Analysis of two duplications of the LDL receptor gene affectingintracellular transport, catabolism, and surface binding of the LDLreceptor

Pharmacologic rescue of conformationally-defective proteins: implications for the treatment of human disease

The process of quality control in the endoplasmic reticulum involves a variety of mechanisms which ensure that only correctly folded proteins enter the secretory pathway. Among these are conformation-screening mechanisms performed by molecular chaperones that assist in protein folding and prevent non-native (or misfolded) proteins from interacting with other misfolded proteins. Chaperones play a central role in the triage of newly formed proteins prior to their entry into the secretion, retention, and degradation pathways. Despite this stringent quality control mechanism, gain- or loss-of-function mutations that affect protein folding in the endoplasmic reticulum can manifest themselves as profound effects on the health of an organism. Understanding the molecular, cellular, and energetic mechanisms of protein routing could prevent or correct the structural abnormalities associated with disease-causing misfolded proteins. Rescue of misfolded, "trafficking-defective", but otherwise functional, proteins is achieved by a variety of physical, chemical, genetic, and pharmacological approaches. Pharmacologic chaperones (or "pharmacoperones") are template molecules that may potentially arrest or reverse diseases by inducing mutant proteins to adopt native-type-like conformations instead of improperly folded ones. Such restructuring leads to a normal pattern of cellular localization and function. This review focuses on protein misfolding and misrouting related to various disease states and describes promising approaches to overcoming such defects. Special attention is paid to the gonadotropin-releasing hormone receptor, since there is a great deal of information about this receptor, which has recently emerged as a particularly instructive model.

Misrouted cell surface receptors as a novel disease aetiology and potential therapeutic target: the case of hypogonadotropic hypogonadism due to gonadotropin-releasing hormone resistance

Molecules that are incorrectly folded or defectively assembled are recognised by cellular quality control mechanisms. This leads such conformationally abnormal molecules to intracellular retention and eventual degradation. A number of diseases caused by mutations that interfere with proper processing and intracellular trafficking of key cell surface proteins have been described. These include a particular variant of hypogonadotropic hypogonadism, which results from mislocalisation of the gonadotropin-releasing hormone (GnRH) receptor. It has been shown recently that membrane expression and function of misfolded GnRH receptor mutants can be rescued by a peptidomimetic antagonist of GnRH (IN3) that permeates into the cell and reaches the abnormally manufactured nascent receptor, stabilising a conformation compatible with cell-surface transport and reversing intracellular retention. This approach seems applicable for the development of defined therapeutic strategies for an array of diseases caused by incorrectly routed cell surface or secreted proteins.

An unequal crossover event in RCCX modules of the human MHC resulting in the formation of a TNXB/TNXA hybrid and deletion of the CYP21A

The central region of the human major histocompatibility complex contains tandemly arranged genes of RP, C4, CYP21, and TNX. The C4 gene region is prone to rearrangements that generates duplications, conversions, and deletions. Diversity in gene number and size causes reorganization and may lead to genetic disorders. The RP, C4, CYP21, and TNX genes form a genetic unit called RCCX. We describe molecular studies on RCCX haplotypes revealing a unique recombination giving rise to a TNXB/TNXA hybrid gene, CYP21A deletion and CYP21B duplication on one chromosome of the propositus. His other chromosome carries a deletion of CYP21A-TNXA-RP2-C4B genes, resulting in the total absence of CYP21A genes and the presence of three CYP21B genes in the genome.

Clinical expression of familial hypercholesterolemia in clusters of mutations of the LDL receptor gene that cause a receptor-defective or receptor-negative phenotype

Seventy-one mutations of the low density lipoprotein (LDL) receptor gene were identified in 282 unrelated Italian familial hypercholesterolemia (FH) heterozygotes. By extending genotype analysis to families of the index cases, we identified 12 mutation clusters and localized them in specific areas of Italy. To evaluate the impact of these mutations on the clinical expression of FH, the clusters were separated into 2 groups: receptor-defective and receptor-negative, according to the LDL receptor defect caused by each mutation. These 2 groups were comparable in terms of the patients' age, sex distribution, body mass index, arterial hypertension, and smoking status. In receptor-negative subjects, LDL cholesterol was higher (+18%) and high density lipoprotein cholesterol lower (-5%) than the values found in receptor-defective subjects. The prevalence of tendon xanthomas and coronary artery disease (CAD) was 2-fold higher in receptor-negative subjects. In patients >30 years of age in both groups, the presence of CAD was related to age, arterial hypertension, previous smoking, and LDL cholesterol level. Independent contributors to CAD in the receptor-defective subjects were male sex, arterial hypertension, and LDL cholesterol level; in the receptor-negative subjects, the first 2 variables were strong predictors of CAD, whereas the LDL cholesterol level had a lower impact than in receptor-defective subjects. Overall, in receptor-negative subjects, the risk of CAD was 2.6-fold that of receptor-defective subjects. Wide interindividual variability in LDL cholesterol levels was found in each cluster. Apolipoprotein E genotype analysis showed a lowering effect of the epsilon2 allele and a raising effect of the epsilon4 allele on the LDL cholesterol level in both groups; however, the apolipoprotein E genotype accounted for only 4% of the variation in LDL cholesterol. Haplotype analysis showed that all families of the major clusters shared the same intragenic haplotype cosegregating with the mutation, thus suggesting the presence of common ancestors.

Influence of beta(0)-thalassemia on the phenotypic expression of heterozygous familial hypercholesterolemia : a study of patients with familial hypercholesterolemia from Sardinia

One of the genetic features of the Sardinian population is the high prevalence of hemoglobin disorders. It has been estimated that 13% to 33% of Sardinians carry a mutant allele of the alpha-globin gene (alpha-thalassemia trait) and that 6% to 17% are beta-thalassemia carriers. In this population, a single mutation of beta-globin gene (Q39X, beta(0) 39) accounts for >95% of beta-thalassemia cases. Because previous studies have shown that Sardinian beta-thalassemia carriers have lower total and low density lipoprotein (LDL) cholesterol than noncarriers, we wondered whether this LDL-lowering effect of the beta-thalassemia trait was also present in subjects with familial hypercholesterolemia (FH). In a group of 63 Sardinian patients with the clinical diagnosis of FH, we identified 21 unrelated probands carrying 7 different mutations of the LDL receptor gene, 2 already known (313+1 g>a and C95R) and 5 not previously reported (D118N, C255W, A378T, T413R, and Fs572). The 313+1 g>a and Fs572 mutations were found in several families. In cluster Fs572, the plasma LDL cholesterol level was 5.76+/-1.08 mmol/L in subjects with beta(0)-thalassemia trait and 8.25+/-1.66 mmol/L in subjects without this trait (P<0.001). This LDL-lowering effect was confirmed in an FH heterozygote of the same cluster who had beta(0)-thalassemia major and whose LDL cholesterol level was below the 50th percentile of the distribution in the normal Sardinian population. The hypocholesterolemic effect of beta(0)-thalassemia trait emerged also when we pooled the data from all FH subjects with and without beta(0)-thalassemia trait, regardless of the type of mutation in the LDL receptor gene. The LDL-lowering effect of beta(0)-thalassemia may be related to (1) the mild erythroid hyperplasia, which would increase the LDL removal by the bone marrow, and (2) the chronic activation of the monocyte-macrophage system, causing an increased secretion of some cytokines (interleukin-1, interleukin-6, and tumor necrosis factor-alpha) known to affect the hepatic secretion and the receptor-mediated removal of apolipoprotein B-containing lipoproteins. The observation that our FH subjects with beta(0)-thalassemia trait (compared with noncarriers) have an increase of blood reticulocytes (40%) and plasma levels of interleukin-6 (+60%) supports these hypotheses. The lifelong LDL-lowering effect of beta(0)-thalassemia trait might slow the development and progression of coronary atherosclerosis in FH.