Clinical Aspects of Genetic and Non-Genetic Cardiovascular Risk Factors in Familial Hypercholesterolemia

Familial hypercholesterolemia (FH) is the most common monogenic metabolic disorder characterized by considerably elevated low-density lipoprotein cholesterol (LDL-C) levels leading to enhanced atherogenesis, early cardiovascular disease (CVD), and premature death. However, the wide phenotypic heterogeneity in FH makes the cardiovascular risk prediction challenging in clinical practice to determine optimal therapeutic strategy. Beyond the lifetime LDL-C vascular accumulation, other genetic and non-genetic risk factors might exacerbate CVD development. Besides the most frequent variants of three genes (LDL-R, APOB, and PCSK9) in some proband variants of other genes implicated in lipid metabolism and atherogenesis are responsible for FH phenotype. Furthermore, non-genetic factors, including traditional cardiovascular risk factors, metabolic and endocrine disorders might also worsen risk profile. Although some were extensively studied previously, others, such as common endocrine disorders including thyroid disorders or polycystic ovary syndrome are not widely evaluated in FH. In this review, we summarize the most important genetic and non-genetic factors that might affect the risk prediction and therapeutic strategy in FH through the eyes of clinicians focusing on disorders that might not be in the center of FH research. The review highlights the complexity of FH care and the need of an interdisciplinary attitude to find the best therapeutic approach in FH patients.

Molecular genetic testing for familial hypercholesterolemia: spectrum of LDL receptor gene mutations in The Netherlands

Mutations in the LDL receptor are responsible for familial hypercholesterolemia (FH). At present, more than 600 mutations of the LDL receptor gene are known to underlie FH. However, the array of mutations varies considerably in different populations. Therefore, the delineation of essentially all LDL receptor gene mutations in a population represents a prerequisite for the implementation of nation-wide genetic testing for FH. In this study, the frequency and geographical distribution of 13 known mutations were evaluated in a cohort of 1223 FH patients. We identified 358 mutation carriers, representing 29% of the FH cohort. Four mutations (N543H-2393de19, 1359--1G-->A, 313 + 1 G-->A and W23X) occurred with a relatively high frequency, accounting for 22.4% of the entire study cohort. Two of these common FH mutations (N543H-2393de19 and 1359 - 1G-->A) showed a preferential geographic distribution. Second, to further expand the array of LDL receptor gene mutations, we conducted mutation analysis by denaturing gradient gel electrophoresis (DGGE) in 141 children with definite FH. A mutation was identified in 111 patients, involving 16 new single base substitutions and four small deletions and insertions, which brings the number of different FH-causing mutations in our country up to 61. Our data indicate that an estimate of the prevalence of specific mutations, as well as the compilation of a database of all FH-causing mutations in a given country, can facilitate selection of the most appropriate molecular diagnostic approach.

Inheritance of two different alleles of the low-density lipoprotein (LDL)-receptor gene carrying the recurrent Pro664Leu mutation in a patient with homozygous familial hypercholesterolaemia

Familial hypercholesterolaemia (FH) is caused by mutations in the low-density lipoprotein (LDL)-receptor gene that result in impaired clearance of plasma LDL and increased risk of coronary heart disease. Numerous different mutations have been found in FH patients worldwide, the majority of which are infrequent in out-bred populations and account for 2% or less of patients with the disorder in large cohorts. Thus, it was surprising to find that two homozygous FH patients referred to a single hospital in the UK were both apparently homozygous for the Pro664Leu mutation. One, an Asian patient, was a true homozygote. The other, of English origin, had inherited two different alleles of the LDL-receptor gene with the same mutation from unrelated parents, as inferred from the haplotype of polymorphic markers. A third, clinically homozygous FH patient, despite being the offspring of first cousins, had inherited one 'Asian' Pro664Leu allele, but an allele with a 1-bp deletion in exon 5 from the other parent. The Pro664Leu mutation in the LDL-receptor gene has now been described in heterozygous patients of very different ethnic origin and is associated with different haplotypes, suggesting that the same base change at a CpG may have recurred as many as six times.

Genetic diagnosis of familial hypercholesterolemia in affected relatives using pedigree tracing

OBJECTIVES

To describe a process of diagnosing familial hypercholesterolemia (FH) at the DNA level in selected family members of affected individuals.

DESIGN AND METHODS

A 63-year-old male patient presented with cholesterol elevations consistent with heterozygous familial hypercholesterolemia. Through participation with the international "MEDPED FH" project to detect affected relatives and to identify their LDL-receptor mutation, the patient was discovered to carry the Lebanese mutation, whereby the codon for cysteine at residue 660 instead codes for a premature termination (C660X), thus truncating the protein product. This mutation also created a new restriction recognition site for the endonuclease Hinfl, which permitted rapid detection of the mutation in selected family members using restriction fragment-length polymorphisms.

RESULTS

The patient's son, who had cholesterol levels consistent with heterozygous FH, was also found to be a heterozygote for the C660X variant of the LDL-receptor.

CONCLUSIONS

Diagnosis of familial hypercholesterolemia at the DNA level is possible as a relatively rapid screening technique in families with a known LDL-receptor mutation, established through participation with the MED-PED FH project.

Apolipoprotein(a) phenotypes and lipoprotein(a) concentrations in patients with hyperthyroidism

Lipoprotein(a) [Lp(a)] is a low-density lipoprotein (LDL) particle in which apolipoprotein B-100 (apoB) is attached to a glycoprotein called apolipoprotein(a) [apo(a)]. Apo(a) has several genetically determined phenotypes differing in molecular weight, to which Lp(a) concentrations in plasma are inversely correlated. High plasma levels of Lp(a) are associated with atherosclerotic diseases. It is therefore of interest to study whether factors other than the apo(a) gene locus are involved in the regulation of Lp(a) concentrations. We measured plasma concentrations of Lp(a) and other lipoproteins and determined apo(a) phenotypes in 31 patients with hyperthyroidism, before and after the patients had become euthyroid by treatment. The mean concentration of LDL cholesterol rose from 2.67 to 3.88 mmol/l (P < 0.01), apoB rose from 0.79 to 1.03 g/l (P < 0.01), and the median Lp(a) concentration increased from 9.74 to 18.97 mg/dl (P < 0.01) on treatment. Lp(a) concentrations were inversely associated to the size of the apo(a) molecule both before (P < 0.01) and after treatment (P < 0.01). The increase in Lp(a) was significant in patients with high molecular weight apo(a) phenotypes (n = 9; P < 0.01) and in patients with low molecular weight apo(a) phenotypes (n = 16; P < 0.01), but not in those with apo(a) "null types" (n = 6; P = 0.5). The low levels LDL cholesterol and apoB in untreated hyperthyroidism may result from increased LDL receptor activity. The increase in Lp(a) levels were not correlated with the increase in LDL cholesterol or apoB.(ABSTRACT TRUNCATED AT 250 WORDS)