Familial hypercholesterolaemia and LDL receptor mutations

Prognostic value of exercise tolerance test for predicting cardiovascular disease in asymptomatic individuals with heterozygous familiar hypercholesterolemia

Heterozygous familiar hypercholesterolemia (hFH) is an autosomal dominant inherited dyslipidemia, associated with premature cardiovascular disease (CVD). Aim of the study was to define prognostic factors for cardiovascular events (CVE) in asymptomatic individuals with hFH. All participants with recent diagnosis of hFH were recruited from the outpatient lipid clinic from 1987 to 2016, without any previous clinical evidence of CVD. A detailed clinical evaluation and laboratory investigation was obtained. Exercise tolerance test (ETT) was performed until maximum exercise capacity was achieved, without evidence of ischemia. Primary endpoint of the study was the first CVE. Four hundred fifty one participants were followed up for 10 ± 8 years, with 68 recorded cases of CVD (15%). Cumulative incidence of CVD was 15%, 24% and 32% for the 3 decades, respectively. In univariate analysis, male gender (p = 0.016), progression of age (p < 0.001), menopause (p = 0.030), waist-hip ratio (p = 0.043) and increased levels of Lp(α) (p = 0.014) were significantly associated with increased CVD incidence; whereas, exercise capacity (p = 0.025), low variation of heart rate (HR) during all stages of ETT compared to resting state (p = 0.020), maximum systolic (p = 0.014) and diastolic (p < 0.001) blood pressure were inversely associated with CVD. In multi-adjusted analysis, male gender (p < 0.001), duration of ETT (p = 0.023), estimated HR (p = 0.029), variation of HR during ETT compared to resting state (p < 0.05) and maximum diastolic pressure (p = 0.044) were significantly associated with CVD. Parameters of ETT in asymptomatic individuals with hFH, without any evidence of ischemia, may predict CVD in these high-risk patients after decades of observation.

NARC-1/PCSK9 and Its Natural Mutants

The discovery of autosomal dominant hypercholesterolemic patients with mutations in the PCSK9 gene, encoding the proprotein convertase NARC-1, resulting in the missense mutations suggested a role in low density lipoprotein (LDL) metabolism. We show that the endoplasmic reticulum-localized proNARC-1 to NARC-1 zymogen conversion is Ca2+-independent and that within the zymogen autocatalytic processing site SSVFAQ [downward arrow]SIP Val at P4 and Pro at P3' are critical. The S127R and D374Y mutations result in approximately 50-60% and > or =98% decrease in zymogen processing, respectively. In contrast, the double [D374Y + N157K], F216L, and R218S natural mutants resulted in normal zymogen processing. The cell surface LDL receptor (LDLR) levels are reduced by 35% in lymphoblasts of S127R patients. The LDLR levels are also reduced in stable HepG2 cells overexpressing NARC-1 or its natural mutant S127R, and this reduction is abrogated in the presence of 5 mm ammonium chloride, suggesting that overexpression of NARC-1 increases the turnover rate of the LDLR. Adenoviral expression of wild type human NARC-1 in mice resulted in a maximal approximately 9-fold increase in circulating LDL cholesterol, while in LDLR-/- mice a delayed approximately 2-fold increase in LDL cholesterol was observed. In conclusion, NARC-1 seems to affect both the level of LDLR and that of circulating apoB-containing lipoproteins in an LDLR-dependent and -independent fashion.

Low-density lipoprotein receptor-knockout mice display impaired spatial memory associated with a decreased synaptic density in the hippocampus

The low-density lipoprotein receptor (LDLR) is the first described receptor for apolipoprotein E (apoE). We hypothesize that the absence of the LDLR, similar to the absence of apoE, results in impaired learning and memory processes. Six-month-old homozygous Ldlr-/- and wild-type littermates (Ldlr+/+), maintained on a standard lab chow diet, were used. Unlike humans, Ldlr-/- mice, under these conditions, do not develop atherosclerosis. The results of the Morris water escape task revealed an impaired spatial memory in the Ldlr-/- mice in comparison with Ldlr+/+ mice. Also in a T-maze task, the working memory performance of the Ldlr-/- mice was impaired. Furthermore, Ldlr-/- mice, in comparison with Ldlr+/+ mice, display a decreased number of synaptophysin-immunoreactive presynaptic boutons in the hippocampus CA1. In conclusion, the results show in mice deficiency for the LDLR results in impaired hippocampal-dependent memory functions. A decrease in the number of presynaptic boutons may underlay these behavioral alterations. Therefore, the LDLR may be an important receptor for apoE in the central nervous system.

Modification of brain aging and neurodegenerative disorders by genes, diet, and behavior

Multiple molecular, cellular, structural, and functional changes occur in the brain during aging. Neural cells may respond to these changes adaptively, or they may succumb to neurodegenerative cascades that result in disorders such as Alzheimer's and Parkinson's diseases. Multiple mechanisms are employed to maintain the integrity of nerve cell circuits and to facilitate responses to environmental demands and promote recovery of function after injury. The mechanisms include production of neurotrophic factors and cytokines, expression of various cell survival-promoting proteins (e.g., protein chaperones, antioxidant enzymes, Bcl-2 and inhibitor of apoptosis proteins), preservation of genomic integrity by telomerase and DNA repair proteins, and mobilization of neural stem cells to replace damaged neurons and glia. The aging process challenges such neuroprotective and neurorestorative mechanisms. Genetic and environmental factors superimposed upon the aging process can determine whether brain aging is successful or unsuccessful. Mutations in genes that cause inherited forms of Alzheimer's disease (amyloid precursor protein and presenilins), Parkinson's disease (alpha-synuclein and Parkin), and trinucleotide repeat disorders (huntingtin, androgen receptor, ataxin, and others) overwhelm endogenous neuroprotective mechanisms; other genes, such as those encoding apolipoprotein E(4), have more subtle effects on brain aging. On the other hand, neuroprotective mechanisms can be bolstered by dietary (caloric restriction and folate and antioxidant supplementation) and behavioral (intellectual and physical activities) modifications. At the cellular and molecular levels, successful brain aging can be facilitated by activating a hormesis response in which neurons increase production of neurotrophic factors and stress proteins. Neural stem cells that reside in the adult brain are also responsive to environmental demands and appear capable of replacing lost or dysfunctional neurons and glial cells, perhaps even in the aging brain. The recent application of modern methods of molecular and cellular biology to the problem of brain aging is revealing a remarkable capacity within brain cells for adaptation to aging and resistance to disease.

An extracellular beta-propeller module predicted in lipoprotein and scavenger receptors, tyrosine kinases, epidermal growth factor precursor, and extracellular matrix components

An abundant, widely dispersed, extracellular sequence repeat that contains a consensus YWTD motif is shown here to occur in groups of six contiguous repeats. Thirteen lines of evidence, including experimental and computational data, predict with p<3x10(-9) that the repeats do not form tandem domains, but rather each group of six repeats folds into a compact beta-propeller structure. The six beta-sheets are arranged about a 6-fold pseudosymmetry axis, and each repeat contributes loops to the faces surrounding the pseudosymmetry axis. Seven different endocytic receptors that contain from one to eight YWTD beta-propeller domains act as lipoprotein, vitellogenin, and scavenger receptors. In the low density lipoprotein receptor (LDLR), the many mutations in familial hypercholesterolaemia that map to the YWTD domain can now be interpreted. In the extracellular matrix component nidogen, the YWTD domain functions to bind laminin. Three YWTD domains and interspersed fibronectin type III (FN3) domains constitute almost the entire extracellular domain of the sevenless and c-ros receptor tyrosine kinases. YWTD domains often are bounded by epidermal growth factor (EGF) modules, including in the EGF precursor itself. YWTD beta-propellers have a circular folding pattern that brings neighboring modules into close proximity, and may have important consequences for the architecture of multi-domain proteins.

Structural studies of detergent-solubilized and vesicle-reconstituted low-density lipoprotein (LDL) receptor

The low-density lipoprotein (LDL) receptor plays a key role in maintaining circulating and cellular cholesterol homeostasis. The LDL receptor is a transmembrane glycoprotein whose biochemical and genetic properties have been extensively studied notably by Brown, Goldstein and colleagues [Brown, M. S., & Goldstein, J. L., (1986) Science 232, 34-47]. However, few if any structural studies of the LDL receptor have been reported, and details of its secondary and tertiary structure are lacking. In an attempt to determine the low-resolution structure of the LDL receptor, we have purified the receptor from bovine adrenal cortices using modifications of the method of Schneider et al. [Schneider, W. J., Goldstein, J. L., & Brown, M. S. (1985) Methods in Enzymol.109, 405-417]. Using circular dichroism, the secondary structure of the detergent-solubilized bovine LDL receptor at 25 degrees C was shown to be 19% alpha-helix, 42% beta-sheet, and 39% random coil. Interestingly, the detergent-solubilized receptor appeared to be quite resistant to changes in secondary structure over the temperature range 10-90 degrees C, with only minor but reversible changes being observed. In contrast, a more pronounced unfolding of the detergent-solubilized receptor was observed in the presence of guanidinium hydrochloride. Using the complete sequence of the human LDL receptor, sequence analysis by the Chou-Fasman prediction algorithm showed quite good agreement with the experimentally determined secondary structure of the bovine LDL receptor at 25 degrees C. Finally, the purified, bovine LDL receptor was reconstituted into large unilamellar vesicles of egg yolk phosphatidylcholine using a procedure exploiting preformed vesicles and detergent dialysis. We showed previously using negative stain electron microscopy that reconstituted vesicles bind LDL. Now, using cryoelectron microscopy of frozen hydrated reconstituted vesicles evidence of an extended, stick-like morphology (length approximately 120 A) for the extracellular domain of the LDL receptor has been obtained. Successful purification of the receptor, its incorporation into single bilayer vesicles, and its direct visualization by cryoelectron microscopy pave the way for more detailed structural studies of the LDL receptor and the receptor-LDL complex.

Cellular effects of hypercholesterolemia in modulation of cancer growth and metastasis: a review of the evidence

Hypercholesterolemia and increased cancer risk have been associated, particularly with the high fat diets characteristic of Western societies. We were interested in the possible association between preexisting hypercholesterolemia and the rapidity and extent of tumor metastases in these patients. To date there has been only a few studies that have suggested and explored this determinant of cancer metastases although it may play a role in a subset of patients who develop cancers. This article will review the literature on the effects of LDL-cholesterol on cell proliferation and differentiation and speculate on mechanisms of involvement of a hypercholesterolemic milieu on cancer progression and enhancement of metastatic potential.

Surface expression of low density lipoprotein receptor in EBV-transformed lymphocytes: characterization and use for studying familial hypercholesterolemia

The objectives of the present study were to characterize the surface expression of low density lipoprotein receptor (LDL-R) in Epstein-Barr virus transformed lymphocytes (EBV-L) and to determine the applicability of the cellular system for the study of familial hypercholesterolemia. The EBV-L subsets and LDL-R expression were determined by immuno-cytofluorimetry. The LDL-R expression in EBV-L which consisted of mostly B cells was no different among antigenic subsets. EBV-L cultured in lipoprotein deficient serum demonstrated a 9.3-fold higher LDL-R expression than primary lymphocytes. Lovastatin caused an additional 1.9-and 1.4-fold increase in EBV-L and primary lymphocytes respectively. This difference in lovastatin response is statistically significant (paired t-test, P < 0.001). 54% of the high LDL-R expression in EBV-L was related to the changes in proliferation measured as stimulation index (SI). LDL and lovastatin modulated the LDL-R expression without affecting SI. FH subjects demonstrated 2% (homozygote, n = 1) and 44.6 +/- 12.3% (heterozygotes, n = 35) in LDL-R expression of controls (n = 30). This maintenance of the FH phenotype and the intrinsically high LDL-R expression in EBV-L make the cellular system suitable for the study of FH as well as the regulation of LDL-R.

Mutations of the low-density-lipoprotein receptor gene and familial hypercholesterolemia

Familial hypercholesterolemia (FH), an autosomal dominant disorder caused by mutation of the low-density-lipoprotein (LDL) receptor, occurs in about one in 500 individuals. The evaluation of naturally occurring mutants has permitted an extensive structure-function analysis of this receptor that has provided insight into the biochemistry and cell biology of cell-surface receptors in general. Novel gene therapeutic approaches to the management of FH are a developing outgrowth of this research.

Receptor cloning and heterologous expression--towards a new tool for drug discovery

The explosion in the number of cloned receptors presents the pharmaceutical industry with challenges to discover new drugs targeting those receptors; to find more-selective drugs for all novel receptor subtypes; and to learn more about the function of the receptors in order to discern the conditions where such drugs may be applied usefully as therapeutics. At the same time, receptor cloning affords an unprecedented opportunity to address these challenges: heterologously expressed recombinant human receptors can be used for drug screening and - through an improved understanding of structure-function relationship - possibly for drug design, while the receptor clones permit mobilization of the full power of molecular biology to elucidate the function of the receptors in health and disease.

The genetic basis of lipoprotein disorders. Introduction and overview

In order to elucidate the genetic abnormalities underlying lipoprotein disorders associated with susceptibility to coronary heart disease, researchers have looked for candidate genes. The studies have focused particularly on the lipoprotein transport genes. Relatively common as well as rare mutations have already been identified in several of these genes. In addition, further metabolic and genetic studies indicate that some of these loci harbour significant, but as yet undefined, genetic variation. In the next few years, it is not unreasonable to expect that all or most of the significant mutations at these loci will be catalogued. It is too early to know whether this will be sufficient to explain the genetic basis of altered lipoprotein levels, or whether new loci will need to be investigated. Additional candidate gene loci might be those coding for genes involved in intracellular cholesterol metabolism, cholesterol absorption or insulin resistance. New loci may also be revealed by the technique of reverse genetics. A more complete understanding of the genetics of susceptibility to atheroscerosis will probably also entail the identification of variants at genetic loci that control both the reaction of the blood vessel wall to atherogenic lipoproteins and the thrombosis system. Investigation of the genetic basis of susceptibility to coronary heart disease remains a worthwhile and lively field, with important implications for clinical and public health.