A genome scan for familial combined hyperlipidemia reveals evidence of linkage with a locus on chromosome 11

The clinical utility of polygenic risk scores for combined hyperlipidemia

PURPOSE OF REVIEW

Combined hyperlipidemia is the most common lipid disorder and is strongly polygenic. Given its prevalence and associated risk for atherosclerotic cardiovascular disease, this review describes the potential for utilizing polygenic risk scores for risk prediction and management of combined hyperlipidemia.

RECENT FINDINGS

Different diagnostic criteria have led to inconsistent prevalence estimates and missed diagnoses. Given that individuals with combined hyperlipidemia have risk estimates for incident coronary artery disease similar to individuals with familial hypercholesterolemia, early identification and therapeutic management of those affected is crucial. With diagnostic criteria including traits such apolipoprotein B, low-density lipoprotein cholesterol, and triglyceride, polygenic risk scores for these traits strongly associate with combined hyperlipidemia and could be used in combination for clinical risk prediction models and developing specific treatment plans for patients.

SUMMARY

Polygenic risk scores are effective tools in risk prediction of combined hyperlipidemia, can provide insight into disease pathophysiology, and may be useful in managing and guiding treatment plans for patients. However, efforts to ensure equitable polygenic risk score performance across different genetic ancestry groups is necessary before clinical implementation in order to prevent the exacerbation of racial disparities in the clinic.

Hypertriglyceridemia-induced acute pancreatitis in children: A mini-review

Severe hypertriglyceridemia (HTG) is a known metabolic cause of acute pancreatitis (AP) in pediatric patients. The incidence of hypertriglyceridemia-induced acute pancreatitis (HTG-AP) is less well established in pediatric compared to adult patients. Studies in adults suggest that higher risk of AP occurs when triglyceride levels (TG) are >1,000 mg/dL. Most common etiologies for severe HTG in pediatric patients are either from primary hypertriglyceridemia, underlying genetic disorders of lipid and TG metabolism, or secondary hypertriglyceridemia, separate disease or exposure which affects TG metabolism. Most common theories for the pathophysiology of HTG-AP include hydrolysis of TG by pancreatic lipase to free fatty acids leading to endothelial and acinar cell damage and ischemia, as well as hyperviscosity related to increased chylomicrons. Though there are varying reports of HTG-AP severity compared to other causes of AP, a steadily growing body of evidence suggests that HTG-AP can be associated with more severe course and complications. Therapeutic interventions for HTG-AP typically involve inpatient management with dietary restriction, intravenous fluids, and insulin; select patients may require plasmapheresis. Long term interventions generally include dietary modification, weight management, control of secondary causes, and/or antihyperlipidemic medications. Though some therapeutic approaches and algorithms exist for adult patients, evidence-based management guidelines have not been well established for pediatric patients.

Polygenic architecture and cardiovascular risk of familial combined hyperlipidemia

BACKGROUND AND AIMS

Familial combined hyperlipidemia (FCHL) is one of the most common inherited lipid phenotypes, characterized by elevated plasma concentrations of apolipoprotein B-100 and triglycerides. The genetic inheritance of FCHL remains poorly understood. The goals of this study were to investigate the polygenetic architecture and cardiovascular risk associated with FCHL.

METHODS AND RESULTS

We identified individuals with an FCHL phenotype among 349,222 unrelated participants of European ancestry in the UK Biobank using modified versions of 5 different diagnostic criteria. The prevalence of the FCHL phenotype was 11.44% (n = 39,961), 5.01% (n = 17,485), 1.48% (n = 5,153), 1.10% (n = 3,838), and 0.48% (n = 1,688) according to modified versions of the Consensus Conference, Dutch, Mexico, Brunzell, and Goldstein criteria, respectively. We performed discovery, case-control genome-wide association studies for these different FCHL criteria and identified 175 independent loci associated with FCHL at genome-wide significance. We investigated the association of genetic and clinical risk with FCHL and found that polygenic susceptibility to hypercholesterolemia or hypertriglyceridemia and features of metabolic syndrome were associated with greater prevalence of FCHL. Participants with an FCHL phenotype had a similar risk of incident coronary artery disease compared to participants with monogenic familial hypercholesterolemia (adjusted hazard ratio vs controls [95% confidence interval]: 2.72 [2.31-3.21] and 1.90 [1.30-2.78]).

CONCLUSIONS

These results suggest that, rather than being a single genetic entity, the FCHL phenotype represents a polygenic susceptibility to dyslipidemia in combination with metabolic abnormalities. The cardiovascular risk associated with an FCHL phenotype is similar to that of monogenic familial hypercholesterolemia, despite being ∼5x more common.

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.

Association of C5L2 genetic polymorphisms with coronary artery disease in a Han population in Xinjiang, China

Background

C5aR-like receptor 2 (C5L2) has been identified as a receptor for the inflammatory factor Complement 5a (C5a) and acylation-stimulating protein (ASP). ASP binding to C5L2 leading to a net accumulation of TG stores and glucose transporter. The aim of the present study is to evaluate the association of the SNPs of C5L2 gene with coronary artery disease (CAD) in a Chinese population.

Methods

We examined the role of the tagging single nucleotide polymorphisms (SNPs) of C5L2 gene for CAD using a case-control design. We determined the prevalence of C5L2 genotypes in 505 CAD patients and 469 age and sex-matched healthy control subjects of Han population.

Results

There was significant difference in genotype distributions of rs2972607 and rs8112962 between CAD patients and control subjects. The rs2972607 was found to be associated with CAD in a dominant model (AA vs. AG + GG, P<0.001). Similarly, the rs8112962 was found to be associated with CAD in a dominant model (TT vs CT + CC, P=0.016). The difference remained statistically significant after multivariate adjustment (OR =1.401, 95% confidence interval [CI]:1.026~1.914, P=0.034; OR = 1.541, 95%CI:1.093~ 2.172, P=0.014; respectively).

Conclusion

The results of this study indicate that both rs2972607 and rs8112962 of C5L2 gene are associated with CAD in a Han population of China.

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.

Primary hypertriglyceridemia in children and adolescents

Primary disorders of lipid metabolism causing hypertriglyceridemia (HyperTG) result from genetic defects in triglyceride synthesis and metabolism. With the exception of lipoprotein lipase deficiency, these primary HyperTG disorders usually present in adulthood. However, some are unmasked earlier by precipitating factors, such as obesity and insulin resistance, and can be diagnosed in adolescence. Physical findings may be present and can include eruptive, palmer, or tuberoeruptive xanthomas. Triglyceride levels are very high to severe and can occur in the absence or the presence of other lipid abnormalities. Each of the causes of HyperTG is associated with an increased risk to develop recurrent pancreatitis and some may increase the risk of premature cardiovascular disease. Adoption of a healthy lifestyle that includes a low-fat diet, optimizing body weight, smoking avoidance/cessation, and daily physical activity is the first line of therapy. Pharmacologic therapies are available and can be beneficial in select disorders. Here, we review the causes of primary HyperTG in children and adolescents, discuss their clinical presentation and associated complications including the risk of pancreatitis and premature cardiovascular disease, and conclude with management and novel therapies currently in development. The goal of this article is to provide a useful resource for clinicians who may encounter primary HyperTG in the pediatric population.

A novel mutation in C5L2 gene was associated with hyperlipidemia and retinitis pigmentosa in a Chinese family

Background

Previous studies indicated that hyperlipidemia was associated with retinitis pigmentosa (RP). We aimed to identify the mutations in the C5L2 gene which was reported to be associated with hyperlipidemia in a Chinese family with (RP).

Methods

The Proband from the family was screened for mutations in the C5L2 gene that was known to cause hyperlipidemia. Cosegregation analysis was performed in the available family members. Linkage analysis was performed for one missense mutation to calculate the likelihood of its pathogenicity. One hundred and fifty unrelated, healthy Chinese subjects were screened to exclude nonpathogenic polymorphisms.

Results

By direct sequencing method, we identified a novel mutation (Thr196Asn) in C5L2 gene. In this family, each affected family members with RP showed a heterozygous mutation in the C5L2 gene. And all the carriers with heterozygous mutation have increased serum lipid levels in this family.

Conclusions

The present study has extended the mutation spectrum of C5L2, and Thr196Asn mutations in C5L2 were associated with RP and serum lipid levels.

The genetics of familial combined hyperlipidaemia

Almost 40 years after the first description of familial combined hyperlipidaemia (FCHL) as a discrete entity, the genetic and metabolic basis of this prevalent disease has yet to be fully unveiled. In general, two strategies have been applied to elucidate its complex genetic background, the candidate-gene and the linkage approach, which have yielded an extensive list of genes associated with FCHL or its related traits, with a variable degree of scientific evidence. Some genes influence the FCHL phenotype in many pedigrees, whereas others are responsible for the affected state in only one kindred, thereby adding to the genetic and phenotypic heterogeneity of FCHL. This Review outlines the individual genes that have been described in FCHL and how these genes can be incorporated into the current concept of metabolic pathways resulting in FCHL: adipose tissue dysfunction, hepatic fat accumulation and overproduction, disturbed metabolism and delayed clearance of apolipoprotein-B-containing particles. Genes that affect metabolism and clearance of plasma lipoprotein particles have been most thoroughly studied. The adoption of new traits, in addition to the classic plasma lipid traits, could aid in the identification of new genes implicated in other pathways in FCHL. Moreover, systems genetic analysis, which integrates genetic polymorphisms with data on gene expression levels, lipidomics or metabolomics, will attribute functions to genetic variants in addition to revealing new genes.

Genome-wide association studies in atherosclerosis

Cardiovascular disease remains the major cause of worldwide morbidity and mortality. Its pathophysiology is complex and multifactorial. Because the phenotype of cardiovascular disease often shows a marked heritable pattern, it is likely that genetic factors play an important role. In recent years, large genome-wide association studies have been conducted to decipher the molecular mechanisms underlying this heritable and prevalent phenotype. The emphasis of this review is on the recently identified 17 susceptibility loci for coronary artery disease. Implications of their discovery for biology and clinical medicine are discussed. A description of the landscape of human genetics in the near future in the context of next-generation sequence technologies is provided at the conclusion of this review.

Relationship between a novel polymorphism of the C5L2 gene and coronary artery disease

Background

C5L2 has been demonstrated to be a functional receptor of acylation-stimulating protein (ASP), which is a stimulator of triglyceride synthesis or glucose transport. However, little is known about the variations in the coding region of the C5L2 gene and their association with coronary artery disease (CAD).

Methodology/Principal Findings

We identified a novel single nucleotide polymorphism (SNP), 698C>T (P233L), in exon 2 using a polymerase chain reaction direct-sequencing method. This nucleotide change causes the amino-acid order from proline to leucine at codon 233. We examined the role of this SNP for CAD using two independent case–control studies: one was in the Han population (492 CAD patients and 577 control subjects) and the other was in the Uygur population (319 CAD patients and 554 control subjects). Heterozygote carriers of the 698CT genotype were more frequent among CAD patients than among controls not only in the Han population (7.3% versus 1.7%) but also in the Uygur population (4.7% versus 1.6%). The odds ratio (OR) for carriers of the 698CT genotype for CAD was 4.484 (95% confidence interval (CI): 2.197–9.174) in the Han group and 2.989 (95% CI: 1.292–6.909) in the Uygur population. After adjustment of confounding factors such as sex, age, smoking, alcohol consumption, hypertension, diabetes, as well as serum levels of triglyceride, total cholesterol, high-density lipoprotein, the difference remained significant in the Han group (P<0.001, OR = 6.604, 95% CI: 2.776–15.711) and in the Uygur group (P = 0.047, OR = 2.602, 95% CI: 1.015–6.671).

Conclusion/Significance

The 698CT genotype of C5L2 may be a genetic maker of CAD in the Han and Uygur population in western China.

Familial combined hyperlipidemia is associated with alterations in the cholesterol synthesis pathway

OBJECTIVE

Familial combined hyperlipidemia (FCH) is a common familial lipid disorder characterized by increases in plasma total cholesterol, triglyceride, and apolipoprotein B-100 levels. In light of prior metabolic and genetic research, our purpose was to ascertain whether FCH cases had significant abnormalities of plasma markers of cholesterol synthesis and absorption as compared to unaffected kindred members.

METHODS AND RESULTS

Plasma levels of squalene, desmosterol, and lathosterol (cholesterol synthesis markers) and campesterol, sitosterol, and cholestanol (cholesterol absorption markers) were measured by gas-liquid chromatography in 103 FCH patients and 240 normolipidemic relatives (NLR). Squalene, desmosterol, and lathosterol levels were 6% (0.078), 31%, (P<0.001) and 51% (P<0.001) higher in FCH as compared to NLR, and these differences were especially pronounced in women. An interaction with obesity was also noted for a subset of these markers. We did not observe any apparent differences for the cholesterol absorption markers among FCH patients and NLR.

CONCLUSIONS

Our data indicate that both men and women with FCH have alterations in the cholesterol synthesis pathway, resulting in 51% higher levels of lathosterol (and additionally desmosterol in women). Plasma levels of the cholesterol precursor sterol squalene were only slightly increased (6%), suggesting enhanced conversion of squalene to lathosterol in this disorder.

A nonsynonymous SNP within PCDH15 is associated with lipid traits in familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is a common lipid disorder characterized by the presence of multiple lipoprotein phenotypes that increase the risk of premature coronary heart disease. In a previous study, we identified an intragenic microsatellite marker within the protocadherin 15 (PCDH15) gene to be associated with high triglycerides (TGs) in Finnish dyslipidemic families. In this study we analyzed all four known nonsynonymous SNPs within PCDH15 in 1,268 individuals from Finnish and Dutch multigenerational families with FCHL. Association analyses of quantitative traits for SNPs were performed using the QTDT test. The nonsynonymous SNP rs10825269 resulted in a P = 0.0006 for the quantitative TG trait. Additional evidence for association was observed with the same SNP for apolipoprotein B levels (apo-B) (P = 0.0001) and total cholesterol (TC) levels (P = 0.001). None of the other three SNPs tested showed a significant association with any lipid-related trait. We investigated the expression of PCDH15 in different human tissues and observed that PCDH15 is expressed in several tissues including liver and pancreas. In addition, we measured the plasma lipid levels in mice with loss-of-function mutations in Pcdh15 (Pcdh15(av-Tg) and Pcdh15(av-3J)) to investigate possible abnormalities in their lipid profile. We observed a significant difference in plasma TG and TC concentrations for the Pcdh15(av-3J) carriers when compared with the wild type (P = 0.013 and P = 0.044, respectively). Our study suggests that PCDH15 is associated with lipid abnormalities.

Galanin preproprotein is associated with elevated plasma triglycerides

OBJECTIVE

There is increasing physiological evidence in rodents connecting the neuropeptide galanin to triglyceride (TG) levels. We hypothesized that variation in the galanin preproprotein (GAL) gene may contribute to hypertriglyceridemia (HTG) in humans.

METHODS AND RESULTS

We investigated GAL as a TG candidate gene by genotyping 4 tagSNPs in Dutch, Finnish, and Mexican familial combined hyperlipidemia (FCHL) families as well as in white combined hyperlipidemia cases/controls (n=2471). The common allele of rs2187331, residing in the promoter region of GAL, was significantly associated with HTG (probability value=0.00038). In an unascertained population sample of 4463 Finnish males, the rare allele of rs2187331 was associated with higher TGs (probability value=0.0028 to 0.00016). We also observed an allele specific difference with rs2187331 in reporter gene expression and nuclear factor binding in vitro. Furthermore, we detected differential expression of many key lipid genes in adipose tissue based on rs2187331 genotypes.

CONCLUSIONS

The SNP rs2187331 is associated with HTG in FCHL and white combined hyperlipidemia cases/controls and influences TG levels in the population. Further studies are warranted to elucidate the allelic difference observed between FCHL and the general population. Functional evidence shows that rs2187331 has an allele specific cis-regulatory function and influences the expression of lipid related genes in adipose.

Abnormal Igf2 gene in Prague hereditary hypertriglyceridemic rats: its relation to blood pressure and plasma lipids

Prague hypertriglyceridemic (HTG) rats represent a suitable model of metabolic syndrome. We have established the set of F(2) hybrids derived from HTG and Lewis progenitors to investigate the relationship between respective polymorphism(s) of Igf2 gene and blood pressure (BP) or other cardiovascular phenotypes. HTG rats had elevated systolic BP and plasma triglycerides but lower plasma cholesterol compared to Lewis rats of both genders. In males, there was higher mean arterial pressure, diastolic BP and relative heart weight in HTG than in Lewis rats. The results obtained in the total population of F(2) hybrids indicated strong segregation of Igf2 genotype with plasma triglycerides. There was no segregation of Igf2 genotype with any BP component except BP changes occurring after the blockade of either renin-angiotensin system (RAS) or NO synthase. When F(2) population was analyzed according to gender, male F(2) progeny homozygous for HTG Igf2 allele had significantly higher plasma triglycerides and greater BP changes after NO synthase blockade than those homozygous for Lewis allele. On the contrary, male F(2) progeny homozygous for HTG Igf2 allele had significantly lower plasma cholesterol and smaller BP changes after RAS blockade. PCR analysis of Igf2 gene by using of microsatelite D1Mgh22 has shown polymorphism between HTG and Lewis rats. Sequence analysis of cDNA revealed insertion of 14 nucleotides in HTG gene. In conclusion, polymorphism in Igf2 gene may be responsible for differences in lipid metabolism between HTG and Lewis rats. It remains to determine how these abnormalities could be involved in BP regulation by particular vasoactive systems.

A pleiotropic QTL on 2p influences serum Lp-PLA2 activity and LDL cholesterol concentration in a baboon model for the genetics of atherosclerosis risk factors

Lipoprotein-associated phospholipase A(2) (Lp-PLA(2)), the major portion of which is bound to low-density lipoprotein, is an independent biomarker of cardiovascular disease risk. To search for common genetic determinants of variation in both Lp-PLA(2) activity and LDL cholesterol (LDL-C) concentration, we assayed these substances in serum from 679 pedigreed baboons. Using a maximum likelihood-based variance components approach, we detected significant evidence for a QTL affecting Lp-PLA(2) activity (LOD=2.79, genome-wide P=0.039) and suggestive evidence for a QTL affecting LDL-C levels (LOD=2.16) at the same location on the baboon ortholog of human chromosome 2p. Because we also found a significant genetic correlation between the two traits (rho(G)=0.50, P<0.00001), we conducted bivariate linkage analyses of Lp-PLA(2) activity and LDL-C concentration. These bivariate analyses improved the evidence (LOD=3.19, genome-wide P=0.015) for a QTL at the same location on 2p, corresponding to the human cytogenetic region 2p24.3-p23.2. The QTL-specific correlation between the traits (rho(Q)=0.62) was significantly different from both zero and 1 (P[rho(Q)=0]=0.047; P[rho(Q)=1]=0.022), rejecting the hypothesis of co-incident linkage and consistent with incomplete pleiotropy at this locus. We conclude that polymorphisms at the QTL described in this study exert some genetic effects that are shared between Lp-PLA(2) activity and LDL-C concentration.

Detailed molecular characterization of cord blood-derived endothelial progenitors

OBJECTIVE

Given their involvement in pathological and physiological angiogenesis, there has been growing interest in understanding and manipulating endothelial progenitor cells (EPC) for therapeutic purposes. However, detailed molecular analysis of EPC before and during endothelial differentiation is lacking and is the subject of the present study.

MATERIALS AND METHODS

We report a detailed microarray gene-expression profile of freshly isolated (day 0) human cord blood (CB)-derived EPC (CD133+KDR+ or CD34+KDR+), and at different time points during in vitro differentiation (early: day 13; late: day 27).

RESULTS

Data obtained reflect an EPC transcriptome enriched in genes related to stem/progenitor cells properties (chromatin remodeling, self-renewal, signaling, cytoskeleton organization and biogenesis, recruitment, and adhesion). Using a complementary DNA microarray enriched in intronic transcribed sequences, we observed, as well, that naturally transcribed intronic noncoding RNAs were specifically expressed at the EPC stage.

CONCLUSION

Taken together, we have defined the global gene-expression profile of CB-derived EPC during the process of endothelial differentiation, which can be used to identify genes involved in different vascular pathologies.

Haplotype analyses of the APOA5 gene in patients with familial combined hyperlipidemia

BACKGROUND

Familial combined hyperlipidemia (FCH) is the most common genetic lipid disorder with an undefined genetic etiology. Apolipoprotein A5 gene (APOA5) variants were previously shown to contribute to FCH. The aim of the present study was to evaluate the association of APOA5 variants with FCH and its related phenotypes in Dutch FCH patients. Furthermore, the effects of variants in the APOA5 gene on carotid intima-media thickness (IMT) and cardiovascular disease (CVD) were examined.

MATERIALS AND METHODS

The study population consisted of 36 Dutch families, including 157 FCH patients. Two polymorphisms in the APOA5 gene (-1131T>C and S19W) were genotyped.

RESULTS

Haplotype analysis of APOA5 showed an association with FCH (p=0.029), total cholesterol (p=0.031), triglycerides (p<0.001), apolipoprotein B (p=0.011), HDL-cholesterol (p=0.013), small dense LDL (p=0.010) and remnant-like particle cholesterol (p=0.001). Compared to S19 homozygotes, 19W carriers had an increased risk of FCH (OR=1.6 [1.0-2.6]; p=0.026) and a more atherogenic lipid profile, reflected by higher triglyceride (+22%) and apolipoprotein B levels (+5%), decreased HDL-cholesterol levels (-7%) and an increased prevalence of small dense LDL (16% vs. 26%). In carriers of the -1131C allele, small dense LDL was more prevalent than in -1131T homozygotes (29% vs. 16%). No association of the APOA5 gene with IMT and CVD was evident.

CONCLUSION

In Dutch FCH families, variants in the APOA5 gene are associated with FCH and an atherogenic lipid profile.

The involvement of upstream stimulatory factor 1 in Dutch patients with familial combined hyperlipidemia

Recently, the upstream stimulatory factor 1 gene (USF1) was proposed as a candidate gene for familial combined hyperlipidemia (FCH). In this study, we examined the previously identified risk haplotype of USF1 with respect to FCH and its related phenotypes in 36 Dutch FCH families. The diagnosis of FCH was based on both the traditional diagnostic criteria and a nomogram. The two polymorphisms, USF1s1 and USF1s2, were in complete linkage disequilibrium. No association was found for the individual single nucleotide polymorphisms (SNPs) with FCH defined by the nomogram (USF1s1, P = 0.53; USF1s2, P = 0.53), whereas suggestive associations were found when using the traditional diagnostic criteria for FCH (USF1s1, P = 0.08; USF1s2, P = 0.07). USF1 was associated with total cholesterol (USF1s1, P = 0.05; USF1s2, P = 0.04) and apolipoprotein B (USF1s1, P = 0.06; USF1s2, P = 0.04). Small dense LDL showed a suggestive association (USF1s1, P = 0.10; USF1s2, P = 0.09). The results from the haplotype analyses supported the results obtained for the individual SNPs. In conclusion, the previously identified risk haplotype of USF1 showed a suggestive association with FCH and contributed to the related lipid traits in our Dutch FCH families.

Identification of a Novel C5L2 Variant (S323I) in a French Canadian Family With Familial Combined Hyperlipemia

Objective— A functional acylation stimulating protein (ASP) receptor, C5L2, has been recently identified in ASP-responsive cells. Impaired ASP-mediated triglyceride synthesis has previously been described in a subset of hyperapolipoprotein B/familial combined hyperlipidemia subjects.

Methods and Results— DNA sequencing of C5L2 coding region in 61 unrelated probands identified a heterozygous variant (G968→T) in 1 subject, resulting in Ser323→Ile substitution in the carboxyl terminal region. This variant was not detected in 2176 additional chromosomes by restriction fragment length polymorphism or fluorescence polarization genotyping. Eight family members of the proband were identified with one altered (+/−)C5L2 allele. Nine other family members had the wild-type (+/+)C5L2 sequence. The abnormal allele was associated with increased plasma triglyceride, plasma cholesterol, low-density lipoprotein (LDL) cholesterol, apolipoprotein B and ASP. Of 23 subjects tested in cell-based ASP bioactivity assays, those with C5L2(+/−) variant (n=2) had a 50% reduction in ASP-stimulated triglyceride synthesis, glucose transport and marked reduction in maximal binding (B max ). By contrast, a C5L2(+/+) family member responded normally, as did hyperapolipoprotein B normal ASP subjects compared with C5L2(+/+) controls (n=6).

Conclusion— The S323I variant may alter C5L2 function and might be one molecular basis contributing to familial combined hyperlipidemia.

Genetics of familial combined hyperlipidemia

PURPOSE OF REVIEW

To provide an overview of recent advances that have defined the first putative genes behind familial combined hyperlipidemia, the most common genetic dyslipidemia and a major risk factor for early coronary heart disease.

RECENT FINDINGS

The first locus for familial combined hyperlipidemia on 1q21-23 revealed a gene encoding a transcription factor critical in lipid and glucose metabolism, USF1. All the associated variants represent noncoding single nucleotide polymorphisms, one of which affects the binding site of nuclear proteins with a putative effect on transcript levels of USF1. Transcript analyses of fat biopsies have exposed risk-allele related changes in the downstream genes. Another recent clue to the molecular pathogenesis of familial combined hyperlipidemia is the association of the high triglyceride trait with the APOA5 gene, located on 11q. More familial combined hyperlipidemia genes are expected to be found, since linkage evidence exists for additional loci on 16q24 and 20q12-q13.1.

SUMMARY

Genetic research of familial combined hyperlipidemia families has revealed several linked loci guiding to susceptibility genes. The USF1 transcription factor is the major gene underlying the 1q21-23 linkage. Modifying genes, especially influencing the high triglyceride trait, include APOC3 and APOA5, the latter representing a downstream target of USF1 and implying a USF1-dependent pathway in the molecular pathogenesis of dyslipidemias.

Augmented levels of CD44 in macrophages from atherosclerotic subjects: a possible IL-6-CD44 feedback loop?

The cell-adhesion molecule CD44 likely participates in atherosclerosis development. We have shown previously that pro-inflammatory cytokines affect CD44 expression. Therefore, this work examined the role of elevated CD44 levels in human macrophages. Macrophages from human atherosclerotic subjects (n=15) showed elevated levels of CD44 transcript and protein (1.5-fold) compared to matched controls (n=15) (P=0.050 and 0.044, respectively). To test whether genetic factors influence CD44 expression, two single nucleotide polymorphisms in the CD44 gene were analyzed but these were not associated with coronary artery disease. We also examined the potential connection between plasma cytokine levels and CD44 expression. In atherosclerotic subjects, elevated CD44 expression correlates (P=0.012) with enhanced macrophage IL-6 secretion (3.13+/-2.5 pg/mL versus 0.32+/-0.16 pg/mL in controls, P=0.021). Additionally, CD44-deficient mice exhibit less circulating IL-6 than wild-type controls (9.8+/-0.7 pg/mL versus 14.3+/-0.7 pg/mL; P=0.032). Furthermore, IL-6 augments CD44 expression in primary human macrophages after 24 h (P=0.038) and 48 h (P=0.015). Taken together, our data show an IL-6-CD44 feedback loop in macrophages. Such a positive feedback loop may aggravate atherosclerosis development.

Familial combined hyperlipidemia: upstream transcription factor 1 and beyond

PURPOSE OF REVIEW

Familial combined hyperlipidemia is a common complex disease that accounts for up to 20% of premature coronary heart disease. The upstream transcription factor 1, located on 1q21, was recently shown to be linked and associated with familial combined hyperlipidemia in Finnish families. Upstream transcription factor 1 is the first gene identified by positional cloning for familial combined hyperlipidemia. Replication studies are critical to investigation of complex diseases because only they can verify the importance of the original findings. We review recent studies that examine the genetic contribution and functional consequence of upstream transcription factor 1 variants to familial combined hyperlipidemia and type 2 diabetes mellitus. Aiming beyond upstream transcription factor 1, we also evaluate novel strategies that have made it possible to globally examine the genome and the transcriptome.

RECENT FINDINGS

Three independent studies support the role of upstream transcription factor 1 in familial combined hyperlipidemia. The results for type 2 diabetes mellitus and the metabolic syndrome have been less conclusive highlight novel strategies for gene identification in familial combined hyperlipidemia.

SUMMARY

Currently, genetic and functional evidence is supportive of a role for upstream transcription factor 1 in the etiology of familial combined hyperlipidemia and its component traits, although the mechanism of causality still remains largely unknown.

Linkage analysis of quantitative traits for obesity, diabetes, hypertension, and dyslipidemia on the island of Kosrae, Federated States of Micronesia

Obesity, diabetes, hypertension, and heart disease are highly heritable conditions that in aggregate are the major causes of morbidity and mortality in the developed world and are growing problems in developing countries. To map the causal genes, we conducted a population screen for these conditions on the Pacific Island of Kosrae. Family history and genetic data were used to construct a pedigree for the island. Analysis of the pedigree showed highly significant heritability for the metabolic traits under study. DNA samples from 2,188 participants were genotyped with 405 microsatellite markers with an average intermarker distance of 11 cM. A protocol using loki, a Markov chain Monte Carlo sampling method, was developed to analyze the Kosraen pedigree for height, a model quantitative trait. Robust quantitative trait loci for height were found on 10q21 and 1p31. This protocol was used to map a set of metabolic traits, including plasma leptin to chromosome region 5q35; systolic blood pressure to 20p12; total cholesterol to 19p13, 12q24, and 16qter; hip circumference to 10q25 and 4q23; body mass index to 18p11 and 20q13; apolipoprotein B to 2p24-25; weight to 18q21; and fasting blood sugar to 1q31-1q43. Several of these same chromosomal regions have been identified in previous studies validating the use of loki. These studies add information about the genetics of the metabolic syndrome and establish an analytical approach for linkage analysis of complex pedigrees. These results also lay the foundation for whole genome scans with dense sets of SNPs aimed to identifying causal genes.

Unraveling the complex genetics of familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) constitutes a substantial risk factor for atherosclerosis since it is observed in about 20% of coronary heart disease (CHD) patients under 60 years. FCHL, characterized by elevated levels of total cholesterol (TC) and triglycerides (TGs), or both, is also one of the most common familial hyperlipidemias with a prevalence of 1%-6% in Western populations. Numerous studies have been performed to identify genes contributing to FCHL. The recent linkage and association studies and their replications are beginning to elucidate the genetic variations underlying the susceptibility to FCHL. Three chromosomal regions on 1q21-23, 11p and 16q22-24.1 have been replicated in different study samples, offering targets for gene hunting. In addition, several candidate gene studies have replicated the influence of the lipoprotein lipase (LPL) gene and apolipoprotein A1/C3/A4/A5 (APOA1/C3/A4/A5) gene cluster in FCHL. Recently, the linked region on chromosome 1q21 was successfully fine-mapped and the upstream transcription factor 1 (USF1) gene identified as the underlying gene for FCHL. This finding has now been replicated in independent FCHL samples. However, the total number of variants, the risk related to each variant and their relative contributions to the disease susceptibility are not known yet.

Up-regulation of CD36/FAT in preadipocytes in familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) shows many features of the metabolic syndrome. The strong genetic component makes it an excellent model to study the genetic background of metabolic syndrome and insulin resistance. Adipose tissue is believed to contribute to, or even underlie, the FCHL phenotype and is an interesting target tissue for gene expression studies. However, interpretation of adipose tissue gene expression experiments is complex since expression differences cannot only arise as a direct consequence of a genetic trait, but may also reflect an adaptation to metabolic influences at the cellular level. In the present study, we measured gene expression levels in cultured primary human preadipocytes from FCHL and control subjects. Since isolated preadipocytes were allowed to replicate for weeks under standardized conditions, the contribution of previous metabolic influences is rather small whereas genetic defects are preserved and expressed in vitro. The main finding was up-regulation of CD36/FAT in FCHL preadipocytes, confirmed in two independent groups of subjects, and a concomitant increase in CD36/FAT-mediated fatty acid uptake. CD36/FAT overexpression has previously been shown to be associated with other insulin-resistant states. The present data suggest that CD36/FAT overexpression in FCHL occurs very early in adipocyte differentiation and may be of genetic origin.

Analysis of quantitative lipid traits in the genetics of NIDDM (GENNID) study

Coronary heart disease (CHD) is the leading cause of death among individuals with type 2 diabetes. Dyslipidemia contributes significantly to CHD in diabetic patients, in whom lipid abnormalities include hypertriglyceridemia, low HDL cholesterol, and increased levels of small, dense LDL particles. To identify genes for lipid-related traits, we performed genome-wide linkage analyses for levels of triglycerides and HDL, LDL, and total cholesterol in Caucasian, Hispanic, and African-American families from the Genetics of NIDDM (GENNID) study. Most lipid traits showed significant estimates of heritability (P < 0.001) with the exception of triglycerides and the triglyceride/HDL ratio in African Americans. Variance components analysis identified linkage on chromosome 3p12.1-3q13.31 for the triglyceride/HDL ratio (logarithm of odds [LOD] = 3.36) and triglyceride (LOD = 3.27) in Caucasian families. Statistically significant evidence for linkage was identified for the triglyceride/HDL ratio (LOD = 2.45) on 11p in Hispanic families in a region that showed suggestive evidence for linkage (LOD = 2.26) for triglycerides in this population. In African Americans, the strongest evidence for linkage (LOD = 2.26) was found on 19p13.2-19q13.42 for total cholesterol. Our findings provide strong support for previous reports of linkage for lipid-related traits, suggesting the presence of genes on 3p12.1-3q13.31, 11p15.4-11p11.3, and 19p13.2-19q13.42 that may influence traits underlying lipid abnormalities associated with type 2 diabetes.

Multiple QTLs influencing triglyceride and HDL and total cholesterol levels identified in families with atherogenic dyslipidemia

We conducted a genome-wide scan using variance components linkage analysis to localize quantitative-trait loci (QTLs) influencing triglyceride (TG), high density lipoprotein-cholesterol (HDL-C), low density lipoprotein-cholesterol, and total cholesterol (TC) levels in 3,071 subjects from 459 families with atherogenic dyslipidemia. The most significant evidence for linkage to TG levels was found in a subset of Turkish families at 11q22 [logarithm of the odds ratio (LOD)=3.34] and at 17q12 (LOD=3.44). We performed sequential oligogenic linkage analysis to examine whether multiple QTLs jointly influence TG levels in the Turkish families. These analyses revealed loci at 20q13 that showed strong epistatic effects with 11q22 (conditional LOD=3.15) and at 7q36 that showed strong epistatic effects with 17q12 (conditional LOD=3.21). We also found linkage on the 8p21 region for TG in the entire group of families (LOD=3.08). For HDL-C levels, evidence of linkage was identified on chromosome 15 in the Turkish families (LOD=3.05) and on chromosome 5 in the entire group of families (LOD=2.83). Linkage to QTLs for TC was found at 8p23 in the entire group of families (LOD=4.05) and at 5q13 in a subset of Turkish and Mediterranean families (LOD=3.72). These QTLs provide important clues for the further investigation of genes responsible for these complex lipid phenotypes. These data also indicate that a large proportion of the variance of TG levels in the Turkish population is explained by the interaction of multiple genetic loci.

Familial Combined Hyperlipidemia in Mexicans

OBJECTIVE

To investigate the largely unknown genetic component of the common lipid disorder, familial combined hyperlipidemia (FCHL) in Mexicans, we analyzed the upstream transcription factor 1 (USF1) gene that was recently associated with FCHL and high triglycerides (TG) in Finns. We also analyzed the Mexican FCHL families for 26 microsatellite markers residing in the seven chromosomal regions on 2p25.1, 9p23, 10q11.23, 11q13, 16q24.1, 19q13, and 21q21, previously linked to FCHL in whites.

METHODS AND RESULTS

We genotyped 314 individuals in 24 Mexican families for 13 SNPs spanning an 88-kb region, including USF1. The FCHL and TG traits showed significant evidence for association with 3 SNPs, hCV1459766, rs3737787, and rs2073658, and haplotype analyses further supported these findings (probability values of 0.05 to 0.0009 for SNPs and their haplotypes). Of these SNPs, hCV1459766 is located in the F11 receptor (F11R) gene, located next to USF1, making it difficult to exclude. Importantly, the association was restricted to a considerably smaller region than in the Finns (14 kb versus 46 kb), possibly because of a different underlying linkage disequilibrium structure. In addition, 1 of the 7 regions, 16q24.1, showed suggestive evidence for linkage (a lod score of 2.6) for total cholesterol in Mexicans.

CONCLUSIONS

This study, the first to extensively investigate the genetic component of the common FCHL disorder in Mexicans, provides independent evidence for the role of USF1 in FCHL in an outbred population and links the 16q24.1 region to an FCHL-component trait in Mexicans.

Genome-wide linkage scans for prediabetes phenotypes in response to 20 weeks of endurance exercise training in non-diabetic whites and blacks: the HERITAGE Family Study

AIMS/HYPOTHESIS

Impaired insulin secretion, insulin action, insulin-independent glucose effectiveness, glucose tolerance and the associated abnormalities in insulin and glucose metabolism phenotypes are precursors of type 2 diabetes. Genome-wide multipoint variance component linkage scans were carried out using 654 markers to identify quantitative trait loci for insulin sensitivity, acute insulin response to glucose, disposition index and glucose effectiveness training responses in whites and blacks in the HERITAGE Family Study.

METHODS

These phenotypes were obtained from an IVGTT with the minimal model. The distributions of insulin sensitivity, acute insulin response to glucose and disposition index training responses (post-training minus baseline) were approximately normalised using a square-root transformation. All phenotypes were adjusted for the effects of age, BMI and their respective baseline values within sex and generation by race prior to linkage scans.

RESULTS

In blacks, a promising linkage with a maximum lod score of 3.1 on 19q (54-62 Mb) for glucose effectiveness training response was found. Six interesting linkages with lod scores of at least 1.0 were found for disposition index training response in whites. They included 1p (30 Mb), 3q (152 Mb), 6p (23-42 Mb), 7q (95-96 Mb), 10p (15 Mb) and 12q (119-126 Mb).

CONCLUSIONS/INTERPRETATION

Quantitative trait loci for 20 weeks of endurance exercise training responses in insulin action and glucose metabolism phenotypes were found on chromosome 19q as well as 6p and 7q, with nominal (6p, 7q) but consistent (6p) linkages across the races.

Interaction between obesity-susceptibility loci in chromosome regions 2p25-p24 and 13q13-q21

One of the chief complexities of genetic influences on human obesity appears to be gene-gene interactions. Here, we employed model-free approaches to look for gene-gene interaction effects in human obesity using genome scan data from 260 European American families. We found consistent evidence for statistical interaction between 2p25-p24 (18-38 cM) and 13q13-q21 (26-47 cM). For discrete traits, the positive correlations were significant at P<0.0001 (P</=0.0023 after correction for multiple tests) in both IBD-based and NPL-based analyses for BMI>/=40 kg/m(2). Other analytic approaches gave consistent, supportive results. For quantitative traits, interaction effects were significant for BMI (P=0.0012), percent fat (P=0.0265) and waist circumference (P=0.0023) in a Haseman-Elston regression model, and for BMI (P=0.0043) in variance component analysis. Our findings suggest that obesity-susceptibility loci in chromosome regions 2p25-p24 and 13q13-21 may interact to influence extreme human obesity. The identification of gene-gene interactions may prove crucial to understanding the contributions of genes, which, by themselves, have relatively small effects on obesity susceptibility and resistance.

Quantitative Trait Loci for Apolipoprotein B, Cholesterol, and Triglycerides in Familial Combined Hyperlipidemia Pedigrees

OBJECTIVE

Familial combined hyperlipidemia (FCHL) is a genetically complex lipid disorder that is diagnosed in families by combinations of increased cholesterol, triglycerides, and/or apolipoprotein B (apoB) levels in patients and their first-degree relatives. Identifying the predisposing genes promises to reveal the primary risk factors and susceptibility pathways and suggest methods of prevention and treatment. As with most genetically complex disorders, a clinical definition of disease may not be the most useful phenotype for finding the complement of predisposing genes, and the quantitative traits used to define the disorder can provide important information. This is a report of a quantitative trait loci (QTL) analysis of FCHL.

METHODS AND RESULTS

A full genome scan of 377 multi-allelic markers genotyped at approximately 10 centimorgan (cM) intervals was conducted in 150 sibling pairs from 22 nuclear families in FCHL pedigrees. These data were analyzed by 2 multipoint QTL linkage methods using the nonparametric and Haseman-Elston procedures of the Genehunter software. Using a criterion of P<0.001 by the nonparametric analysis, we found evidence of 2 apoB QTL at 1p21-31 (P<0.000009) and 17p11-q21 (P<0.000009), a total serum cholesterol QTL at 12p13 (P<0.0001), and a serum triglycerides QTL at 4p15-16 (P<0.0002). Using the criterion of P<0.03 for at least 2 traits at the same locus, additional evidence for cholesterol (P<0.01) and a triglycerides P<0.02) was observed at 17p11-21, as well as suggestive evidence for apoB (P<0.02) and triglycerides (P<0.01) at 4q34-35, and cholesterol (P<0.01) and triglycerides (P<0.02) and a binary FCHL trait (lod=1.5) at 16p12-13.

CONCLUSIONS

QTL analyses of the traits that define FCHL are effective for localizing disease-predisposing genes.

Compendium of genome-wide scans of lipid-related phenotypes: adding a new genome-wide search of apolipoprotein levels

The genetic dissection of complex inherited diseases is a major challenge. Despite limited success in finding genes, substantial data based on genome-wide scan strategies is now available for a variety of diseases and related phenotypes. This can perhaps best be appreciated in the field of lipid and lipoprotein levels, where the amount of information generated is becoming overwhelming. We have created a database containing the results from whole-genome scans of lipid-related phenotypes undertaken to date. The usefulness of this database is demonstrated by performing a new autosomal genomic scan on apolipoprotein B (apoB), LDL-apoB, and apoA-I levels, measured in 679 subjects of 243 nuclear families. Linkage was tested using both allele-sharing and variance-component methods. Only two loci provided support for linkage with both methods: a LDL-apoB locus on 18q21.32 and an apoA-I locus on 3p25.2. Adding those findings to the database highlighted the fact that the former is reported as a lipid-related locus for the first time, whereas the latter has been observed before. However, concerns arise when displaying all data on the same map, because a large portion of the genome is now covered with loci supported by at least suggestive evidence of linkage.

Contribution of Chromosome 1q21-q23 to Familial Combined Hyperlipidemia in Mexican Families

Familial combined hyperlipidemia (FCHL) is the most common familial dyslipidemia, with a prevalence of 1-2% in the general population. A major locus for FCHL has been mapped to chromosome 1q21-q23 in Finnish, Chinese, German and US families. We studied seven extended Mexican families with 153 members, including 64 affected subjects. A total of 11 markers were genotyped, including D1S104 which has been linked to FCHL in other studies. Two point linkage analysis for the FCHL phenotype, and for the elevated triglyceride (TG) trait, allowing for heterogeneity, gave a maximum HLOD of 1.67 (alpha = 0.49) and 1.93 (alpha = 0.43) at D1S2768 (2.69 cM proximal to D1S104) respectively. Heterogeneity and non-parametric (NPL) multipoint analyses for the FCHL phenotype and the TG trait showed maximum HLODs of 1.27 (alpha = 0.46) and 1.64 (alpha = 0.38), and NPLs of 4.00 (P = 0.0001) and 3.68 (P = 0.0003) near D1S2768, respectively. In addition, analysis of four candidate genes putatively involved in the expression of FCHL showed no evidence of linkage for the LCAT gene or the APOA1/C3/A4/A5 gene cluster. However, we cannot exclude the participation of these genes, or the LIPC and LPL genes, as minor susceptibility loci in the expression of FCHL, or the TG or elevated total cholesterol (TC) traits in our families. In conclusion, our data confirm the involvement of a major susceptibility locus on chromosome 1q21-q23 in FCHL Mexican families, consistent with findings in other populations.

A genome-wide scan of serum lipid levels in the Old Order Amish

Elevated serum low density lipoprotein cholesterol (LDL-C) and triglyceride (TG) and decreased high density lipoprotein cholesterol (HDL-C) levels are established risk factors for cardiovascular disease (CVD). To identify quantitative trait loci influencing lipid levels, we conducted genome-wide linkage analyses of total serum cholesterol (TSC), HDL-C, ln-transformed TG (LNTG) and LDL-C levels in 612 individuals from 28 families of the Amish Family Diabetes Study (AFDS). Subjects were genotyped for 373 microsatellite markers covering all 22 autosomes and the X chromosome at an average density of 9.7 centimorgans. All lipid traits exhibited moderate estimated heritability (h2 +/- S.E.): TSC, 0.63 +/- 0.11; HDL-C, 0.54 +/- 0.08; LNTG, 0.37 +/- 0.08; LDL-C, 0.62 +/- 0.10. The highest logarithm of the odds (LOD) score observed was 2.47 (P = 0.0003), at 3p25 for LDL-C. LOD scores exceeding 2.0 (P < 0.001) were also observed at 2p23 (LOD = 2.17) and 19p13 (LOD = 2.23) for LDL-C, and at 11q23 (LOD = 2.03) for LNTG. Three additional regions exhibited LOD scores greater than 1.5, corresponding to a P-value of <0.005. Many of the regions suggestively linked in this genome-wide scan contain genes encoding proteins with established roles in lipid metabolism, including apolipoproteins, peroxisome proliferater-activated receptor-gamma and the LDL receptor.

Apolipoprotein A-II, genetic variation on chromosome 1q21-q24, and disease susceptibility

PURPOSE OF REVIEW

Apolipoprotein (apo) A-II is the second most abundant HDL apolipoprotein; however its function remains largely unknown. Owing to the lack of consequences of apoA-II deficiency in humans, it has long been considered an apolipoprotein of minor importance. Overexpression of apoA-II in transgenic mice, however, causes combined hyperlipidemia and, in some cases, insulin resistance. This, and the location of the apoA-II gene in chromosome 1q23, a hot region in the search for genes associated with familial combined hyperlipidemia, insulin resistance and type 2 diabetes mellitus, has greatly increased interest in this protein.

RECENT FINDINGS

ApoA-II is biochemically and genetically linked to familial combined hyperlipidemia. Given that the chromosome 1q21-q24 region is associated with insulin resistance or type 2 diabetes, this region is a now a focus of interest in the study of these complex, often overlapping diseases. However, no polymorphisms that increase apoA-II levels have been identified to date in humans. Other nonstructural loci may regulate apoA-II plasma concentration. Further, plasma apoA-II concentration is increased by saturated fat intake. Several reports have added to our understanding of the relationship between apoA-II mutations and amyloidosis both in humans and mice.

SUMMARY

An increased plasma concentration of apoA-II might contribute to familial combined hyperlipidemia or type 2 diabetes mellitus expression, which emphasizes the need to understand its function and metabolism. Genetic studies in well characterized patients and genomic and proteomic approaches in cell and mouse models may help to achieve this understanding.

Genetic contributors to lipoprotein cholesterol levels in an intercross of 129S1/SvImJ and RIIIS/J inbred mice

To determine the genetic contribution to variation among lipoprotein cholesterol levels, we performed quantitative trait locus (QTL) analyses on an intercross between mouse strains RIIIS/J and 129S1/SvImJ. Male mice of the parental strains and the reciprocal F1and F2populations were fed a high-cholesterol, cholic acid-containing diet for 8–12 wk. At the end of the feeding period, plasma total, high-density lipoprotein (HDL), and non-HDL cholesterol were determined. For HDL cholesterol, we identified three significant QTLs on chromosomes (Chrs) 1 ( D1Mit507, 88 cM, 72–105 cM, 4.8 LOD), 9 ( D11Mit149, 14 cM, 10–25 cM, 9.4 LOD), and 12 ( D12Mit60, 20 cM, 0–50 cM, 5.0 LOD). These QTLs were considered identical to QTLs previously named Hdlq5, Hdlq17, and Hdlq18, respectively, in crosses sharing strain 129. For total cholesterol, we identified two significant QTLs on Chrs 1 and 9, which were named Chol10 ( D1Mit507, 88 cM, 10–105 cM, 3.9 LOD) and Chol11 ( D11Mit149, 14 cM, 0–30 cM, 4.4 LOD), respectively. In addition, for total cholesterol, we identified two suggestive QTLs on Chrs 12 (distal) and 17, which remain unnamed. For non-HDL cholesterol, we identified and named one new QTL on Chr 17, Nhdlq3 ( D17Mit221, 58 cM, 45–60 cM, 3.4 LOD). Nhdlq3 colocalized with orthologous human QTLs for lipoprotein phenotypes, and with Abcg5 and Abcg8. Overall, we detected eight QTLs for lipoprotein cholesterol concentrations on Chrs 1, 9, 12, and 17 (each two per chromosome), including a new QTL for non-HDL cholesterol, Nhdlq3, on Chr 17.

Rat model of familial combined hyperlipidemia as a result of comparative mapping

Total genome scan was carried out in 266 F2intercrosses from the Prague hypertriglyceridemic (HTG) rat that shares several clinical characteristics with human metabolic syndrome. Two loci for plasma triglycerides (TG) were localized on chromosome 2 (Chr 2) (LOD 4.4, 3.2). The first locus overlapped with the rat syntenic region of the human locus for the metabolic syndrome and for small, dense LDL, while the second overlapped with the syntenic region of another locus for small, dense LDL in humans by the comparative mapping approach. Loci for TG on rat Chr 13 (LOD 3.3) and Chr 1 (LOD 2.7) overlapped with the syntenic region of loci for human familial combined hyperlipidemia (FCHL) in Finnish and Dutch populations, respectively. The concordances of loci for TG localized in this study with previously reported loci for FCHL and its related phenotypes are underlying the generalized importance of these loci in dyslipidemia. These data suggest the close relationship between dyslipidemia in HTG rats and human FCHL, establishing a novel animal model for exploration of pathophysiology and therapy based on genomic determinants.

Empirically derived phenotypic subgroups - qualitative and quantitative trait analyses

Background

The Framingham Heart Study has contributed a great deal to advances in medicine. Most of the phenotypes investigated have been univariate traits (quantitative or qualitative). The aims of this study are to derive multivariate traits by identifying homogeneous groups of people and assigning both qualitative and quantitative trait scores; to assess the heritability of the derived traits; and to conduct both qualitative and quantitative linkage analysis on one of the heritable traits.

Methods

Multiple correspondence analysis, a nonparametric analogue of principal components analysis, was used for data reduction. Two-stage clustering, using both k-means and agglomerative hierarchical clustering, was used to cluster individuals based upon axes (factor) scores obtained from the data reduction. Probability of cluster membership was calculated using binary logistic regression. Heritability was calculated using SOLAR, which was also used for the quantitative trait analysis. GENEHUNTER-PLUS was used for the qualitative trait analysis.

Results

We found four phenotypically distinct groups. Membership in the smallest group was heritable (38%, p < 1 × 10^-6) and had characteristics consistent with atherogenic dyslipidemia. We found both qualitative and quantitative LOD scores above 3 on chromosomes 11 and 14 (11q13, 14q23, 14q31). There were two Kong & Cox LOD scores above 1.0 on chromosome 6 (6p21) and chromosome 11 (11q23).

Conclusion

This approach may be useful for the identification of genetic heterogeneity in complex phenotypes by clarifying the phenotype definition prior to linkage analysis. Some of our findings are in regions linked to elements of atherogenic dyslipidemia and related diagnoses, some may be novel, or may be false positives.

Search for genetic factors predisposing to atherogenic dyslipidemia

Background

Atherogenic dyslipidemia (AD) is a common feature in persons with premature coronary heart disease. While several linkage studies have been carried out to dissect the genetic etiology of lipid levels, few have investigated the AD lipid triad comprising elevated serum triglyceride, small low density lipoprotein (LDL) particles, and reduced high density lipoprotein (HDL) cholesterol levels. Here we report the results of a whole-genome screen for AD using the Framingham Heart Study population.

Results

Our analyses provide some evidence for linkage to AD on chromosomes 1q31, 3q29, 10q26, 14p12, 14q13, 16q24, 18p11, and 19q13.

Conclusion

AD susceptibility is modulated by multiple genes in different chromosomes. Our study confirms results from other populations and suggests new areas of potential importance.

Genome-wide linkage scan reveals multiple susceptibility loci influencing lipid and lipoprotein levels in the Quebec Family Study

A genome-wide linkage study was performed to identify chromosomal regions harboring genes influencing lipid and lipoprotein levels. Linkage analyses were conducted for four quantitative lipoprotein/lipid traits, i.e., total cholesterol, triglyceride, HDL-cholesterol (HDL-C), and LDL-C concentrations, in 930 subjects enrolled in the Québec Family Study. A maximum of 534 pairs of siblings from 292 nuclear families were available. Linkage was tested using both allele-sharing and variance-component linkage methods. The strongest evidence of linkage was found on chromosome 12q14.1 at marker D12S334 for HDL-C, with a logarithm of the odds (LOD) score of 4.06. Chromosomal regions harboring quantitative trait loci (QTLs) for LDL-C included 1q43 (LOD = 2.50), 11q23.2 (LOD = 3.22), 15q26.1 (LOD = 3.11), and 19q13.32 (LOD = 3.59). In the case of triglycerides, three markers located on 2p14, 11p13, and 11q24.1 provided suggestive evidence of linkage (LOD > 1.75). Tests for total cholesterol levels yielded significant evidence of linkage at 15q26.1 and 18q22.3 with the allele-sharing linkage method, but the results were nonsignificant with the variance-component method. In conclusion, this genome scan provides evidence for several QTLs influencing lipid and lipoprotein levels. Promising candidate genes were located in the vicinity of the genomic regions showing evidence of linkage.

Genome-wide linkage scan for the metabolic syndrome in the HERITAGE Family Study

The metabolic syndrome involves multiple and interactive effects of genes and environmental factors. To identify chromosomal regions encoding genes possibly predisposing to the metabolic syndrome, we performed a genome-wide scan with 456 white and 217 black participants from 204 nuclear families of the HERITAGE Family Study, using regression-based, single- and multipoint linkage analyses on 509 markers. A principal component analysis was performed on 7 metabolic syndrome-related phenotypes. Two principal components, PC1 and PC2 (55% of the variance), were used as metabolic syndrome phenotypes. ANOVA was used to quantify the familial aggregation of PC1 and PC2. Family membership contributed significantly (P < 0.0023) to the variance in PC1 (r(2) = 0.38 in whites; r(2) = 0.55 in blacks) and PC2 (r(2) = 0.51; r(2) = 0.48). In whites, promising evidence for linkage (P < 0.0023) was found for PC1 (2 markers on 10p11.2) and PC2 (a marker on 19q13.4). Suggestive evidence of linkage (0.01 > P > 0.0023) appeared for PC1 (1q41 and 9p13.1) and PC2 (2p22.3). In blacks, promising linkage was found for PC2 on 1p34.1, and suggestive linkage was found on 7q31.3 and 9q21.1. The genome-wide scan revealed evidence for quantitative trait loci on chromosomal regions that have been previously linked with individual cardiovascular disease and type 2 diabetes risk factors. Some of these chromosomal regions harbor promising potential candidate genes.

Effects of atorvastatin on fasting and postprandial complement component 3 response in familial combined hyperlipidemia

VLDL overproduction by enhanced hepatic FFA flux is a major characteristic of familial combined hyperlipidemia (FCHL). The postprandial complement component 3 (C3) response has been associated with impaired postprandial FFA metabolism in FCHL. We investigated the effects of 16 weeks of treatment with atorvastatin on postprandial C3 and lipid changes in 12 FCHL patients. Atorvastatin significantly lowered fasting plasma C3 and triglyceride (TG) in FCHL. Fasting TG and insulin sensitivity were the best predictors of fasting and postprandial C3. Postprandial triglyceridemia and C3 response, estimated as area under the curve (AUC), were significantly lowered by atorvastatin by 19% and 12%, respectively, albeit still elevated, compared with 10 matched controls. Postprandial FFA-AUC and postheparin plasma lipolytic activities remained unchanged after atorvastatin, suggesting no major effect on lipolysis. After atorvastatin, postprandial hydroxybutyric acid-AUC, which was elevated in untreated FCHL patients, was decreased, reaching values similar to those in controls. The present data show reduction of postprandial hepatic FFA flux in FCHL by atorvastatin, providing an additional mechanistic explanation for the reduction of VLDL secretion reported previously for atorvastatin. This was accompanied by a decrease in fasting plasma C3 concentrations and a blunted postprandial C3 response to an acute oral fat load.

Confirmed Locus on Chromosome 11p and Candidate Loci on 6q and 8p for the Triglyceride and Cholesterol Traits of Combined Hyperlipidemia

Background- Combined hyperlipidemia is a common disorder characterized by a highly atherogenic lipoprotein profile and increased risk of coronary heart disease. The etiology of the lipid abnormalities (increased serum cholesterol and triglyceride or either lipid alone) is unknown.

METHODS AND RESULTS

We assembled 2 large cohorts of families with familial combined hyperlipidemia (FCHL) and performed disease and quantitative trait linkage analyses to evaluate the inheritance of the lipid abnormalities. Chromosomal regions 6q16.1-q16.3, 8p23.3-p22, and 11p14.1-q12.1 produced evidence for linkage to FCHL. Chromosomes 6 and 8 are newly identified candidate loci that may respectively contribute to the triglyceride (logarithm of odds [LOD], 1.43; P=0.005) and cholesterol (LOD, 2.2; P=0.0007) components of this condition. The data for chromosome 11 readily fulfil the guidelines required for a confirmed linkage. The causative alleles may contribute to the inheritance of the cholesterol (LOD, 2.04 at 35.2 cM; P=0.0011) component of FCHL as well as the triglyceride trait (LOD, 2.7 at 48.7 cM; P=0.0002).

CONCLUSIONS

Genetic analyses identify 2 potentially new loci for FCHL and provide important positional information for cloning the genes within the chromosome 11p14.1-q12.1 interval that contributes to the lipid abnormalities of this highly atherogenic disorder.

Evidence for a QTL on chromosome 19 influencing LDL cholesterol levels in the general population

The genetic basis of cardiovascular disease (CVD) with its complex etiology is still largely elusive. Plasma levels of lipids and apolipoproteins are among the major quantitative risk factors for CVD and are well-established intermediate traits that may be more accessible to genetic dissection than clinical CVD end points. Chromosome 19 harbors multiple genes that have been suggested to play a role in lipid metabolism and previous studies indicated the presence of a quantitative trait locus (QTL) for cholesterol levels in genetic isolates. To establish the relevance of genetic variation at chromosome 19 for plasma levels of lipids and apolipoproteins in the general, out-bred Caucasian population, we performed a linkage study in four independent samples, including adolescent Dutch twins and adult Dutch, Swedish and Australian twins totaling 493 dizygotic twin pairs. The average spacing of short-tandem-repeat markers was 6-8 cM. In the three adult twin samples, we found consistent evidence for linkage of chromosome 19 with LDL cholesterol levels (maximum LOD scores of 4.5, 1.7 and 2.1 in the Dutch, Swedish and Australian sample, respectively); no indication for linkage was observed in the adolescent Dutch twin sample. The QTL effects in the three adult samples were not significantly different and a simultaneous analysis of the samples increased the maximum LOD score to 5.7 at 60 cM pter. Bivariate analyses indicated that the putative LDL-C QTL also contributed to the variance in ApoB levels, consistent with the high genetic correlation between these phenotypes. Our study provides strong evidence for the presence of a QTL on chromosome 19 with a major effect on LDL-C plasma levels in outbred Caucasian populations.

Linkage and association between distinct variants of the APOA1/C3/A4/A5 gene cluster and familial combined hyperlipidemia

OBJECTIVE

Combined hyperlipidemia is a common disorder, characterized by a highly atherogenic lipoprotein profile and a substantially increased risk of coronary heart disease. The purpose of this study was to establish whether variations of apolipoprotein A5 (APOA5), a newly discovered gene of lipid metabolism located 30 kbp downstream of the APOA1/C3/A4 gene cluster, contributes to the transmission of familial combined hyperlipidemia (FCHL).

METHODS AND RESULTS

We performed linkage and association tests on 128 families. Two independent alleles, APOA5c.56G and APOC3c.386G, of the APOA1/C3/A4/A5 gene cluster were overtransmitted in FCHL (P=0.004 and 0.007, respectively). This was paired with reduced transmission of the common APOA1/C3/A4/A5 haplotype (frequency 0.4461) to affected subjects (P=0.012). The APOA5c.56G genotype accounted for 7.3% to 13.8% of the variance in plasma triglyceride levels in probands (P<0.004). The APOC3c.386G genotypes accounted for 4.4% to 5.1% of the variance in triglyceride levels in FCHL spouses (P<0.007), suggesting that this allele marks a FCHL quantitative trait as well as representing a susceptibility locus for the condition.

CONCLUSIONS

A combined linkage and association analysis establishes that variation at the APOA1/C3/A4/A5 gene cluster contributes to FCHL transmission in a substantial proportion of northern European families.