A.C. Alves

Genetic diagnosis of familial hypercholesterolaemia: the importance of functional analysis of potential splice-site mutations

Familial hypercholesterolemia (FH) results from defective low-density lipoprotein receptor (LDLR) activity, mainly due to LDLR gene defects. Of the many different LDLR mutations found in patients with FH, about 6% of single base substitutions are located near or within introns, and are predicted to result in exon skipping, retention of an intron, or activation of cryptic sites during mRNA splicing. This paper reports on the Portuguese FH Study, which found 10 such mutations, 6 of them novel. For the mutations that have not been described before or those whose effect on function have not been analysed, their effect on splicing was investigated, using reverse transcriptase PCR analysis of LDLR mRNA from freshly isolated blood mononuclear cells. Two of these variants (c.313+6 T→C, c.2389G→T (p.V776L)) caused exon skipping, and one caused retention of an intron (c.1359−5C→G), whereas two others (c.2140+5 G→A and c.1061−8T→C) had no apparent effect. Any effect of c.1185G→C (p.V374V) on splicing could not be determined because it was on an allele with a promoter mutation (−42C→G) that was probably not transcribed. Variants in four patients lost to follow-up could not be tested experimentally, but they almost certainly affect splicing because they disrupt the invariant AG or GT in acceptor (c.818−2A→G) or donor (c.1060+1G→A, c.1845+1delG and c.2547+1G→A) spice sites. These findings emphasise that care must be taken before reporting the presence or absence of a splice-site mutation in the LDLR gene for diagnostic purposes. The study also shows that relatively simple, quick and inexpensive RNA assays can evaluate putative splicing mutations that are not always predictable by available software, thereby reducing genetic misdiagnosis of patients with FH.

How good is controlled attenuation parameter and fatty liver index for assessing liver steatosis in general population: correlation with ultrasound

Liver steatosis measurement by controlled attenuation parameter (CAP) is a non‐invasive method for diagnosing steatosis, based on transient elastography. Its usefulness as screening procedure for hepatic steatosis in general population has not been previously evaluated. The aim of this study was to evaluate the diagnostic accuracy of CAP and fatty liver index (FLI) for detection and quantification of steatosis in general population.

Update of the Portuguese Familial Hypercholesterolaemia Study

The main aim of the Portuguese Familial Hypercholesterolaemia Study is to identify the genetic cause of hypercholesterolaemia in individuals with a clinical diagnosis of Familial Hypercholesterolaemia (FH). A total of 1340 blood samples were collected from 482 index patients and 858 relatives with the collaboration of clinicians from several hospitals all over the country. The genetic diagnosis of FH in this study is based on the analyses of three genes: LDLR, APOB and PCSK9. In the last 10 years, the Portuguese FH Study identified a genetic defect in a total of 171 index patients, corresponding to an overall of 48% of the total received cases with clinical diagnosis of FH. Although the Simon Broome FH register criteria have been adapted to our study, 59 patients that did not fulfil all criteria were included in the study and a mutation causing disease was identified in 8 of these patients. In the LDLR gene were found 80 different mutations in 165 unrelated index patients: 159 heterozygous, 3 compounds heterozygous and 3 true homozygous. The APOB p.Arg3527Gln and the PCSK9 p.Asp374His mutation were not found in any of our patients since our last report, but a novel mutation in the APOB gene, predicted to cause a single amino acid substitution p.Tyr3560Cys, was found in one patient. The cascade screening in relatives of these 171 index patients allowed the identification and genetic characterization of a total of 404 FH patients in Portugal.

Novel functional APOB mutations outside LDL-binding region causing familial hypercholesterolaemia

Familial hypercholesterolaemia (FH) is characterized by increased circulating low-density lipoprotein (LDL) cholesterol leading to premature atherosclerosis and coronary heart disease. Although FH is usually caused by mutations in LDLR, mutations in APOB and PCSK9 also cause FH but only a few mutations have been reported, APOB p.R3527Q being the most common. However, 30-80% of clinical FH patients do not present an identifiable mutation in any of the described genes. To identify the genetic cause of the hypercholesterolaemia in 65 patients without mutations in LDLR, PCSK9 or in fragments of exon 26 and 29 of APOB currently analysed, we performed whole sequencing of APOB by pyrosequencing. A total of 10 putative mutations in APOB were identified. Flow cytometry with fluorescently labelled LDL from patients and relatives showed that p.Arg1164Thr (exon 22) and p.Gln4494del (exon 29) presented a 40% decrease in internalization in lymphocytes and HepG2 cells, very similar to APOB3527. The proliferation assays with U937 cells showed reduced growth for both cases. The variant p.Tyr1247Cys was found to be neutral and other three alterations were considered polymorphisms. Our results emphasize the need to study the whole APOB in routine protocols to improve patient identification and cardiovascular risk assessment.

Mutational analysis of a cohort with clinical diagnosis of familial hypercholesterolemia: considerations for genetic diagnosis improvement

Purpose:Familial hypercholesterolemia (FH) is a common autosomal dominant disorder of lipid metabolism caused by mutations in LDLR, APOB, and PCSK9. To fulfill the World Health Organization recommendation, the Portuguese FH Study was established. Here, we report the results of the past 15 years and present practical considerations concerning the genetic diagnosis of FH based on our experience.Methods:Our approach comprises a biochemical and molecular study and is divided into five phases, including the study of whole APOB and functional assays.Results:A total of 2,122 individuals were enrolled. A putative pathogenic variant was identified in 660 heterozygous patients: LDLR (623), APOB (33), and PCSK9 (4); 8 patients presented with homozygous FH. A detection rate of 41.5% was observed. A stricter biochemical criteria was shown to improve patient identification. Overall, we have identified 3.4% and 80% of all heterozygous and homozygous patients, respectively, estimated to exist in our country.Conclusion:The Portuguese FH Study has established the genetic diagnosis of FH in Portugal and is committed to continue the investigation of the genetic complexity of FH. Genetic diagnosis of FH should be expanded to include the study of all coding/flanking regions of APOB and functional in vitro studies, to improve the correct patient identification, and to avoid misdiagnosis.Genet Med 18 4, 316–324.

Advantages and Versatility of Fluorescence-Based Methodology to Characterize the Functionality of LDLR and Class Mutation Assignment

Familial hypercholesterolemia (FH) is a common autosomal codominant disease with a frequency of 1∶500 individuals in its heterozygous form. The genetic basis of FH is most commonly mutations within the LDLR gene. Assessing the pathogenicity of LDLR variants is particularly important to give a patient a definitive diagnosis of FH. Current studies of LDLR activity ex vivo are based on the analysis of 125I-labeled lipoproteins (reference method) or fluorescent-labelled LDL. The main purpose of this study was to compare the effectiveness of these two methods to assess LDLR functionality in order to validate a functional assay to analyse LDLR mutations. LDLR activity of different variants has been studied by flow cytometry using FITC-labelled LDL and compared with studies performed previously with 125I-labeled lipoproteins. Flow cytometry results are in full agreement with the data obtained by the 125I methodology. Additionally confocal microscopy allowed the assignment of different class mutation to the variants assayed. Use of fluorescence yielded similar results than 125I-labeled lipoproteins concerning LDLR activity determination, and also allows class mutation classification. The use of FITC-labelled LDL is easier in handling and disposal, cheaper than radioactivity and can be routinely performed by any group doing LDLR functional validations.

Low-density lipoprotein receptor mutational analysis in diagnosis of familial hypercholesterolemia

Purpose of review To present up to date evidence on the pathogenicity of low-density lipoprotein receptor (LDLR) variants and to propose a strategy that is suitable for implementation in the clinical work-up of familial hypercholesterolaemia. Recent findings More than 1800 variants have been described in the LDLR gene of patients with a clinical diagnosis of familial hypercholesterolaemia; however, less than 15% have functional evidence of pathogenicity. Summary The spectrum of variants in the LDLR identified in patients with clinical familial hypercholesterolaemia is increasing as novel variants are still being reported. However, over 50% of all LDLR variants need further evidence before they can be confirmed as mutations causing disease. Even with applying the recent American College of Medical Genetics variant classification, a large number of variants are still considered variants of unknown significance. Before obtaining an undisputable confirmation of the effect on the expression and activity of the LDLR, reporting these variants as part of a clinical diagnosis to the patient holds the risk that it might need to be withdrawn in a later stage. An investment should be made to develop functional assays to characterize LDLR variants of unknown significance for a better patient diagnosis and to prevent confusion in the physicians office.

Structural analysis of APOB variants, p.(Arg3527Gln), p.(Arg1164Thr) and p.(Gln4494del), causing Familial Hypercholesterolaemia provides novel insights into variant pathogenicity

Familial hypercholesterolaemia (FH) is an inherited autosomal dominant disorder resulting from defects in the low-density lipoprotein receptor (LDLR), in the apolipoprotein B (APOB) or in the proprotein convertase subtilisin/kexin type 9 (PCSK9) genes. In the majority of the cases FH is caused by mutations occurring within LDLR, while only few mutations in APOB and PCSK9 have been proved to cause disease. p.(Arg3527Gln) was the first mutation in APOB being identified and characterized. Recently two novel pathogenic APOB variants have been described: p.(Arg1164Thr) and p.(Gln4494del) showing impaired LDLR binding capacity, and diminished LDL uptake. The objective of this work was to analyse the structure of p.(Arg1164Thr) and p.(Gln4494del) variants to gain insight into their pathogenicity. Secondary structure of the human ApoB100 has been investigated by infrared spectroscopy (IR) and LDL particle size both by dynamic light scattering (DLS) and electron microscopy. The results show differences in secondary structure and/or in particle size of p.(Arg1164Thr) and p.(Gln4494del) variants compared with wild type. We conclude that these changes underlie the defective binding and uptake of p.(Arg1164Thr) and p.(Gln4494del) variants. Our study reveals that structural studies on pathogenic variants of APOB may provide very useful information to understand their role in FH disease.

The importance of an integrated analysis of clinical, molecular, and functional data for the genetic diagnosis of familial hypercholesterolemia

Purpose:Familial hypercholesterolemia (FH) is one of the most common monogenic disorders, and the high concentrations of low-density lipoprotein (LDL) cholesterol presented since birth confers on these patients an increased cardiovascular risk. More than 1,600 alterations have been described in the LDL receptor gene (LDLR), but a large number need to be validated as mutations causing disease to establish a diagnosis of FH. This study aims to characterize, both at the phenotypic and genotypic levels, families with a clinical diagnosis of FH and present evidence for the importance of the integration of clinical, molecular, and functional data for the correct diagnosis of patients with FH.Methods:A detailed analysis of the phenotype and genotype presented by 55 families with 13 different alterations in the LDLR was conducted. For eight of these, an extensive functional characterization was performed by flow cytometry, confocal microscopy, and reverse transcriptase polymerase chain reaction.Results:Carriers of neutral alterations presented a significantly lower incidence of premature cardiovascular disease, lower levels of atherogenic lipoproteins and a large number of these individuals had LDL-cholesterol values below the 75th percentile. presented a significantly lower incidence of premature cardiovascular disease, lower levels of atherogenic lipoproteins and a large number of these individuals had LDL-cholesterol values below the 75th percentile However, the functional study was essential to determine the pathogenicity of variants.Conclusion:The data collected illustrate the importance of this integrated analysis for the correct assessment of patients with FH who can otherwise be misdiagnosed.Genet Med 17 12, 980–988.

Cardiovascular risk assessment of dyslipidemic children: analysis of biomarkers to identify monogenic dyslipidemia

The distinction between a monogenic dyslipidemia and a polygenic/environmental dyslipidemia is important for the cardiovascular risk assessment, counseling, and treatment of these patients. The present work aims to perform the cardiovascular risk assessment of dyslipidemic children to identify useful biomarkers for clinical criteria improvement in clinical settings. Main cardiovascular risk factors were analyzed in a cohort of 237 unrelated children with clinical diagnosis of familial hypercholesterolemia (FH). About 40% carried at least two cardiovascular risk factors and 37.6% had FH, presenting mutations in LDLR and APOB. FH children showed significant elevated atherogenic markers and lower concentration of antiatherogenic particles. Children without a molecular diagnosis of FH had higher levels of TGs, apoC2, apoC3, and higher frequency of BMI and overweight/obesity, suggesting that environmental factors can be the underlying cause of their hypercholesterolem≥ia. An apoB/apoA1 ratio ≥0.68 was identified as the best biomarker (area under the curve = 0.835) to differentiate FH from other dyslipidemias. The inclusion in clinical criteria of a higher cut-off point for LDL cholesterol or an apoB/apoA1 ratio ≥0.68 optimized the criteria sensitivity and specificity. The correct identification, at an early age, of all children at-risk is of great importance so that specific interventions can be implemented. apoB/apoA1 can improve the identification of FH patients.