New Canadian research provides the strongest evidence to date that there are numerous forms of hypertension, and that an individual's susceptibility to high blood pressure is programmed into his genes.
"Hypertension is not one disease, it's several diseases," said Dr. Pavel Hamet, director of research at the Centre hospitalier de l'Université de Montréal. "People with different types of hypertension have different ancestors, a different genetic makeup." The finding is important, he said, because it means scientists should be able to develop genetic tests to identify what form of hypertension a person suffers from and then tailor the treatment.
"Right now, we don't know what medication is best for whom," he said. "We have to treat patients, unfortunately, by trial and error."
About five million Canadians have high blood pressure, and only a fraction have the condition under control; being hypertensive triples the risk of heart attack and stroke. High blood pressure is largely a result of lifestyle. Dr. Hamet estimated hypertension is 30-per-cent genetic, 70-per-cent environmental.
Smoking, being overweight, inactivity, excessive salt intake and alcohol consumption are all risk factors.
But the new research confirms anecdotal evidence that the impact of these risk factors on blood pressure can vary markedly, depending on a person's genes. For example, people with a cluster of markers on chromosome 1 suffer from obesity and high blood pressure, while those with a different cluster on chromosome 3 suffer high blood pressure but are not obese.
What is clear is that there is not a single gene responsible for high blood pressure, said Dr. Allen Cowley, chairman of the department of physiology at the Medical College of Wisconsin in Milwaukee, and another member of the research team.
"We're not just looking for a gene or two that make your blood pressure go up. We're looking for a host of genes that make you more susceptible to environmental factors that affect blood pressure," he said.
The scientists stressed that treatments tailored to individuals are not around the corner.
The research was conducted on a group of 120 families living in the Saguenay-Lac St-Jean region of Quebec, where much of the population is descended form a small number of founding families, and where genealogical records are impeccable. A parallel study is being done among black families in the Milwaukee area, where there is a similar founder effect.
Dr. Daniel Gaudet, a professor of genomics at the University of Montreal, stressed that families in the Saguenay region do not have elevated rates of high blood pressure, but that the detailed knowledge of their backgrounds made it much easier to tease out genetic susceptibility.
Dr. Hamet said that medication to treat high blood pressure costs, on average, about $1,500 a year, and that drugs work only about half the time on specific patients, leading to many side effects.
Taking the guesswork out of prescription with genetic tests and tailored treatment would greatly reduce costs and adverse events, he said, but he predicted that this kind of tailored treatment is at least five years away.
Hypertension at a glance
Healthy adults should have a blood pressure of 120/80 millimetres of mercury (mmHg) or better. A person is considered hypertensive with a blood pressure reading of 140/90 or higher.
Blood pressure is a measure of the force of the blood against the walls of blood vessels. It is expressed in two numbers:
Systolic pressure, the higher number, is the pressure when the heart contracts;
Diastolic pressure, the lower number, is the pressure when the heart is relaxed.
Five types of drugs are used to treat hypertension: diuretics (water pills); beta-blockers, calcium channel blockers; angiotensin converting enzyme (ACE) inhibitors; angiotensin II receptor blockers (ARBs). Many people with hypertension are also prescribed a daily low dose of acetylsalicylic acid (ASA, or Aspirin).
