Genetic testing for risk of Heart Disease: fact or fiction?
By news editor, on 28 February 2012
Professor Steve Humphries (UCL Institute of Human Genetics and Health) delivered an insightful and comprehensive Lunch Hour Lecture, on 23 February, regarding the issues surrounding genetic testing for risk of heart disease.
The lecture was broken down into four manageable chunks: (1) The causes and mechanisms of heart disease (2) What is a gene? (3) What is SNP and how is it useful? (4) How can we use DNA tests?
The causes and mechanisms of heart disease
To begin the lecture Professor Humphries challenged the audience to suggest known risk factors for heart disease. This was effective in engaging the audience and a wide-ranging list was compiled, including: high blood pressure, obesity, smoking, diabetes, age and male gender. The risk factors could be genetic or environmental, but in reality, many are a combination of both.
Professor Humphries then proceeded to explain briefly the mechanisms of heart disease illustrating his points with some supportive graphics.
To summarise, he described the fact that after being born with clean arteries there can be the process of ‘silent build up’, which takes place in early adulthood. Although this can be reversed, in many it will lead on to fatty streaks, atherosclerotic plaques and in the unfortunate; chest pain, plaque rupture and myocardial infarction.
“Nature loads the gun and nurture pulls the trigger” was a particularly clear quote used by Professor Humphries, highlighting the contribution of both genes and environment in relation to heart disease.
The current UK NICE guidelines mean that only people who have a greater than 20% chance of developing heart disease within the next 10 years – as calculated using risk score algorithms – will be prescribed statins.
Anyone with a lower than 20% chance of developing heart disease within the next 10 years will simply be encouraged to improve their lifestyle.
Although this may seem like a reasonable guideline to have, a study carried out by Professor Humphries et al. (2006) highlighted a major weakness.
With a sample of 3,012 men, of which more than 200 developed heart disease in the first 10 years, the conventional risk factor assessment would only have found 14% of these men to be above the cut-off and, therefore, prescribed them statins.
The study emphasises just how many cases would not be classified as high risk based on a conventional risk factor assessment. When the risk estimates also incorporated genotype-risk factors they were more accurate than risk estimates that used solely conventional risk factors.
What is a gene?
Professor Humphries concisely ran through some basic information regarding genes and gene mutations, which ensured the lecture was coherent for anyone in the audience from a non-scientific background.
He described the effects of a gene mutation by referring to a mythical umbrella-making machine. When it rained, a properly functioning machine should make three umbrellas. However, some machines may malfunction and only produce one – which would not be sufficient.
Other machines may produce three umbrellas – but they may not open. Further machines may make three working umbrellas, but only when it wasn’t raining.
This clear, imagery-led description, although slightly bizarre, really clarified in a simple way the various different possible gene mutations.
What is SNP and how is it useful?
SNP stands for single nucleotide polymorphism; essentially, this is when a single nucleotide base (A, T, C and G) differs between the paired chromosomes. These tend to occur in the non-coding sections of the DNA sequence.
The SNPs that occur more frequently in individuals with heart disease – compared to controls – can be identified and possibly flagged up as genetic risk factors.
One particular SNP seemed to be most strongly associated with risk. A study by Talmud et al. (2008) found when comparing classic risk factors alone to the classic factors combined with the specific single SNP, that there was a three per cent improvement.
However, the results were not statistically significant, which indicates that risk has a multifactorial origin – consequently other SNPs needed to also be involved.
How can we use DNA tests?
Professor Humphries was keen to keep the audience grounded, pointing out that really this knowledge is just the tip of the iceberg. It is known that the heritability of heart disease is 45-55%, but only 10% of possible genetic causes have been found.
Despite this, genetic testing for heart disease is ready and can be of use. However, with the NHS in the state it is currently in, widespread use of this testing is not imminent. Professor Humphries suggested that this sort of technology might be of more use to organisations such as BUPA.
In the last few minutes of the lecture, Professor Humphries also managed to touch on some of the psychological issues with such testing.
He broached the idea of ‘genetic fatalism’. This was illustrated by an amusing cartoon, two women sat at a bar enjoying a cigarette. One remarking, “I haven’t got the gene, so it’s safe for me to smoke” and the other, “I’ve got the gene, so I’m doomed anyway!”
Although an important consideration, the audience was reassured that in this case the concern may be unjustified. There is the potential for positive change: if people know that they may be at higher risk, they may be more likely to change their behaviour and adhere to taking their medication.
By Jessica Lowrie, intern in UCL Communications & Marketing