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Doubt Cast on the 'Good' in 'Good Cholesterol' - 2 Lancet articles below
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Download the PDF here
Download the PDF here
NY Times By GINA KOLATA
Published: May 16, 2012
The name alone sounds so encouraging: HDL, the "good cholesterol." The more of it in your blood, the lower your risk of heart disease. So bringing up HDL levels has got to be good for health.
Or so the theory went.
Now, a new study that makes use of powerful databases of genetic information has found that raising HDL levels may not make any difference to heart disease risk. People who inherit genes that give them naturally higher HDL levels throughout life have no less heart disease than those who inherit genes that give them slightly lower levels. If HDL were protective, those with genes causing higher levels should have had less heart disease.
Researchers not associated with the study, published online Wednesday in The Lancet, found the results compelling and disturbing. Companies are actively developing and testing drugs that raise HDL, although three recent studies of such treatments have failed. And patients with low HDL levels are often told to try to raise them by exercising or dieting or even by taking niacin, which raised HDL but failed to lower heart disease risk in a recent clinical trial.
"I'd say the HDL hypothesis is on the ropes right now," said Dr. James A. de Lemos, a professor at the University of Texas Southwestern Medical Center, who was not involved in the study.
Dr. Michael Lauer, director of the division of cardiovascular sciences at the National Heart, Lung and Blood Institute, agreed.
"The current study tells us that when it comes to HDL we should seriously consider going back to the drawing board, in this case meaning back to the laboratory," said Dr. Lauer, who also was not connected to the research. "We need to encourage basic laboratory scientists to figure out where HDL fits in the puzzle - just what exactly is it a marker for."
But Dr. Steven Nissen, chairman of cardiovascular medicine at the Cleveland Clinic, who is helping conduct studies of HDL-raising drugs, said he remained hopeful. HDL is complex, he said, and it is possible that some types of HDL molecules might in fact protect against heart disease.
"I am an optimist," Dr. Nissen said.
The study's authors emphasize that they are not questioning the well-documented finding that higher HDL levels are associated with lower heart disease risk. But the relationship may not be causative. Many assumed it was because the association was so strong and consistent. Researchers also had a hypothesis to explain how HDL might work. From studies with mice and with cells grown in the laboratory, they proposed that HDL ferried cholesterol out of arteries where it did not belong.
Now it seems that instead of directly reducing heart disease risk, high HDL levels may be a sign that something else is going on that makes heart disease less likely. To investigate the relationship between HDL and cardiovascular risk, the researchers, led by Dr. Sekar Kathiresan, director of preventive cardiology at Massachusetts General Hospital and a geneticist at the Broad Institute of M.I.T. and Harvard, used a method known as Mendelian randomization. It is a study design that has recently become feasible with the advent of quick and lower-cost genetic analyses.
The idea is that people inherit any of a wide variety of genetic variations that determine how much HDL they produce. The result is that people are naturally and randomly assigned by these variations in their inherited genes to make more, or less, HDL, throughout their lives. If HDL reduces the risk of heart disease, then those who make more should be at lower risk.
For purposes of comparison, the researchers also examined inherited variations in 13 genes that determine levels of LDL, the so-called bad cholesterol. It is well known and widely accepted that lowering LDL levels by any means - diet and exercise, statin drugs - reduces risk. Clinical trials with statins established with certainty that reducing LDL levels is protective. So, the researchers asked, did people who inherited gene variations that affected their LDL levels, have correspondingly higher or lower heart disease risk?
The study found, as expected, that gene variations that raise LDL increase risk and those that lower LDL decrease risk. The gene effects often were tiny, altering LDL levels by only a few percent. But the data, involving tens of thousands of people, clearly showed effects on risk.
"That speaks to how powerful LDL is," Dr. Kathiresan said.
But the HDL story was very different. First the investigators looked at variations in a well-known gene, endothelial lipase, that affects only HDL. About 2.6 percent of the population has a variation in that gene that raises their HDL levels by about 6 points. The investigators looked at 116,000 people, asking if they had the variant and if those who carried the HDL-raising variant had lower risk for heart disease.
"We found absolutely no association between the HDL-boosting variant and risk for heart disease," Dr. Kathiresan said. "That was very surprising to us."
Then they looked at a group of 14 gene variants that also affect HDL levels, asking if there was a relationship between these variants and risk for heart disease. The data included genetic data on 53,500 people. Once again, there was no association between having the variants that increased HDL and risk of heart disease.
Dr. Lauer explains what that means with an analogy.
"One might think of a highway accident that causes a massive traffic jam," he said. "Stewing in the jam many miles away, I might be tempted to strike the sign that says 'accident ahead,' but that won't do any good. The 'accident-ahead' sign is not the cause of the traffic jam - the accident is. Analogously, targeting HDL won't help if it's merely a sign."
Dr. Kathiresan said there were many things HDL might indicate. "The number of factors that track with low HDL is a mile long," he said. "Obesity, being sedentary, smoking, insulin resistance, having small LDL particles, having increased cholesterol in remnant particles, and having increased amounts of coagulation factors in the blood," he said. "Our hypothesis is that much of the association may be due to these other factors."
"I often see patients in the clinic with low HDL levels who ask how they can raise it," Dr. Kathiresan said. "I tell them, 'It means you are at increased risk, but I don't know if raising it will affect your risk.' "
That often does not go over well, he added. The notion that HDL is protective is so entrenched that the study's conclusions may prove hard to accept, he and other researchers said.
"When people see numbers in the abnormal range they want to do something about it," Dr. Kathiresan said. "It is very hard to get across the concept that the safest thing might be to leave people alone."
This article has been revised to reflect the following correction:
Correction: May 16, 2012
Because of an editing error, a correction with an earlier version of this article was appended mistakenly, and described niacin incorrectly. While niacin is a vitamin, as the correction noted, it is considered a drug when given at pharmaceutical doses, as in a recent trial in which it was shown to raise HDL without lowering heart disease risk.
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The Lancet, Early Online Publication, 17 May 2012
"These results show that some ways of raising HDL cholesterol might not reduce risk of myocardial infarction in human beings. Therefore, if an intervention such as a drug raises HDL cholesterol, we cannot automatically assume that risk of myocardial infarction will be reduced."
"These results challenge several established views about plasma HDL cholesterol. First, these data question the concept that raising of plasma HDL cholesterol should uniformly translate into reductions in risk of myocardial infarction. We raise the strong possibility that a specific means of raising of HDL cholesterol in human beings-namely, inhibition of endothelial lipase-will not reduce risk of myocardial infarction. In animal models, inhibition or deletion of the endothelial lipase gene increases HDL cholesterol concentrations,27 but there has been debate as to the consequent effect on atherosclerosis. One report suggested that mice deleted for Lipg on an Apoe knockout genetic background have decreased aortic atherosclerosis,28 but a subsequent study showed no effect of Lipg deletion on aortic atherosclerosis.29
Second, these findings emphasise the potential limitation of plasma HDL cholesterol as a surrogate measure for risk of myocardial infarction in intervention trials."
Mendelian randomisation, lipids, and cardiovascular disease
Seamus C Harrison a, Michael V Holmes b, Steve E Humphries a
In The Lancet, Benjamin Voight and colleagues1 use mendelian randomisation analysis to investigate the relation between HDL cholesterol and coronary heart disease. High HDL cholesterol concentration is associated with reduced risk of coronary heart disease in observational studies,2 but whether the association is causal cannot be unequivocally ascertained from these studies alone, and whether raising of HDL cholesterol would be an effective means to reduce risk of coronary heart disease remains uncertain. The ILLUMINATE trial of torcetrapib, a cholesteryl ester transport protein (CETP) inhibitor that raises HDL cholesterol, was stopped early because of an increase in the number of cardiovascular events.3 This outcome might have been due to an off-target action of torcetrapib on blood pressure that appears not to be shared by newer drugs from the same class, and the effect of these drugs on risk of coronary heart disease is now being evaluated in phase 3 trials.
Mendelian randomisation exploits genetic information to investigate associations between exogenous or endogenous exposures (or both) and disease outcomes.4 The random allocation of genotype at gametogenesis (like the randomised allocation to a drug in a clinical trial) minimises confounding. If HDL-cholesterol-mediated pathways were causal for coronary heart disease, carriers of genetic variants associated with high concentrations of HDL cholesterol (representing lifelong exposure to high HDL cholesterol) would be expected to have a reduced risk of coronary heart disease.
Voight and colleagues used a rare non-synonymous variant in the endothelial lipase gene (LIPG Asn396Ser, minor allele frequency about 2.6%) that showed a consistent association with high HDL cholesterol concentrations, but no association with LDL cholesterol or triglycerides. In view of the observed effect of the LIPG variant on HDL cholesterol, this allele was expected to result in roughly a 13% reduced risk of coronary heart disease. However, in pooled data from 20913 cases and 95407 controls, the observed odds ratio for coronary heart disease was 0.99 (95% CI 0.88-1.11), suggesting that LIPG-mediated increases in HDL cholesterol do not reduce risk of the disease. In Voight and colleagues' analysis, a panel of 14 common variants, each with a small effect on HDL cholesterol,5 was combined into a genetic risk score. The risk score was also not associated with coronary heart disease in pooled data from 12482 cases and 41331 controls (odds ratio per SD increase in weighted genetic risk 0.93, 95% CI 0.68-1.26). On the basis of these results, genetically raised HDL cholesterol concentrations do not seem to reduce risk of coronary heart disease-an observation that calls into question whether raising of HDL cholesterol therapeutically would translate into the expected clinical benefit.
The validity of a mendelian randomisation analysis is determined by various factors. First, the intermediate phenotype (HDL cholesterol) must associate with the outcome (coronary heart disease). Second, the genetic instrument must associate with the outcome only through effects on the intermediate phenotype. Third, the genetic instrument should be sufficiently strongly associated with the intermediate phenotype to avoid weak instrument bias.6 The LIPG variant used here had a fairly large effect on HDL cholesterol, but is rare in the population and so might not represent a strong instrument. We should note, however, that the case-control analysis of this variant was adequately powered to detect even a small effect, and the negative association of this variant can be regarded as definitive. For the genetic risk score, variants with the largest effects were excluded because of associations with other lipid fractions, and although each of the risk score variants showed association with HDL cholesterol, individual effect sizes were small, which might not translate into a powerful instrument even when combined.
These findings are consistent with previous mendelian randomisation analyses that also refute a causal role of HDL cholesterol in coronary heart disease.7, 8 Furthermore, they are supported by rare mendelian disorders such as Tangier disease, caused by mutations in ABCA1, resulting in very low HDL cholesterol concentrations, which do not show convincing associations with premature coronary disease.9 One notable exception is that variants in CETP that raise HDL cholesterol have been reported to be associated with lowered risk of coronary heart disease,10 a finding replicated by Voight and colleagues. The CETP variant, however, has effects on both HDL and LDL cholesterol and therefore attribution of its protective effect solely to HDL cholesterol might not be valid. For the same reason, ongoing randomised trials of CETP inhibitors might not provide definitive evidence about a causal role for HDL cholesterol in coronary heart disease. However, even if HDL cholesterol concentration is not validated as a causal factor, further investigation into the mechanisms of HDL cholesterol dysfunction11 and its role in coronary heart disease is warranted.
This report adds to a growing number of mendelian randomisation analyses investigating biomarkers of coronary heart disease. Previous reports suggest that genetic variants underlying lipoprotein (a)12 and triglycerides are likely to be causal,13 whereas those in C-reactive protein are not.14 These studies have focused on single variants as instruments, but combination of variants into a score represents a potential improvement of the technique. As the research area matures, a consensus for methodology and reporting will be important, particularly when the potentially powerful, but also complex, genetic risk score approach is used. Taken together with adequately powered studies, mendelian randomisation is likely to yield insights that can both guide public health policy and prioritise potential therapeutic targets.
SEH is medical director and minority shareholder of the UCL start-up coronary heart disease risk genetic testing company Storegene, and has received honoraria for speaking at educational meetings with a pharmaceutical sponsor, donating all sums to medical charities. MVH and SCH declare that they have no conflicts of interest.
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The Lancet, Early Online Publication, 17 May 2012
Plasma HDL cholesterol and risk of myocardial infarction: a mendelian randomisation study
Summary
Background
High plasma HDL cholesterol is associated with reduced risk of myocardial infarction, but whether this association is causal is unclear. Exploiting the fact that genotypes are randomly assigned at meiosis, are independent of non-genetic confounding, and are unmodified by disease processes, mendelian randomisation can be used to test the hypothesis that the association of a plasma biomarker with disease is causal.
Methods
We performed two mendelian randomisation analyses. First, we used as an instrument a single nucleotide polymorphism (SNP) in the endothelial lipase gene (LIPG Asn396Ser) and tested this SNP in 20 studies (20 913 myocardial infarction cases, 95 407 controls). Second, we used as an instrument a genetic score consisting of 14 common SNPs that exclusively associate with HDL cholesterol and tested this score in up to 12 482 cases of myocardial infarction and 41 331 controls. As a positive control, we also tested a genetic score of 13 common SNPs exclusively associated with LDL cholesterol.
Findings
Carriers of the LIPG 396Ser allele (2·6% frequency) had higher HDL cholesterol (0·14 mmol/L higher, p=8x10-13) but similar levels of other lipid and non-lipid risk factors for myocardial infarction compared with non-carriers. This difference in HDL cholesterol is expected to decrease risk of myocardial infarction by 13% (odds ratio [OR] 0·87, 95% CI 0·84-0·91). However, we noted that the 396Ser allele was not associated with risk of myocardial infarction (OR 0·99, 95% CI 0·88-1·11, p=0·85). From observational epidemiology, an increase of 1 SD in HDL cholesterol was associated with reduced risk of myocardial infarction (OR 0·62, 95% CI 0·58-0·66). However, a 1 SD increase in HDL cholesterol due to genetic score was not associated with risk of myocardial infarction (OR 0·93, 95% CI 0·68-1·26, p=0·63). For LDL cholesterol, the estimate from observational epidemiology (a 1 SD increase in LDL cholesterol associated with OR 1·54, 95% CI 1·45-1·63) was concordant with that from genetic score (OR 2·13, 95% CI 1·69-2·69, p=2x10-10).
Interpretation
Some genetic mechanisms that raise plasma HDL cholesterol do not seem to lower risk of myocardial infarction. These data challenge the concept that raising of plasma HDL cholesterol will uniformly translate into reductions in risk of myocardial infarction.
Funding
US National Institutes of Health, The Wellcome Trust, European Union, British Heart Foundation, and the German Federal Ministry of Education and Research.
Introduction
Cholesterol fractions such as LDL and HDL cholesterol are among the most commonly measured biomarkers in clinical medicine.1 Observational studies have shown that LDL and HDL cholesterol have opposing associations with risk of myocardial infarction, with LDL cholesterol being positively associated and HDL cholesterol being inversely associated.2, 3 However, observational studies cannot distinguish between a causal role in the pathological process and a marker of the underlying pathophysiology. These two possibilities can be distinguished in human beings by changes of the cholesterol fractions in large-scale randomised trials or by studies of inherited DNA variation. For LDL cholesterol, the results of both randomised trials of LDL-cholesterol-lowering treatments4 and from human mendelian diseases5, 6 are concordant and suggest that plasma LDL cholesterol is causally related to risk of myocardial infarction. However, the available evidence for the causal relevance of HDL cholesterol from randomised trials or mendelian diseases is scarce and inconsistent.7, 8
If a particular plasma biomarker is directly involved in an underlying pathological process, then inherited variation changing plasma concentrations of this biomarker should affect risk of disease in the direction and magnitude predicted by the plasma concentrations. Referred to as mendelian randomisation,9-11 this analytical approach has been previously applied to plasma HDL cholesterol, albeit with restricted sample sizes, a small number of single nucleotide polymorphisms (SNPs) at a few genes, and with SNPs that affect multiple lipid fractions.8,12-15 Hence, these studies have not been able to resolve fully the possible causal relevance of HDL cholesterol concentrations for risk of myocardial infarction.
Recently, we have used the genome-wide association approach to identify SNPs that affect blood lipid concentrations.16, 17 Additionally, through resequencing, we identified a loss-of-function coding SNP at the endothelial lipase gene (LIPG Asn396Ser) that affects plasma HDL cholesterol in isolation.18, 19 Here, we use these SNPs in case-control studies and prospective cohort studies to test the hypothesis that genetically raised plasma HDL cholesterol might be protective for myocardial infarction.
Discussion
For a biomarker directly involved in disease pathogenesis, we expect a genetic variant that modulates the biomarker to likewise confer risk of disease. We tested this hypothesis for two plasma biomarkers: LDL and HDL cholesterol. SNPs affecting LDL cholesterol were consistently related to risk of myocardial infarction. However, we unexpectedly found that LIPG Asn396Ser, a genetic variant that specifically and substantially increases plasma HDL cholesterol, did not reduce risk of myocardial infarction. A genetic score combining 14 variants exclusively related to HDL cholesterol also showed no association with risk of myocardial infarction (Panel).
Panel
Research in context
Systematic review
Electronic searches of Medline and PubMed, supplemented by hand searches of reference lists of other review articles, identified reports from three large mendelian randomisation studies for plasma HDL cholesterol.7, 12, 15 In each of these previous reports, genetically increased plasma HDL cholesterol was not associated with risk of ischaemic heart disease.
Interpretation
The present study tested a naturally occurring loss-of-function variant in the endothelial lipase gene and, with this new instrument, we confirm that genetically raised plasma HDL cholesterol is not associated with risk of myocardial infarction. The study further extends previous work by testing an instrument consisting of 14 common variants exclusively associated with plasma HDL cholesterol. A genetic score consisting of these 14 variants was not associated with risk of myocardial infarction. These results show that some ways of raising HDL cholesterol might not reduce risk of myocardial infarction in human beings. Therefore, if an intervention such as a drug raises HDL cholesterol, we cannot automatically assume that risk of myocardial infarction will be reduced.
These results challenge several established views about plasma HDL cholesterol. First, these data question the concept that raising of plasma HDL cholesterol should uniformly translate into reductions in risk of myocardial infarction. We raise the strong possibility that a specific means of raising of HDL cholesterol in human beings-namely, inhibition of endothelial lipase-will not reduce risk of myocardial infarction. In animal models, inhibition or deletion of the endothelial lipase gene increases HDL cholesterol concentrations,27 but there has been debate as to the consequent effect on atherosclerosis. One report suggested that mice deleted for Lipg on an Apoe knockout genetic background have decreased aortic atherosclerosis,28 but a subsequent study showed no effect of Lipg deletion on aortic atherosclerosis.29
Second, these findings emphasise the potential limitation of plasma HDL cholesterol as a surrogate measure for risk of myocardial infarction in intervention trials. The data presented here using mendelian randomisation are consistent with results from completed randomised controlled trials focused on raising plasma HDL cholesterol. Hormone replacement therapy raised plasma HDL cholesterol but did not lower risk of myocardial infarction.30 In the Atherothrombosis Intervention in Metabolic Syndrome with Low HDL Cholesterol/High Triglyceride and Impact on Global Health Outcomes (AIM-HIGH) trial,31 the addition of long-acting niacin to background simvastatin increased HDL cholesterol and lowered triglycerides but did not lower risk of cardiovascular events.
Of note, at the cholesterol ester transfer protein (CETP) gene, we did find that common genetic variation reduces risk of myocardial infarction by 4%, a result in line with an earlier meta-analysis.32 However, the CETP variant both increases HDL cholesterol and lowers LDL cholesterol17 in a manner similar to pharmacological inhibitors of CETP.33 As such, whether the protection afforded by the CETP variant is due to the change in HDL or LDL cholesterol is difficult to distinguish.
Third, biomarkers that assay HDL function might overcome some limitations of standard HDL cholesterol assays. However, a challenge will remain-namely, to prove that new functional HDL biomarkers reflect a causal association with myocardial infarction rather than an indirect one, as seems to be the case with plasma HDL cholesterol. For example, using a new in-vitro measure involving mouse macrophages and human serum, Khera and colleagues34 showed an inverse correlation between a specific functional property of HDL, cholesterol efflux capacity, and coronary artery disease status. The present study suggests that a fruitful approach to the causal evaluation of such functional measures in human beings might be large-scale study of relevant inherited DNA variation of HDL function.
There are inherent limitations to the mendelian randomisation study design. Naturally occurring genetic variation could be a useful instrument to assess causality provided that several requirements have been satisfied.35, 36 First, one needs suitable genetic variants for the study of the modifiable exposure of interest (in our case, plasma HDL cholesterol). Although many loci are associated with plasma HDL cholesterol, we found LIPG Asn396Ser to be particularly informative because it is specifically associated with substantial increases in HDL cholesterol. Additionally, we evaluated a set of 14 common genetic variants that also exclusively affected HDL cholesterol. Both instruments, LIPG Asn396Ser and the genetic score, produced similar results.
Second, reliable genotype-to-intermediate-phenotype and intermediate-phenotype-to-disease effect estimates are needed. To obtain as precise estimates as possible, we derived SNP-to-lipid effect estimates from analysis of a large sample involving more than 24 000 participants. Estimates of plasma lipid to myocardial infarction were derived from meta-analysis of four large cohort studies involving more than 25 000 participants.
Third, there must not be pleiotropic effects of the genetic variants of interest. We cannot exclude all potential pleiotropic effects of the LIPG Asn396Ser SNP; however, we have assessed but did not detect pleiotropic effects on other cardiovascular risk factors including LDL cholesterol, small LDL particle concentration, triglycerides, body-mass index, systolic blood pressure, plasma C-reactive protein, and type 2 diabetes status.
Finally, the absence of association of individual SNPs with myocardial infarction could be due to low statistical power. However, for the crucial SNP in the mendelian randomisation study for plasma HDL cholesterol, we had sufficient power. In this study, LIPG Asn396Ser has been tested in 20 913 myocardial infarction cases and 95 407 myocardial-infarction-free participants. We had 90% power to detect the expected 13% reduction in risk of myocardial infarction for the LIPG Asn396Ser variant (at a two-sided α of 0·05).
In summary, our results showed that polymorphisms related to plasma LDL cholesterol were consistently associated with risk of myocardial infarction, whereas this was not the case for variants related to plasma HDL cholesterol. A polymorphism in the endothelial lipase gene and a genetic score of 14 common SNPs that specifically raised HDL cholesterol were not associated with myocardial infarction, suggesting that some genetic mechanisms that raise HDL cholesterol do not lower risk of myocardial infarction. Hence, interventions (lifestyle or pharmacological) that raise plasma HDL cholesterol cannot be assumed ipso facto to lead to a corresponding benefit with respect to risk of myocardial infarction.
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