Lipoprotein (a): Coming of Age at Last

In 1963, the geneticist Kare Berg was seeking to define lipoprotein differences between individual human sera, and through a simple but ingenious set of immunological investigations of human sera, discovered a new antigen that was associated with low density lipoproteins (LDL) (1Berg K. A new serum type system in man–the LP system.Acta Pathol. Microbiol. Scand. 1963; 59: 369-382Crossref PubMed Scopus (1044) Google Scholar). He shortly showed that this new antigen was a genetic trait and proposed it be called Lp(a): the Lp referring to the lipoprotein, and the small “a” in brackets as this was the accepted terminology at that time for naming antigens in human immunogenetics. Thus, as originally intended, the term Lp(a) referred to the antigenic structure of the new antigen and not the Lp(a) lipoprotein as it is used today. This terminology has subsequently led to much confusion in the literature and among clinicians because of the similarity of “apolipoprotein A” (apoA-I of HDL) with the “apolipoprotein (a)” of Lp(a). However, the use of “a” to describe the antigen protein linked to LDL predated the development of the apolipoprotein terminology by a number of years. Berg's very first paper was remarkably prescient, as he already described family studies showing that Lp(a) behaved as a genetic trait, and this work was rapidly extended (2Berg K. Mohr J. Genetics of the LP system.Acta Genet. Stat. Med. 1963; 13: 349-360PubMed Google Scholar). Early work using relatively insensitive techniques led to the concept that people were either Lp(a)+ or Lp(a)− and as early as 1974, Berg and colleagues reported a higher frequency of the Lp(a)+ phenotype in patients with coronary heart disease compared with healthy controls (3Berg K. Dahlen G. Frick M.H. Lp(a) lipoprotein and pre-beta1-lipoprotein in patients with coronary heart disease.Clin. Genet. 1974; 6: 230-235Crossref PubMed Scopus (265) Google Scholar). As techniques for measuring Lp(a) improved, the association of Lp(a) with cardiovascular disease (CVD) was more fully developed, so that by the late 1980s it was clear that Lp(a) represented a highly prevalent inherited risk factor for CVD, but unlike most classical risk factors, Lp(a) levels were relatively unresponsive to either diet, environmental variables, or available drugs (4Djurovic S. Berg K. Epidemiology of Lp(a) lipoprotein: its role in atherosclerotic/thrombotic disease.Clin. Genet. 1997; 52: 281-292Crossref PubMed Scopus (68) Google Scholar). The cloning and sequencing of apo(a) in 1987 by Lawn and colleagues (5Eaton D.L. Fless G.M. Kohr W.J. McLean J.W. Xu Q.T. Miller C.G. Lawn R.M. Scanu A.M. Partial amino acid sequence of apolipoprotein(a) shows that it is homologous to plasminogen.Proc. Natl. Acad. Sci. USA. 1987; 84: 3224-3228Crossref PubMed Scopus (353) Google Scholar, 6McLean J.W. Tomlinson J.E. Kuang W.J. Eaton D.L. Chen E.Y. Fless G.M. Scanu A.M. Lawn R.M. cDNA sequence of human apolipoprotein(a) is homologous to plasminogen.Nature. 1987; 330: 132-137Crossref PubMed Scopus (1594) Google Scholar) revealed many surprises and kindled a new burst of interest in Lp(a). They showed that the apolipoprotein (a) gene, termed LPA, evolved from the plasminogen (PLG) gene and that it contained multiple so-called kringle (K) domains common in coagulation factors, as well as a mutated protease domain that lacked the proteolytic activity of plasmin. Their seminal work provided new insights into the complexity of the genetics of LPA, which helped to explain the widespread variation in plasma Lp(a) levels. Among the various Ks derived from the PLG gene, the KIV had expanded and diversified by mutation into 10 different types termed KIV1–10. One of these, the KIV-2, was found to exist in multiple copies such that few individuals have the same two alleles. Indeed more than 95% of the population has heterozygosity for the copy number variation (CNV) of KIV-2, which can vary from as few as 3 to > 40 copies, leading to different sized apo(a) proteins (7Koschinsky M.L. Beisiegel U. Henne-Bruns D. Eaton D.L. Lawn R.M. Apolipoprotein(a) size heterogeneity is related to variable number of repeat sequences in its mRNA.Biochemistry. 1990; 29: 640-644Crossref PubMed Scopus (123) Google Scholar). Thus, most individuals generate two apo(a) proteins of different size. The apo(a) allele with a small CNV of KIV-2 will yield an Lp(a) with a shorter apo(a) protein, which is associated with high plasma levels, while an apo(a) allele with a high CNV will yield an Lp(a) with a longer apo(a), associated with lower plasma Lp(a) levels. Indeed, it was shown soon after in a small population that the KIV-2 CNV predicts Lp(a) levels and CVD risk (8Kraft H.G. Lingenhel A. Kochl S. Hoppichler F. Kronenberg F. Abe A. Muhlberger V. Schonitzer D. Utermann G. Apolipoprotein(a) kringle IV repeat number predicts risk for coronary heart disease.Arterioscler. Thromb. Vasc. Biol. 1996; 16: 713-719Crossref PubMed Scopus (210) Google Scholar). Over the next few decades, extensive studies undertook to define the mechanisms by which Lp(a) promoted atherosclerosis, and in a broad sense these focused on the possibility that apo(a) might somehow interfere with the role of plasminogen in promoting fibrinolysis on the one hand, and on the other, on the possibility that its atherogenicity depended on properties associated with the LDL moiety or proatherogenic properties of apo(a) itself (9Dubé J.B. Boffa M.B. Hegele R.A. Koschinsky M.L. Lipoprotein(a): more interesting than ever after 50 years.Curr. Opin. Lipidol. 2012; 23: 133-140Crossref PubMed Scopus (95) Google Scholar). More recently, the discovery that Lp(a) among all lipoproteins was a carrier of proinflammatory oxidized phospholipids (OxPLs) added yet another possible explanation for its atherogenicity (10Tsimikas S. Bergmark C. Beyer R.W. Patel R. Pattison J. Miller E. Juliano J. Witztum J.L. Temporal increases in plasma markers of oxidized low-density lipoprotein strongly reflect the presence of acute coronary syndromes.J. Am. Coll. Cardiol. 2003; 41: 360-370Crossref PubMed Scopus (315) Google Scholar, 11Bergmark C. Dewan A. Orsoni A. Merki E. Miller E.R. Shin M.J. Binder C.J. Horkko S. Krauss R.M. Chapman M.J. et al.A novel function of lipoprotein [a] as a preferential carrier of oxidized phospholipids in human plasma.J. Lipid Res. 2008; 49: 2230-2239Abstract Full Text Full Text PDF PubMed Scopus (232) Google Scholar). Indeed, it is popular to say that Lp(a) promotes “atherothrombosis,” but as with so many other aspects of Lp(a) biology, whether Lp(a) interferes with normal coagulation properties in vivo and the mechanisms by which it promotes atherosclerosis are still unresolved issues. Indeed, there are a whole host of fundamental questions regarding the biology and metabolism of Lp(a) that remain to this day unanswered, such fundamental questions as: what governs its synthesis and regulation, how and where is apo(a) covalently linked to apoB-100 of LDL to form Lp(a), and what governs Lp(a) metabolism and clearance. There is indeed much to be learned. Despite ongoing advances in epidemiological studies and insights from mechanistic studies, interest in Lp(a) as a CVD risk factor waned until the recent decade. Since the 1990s, Lp(a) remained for the most part a focus of academic study among a limited cohort of investigators. Even to the present time, it is not widely known as a CVD risk factor among most cardiologists, clinicians, or even endocrinologists, and even fewer understand what it is, what are its metabolic properties, what regulates its genetics, and what factors govern its levels. At the clinical level, with the exception of some dedicated lipidologists and cardiologists, physicians are not measuring Lp(a) as a cardiac risk factor in their patients, even in those at high risk or with established CVD. In part, this has stemmed from the fact that there are no standardized ways to measure and report Lp(a) levels, and cut points that represent “normal” or sufficiently elevated levels to require treatment are not well defined. For the most part, measurements are not available in routine laboratories and must be sent away to specialty referral laboratories at considerable expense. Most likely, the primary reason clinicians fail to measure Lp(a) is that there is no specific therapy to substantially lower elevated levels. Although niacin is known to be capable of lowering plasma Lp(a) levels 25–40% or even more in some patients, its use is associated with considerable side effects and has been greatly restricted due to the perception that it is unsafe and does not reduce CVD risk. However, in recent years a remarkable resurgence of interest in Lp(a) has occurred. This has been fueled by meta-analyses of by now a large body of epidemiological studies, which have consistently shown modest associations of Lp(a) with nonfatal myocardial infarction and coronary death (12Erqou S. Kaptoge S. Perry P.L. Di Angelantonio E. Thompson A. White I.R. Marcovina S.M. Collins R. Thompson S.G. Danesh J. Lipoprotein(a) concentration and the risk of coronary heart disease, stroke, and nonvascular mortality.JAMA. 2009; 302: 412-423Crossref PubMed Scopus (1156) Google Scholar, 13Erqou S. Thompson A. Di Angelantonio E. Saleheen D. Kaptoge S. Marcovina S. Danesh J. Apolipoprotein(a) isoforms and the risk of vascular disease: systematic review of 40 studies involving 58,000 participants.J. Am. Coll. Cardiol. 2010; 55: 2160-2167Crossref PubMed Scopus (247) Google Scholar). Moreover, a series of genome-wide association studies (GWASs) and Mendelian randomization studies, where confounding variables that limit epidemiological studies are minimized, show a linear and potent relationship to CVD risk with odds ratios for the highest Lp(a) values that are 3-fold above the lowest reference Lp(a) values (14Clarke R. Peden J.F. Hopewell J.C. Kyriakou T. Goel A. Heath S.C. Parish S. Barlera S. Franzosi M.G. Rust S. et al.Genetic variants associated with Lp(a) lipoprotein level and coronary disease.N. Engl. J. Med. 2009; 361: 2518-2528Crossref PubMed Scopus (1031) Google Scholar, 15Kamstrup P.R. Tybjaerg-Hansen A. Steffensen R. Nordestgaard B.G. Genetically elevated lipoprotein(a) and increased risk of myocardial infarction.JAMA. 2009; 301: 2331-2339Crossref PubMed Scopus (859) Google Scholar, 16Kamstrup P.R. Tybjaerg-Hansen A. Nordestgaard B.G. Genetic evidence that lipoprotein(a) associates with atherosclerotic stenosis rather than venous thrombosis.Arterioscler. Thromb. Vasc. Biol. 2012; 32: 1732-1741Crossref PubMed Scopus (123) Google Scholar). In such studies, as for example, reflected by single nucleotide polymorphisms that are associated with life-long elevated plasma Lp(a) levels, elevated Lp(a) appears to be a key driver of CVD. Indeed, these modern, high-throughput assays applied to exceedingly large populations have provided high confidence in observations of the strong relationship of Lp(a) to CVD made much earlier in smaller populations, and to borrow a phrase used by Sekar Kathiresan, are “more ‘redux’ than new” (17Kathiresan S. Lp(a) lipoprotein redux–from curious molecule to causal risk factor.N. Engl. J. Med. 2009; 361: 2573-2574Crossref PubMed Scopus (16) Google Scholar), reflecting the strong scholarship of the earlier investigators. Moreover, recent GWASs have revealed that the LPA gene is also the single most powerful genetic risk factor for calcific aortic valve stenosis (CAVS), with ever-new studies confirming this association (18Thanassoulis G. Campbell C.Y. Owens D.S. Smith J.G. Smith A.V. Peloso G.M. Kerr K.F. Pechlivanis S. Budoff M.J. Harris T.B. et al.Genetic associations with valvular calcification and aortic stenosis.N. Engl. J. Med. 2013; 368: 503-512Crossref PubMed Scopus (626) Google Scholar, 19Kamstrup P.R. Tybjaerg-Hansen A. Nordestgaard B.G. Elevated lipoprotein(a) and risk of aortic valve stenosis in the general population.J. Am. Coll. Cardiol. 2014; 63: 470-477Crossref PubMed Scopus (351) Google Scholar, 20Capoulade R. Chan K.L. Yeang C. Mathieu P. Bosse Y. Dumesnil J.G. Tam J.W. Teo K.K. Mahmut A. Yang X. et al.Oxidized Phospholipids, Lipoprotein(a), and Progression of Calcific Aortic Valve Stenosis.J. Am. Coll. Cardiol. 2015; 66: 1236-1246Crossref PubMed Scopus (239) Google Scholar). Based on the emerging epidemiological, GWAS, and Mendelian randomization data supporting a causal relationship of Lp(a) levels to CVD and CAVS, a European consensus panel has proposed that Lp(a) values above ∼50 mg/dl (or 125 nmol/L) pose a major risk (21Nordestgaard B.G. Chapman M.J. Ray K. Boren J. Andreotti F. Watts G.F. Ginsberg H. Amarenco P. Catapano A. Descamps O.S. et al.Lipoprotein(a) as a cardiovascular risk factor: current status.Eur. Heart J. 2010; 31: 2844-2853Crossref PubMed Scopus (1255) Google Scholar). Based on preliminary prevalence data, adoption of this cutoff indicates that up to 20% of populations in the United States and Europe are at risk. However, Lp(a) levels vary considerably in different ethnic populations and much effort will be required to both standardize measurements and develop ethnic appropriate cutoff values. As noted above, even the recognition that Lp(a) is a major risk factor in and of itself would not likely lead to concerted efforts to push for widespread adoption of recommendations to have Lp(a) levels measured if a therapy for Lp(a) were not available. Indeed, proponents of evidence-based medical practice have argued against measurement of Lp(a) in the absence of effective therapy, even if one understands that this may place a given subject at higher CVD risk. Until now there has not been specific therapy and, even for the minority of patients who can tolerate relatively high doses of niacin, reductions in Lp(a) of 25–40% are usually the best that can be expected. Although newer or experimental agents such as mipomersen (22Santos R.D. Raal F.J. Catapano A.L. Witztum J.L. Steinhagen-Thiessen E. Tsimikas S. Mipomersen, an antisense oligonucleotide to apolipoprotein B-100, reduces lipoprotein(a) in various populations with hypercholesterolemia: results of 4 phase III trials.Arterioscler. Thromb. Vasc. Biol. 2015; 35: 689-699Crossref PubMed Scopus (143) Google Scholar), monoclonal antibodies to PCSK9 (23Raal F.J. Giugliano R.P. Sabatine M.S. Koren M.J. Langslet G. Bays H. Blom D. Eriksson M. Dent R. Wasserman S.M. et al.Reduction in lipoprotein(a) with PCSK9 monoclonal antibody evolocumab (AMG 145): a pooled analysis of more than 1,300 patients in 4 phase II trials.J. Am. Coll. Cardiol. 2014; 63: 1278-1288Crossref PubMed Scopus (289) Google Scholar), or CETP inhibitors (24Cannon C.P. Shah S. Dansky H.M. Davidson M. Brinton E.A. Gotto A.M. Stepanavage M. Liu S.X. Gibbons P. Ashraf T.B. et al.Safety of anacetrapib in patients with or at high risk for coronary heart disease.N. Engl. J. Med. 2010; 363: 2406-2415Crossref PubMed Scopus (681) Google Scholar) can reduce Lp(a), the extent of lowering is in general similar to or less than that achieved by niacin. However, for the first time, there appears to be a therapeutic strategy that offers the promise of both specific and effective therapy to lower plasma Lp(a). Recently, the use of antisense therapy that targets hepatic LPA mRNA was shown to be highly effective in reducing Lp(a) levels in humans (25Tsimikas S. Viney N.J. Hughes S.G. Singleton W. Graham M.J. Baker B.F. Burkey J.L. Yang Q. Marcovina S.M. Geary R.S. et al.Antisense therapy targeting apolipoprotein(a): a randomised, double-blind, placebo-controlled phase 1 study.Lancet. 2015; 386: 1472-1483Abstract Full Text Full Text PDF PubMed Scopus (342) Google Scholar). Further refinements in this methodology promise even more effective and well-tolerated therapies. Now we can begin to envision agents sufficiently specific and effective to allow clinical trials to be designed and conducted to test the efficacy of lowering Lp(a) to prevent the enhanced risk associated with CVD and CAVS. It is against this background of exciting new data and the possible availability of specific and effective therapy that we suggest that Lp(a) is at last coming of age. In recognition of the surge in interest and new information in the Lp(a) field, we commissioned a comprehensive set of reviews of both basic and clinical aspects of Lp(a) biology to inform our readership. This issue marks the beginning of a new Thematic Series entitled “Lp(a): Coming of Age at Last”. It will consist of a total of 14 articles published over the next year and authored by some of the foremost experts in Lp(a) biology. The development of efficacious therapy to lower Lp(a) levels has the potential to transform the Lp(a) from the to the at The first in this Thematic which is in this will describe the development of antisense therapy to apo(a) with an review of and for this exciting This is authored by and Tsimikas and is entitled of apolipoprotein (a) to lower plasma lipoprotein (a) levels in this development is an appropriate to the The possibility to substantially and lower Lp(a) levels in humans that at last we may be in a to the of Lp(a) to human biology and In the Marcovina and who have been in the of efforts to develop specific and standardized measurements of Lp(a), will present a of where we are and where we to to have measurements of Lp(a) in their Thematic entitled measurements for clinical This will be in by an on the role of Lp(a) in coagulation by Boffa and a factor in cardiovascular In who led the that made the seminal that Lp(a) was a genetic risk factor for CAVS, will present in calcific aortic valve disease: from to novel for aortic review will up to on this and the for the that lowering Lp(a) might the development of CAVS. In a Nordestgaard and studies in the population have greatly our of the of Lp(a), will review the and genetic evidence that Lp(a) in CVD and CAVS. In coming and colleagues will the of and environmental factors that Lp(a) levels. Utermann will present a review of the of genetics on the structure and function of Lp(a) and Ginsberg will review what is known and what to be Lp(a) and colleagues will present a review on all the therapeutic and that Lp(a) from antisense therapy, and and will present a on the role of LDL for example, is now widely used in to lower Lp(a) in those with CVD. Hopewell and will review Lp(a) metabolism in disease and the possible role of Lp(a) in the enhanced risk of CVD associated with and Tsimikas and Witztum will review what is known the mechanisms by which Lp(a) promotes with a on the emerging role of oxidized phospholipids in this the discovery of Lp(a) by Kare the Lp(a) has as the of by many both basic and as so there are key the both those the of and as for example, what the cloning of apo(a), as well as those discovered by for example, the that Lp(a) was the lipoprotein carrier of that to new insights and new for in the Thematic series will be by who will to a of the Lp(a) and to into the As two who have and to the Lp(a) field, we are the of Lp(a) and clinical and the to at last clinical trials that will to the test the many on the role of Lp(a) in CVD and that have been over the years. are this Thematic Series will the of the Lp(a) and will both the and who to into this exciting