Identification of TNFRSF1B as a novel modifier gene in familial combined hyperlipidemia

The Genetic Basis of Hypertriglyceridemia

PURPOSE OF REVIEW

Hypertriglyceridemia is a common dyslipidemia associated with an increased risk of cardiovascular disease and pancreatitis. Severe hypertriglyceridemia may sometimes be a monogenic condition. However, in the vast majority of patients, hypertriglyceridemia is due to the cumulative effect of multiple genetic risk variants along with lifestyle factors, medications, and disease conditions that elevate triglyceride levels. In this review, we will summarize recent progress in the understanding of the genetic basis of hypertriglyceridemia.

RECENT FINDINGS

More than 300 genetic loci have been identified for association with triglyceride levels in large genome-wide association studies. Studies combining the loci into polygenic scores have demonstrated that some hypertriglyceridemia phenotypes previously attributed to monogenic inheritance have a polygenic basis. The new genetic discoveries have opened avenues for the development of more effective triglyceride-lowering treatments and raised interest towards genetic screening and tailored treatments against hypertriglyceridemia. The discovery of multiple genetic loci associated with elevated triglyceride levels has led to improved understanding of the genetic basis of hypertriglyceridemia and opened new translational opportunities.

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.

Weighted gene coexpression network analysis identifies specific transcriptional modules and hub genes related to intramuscular fat traits in chicken breast muscle

Intramuscular fat (IMF) traits are important factors that influence meat quality. However, the molecular regulatory mechanisms that underlie this trait in chickens are still poorly understood at the gene coexpression level. Here, we performed a weighted gene coexpression network analysis between IMF traits and transcriptome profile in breast muscle in the Chinese domestic Gushi chicken breed at 6, 14, 22, and 30 weeks. A total of 26 coexpressed gene modules were identified. Six modules, which included the dark gray, purple, cyan, pink, light cyan, and blue modules, showed a significant positive correlation (P < 0.05) with IMF traits. The strongest correlation was observed between the dark gray module and IMF content (r = 0.85; P = 4e-04) and between the blue module and different fatty acid content (r = 0.87~0.91; P = 5e-05~2e-04). Enrichment analysis showed that the enrichment of biological processes, such as fatty acid metabolic process, fat cell differentiation, acylglycerol metabolic process, and glycerolipid metabolism were significantly different in the six modules. In addition, the 32, 24, 4, 7, 6, and 25 hub genes were identified from the blue, pink, light cyan, cyan, dark gray, and purple modules, respectively. These hub genes are involved in multiple links to fatty acid metabolism, phospholipid metabolism, cholesterol metabolism, diverse cellular behaviors, and cell events. These results provide novel insights into the molecular regulatory mechanisms for IMF-related traits in chicken and may also help to uncover the formation mechanism for excellent meat quality traits in local breeds of Chinese chicken.

Genetic Variability of Tumor Necrosis Factor Receptors Type I and II in Lymphoproliferative Diseases in the Serbian Population –

TNF-alpha and LT-alpha are involved in the pathogenesis of established lymphoproliferative diseases. Both molecules bind to TNFRI and TNFRII. TNFRI is the major mediator of the TNF pro-apoptotic and proliferative effects and TNFRII might enhance these effects. TNF receptors I and II are normally present on hematopoetic cells. TNFR II is characteristic only on immune cells, especially on peripheral leukocytes. Neoplastic B cells and activated B lymphocytes have increased expression of surface TNFR I. In this study, we have analyzed polymorphisms in the TNFRI gene (TNFRI+36A/G SNP) and polymorphism in the TNFRII gene (TNFRII + 676 T/G). All these polymorphisms were studied in patients with chronic lymphocytic leukemia (CLL), patients with non-Hodkin’s lymphoma (NHL) and in healthy controls. The present study was undertaken to investigate the genetic association of these polymorphisms with lymproproliferative disease development.

A total of 68 patients (49-CLL, 19-HNL) were diagnosed at the Clinic of Hematology, Clinical centre Niš, Serbia, using clinical findings and conventional morphological, cytochemical and immunological tests. Genomic DNA was isolated from isolated lymphocytes by proteinase K/phenol/chloroform method, and genotyped for TNFR I (A36G) and TNFR II (T676G) using the PCR-RFLP method.

No significant differences in allele frequencies of TNFR1 polymorphism were found between the patients with lymphoproliferative disease and healthy individuals. In a group of healthy individuals, the study has revealed for the first time significantly higher TNFRI G/G genotype compared to the patients with lymphoproliferative disease (χ22 = 5.66; p = 0.017). Also, we reported the implication of TNFRII T allele in NHL pathogenesis, respectively (χ22 = 10.77; p = 0.001; Mantel-Haenszel: χ22 = 10.64; p = 0.0011).

Our data showed that TNFRII T676G polymorphisms have an important role in NHL pathogenesis but not in CLL patients. A/A polymorphism in TNFRI was not associated with CLL and NHL patients in the Serbian population. Investigated polymorphisms on TNFR genes in leukemic cells of CLL and NHL patients have not showed a correlation with increased proliferation of B lymphocytes and increased expression of TNF R II on B CLL lymphocytes.

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.

Detection and impact of rare regulatory variants in human disease

Advances in genome sequencing are providing unprecedented resolution of rare and private variants. However, methods which assess the effect of these variants have relied predominantly on information within coding sequences. Assessing their impact in non-coding sequences remains a significant contemporary challenge. In this review, we highlight the role of regulatory variation as causative agents and modifiers of monogenic disorders. We further discuss how advances in functional genomics are now providing new opportunity to assess the impact of rare non-coding variants and their role in disease.

Abdominal Obesity and Expression of Familial Combined Hyperlipidemia

OBJECTIVE

To investigate the role of abdominal and body obesity on the prevalence of hyperlipidemia, in particular, hypertriglyceridemia, hypercholesterolemia, and high apolipoprotein B levels, in familial combined hyperlipidemia (FCHL) relatives and their spouses.

RESEARCH METHODS AND PROCEDURES

In FCHL relatives (n = 618) and spouses (n = 297), prevalence data of hyperlipidemia and high apolipoprotein B levels and their age and gender-corrected odds ratios (ORs) were calculated for sex-adjusted categories of waist-to-hip ratio (WHR), waist circumference, and BMI.

RESULTS

Increments of BMI, waist circumference, and WHR increased the frequency of hyperlipidemia. In the whole study population (relatives and spouses combined), frequency of hypertriglyceridemia showed a significant interaction only between WHR categories and FCHL. This was studied further after stratification of relatives by multivariable logistic regression analyses corrected for age and gender. Predominant expression of hypertriglyceridemia was observed with higher categories of WHR in FCHL relatives (prevalence up to 57.6%, OR 8.48 in highest vs. lowest WHR category, p < 0.001) but not in spouses (up to 32.9%, OR 1.05 in highest vs. lowest WHR category, not significant).

DISCUSSION

Both in spouses and FCHL relatives, increments in BMI and waist circumference increased the prevalence of hyperlipidemia. Specifically, in FCHL relatives, WHR was the most informative determinant of the expression of hyperlipidemia, in particular, hypertriglyceridemia. The data indicate that FCHL develops against a background of abdominal obesity.

The methionine 196 arginine polymorphism of the TNF receptor 2 gene (TNFRSF1B) is not associated with worse outcomes in heart failure

Tumor necrosis factor α (TNFα) may contribute to the pathologic process of congestive heart failure (CHF). TNFα signaling occurs through two receptors; TNFR1 (TNFRSF1A) and TNFRII (TNFRSF1B). In humans a single nucleotide polymorphism (rs1061622 in TNFRSF1B exon 6; T587G) encodes two different amino acids (M196R) in the transmembrane region. The 587G allele is associated with greater severity and/or prevalence of some inflammatory diseases, but its role in CHF in unknown. This study sought to test the hypothesis that the 587G allele is associated with a worse outcome or more severe phenotype in CHF. Peripheral blood DNA was isolated and genotyped from 379 heart failure patients enrolled in a genetic outcome study (GRACE); (44.7% ischemic, 70.4% male, 8.5% black race, age 55.6 ± 11.7 yr (SD), LVEF 24.5 ± 8.3%, NYHA 2.53 ± 0.64). Genotyping was performed by PCR-RFLP. Cardiac function was assessed from medical records at study entry. The distribution of genotypes in this population was 54% T/T, 38.4% G/T and 7.7% G/G. Mean LV ejection fraction (T/T 24.4 ± 8.2, T/G 25.0 ± 8.4, G/G 23.3 ± 8.6, n=352, p=ns) and LV end-diastolic dimensions (T/T 6.57 ± 0.93, T/G 6.53 ± 1.0, G/G 6.57 ± 0.78, n=211, p=ns) were comparable in all groups. Transplant-free survival (median 23 months (range 1-62 months) did not vary by genotype (p=0.95). A lack of effect (p=0.74) on transplant-free survival was also observed in a subset of patients with ischemic heart failure (n=169). The TNFRSF1B 587G allele is not associated with the severity of heart failure phenotype or clinical outcomes in patients with chronic CHF.

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.

No association of TNFRSF1B variants with type 2 diabetes in Indians of Indo-European origin

BACKGROUND

There has been no systematic evaluation of the association between genetic variants of type 2 receptor for TNFα (TNFR2) and type 2 diabetes, despite strong biological evidence for the role of this receptor in the pathogenesis of this complex disorder. In view of this, we performed a comprehensive association analysis of TNFRSF1B variants with type 2 diabetes in 4,200 Indo-European subjects from North India.

METHODS

The initial phase evaluated association of seven SNPs viz. rs652625, rs496888, rs6697733, rs945439, rs235249, rs17883432 and rs17884213 with type 2 diabetes in 2,115 participants (1,073 type 2 diabetes patients and 1,042 control subjects). Further, we conducted replication analysis of three associated SNPs in 2,085 subjects (1,047 type 2 diabetes patients and 1,038 control subjects).

RESULTS

We observed nominal association of rs945439, rs235249 and rs17884213 with type 2 diabetes (P < 0.05) in the initial phase. Haplotype CC of rs945439 and rs235249 conferred increased susceptibility for type 2 diabetes [OR = 1.19 (95%CI 1.03-1.37), P = 0.019/Pperm = 0.076] whereas, TG haplotype of rs235249 and rs17884213 provided protection against type 2 diabetes [OR = 0.83 (95%CI 0.72-0.95, P = 7.2 × 10-3/Pperm = 0.019]. We also observed suggestive association of rs496888 with plasma hsCRP levels [P = 0.042]. However, the association of rs945439, rs235249 and rs17884213 with type 2 diabetes was not replicated in the second study population. Meta-analysis of the two studies also failed to detect any association with type 2 diabetes.

CONCLUSIONS

Our two-stage association analysis suggests that TNFRSF1B variants are not the determinants of genetic risk of type 2 diabetes in North Indians.

Identification of MicroRNAs that control lipid droplet formation and growth in hepatocytes via high-content screening

Hepatic lipid droplets (LDs) are associated with metabolic syndrome, type 2 diabetes, hepatitis C, and both alcoholic and nonalcoholic fatty liver disease. MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression at the level of translation. Approximately 1000 different miRNA species are encoded within the human genome, and many are differentially expressed by healthy and diseased liver. However, few studies have investigated the role of miRNAs in regulating LD expression. Accordingly, a high-content assay (HCA) was performed in which human hepatocytes (Huh-7 cells) were transiently transfected with 327 unique human miRNAs; the cells were then fixed, labeled for nuclei and lipid droplets, and imaged with an automated digital microscopy workstation. LD expression was analyzed on a cell-by-cell basis, using automated image analysis. Eleven miRNAs were identified that altered LDs. MiR-181d was the most efficacious inhibitor, decreasing LDs by about 60%. miRNA-181d was also confirmed to reduce cellular triglycerides and cholesterol ester via biochemical assays. Furthermore, a series of proteins was identified via miRNA target analysis, and siRNAs directed against many of these proteins also modified LDs. Thus, HCA-based screening identified novel miRNA and protein regulators of LDs and cholesterol metabolism that may be relevant to hepatic diseases arising from obesity and alcohol abuse.

A novel locus for arterial hypertension on chromosome 1p36 maps to a metabolic syndrome trait cluster in the Sorbs, a Slavic population isolate in Germany

OBJECTIVE

Genome-wide linkage studies and genome-wide association studies have not as yet identified major genes contributing to primary hypertension in the general population. This state-of-affairs suggests considerable heterogeneity with small contributing effects for primary hypertension, or other complex genetic traits, in outbred populations. Isolated populations, as recent data from Iceland and French Canada suggest, could offer a solution to this problem.

METHODS

We studied a Slavic isolate in Germany, the Sorbs, and genotyped 1040 polymorphic microsatellite markers in 87 multigeneration families.

RESULTS

Our genome-wide linkage scan revealed a locus on chromosome 1p36.13 at D1S3669-D1S2826 (40.95 cM Marshfield coordinates; logarithm of the odds = 3.45, nominal P = 0.00003) that reached genome-wide significance (P = 0.004), indicating the increased power in isolated populations. The chromosome 1 locus maps to a region in which traits such as diabetes, hyperlipidemia, obesity and BMI cluster.

CONCLUSION

Our results suggest that this locus contributes to the metabolic syndrome, and that further attention in this and other populations is warranted.

Association analysis of TNFRSF1B polymorphisms with type 2 diabetes and its related traits in North India

Inflammation plays a crucial role in the pathogenesis of type 2 diabetes and various lines of evidences suggest an important contribution of type 2 receptor for TNFalpha (TNFR2), a mediator of inflammatory responses. Though genetic association of TNFRSF1B (encoding TNFR2) polymorphisms have been investigated in various studies, their involvement is not clear because of inconsistent findings. Because of high susceptibility of Indian population to type 2 diabetes and its complications, we evaluated the association of TNFRSF1B polymorphisms-rs1061622 (M196R; exon6) and rs3397 (3'UTR) and (CA)( n ) repeat (intron 4) in 1,852 subjects including 1,040 cases and 812 controls with type 2 diabetes and its associated peripheral neuropathy and hypertension in North Indians of Indo-European ethnicity. The allelic and genotypic distributions of these polymorphisms were comparable among healthy control vs. type 2 diabetes, peripheral neuropathy vs. non-neuropathy and hypertensive vs. normotensive groups. (CA)( n ) polymorphism has been shown to be associated with diabetic neuropathy in Caucasians, however, this could not be replicated in our study (P = 0.27). None of the polymorphisms were found to influence the 14 anthropometric and biochemical traits related to type 2 diabetes studied here. Thus, we conclude that TNFRSF1B is not a major contributing factor to the genetic risk of type 2 diabetes, its associated peripheral neuropathy and hypertension and related metabolic traits in North Indians.

Polymorphisms of the tumor necrosis factor-alpha receptor 2 gene are associated with obesity phenotypes among 405 Caucasian nuclear families

The plasma level of the tumor necrosis factor-alpha receptor 2 (TNFR2) is associated with obesity phenotypes. However, the genetic polymorphisms for such an association have rarely been explored and are generally unknown. In this study, by employing a large sample of 1,873 subjects from 405 Caucasian nuclear families, we explored the association of 12 SNPs of the TNFR2 gene and obesity-related phenotypes, including body mass index (BMI), fat mass, and percentage fat mass (PFM). The within-family quantitative transmission disequilibrium test, which is robust to sample stratification, was implemented to evaluate the association of TNFR2 gene with obesity phenotypes. Evidence of association was obtained at SNP9 (rs5746059) with fat mass (P = 0.0002), BMI (P = 0.002), and PFM (P = 0.0006). The contribution of this polymorphism to the variation of fat mass and PFM was 6.24 and 7.82%, respectively. Individuals carrying allele A at the SNP9 site had a 4.6% higher fat mass and a 2.5% increased PFM compared to noncarriers. The results remained significant even after correction for multiple testing. Evidence of association between the TNFR2 gene and obesity phenotypes are also found in 700 independent Chinese Han and 1,000 random Caucasians samples. The results suggest that the TNFR2 gene polymorphisms contribute to the variation of obesity phenotypes.

Plasma levels of soluble tumor necrosis factor-alpha receptors are related to total and LDL-cholesterol in lean, but not in obese subjects

Background

Tumor necrosis factor-α (TNFα) is a mediator of insulin resistance. Plasma levels of soluble TNFα receptors (sTNFR1 and sTNFR2) probably reflect paracrine action of the cytokine. TNFα is also a regulator of lipid metabolism, however, data about impact of obesity on the relationships between TNFα and plasma lipids remain controversial.

Aim

The purpose of the present study was to examine the associations of TNFα system with plasma lipids in lean and obese subjects with normal glucose metabolism.

Methods

We examined 63 subjects, 33 lean (BMI<25 kg × m^-2) and 30 with marked overweight or obesity (BMI>27.8 kg × m^-2). Anthropometric and biochemical parameters were measured. Oral glucose tolerance test and euglycemic hyperinsulinemic clamp were also performed.

Results

Obese subjects were markedly more insulin resistant and had higher levels of both TNFα receptors. Total (TC) and LDL-cholesterol (LDL-C), triglycerides (TG) and non-esterified fatty acids (NEFA) were also higher in the obese group. In obese subjects, both receptors were significantly related to TG and HDL-cholesterol (HDL-C), while sTNFR2 was also associated with NEFA. All those correlations disappeared after controlling for insulin sensitivity. In lean subjects, both receptors were related to TC, HDL-C and LDL-C. In that group, sTNFR1 predicted values of all those parameters independently of BMI, plasma glucose and insulin, and insulin sensitivity.

Conclusion

We conclude that TNFα receptors are associated with plasma lipids in different way in lean and in obese subjects. TNFα system is probably important in determining cholesterol levels in lean subjects, while in obese this effect might be masked by other metabolic abnormalities.

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.

A genomewide search finds major susceptibility loci for gallbladder disease on chromosome 1 in Mexican Americans

Gallbladder disease (GBD) is one of the major digestive diseases. Its risk factors include age, sex, obesity, type 2 diabetes, and metabolic syndrome (MS). The prevalence of GBD is high in minority populations, such as Native and Mexican Americans. Ethnic differences, familial aggregation of GBD, and the identification of susceptibility loci for gallstone disease by use of animal models suggest genetic influences on GBD. However, the major susceptibility loci for GBD in human populations have not been identified. Using ultrasound-based information on GBD occurrence and a 10-cM gene map, we performed multipoint variance-components analysis to localize susceptibility loci for GBD. Phenotypic and genotypic data from 715 individuals in 39 low-income Mexican American families participating in the San Antonio Family Diabetes/Gallbladder Study were used. Two GBD phenotypes were defined for the analyses: (1) clinical or symptomatic GBD, the cases of cholecystectomies due to stones confirmed by ultrasound, and (2) total GBD, the clinical GBD cases plus the stone carriers newly diagnosed by ultrasound. With use of the National Cholesterol Education Program/Adult Treatment Panel III criteria, five MS risk factors were defined: increased waist circumference, hypertriglyceredemia, low high-density lipoprotein cholesterol, hypertension, and high fasting glucose. The MS risk-factor score (range 0-5) for a given individual was used as a single, composite covariate in the genetic analyses. After accounting for the effects of age, sex, and MS risk-factor score, we found stronger linkage signals for the symptomatic GBD phenotype. The highest LOD scores (3.7 and 3.5) occurred on chromosome 1p between markers D1S1597 and D1S407 (1p36.21) and near marker D1S255 (1p34.3), respectively. Other genetic locations (chromosomes 2p, 3q, 4p, 8p, 9p, 10p, and 16q) across the genome exhibited some evidence of linkage (LOD >or=1.2) to symptomatic GBD. Some of these chromosomal regions corresponded with the genetic locations of Lith loci, which influence gallstone formation in mouse models. In conclusion, we found significant evidence of major genetic determinants of symptomatic GBD on chromosome 1p in Mexican Americans.

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.

Haplotype analysis of tumour necrosis factor receptor genes in 1p36: no evidence for association with systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease with partially understood aetiology. The 1p36 region has been previously linked with SLE and harbours tumour necrosis factor receptor (TNFR) genes. Functional and genetic data implicate their gene products in SLE and other autoimmune diseases. In all, single-nucleotide polymorphisms (SNPs) across TNFRSF14 (HVEM), and 43 SNPs across the TNFRSF8 (CD30) and TNFRSF1B (CD120B) locus were investigated for linkage disequilibrium (LD) and haplotype analysis in European-Caucasians. Strong LD was observed across HVEM and CD120B, and little LD and recombination across CD30. We also examined the association of SNPs and haplotypes in HVEM, CD30 and CD120B with SLE in European-Caucasians. There was no evidence of association for these genes in 456 European-Caucasian families with SLE from UK. Haplotype tagging SNPs are made known across areas of strong LD, which will facilitate analysis for susceptibility in other diseases.

Tumor necrosis factor alpha (TNFalpha) and its soluble receptor p75 (sTNF-R p75) in familial combined hyperlipidemia (FCHL)

BACKGROUND AND AIM

Familial combined hyperlipidemia (FCHL) is a genetic disorder of lipid metabolism associated with insulin resistance and abnormalities in fatty acid metabolism whose underlying mechanisms are largely unknown. Perturbations in the TNFalpha/TNF-R pathway may play a role in these abnormalities.

METHODS AND RESULTS

We determined plasma levels of TNFalpha and sTNF-R p75 in 85 FCHL patients (TC 245+/-45 mg/dl; TG 260+/-148 mg/dl; apoB 148+/-37 mg/dl) and in 29 age- and sex-matched normolipemic relatives (NL) (TC 187+/-22.8 mg/dl; TG 115+/-37 mg/dl; apoB 106+/-16 mg/dl). Thirty-four normolipemic subjects (TC 180+/-34 mg/dl; TG 107+/-42 mg/dl; apoB 95+/-22 mg/dl) were also included as unrelated controls (NC). Plasma free fatty acids (NEFA) were also measured and insulin sensitivity was evaluated by HOMA. Levels of sTNF-R p75 were significantly reduced in FCHL compared to NL (2.30+/-0.55 ng/ml vs. 2.64+/-0.88 ng/ml, p<0.05) but not compared to NC (2.35+/-0.68 ng/ml). HOMA values were comparable in all groups and did not show any relation with plasma levels of sTNF-R p75. Logistic analysis demonstrated that a low concentration of sTNF-R p75 was an independent predictor of the affected status within FCHL families, but this role was no longer evident when FCHL patients were compared to NC. In FCHL, age (p<0.001) was positively, and TG (p=0.029) and HDL-C (p=0.025) were negatively correlated with plasma concentrations of sTNF-R p75. In the other groups, age (in NL) and non-HDL-C (in NC) were significantly correlated with sTNF-R p75.

CONCLUSIONS

Although our data do not support a causative role of TNFalpha/TNF-R alterations in FCHL, they confirm that variation in TNF-R shedding may influence lipid phenotypic expression in FCHL families.

No association with hypertension of CLCNKB and TNFRSF1B polymorphisms at a hypertension locus on chromosome 1p36

OBJECTIVE

Chromosome 1p36 has been linked to essential hypertension and systolic blood pressure. This locus contains the chloride channel-Kb gene (CLCNKB) and the tumour necrosis factor receptor 2 gene (TNFRSF1B). Polymorphisms of each of these have shown association with hypertension, and a CLCNKB T481S variant alters receptor function. Here we performed association studies in a well-characterized cohort of hypertensives and normotensives whose blood pressure status matched that of both their parents.

METHODS

The study involved 196 essential hypertensives and 321 normotensives. These were genotyped for TNFRSF1B variants T-1710A upstream, A257G in exon 2, a CA-repeat polymorphism in intron 4, E232K and M196R in exon 6, and T1668G and T1690C in the 3'-untranslated region, and the T481S variant of CLCNKB.

RESULTS

The CLCNKB T481S variant showed no association with hypertension. Thermodynamic modelling of the 3'-untranslated region of TNFRSF1B mRNA predicted that the T1668G variant alters the stem-loop structure and thus the mRNA stability and expression. However, neither this nor the other TNFRSF1B polymorphisms, either alone or after haplotype analysis, were associated with hypertension. Moreover, for each gene the blood pressure, body mass index, plasma sodium and plasma lipid concentrations were generally similar across genotypes.

CONCLUSION

Our data fail to support previous association findings for TNFRSF1B and CLCNKB at the chromosome 1p36 locus implicated in hypertension.

Dyslipidemia and inflammation: an evolutionary conserved mechanism

Inflammation leads to changes in lipid metabolism aimed at decreasing the toxicity of a variety of harmful agents and tissue repair by redistributing nutrients to cells involved in host defence. Acute phase response, mediated by cytokines, preserves the host from acute injury. When this inflammation becomes chronic, it might lead to chronic disorders as atherosclerosis and the metabolic syndrome. The activation of the inflammatory cascade will induce a decrease in HDL-cholesterol (HDL-C), with impairment in reverse cholesterol transport, and parallel changes in apolipoproteins, enzymes, anti-oxidant capacity and ATP binding cassette A1-dependent efflux. This decrease in HDL-C and phospholipids could stimulate compensatory changes, as synthesis and accumulation of phospholipid-rich VLDL which binds bacterial products and other toxic substances, resulting in hypertriglyceridemia. The final consequence is an increased accumulation of cholesterol in cells. When the compensatory response (inflammation) is not able to repair injury, it turns into a harmful reaction, and the lipid changes will become chronic, either by repeated or overwhelming stimulus, enhancing the formation of atherosclerotic lesions. Thus, the classical lipid changes associated with the metabolic syndrome (increased triglycerides and decreased HDL-C) may be envisioned as a highly conserved evolutionary response aimed at tissue repair. Under this assumption, the problem is not the response but the persistence of the stimulus.

A polymorphism in the 3' untranslated region of the gene for tumor necrosis factor receptor 2 modulates reporter gene expression

The gene encoding the human TNF alpha receptor (TNFR) 2 contains polymorphisms in the 3' untranslated region (UTR). Previous studies have shown that some variant alleles in this region are associated with obesity and insulin resistance. However, the effect of these polymorphisms on the expression of TNFR2 has not been studied to date. To examine the role played by different haplotypes in the control of TNFR2 expression (haplotypes A1-A5, referring to nucleotides 1663 G/A, 1668 T/G, and 1690 T/C), we introduced these sequences into the 3'-UTR of a heterologous reporter gene and expressed the corresponding constructs in a human T-cell line. We demonstrate that a 485-nt fragment of the TNFR2 3'-UTR that contains a U-rich region decreases reporter expression and that haplotypes A1-A4 exert a stronger effect than A5. Furthermore, time-course assays of mRNA stability using actinomycin D revealed that haplotypes A1-A4 destabilize the mRNA. The proximal TNFR2 3'-UTR, independently of haplotype differences, responded to T-cell activation by increasing mRNA decay. Electromobility shift analysis demonstrated that protein(s) found in T-cell extracts bind to the 485-nt fragment. We suggest that an increased rate of TNFR2 mRNA decay protects cells from unrestrained TNF alpha effects and that this protection is weakened in A5 subjects. These findings may explain the association of this haplotype with obesity and increased leptin levels.

Tumour necrosis factor receptors (TNFRs) in Type 2 diabetes. Analysis of soluble plasma fractions and genetic variations of TNFR2 gene in a case-control study

AIMS

We have studied the relationships between soluble fractions of tumour necrosis factor receptors (sTNFR1 and sTNFR2) in Type 2 diabetes (DM2) and its chronic microvascular complications. Likewise, we have analysed the genetic susceptibility of 196T > G exon6/CA-repeat intron 4 mutations in the TNFR2 gene in this population.

METHODS

A case-control study was conducted to examine the role of sTNFRs in 345 DM2 patients and 173 healthy subjects. The mutations were studied in all healthy subjects and in a subset of 232 patients.

RESULTS

sTNFRs levels were similar in healthy and DM2 patients. A positive correlation between age and both sTNFRs was observed in healthy subjects. In DM2 patients, sTNFR1 showed a positive correlation with age, systolic blood pressure and leptin levels (r = 0.53, P < 0.0001; r = 0.28, P = 0.005; r = 0.46, P < 0.0001, respectively) and sTNFR2 was positively correlated with age, triglycerides and leptin levels (r = 0.34, P < 0.0001; r = 0.21, P < 0.0001; r = 0.28, P = 0.002, respectively). Patients with micro- or macroalbuminuria showed higher plasma levels of sTNFR1 and sTNFR2 than normoalbuminuric patients, after adjusting for confounding variables (B = 0.85, P = 0.022, 95% CI: 0.12-1.58 for sTNFR1 and B = 1.50, P < 0.001, 95% CI: 0.67-2.33 for sTNFR2). In DM2 patients, TT-exon 6 homozygous showed lower levels of sTNFR1 [2,4 (1.1) vs. 3.4 (1.2) ng/ml], and the CA273-allele tracked with elevated plasma HDL-cholesterol [1.8 (0.7), 1.4 (0.3) and 1.3 (0.3) mm, for CA273/273, CA273/- and CA-/-, respectively]. No association was seen with other analysed variables.

CONCLUSIONS

Our findings suggest that chronic TNF activation may have some pathogenic role in diabetic nephropathy in DM2 patients. Genetic variations in exon 6/intron 4 of the TNFR2 gene do not predispose to a major risk for DM2 or its microvascular complications.

The molecular-genetic basis of functional hyperandrogenism and the polycystic ovary syndrome

The genetic mechanisms underlying functional hyperandrogenism and the polycystic ovary syndrome (PCOS) remain largely unknown. Given the large number of genetic variants found in association with these disorders, the emerging picture is that of a complex multigenic trait in which environmental influences play an important role in the expression of the hyperandrogenic phenotype. Among others, genomic variants in genes related to the regulation of androgen biosynthesis and function, insulin resistance, and the metabolic syndrome, and proinflammatory genotypes may be involved in the genetic predisposition to functional hyperandrogenism and PCOS. The elucidation of the molecular genetic basis of these disorders has been burdened by the heterogeneity in the diagnostic criteria used to define PCOS, the limited sample size of the studies conducted to date, and the lack of precision in the identification of ethnic and environmental factors that trigger the development of hyperandrogenic disorders. Progress in this area requires adequately sized multicenter collaborative studies after standardization of the diagnostic criteria used to classify hyperandrogenic patients, in whom modifying environmental factors such as ethnicity, diet, and lifestyle are identified with precision. In addition to classic molecular genetic techniques such as linkage analysis in the form of a whole-genome scan and large case-control studies, promising genomic and proteomic approaches will be paramount to our understanding of the pathogenesis of functional hyperandrogenism and PCOS, allowing a more precise prevention, diagnosis, and treatment of these prevalent disorders.

TNF and TNFR polymorphisms in severe sepsis and septic shock: a prospective multicentre study

Tumour necrosis factor (TNF) is an important pro-inflammatory cytokine produced in sepsis. Studies examining the association of individual TNF single nucleotide polymorphisms with sepsis have produced conflicting results. This study investigated whether common polymorphisms of the TNF locus and the two receptor genes, TNFRSF1A and TNFRSF1B, influence circulating levels of encoded proteins, and whether individual polymorphisms or extended haplotypes of these genes are associated with susceptibility, severity of illness or outcome in adult patients with severe sepsis or septic shock. A total of 213 Caucasian patients were recruited from eight intensive care units (ICU) in the UK and Australia. Plasma levels of TNF (P = 0.02), sTNFRSF1A (P = 0.005) and sTNFRSF1B (P = 0.01) were significantly higher in those who died on ICU compared to those who survived. There was a positive correlation between increasing soluble receptor levels and organ dysfunction (increasing SOFA score) (sTNFRSF1A R = 0.51, P < 0.001; sTNFRSF1B R = 0.53, P < 0.001), and in particular with the degree of renal dysfunction. In this study, there were no significant associations between the selected candidate TNF or TNF receptor polymorphisms, or their haplotypes, and susceptibility to sepsis, illness severity or outcome. The influence of polymorphisms of the TNF locus on susceptibility to, and outcome from sepsis remains uncertain.

Comparative study of the haplotype structure and linkage disequilibrium of chromosome 1p36.2 region in the Korean and Japanese populations

The patterns of linkage disequilibrium (LD) in the human genome provide important information for disease gene mapping. LDs may vary depending on chromosomal regions and populations. We have compared LD and haplotypes defined by SNPs in the chromosome 1p36.2 region of the Korean and Japanese populations. Fifty-eight SNPs in about 418 kb ranging from tumor necrosis factor receptor 2 (TNFR2:TNFRSF1B) to procollagen-lysine, 2-oxoglutarate 5-dioxygenase (PLOD) gene were examined in 96 healthy Koreans and Japanese each by direct sequencing and fluorescence correlation spectroscopy combined with the PCR-sequence specific primer method (PCR-SSP-FCS), respectively. Upon pair-wise LD analysis, a total of 25 and 16 out of 58 SNPs greater than MAF 10% were included in LD blocks, encompassing almost 81 kb and 55 kb in total, in Koreans and Japanese, respectively. Both similarities and differences were observed in LD strength and haplotype frequencies between the populations. Considerable similarities were observed in the telomeric region where a long-range block of approximately 80 kb including three genes was found to have strong LDs in both Koreans and Japanese. Significant difference in LD strength was present near the TNFR2 region between the Japanese and Korean populations.

Essential hypertension: genes and dreams

Herein we review all of the data from linkage by genome scanning and from association studies in essential hypertension. Genome scans have yielded loci linked to hypertension on almost every chromosome. We tabulate all of these loci to highlight the striking inconsistency. Similarly, association studies have implicated > 66 genes to date, which we also list, but virtually all have failed to show consistent replication in other settings. Nevertheless, we believe that molecular genetics should eventually find all of the major gene variants for essential hypertension. This will be a great scientific achievement and lead to new treatments. The dream, however, of using this information in clinical genetic testing could turn out to be a nightmare. Thus at present the hype surrounding genes for complex polygenic diseases like hypertension far exceeds the reality.

Insulin resistance and chronic cardiovascular inflammatory syndrome

Insulin resistance is increasingly recognized as a chronic, low-level, inflammatory state. Hyperinsulinemia and insulin action were initially proposed as the common preceding factors of hypertension, low high-density lipoprotein cholesterol, hypertriglyceridemia, abdominal obesity, and altered glucose tolerance, linking all these abnormalities to the development of coronary heart disease. The similarities of insulin resistance with another inflammatory state, atherosclerosis, have been described only in the last few decades. Atherosclerosis and insulin resistance share similar pathophysiological mechanisms, mainly due to the actions of the two major proinflammatory cytokines, TNF-alpha and IL-6. Genetic predisposition to increased transcription rates of these cytokines is associated with metabolic derangement and simultaneously with coronary heart disease. Dysregulation of the inflammatory axis predicts the development of insulin resistance and type 2 diabetes mellitus. The knowledge of how interactions between metabolic and inflammatory pathways occur will be useful in future therapeutic strategies. The effective administration of antiinflammatory agents in the treatment of insulin resistance and atherosclerosis is only the beginning of a promising approach in the management of these syndromes.

A polymorphism in the promoter of the tumor necrosis factor-alpha gene (-308) is associated with coronary heart disease in type 2 diabetic patients

BACKGROUND

Tumor necrosis factor-alpha (TNF-alpha) is a key cytokine in the inflammation process of atherosclerosis. Through its effects on lipid metabolism, insulin resistance and endothelial function, it might be involved in coronary heart disease (CHD). A biallelic polymorphism within the promoter of TNF-alpha locus at the position -308 has been reported to be associated with TNF production. We have studied the association of this polymorphism with CHD in a Mediterranean non-diabetic and type 2 diabetic population.

METHODS

Three hundred and forty one CHD patients (106 with type 2 diabetes), 207 healthy matched control subjects and 135 type 2 diabetic patients without CHD were evaluated. A single nucleotide polymorphism at the promoter TNF-alpha (-308) was analyzed by RFLP-PCR.

RESULTS

TNF-alpha (-308) genotype and allele frequencies for A carriers were higher in CHD patients than those observed in the control group (32.3 vs. 23.2%, P=0.03; and 18.8 vs. 12.1%, P=0.0047; respectively) independently of other risk factors. Genotypic analysis revealed that CHD patients with type 2 DM displayed a greater prevalence of the -308 TNF-alpha A allele (40.6%) than controls (23.2%) or CHD patients without type 2 DM (28.5%) (P=0.0056). The odds ratio for CHD in type 2 diabetic patients in presence of -308 TNF-alpha A allele was 2.86 (CI 95%: 1.55-5.32). This difference was observed mainly in diabetic women for the A allele carriers (OR: 4.29; CI 95%: 1.6-11.76).

CONCLUSIONS

These results suggest that -308 TNF-alpha gene polymorphism may contribute to CHD risk in patients with type 2 diabetes and it could constitute an useful predictive marker for CHD in type 2 diabetic women.

A decade of high-resolution liquid chromatography of nucleic acids on styrene-divinylbenzene copolymers

The introduction of alkylated, nonporous poly-(styrene-divinylbenzene) microparticles in 1992 enabled the subsequent development of denaturing HPLC that has emerged as the most sensitive screening method for mutations to date. Denaturing HPLC has provided unprecedented insight into human origins and prehistoric migrations, accelerated the cloning of genes involved in mono- and polygenic traits, and facilitated the mutational analysis of more than a hundred candidate genes of human disease. A significant step toward increased sample-throughput and information content was accomplished by the recent introduction of monolithic poly(styrene-divinylbenzene) capillary columns. They have enabled the construction of capillary arrays amenable to multiplex analysis of fluorescent dye-labeled nucleic acids by laser-induced fluorescence detection. Hyphenation of denaturing HPLC with electrospray ionization mass spectrometry, on the other hand, has allowed the direct elucidation of the chemical nature of DNA variation and determination of phase of multiple alleles on a chromosome.

Variations in immune response genes and their associations with multifactorial immune disorders

There are three genetic methods often used for detecting genes contributing to susceptibility or resistance to multifactorial diseases: nonparametric linkage analysis, case-control association analysis, and transmission disequilibrium test. In this review, we present the theoretical basis that the case-control association study has the highest power of detecting disease genes if there is no population stratification between patients and controls. Taking advantage of the high power, we have carried out extensive case-control association analyses of candidate genes for the search of susceptibility genes to rheumatic diseases in the Japanese as well as in some other populations. Several new associations have been disclosed, including those of TNFR2, FCGR2B, and CD19 gene polymorphisms with systemic lupus erythematosus, in addition to some unexpected findings such as the common occurrence of NKG2-C null allele in the healthy population. Genome-wide association studies using single nucleotide polymorphisms (SNPs) or microsatellite polymorphisms have become realistic, and development of new high-throughput and cost-effective SNP typing technologies is urgently needed. At the same time, our observations may indicate that the 'classical' candidate gene approach will remain a strong alternative, even in the age of 'post genome-sequence'.

Identification of the PPARA locus on chromosome 22q13.3 as a modifier gene in familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is a common genetic lipid disorder that is present in 10% of patients with premature coronary artery disease (CAD). It was the objective of the present study to evaluate the possible involvement of the PPARA locus in the pathophysiology of FCHL. Mutation detection analyses of the six coding PPARA exons resulted in the identification of four novel variants, [C/T] intron 3, S234G, [G/A] intron 5, and [C/A] 3(') UTR in three FCHL probands, whereas no novel variants were identified in spouses. In a case-control study, markers D22S275 and D22S928 were shown not to be associated with FCHL. However, D22S928, mapped within 1Mb of the PPARA gene, was shown to have a modifying effect on plasma apoCIII concentrations (P=0.011) and the combined hyperlipidemic FCHL phenotype (P=0.038). In addition two PPARA polymorphisms in intron 2 and 7 were studied, but these were not associated with FCHL. The frequency of the L162V variant was less in FCHL probands (1.98%) compared to that in spouses (4.84%). These results clearly demonstrate the genetically complex nature of FCHL and identify the PPARA gene as a modifier of the FCHL phenotype.

Familial combined hyperlipidemia plasma stimulates protein secretion by HepG2 cells: identification of fibronectin in the differential secretion proteome

The aim of this study was to evaluate whether soluble factors in plasma of familial combined hyperlipidemia (FCHL) patients affect hepatic protein secretion. Cultured human hepatocytes, i.e., HepG2 cells, were incubated with fasting plasma (20%, v/v, in DMEM) from untreated FCHL patients or normolipidemic controls. Overall protein secretion was 10-15% higher after incubation with FCHL plasma. This was specifically caused by an increase in four secreted proteins, with estimated sizes of 240, 180, 120, and <40 kD (P < 0.001, P < 0.006, P < 0.002, P < 0.02, respectively). The 240 kD protein in the secretion proteome was identified as fibronectin by mass spectrometry. Plasma fibronectin concentrations were elevated in FCHL patients, confirming biological relevance of these data. Overall protein secretion by HepG2 cells correlated with concentrations of triglycerides (r = 0.61, P < 0.001) in the applied plasma samples. VLDL+IDL isolated from FCHL patients, induced a higher protein secretion than lipoproteins isolated from controls (P < 0.001). Remarkably, secretion of apoB, the structural protein of VLDL, was stimulated to a similar extent by FCHL and control plasma. FCHL plasma did not induce excess secretion of apoB by HepG2 cells compared with control plasma. FCHL plasma did stimulate secretion of several distinct hepatic proteins, among which fibronectin was identified.

Clinical relevance of the biochemical, metabolic, and genetic factors that influence low-density lipoprotein heterogeneity

Traditional risk factors for coronary artery disease (CAD) predict about 50% of the risk of developing CAD. The Adult Treatment Panel (ATP) III has defined emerging risk factors for CAD, including small, dense low-density lipoprotein (LDL). Small, dense LDL is often accompanied by increased triglycerides (TGs) and low high-density lipoprotein (HDL). An increased number of small, dense LDL particles is often missed when the LDL cholesterol level is normal or borderline elevated. Small, dense LDL particles are present in families with premature CAD and hyperapobetalipoproteinemia, familial combined hyperlipidemia, LDL subclass pattern B, familial dyslipidemic hypertension, and syndrome X. The metabolic syndrome, as defined by ATP III, incorporates a number of the components of these syndromes, including insulin resistance and intra-abdominal fat. Subclinical inflammation and elevated procoagulants also appear to be part of this atherogenic syndrome. Overproduction of very low-density lipoproteins (VLDLs) by the liver and increased secretion of large, apolipoprotein (apo) B-100-containing VLDL is the primary metabolic characteristic of most of these patients. The TG in VLDL is hydrolyzed by lipoprotein lipase (LPL) which produces intermediate-density lipoprotein. The TG in intermediate-density lipoprotein is hydrolyzed further, resulting in the generation of LDL. The cholesterol esters in LDL are exchanged for TG in VLDL by the cholesterol ester tranfer proteins, followed by hydrolysis of TG in LDL by hepatic lipase which produces small, dense LDL. Cholesterol ester transfer protein mediates a similar lipid exchange between VLDL and HDL, producing a cholesterol ester-poor HDL. In adipocytes, reduced fatty acid trapping and retention by adipose tissue may result from a primary defect in the incorporation of free fatty acids into TGs. Alternatively, insulin resistance may promote reduced retention of free fatty acids by adipocytes. Both these abnormalities lead to increased levels of free fatty acids in plasma, increased flux of free fatty acids back to the liver, enhanced production of TGs, decreased proteolysis of apo B-100, and increased VLDL production. Decreased removal of postprandial TGs often accompanies these metabolic abnormalities. Genes regulating the expression of the major players in this metabolic cascade, such as LPL, cholesterol ester transfer protein, and hepatic lipase, can modulate the expression of small, dense LDL but these are not the major defects. New candidates for major gene effects have been identified on chromosome 1. Regardless of their fundamental causes, small, dense LDL (compared with normal LDL) particles have a prolonged residence time in plasma, are more susceptible to oxidation because of decreased interaction with the LDL receptor, and enter the arterial wall more easily, where they are retained more readily. Small, dense LDL promotes endothelial dysfunction and enhanced production of procoagulants by endothelial cells. Both in animal models of atherosclerosis and in most human epidemiologic studies and clinical trials, small, dense LDL (particularly when present in increased numbers) appears more atherogenic than normal LDL. Treatment of patients with small, dense LDL particles (particularly when accompanied by low HDL and hypertriglyceridemia) often requires the use of combined lipid-altering drugs to decrease the number of particles and to convert them to larger, more buoyant LDL. The next critical step in further reduction of CAD will be the correct diagnosis and treatment of patients with small, dense LDL and the dyslipidemia that accompanies it.

Evidence of insulin resistant lipid metabolism in adipose tissue in familial combined hyperlipidemia, but not type 2 diabetes mellitus

In patients with familial combined hyperlipidemia (FCHL) and type 2 diabetes (DM2) organ-specific differences in insulin resistance may exist. In FCHL and DM2 in vivo insulin mediated muscle glucose uptake and inhibition of lipolysis were studied by euglycemic hyperinsulinemic clamp. Insulin mediated glucose uptake was impaired to the same extent in both FCHL and DM2. Only FCHL subjects showed no reduction in plasma glycerol concentrations during insulin infusion and incomplete suppression of plasma free fatty acid (FFA) concentrations combined. This finding indicated that insulin-induced suppression of lipolysis, or glycerol/FFA utilization, or both, were impaired in FCHL, in contrast to DM2 or control subjects. To analyze these possibilities in more detail, control, FCHL, and DM2 adipocytes were studied in vitro. In contrast to adipocytes from DM2 or control subjects, no reduction in medium FFA concentration was detected with FCHL adipocytes after incubation with insulin. This finding indicated impaired intracellular FFA utilization, most likely impaired FFA re-esterification. Genetic linkage analysis in 18 Dutch families with FCHL revealed no evidence for involvement of LIPE, the hormone sensitive lipase gene, indicating that genetic variation in adipocyte lipolysis by LIPE is not the key defect in FCHL. In conclusion, FCHL as well as DM2 subjects exhibited in vivo insulin resistance to glucose disposal, which occurs mainly in muscle. FCHL subjects showed insulin resistant adipose tissue lipid metabolism, in contrast to DM2 and controls. The different pattern of organ-specific insulin resistance in FCHL versus DM2 advances our understanding of differences and similarities in phenotypes between these disorders.

Comment: the methionine 196 arginine polymorphism in exon 6 of the TNF receptor 2 gene (TNFRSF1B) is associated with the polycystic ovary syndrome and hyperandrogenism

Inflammatory cytokines such as TNF alpha may play a role in the pathogenesis of common metabolic disorders, including hyperandrogenism and the polycystic ovary syndrome (PCOS). The TNF receptor 2 mediates most of the metabolic effects of TNF alpha. In the present study, we have evaluated serum soluble TNF receptor 2 levels, and several common polymorphisms in the TNF receptor 2 gene (TNFRSF1B), in women presenting with PCOS or hyperandrogenic disorders. Initial studies included 103 hyperandrogenic patients (42 presenting with PCOS) and 36 controls from Spain. The 196R alleles of the M196R (676 T-->G) variant in exon 6 of TNFRSF1B, which is in linkage disequilibrium with a CA-repeat microsatellite polymorphism in intron 4 of TNFRSF1B, tended to be more frequent in hyperandrogenic patients than in controls (P = 0.056), reaching statistical significance when the analysis was restricted to include only PCOS patients (P < 0.03). Extended analysis including another 11 hyperandrogenic patients from Spain and 64 patients and 29 controls from Italy confirmed the association between 196R alleles of the M196R variant and hyperandrogenic disorders (P < 0.05), which was maintained when restricting the analysis to PCOS patients (P < 0.02). On the contrary, the 3'-untranslated region (exon 10) variants 1663 G-->A, 1668 T-->G, and 1690 T-->C were not associated with hyperandrogenism. The soluble TNF receptor 2 levels were not different between patients and controls but were increased in obese subjects, compared with lean individuals, and were affected by the interaction between the 1663 G-->A and 1668 T-->G variants in the 3'-untranslated region of TNFRSF1B. The TNFRSF1B genotype did not influence any clinical or biochemical variable related to hyperandrogenism or insulin sensitivity and was not associated with obesity, both in hyperandrogenic patients and healthy controls considered separately. In conclusion, the M196R (676 T-->G) variant in exon 6 of TNFRSF1B is associated with hyperandrogenism and PCOS, further suggesting a role for inflammatory cytokines in the pathogenesis of these disorders.

Mutation in the ARH gene and a chromosome 13q locus influence cholesterol levels in a new form of digenic-recessive familial hypercholesterolemia

We studied a Syrian family with 3 children who had low-density lipoprotein cholesterol (LDL) concentrations of 13.3, 12.2, and 8.6 mmol/L, respectively. Three other siblings and the parents all had LDL values <4.52 mmol/L, suggesting an autosomal-recessive mode of inheritance. The extended pedigree had 66 additional persons with normal LDL values. A genome-wide scan in the core family with 427 markers showed support for linkage on both chromosomes 1 and 13. Markers on chromosome 1 revealed a 3.07 multipoint LOD score between 1p36.1-p35, an 18-cM interval. Surprisingly, we also found linkage to 13q22-q32, a 14-cM interval, with a 3.08 LOD score. We had identified this locus earlier as containing a gene strongly influencing LDL in another Arab family with autosomal-dominant familial hypercholesterolemia and in normal dizygotic twins. We found evidence for an interaction between these loci. We next genotyped our twin panel and confirmed linkage of the 1p36.1-p35 locus to LDL (P<0.002) in this normal population. Elucidation of ARH, the LDL receptor adaptor protein at chromosome 1p35, caused us to sequence that gene. We first identified the genomic structure of ARH gene and then sequenced the gene in our family. We found an intron 1 acceptor splice-site mutation. This mutation was not found in any other family members, in 31 nonrelated Syrian persons, or in 30 Germans. Our results underscore the importance of ARH on chromosome 1 and the chromosome 13q locus to LDL, not only in families with unusual illnesses, but also to the general population.

Genetic dissection of familial combined hyperlipidemia

Familial combined hyperlipidemia (FCHL) is the most common genetic hyperlipidemia in man. FCHL is characterized by familial clustering of hyperlipidemia and clinical manifestations of premature coronary heart disease, i.e., before the age of 60. Although FCHL was delineated about 25 years ago, at present the FCHL phenotype and its complex genetics are not fully understood. Initially, the familial aggregation of high plasma total cholesterol and triglyceride levels, with a bimodal distribution of triglycerides, was taken as evidence of a dominant mode of inheritance. However, it is now clear that the genetics of FCHL is more complex, and it has been suggested that FCHL is heterogeneous. Several approaches can be taken to identify genes contributing to the disease phenotype in complex genetic disorders either by studying the disease in the human situation or by using animal models. Recent reports have shown that a combination of genetic linkage studies, association studies, and differential gene expression studies provides a useful tool for the genetic dissection of complex diseases. Therefore, the genetic strategies that will be used to dissect the genetic background of FCHL are reviewed.

TNFRSF1B in genetic predisposition to clinical neuropathy and effect on HDL cholesterol and glycosylated hemoglobin in type 2 diabetes

OBJECTIVE

Genetic variation in the tumor necrosis factor (TNF) receptor 2 gene (TNFRSF1B) has shown association with insulin resistance in type 2 diabetes, hypercholesterolemia, coronary artery disease, and essential hypertension. Here we tested the TNFRSF1B marker used in the latter studies in type 2 diabetes patients.

RESEARCH DESIGN AND METHODS

A case-control study of a microsatellite marker with five alleles (CA13- CA17) in intron 4 of TNFRSF1B was performed in 357 well-characterized white patients and 183 healthy control subjects.

RESULTS

The CA16 allele was associated with clinical neuropathy (frequency = 27% in 69 patients with the condition versus 16% in 230 subjects without the condition; chi2 = 9.0, P = 0.011; odds ratio = 2.1 [95% CI 1.2-3.8]). No association was seen with other complications or diabetes itself. The CA16 allele tracked with elevation plasma HDL cholesterol (1.3 +/- 0.2, 1.2 +/- 0.4, and 1.1 +/- 0.2 for CA16/CA16, CA16/-, and -/-, respectively; n = 9, 110, and 218, respectively; P = 0.009) and reduction in plasma glycosylated hemoglobin (6.6 +/- 0.3, 8.3 +/- 0.2, and 8.1 +/- 0.1 for CA16/CA16, CA16/-, and -/-, respectively; n = 9, 102, 205, respectively; P = 0.007) Significance remained after Bonferroni correction for multiple testing.

CONCLUSIONS

Genetic variation in or near TNFRSF1B may predispose clinical neuropathy, reduced glycosylated hemoglobin, and increased HDL cholesterol in type 2 diabetes patients. The latter could be part of a protective response.