of some psychiatric diseases suggests that… we are dealing with a conflict between our contemporary lifestyle and our slowly adapting genome.”
If you want to influence your genes, and therefore the diseases you may be susceptible to, diet is a powerful way to do so. Dietary interventions can alter the course of certain diseases, via crosstalk between diet and genome. Genetics show only susceptibility — environmental factors determine “which genetically susceptible individuals will be affected.”
Red pill, blue pill?
The weak association between genes and disease can be viewed one of two ways: a) disappointing, because who doesn’t like a unifying theory of disease that simplifies everything and means we don’t have to do anything different; or b) encouraging, because it means we can take action to reduce our chances of succumbing to chronic disease.
If we opt for b), we can look at elements of what we eat and ask: Will my ancient genome recognise this as food? Should I rethink certain aspects of my diet and lifestyle to avoid switching on gene variants that could cause disease?
In short, if you are considering testing for susceptibility to particular diseases, it is advisable to seek professional advice first. Your doctor can advise you on whether genetic testing is appropriate or even expedient in your individual case, or serve only to increase levels of anxiety for no good reason.
But if you just fancy digging up your family history and opening cupboards that rattle with skeletons, go for it. Unearthing the past is much easier than predicting the future.