Geesje M. Dallinga-Thie

Excess of rare variants in genes identified by genome-wide association study of hypertriglyceridemia

Genome-wide association studies (GWAS) have identified multiple loci associated with plasma lipid concentrations. Common variants at these loci together explain <10% of variation in each lipid trait. Rare variants with large individual effects may also contribute to the heritability of lipid traits; however, the extent to which rare variants affect lipid phenotypes remains to be determined. Here we show an accumulation of rare variants, or a mutation skew, in GWAS-identified genes in individuals with hypertriglyceridemia (HTG). Through GWAS, we identified common variants in APOA5, GCKR, LPL and APOB associated with HTG. Resequencing of these genes revealed a significant burden of 154 rare missense or nonsense variants in 438 individuals with HTG, compared to 53 variants in 327 controls (P = 6.2 × 10−8), corresponding to a carrier frequency of 28.1% of affected individuals and 15.3% of controls (P = 2.6 × 10−5). Considering rare variants in these genes incrementally increased the proportion of genetic variation contributing to HTG.

The therapeutic potential of manipulating gut microbiota in obesity and type 2 diabetes mellitus.

Obesity and type 2 diabetes mellitus (T2DM) are attributed to a combination of genetic susceptibility and lifestyle factors. Their increasing prevalence necessitates further studies on modifiable causative factors and novel treatment options. The gut microbiota has emerged as an important contributor to the obesity—and T2DM—epidemic proposed to act by increasing energy harvest from the diet. Although obesity is associated with substantial changes in the composition and metabolic function of the gut microbiota, the pathophysiological processes remain only partly understood. In this review we will describe the development of the adult human microbiome and discuss how the composition of the gut microbiota changes in response to modulating factors. The influence of short‐chain fatty acids, bile acids, prebiotics, probiotics, antibiotics and microbial transplantation is discussed from studies using animal and human models. Ultimately, we aim to translate these findings into therapeutic pathways for obesity and T2DM in humans.

Elevated remnant-like particle cholesterol concentration - A characteristic feature of the atherogenic lipoprotein phenotype

Patients at increased risk of coronary artery disease (CAD) frequently exhibit an atherogenic lipoprotein phenotype characterized by elevated plasma levels of both triglyceride-rich lipoproteins (TRL) and small, dense LDL and low concentrations of HDL cholesterol. Recently, in a large observational study, the calculated non-HDL plasma cholesterol concentration (the sum of the cholesterol contents of LDL, intermediate-density lipoprotein [IDL], and very-low-density lipoprotein [VLDL]) was a stronger predictor of cardiovascular events than plasma cholesterol alone.1–3 Improvement in the predictability of CAD on inclusion of VLDL and IDL cholesterol emphasizes the proatherogenic nature of TRL and their remnant particles.nnThe atherogenic lipoprotein phenotype has been defined by Austin et al4 as the presence of a predominance of small, dense LDL particles, elevated plasma triglyceride (TG) levels, and low plasma HDL cholesterol levels in the lipoprotein profile, which is associated with an approximately 3-fold increased risk of atherosclerotic disease.5–8 It is now commonly accepted that small, dense LDL particles are the products of the intravascular remodeling of TG-rich VLDL particles after interaction primarily with lipoprotein lipase, hepatic lipase, and cholesterol ester transfer protein9,10 (Figure). The atherogenic lipoprotein phenotype is strongly linked to obesity, insulin resistance, familial combined hyperlipidemia (FCHL), hypertension, and abnormalities in postprandial lipid metabolism.7,11,12 Epidemiological data from the Framingham study have already revealed that plasma TG concentration is an important independent risk indicator of CAD in women13; additional evidence supporting this observation was obtained by Yarnell et al14 in a 10-year follow-up study and confirmed by others.15,16 In the PROCAM (Prospective Cardiovascular Munster) study,17 this relationship was dependent on plasma HDL cholesterol concentration. Criqui et al,18 however, could not demonstrate an independent relationship between plasma TG and cardiovascular mortality in a North American population participating in the Lipid Research Clinics Follow-up. …

Genetic Variation at the Phospholipid Transfer Protein Locus Affects Its Activity and High-Density Lipoprotein Size and Is a Novel Marker of Cardiovascular Disease Susceptibility

Background— In contrast to clear associations between variants in genes participating in low-density lipoprotein metabolism and cardiovascular disease risk, such associations for high-density lipoprotein (HDL)–related genes are not well supported by recent large studies. We aimed to determine whether genetic variants at the locus encoding phospholipid transfer protein (PLTP), a protein involved in HDL remodeling, underlie altered PLTP activity, HDL particle concentration and size, and cardiovascular disease risk. Methods and Results— We assessed associations between 6 PLTP tagging single nucleotide polymorphisms and PLTP activity in 2 studies (combined n=384) and identified 2 variants that show reproducible associations with altered plasma PLTP activity. A gene score based on these variants is associated with lower hepatic PLTP transcription (P=3.2×10−18) in a third study (n=957) and with an increased number of HDL particles of smaller size (P=3.4×10−17) in a fourth study (n=3375). In a combination of 5 cardiovascular disease case-control studies (n=4658 cases and 11 459 controls), a higher gene score was associated with a lower cardiovascular disease risk (per-allele odds ratio, 0.94; 95% confidence interval, 0.90 to 0.98; P=1.2×10−3; odds ratio for highest versus lowest gene score, 0.69; 95% confidence interval, 0.55 to 0.86; P=1.0×10−3). Conclusions— A gene score based on 2 PLTP single nucleotide polymorphisms is associated with lower PLTP transcription and activity, an increased number of HDL particles, smaller HDL size, and decreased risk of cardiovascular disease. These findings indicate that PLTP is a proatherogenic entity and suggest that modulation of specific elements of HDL metabolism may offer cardiovascular benefit.

Lipoprotein lipase gene mutations D9N and N291S in four pedigrees with familial combined hyperlipidaemia

Abstract. The role of the lipoprotein lipase (LPL) gene in familial combined hyperlipidaemia (FCH) is unclear at present. We screened a group of 28 probands with familial combined hyperlipidaemia and a group of 91 population controls for two LPL gene mutations. D9N and N291S. LPL‐D9N was found in two probands and one normolipidaemic population control. LPL‐N291S was found in four probands and four population controls. Subsequently, two pedigrees from probands with the D9N mutation and two pedigrees from probands with the N291S mutation were studied, representing a total of 24 subjects. Both LPL gene mutations were associated with a significant effect on plasma lipids and apolipoproteins. Presence of the D9N mutation (n = 7) was associated with hypertriglyceridaemia [2.69± 1.43 (SD) mmol L‐1] and reduced plasma high‐density lipoprotein cholesterol (HDL‐C) concentrations (0.92± 0.21 mmol L‐1) compared with 11 non‐carriers (triglyceride 1.75± 0.64 mmol L‐1; HDL‐C 1.23± 0.30 mmol L‐1, P= 0.03 and P= 0.025 respectively). LPL‐D9N carriers had higher diastolic blood pressures than non‐carriers. LPL‐N291S carriers (n= 6) showed significantly higher (26%) apo B plasma concentrations (174± 26 mg dL‐1) than non‐carriers (138± 26 mg dL‐1; P= 0.023), with normal post‐heparin plasma LPL activities. Linkage analysis revealed no significant relationship between the D9N or N291S LPL gene mutations and the FCH phenotype (hypertriglyceridaemia, hypercholesterolaemia or increased apo B concentrations). It is concluded that the LPL gene did not represent the major single gene causing familial combined hyperlipidaemia in the four pedigrees studied, but that the LPL‐D9N and LPL‐N291S mutations had significant additional effects on lipid and apolipoprotein phenotype.

Abnormal Patterns of Lipoprotein Lipase Release into the Plasma in GPIHBP1-deficient Mice

GPIHBP1-deficient mice (Gpihbp1–/–) exhibit severe chylomicronemia. GPIHBP1 is located within capillaries of muscle and adipose tissue, and expression of GPIHBP1 in Chinese hamster ovary cells confers upon those cells the ability to bind lipoprotein lipase (LPL). However, there has been absolutely no evidence that GPIHBP1 actually interacts with LPL in vivo. Heparin is known to release LPL from its in vivo binding sites, allowing it to enter the plasma. After an injection of heparin, we reasoned that LPL bound to GPIHBP1 in capillaries would be released very quickly, and we hypothesized that the kinetics of LPL entry into the plasma would differ in Gpihbp1–/– and control mice. Indeed, plasma LPL levels peaked very rapidly (within 1 min) after heparin in control mice. In contrast, plasma LPL levels in Gpihbp1–/– mice were much lower 1 min after heparin and increased slowly over 15 min. In keeping with that result, plasma triglycerides fell sharply within 10 min after heparin in wild-type mice, but were negligibly altered in the first 15 min after heparin in Gpihbp1–/– mice. Also, an injection of Intralipid released LPL into the plasma of wild-type mice but was ineffective in releasing LPL in Gpihbp1–/– mice. The observed differences in LPL release cannot be ascribed to different tissue stores of LPL, as LPL mass levels in tissues were similar in Gpihbp1–/– and control mice. The differences in LPL release after intravenous heparin and Intralipid strongly suggest that GPIHBP1 represents an important binding site for LPL in vivo.

Targeting macrophage Histone deacetylase 3 stabilizes atherosclerotic lesions

Macrophages are key immune cells found in atherosclerotic plaques and critically shape atherosclerotic disease development. Targeting the functional repertoire of macrophages may hold novel approaches for future atherosclerosis management. Here, we describe a previously unrecognized role of the epigenomic enzyme Histone deacetylase 3 (Hdac3) in regulating the atherosclerotic phenotype of macrophages. Using conditional knockout mice, we found that myeloid Hdac3 deficiency promotes collagen deposition in atherosclerotic lesions and thus induces a stable plaque phenotype. Also, macrophages presented a switch to anti‐inflammatory wound healing characteristics and showed improved lipid handling. The pro‐fibrotic phenotype was directly linked to epigenetic regulation of the Tgfb1 locus upon Hdac3 deletion, driving smooth muscle cells to increased collagen production. Moreover, in humans, HDAC3 was the sole Hdac upregulated in ruptured atherosclerotic lesions, Hdac3 associated with inflammatory macrophages, and HDAC3 expression inversely correlated with pro‐fibrotic TGFB1 expression. Collectively, we show that targeting the macrophage epigenome can improve atherosclerosis outcome and we identify Hdac3 as a potential novel therapeutic target in cardiovascular disease.

An Increased Burden of Common and Rare Lipid-Associated Risk Alleles Contributes to the Phenotypic Spectrum of Hypertriglyceridemia

Objective—Earlier studies have suggested that a common genetic architecture underlies the clinically heterogeneous polygenic Fredrickson hyperlipoproteinemia (HLP) phenotypes defined by hypertriglyceridemia (HTG). Here, we comprehensively analyzed 504 HLP-HTG patients and 1213 normotriglyceridemic controls and confirmed that a spectrum of common and rare lipid-associated variants underlies this heterogeneity. Methods and Results—First, we demonstrated that genetic determinants of plasma lipids and lipoproteins, including common variants associated with plasma triglyceride (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C) from the Global Lipids Genetics Consortium were associated with multiple HLP-HTG phenotypes. Second, we demonstrated that weighted risk scores composed of common TG-associated variants were distinctly increased across all HLP-HTG phenotypes compared with controls; weighted HDL-C and LDL-C risk scores were also increased, although to a less pronounced degree with some HLP-HTG phenotypes. Interestingly, decomposition of HDL-C and LDL-C risk scores revealed that pleiotropic variants (those jointly associated with TG) accounted for the greatest difference in HDL-C and LDL-C risk scores. The APOE E2/E2 genotype was significantly overrepresented in HLP type 3 versus other phenotypes. Finally, rare variants in 4 genes accumulated equally across HLP-HTG phenotypes. Conclusion—HTG susceptibility and phenotypic heterogeneity are both influenced by accumulation of common and rare TG-associated variants.

Improvement of Insulin Sensitivity after Lean Donor Feces in Metabolic Syndrome Is Driven by Baseline Intestinal Microbiota Composition

The intestinal microbiota has been implicated in insulin resistance, although evidence regarding causality in humans is scarce. We therefore studied the effect of lean donor (allogenic) versus own (autologous) fecal microbiota transplantation (FMT) to male recipients with the metabolic syndrome. Whereas we did not observe metabolic changes at 18xa0weeks after FMT, insulin sensitivity at 6xa0weeks after allogenic FMT was significantly improved, accompanied by altered microbiota composition. We also observed changes in plasma metabolites such as γ-aminobutyric acid and show that metabolic response upon allogenic FMT (defined as improved insulin sensitivity 6xa0weeks after FMT) is dependent on decreased fecal microbial diversity at baseline. In conclusion, the beneficial effects of lean donor FMT on glucose metabolism are associated with changes in intestinal microbiota and plasma metabolites and can be predicted based on baseline fecal microbiota composition.

Intestinal microbiota and faecal transplantation as treatment modality for insulin resistance and type 2 diabetes mellitus

The prevalence of obesity and diabetes mellitus type 2 is increasing rapidly around the globe. Recent insights have generated an entirely new perspective that the intestinal microbiota may play a significant role in the development of these metabolic disorders. Alterations in the intestinal microbiota composition promote systemic inflammation that is a hallmark of obesity and subsequent insulin resistance. Thus, it is important to understand the reciprocal relationship between intestinal microbiota composition and metabolic health in order to eventually prevent disease progression. In this respect, faecal transplantation studies have implicated that butyrate‐producing intestinal bacteria are crucial in this process and be considered as key players in regulating diverse signalling cascades associated with human glucose and lipid metabolism.