Joseph B. Dubé

Lipoprotein(a): more interesting than ever after 50 years.

Purpose of review Lipoprotein(a) [Lp(a)] is a risk factor for cardiovascular disease; we highlight the most recent research initiatives that have sought to define Lp(a)-dependent pathogenicity as well as pharmacologic approaches to lowering Lp(a). Recent findings Recent large-scale meta-analyses have confirmed elevated Lp(a) concentrations to be a moderate but consistent prospective coronary heart disease (CHD) risk factor. The Mendelian randomization approach has also associated LPA variants with Lp(a) concentration and CHD risk. Discoveries linking Lp(a) to oxidized phospholipid burden have implicated a proinflammatory role for Lp(a) hinting at a new mechanism underlying the association with CHD risk, which adds to previous atherogenic and thrombogenic mechanisms. Most existing Lp(a)-lowering drug treatments almost always show simultaneous effects on other lipoproteins, making it difficult to assign any clinical outcome specifically to the effects of Lp(a) lowering. Early experiments with antisense oligonucleotides targeting apolipoprotein(a) mRNA seem to indicate the pleiotropic effects of Lp(a) reduction on LDL and HDL in mice. The mechanism linking Lp(a) concentration with concentrations of other blood lipids remains unknown but may provide an insight into Lp(a) metabolism. Summary Despite the wealth of epidemiologic evidence supporting Lp(a) concentration as a CHD risk factor, the lack of a definitive functional mechanism involving an Lp(a)-dependent pathway in CHD pathogenesis has limited the potential clinical connotation of Lp(a). However, the application of novel technologies to the long-standing mysteries of Lp(a) biology seems to provide the opportunity for expanding our understanding of Lp(a) and its complex role in cardiovascular health.

LipidSeq: a next-generation clinical resequencing panel for monogenic dyslipidemias

We report the design of a targeted resequencing panel for monogenic dyslipidemias, LipidSeq, for the purpose of replacing Sanger sequencing in the clinical detection of dyslipidemia-causing variants. We also evaluate the performance of the LipidSeq approach versus Sanger sequencing in 84 patients with a range of phenotypes including extreme blood lipid concentrations as well as additional dyslipidemias and related metabolic disorders. The panel performs well, with high concordance (95.2%) in samples with known mutations based on Sanger sequencing and a high detection rate (57.9%) of mutations likely to be causative for disease in samples not previously sequenced. Clinical implementation of LipidSeq has the potential to aid in the molecular diagnosis of patients with monogenic dyslipidemias with a high degree of speed and accuracy and at lower cost than either Sanger sequencing or whole exome sequencing. Furthermore, LipidSeq will help to provide a more focused picture of monogenic and polygenic contributors that underlie dyslipidemia while excluding the discovery of incidental pathogenic clinically actionable variants in nonmetabolism-related genes, such as oncogenes, that would otherwise be identified by a whole exome approach, thus minimizing potential ethical issues.

Sortilin: An unusual suspect in cholesterol metabolism

The concentration of low-density lipoprotein (LDL) cholesterol (C) in plasma is a key determinant of cardiovascular disease risk and human genetic studies have long endeavoured to elucidate the pathways that regulate LDL metabolism. Massive genome-wide association studies (GWASs) of common genetic variation associated with LDL-C in the population have implicated SORT1 in LDL metabolism. Using experimental paradigms and standards appropriate for understanding the mechanisms by which common variants alter phenotypic expression, three recent publications have presented divergent and even contradictory findings. Interestingly, although these reports each linked SORT1 to LDL metabolism, they did not agree on a mechanism to explain the association. Here, we review recent mechanistic studies of SORT1 - the first gene identified by GWAS as a determinant of plasma LDL-C to be evaluated mechanistically.

Western Database of Lipid Variants (WDLV): a catalogue of genetic variants in monogenic dyslipidemias.

BACKGROUND Next-generation sequencing (NGS) is nearing routine clinical application, especially for diagnosis of rare monogenic cardiovascular diseases. But NGS uncovers so much variation in an individual genome that filtering steps are required to streamline data management. The first step is to determine whether a potential disease-causing variant has been observed previously in affected patients. METHODS To facilitate this step for lipid disorders, we developed the Western Database of Lipid Variants (WDLV) of 2776 variants in 24 genes that cause monogenic dyslipoproteinemias, including conditions characterized primarily by either high or low low-density lipoprotein cholesterol, high or low high-density lipoprotein cholesterol, high triglyceride, and some miscellaneous disorders. We incorporated quality-control steps to maximize the likelihood that a listed mutation was disease causing. RESULTS The details of each mutation found in a dyslipidemia, together with a mutation map of the causative genes, are shown in graphical display items. CONCLUSIONS WDLV will help clinicians and researchers determine the potential pathogenicity of mutations discovered by DNA sequencing of patients or research participants with lipid disorders.

Common Low-Density Lipoprotein Receptor p.G116S Variant Has a Large Effect on Plasma Low-Density Lipoprotein Cholesterol in Circumpolar Inuit Populations

Background—Inuit are considered to be vulnerable to cardiovascular disease because their lifestyles are becoming more Westernized. During sequence analysis of Inuit individuals at extremes of lipid traits, we identified 2 nonsynonymous variants in low-density lipoprotein receptor (LDLR), namely p.G116S and p.R730W. Methods and Results—Genotyping these variants in 3324 Inuit from Alaska, Canada, and Greenland showed they were common, with allele frequencies 10% to 15%. Only p.G116S was associated with dyslipidemia: the increase in LDL cholesterol was 0.54 mmol/L (20.9 mg/dL) per allele (P=5.6×10−49), which was >3× larger than the largest effect sizes seen with other common variants in other populations. Carriers of p.G116S had a 3.02-fold increased risk of hypercholesterolemia (95% confidence interval, 2.34–3.90; P=1.7×10−17), but did not have classical familial hypercholesterolemia. In vitro, p.G116S showed 60% reduced ligand-binding activity compared with wild-type receptor. In contrast, p.R730W was associated with neither LDL cholesterol level nor altered in vitro activity. Conclusions—LDLR p.G116S is thus unique: a common dysfunctional variant in Inuit whose large effect on LDL cholesterol may have public health implications.Inuit were long-believed to have lower CVD risk than non-indigenous populations.1–3 However, re-evaluation of population studies indicates that ischemic heart disease rates are similar between Inuit and non-Indigenous people.4 Furthermore, ongoing Westernization in many Inuit communities has intensified their exposure to CVD risk factors such as smoking, calorie-dense processed foods, and a more comfortable but also sedentary lifestyle, all of which affect CVD risk and prevalence.4–10 Among classical CVD risk factors, Inuit adults tend to have higher plasma concentrations of LDL cholesterol than non-indigenous populations.11–15 The predominant monogenic cause of elevated LDL cholesterol concentration in most global populations is familial hypercholesterolemia (FH, Online Mendelian Inheritance in Man [OMIM] 143890).16 Heterozygous FH (HeFH) prevalence may be as high as 1:200 in certain European populations, and it is a potent predisposition state for early CVD.11–13 To date, DNA sequencing and biochemical studies have identified >1,600 rare loss-of-function mutations in the gene encoding the LDL receptor (LDLR), which can increase LDL cholesterol levels by 100% or more, and underlie >95% of cases of molecularly diagnosed FH.16 But despite the relatively high levels of LDL cholesterol observed in some Inuit, the role of LDLR gene variation has not been systematically studied.13–15 We thus investigated the LDLR locus in Inuit and tested for association of variants therein with plasma lipids. Through Sanger sequencing and targeted genotyping, we found two new LDLR variants common to five Inuit subgroups from across North America and Greenland: 1) p.G116S was both dysfunctional in vitro and associated with a relatively large increase in plasma LDL cholesterol levels; while 2) p.R730W had minimal dysfunction and impact on the lipid profile.

The application of gene therapy in lipid disorders: where are we now?

Abstract Lipid disorders or dyslipidemias result from perturbations within the biochemical pathways that regulate lipid metabolism. The dramatically altered lipid profiles that sometimes result from defects in these pathways can greatly enhance the risk of cardiovascular disease or other complications, with an attendant negative impact on morbidity and mortality. Patients with rare monogenic forms of dyslipidemia often live highly restrictive lifestyles, and options among existing pharmaceutical treatments are limited. Gene therapy provides a potential treatment option for monogenic dyslipidemias and perhaps even a long-term stable cure. In this article, we review the gene therapies applied to two types of monogenic dyslipidemias: homozygous familial hypercholesterolemia and familial LPL deficiency. We discuss the limitations of this approach, consider some future directions of gene therapy for monogenic dyslipidemias and possibilities for polygenic dyslipidemias.

Common Low-Density Lipoprotein Receptor p.G116S Variant Has a Large Effect on Plasma Low-Density Lipoprotein Cholesterol in Circumpolar Inuit PopulationsCLINICAL PERSPECTIVE

Background—Inuit are considered to be vulnerable to cardiovascular disease because their lifestyles are becoming more Westernized. During sequence analysis of Inuit individuals at extremes of lipid traits, we identified 2 nonsynonymous variants in low-density lipoprotein receptor (LDLR), namely p.G116S and p.R730W. Methods and Results—Genotyping these variants in 3324 Inuit from Alaska, Canada, and Greenland showed they were common, with allele frequencies 10% to 15%. Only p.G116S was associated with dyslipidemia: the increase in LDL cholesterol was 0.54 mmol/L (20.9 mg/dL) per allele (P=5.6×10−49), which was >3× larger than the largest effect sizes seen with other common variants in other populations. Carriers of p.G116S had a 3.02-fold increased risk of hypercholesterolemia (95% confidence interval, 2.34–3.90; P=1.7×10−17), but did not have classical familial hypercholesterolemia. In vitro, p.G116S showed 60% reduced ligand-binding activity compared with wild-type receptor. In contrast, p.R730W was associated with neither LDL cholesterol level nor altered in vitro activity. Conclusions—LDLR p.G116S is thus unique: a common dysfunctional variant in Inuit whose large effect on LDL cholesterol may have public health implications.Inuit were long-believed to have lower CVD risk than non-indigenous populations.1–3 However, re-evaluation of population studies indicates that ischemic heart disease rates are similar between Inuit and non-Indigenous people.4 Furthermore, ongoing Westernization in many Inuit communities has intensified their exposure to CVD risk factors such as smoking, calorie-dense processed foods, and a more comfortable but also sedentary lifestyle, all of which affect CVD risk and prevalence.4–10 Among classical CVD risk factors, Inuit adults tend to have higher plasma concentrations of LDL cholesterol than non-indigenous populations.11–15 The predominant monogenic cause of elevated LDL cholesterol concentration in most global populations is familial hypercholesterolemia (FH, Online Mendelian Inheritance in Man [OMIM] 143890).16 Heterozygous FH (HeFH) prevalence may be as high as 1:200 in certain European populations, and it is a potent predisposition state for early CVD.11–13 To date, DNA sequencing and biochemical studies have identified >1,600 rare loss-of-function mutations in the gene encoding the LDL receptor (LDLR), which can increase LDL cholesterol levels by 100% or more, and underlie >95% of cases of molecularly diagnosed FH.16 But despite the relatively high levels of LDL cholesterol observed in some Inuit, the role of LDLR gene variation has not been systematically studied.13–15 We thus investigated the LDLR locus in Inuit and tested for association of variants therein with plasma lipids. Through Sanger sequencing and targeted genotyping, we found two new LDLR variants common to five Inuit subgroups from across North America and Greenland: 1) p.G116S was both dysfunctional in vitro and associated with a relatively large increase in plasma LDL cholesterol levels; while 2) p.R730W had minimal dysfunction and impact on the lipid profile.

The common LDLR p.G116S variant has a large effect on plasma LDL cholesterol in circumpolar Inuit populations

Background—Inuit are considered to be vulnerable to cardiovascular disease because their lifestyles are becoming more Westernized. During sequence analysis of Inuit individuals at extremes of lipid traits, we identified 2 nonsynonymous variants in low-density lipoprotein receptor (LDLR), namely p.G116S and p.R730W. Methods and Results—Genotyping these variants in 3324 Inuit from Alaska, Canada, and Greenland showed they were common, with allele frequencies 10% to 15%. Only p.G116S was associated with dyslipidemia: the increase in LDL cholesterol was 0.54 mmol/L (20.9 mg/dL) per allele (P=5.6×10−49), which was >3× larger than the largest effect sizes seen with other common variants in other populations. Carriers of p.G116S had a 3.02-fold increased risk of hypercholesterolemia (95% confidence interval, 2.34–3.90; P=1.7×10−17), but did not have classical familial hypercholesterolemia. In vitro, p.G116S showed 60% reduced ligand-binding activity compared with wild-type receptor. In contrast, p.R730W was associated with neither LDL cholesterol level nor altered in vitro activity. Conclusions—LDLR p.G116S is thus unique: a common dysfunctional variant in Inuit whose large effect on LDL cholesterol may have public health implications.Inuit were long-believed to have lower CVD risk than non-indigenous populations.1–3 However, re-evaluation of population studies indicates that ischemic heart disease rates are similar between Inuit and non-Indigenous people.4 Furthermore, ongoing Westernization in many Inuit communities has intensified their exposure to CVD risk factors such as smoking, calorie-dense processed foods, and a more comfortable but also sedentary lifestyle, all of which affect CVD risk and prevalence.4–10 Among classical CVD risk factors, Inuit adults tend to have higher plasma concentrations of LDL cholesterol than non-indigenous populations.11–15 The predominant monogenic cause of elevated LDL cholesterol concentration in most global populations is familial hypercholesterolemia (FH, Online Mendelian Inheritance in Man [OMIM] 143890).16 Heterozygous FH (HeFH) prevalence may be as high as 1:200 in certain European populations, and it is a potent predisposition state for early CVD.11–13 To date, DNA sequencing and biochemical studies have identified >1,600 rare loss-of-function mutations in the gene encoding the LDL receptor (LDLR), which can increase LDL cholesterol levels by 100% or more, and underlie >95% of cases of molecularly diagnosed FH.16 But despite the relatively high levels of LDL cholesterol observed in some Inuit, the role of LDLR gene variation has not been systematically studied.13–15 We thus investigated the LDLR locus in Inuit and tested for association of variants therein with plasma lipids. Through Sanger sequencing and targeted genotyping, we found two new LDLR variants common to five Inuit subgroups from across North America and Greenland: 1) p.G116S was both dysfunctional in vitro and associated with a relatively large increase in plasma LDL cholesterol levels; while 2) p.R730W had minimal dysfunction and impact on the lipid profile.