Regulation of the expression of the apolipoprotein(a) gene: evidence for a regulatory role of the 5' distal apolipoprotein(a) transcription control region enhancer in yeast artificial chromosome transgenic mice

Lipoprotein(a): Just an Innocent Bystander in Arterial Hypertension?

Elevated plasma lipoprotein(a) [Lp(a)] is a relatively common and highly heritable trait conferring individuals time-dependent risk of developing atherosclerotic cardiovascular disease (CVD). Following its first description, Lp(a) triggered enormous scientific interest in the late 1980s, subsequently dampened in the mid-1990s by controversial findings of some prospective studies. It was only in the last decade that a large body of evidence has provided strong arguments for a causal and independent association between elevated Lp(a) levels and CVD, causing renewed interest in this lipoprotein as an emerging risk factor with a likely contribution to cardiovascular residual risk. Accordingly, the 2022 consensus statement of the European Atherosclerosis Society has suggested inclusion of Lp(a) measurement in global risk estimation. The development of highly effective Lp(a)-lowering drugs (e.g., antisense oligonucleotides and small interfering RNA, both blocking LPA gene expression) which are still under assessment in phase 3 trials, will provide a unique opportunity to reduce "residual cardiovascular risk" in high-risk populations, including patients with arterial hypertension. The current evidence in support of a specific role of Lp(a) in hypertension is somehow controversial and this narrative review aims to overview the general mechanisms relating Lp(a) to blood pressure regulation and hypertension-related cardiovascular and renal damage.

Significant differentiation in the apolipoprotein(a)/lipoprotein(a) trait between chimpanzees from Western and Central Africa

Elevated Lipoprotein(a) (Lp(a)) plasma concentrations are a risk factor for cardiovascular disease in humans, largely controlled by the LPA gene encoding apolipoprotein(a) (apo(a)). Lp(a) is composed of low-density lipoprotein (LDL) and apo(a) and restricted to Catarrhini. A variable number of kringle IV (KIV) domains in LPA lead to a size polymorphism of apo(a) that is inversely correlated with Lp(a) concentrations. Smaller apo(a) isoforms and higher Lp(a) levels in central chimpanzees (Pan troglodytes troglodytes [PTT]) compared to humans from Europe had been reported. We studied apo(a) isoforms and Lp(a) concentrations in 75 western (Pan troglodytes verus [PTV]) and 112 central chimpanzees, and 12 bonobos (Pan paniscus [PPA]), all wild born and living in sanctuaries in Sierra Leone, Republic of the Congo, and DR Congo, respectively, and 116 humans from Gabon. Lp(a) levels were severalfold higher in western than in central chimpanzees (181.0 ± 6.7 mg/dl vs. 56.5 ± 4.3 mg/dl), whereas bonobos showed intermediate levels (134.8 ± 33.4 mg/dl). Apo(a) isoform sizes differed significantly between subspecies (means 20.9 ± 2.2, 22.9 ± 4.4, and 23.8 ± 3.8 KIV repeats in PTV, PTT, and PPA, respectively). However, far higher isoform-associated Lp(a) concentrations for all isoform sizes in western chimpanzees offered the main explanation for the higher overall Lp(a) levels in this subspecies. Human Lp(a) concentrations (mean 47.9 ± 2.8 mg/dl) were similar to those in central chimpanzees despite larger isoforms (mean 27.1 ± 4.9 KIV). Lp(a) and LDL, apoB-100, and total cholesterol levels only correlated in PTV. This remarkable differentiation between chimpanzees from different African habitats and the trait's similarity in humans and chimpanzees from Central Africa poses the question of a possible impact of an environmental factor that has shaped the genetic architecture of LPA. Overall, studies on the cholesterol-containing particles of Lp(a) and LDL in chimpanzees should consider differentiation between subspecies.

Lipoprotein(a): a promising marker for residual cardiovascular risk assessment

Atherosclerotic cardiovascular diseases (CVD) are still the leading cause of morbidity and mortality worldwide, although optimal medical therapy has been prescribed for primary and secondary preventions. Residual cardiovascular risk for some population groups is still considerably high although target low density lipoprotein-cholesterol (LDL-C) level has been achieved. During the past few decades, compelling pieces of evidence from clinical trials and meta-analyses consistently illustrate that lipoprotein(a) (Lp(a)) is a significant risk factor for atherosclerosis and CVD due to its proatherogenic and prothrombotic features. However, the lack of effective medication for Lp(a) reduction significantly hampers randomized, prospective, and controlled trials conducting. Based on previous findings, for patients with LDL-C in normal range, Lp(a) may be a useful marker for identifying and evaluating the residual cardiovascular risk, and aggressively lowering LDL-C level than current guidelines' recommendation may be reasonable for patients with particularly high Lp(a) level.

Exaptation of transposable elements into novel cis-regulatory elements: is the evidence always strong?

Transposable elements (TEs) are mobile genetic sequences that can jump around the genome from one location to another, behaving as genomic parasites. TEs have been particularly effective in colonizing mammalian genomes, and such heavy TE load is expected to have conditioned genome evolution. Indeed, studies conducted both at the gene and genome levels have uncovered TE insertions that seem to have been co-opted--or exapted--by providing transcription factor binding sites (TFBSs) that serve as promoters and enhancers, leading to the hypothesis that TE exaptation is a major factor in the evolution of gene regulation. Here, we critically review the evidence for exaptation of TE-derived sequences as TFBSs, promoters, enhancers, and silencers/insulators both at the gene and genome levels. We classify the functional impact attributed to TE insertions into four categories of increasing complexity and argue that so far very few studies have conclusively demonstrated exaptation of TEs as transcriptional regulatory regions. We also contend that many genome-wide studies dealing with TE exaptation in recent lineages of mammals are still inconclusive and that the hypothesis of rapid transcriptional regulatory rewiring mediated by TE mobilization must be taken with caution. Finally, we suggest experimental approaches that may help attributing higher-order functions to candidate exapted TEs.

Lipoprotein(a) metabolism: potential sites for therapeutic targets

Lipoprotein(a) [Lp(a)] resembles low-density lipoprotein (LDL), with an LDL lipid core and apolipoprotein B (apoB), but contains a unique apolipoprotein, apo(a). Elevated Lp(a) is an independent risk factor for coronary and peripheral vascular diseases. The size and concentration of plasma Lp(a) are related to the synthetic rate, not the catabolic rate, and are highly variable with small isoforms associated with high concentrations and pathogenic risk. Apo(a) is synthesized in the liver, although assembly of apo(a) and LDL may occur in the hepatocytes or plasma. While the uptake and clearance site of Lp(a) is poorly delineated, the kidney is the site of apo(a) fragment excretion. The structure of apo(a) has high homology to plasminogen, the zymogen for plasmin and the primary clot lysis enzyme. Apo(a) interferes with plasminogen binding to C-terminal lysines of cell surface and extracellular matrix proteins. Lp(a) and apo(a) inhibit fibrinolysis and accumulate in the vascular wall in atherosclerotic lesions. The pathogenic role of Lp(a) is not known. Small isoforms and high concentrations of Lp(a) are found in healthy octogenarians that suggest Lp(a) may also have a physiological role. Studies of Lp(a) function have been limited since it is not found in commonly studied small mammals. An important aspect of Lp(a) metabolism is the modification of circulating Lp(a), which has the potential to alter the functions of Lp(a). There are no therapeutic drugs that selectively target elevated Lp(a), but a number of possible agents are being considered. Recently, new modifiers of apo(a) synthesis have been identified. This review reports the regulation of Lp(a) metabolism and potential sites for therapeutic targets.

Tissue engineering: a new frontier in physiological genomics

Considerable progress has been made in the last decade in the engineering and construction of a number of artificial tissue types. These constructs are typically viewed from the perspective of possible sources for implant and transplant materials in the clinical arena. However, incorporation of engineered tissues, often referred to as three-dimensional (3D) cell culture, also offers the possibility for significant advancements in research for physiological genomics. These 3D systems more readily mimic the in vivo setting than traditional 2D cell culture, and offer distinct advantages over the in vivo setting for some organ systems. As an example, cardiac cells in 3D culture 1) are more accessible for siRNA studies, 2) can be engineered with specific cell types, and 3) offer the potential for high-throughput screening of gene function. Here the state-of-the-art is reviewed and the applications for engineered tissue in genomics research are proposed. The ability to use engineered tissue in combination with genomics creates a bridge between traditional cellular and in vivo studies that is critical to enabling the transition of genetic information into mechanistic understanding of disease processes.

Conditional animal models for the study of lipid metabolism and lipid disorders

The advent of technologies that allow conditional mutagenesis has revolutionized our ability to explore gene functions and to establish animal models of human diseases. Both aspects have proven to be of particular importance in the study of lipid-related disorders. Classical approaches to gene inactivation by conventional gene targeting strategies have been successfully applied to generate animal models like the LDL receptor- and the apolipoprotein E-knockout mice, which are still widely used to study diverse aspects of atherosclerosis, lipid transport, and neurodegenerative disease. In many cases, however, simply inactivating the gene of interest has resulted in early lethal or complex phenotypes which are difficult to interpret. In recent years, additional tools have therefore been developed that allow the spatiotemporally controlled manipulation of the genome, as described in detail in Part I of this volume. Our aim is to provide an exemplary survey of the application of different conditional mutagenesis techniques in lipid research in order to illustrate their potential to unravel physiological functions of a broad range of genes involved in lipid homeostasis.

Detection of variability in apo(a) gene transcription regulatory sequences using the DGGE method

BACKGROUND

Increased lipoprotein(a), Lp(a), concentration is an independent risk factor for premature atherosclerosis. Apolipoprotein(a), apo(a), determines properties of the lipoprotein and its production rate is the limiting step in Lp(a) particle formation.

METHODS

Subjects covering the whole range of Lp(a) concentration were separated into quintiles. A randomly chosen sample from each quintile was derived, there being a total number of 713 individuals. The DGGE method was used to scan the known transcription regulatory regions of apo(a) gene (promoter; DHII and DHIII enhancers) for variability and its distribution across quintiles.

RESULTS

Besides 5 previously reported nucleotide substitutions (+121 G>A; +93 C>T; -1712 G>T; -1617 C>A; -1230 A>G) 16 unreported rare sequence variants were detected. All polymorphic variants were distributed throughout the quintiles with several significant differences. The novel +62 C variant was found only among individuals with Lp(a) levels over 16 mg/dl.

CONCLUSION

The apo(a) gene transcription regulatory regions were not revealed to be extremely polymorphic. However, we should consider a combined effect of all polymorphic sites from the whole apo(a) gene locus, including the apo(a) gene length polymorphism, when dealing with high population variability of Lp(a) levels.

Reduced LPA expression after peroxisome proliferator-activated receptor alpha (PPARα) activation in LPA-YAC transgenic mice

BACKGROUND:: Apolipoprotein(a) (apo(a)), which is part of the atherogenic lipoprotein Lp(a), shares structural homology with plasminogen (plg). Genes coding for plasminogen (PLG) and apo(a) (LPA) are linked and situated 40kb apart in the telomeric region of the long arm of chromosome 6. LPA is naturally expressed only in primates and hedgehogs. Thus, access to knowledge regarding the mechanism by which LPA expression is regulated is limited due to shortage of appropriate animal models. However, mice transgenic for the human LPA gene have been produced. Lp(a) levels in man are genetically determined and not altered significantly by dietary changes. In contrast, mice transgenic for LPA-yeast artificial chromosome (LPA-YAC) have markedly reduced apo(a) levels after maintenance on a high-fat diet. LPA-YAC carries the 40kb LPA-PLG intergenic region, which includes a putative binding site for peroxisome proliferator-activated receptor alpha (PPARalpha). Therefore, we examined if fibrates, which exert their effect via PPARalpha, could alter LPA expression in transgenic mice. METHODS:: Two LPA transgenic mouse lines with or without the LPA-PLG intergenic region we fed either PPARalpha agonist fenofibrate (FF) or 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY 14643) containing diets for 3 weeks. For the study of serum apo(a) levels, blood were sampled prior the experiment and when the animals were sacrificed. For the study of gene expression pattern pieces of livers were collected and submerged in RNAlater buffer and stored at -70 degrees C until analysis by quantitative PCR. RESULTS AND CONCLUSIONS:: The results showed that fibrates reduce LPA expression in LPA-YAC transgenic mice, but have no impact on hepatic apo(a) mRNA or serum apo(a) protein levels in LPA-cDNA transgenic mice, which lack the LPA-PLG intergenic region. This suggests that the effect of fibrates on LPA expression is mediated upstream of the LPA gene. However, on the basis of current data it is not possible to conclude that PPARalpha is the primary factor that represses LPA expression in LPA-YAC transgenic mice. Negative correlation between FXR and apo(a) mRNA levels, in addition to putative FXR DNA binding sequence in LPA-PLG intergenic region, suggest that it is equally likely that reduced expression of LPA could be a secondary consequence of PPARalpha activation on other genes, such as FXR.

Atherogenesis and vascular calcification in mice expressing the human LPA gene

Background: Lp(a) lipoprotein (Lp(a)) contains polymorphic glycoprotein, apolipoprotein(a) (apo(a)) and low density lipoprotein (LDL). The extensive homology between apo(a) and plasminogen is believed to contribute to the pathogenicity of apo(a), but the precise mechanisms by which Lp(a) participates in atherogenesis is still unknown. We used LPA-yeast artificial chromosome (LPA-YAC) transgenic mice with or without the human APOB (hAPOB) gene to study pathogenicity of apo(a)/Lp(a) and illucidate its role in regulation of serum lipid levels. Methods: Middle-aged (1-year-old) mice were fed a control (AIN-76), a high-cholesterol (HC) or a high-cholesterol/high-fat (HCHF) diet for 7 weeks. For the study of serum total apo(a) and lipid levels, mice were sampled prior to the experiment, at 2 weeks and at 7 weeks when the animals were sacrificed. Hearts with ascending aorta were fixed in formalin, embedded in gelatine and prepared for sections on a cryostat. Livers were washed in ice cold saline and submerged in RNAlater trade mark buffer and stored at -70 degrees C until mRNA analysis. Results: Wild type mice fed the control diet did not develop aortic lesions. Presence of the LPA gene was sufficient to induce development of aortic lesions, but neither coexpression of the hAPOB gene nor feeding the HC diet or the HCHF diet augmented the development of aortic lesions in LPA-YAC transgenic mice. On the control diet transgenic females had larger aortic lesion size than transgenic males. Furthermore, aortic lesions in transgenic females were associated with calcification more often than in transgenic males. Serum total cholesterol levels were higher both in wild type and LPA-YAC transgenic males than in females mainly because of higher serum high-density lipoprotein cholesterol levels. HC and HCHF feeding had more pronounced effect on total cholesterol levels in LPA-YAC/hAPOB transgenic mice than in either wild type or LPA-YAC transgenic mice, due to increased low density lipoprotein cholesterol levels. Furthermore, these diets reduced serum total apo(a) levels in both transgenic mouse lines. Conclusion: Expression of the human LPA gene in mice is sufficient to trigger development of aortic lesions. Similar frequency of calcified lesions in LPA-YAC transgenic mice with or without hAPOB gene may suggest that apo(a) is the part of the Lp(a) molecule that causes aortic calcification. The basis for reduced serum total apo(a) level in response to cholesterol feeding is not clear, but interplay between LPA and factors involved in cholesterol or bile acid homeostasis is worth of future studies.

Human apoB contributes to increased serum total apo(a) level in LPA transgenic mice

Background

The Lp(a) lipoprotein (Lp(a)) consists of the polymorphic glycoprotein apolipoprotein(a) (apo(a)), which is attached by a disulfide bond to apolipoprotein B (apoB). Apo(a), which has high homology with plasminogen, is present only in primates and hedgehogs. However, transgenic mice and rabbits with high serum apo(a) levels exist. Liver is the main site for apo(a) synthesis, but the site of removal is uncertain. To examine differences between transgenic mice expressing the LPA gene and mice capable of forming Lp(a) particles, LPA-YAC transgenic mice and hAPOB transgenic mice were crossed and their offspring examined.

Results

Comparison of LPA-YAC with LPA-YAC/hAPOB transgenic mice showed that LPA-YAC/hAPOB transgenic mice have higher serum total apo(a) and total cholesterol level than mice lacking the hAPOB gene. However, hepatic apo(a) mRNA level was higher in LPA-YAC transgenic mice than in LPA-YAC/hAPOB transgenic mice. Feeding of a high-cholesterol/high-fat diet to male LPA-YAC transgenic mice with or without the hAPOB gene resulted in reduced serum total apo(a) and hepatic apo(a) mRNA level.

Conclusion

In conclusion, the higher serum total apo(a) level in LPA-YAC/hAPOB transgenic mice than in LPA-YAC transgenic mice is not caused by increased apo(a) synthesis. Lower hepatic apo(a) mRNA level in LPA-YAC/hAPOB than in LPA-YAC transgenic mice may suggest that the increase in total apo(a) level is a result of apo(a) accumulation in serum. Furthermore, observed higher serum total cholesterol level in LPA-YAC/hAPOB transgenic mice than either in wild type or LPA-YAC transgenic mice may further suggest that human APOB transgenicity is a factor that contributes to increased serum total apo(a) and cholesterol levels. Our results on reduced serum total apo(a) and hepatic apo(a) mRNA levels in HCHF fed male LPA-YAC transgenic mice confirm earlier findings in females, and show that there are no sex difference in mechanisms for lowering apo(a) level in response to HCHF feeding.