Does lowering LDL matter? Insights from recent Mendelian Randomization studies

Meta-analyses of numerous randomized trials have demonstrated that lowering low density lipoprotein (LDL) by inhibiting HMG-CoA reductase (HMGCR) with a statin reduces the risk of major cardiovascular events by approximately 20% for each mmol/L lower LDL.¹ Remarkably however, there are still those who debunk the “lipoprotein theory of atherosclerosis” and heavily criticize statin therapy.  The sceptics often cite the several randomized trials which have failed to demonstrate that further lowering LDL by adding niacin, a fibrate or a CETP inhibitor to a statin further reduces the risk of cardiovascular events.^{2 3 4 5 6}. Naturally randomized genetic data from recent Mendelian randomization studies may help to resolve this uncertainty. ^{7 14}

Numerous polymorphisms* in the genes that encode the targets of various LDL lowering medications, including the statins, ezetimibe and the PCSK9 inhibitors, are associated with lower LDL; and each of these polymorphisms is inherited approximately randomly at the time of conception in a process sometimes referred to as Mendelian randomization.

Therefore, inheriting a LDL lowering allele in one of these genes is analogous to being randomly allocated to treatment with a LDL lowering therapy, while inheriting the other allele is analogous to being randomly allocated to usual care. If the polymorphism under study is associated with only LDL but not with other lipid or non-lipid pleiotropic effects, and if allocation is indeed random, then comparing the risk of CVD among persons with and without such a polymorphism should provide a naturally randomized and unconfounded estimate of the causal effect of lower LDL on the risk of cardiovascular disease (CVD) in a manner analogous to a long-term randomized trial.

Mendelian randomization studies have demonstrated that polymorphisms in multiple different genes are associated with both lower LDL and a lower risk of CVD^{8 15 9}, providing confirmation that LDL is causally associated with the risk of CVD. These studies have included not only polymorphisms in the genes that encode the targets of statins and ezetimibe, but also both common polymorphisms and the less common “loss-of-function” mutation* in the PCSK9 gene that motivated the discovery of monoclonal antibodies directed against PCSK9. Taken together, these studies can be thought of as a portfolio of “naturally randomized trials”, each evaluating a different mechanism of lowering LDL.

In these studies, polymorphisms with the largest effect on LDL were also associated with the greatest corresponding reduction in CVD risk.^{10 16} Indeed, when the effect of each polymorphism on LDL is plotted against its effect on the risk of CVD, there appears to be a log-linear association between genetically mediated lower LDL and the risk of CVD, independent of the mechanism by which LDL is lowered (Figure). Furthermore, when adjusted for a standard decrement in LDL change, each of these polymorphisms appears to have a remarkably similar effect on the risk of CVD per unit lower LDL (Figure). Therefore, the naturally randomized genetic evidence strongly argues that the effect of lower LDL on the risk of CVD is independent of the mechanism by which LDL is lowered.

The totality of the genetic evidence suggests that the effect of lower LDL on the risk of CVD appears to be determined by the absolute magnitude of exposure to lower LDL, regardless of how LDL is lowered.

This hypothesis was directed tested in a recent Mendelian randomization study that compared the effect of lower LDL on the risk of CVD mediated by polymorphisms in the NPC1L1 gene (the target of ezetimibe), the HMGCR gene (the target of statins) or both (the targets of combination therapy) in a “naturally randomized IMPROVE-IT Trial”. This study found that polymorphisms that mimic the effect of ezetimibe and polymorphisms that mimic the effect of statins had approximately the same effect on the risk of CVD per unit lower LDL, and when present together they had independent linearly additive effects on LDL and a log-linearly additive effects on CVD risk.¹¹ The naturally randomized genetic data from this study therefore predicted that adding ezetimibe to a statin should reduce the risk of CVD proportional to the absolute achieved reduction in LDL.

Indeed, the naturally randomized genetic data precisely predicted the results of the recently completed IMPROVE-IT trial. In IMPROVE-IT, adding ezetimibe to a statin resulted in a linearly additive 15 mg/dl further reduction in LDL and a log-linearly additive 6.4% lower risk of the primary composite endpoint and a 10% lower risk of the secondary composite endpoint of CVD death, MI or stroke. ¹² The magnitude of this risk reduction is approximately what would be expected based on the absolute reduction in LDL observed during the trial as estimated by the Cholesterol Treatment Trialists’ Collaborators meta-analysis of statin trials.¹³

The close agreement between the results of the Mendelian randomization studies and the results of landmark IMPROVE-IT trial suggest that the effect of both genetically and pharmacologically mediated lower LDL on the risk of CVD appears to be determined by the absolute magnitude of exposure to lower LDL, regardless of how LDL is lowered (Figure).

This finding may explain the failure of the niacin, fibrate and dalcetrapib CETP inhibitor trials. In these studies, the absolute magnitude of the achieved LDL reduction was too small and the number of events accrued too few to reliably demonstrate a numerically stable reduction in the risk of CVD. Furthermore, the close agreement between the naturally randomized genetic data and the results of IMPROVE-IT strongly suggest that lowering LDL with a statin or with ezetimibe or with combination therapy or with any other method of lowering LDL should each reduce the risk of CVD by approximately the same amount per unit lower LDL regardless of which treatment is used.

Importantly, these data therefore also suggest that inhibiting PCSK9 with a monoclonal antibody should reduce the risk of CVD by approximately the same amount as do statins per unit lower LDL. Indeed, based on previous (and ongoing) Mendelian randomization studies one would predict that the eagerly anticipated PCSK9 outcome trials are likely to demonstrate that further lowering LDL cholesterol by adding a PCSK9 inhibitor to a statin will reduce the risk of CVD by approximately 20% for each mmol/L reduction in LDL cholesterol observed in the trial, because the effect of lower LDL on the risk of CHD appears to be independent of the mechanism by which LDL is lowered.

* What is the difference between “polymorphism” and “mutation”?

A mutation is generally defined as a change in DNA sequence away from normal, thus implying that there is a normal allele in the population and that the mutation changes it to an abnormal variant. By contrast, a polymorphism is a change in DNA sequence that is common, where no single allele is regarded as the normal allele. In general, mutations are rare while polymorphisms, by definition, are common.  

References:

1, 13.		Cholesterol Treatment Trialists’ (CTT) Collaboration. Efficacy and safety of more intensive lowering of LDL cholesterol: a meta-analysis of data from 170,000 participants in 26 randomised trials. Lancet 2010;376:1670-81.
2.		AIM-HIGH Investigators. Niacin in patients with low HDL cholesterol levels receiving intensive statin therapy. N Engl J Med. 2011;365:2255-67.
3.		HPS2-THRIVE Collaborative Group. Effects of extended-release niacin with laropiprant in high-risk patients. N Engl J Med. 2014;371:203-12.
4.		ACCORD Study Group. Effects of combination lipid therapy in type 2 diabetes mellitus. N Engl J Med. 2010;362:1563-74.
5.		dal-OUTCOMES Investigators. Effects of dalcetrapib in patients with a recent acute coronary syndrome. N Engl J Med. 2012;367:2089-99.
6.		Stone NJ, Robinson JG, Lichtenstein AH, et al. 2013 ACC/AHA guideline on the treatment of blood cholesterol to reduce atherosclerotic cardiovascular risk in adults: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 2014;63(25 Pt B):2889-934.
7, 8, 10.		Ference BA, Yoo W, Alesh I, et al. Effect of long-term exposure to lower low-density lipoprotein cholesterol beginning early in life on the risk of coronary heart disease: a Mendelian randomization analysis. J Am Coll Cardiol. 2012;60:2631-9.
9.		Global Lipids Genetics Consortium. Discovery and refinement of loci associated with lipid levels. Nat Genet. 2013;45:1274-83.
11, 14, 15, 16.		Ference BA, Majeed F, Penumetcha R, Flack JM, Brook RD. Effect of naturally randomvallocation to lower low-density lipoprotein cholesterol on the risk of coronary heart disease mediated by polymorphisms in NPC1L1, HMGCR, or both: a 2 x 2 factorial Mendelian randomization study. J Am Coll Cardiol 2015;65:1552–61.
12.		Cannon CP, for the IMPROVE-IT Investigators. IMPROVE-IT Trial: A Comparison of Ezetimibe/Simvastatin versus Simvastatin Monotherapy on Cardiovascular Outcomes After Acute Coronary Syndromes. Presented at: American Heart Association Scientific Sessions; November, 17 2014; Chicago, IL.