The Role of Lipoprotein(a) in Calcific AV Disease and Insights Into Its Medical Therapies

Calcific aortic valve (AV) disease, including calcific AV stenosis and its antecedent preclinical phase aortic sclerosis, is a result of progressive pro-inflammatory and fibro-calcific changes resulting in calcium deposition and ectopic bone formation in the AV causing restricted mobility of its leaflets. No effective medical therapy exists for the ~12% of the population over the age of 75 with calcific AV stenosis and 3% with severe calcific AV stenosis. The incidence is projected increase due to the aging of the population.1 The natural history of calcific AV stenosis is progression from development of symptoms to heart failure and ultimately death. Currently, transcatheter AV replacement (TAVR) and surgical AV replacement (SAVR) are effective options for those who develop severe symptomatic calcific AV stenosis. However, both procedures are performed at relatively late stages of the natural history of calcific AV stenosis and can also be associated with a low but not insignificant rate of complications including disabling stroke, life-threatening bleed, vascular complications, need for permanent pacing, and death. Over 20 studies (Table 1) have established lipoprotein(a) [Lp(a)] as a genetically determined and likely causal risk factor for calcific AV stenosis and a predictor of faster calcific AV stenosis progression. These studies have bolstered the need to further understand the role of Lp(a) in the disease and as a potential therapeutic target.

Table 1: Published Studies Demonstrating the Association Between Lp(a) and Calcific AV Disease

Table 1

Lp(a) consists of a particle of low-density lipoprotein (LDL) bound covalently to apolipoprotein(a) [apo(a)], encoded by the LPA gene. The LPA gene arose from duplication and modification of its neighboring PLG gene, and apo(a) contains kringle domains KIV and KV and an inactive protease domain, which are highly homologous to those on plasminogen. However, apo(a) has 10 types of KIV domains, with KIV2 present at varying copy numbers ranging from 1 to >40 on each allele. Only one copy of KIV1 and KIV3-10 are present, but the number of KIV2 repeats determines the apo(a) isoform size in each individual. Importantly, two key properties on KIV10 of apo(a) are relevant to calcific AV stenosis.2 First, Lp(a) is the major lipoprotein carrier of pro-inflammatory oxidized phospholipids (OxPL), a large proportion of which are covalently bound to apo(a), likely on key histidine residues on KIV10, but also present in the LDL moiety. Second, strong lysine and fibrin binding sites are present on apo(a) KIV10 that allow Lp(a) to bind tightly to valvular and vascular subendothelial spaces, resulting in its accumulation. In concert with accumulation, Lp(a) can then deliver its cargo of both unoxidized and oxidized lipids to induce pro-inflammatory and pro-calcifying changes in valve leaflets.

Lp(a) levels are highly heritable3 in an autosomal co-dominant pattern. Readily measurable aspects of Lp(a) genetics, such as apo(a) isoforms and SNPs, have been instrumental in establishing potential causality for Lp(a) in calcific AV disease that is not confounded by environmental and lifestyle factors. Apo(a) isoform size is inversely related to plasma Lp(a) levels, with larger isoforms less efficiently secreted from hepatocytes. SNPs such as rs3798220 and rs10455872, present in ~3% and ~15% of European populations, respectively, in various regions of the LPA gene are associated with higher plasma Lp(a) levels, whereas others, including rs41272114 and rs143431368, are associated with lower plasma Lp(a) levels. Mendelian randomization analysis links apo(a) isoform size or LPA SNPs, plasma Lp(a) levels, and calcific AV disease risk, thereby eliminating reverse causality, and has complemented traditional observational studies in providing strong evidence for Lp(a) as a likely causal risk factor for calcific AV disease (Table 1).

The mechanisms by which Lp(a) mediates calcific AV disease are under investigation, but based on currently available data, a working model is that Lp(a) along with its lipid and protein cargo accumulate within the AV, mediating inflammation and calcification that ultimately results in calcific AV stenosis. Lp(a), as well as its OxPL, the enzyme ATX that generates pro-calcifying lysophosphatidic acid from OxPL, and the protein apolipoprotein C-III (apoC-III) associated with elevated triglycerides and metabolic syndrome are found in proximity to calcified regions of human stenotic AV leaflets (Figure 1).4,5 Animal models of OxPL neutralization with a natural antibody E066 and of diet-fed lysophosphatidic acid, the enzymatic product of ATX,7 have demonstrated their respective roles in augmenting AV calcification in vivo. Although animal models interrogating the role of Lp(a) and calcific AV disease are ongoing,8 in vitro studies have suggested that Lp(a) promotes valvular calcification.9 Lp(a) or recombinant apo(a) exposure to valvular interstitial cells derived from human AVs results in upregulation of genes involved in osteogenic differentiation including IL-6, BMP2, and RUNX2, in addition to augmenting calcium deposition in vitro.9,10 It is interesting to note that OxPL neutralization with E06 and recombinant apo(a) lacking OxPL both resulted in significantly attenuated transcriptional osteogenic changes,10 suggesting a key role for the OxPL on Lp(a).

Figure 1

Figure 1
Working model for how Lp(a) drives progression of calcific AV disease. Lp(a) and its OxPL, apoC-III, and ATX components accumulate in human AVs. Over time, Lp(a) mediates AV calcification, hemodynamic progression of calcific AV stenosis, and increased risk of AV replacement and death. Alizarin red = calcium stain; E06 = monoclonal antibody specific for OxPL; LPA4 = monoclonal antibody specific for apo(a). Modified under creative commons from Torzewski et al.,4 Capoulade et al.,5 and Zheng et al.10

The pro-calcific mechanisms attributed to Lp(a) have translated well into clinical observations. Persons with elevated Lp(a) above 35 mg/dL10 or above 75 nM11 had increase in AV microcalcifications as detected by 18F-NaF positron emission tomography computed tomography, a predictor of developing calcific AV disease. Moreover, elevated Lp(a) (>35 mg/dL) was associated with 3 times faster progression of AV calcification compared to those with Lp(a) <35 mg/dL, with median (interquartile range) (309 [142 - 483 arbitrary units/yr] vs. 93 [56 - 296] arbitrary units/yr; p = 0.015) in 51 patients with calcific AV stenosis who had baseline and 2-year follow-up computed tomography scans.10 Like Lp(a), those with elevated OxPL-apoB—a measurement of OxPL on apoB containing lipoproteins reflecting in large part (50-80%) the OxPL content of Lp(a)—also had increased AV microcalcifications and faster progression of macrocalcifications.10 In line with the key role of AV calcification in the clinical progression of calcific AV disease,12 patients with elevated Lp(a) in the top tertile of patients with mild-moderate calcific AV stenosis and followed prospectively had more rapid echocardiographically determined hemodynamic progression, as measured by peak velocity across the AV (Vmax). Vmax progression (mean ± SD) in the ASTRONOMER study (n = 220) was 0.26 ± 0.26 m/s/yr with Lp(a) >58.5 mg/dL versus 0.17 ± 0.21 m/s/yr with Lp(a) <58.5 mg/dL; p = 0.005.13 In the SALTIRE and Ring of Fire studies (n = 129 with serial echocardiogram studies), Vmax progression was 0.23 ± 0.2 m/s/yr with Lp(a) >35 mg/dL versus 0.14 ± 0.2 ms/s/y with Lp(a) <35 mg/dL; p = 0.019.10 There was a linear relationship between Lp(a) levels and the annual rate of Vmax increase in the ASTRONMER study, with 1.10 OR per 10 mg/dL (95% CI, 1.03-1.19; p = 0.006).14 Of even greater importance, elevated Lp(a) was associated with a higher risk for AVR or CV death in ASTRONOMER13 and the SALTIRE and Ring of Fire10 studies. Moreover, similar findings of faster hemodynamic progression and hard clinical endpoints were observed in individuals with the highest tertiles of OxPL-apoB10,13 and Lp(a)-associated apoC-III,5 again suggesting important roles for these Lp(a) components as mediators of calcific AV disease progression.

The strong association between Lp(a) and calcific AV disease and its clinical progression has two main clinical implications. First, as our understanding regarding the optimal timing of TAVR or SAVR in individuals with asymptomatic calcific AV stenosis evolves, biomarkers including Lp(a) and OxPL-apoB (now clinically available) can guide appropriate frequency and modality of tests and imaging for surveillance. Second, given the lack of effective medical therapies, including LDL cholesterol-lowering with statins, which increases Lp(a),15 to prevent calcific AV disease progression,16 likely causal risk factors such as Lp(a) represent logical therapeutic targets. Clinical trials evaluating calcific AV disease progression with Proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors are ongoing. PCSK9 inhibitors are an attractive candidate because they can dramatically lower LDL cholesterol and also lower Lp(a) by ~14-30%. Unfortunately, they are least effective in subjects with elevated Lp(a) (>50 mg/dL), with only a 14% reduction.17 Furthermore, there is burgeoning evidence that PCSK9 may be directly involved in calcific AV disease. Plasma PCSK9 levels were higher in 112 individuals with calcific AV stenosis versus 32 controls.18 Moreover, individuals with the PCSK9 loss of function mutation R46L, known to have lower PCSK9 plasma levels, had decreased incidence of calcific AV stenosis in over 103,000 prospectively followed Danish individuals.19 Although PCSK9 R46L is also associated with lower LDL cholesterol and Lp(a) levels, AVs from PCSK9-deficient mice have lower calcium content than controls,20 suggesting a direct effect of PCKS9 in development of calcific AV disease. However, much larger reductions in Lp(a) may be needed to effectively slow progression of calcific AV disease, as suggested by mendelian randomization analyses estimating a 67.5-100 mg/dL decrease in Lp(a) required for a 25% risk reduction in coronary artery disease.21,22 Therefore, novel strategies to prevent calcific AV disease progression, such as neutralizing OxPL or apoC-III, or potent Lp(a) lowering with antisense oligonucleotides targeting LPA,23 will be of great interest. Approximately 30-35% of subjects with AS or undergoing TAVR have elevated Lp(a);11,24,25 therefore, identification of subjects at risk for calcific AV stenosis progression can be easily accomplished, and a phase 3 trial can be designed to lower Lp(a) substantially and test the hypothesis that lowering Lp(a) and its bioactive cargo including OxPL can reduce the progression of AS and need for AVR.

References

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Keywords: Heart Valve Diseases, Lipoprotein(a), Apolipoprotein C-III, Kringles, Aortic Valve, Aortic Valve Stenosis, Calcinosis, Receptors, Lysophosphatidic Acid, Calcium, Lysine, Histidine, Apolipoproteins B, Interleukin-6


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