Genetics, clinical phenotype, and molecular cell biology of autosomal recessive hypercholesterolemia

Improved lipid-lowering treatment and reduction in cardiovascular disease burden in homozygous familial hypercholesterolemia: The SAFEHEART follow-up study

AIM

We aimed to describe clinical and genetic characteristics, lipid-lowering treatment and atherosclerotic cardiovascular disease (ASCVD) outcomes over a long-term follow-up in homozygous familial hypercholesterolemia (HoFH).

METHODS

SAFEHEART (Spanish Familial Hypercholesterolaemia Cohort Study) is a long-term study in molecularly diagnosed FH. Data analyzed in HoFH were prospectively obtained from 2004 until 2022. ASCVD events, lipid profile and lipid-lowering treatment were determined.

RESULTS

Thirty-nine HoFH patients were analyzed. The mean age was 42 ± 20 years and nineteen (49%) were women. Median follow-up was 11 years (IQR 6,18). Median age at genetic diagnosis was 24 years (IQR 8,42). At enrolment, 33% had ASCVD and 18% had aortic valve disease. Patients with new ASCVD events and aortic valve disease at follow-up were six (15%), and one (3%), respectively. Median untreated LDL-C levels were 555 mg/dL (IQ 413,800), and median LDL-C levels at last follow-up was 122 mg/dL (IQR 91,172). Most patients (92%) were on high intensity statins and ezetimibe, 28% with PCSK9i, 26% with lomitapide, and 23% with lipoprotein-apheresis. Fourteen patients (36%) attained an LDL-C level below 100 mg/dL, and 10% attained an LDL-C below 70 mg/dL in secondary prevention. Patients with null/null variants were youngers, had higher untreated LDL-C and had the first ASCVD event earlier. Free-event survival is longer in patients with defective variant compared with those patients with at least one null variant (p=0.02).

CONCLUSIONS

HoFH is a severe life threating disease with a high genetic and phenotypic variability. The improvement in lipid-lowering treatment and LDL-C levels have contributed to reduce ASCVD events.

The comparison of morphology and transcriptome in the inner membrane reveals the potential mechanism of the heritable carapace color of the Chinese mitten crab Eriocheir sinensis

Carapace color plays an important role in the communication, reproduction, and self-defense of crustaceans, which is also related to their economic value. Chinese mitten crab (Eriocheir sinensis) is an important aquaculture species in China, and there are different strains with heritable carapace colors, i.e. Green, White, and Red. However, there is a lack of research on the formation mechanism of carapace color of this species. This study was conducted to compare the histology and transcriptome in the inner membrane of three carapace color strains of E. sinensis. Histological comparisons revealed that the inner membrane of green and red carapace crabs contained more melanin, appearing in clusters, and had a higher presence of yellow or orange pigments. In contrast, the inner membrane of white carapace crabs had smaller and fewer melanin particles, as well as a lower presence of yellow or orange pigments. Observation under an electron microscope showed that the inner membrane of E. sinensis contained a large number of collagen fibers and various types of cells, including fibroblasts, melanocytes, and other tissue cells, which exhibited different levels of activity. Transcriptome analysis showed that the Green, Red, and White strains of E. sinensis had approximately 80.3 K, 81.6 K and 80.3 K expressed unigenes in their inner membranes, respectively. When comparing Green and Red crabs, there were 2, 850 upregulated genes and 2, 240 downregulated genes. In the comparison between Red and White crabs, there were 2, 853 upregulated genes and 2, 583 downregulated genes. Furthermore, there were 2, 336 upregulated genes and 2, 738 downregulated genes in the inner membranes between White and Green crabs. Among these genes, some members of the solute carriers family, which are involved in carotenoid transportation, showed differential expression among the three carapace color strains. Additionally, significant differences were observed in the expression of genes related to melanin synthesis, including wingless/integrate, tyrosinase, guanine nucleotide-binding protein inhibitory subunit, cell adhesion molecule, adenylyl cyclase, and creb-binding protein. there were no differences in the gene expression levels of the crustacyanin family. In conclusion, this study identified several candidate genes associated with carapace color in the inner membrane of E. sinensis, suggesting a close relationship between the heritable carapace colors and the transport of the carotenoids as well as the synthesis of melanin.

Unveiling Familial Hypercholesterolemia-Review, Cardiovascular Complications, Lipid-Lowering Treatment and Its Efficacy

Familial hypercholesterolemia (FH) is a genetic disorder primarily transmitted in an autosomal-dominant manner. We distinguish two main forms of FH, which differ in the severity of the disease, namely homozygous familial hypercholesterolemia (HoFH) and heterozygous familial hypercholesterolemia (HeFH). The characteristic feature of this disease is a high concentration of low-density lipoprotein cholesterol (LDL-C) in the blood. However, the level may significantly vary between the two mentioned types of FH, and it is decidedly higher in HoFH. A chronically elevated concentration of LDL-C in the plasma leads to the occurrence of certain abnormalities, such as xanthomas in the tendons and skin, as well as corneal arcus. Nevertheless, a significantly more severe phenomenon is leading to the premature onset of cardiovascular disease (CVD) and its clinical implications, such as cardiac events, stroke or vascular dementia, even at a relatively young age. Due to the danger posed by this medical condition, we have investigated how both non-pharmacological and selected pharmacological treatment impact the course of FH, thereby reducing or postponing the risk of clinical manifestations of CVD. The primary objective of this review is to provide a comprehensive summary of the current understanding of FH, the effectiveness of lipid-lowering therapy in FH and to explain the anatomopathological correlation between FH and premature CVD development, with its complications.

Genetic and molecular architecture of familial hypercholesterolemia

Atherosclerotic cardiovascular disease is the leading cause of death globally. Despite its important risk of premature atherosclerosis and cardiovascular disease, familial hypercholesterolemia (FH) is still largely underdiagnosed worldwide. It is one of the most frequently inherited diseases due to mutations, for autosomal dominant forms, in either of the LDLR, APOB, and PCSK9 genes or possibly a few mutations in the APOE gene and, for the rare autosomal forms, in the LDLRAP1 gene. The discovery of the genes implicated in the disease has largely helped to improve the diagnosis and treatment of FH from the LDLR by Brown and Goldstein, as well as the introduction of statins, to PCSK9 discovery in FH by Abifadel et al., and the very rapid availability of PCSK9 inhibitors. In the last two decades, major progress has been made in clinical and genetic diagnostic tools and the therapeutic arsenal against FH. Improving prevention, diagnosis, and treatment and making them more accessible to all patients will help reduce the lifelong burden of the disease.

Assessing the genetic burden of familial hypercholesterolemia in a large middle eastern biobank

BACKGROUND

The genetic architecture underlying Familial Hypercholesterolemia (FH) in Middle Eastern Arabs is yet to be fully described, and approaches to assess this from population-wide biobanks are important for public health planning and personalized medicine.

METHODS

We evaluate the pilot phase cohort (n = 6,140 adults) of the Qatar Biobank (QBB) for FH using the Dutch Lipid Clinic Network (DLCN) criteria, followed by an in-depth characterization of all genetic alleles in known dominant (LDLR, APOB, and PCSK9) and recessive (LDLRAP1, ABCG5, ABCG8, and LIPA) FH-causing genes derived from whole-genome sequencing (WGS). We also investigate the utility of a globally established 12-SNP polygenic risk score to predict FH individuals in this cohort with Arab ancestry.

RESULTS

Using DLCN criteria, we identify eight (0.1%) 'definite', 41 (0.7%) 'probable' and 334 (5.4%) 'possible' FH individuals, estimating a prevalence of 'definite or probable' FH in the Qatari cohort of ~ 1:125. We identify ten previously known pathogenic single-nucleotide variants (SNVs) and 14 putatively novel SNVs, as well as one novel copy number variant in PCSK9. Further, despite the modest sample size, we identify one homozygote for a known pathogenic variant (ABCG8, p. Gly574Arg, global MAF = 4.49E-05) associated with Sitosterolemia 2. Finally, calculation of polygenic risk scores found that individuals with 'definite or probable' FH have a significantly higher LDL-C SNP score than 'unlikely' individuals (p = 0.0003), demonstrating its utility in Arab populations.

CONCLUSION

We design and implement a standardized approach to phenotyping a population biobank for FH risk followed by systematically identifying known variants and assessing putative novel variants contributing to FH burden in Qatar. Our results motivate similar studies in population-level biobanks - especially those with globally under-represented ancestries - and highlight the importance of genetic screening programs for early detection and management of individuals with high FH risk in health systems.

Diabetes and Familial Hypercholesterolemia: Interplay between Lipid and Glucose Metabolism

Familial hypercholesterolemia (FH) is a genetic disease characterized by high low-density lipoprotein (LDL) cholesterol (LDL-c) concentrations that increase cardiovascular risk and cause premature death. The most frequent cause of the disease is a mutation in the LDL receptor (LDLR) gene. Diabetes is also associated with an increased risk of cardiovascular disease and mortality. People with FH seem to be protected from developing diabetes, whereas cholesterol-lowering treatments such as statins are associated with an increased risk of the disease. One of the hypotheses to explain this is based on the toxicity of LDL particles on insulin-secreting pancreatic β-cells, and their uptake by the latter, mediated by the LDLR. A healthy lifestyle and a relatively low body mass index in people with FH have also been proposed as explanations. Its association with superimposed diabetes modifies the phenotype of FH, both regarding the lipid profile and cardiovascular risk. However, findings regarding the association and interplay between these two diseases are conflicting. The present review summarizes the existing evidence and discusses knowledge gaps on the matter.

Twelve Variants Polygenic Score for Low-Density Lipoprotein Cholesterol Distribution in a Large Cohort of Patients With Clinically Diagnosed Familial Hypercholesterolemia With or Without Causative Mutations

Background

A significant proportion of individuals clinically diagnosed with familial hypercholesterolemia (FH), but without any disease‐causing mutation, are likely to have polygenic hypercholesterolemia. We evaluated the distribution of a polygenic risk score, consisting of 12 low‐density lipoprotein cholesterol (LDL‐C)‐raising variants (polygenic LDL‐C risk score), in subjects with a clinical diagnosis of FH.

Methods and Results

Within the Lipid Transport Disorders Italian Genetic Network (LIPIGEN) study, 875 patients who were FH‐mutation positive (women, 54.75%; mean age, 42.47±15.00 years) and 644 patients who were FH‐mutation negative (women, 54.21%; mean age, 49.73±13.54 years) were evaluated. Patients who were FH‐mutation negative had lower mean levels of pretreatment LDL‐C than patients who were FH‐mutation positive (217.14±55.49 versus 270.52±68.59 mg/dL, P<0.0001). The mean value (±SD) of the polygenic LDL‐C risk score was 1.00 (±0.18) in patients who were FH‐mutation negative and 0.94 (±0.20) in patients who were FH‐mutation positive (P<0.0001). In the receiver operating characteristic analysis, the area under the curve for recognizing subjects characterized by polygenic hypercholesterolemia was 0.59 (95% CI, 0.56–0.62), with sensitivity and specificity being 78% and 36%, respectively, at 0.905 as a cutoff value. Higher mean polygenic LDL‐C risk score levels were observed among patients who were FH‐mutation negative having pretreatment LDL‐C levels in the range of 150 to 350 mg/dL (150–249 mg/dL: 1.01 versus 0.91, P<0.0001; 250–349 mg/dL: 1.02 versus 0.95, P=0.0001). A positive correlation between polygenic LDL‐C risk score and pretreatment LDL‐C levels was observed among patients with FH independently of the presence of causative mutations.

Conclusions

This analysis confirms the role of polymorphisms in modulating LDL‐C levels, even in patients with genetically confirmed FH. More data are needed to support the use of the polygenic score in routine clinical practice.

PCSK9 Gene Participates in the Development of Primary Dyslipidemias

Dyslipidemias are a group of diseases, which are characterized by abnormal blood concentrations of cholesterol, triglycerides and/or low-density lipoprotein-cholesterol (LDL-c). Dyslipidemia is a determinant condition for the progress of an atherosclerotic plaque formation. The resulting atherogenicity is due to at least two mechanisms: first, to the accumulation in the plasma of lipid particles that have the capacity to alter the function of the endothelium and deposit at the atheromatous plaque, and second, at an insufficient concentration of multifactorial type of high density lipoprotein-cholesterol (HDL-c), whose function is to protect against the development of atherosclerosis. Its highest prevalence is encountered among individuals with diabetes, hypertension or overweight. Hyperlipidemia is one of the main predisposing factors for the development of cardiovascular disease. Hyperlipidemia can be the result of a genetic condition, the secondary expression of a primary process or the consequence of exogenous factors (food, cultural, socio-economic, etc.), all of which lead to the elevation of plasma lipid levels. The objective of this study was to carry out an analysis of the genes involved in the development of dyslipidemias that lead to cardiovascular disease with special emphasis on the proprotein convertase subtilin/kexin type 9 (PCSK9) gene. The PCSK9 gene participates in the development of primary dyslipidemias, mainly familial hypercholesterolemia, currently the pharmacological treatment of choice to reduce LDL-c are statins, however, it has been observed that these have been insufficient to eliminate cardiovascular risk, especially in subjects with primary forms of hypercholesterolemia related to genetic mutations, or statin intolerance.

Persistent hypercholesterolemia in child with homozygous autosomal recessive hypercholesterolemia: A decade of lipid management

Autosomal recessive hypercholesterolemia (ARH) is a rare form of genetic hypercholesterolemia caused by mutations in low density lipoprotein receptor adaptor protein 1 (LDLRAP1). The proband first presented with linear eruptive xanthomas over her ankles, knees and elbows, with low density lipoprotein cholesterol (LDL-C) of 16.0 mmol/L (618.7 mg/dL), at 2.5 years old. Next generation sequencing revealed a novel homozygous mutation in LDLRAP1 exon 5 (c.466delG). In the first year, drug regimens of either cholestyramine or simvastatin, reduced her LDL-C to 10.5 mmol/L (406 mg/dL) and 11.7 mmol/L (452.4 mg/dL), respectively. Combination simvastatin and ezetimibe was the mainstay of therapy from age 5 - 10 years. Her lowest achieved LDL-C was 6.3 mmol/L (243.6 mg/dL). Switching to atorvastatin did not lead to further reduction. Carotid intima-media thickness was 0.47 mm (> 97^th percentile) and 0.32 mm (75 - 95^th percentile) at ages 8 years and 11 years, respectively. Addition of monthly injections of evolocumab for 3 months, led to an increase in LDL-C, from 7.0 mmol/L (270.7 mg/dL) to a range of [(8.4 - 9.1) mmol/L or (324.8 - 351.9) mg/dL]. In this report, a decade-long lipid management is described in a patient with ARH. Residual activity of LDLRAP1 is a likely determinant of her response. Clinical management remains sub-optimal and options for the paediatric population are limited. Novel classes of cholesterol-lowering medications are needed for this ultra-rare and severe hypercholesterolemia.

Role of PCSK9 Inhibitors in Patients with Familial Hypercholesterolemia

Patients with familial hypercholesterolemia (FH) are at high or very high risk for cardiovascular disease. Those with heterozygous FH (HeFH) often do not reach low-density lipoprotein cholesterol (LDL-C) targets with statin and ezetimibe therapy, and those with homozygous FH (HoFH) usually require additional lipid-modifying therapies. Drugs that inhibit proprotein convertase subtilisin/kexin type 9 (PCSK9) offer a novel approach to reduce LDL-C. The monoclonal antibodies, alirocumab and evolocumab, given by subcutaneous injection every 2 or 4 weeks produce reductions in LDL-C of 50% to 60% in patients with HeFH, allowing many of them to achieve their LDL-C goals. Patients with HoFH show a reduced and more variable LDL-C response, which appears to depend on residual LDL receptor activity, and those with receptor-negative mutations may show no response. Inclisiran is a long-acting small interfering RNA therapeutic agent that inhibits the synthesis of PCSK9. Subcutaneous doses of 300 mg can reduce LDL-C by more than 50% for at least 6 months and the responses in HeFH and HoFH patients are similar to those achieved with monoclonal antibodies. These PCSK9 inhibitors are generally well tolerated and they provide a new opportunity for effective treatment for the majority of patients with FH.

Why patients with familial hypercholesterolemia are at high cardiovascular risk? Beyond LDL-C levels

Familial hypercholesterolemia (FH) is a common genetic cause of elevated low-density lipoprotein cholesterol (LDL-C) due to defective clearance of circulating LDL particles. All FH patients are at high risk for premature cardiovascular disease (CVD) events due to their genetically determined lifelong exposure to high LDL-C levels. However, different rates of CVD events have been reported in FH patients, even among those with the same genetic mutations and comparable LDL-C levels. Hence, additional CVD risk modifiers, beyond LDL-C, may contribute to increase CVD risk in the FH population. In this review, we discuss the overall CVD risk burden of the FH population. Additionally, we revise the prognostic impact of several traditional and emerging predictors of CVD risk and we provide an overview of the role of specific tools to stratify CVD risk in FH patients in order to ensure them a more personalized treatment approach.

Regulation of glucose and lipid metabolism in health and disease

Glucose and fatty acids are the major sources of energy for human body. Cholesterol, the most abundant sterol in mammals, is a key component of cell membranes although it does not generate ATP. The metabolisms of glucose, fatty acids and cholesterol are often intertwined and regulated. For example, glucose can be converted to fatty acids and cholesterol through de novo lipid biosynthesis pathways. Excessive lipids are secreted in lipoproteins or stored in lipid droplets. The metabolites of glucose and lipids are dynamically transported intercellularly and intracellularly, and then converted to other molecules in specific compartments. The disorders of glucose and lipid metabolism result in severe diseases including cardiovascular disease, diabetes and fatty liver. This review summarizes the major metabolic aspects of glucose and lipid, and their regulations in the context of physiology and diseases.

A case of autosomal recessive hypercholesterolemia caused by a new variant in the LDL receptor adaptor protein 1 gene

We report a new variant in the LDLRAP1 gene associated with autosomal recessive hypercholesterolemia in a woman of central European ancestry.

Genetic Testing and Risk Scores: Impact on Familial Hypercholesterolemia

Familial Hypercholesterolemia (FH) is an inherited lipid disorder affecting 1 in 220 individuals resulting in highly elevated low-density lipoprotein levels and risk of premature coronary disease. Pathogenic variants causing FH typically involve the LDL receptor (LDLR), apolipoprotein B-100 (APOB), and proprotein convertase subtulisin/kexin type 9 genes (PCSK9) and if identified convey a risk of early onset coronary artery disease (ASCVD) of 3- to 10-fold vs. the general population depending on the severity of the mutation. Identification of monogenic FH within a family has implications for family-based testing (cascade screening), risk stratification, and potentially management, and it has now been recommended that such testing be offered to all potential FH patients. Recently, robust genome wide association studies (GWAS) have led to the recognition that the accumulation of common, small effect alleles affecting many LDL-c raising genes can result in a clinical phenotype largely indistinguishable from monogenic FH (i.e., a risk of early onset ASCVD of ~3-fold) in those at the extreme tail of the distribution for these alleles (i.e., the top 8% of the population for a polygenic risk score). The incorporation of these genetic risk scores into clinical practice for non-FH patients may improve risk stratification but is not yet widely performed due to a less robust evidence base for utility. Here, we review the current status of FH genetic testing, potential future applications as well as challenges and pitfalls.

MicroRNAs: New Therapeutic Targets for Familial Hypercholesterolemia?

Familial hypercholesterolemia (FH) is the most common inherited form of dyslipidemia and a major cause of premature cardiovascular disease. Management of FH mainly relies on the efficiency of treatments that reduce plasma low-density lipoprotein (LDL) cholesterol (LDL-C) concentrations. MicroRNAs (miRs) have been suggested as emerging regulators of plasma LDL-C concentrations. Notably, there is evidence showing that miRs can regulate the post-transcriptional expression of genes involved in the pathogenesis of FH, including LDLR, APOB, PCSK9, and LDLRAP1. In addition, many miRs are located in genomic loci associated with abnormal levels of circulating lipids and lipoproteins in human plasma. The strong regulatory effects of miRs on the expression of FH-associated genes support of the notion that manipulation of miRs might serve as a potential novel therapeutic approach. The present review describes miRs-targeting FH-associated genes that could be used as potential therapeutic targets in patients with FH or other severe dyslipidemias.

Autosomal recessive hypercholesterolemia in Spain

BACKGROUND AND AIMS

Autosomal recessive hypercholesterolemia (ARH) is a very rare disease, caused by mutations in LDL protein receptor adaptor 1 (LDLRAP1). It is characterized by high levels of low-density lipoprotein cholesterol (LDL-C) and increased risk of premature cardiovascular disease. We aimed to characterize ARH in Spain.

METHODS

Data were collected from the Dyslipidemia Registry of the Spanish Atherosclerosis Society. A literature search was performed up to June 2017, and all diagnostic genetic studies for familial hypercholesterolemia of Spain were reviewed.

RESULTS

Seven patients with ARH were identified, 6 true homozygous and one compound heterozygous with a novel mutation: c.[863C>T];p.[Ser288Leu]. High genetic heterogeneity was found in this cohort. True homozygous subjects for LDLRAP1 have more severe phenotypes than the compound heterozygous patient, but similar to patients with homozygous familial hypercholesterolemia (HoFH). Cardiovascular disease was present in 14% of the ARH patients. LDL-C under treatment was above 185 mg/dl and the response to PCSK9 inhibitors was heterogeneous. Finally, the estimated prevalence in Spain is very low, with just 1 case per 6.5 million people.

CONCLUSIONS

ARH is a very rare disease in Spain, showing high genetic heterogeneity, similarly high LDL-C concentrations, but lower incidence of ASCVD than HoFH.

Autosomal Recessive Hypercholesterolemia: Long-Term Cardiovascular Outcomes

BACKGROUND

Autosomal recessive hypercholesterolemia (ARH) is a rare lipid disorder characterized by premature atherosclerotic cardiovascular disease (ASCVD). There are sparse data for clinical management and cardiovascular outcomes in ARH.

OBJECTIVES

Evaluation of changes in lipid management, achievement of low-density lipoprotein cholesterol (LDL-C) goals and cardiovascular outcomes in ARH.

METHODS

Published ARH cases were identified by electronic search. All corresponding authors and physicians known to treat these patients were asked to provide follow-up information, using a standardized protocol.

RESULTS

We collected data for 52 patients (28 females, 24 males; 31.1 ± 17.1 years of age; baseline LDL-C: 571.9 ± 171.7 mg/dl). During a mean follow-up of 14.1 ± 7.3 years, there was a significant increase in the use of high-intensity statin and ezetimibe in combination with lipoprotein apheresis; in 6 patients, lomitapide was also added. Mean LDL-C achieved at nadir was 164.0 ± 85.1 mg/dl (-69.6% from baseline), with a better response in patients taking lomitapide (-88.3%). Overall, 23.1% of ARH patients reached LDL-C of <100 mg/dl. During follow-up, 26.9% of patients had incident ASCVD, and 11.5% had a new diagnosis of aortic valve stenosis (absolute risk per year of 1.9% and 0.8%, respectively). No incident stroke was observed. Age (≥30 years) and the presence of coronary artery disease at diagnosis were the major predictors of incident ASCVD.

CONCLUSIONS

Despite intensive treatment, LDL-C in ARH patients remains far from targets, and this translates into a poor long-term cardiovascular prognosis. Our data highlight the importance of an early diagnosis and treatment and confirm the fact that an effective treatment protocol for ARH is still lacking.

The Spectrum of Familial Hypercholesterolemia (FH) in Saudi Arabia: Prime Time for Patient FH Registry

BACKGROUND

Familial hypercholesterolemia (FH) is a life-threatening inherited condition. Untreated patients have the risk to develop raised plasma levels of cholesterol, atherosclerosis and cardiovascular disease (CVD). If diagnosed and treated early in life, the pathological consequences due to atherosclerosis could be avoided and patients with FH can have an anticipated normal life. Mounting evidence suggests that FH is underdiagnosed and undertreated in all populations. The underlying molecular basis of FH is the presence of mutations in one or more genes in the low-density lipoprotein receptor (LDLR), apolipoprotein B (APOB) or proprotein convertase subtilisin/kexin 9 (PCSK9). However, their prevalence is largely unknown in Saudi Arabia but given the high rates of consanguinity, the prevalence appears to be higher. Furthermore, the high prevalence of obesity and diabetes mellitus in Saudi Arabia increases the vascular disease burden in FH cases by adding additional CVD risk factors.

OBJECTIVE

This article explores the spectrum of FH-causing mutations in the highly consanguineous Saudi community, the need for establishing the Saudi FH registry, the challenges in creating gene databases, and cascade screening.

CONCLUSION

The establishment of FH registry and genetic testing should raise awareness not only among healthcare professionals, but the general population as well. It also helps to provide the best treatment regimen in a cost effective manner to this under-recognised population of FH patients.

The genetics and screening of familial hypercholesterolaemia

Familial Hypercholesterolaemia is an autosomal, dominant genetic disorder that leads to elevated blood cholesterol and a dramatically increased risk of atherosclerosis. It is perceived as a rare condition. However it affects 1 in 250 of the population globally, making it an important public health concern. In communities with founder effects, higher disease prevalences are observed.

We discuss the genetic basis of familial hypercholesterolaemia, examining the distribution of variants known to be associated with the condition across the exons of the genes LDLR, ApoB, PCSK9 and LDLRAP1. We also discuss screening programmes for familial hypercholesterolaemia and their cost-effectiveness. Diagnosis typically occurs using one of the Dutch Lipid Clinic Network (DCLN), Simon Broome Register (SBR) or Make Early Diagnosis to Prevent Early Death (MEDPED) criteria, each of which requires a different set of patient data. New cases can be identified by screening the family members of an index case that has been identified as a result of referral to a lipid clinic in a process called cascade screening. Alternatively, universal screening may be used whereby a population is systematically screened.

It is currently significantly more cost effective to identify familial hypercholesterolaemia cases through cascade screening than universal screening. However, the cost of sequencing patient DNA has fallen dramatically in recent years and if the rate of progress continues, this may change.

The history of Autosomal Recessive Hypercholesterolemia (ARH). From clinical observations to gene identification

The most frequent form of monogenic hypercholesterolemia, also known as Familial Hypercholesterolemia (FH), is characterized by plasma accumulation of cholesterol transported in Low Density Lipoproteins (LDLs). FH has a co-dominant transmission with a gene-dosage effect. FH heterozygotes have levels of plasma LDL-cholesterol (LDL-C) twice normal and present xanthomas and coronary heart disease (CHD) in adulthood. In rare FH homozygotes plasma LDL-C level is four times normal, while xanthomas and CHD are present from infancy. Most FH patients are carriers of mutations of the LDL receptor (LDLR); a minority of them carry either mutations in the Apolipoprotein B (ApoB), the protein constituent of LDLs which is the ligand for LDLR, or gain of function mutations of PCSK9, the protein responsible for the intracellular degradation of the LDLR. From 1970 to the mid 90s some publications described children with the clinical features of homozygous FH, who were born from normocholesterolemic parents, strongly suggesting a recessive transmission of FH. In these patients the involvement of LDLR and APOB genes was excluded. Interestingly, several patients were identified in the island of Sardinia (Italy), whose population has a peculiar genetic background due to geographical isolation. In this review, starting from the early descriptions of patients with putative recessive hypercholesterolemia, we highlight the milestones that led to the identification of a novel gene involved in LDL metabolism and the characterization of its encoded protein. The latter turned out to be an adaptor protein required for the LDLR-mediated endocytosis of LDLs in hepatocytes. The loss of function of this protein is the cause of Autosomal Recessive Hypercholesterolemia (ARH).

Mechanism of action of anti-hypercholesterolemia drugs and their resistance

Coronary artery disease is one of the leading causes of death worldwide. One of the significant causes of this disease is hypercholesterolemia which is the result of various genetic alterations that are associated with the accumulation of specific classes of lipoprotein particles in plasma. A number of drugs are used to treat hypercholesterolemia like statin, fibrate, bile acid sequestrants, niacin, ezetimibe, omega-3 fatty acids and natural extracts. It has been observed that these drugs show diverse response in different individuals. The present review explains the mechanism of action of these drugs as well as mechanism of its lesser effectiveness or resistance in some individuals. There are various identified genetic variations that are associated with diversity in the drugs response. Therefore, present study helps to understand the ethiology of drug mechanism and resistance developed against drugs used to treat hypercholesterolemia.

Reciprocal regulation of endocytosis and metabolism

The cellular uptake of many nutrients and micronutrients governs both their cellular availability and their systemic homeostasis. The cellular rate of nutrient or ion uptake (e.g., glucose, Fe(3+), K(+)) or efflux (e.g., Na(+)) is governed by a complement of membrane transporters and receptors that show dynamic localization at both the plasma membrane and defined intracellular membrane compartments. Regulation of the rate and mechanism of endocytosis controls the amounts of these proteins on the cell surface, which in many cases determines nutrient uptake or secretion. Moreover, the metabolic action of diverse hormones is initiated upon binding to surface receptors that then undergo regulated endocytosis and show distinct signaling patterns once internalized. Here, we examine how the endocytosis of nutrient transporters and carriers as well as signaling receptors governs cellular metabolism and thereby systemic (whole-body) metabolite homeostasis.

Management of hypercholesterolemia in children

Cardiovascular disease (CVD) remains the leading cause of death and morbidity in our society. One of the major risk factors for CVD is hypercholesterolemia. Hypercholesterolemia in children can be caused by a hereditary disorder or can be secondary to other diseases or drugs. In order to prevent CVD later in life, children with hypercholesterolemia should be identified and treated as early as possible. Currently, several different screening strategies have been developed, using either universal screening or case finding to search for children at risk. Once those children are identified, the first step in treatment is lifestyle adjustment. If cholesterol levels remain elevated, the drugs of first choice are statins. Other pharmacological options are ezetimibe or bile acid sequestrants. These agents have all proven to be safe and effective in lowering low-density lipoprotein cholesterol levels and improving surrogate markers of CVD. However, there is a need for long-term follow-up studies to answer the question as to whether it is safe to initiate treatment at a young age to prevent CVD later in life.

Post-prandial remnant lipoprotein metabolism in autosomal recessive hypercholesterolaemia

BACKGROUND

Phenotype of autosomal recessive hypercholesterolaemia (ARH), a rare lipid disorder, is known to be milder than that of homozygous familial hypercholesterolaemia (FH) with LDL receptor gene mutation. However, few data exist regarding the functional differences in ARH and FH particularly in terms of remnant-like particles' (RLP) metabolism.

MATERIALS AND METHODS

Blood sampling was performed up to 6h after OFTT cream loading (50 g/body surface area) with 2-h intervals in a single ARH proband, four heterozygous FH patients with LDL receptor gene mutation and four normal controls. Plasma lipoprotein and RLP fraction were determined by HPLC system. The area under curve (AUC) of each lipoprotein including RLP fractions was evaluated.

RESULTS

The AUC of TG, RLP cholesterol (RLP-C) and RLP triglyceride (RLP-TG) levels of heterozygous FH subjects was significantly higher than those of controls (466±71 mg/dL×h vs. 303±111 mg/dL×h, P<0·05; 35±7 mg/dL×h vs. 21±8 mg/dL×h, P<0·05; 124±57 mg/dL×h vs. 51±13 mg/dL×h, P<0·05, respectively). Under these conditions, those values of ARH were close to those of controls (310 mg/dL×h, 22 mg/dL×h, 23 mg/dL×h, respectively).

CONCLUSION

These data demonstrate that unlike in FH, RLP clearance is preserved in ARH. The preservation of post-prandial RLP clearance may contribute to the mild phenotype of ARH compared with FH.

Miscellaneous non-inflammatory musculoskeletal conditions. Rare thesaurismosis and xanthomatosis

The focus will be on xanthomatosis, a tissue danger signal which needs to be recognized by the clinician, and its relationship with monogenetic lipoprotein disorders (cholesterol, triglycerides), bile acid and sterol metabolism, particularly on metabolic pathways and genetics as well as on musculoskeletal and cardiovascular involvement, and their implications for clinical management. The critical question is to assess coronary heart disease risk, requiring correct identification of the pattern of lipoprotein disorders and of the causes (primary or secondary). Familial hypercholesterolemia must be suspected in adults and children with raised total cholesterol, especially when there is a personal or a family history of premature coronary heart disease, usually requiring potent statins to achieve adequate LDL-cholesterol lowering, even if we do not know safety of long-term therapy and whether treatments of dyslipidemia early in life prevent cardiovascular diseases in adulthood. Cerebrotendinous xanthomatosis is a treatable disease and must be suspected if there is a history of infantile chronic diarrhea and/or juvenile cataracts, even in the absence of tendon xanthomas. Current evidence for the prevention and screening, diagnosis, and treatment of dyslipidemia are available for the clinicians.

Premature senescence in cells from patients with autosomal recessive hypercholesterolemia (ARH): evidence for a role for ARH in mitosis

OBJECTIVE

The defective gene causing autosomal recessive hypercholesterolemia (ARH) encodes ARH, a clathrin-associated adaptor protein required for low-density-lipoprotein receptor endocytosis in most cells but not in skin fibroblasts. The aim here was to elucidate why ARH fibroblasts grow slowly and undergo premature senescence.

METHODS AND RESULTS

Knockdown of ARH by RNA interference in IMR90 cells produces the same phenotype, indicated by increased p16 expression, γ-H2AX-positive foci, and enlarged flattened morphology. We showed that ARH contributes to several aspects of mitosis: it localizes to mitotic microtubules, with lamin B1 on the nuclear envelope and spindle matrix, and with clathrin heavy chain on mitotic spindles. Second, ARH is phosphorylated in G(2)/M phase by a roscovitine-sensitive kinase, probably cdc2. Third, cells lacking ARH show disfigured nuclei and defective mitotic spindles. Defects are most marked in ARH W22X cells, where translation starts at Met46, so the protein lacks a phosphorylation site at Ser14, identified by mass spectrometry of wild-type ARH.

CONCLUSIONS

The ARH protein is involved in cell cycle progression, possibly by affecting nuclear membrane formation through interaction with lamin B1 or other mitotic proteins, and its absence affects cell proliferation and induces premature senescence, which may play a role in the development of atherosclerosis in ARH.

Familial hypercholesterolemia: the lipids or the genes?

Familial Hypercholesterolemia (FH) is a common cause of premature cardiovascular disease and is often undiagnosed in young people. Although the disease is diagnosed clinically by high LDL cholesterol levels and family history, to date there are no single internationally accepted criteria for the diagnosis of FH. Several genes have been shown to be involved in FH; yet determining the implications of the different mutations on the phenotype remains a hard task. The polygenetic nature of FH is being enhanced by the discovery of new genes that serve as modifiers. Nevertheless, the picture is still unclear and many unknown genes contributing to the phenotype are most likely involved. Because of this evolving polygenetic nature, the diagnosis of FH by genetic testing is hampered by its cost and effectiveness.

In this review, we reconsider the clinical versus genetic nomenclature of FH in the literature. After we describe each of the genetic causes of FH, we summarize the known correlation with phenotypic measures so far for each genetic defect. We then discuss studies from different populations on the genetic and clinical diagnoses of FH to draw helpful conclusions on cost-effectiveness and suggestions for diagnosis.

LDLR-Gene therapy for familial hypercholesterolaemia: problems, progress, and perspectives

Coronary artery diseases (CAD) inflict a heavy economical and social burden on most populations and contribute significantly to their morbidity and mortality rates. Low-density lipoprotein receptor (LDLR) associated familial hypercholesterolemia (FH) is the most frequent Mendelian disorder and is a major risk factor for the development of CAD. To date there is no cure for FH. The primary goal of clinical management is to control hypercholesterolaemia in order to decrease the risk of atherosclerosis and to prevent CAD. Permanent phenotypic correction with single administration of a gene therapeutic vector is a goal still needing to be achieved. The first ex vivo clinical trial of gene therapy in FH was conducted nearly 18 years ago. Patients who had inherited LDLR gene mutations were subjected to an aggressive surgical intervention involving partial hepatectomy to obtain the patient's own hepatocytes for ex vivo gene transfer with a replication deficient LDLR-retroviral vector. After successful re-infusion of transduced cells through a catheter placed in the inferior mesenteric vein at the time of liver resection, only low-level expression of the transferred LDLR gene was observed in the five patients enrolled in the trial. In contrast, full reversal of hypercholesterolaemia was later demonstrated in in vivo preclinical studies using LDLR-adenovirus mediated gene transfer. However, the high efficiency of cell division independent gene transfer by adenovirus vectors is limited by their short-term persistence due to episomal maintenance and the cytotoxicity of these highly immunogenic viruses. Novel long-term persisting vectors derived from adeno-associated viruses and lentiviruses, are now available and investigations are underway to determine their safety and efficiency in preparation for clinical application for a variety of diseases. Several novel non-viral based therapies have also been developed recently to lower LDL-C serum levels in FH patients. This article reviews the progress made in the 18 years since the first clinical trial for gene therapy of FH, with emphasis on the development, design, performance and limitations of viral based gene transfer vectors used in studies to ameliorate the effects of LDLR deficiency.

The connecdenn family, Rab35 guanine nucleotide exchange factors interfacing with the clathrin machinery

Rabs constitute the largest family of monomeric GTPases, yet for the majority of Rabs relatively little is known about their activation and recruitment to vesicle-trafficking pathways. We recently identified connecdenn (DENND1A), which contains an N-terminal DENN (differentially expressed in neoplastic versus normal cells) domain, a common and evolutionarily ancient protein module. Through its DENN domain, connecdenn functions enzymatically as a guanine-nucleotide exchange factor (GEF) for Rab35. Here we identify two additional connecdenn family members and demonstrate that all connecdenns function as Rab35 GEFs, albeit with different levels of activity. The DENN domain of connecdenn 1 and 2 binds Rab35, whereas connecdenn 3 does not, indicating that Rab35 binding and activation are separable functions. Through their highly divergent C termini, each of the connecdenns binds to clathrin and to the clathrin adaptor AP-2. Interestingly, all three connecdenns use different mechanisms to bind AP-2. Characterization of connecdenn 2 reveals binding to the beta2-ear of AP-2 on a site that overlaps with that used by the autosomal recessive hypercholesterolemia protein and betaarrestin, although the sequence used by connecdenn 2 is unique. Loss of connecdenn 2 function through small interference RNA knockdown results in an enlargement of early endosomes, similar to what is observed upon loss of Rab35 activity. Our studies reveal connecdenn DENN domains as generalized GEFs for Rab35 and identify a new AP-2-binding motif, demonstrating a complex link between the clathrin machinery and Rab35 activation.

Prevalence and clinical features of heterozygous carriers of autosomal recessive hypercholesterolemia in Sardinia

OBJECTIVE

Autosomal recessive hypercholesterolemia (ARH) is a lipid disorder caused by mutations in a specific adaptor protein for the LDL receptor. ARH is rare except in Sardinia where three alleles (ARH1, ARH2 and ARH3) explain most of cases. The prevalence of ARH heterozygotes in Sardinia is not well determined as well as inconclusive data are available on the effect of the ARH carrier status on LDL cholesterol (LDL-C) and coronary risk.

METHODS

3410 Sardinians (986 blood donors, 1709 with hypertension and 715 with myocardial infarction (MI)) were screened for ARH alleles. For comparison purposes, lipid data of 60 ARH heterozygous carriers and 60 non-carriers identified within 24 ARH families were also considered.

RESULTS

In the whole study cohort, no ARH homozygotes were found, but 15 ARH1 (0.44%) and 9 ARH2 (0.26%) heterozygous carriers were identified. The frequency of ARH alleles in blood donors was 0.0030, not different from that in hypertensive subjects (0.0032). ARH alleles tended to be more common in MI patients (0.0049), but no association between ARH carrier status and MI risk was detected after controlling for conventional risk factors. ARH carriers and non-carriers showed similar LDL-C levels. This result was confirmed when ARH carriers and non-carriers identified throughout family-based and population-based screenings were combined and compared (141.0+/-41 mg/dl vs. 137.0+/-41 mg/dl, respectively; p=0.19).

CONCLUSIONS

These data indicate that the frequency of ARH heterozygotes in Sardinia is approximately 1:143 individuals, thus making this condition one of the most common in the Sardinian population. However, ARH carrier status does not influence LDL-C concentration and coronary risk, thus suggesting that ARH can be regarded as a truly recessive disorder.

[Familial hypercholesterolemia in Tunisia]

Familial hypercholesterolemia or autosomal dominant hypercholesterolemia is characterized by raised serum LDL (low density lipoproteins)-cholesterol levels, which result in excess deposition of cholesterol in tissues, leading to accelerated atherosclerosis and increased risk of premature coronary heart disease. Familial hypercholesterolemia results from defects in the hepatic uptake and degradation of LDL via the LDL receptor pathway. Familial hypercholesterolemia is commonly caused by a loss of function in the LDL receptor gene, or by a mutation in the gene encoding apolipoprotein B (APOB) or PCSK9 gene. In Tunisia, the frequency of this disease is about one of 165 for heterozygote. It is a higher frequency compared to most European countries, which is about one of 500 for heterozygote. Only five mutations in the LDLR gene were reported in this population. No mutations in the APOB or PCSK9 gene were reported.

Monogenic pediatric dyslipidemias: classification, genetics and clinical spectrum

Monogenic disorders that cause abnormal levels of plasma cholesterol and triglycerides have received much attention due to their role in metabolic dysfunction and cardiovascular disease. While these disorders often present clinically during adulthood, some present most commonly in the pediatric population and can have serious consequences if misdiagnosed or untreated. This review provides an overview of monogenic lipid disorders that present with unusually high or low levels of plasma cholesterol and/or triglycerides during infancy, childhood and adolescence. Biochemical and genetic findings, clinical presentation and treatment options are discussed with an emphasis upon recent advances in our understanding and management of these monogenic disorders.