Author + information
- Nalini M. Rajamannan, MD∗ ()
- Most Sacred Heart of Jesus Cardiology and Valvular Institute, Sheboygan, Wisconsin
- Department of Molecular Biology and Biochemistry, Mayo Clinic School of Medicine, Rochester, Minnesota
- ↵∗Reprint requests and correspondence:
Dr. Nalini M. Rajamannan, Mayo Clinic, Department of Biochemistry and Molecular Biology, 1601 Guggenheim, 200 First St SW, Rochester, Minnesota 55905.
Calcific aortic valve disease (CAVD) is the most common indication worldwide for valve intervention. For years, the mechanism for this calcification was thought to be due to a passive degenerative process. However, in the 21st century, the National Heart, Lung, and Blood Institute of the National Institutes of Health recognized that CAVD is an active biologic osteogenic process (1). Initiation of osteogenesis in the aortic valve depends on risk factors similar to those known to promote coronary artery disease, which cause myofibroblasts to differentiate via an osteogenic gene activation that results in valve calcification (1,2).
In this issue of the Journal, a study from the Netherlands by ten Kate et al. (3) tested the prevalence, extent, and risk modifiers of CAVD in patients with heterozygous familial hypercholesterolemia (he-FH). Clinically, the he-FH phenotype is encountered more often than the homozygous phenotype due to rapid progression of coronary artery disease in the homozygous patient population. The investigators therefore sought to determine the prevalence of CAVD in patients with he-FH by measuring the amount of calcification burden via computed tomography measurements of the coronary artery and aortic valve, low-density lipoprotein receptor (LDLR) function, and lipid levels and assessing their association with CAVD.
LDL Receptor Density
The investigators discovered that the prevalence of aortic valve calcification (AoVC) and the AoVC score (median [interquartile range]) were both higher in patients with he-FH than in control subjects: 41% versus 21%, respectively (p < 0.001) and 51 (9 to 117) versus 21 (3 to 49) (p = 0.007) (3). LDLR-negative mutational he-FH was the strongest predictor of the AoVC score (odds ratio: 4.81; 95% confidence interval: 2.22 to 10.40; p < 0.001). He-FH was associated with a high prevalence and a large extent of subclinical AoVC, especially in patients with LDLR-negative mutations, compared with the control subjects. Moreover, the AoVC scores increased faster with age in the LDLR-negative he-FH patients than in the LDLR-defective he-FH patients.
The LDLR-negative mutation carrier status was a strong predictor of the extent of AoVC (3). The association between coronary artery calcification and AoVC was associated with a higher prevalence of AoVC, both in patients with he-FH and in control subjects. The authors hypothesized that the high level of coronary artery calcification may be due to confounding variables such as differences in statin therapy in the he-FH population versus the control population. The concept of 2 different phases of AoVC progression is not only essential but could explain the discordant findings. The National Heart, Lung, and Blood Institute Aortic Stenosis Working Group for CAVD (1) also emphasized this concept in early valve sclerosis versus late valve stenosis.
Compared with he-FH patients with LDLR-defective mutations, patients with LDLR-negative mutational he-FH had higher levels of total cholesterol and maximum untreated low-density lipoprotein cholesterol (3). In addition, he-FH patients with LDLR-negative mutations began statin treatment at a younger age and used statins for a longer period of time. Figure 1 illustrates the results of the study in patients with he-FH, including the effect of functional low-density lipoprotein (LDL) receptors and proportional increases in LDL with the degree of AoVC.
To the best of our knowledge, this study by ten Kate et al. (3) is the first to correlate in patients the role of LDL and the effect of the LDL receptor genetic contribution in terms of phenotypic expression of calcification in the valve and in the coronary arteries. The LDL-density theories (4–6) provide a hemodynamic explanation for why abnormal calcification develops secondary to high LDL density concentration up-regulating osteogenesis. The effect of fluid flow in the heart is responsible for the variable phenotype expression, depending on the radius of the specific anatomic location in the heart (i.e., artery vs. valve).
Fluid hemodynamics in the heart depends on multiple factors, as derived by the Bernoulli equation for fluid flow (7). Bernoulli described flow through a column as being directly proportional to the change in pressure across the column and indirectly proportional to the resistance. The formula for flow through the heart is similar to Ohm’s law for electricity, as shown in Equation 1.Equation 1The entire formula for resistance for steady-state flow through a circular tube is shown in Equation 2, where η = viscosity and r = radius of the tube.Equation 2Equations 1 and 2 can be combined to provide the flow rate through a circular tube in terms of a pressure drop, which is described as Poiseuille’s law:Equation 3
The differences in the rate of fluid flow depend on the radius of the anatomic structure, which is inversely proportional to the resistance. In addition, it is important to note the inverse r4 dependence of the resistance to fluid flow. If the radius of the tube is halved, the pressure drop for a given flow rate and viscosity is increased by a factor of 16, because the flow rate is then proportional to the fourth power of the radius. The LDL-Density-Radius Theory (4) and the LDL-Density-Pressure Theory (5) provide the molecular hypothesis of the role of lipids in the differentiation of valve myofibroblasts into osteoblast-like cells responsible for the calcifying phenotype. Expression of the calcification in the coronary artery (8) occurs at a faster rate than the aortic valve secondary to the effect of the radius in these 2 anatomic locations in the heart.
The present study (3) measured the level of calcification, and the results correlate the LDL concentration, LDL receptor gene expression, and finally a Mendelian randomization analysis to suggest a causal role of LDL-C in beginning aortic valve pathology. The first case report to demonstrate by histology the presence of atherosclerosis in the aortic valve is in a post-mortem analysis of a patient's aortic valve who had the diagnosis of familial hypercholesterolemia (9).
If atherosclerosis is an initiating event in this patient population, would lipid-lowering strategies be effective for the slowing of disease progression? In the present study (3), patients with he-FH were exposed to extremely high levels of LDL before statin treatment, especially those with mutational he-FH. Since these patients were first diagnosed with hypercholesterolemia, they have been treated with statins; this approach dramatically lowered LDL-C levels and thereby reduced the predictive value of LDL-C toward AoVC. The authors (3) proposed that the benefits of the statins are for patients who received the statins early in the atherosclerotic process (9), before the development of calcification and eventually severe stenosis. Furthermore, they hypothesized that the results of randomized controlled trials (6) that tested the effect of statins in CAVD may be due to the initiation of treatment in patients with advanced calcific disease.
In conclusion, the present study (3) is the first to combine biochemical analysis with genetic LDL receptor function and the calcifying phenotype in the heart. The study further confirms the hypothesis regarding the possible modification and slowing of CAVD progression with the use of long-term lipid lowering if the therapy is initiated in the early stages of pre-clinical CAVD, the atherosclerotic phase (9).
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Rajamannan is the inventor on a patent for methods to slow progression of aortic valve disease; the patent is owned by the Mayo Clinic, with no royalty payments to the inventor.
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