Using inactivating mutations to provide insight into drug action

Cardiovascular diseases (CVDs) are the primary cause of death globally 1]. In 2008, 30% of all global deaths were attributed to CVDs, including an estimated
7.3 million deaths caused by coronary heart disease (CHD) 1]. As a major risk factor for CHD, increased circulating cholesterol – particularly
low-density lipoprotein cholesterol (LDL-C) – is a well-established target for clinical
intervention. Although treatment with 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCR)
inhibitors (statins) has been shown both to lower LDL-C levels and to reduce major
cardiovascular events, in some cases statin therapy alone is insufficient to achieve
optimal LDL-C levels 2]. Currently, ezetimibe, which inhibits the function of the NPC1L1 protein, can be
prescribed alongside statins in order to achieve further reductions in LDL-C or as
an alternative in instances where statins are contraindicated. Whilst the ability
of ezetimibe to independently and additively lower LDL-C beyond the levels achieved
by statins alone does not appear to be in question, the degree to which the drug contributes
to a reduction in the risk of clinically relevant cardiovascular outcomes such as
CHD is unclear.

Numerous trials have been conducted to assess the clinical utility of LDL-C-lowering
therapies in reducing the incidence of CVD. The Pravastatin or Atorvastatin Evaluation
and Infection Therapy Trial (PROVE-IT) demonstrated that more intensive lipid lowering
achieved through an increased statin dose clinically benefited patients who had previously
suffered an acute coronary syndrome 3]. However, whether the same benefits can be achieved by prescribing ezetimibe alongside
statins to achieve similar reductions in LDL-C remains uncertain owing to inconsistent
trial outcomes, particularly where endpoints such as carotid intimal thickening and
vascular reactivity have been used as surrogates for CVD risk 2].

Two of the largest randomized control trials (RCTs) designed to determine whether
adding ezetimibe to statins provides clinical benefit (over and above statin monotherapy)
carried out to date are the Ezetimibe and Simvastatin in Hypercholesterolemia Enhances
Atherosclerosis Regression (ENHANCE) trial and IMProved Reduction of Outcomes: Vytorin
Efficacy International Trial (IMPROVE-IT). The ENHANCE trial (in which patients with
heterozygous familial hypercholesterolemia received simvastatin with or without ezetimibe)
was designed to study the effect of ezetimibe on the progression of atherosclerosis,
using carotid intima-media thickness (IMT) as a target endpoint. In this trial, the
addition of ezetimibe to simvastatin therapy in the treatment of familial hypercholesterolemia
did not produce a reduction in carotid IMT, despite achieving a differential reduction
in LDL-C 4]. This result was contrary to findings of similar trials conducted around the same
time 2]. In November 2014, preliminary results of the landmark IMPROVE-IT, designed to determine
whether adding ezetimibe to simvastatin in patients presenting with acute coronary
syndromes adds clinical benefit by further reducing major cardiovascular events compared
with simvastatin monotherapy 5], were presented at the American Heart Association Annual Meeting (Chicago, IL, Nov.
15-19, 2014). Although a full description of the findings is yet to be published,
preliminary results apparently suggest a modest benefit in reducing cardiovascular
events with the addition of ezetimibe to simvastatin in this population of around
18,000 patients from 39 countries. However, overall, the discordant results of studies
carried out to date has generated a degree of scepticism about whether ezetimibe offers
any health benefits above and beyond those afforded by statin therapy.

Using a population-based approach theoretically derived from the concept of genocopy,
Stitziel et al.6] offer evidence to strengthen inference from existing observational and RCT studies
and to inform future research. The term ‘genocopy’ refers to genetic variation that
generates an outcome similar to that produced by an environmental exposure 7]. An illustrative example of this phenomenon is the autosomal recessive condition
of Hartnup disease. This disease is caused by a mutation in solute carrier family
6 member 19 (SLC6A19; the ‘genocopy’) but a very similar clinical manifestation occurs in cases of dietary
niacin deficiency, a condition known as pellagra (the ‘phenocopy’). Using a Mendelian
randomization approach that takes advantage of the properties of genetic variation
and follows similar logic (such that mutations in NCP1L1 act as genocopies mimicking the action of ezetimibe), Stitziel et al.6] attempt to separate causation from association, providing validation of NCP1L1 as a therapeutic target.