Mutations in the low-density-lipoprotein receptor gene in German patients with familial hypercholesterolaemia

Genetic Testing for Familial Hypercholesterolemia in a Pediatric Group: A Romanian Showcase

Familial hypercholesterolemia (FH) is a genetic disease marked by high levels of LDL-cholesterol. This condition has long-term clinical implications, such as cardiovascular events, that are evident during adult life. Here, we report on a single-center cross-sectional showcase study of genetic testing for FH in a Romanian pediatric group. Genetic testing for FH was performed on 20 Romanian pediatric patients, 10 boys and 10 girls, admitted with LDL-cholesterol levels over 130 mg/mL to the National Institute for Mother and Child Health "Alesssandrescu-Rusescu" in 2020. Genetic testing was performed using the Illumina TruSight Cardio panel. We identified pathogenic/likely pathogenic variants that could explain the phenotype in 5/20 cases. The involved genes were LDLR and APOB. Clinical signs that suggest the diagnosis of FH are scarce for the pediatric patient, although it can be diagnosed early during childhood by lipid panel screening. Prevention could prove lifesaving for some of these patients.

Establishing the Mutational Spectrum of Hungarian Patients with Familial Hypercholesterolemia

Familial hypercholesterolemia (FH) is one of the most common autosomal, dominantly inherited diseases affecting cholesterol metabolism, which, in the absence of treatment, leads to the development of cardiovascular complications. The disease is still underdiagnosed, even though an early diagnosis would be of great importance for the patient to receive proper treatment and to prevent further complications. No studies are available describing the genetic background of Hungarian FH patients. In this work, we present the clinical and molecular data of 44 unrelated individuals with suspected FH. Sequencing of five FH-causing genes (LDLR, APOB, PCSK9, LDLRAP1 and STAP1) has been performed by next-generation sequencing (NGS). In cases where a copy number variation (CNV) has been detected by NGS, confirmation by multiplex ligation-dependent probe amplification (MLPA) has also been performed. We identified 47 causal or potentially causal (including variants of uncertain significance) LDLR and APOB variants in 44 index patients. The most common variant in the APOB gene was the c.10580G>A p.(Arg3527Gln) missense alteration, this being in accordance with literature data. Several missense variants in the LDLR gene were detected in more than one index patient. LDLR variants in the Hungarian population largely overlap with variants detected in neighboring countries.

Saudi Familial Hypercholesterolemia Patients With Rare LDLR Stop Gain Variant Showed Variable Clinical Phenotype and Resistance to Multiple Drug Regimen

Familial hypercholesterolemia (FH), a well-known lipid disease caused by inherited genetic defects in cholesterol uptake and metabolism is underdiagnosed in many countries including Saudi Arabia. The present study aims to identify the molecular basis of severe clinical manifestations of FH patients from unrelated Saudi consanguineous families. Two Saudi families with multiple FH patients fulfilling the combined FH diagnostic criteria of Simon Broome Register, and the Dutch Lipid Clinic Network (DLCN) were recruited. LipidSeq, a targeted resequencing panel for monogenic dyslipidemias, was used to identify causative pathogenic mutation in these two families and in 92 unrelated FH cases. Twelve FH patients from two unrelated families were sharing a very rare, pathogenic and founder LDLR stop gain mutation i.e., c.2027delG (p.Gly676Alafs^*33) in both the homozygous or heterozygous states, but not in unrelated patients. Based on the variant zygosity, a marked phenotypic heterogeneity in terms of LDL-C levels, clinical presentations and resistance to anti-lipid treatment regimen (ACE inhibitors, β-blockers, ezetimibe, statins) of the FH patients was observed. This loss-of-function mutation is predicted to alter the free energy dynamics of the transcribed RNA, leading to its instability. Protein structural mapping has predicted that this non-sense mutation eliminates key functional domains in LDLR, which are essential for the receptor recycling and LDL particle binding. In conclusion, by combining genetics and structural bioinformatics approaches, this study identified and characterized a very rare FH causative LDLR pathogenic variant determining both clinical presentation and resistance to anti-lipid drug treatment.

Molecular Genetic Approach and Evaluation of Cardiovascular Events in Patients with Clinical Familial Hypercholesterolemia Phenotype from Romania

This study identifies the genetic background of familial hypercholesterolemia (FH) patients in Romania and evaluates the association between mutations and cardiovascular events. We performed a prospective observational study of 61 patients with a clinical diagnosis of FH selected based on Dutch Lipid Clinic Network (DLCN) and Simon Broome score between 2017 and 2020. Two techniques were used to identify mutations: multiplex ligation-dependent probe amplification (MLPA) and Sanger sequencing. The mutation rate was 37.7%, i.e., 23 patients with mutations were identified, of which 7 subjects had pathogenic mutations and 16 had polymorphisms. Moreover, 10 variants of the low-density lipoprotein receptor (LDLR) gene were identified in 22 patients, i.e., one variant of the proprotein convertase subtilisin/kexin type 9 (PCSK9) gene in six patients, and one variant of the apolipoprotein B (APOB) gene in three patients. Of the LDLR gene variants, four were LDLR pathogenic mutations (c.81C > G, c.502G > A, c.1618G > A mutations in exon 2, exon 4, exon 11, and exon 13–15 duplication). The PCSK9 and APOB gene variants were benign mutations. The pathogenic LDLR mutations were significant predictors of the new cardiovascular events, and the time interval for new cardiovascular events occurrence was significantly decreased, compared to FH patients without mutations. In total, 12 variants were identified, with four pathogenic variants identified in the LDLR gene, whereas 62.3% of the study population displayed no pathological mutations.

Identification of roles for H264, H306, H439, and H635 in acid-dependent lipoprotein release by the LDL receptor

Lipoproteins internalized by the LDL receptor (LDLR) are released from this receptor in endosomes through a process that involves acid-dependent conformational changes in the receptor ectodomain. How acidic pH promotes this release process is not well understood. Here, we assessed roles for six histidine residues for which either genetic or structural data suggested a possible role in the acid-responsiveness of the LDLR. Using assays that measured conformational change, acid-dependent lipoprotein release, LDLR recycling, and net lipoprotein uptake, we show that H635 plays important roles in acid-dependent conformational change and lipoprotein release, while H264, H306, and H439 play ancillary roles in the response of the LDLR to acidic pH.

Clinical characterization and mutation spectrum of German patients with familial hypercholesterolemia

BACKGROUND AND AIMS

Autosomal-dominant familial hypercholesterolemia (FH) is characterized by elevated plasma levels of low-density lipoprotein cholesterol (LDL-C) and a dramatically increased risk to develop cardiovascular disease (CVD). Mutations in three major genes have been associated with FH: the LDL receptor gene (LDLR), the apolipoprotein B gene (APOB), and the proprotein convertase subtilisin/kexin 9 gene (PCSK9). Here we investigated the frequency and the spectrum of FH causing mutations in Germany.

METHODS

We screened 206 hypercholesterolemic patients, of whom 192 were apparently unrelated, for mutations in the coding region of the genes LDLR, PCSK9 and the APOB [c.10580G > A (p.Arg3527Gln)]. We also categorized the patients according to the Dutch Lipid Clinic Network Criteria (DLCNC) in order to allow a comparison between the mutations identified and the clinical phenotypes observed. Including data from previous studies on German FH patients enabled us to analyse data from 479 individuals.

RESULTS

Ninety-eight FH causing variants were found in 92 patients (nine in related patients and 6 patients with two variants and likely two affected alleles), of which 90 were located in the LDLR gene and eight mutations were identified in the APOB gene (c.10580G > A). No mutation was found in the PCSK9 gene. While 48 of the LDLR mutations were previously described as disease causing, we found 9 new LDLR variants which were rated as "pathogenic" or "likely pathogenic" based on the predicted effect on the corresponding protein. The proportions of different types of LDLR mutations and their localization within the gene was similar in the group of patients screened for mutations here and in the combined analysis of 479 patients (current study/cases from the literature) and also to other studies on the LDLR mutation spectrum, with about half of the variants being of the missense type and clustering of mutations in exons 4, 5 and 9. The mutation detection rate in the 35 definite and 45 probable FH patients (according to DLCNC) was 77.1% and 68.9%, respectively. The data show a similar discriminatory power between the DLCNC score (AUC = 0.789 (95% CI 0.721-0,857)) and baseline LDL-C levels (AUC = 0.799 (95% CI = 0.732-0.866)).

CONCLUSIONS

This study further substantiates the mutation spectrum for FH in German patients and confirms the clinical and genetic heterogeneity of the disease.

LDL-receptor mutations in Europe

Familial hypercholesterolemia (FH) is a clinical definition for a remarkable increase of cholesterol serum concentration, presence of xanthomas, and an autosomal dominant trait of either increased serum cholesterol or premature coronary artery disease (CAD). The identification of the low-density lipoprotein (LDL)-receptor (LDLR) as the underlying cause and its genetic characterization in FH patients revealed more insights in the trafficking of LDL, which primarily transports cholesterol to hepatic and peripheral cells. Mutations within LDLR result in hypercholesterolemia and, subsequently, cholesterol deposition in humans to a variable degree. This confirms the pathogenetic role of LDLR and also highlights the existence of additional factors in determining the phenotype. Autosomal dominant FH is caused by LDLR deficiency and defective apolipoprotein B-100 (APOB), respectively. Heterozygosity of the LDLR is relatively common (1:500). Clinical diagnosis is highly important and genetic diagnosis may be helpful, since treatment is usually effective for this otherwise fatal disease. Very recently, mutations in PCSK9 have been also shown to cause autosomal dominant hypercholesterolemia. For autosomal recessive hypercholesterolemia, mutations within the so-called ARH gene encoding a cellular adaptor protein required for LDL transport have been identified. These insights emphasize the crucial importance of LDL metabolism intra- and extracellularly in determining LDL-cholesterol serum concentration. Herein, we focus on the published European LDLR mutation data that reflect its heterogeneity and phenotypic penetrance.

Baseline lipid values partly determine the response to high-dose simvastatin in patients with familial hypercholesterolemia. The examination of probands and relatives in Statin studies with familial hypercholesterolemia (ExPRESS FH)

Statins decrease low-density lipoprotein cholesterol (LDL-C), and additionally, reduce triglycerides (TG) and raise high-density lipoprotein cholesterol (HDL-C) levels. This study evaluated the frequency of abnormal TG and HDL-C levels in patients with classical familial hypercholesterolemia (FH) and assessed therapeutic response at different baseline levels of these lipoproteins after 1 year of statin therapy. A total of 508 FH patients were included and mean LDL-C levels (8.37+/-2.12 mmol l(-1)) were severely elevated. After a washout period of 6 weeks, all patients started monotherapy with 80 mg simvastatin. Remarkably, LDL-C reduction was dependent on baseline LDL-C levels ranging from 51.1 to 45.5% in the top versus the bottom third of the LDL-C distribution. Unexpected in FH, elevated baseline TG levels were seen in 30% and low HDL-C levels in 15% of all patients. Also, changes in these lipoproteins were dependent on baseline levels; TG reduction was 40.7 versus 22.2% in patients with elevated versus normal levels, while HDL-C increase was 29.1 versus 11.4% in patients with low versus normal HDL-C levels. In conclusion, FH patients with the worst lipoprotein profile showed the greatest benefit from high-dose simvastatin treatment, since changes in these parameters were partly determined by baseline lipid levels.