And you measure genetic polymorphisms?
Correct. In this study, we created a score of 46 different polymorphisms associated with lower LDL, and a separate SBP genetic score consisting of 33 polymorphisms associated with lower SBP. These scores were used as instruments to naturally randomize people to higher or lower LDL, higher or lower SBP, or both, in a factorial design, in exactly the way we would in a randomized trial.
How did you lower blood pressure (BP) and lower LDL?
We didn’t intervene to lower BP. It was the scores that randomized people to higher or lower levels. This is referred to as the concept of Mendelian randomization. Perhaps the easiest way to explain that concept is by way of analogy with a randomized trial.
There are numerous polymorphisms, for example, that are associated with lower LDL, and each of those polymorphisms are inherited approximately randomly at the time of conception in a process sometimes referred to as Mendelian randomization. Inheriting an allele associated with lower LDL is analogous to being randomly allocated to an LDL-lowering therapy, while inheriting the other allele is analogous to being randomly allocated to usual care. If allocation is really random, the only difference will be the LDL level. Then we can follow people forward to get an unconfounded estimate of the causal effect of LDL in a manner analogous to a long-term randomized trial.
All randomized trials I know of have entry and exclusion criteria. Were there any entry or exclusion criteria in this methodology?
There’s a fundamental difference between a person enrolled in a naturally (Mendelian) randomized trial and one enrolled in a randomized trial.
Unfortunately, people enrolled in a randomized trial, for convenience, are those with advanced atherosclerotic disease, because the expectation is they will have the greatest number of events in the shortest period to minimize the sample size. Attempts to lower LDL or BP in this population may have minimal impact (or at least less than optimal impact), because of the advanced atherosclerotic and arteriosclerotic burden. By contrast, the approach of the Mendelian randomization contemplates keeping BP and LDL lower beginning much earlier in the atherosclerotic disease process. It therefore examines explicitly what could happen if we began to intervene on risk factors – not necessarily pharmacologically, but with lifestyle factors and other methods – much earlier in the atherosclerotic disease process.
To prevent a cardiovascular event, it’s got to start in childhood, by preventing risk factors from evolving.
Right. I think this is what we tried to asses in this study. I’m not so sure it necessarily has to be childhood. I think in childhood it’s the perfect time to begin introducing healthy lifestyles. However, whether or not we aggressively seek to set targets or lower risk factors that young is hard to say. I think for LDL certainly it’s a little bit different, because it’s fairly stable over the life course. But BP is slightly different; it remains stable into the 20s and 30s, and when it hits an inflection point it begins to rise linearly with age. The key is whether we can identify when BP begins to rise and prevent its rise with age. This study addresses that issue.
What did you find?
The purpose of this study was specifically to evaluate the causal effect of combined exposure to both lower LDL and lower SBP on the risk of cardiovascular disease. We sought to address this topic because previous observational studies have suggested that persons who can maintain ideal risk factor profiles throughout the whole of their lifetime have a very low lifetime risk of cardiovascular disease. Unfortunately, fewer than five percent of the population can maintain ideal risk factors.
By contrast, previous Mendelian randomization studies from our group and others have suggested that both LDL and SBP are not only causally related to cardiovascular disease, but they have a cumulative effect over time. This would suggest a simple strategy that promotes long-term exposure to the combination of both lower LDL and lower SBP may be effective for preventing cardiovascular disease.
Unfortunately, the causal effect of exposure to both lower LDL and lower SBP on events is unknown. That uncertainty was recently increased after the landmark HOPE-3 trial. As you know, HOPE-3 reported that the combination of lipid-lowering therapy plus a modest blood pressure-lowering regimen didn’t reduce cardiovascular events significantly more than lipid-lowering alone. That has caused some people to question this synergy between LDL and BP on the risk of cardiovascular disease, particularly among persons without hypertension. We specifically wanted to determine the causal effect of combined exposure to lower LDL and lower BP.
What was the average age of patients? What did you consider hypertension and hyperlipidemia?
We didn’t define hyperlipidemia or hypertension as a criterion. In fact, I would go so far as to suggest that the “at the moment” way we treat hypertension is slightly backward. Rather than letting arteriosclerotic injury advance over multiple years to prove the point when the person develops a BP of 150 or 160, and then argue about how low we should lower it, we should probably begin to think about the notion of preventing the age-related rise. This study gets to that.
The average age was 60.7 years, the average LDL was about 3.4 mmol (128 mg/dl), and the average BP was about 126 mm Hg.
So it was relatively low blood pressure.
Yes, relatively normal levels. The LDL genetic scores and the SBP scores were used to naturally randomize people into four groups: a reference group, a lower LDL group, a lower SBP group, or both. We found that the combination of exposure to both lower LDL and lower SBP was associated with a significantly greater benefit than lower LDL alone or lower SBP alone. In fact, the effects were independent, multiplicative, and cumulative over time […] and that effect was quite consistent across multiple different composite cardiovascular outcomes and the individual components of the composite, including myocardial infarction and coronary heart disease death. There was an 84 percent reduction in coronary heart disease death, which translated into a smaller but still significant and robust 36 percent reduction in all-cause mortality. In addition, the effect seems to be consistent across all the subgroups we studied.
So the take-home message is that lowering SBP and LDL each lower risk, and lowering the combination lowers risk a bit better than individually. There is also a cumulative effect over time that’s persistent. Anything else?
The last point is salient. If we’d like to prevent cardiovascular disease, we might have to start slightly earlier than we do now.
Editor’s Note: This interview has been edited for print from a transcript.
Ference BA. A naturally randomized trial comparing the effect of long-term exposure to lower LDL-C, lower SBP, or both on the risk of cardiovascular disease. Presented at the European Society of Cardiology Congress, Rome, Italy. August 29, 2016.
Ference BA, Julius S, Mahajan N, Levy PD, Williams KA Sr, Flack JM. Hypertension 2014;63:1182-8.
Ference BA, Majeed F, Penumetcha R, Flack JM, Brook RD. J Am Coll Cardiol 2015;65:1552-61.
Ference BA, Robinson JG, Brook RD, et al. N Engl J Med 2016;375:2144-53.
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