Familial combined hyperlipidemia: controversial aspects of its diagnosis and pathogenesis

Familial combined hyperlipidemia is a polygenic trait

PURPOSE OF REVIEW

: Familial combined hyperlipidemia (FCH), defined by concurrently elevated plasma triglyceride (TG) and low-density lipoprotein (LDL) cholesterol, has long been investigated to characterize its genetic basis. Despite almost half a century of searching, a single gene cause for the phenotype has not yet been identified.

RECENT FINDINGS

: Recent studies using next-generation genetic analytic methods confirm that FCH has a polygenic basis, with a clear large contribution from the accumulation of small-to-moderate effect common single nucleotide polymorphisms (SNPs) throughout the genome that is associated with raising TG, and probably also those raising LDL cholesterol. On the other hand, rare monogenic variants, such as those causing familial hypercholesterolemia, play a negligible role, if any. Genetic profiling suggests that patients with FCH and hypertriglyceridemia share a strong polygenic basis and show a similar profile of multiple TG-raising common SNPs.

SUMMARY

: Recent progress in genomics has shown that most if not all of the genetic susceptibility to FCH is polygenic in nature. Future research should include larger cohort studies, with wider ancestral diversity, ancestry-specific polygenic scores, and investigation of epigenetic and lifestyle factors to help further elucidate the causative agents at play in cases where the genetic etiology remains to be defined.

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.

Prevalence of heterozygous familial hypercholesterolemia and combined hyperlipidemia phenotype in very young survivors of myocardial infarction and their association with the severity of atheromatous burden

BACKGROUND

Heterozygous familial hypercholesterolemia (HeFH) and combined hyperlipidemia (CHL) phenotype are associated with premature myocardial infarction (MI).

OBJECTIVE

To assess the prevalence of HeFH and CHL phenotype among young survivors of MI and compare patients' characteristics with these 2 lipid disorders.

METHODS

We recruited 382 young survivors of MI (≤40 years). Fasting lipids, lipoprotein(a) [Lp(a)], apolipoprotein A-1, and apolipoprotein B (apoB) levels were determined. Using the Dutch Lipid Clinic Network (DLCN) algorithm, patients having definite or probable HeFH were identified. Patients with apoB levels >120 mg/dL and triglyceride levels >170 mg/dL (1.92 mmol/L) [>90th percentile of 326 age and sex-matched healthy controls] were classified as having CHL phenotype. Common carotid artery intima-media thickness (CCA-IMT) was measured by B-mode ultrasonography.

RESULTS

Eighty-one patients (21.2%) had definite/probable HeFH and 62 (16.2%) had CHL phenotype. Twenty-three patients fulfilled the criteria for both HeFH and CHL phenotype and were removed from further comparisons. Patients with HeFH (n = 58) had higher levels of total cholesterol, low-density lipoprotein (LDL)-cholesterol, Lp(a), and apoB, whereas patients with CHL phenotype (n = 39) had higher levels of triglycerides and lower high-density lipoprotein (HDL)-cholesterol levels. The prevalence of metabolic syndrome was higher in patients with CHL phenotype compared to those with HeFH (67.0% vs 16.4%, P < .001). Patients with HeFH had more extensive coronary artery disease (3-vessel disease: 36.2% vs 12.8%, P = .011) and greater right CCA-IMT (0.67 ± 0.11 mm vs 0.56 ± 0.09 mm, P < .001) and left CCA-IMT (0.68 ± 0.10 mm vs 0.56 ± 0.08 mm, P < .001) compared to CHL phenotype patients.

CONCLUSIONS

Both HeFH and CHL phenotype are common among patients with premature MI. CHL phenotype compared to HeFH is associated with less atheromatous burden in coronary and carotid arteries at the time of first MI.

Factors associated with postprandial lipemia and apolipoprotein A-V levels in individuals with familial combined hyperlipidemia

BACKGROUND

Alterations in postprandial metabolism have been described in familial combined hyperlipidemia (FCH); however, their underlying mechanisms are not well characterized. We aimed to identify factors related to the magnitude of postprandial lipemia and apolipoprotein (apo) A-V levels in subjects with FCH.

METHODS

FCH cases (n = 99) were studied using a standardized meal test. Abdominal obesity was assessed using the waist to hip ratio (WHR). A linear regression model was performed to investigate the variables associated with the triglycerides incremental area under the curve (iAUC). Independent associations between metabolic variables and apo A-V iAUC were also investigated in a randomly selected subgroup (n = 44). The study sample was classified according to the presence of fasting hypertriglyceridemia (≥150 mg/dL) and abdominal obesity (WHR ≥0.92 in men and ≥0.85 in women) to explore differences in parameters.

RESULTS

The fasting apo B-48 levels (r = 0.404), and the WHR (r = 0.359) were independent factors contributing to the triglycerides iAUC (r2 = 0.29, P < 0.001). The triglycerides iAUC was independently associated with the apo A-V iAUC (r2 = 0.54, P < 0.01). Patients with both hypertriglyceridemia and abdominal obesity showed the most robust triglycerides and apo A-V postprandial responses.

CONCLUSIONS

In patients with FCH the fasting apo B-48 level is the main factor associated with postprandial lipemia. Abdominal obesity also contributes to the magnitude of the postprandial response.The triglycerides postprandial increment is the principal factor associated with the apo A-V postprandial response.

Statin therapy and risk of diabetes in patients with heterozygous familial hypercholesterolemia or familial combined hyperlipidemia

OBJECTIVE

Controversial findings exist regarding potential influence of statin therapy on diabetic incidence. Aim of this study was to investigate the role of long duration statin treatment on diabetes mellitus (DM) incidence of Heterozygous Familial Hypercholesterolemia (hFH) and Familial Combined Hyperlipidemia (FCH) patients.

METHODS

Study population consisted of 212 hFH and 147 FCH patients that visited Lipid Outpatient Department (mean follow up of 11 and 10 years respectively). Several clinical data such as history of DM, cardiovascular disease, thyroid function, metabolic syndrome, glucose levels, lipid profile and lifestyle data were obtained. In order to compare the effects of different doses of different types of statins, a "statin treatment intensity product" was used.

RESULTS

14% of FCH and only 1% of hFH patients developed DM during follow up. Although univariate analysis showed a statistical trend (p = 0.06) in the association between new onset DM and statin treatment intensity (STI) in the FCH subgroup of patients with normal baseline glucose levels, this was no longer significant after adjusting for several confounders. Furthermore, the type of statins used did not seem to play a role in the development of DM either in hFH or FCH patients.

CONCLUSION

Long duration of high STI does not seem to be associated with diabetic risk in hFH patients. High STI used in the FCH population is not associated with increased risk of new onset DM compared to low STI. Further studies are required in order to clarify the potential diabetogenic effects of statins in these high risk populations.

Determinants of VLDL composition and apo B-containing particles in familial combined hyperlipidemia

BACKGROUND

In familial combined hyperlipidemia (FCHL) the severity of the dyslipidemia is determined by an overproduction of VLDL (very low density lipoprotein) particles and by its abnormal lipid composition. However, few are known regarding the metabolic factors that determine these abnormalities. We investigated the impact of metabolic factors on the number of atherogenic particles (apolipoprotein B level (apoB)) and the triglyceride content of very low-density lipoproteins (VLDLs-TG).

METHODS

A cross-sectional study done in FCHL subjects and gender and age-matched healthy subjects. A clinical assessment, lipid profile and plasma concentrations of insulin, apolipoprotein CIII (apo CIII), apolipoprotein AII (apo AII), high sensitive C-reactive protein (HS-CRP), adiponectin and leptin were documented in 147 FCHL patients and 147 age-matched healthy subjects. Multivariate regression models were performed to investigate the independent determinants of VLDL-TG and apo B levels adjusting for confounding factors.

RESULTS

The variables that determined the VLDL-triglyceride content as a surrogate of VLDL composition were apo CIII (β=0.365, p<0.001), insulin (β=0.281, p<0.001), Apo AII (β=0.145, p<0.035), and adiponectin levels (β=-0.255, p<0.001). This model explained 34% of VLDL composition (VLDL-TG) variability. However, none of these variables were independent contributors of apo B-containing particles.

CONCLUSIONS

In patients with FCHL apo CIII, apo AII and adiponectin are major novel factors determining the VLDL particle composition. However, such factors do not explain apo B-containing particles.

Genetic and environmental determinants of the susceptibility of Amerindian derived populations for having hypertriglyceridemia

Here, we discuss potential explanations for the higher prevalence of hypertriglyceridemia in populations with an Amerindian background. Although environmental factors are the triggers, the search for the ethnic related factors that explain the increased susceptibility of the Amerindians is a promising area for research. The study of the genetics of hypertriglyceridemia in Hispanic populations faces several challenges. Ethnicity could be a major confounding variable to prove genetic associations. Despite that, the study of hypertriglyceridemia in Hispanics has resulted in significant contributions. Two GWAS reports have exclusively included Mexican mestizos. Fifty percent of the associations reported in Caucasians could be generalized to the Mexicans, but in many cases the Mexican lead SNP was different than that reported in Europeans. Both reports included new associations with apo B or triglycerides concentrations. The frequency of susceptibility alleles in Mexicans is higher than that found in Europeans for several of the genes with the greatest effect on triglycerides levels. An example is the SNP rs964184 in APOA5. The same trend was observed for ANGPTL3 and TIMD4 variants. In summary, we postulate that the study of the genetic determinants of hypertriglyceridemia in Amerindian populations which have major changes in their lifestyle, may prove to be a great resource to identify new genes and pathways associated with hypertriglyceridemia.

Biochemical Manifestation of HIV Lipodystrophy Syndrome

OBJECTIVES

Highly active anti-retroviral therapy (HAART), including protease inhibitors (PI) have led to dramatic improvements in the quality and quantity of life in patients with acquired immunodeficiency syndrome (AIDS). However, a significant number of AIDS patients on HAART develop characteristic changes in body fat redistribution referred to as lipodystrophy syndrome (LDS). Features of LDS include hypertrophy in the neck fat pad (buffalo hump), increased fat in the abdominal region (protease paunch), gynecomastia and loss of fat in the mid-face and extremities.

METHODS

The aim of this paper is to review the current knowledge regarding this syndrome. This article reviews the published investigations on biochemical manifestation of HIV lipodystrophy syndrome.

RESULTS

It is estimated that approximately 64% of patients treated with PI will experience this syndrome. Biochemically, these patients have increased triglycerides (Trig), total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C) and extremely low high-density lipoprotein-cholesterol (HDL-C).

CONCLUSIONS AND PUBLIC HEALTH IMPLICATIONS

It is hoped that awareness of this syndrome would aid in early diagnosis and better patient management, possibly leading to a lower incidence of cardiovascular complications among these patients.

Calculated low-density lipoprotein cholesterol remains a viable and important test for screening and targeting therapy

BACKGROUND

Most clinical laboratories use calculated (C) low-density lipoprotein cholesterol (LDL-C) for measurement. Some studies have questioned the linearity of CLDL-C in the clinically useful low range. Moreover, it is generally believed that calculation leads to poor precision such that variation in CLDL-C is greater than the 4% guideline since the calculation is dependent on three primary variables. Actually, the degree of variability of a calculated value will be small if the variability of each primary value is small as compared to its contribution to the calculated value. When LDL-C is low, high-density lipoprotein cholesterol (HDL-C), that has poorer precision, becomes more important in defining the precision of CLDL-C. New homogeneous (direct) HDL-C (dHDL) methods show better precision than the older heterogeneous methods. We hypothesized that a direct homogeneous HDL-C method would substantially improve the low range precision of LDL-C as compared to older heterogeneous HDL-C methods.

METHODS

We compared CLDL-C to a standardized electrophoretic method that shows very high precision. We also compared the precision of CLDL-C calculated using a homogeneous dHDL and a heterogeneous indirect method.

RESULTS

We found good linearity for CLDL-C down to 500 mg/L (x0.002586). The main source of CLDL-C variation was HDL-C. Precision was within guidelines when the dHDL method was used. Using our automated methods for lipoprotein lipids, assuming our reference method is accurate, the formula that calculated CLDL-C (mg/dL) using triglyceride (mg/dL) (x0.001129) x0.2 suggested by some gave more accurate results than the formula using triglyceride (mg/dL) x0.16 suggested by others.

CONCLUSIONS

Given the potential for CLDL-C to meet the precision guidelines, until direct LDL-C methods can be refined, CLDL-C should continue to be the primary test used for assessing LDL-C clinically. Standardized testing for CLDL-C for manufacturers should be available so that the formula used for each instrument can provide well-defined accuracy.

Familial dyslipidaemias: an overview of genetics, pathophysiology and management

Plasma lipid disorders can occur either as a primary event or secondary to an underlying disease or use of medications. Familial dyslipidaemias are traditionally classified according to the electrophoretic profile of lipoproteins. In more recent texts, this phenotypic classification has been replaced with an aetiological classification. Familial dyslipidaemias are generally grouped into disorders leading to hypercholesterolaemia, hypertriglyceridaemia, a combination of hyper-cholesterolaemia and hypertriglyceridaemia, or abnormal high-density lipoprotein-cholesterol (HDL-C) levels. The management of these disorders requires an understanding of plasma lipid and lipoprotein metabolism. Lipid transport and metabolism involves three general pathways: (i) the exogenous pathway, whereby chylomicrons are synthesised by the small intestine, and dietary triglycerides (TGs) and cholesterol are transported to various cells of the body; (ii) the endogenous pathway, whereby very low-density lipoprotein-cholesterol (VLDL-C) and TGs are synthesised by the liver for transport to various tissues; and (iii) the reverse cholesterol transport, whereby HDL cholesteryl ester is exchanged for TGs in low-density lipoptrotein (LDL) and VLDL particles through cholesteryl ester transfer protein in a series of steps to remove cholesterol from the peripheral tissues for delivery to the liver and steroidogenic organs. The plasma lipid profile can provide a framework to guide the selection of appropriate diet and drug treatment. Many patients with hyperlipoproteinaemia can be treated effectively with diet. However, dietary regimens are often insufficient to bring lipoprotein levels to within acceptable limits. In this article, we review lipid transport and metabolism, discuss the more common lipid disorders and suggest some management guidelines. The choice of a particular agent depends on the baseline lipid profile achieved after 6-12 weeks of intense lifestyle changes and possible use of dietry supplements such as stanols and plant sterols. If the predominant lipid abnormality is hypertriglyceridaemia, omega-3 fatty acids, a fibric acid derivative (fibrate) or nicotinic acid would be considered as the first choice of therapy. In subsequent follow-up, when LDL-C is >130 mg/dL (3.36 mmol/L) then an HMG-CoA reductase inhibitor (statin) should be added as a combination therapy. If the serum TG levels are <500 mg/dL (2.26 mmol/L) and the LDL-C values are over 130 mg/dL (3.36 mmol/L) then a statin would be the first drug of choice. The statin dose can be titrated up to achieve the therapeutic goal or, alternatively, ezetimibe can be added. A bile acid binding agent is an option if the serum TG levels do not exceed 200 mg/dL (5.65 mmol/L), otherwise a fibrate or nicotinic acid should be considered. The decision to treat a particular person has to be individualised.

Genetic lipoprotein disorders and coronary atherosclerosis

Familial lipoprotein disorders are seen frequently in subjects with premature coronary artery disease. The genetic basis for several dyslipoproteinemias has been elucidated in the past two decades. The majority of lipoprotein disorders result from a combination of polygenic predisposition and poor lifestyle habits, including physical inactivity, increased visceral adipose tissue, increased caloric intake, and cigarette smoking. Monogenic disorders are seen in approximately 5% of premature coronary artery disease cases, and this prevalence is higher in populations with the Founder effect. Unraveling the genetic basis of many lipoprotein disorders has allowed fundamental discoveries in molecular cellular physiology and has paved the way for novel therapeutic approaches.