Familial combined hyperlipidemia: from molecular insights to tailored therapy

Genetic Lipid Disorders Associated with Atherosclerotic Cardiovascular Disease: Molecular Basis to Clinical Diagnosis and Epidemiologic Burden

Genetic lipid disorders, ranging from common dyslipidemias such as familial hypercholesterolemia, lipoprotein (a), and familial combined hyperlipidemia to rare disorders including familial chylomicronemia syndrome and inherited hypoalphalipoproteinemias (ie, Tangier and fish eye diseases), affect millions of individuals in the United States and tens of millions around the world and are often undiagnosed in the general population. Clinicians should take into consideration the potential of inherited lipid disorders or syndromes when severe derangements in lipid parameters are observed. Patients' combined genotype and phenotype should be evaluated in conjunction with a host of environmental factors impacting their risk of atherosclerotic cardiovascular disease.

Dietary Recommendations for Familial Hypercholesterolaemia: an Evidence-Free Zone

We have evaluated dietary recommendations for people diagnosed with familial hypercholesterolaemia (FH), a genetic condition in which increased low-density lipoprotein cholesterol (LDL-C) is associated with an increased risk for coronary heart disease (CHD). Recommendations for FH individuals have emphasised a low saturated fat, low cholesterol diet to reduce their LDL-C levels. The basis of this recommendation is the 'diet-heart hypothesis', which postulates that consumption of food rich in saturated fat increases serum cholesterol levels, which increases risk of CHD. We have challenged the rationale for FH dietary recommendations based on the absence of support for the diet-heart hypothesis, and the lack of evidence that a low saturated fat, low cholesterol diet reduces coronary events in FH individuals. As an alternative approach, we have summarised research which has shown that the subset of FH individuals that develop CHD exhibit risk factors associated with an insulin-resistant phenotype (elevated triglycerides, blood glucose, haemoglobin A1c (HbA1c), obesity, hyperinsulinaemia, high-sensitivity C reactive protein, hypertension) or increased susceptibility to develop coagulopathy. The insulin-resistant phenotype, also referred to as the metabolic syndrome, manifests as carbohydrate intolerance, which is most effectively managed by a low carbohydrate diet (LCD). Therefore, we propose that FH individuals with signs of insulin resistance should be made aware of the benefits of an LCD. Our assessment of the literature provides the rationale for clinical trials to be conducted to determine if an LCD would prove to be effective in reducing the incidence of coronary events in FH individuals which exhibit an insulin-resistant phenotype or hypercoagulation risk.

Case Studies in Pediatric Lipid Disorders and Their Management

CONTEXT

Identification of modifiable risk factors, including genetic and acquired disorders of lipid and lipoprotein metabolism, is increasingly recognized as an opportunity to prevent premature cardiovascular disease (CVD) in at-risk youth. Pediatric endocrinologists are at the forefront of this emerging public health concern and can be instrumental in beginning early interventions to prevent premature CVD-related events during adulthood.

AIM

In this article, we use informative case presentations to provide practical approaches to the management of pediatric dyslipidemia.

CASES

We present 3 scenarios that are commonly encountered in clinical practice: isolated elevation of low-density lipoprotein cholesterol (LDL-C), combined dyslipidemia, and severe hypertriglyceridemia. Treatment with statin is indicated when the LDL-C is ≥190 mg/dL (4.9 mmol/L) in children ≥10 years of age. For LDL-C levels between 130 and 189 mg/dL (3.4-4.89 mmol/L) despite dietary and lifestyle changes, the presence of additional risk factors and comorbid conditions would favor statin therapy. In the case of combined dyslipidemia, the primary treatment target is LDL-C ≤130 mg/dL (3.4 mmol/L) and the secondary target non-high-density lipoprotein cholesterol <145 mg/dL (3.7 mmol/L). If the triglyceride is ≥400 mg/dL (4.5 mmol/L), prescription omega-3 fatty acids and fibrates are considered. In the case of triglyceride >1000 mg/dL (11.3 mmol/L), dietary fat restriction remains the cornerstone of therapy, even though the landscape of medications is changing.

CONCLUSION

Gene variants, acquired conditions, or both are responsible for dyslipidemia during childhood. Extreme elevations of triglycerides can lead to pancreatitis. Early identification and management of dyslipidemia and cardiovascular risk factors is extremely important.

Familial combined hyperlipidaemia/polygenic mixed hyperlipidaemia

Familial combined hyperlipidaemia (FCH) is the most prevalent form of familial hyperlipidaemia with a multigenic origin and a complex pattern of inheritance. In this respect, FCH is an oligogenic primary lipid disorder due to interaction of genetic variants and mutations with environmental factors. Patients with FCH are at increased risk of cardiovascular disease and often have other associated metabolic conditions. Despite its relevance in cardiovascular prevention, FCH is frequently underdiagnosed and very often undertreated. In this review, emphasis is placed on the most recent advances in FCH, in order to increase its awareness and ultimately contribute to improving its clinical control.

Pediatric Dyslipidemia-Beyond Familial Hypercholesterolemia

Dyslipidemia is seen with increasing prevalence in young Canadians, mainly mild to moderate hypertriglyceridemia secondary to obesity. This review focuses on pediatric dyslipidemias excluding familial hypercholesterolemia (FH), but including both severe and mild to moderate hypertriglyceridemia, combined hyperlipidemia, and elevated lipoprotein(a) [Lp(a)]. We suggest that for Canadian children and adolescents with dyslipidemia, atherosclerotic cardiovascular disease (ASCVD) risk assessment should include both low-density lipoprotein cholesterol and triglyceride measurement. To further stratify risk, determination of non-high-density lipoprotein cholesterol is recommended, for both its ability to predict ASCVD and convenience for the patient because fasting is not required. Similarly, apolipoprotein B measurement (fasting or nonfasting), where available, can be helpful. Lp(a) measurement should not be routine in childhood, but it can be considered in special circumstances. After ruling out secondary causes, the foundation for management of pediatric dyslipidemia includes weight regulation, optimizing diet, and increasing activity level. At present, randomized clinical trial data to guide pharmaceutical management of pediatric hypertriglyceridemia or other non-FH pediatric dyslipidemias are scarce. Pharmaceutical management should be reserved for special situations in which risk of complications such as acute pancreatitis or ASCVD over the intermediate term is high and conservative lifestyle-based interventions have been ineffective.

Familial Combined Hyperlipidemia (FCH) Patients with High Triglyceride Levels Present with Worse Lipoprotein Function Than FCH Patients with Isolated Hypercholesterolemia

Lipoprotein characteristics were analyzed in familial combined hyperlipidemia (FCH) patients before and after statin treatment. Twenty-six FCH patients were classified according to the presence (HTG group, n = 13) or absence (normotriglyceridemic (NTG) group, n = 13) of hypertriglyceridemia. Fifteen healthy subjects comprised the control group. Lipid profile, inflammation markers, and qualitative characteristics of lipoproteins were assessed. Both groups of FCH subjects showed high levels of plasma C-reactive protein (CRP), lipoprotein-associated phospholipase A2 (Lp-PLA2) activity and apolipoprotein J. Statins reverted the increased levels of Lp-PLA2 and CRP. Lipoprotein composition alterations detected in FCH subjects were much more frequent in the HTG group, leading to dysfunctional low-density lipoproteins (LDL) and high-density lipoproteins (HDL). In the HTG group, LDL was smaller, more susceptible to oxidation, and contained more electronegative LDL (LDL(-)) compared to the NTG and control groups. Regarding HDL, the HTG group had less Lp-PLA2 activity than the NTG and control groups. HDL from both FCH groups was less anti-inflammatory than HDL from the control group. Statins increased LDL size, decreased LDL(-), and lowered Lp-PLA2 in HDL from HTG. In summary, pro-atherogenic alterations were more frequent and severe in the HTG group. Statins improved some alterations, but many remained unchanged in HTG.

A novel variant in LPL gene is associated with familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is a common genetic disorder characterized by increased fasted serum cholesterol, triglycerides, and apolipoprotein B-100. Molecular genetic techniques such as next generation sequencing have been very successful methods for rare variants finding with a moderate-to large effect. In this study, we characterized a large pedigree from MASHAD study in northeast Iran with coinheritance of FCHL and early-onset coronary heart disease. In this family, we used whole-exome sequencing and Sanger sequencing to determine the disease-associated gene. We identified a novel variant in the LPL gene, leading to a substitution of an asparagine for aspartic acid at position 151. The D151N substitution cosegregated with these characters in all affected family members in the pedigree but it was absent in all unaffected members in this family. We speculated that the mutation D151N in LPL gene might be associated with FCHL and early-onset coronary heart disease in this family. However, the substantial mechanism requires further investigation.

Familial combined hyperlipidemia: An overview of the underlying molecular mechanisms and therapeutic strategies

Among different types of dyslipidemia, familial combined hyperlipidemia (FCHL) is the most common genetic disorder, which is characterized by at least two different forms of lipid abnormalities: hypercholesterolemia and hypertriglyceridemia. FCHL is an important cause of cardiovascular diseases. FCHL is a heterogeneous condition linked with some metabolic defects that are closely associated with FCHL. These metabolic features include dysfunctional adipose tissue, delayed clearance of triglyceride-rich lipoproteins, overproduction of very low-density lipoprotein and hepatic lipids, and defect in the clearance of low-density lipoprotein particles. There are also some genes associated with FCHL such as those affecting the metabolism and clearance of plasma lipoprotein particles. Due to the high prevalence of FCHL especially in cardiovascular patients, targeted treatment is ideal but this necessitates identification of the genetic background of patients. This review describes the metabolic pathways and associated genes that are implicated in FCHL pathogenesis. We also review existing and novel treatment options for FCHL. © 2019 IUBMB Life, 71(9):1221-1229, 2019.

Vascular inflammation and metabolic activity in hematopoietic organs and liver in familial combined hyperlipidemia and heterozygous familial hypercholesterolemia

BACKGROUND

Familial dyslipidemias of either heterozygous (heFH) or combined (FCH) type lead to accelerated atherogenesis and increased cardiovascular risk.

OBJECTIVE

The aim of this study was to investigate in statin-naïve adult patients with familial dyslipidemias whether inflammatory activation and liver, spleen and bone marrow metabolic activity differ compared with normolipidemic subjects and between dyslipidemic groups.

METHODS

Fourteen patients with FCH, 14 with heFH, and 14 normolipidemic individuals were enrolled. Serum lipids, high-sensitivity C-reactive protein, and fibrinogen levels were measured, followed by ¹⁸F-fluorodeoxyglucose positron-emission tomography/computed tomography imaging. Radiotracer uptake in the aortic wall, spleen, bone marrow, and liver was quantified as tissue-to-background ratio (TBR).

RESULTS

Patients with heFH had significantly higher low-density lipoprotein levels compared with those with FCH and controls (P < .001). However, aortic TBRs were higher in FCH compared with heFH patients and controls (P = .02 and P < .001, respectively). FCH patients exhibited higher FDG uptake in the spleen compared with controls (P = .05). In addition, FCH exhibited higher bone marrow FDG uptake compared with heFH patients and controls (P = .03 and P = .02, respectively). FCH had higher liver uptake compared with heFH patients and controls (P < .001 for both). Significant correlations were observed between inflammatory biomarkers and imaging indices as well as between aortic TBR and FDG uptake of hematopoietic organs and liver.

CONCLUSIONS

Systemic, as well as vascular inflammation and spleen, bone marrow, and hepatic metabolic activity are increased in patients with FCH despite lower levels of low-density lipoprotein.

Single Nucleotide Variants Associated With Polygenic Hypercholesterolemia in Families Diagnosed Clinically With Familial Hypercholesterolemia

INTRODUCTION AND OBJECTIVES

Approximately 20% to 40% of clinically defined familial hypercholesterolemia cases do not show a causative mutation in candidate genes, and some of them may have a polygenic origin. A cholesterol gene risk score for the diagnosis of polygenic hypercholesterolemia has been demonstrated to be valuable to differentiate polygenic and monogenic hypercholesterolemia. The aim of this study was to determine the contribution to low-density lipoprotein cholesterol (LDL-C) of the single nucleotide variants associated with polygenic hypercholesterolemia in probands with genetic hypercholesterolemia without mutations in candidate genes (nonfamilial hypercholesterolemia genetic hypercholesterolemia) and the genetic score in cascade screening in their family members.

METHODS

We recruited 49 nonfamilial hypercholesterolemia genetic hypercholesterolemia families (294 participants) and calculated cholesterol gene scores, derived from single nucleotide variants in SORT1, APOB, ABCG8, APOE and LDLR and lipoprotein(a) plasma concentration.

RESULTS

Risk alleles in SORT1, ABCG8, APOE, and LDLR showed a statistically significantly higher frequency in blood relatives than in the 1000 Genomes Project. However, there were no differences between affected and nonaffected members. The contribution of the cholesterol gene score to LDL-C was significantly higher in affected than in nonaffected participants (P = .048). The percentage of the LDL-C variation explained by the score was 3.1%, and this percentage increased to 6.9% in those families with the highest genetic score in the proband.

CONCLUSIONS

Nonfamilial hypercholesterolemia genetic hypercholesterolemia families concentrate risk alleles for high LDL-C. Their contribution varies greatly among families, indicating the complexity and heterogeneity of these forms of hypercholesterolemias. The gene score explains a small percentage of LDL-C, which limits its use in diagnosis.

The Contribution of GWAS Loci in Familial Dyslipidemias

Familial combined hyperlipidemia (FCH) is a complex and common familial dyslipidemia characterized by elevated total cholesterol and/or triglyceride levels with over five-fold risk of coronary heart disease. The genetic architecture and contribution of rare Mendelian and common variants to FCH susceptibility is unknown. In 53 Finnish FCH families, we genotyped and imputed nine million variants in 715 family members with DNA available. We studied the enrichment of variants previously implicated with monogenic dyslipidemias and/or lipid levels in the general population by comparing allele frequencies between the FCH families and population samples. We also constructed weighted polygenic scores using 212 lipid-associated SNPs and estimated the relative contributions of Mendelian variants and polygenic scores to the risk of FCH in the families. We identified, across the whole allele frequency spectrum, an enrichment of variants known to elevate, and a deficiency of variants known to lower LDL-C and/or TG levels among both probands and affected FCH individuals. The score based on TG associated SNPs was particularly high among affected individuals compared to non-affected family members. Out of 234 affected FCH individuals across the families, seven (3%) carried Mendelian variants and 83 (35%) showed high accumulation of either known LDL-C or TG elevating variants by having either polygenic score over the 90^th percentile in the population. The positive predictive value of high score was much higher for affected FCH individuals than for similar sporadic cases in the population. FCH is highly polygenic, supporting the hypothesis that variants across the whole allele frequency spectrum contribute to this complex familial trait. Polygenic SNP panels improve identification of individuals affected with FCH, but their clinical utility remains to be defined.

Familial combined hyperlipidemia (FCH) is a familial dyslipidemia and the most common familial risk factor for premature coronary heart disease. Its genetic architecture is poorly understood. Rare high-impact variants have been identified in some patients, but have not explained a substantial portion of the trait. FCH has previously been speculated to be a polygenic disorder, but genetic data supporting this hypothesis have so far been incomplete. We provide experimental evidence for the polygenicity and heterogeneity of FCH in a large set of affected families using comprehensive genome-wide variant data. Approximately a third of the affected FCH individuals in our sample had high polygenic burden, and only a minority carried high-impact variants identifiable by genotyping. We show that the polygenic burden of affected FCH family members is comparable to that observed in individuals with similar lipid phenotypes in the general population. Genetic variants identified in large-scale population studies can also underlie the typical phenotypes observed in complex familial diseases such as FCH. Advances in genetic diagnosis based on population samples may thus also benefit FCH families. Families without high polygenic burden are good candidates for sequencing studies to identify rare variants not observable with genotyping.

Combined hyperlipidemia: familial but not (usually) monogenic

PURPOSE OF REVIEW

Combined hyperlipidemia (CHL) is a complex phenotype that is commonly encountered clinically and is often associated with the expression of early heart disease. The affixed adjective 'familial' gives the impression that the trait is monogenic, like familial hypercholesterolemia. But despite significant efforts, genetic studies have yielded little evidence of single gene determinants of CHL.

RECENT FINDINGS

Sophisticated linkage studies suggest that individual lipid components of the CHL phenotype - such as elevated LDL and triglyceride - each have several determinants that segregate independently in families. Furthermore, DNA sequencing shows that rare large-effect variants in genes such as LDL receptor (LDLR) and lipoprotein lipase are found in some CHL patients, explaining the elevated LDL cholesterol and triglyceride components, respectively. In addition, multiple common small-effect lipid-altering variants accumulate in an individual's genome, raising the LDL cholesterol and/or triglyceride components by multiple mechanisms. Finally, secondary factors, such as poor diet, obesity,fatty liver or diabetes further modulate the expression of the biochemically defined CHL phenotype.

SUMMARY

Given the current state of genetic understanding, CHL may be best conceptualized as a syndrome with common clinical presentation but multigenic causes, similar to other common conditions such as type 2 diabetes.

Role of the BAHD1 Chromatin-Repressive Complex in Placental Development and Regulation of Steroid Metabolism

BAHD1 is a vertebrate protein that promotes heterochromatin formation and gene repression in association with several epigenetic regulators. However, its physiological roles remain unknown. Here, we demonstrate that ablation of the Bahd1 gene results in hypocholesterolemia, hypoglycemia and decreased body fat in mice. It also causes placental growth restriction with a drop of trophoblast glycogen cells, a reduction of fetal weight and a high neonatal mortality rate. By intersecting transcriptome data from murine Bahd1 knockout (KO) placentas at stages E16.5 and E18.5 of gestation, Bahd1-KO embryonic fibroblasts, and human cells stably expressing BAHD1, we also show that changes in BAHD1 levels alter expression of steroid/lipid metabolism genes. Biochemical analysis of the BAHD1-associated multiprotein complex identifies MIER proteins as novel partners of BAHD1 and suggests that BAHD1-MIER interaction forms a hub for histone deacetylases and methyltransferases, chromatin readers and transcription factors. We further show that overexpression of BAHD1 leads to an increase of MIER1 enrichment on the inactive X chromosome (Xi). In addition, BAHD1 and MIER1/3 repress expression of the steroid hormone receptor genes ESR1 and PGR, both playing important roles in placental development and energy metabolism. Moreover, modulation of BAHD1 expression in HEK293 cells triggers epigenetic changes at the ESR1 locus. Together, these results identify BAHD1 as a core component of a chromatin-repressive complex regulating placental morphogenesis and body fat storage and suggest that its dysfunction may contribute to several human diseases.

Effects of Armolipid Plus on small dense LDL particles in a sample of patients affected by familial combined hyperlipidemia

AIM

The aim of this study was to test small dense LDL changes with Armolipid Plus treatment in patients with familial combined hyperlipidemia (FCHL).

METHODS

After 4 weeks, 30 patients with FCHL were included in an 8-week, randomized, double-blind study and were taking, in addition to the standard diet, either placebo or Armolipid Plus.

RESULTS

The placebo group showed no statistically significant differences in the studied parameters; instead, in the Armolipid Plus group, statistically significant reduction differences were detected in BMI (p = 0.010), LDL score (p = 0.035) and an increase in mean LDL particle diameter (p = 0.040).

CONCLUSION

The combination of a standard diet with Armolipid Plus is able to reduce LDL score and increase LDL particle diameter in a group of FCHL after 8 weeks of treatment.