Apolipoprotein E-C1-C4-C2 gene cluster region and inter-individual variation in plasma lipoprotein levels: a comprehensive genetic association study in two ethnic groups

Mapping the APOE structurally on missense variants in elderly Brazilians

Cardiovascular diseases (CVDs) pose a significant global health threat, with familial hypercholesterolemia (FH) being a key genetic contributor. The apolipoprotein E (APOE) gene plays a vital role in lipid metabolism, and its variants are associated with CVD risk. This study explores prevalent APOE variants (p.R163C, p.R176C, p.R246C and p.V254E) using genetic and structural analyses. The research, initiated by identifying high-frequency APOE variants through the ABraOM database, utilizes homology modeling and molecular dynamics (MD) simulations to understand the structural consequences. The major lipid-binding region, a critical domain for lipid metabolism, was a focal point. Structural dynamics, including principal component analyses and domain movement analyses, highlighted distinct patterns in APOE variants compared to the wild type (WT). Results revealed significant differences in the structural behavior of variants, particularly in the Major lipid-binding region. The identification of an 'elbow' structure with two states (Elbow I and Elbow II) provided insights into conformational changes. Notably, variants exhibited unique patterns in hydrogen bonding (hb) and hydrophobic interactions, indicating potential functional consequences. The study further associated APOE variants with clinical outcomes, including cognitive impairment and cholesterol levels. Specific variants demonstrated correlations with cognitive decline and variations in lipid profiles, emphasizing their relevance to cardiovascular and neurobiological health. In conclusion, this integrated approach enhances our understanding of APOE variants, shedding light on their role in lipid metabolism and cardiovascular health. The identified structural 'elbows' and their association with clinical outcomes offer a nuanced perspective, guiding future research toward targeted interventions for diseases linked to lipid metabolism and neurobiology.Communicated by Ramaswamy H. Sarma.

Accurate and efficient estimation of local heritability using summary statistics and the linkage disequilibrium matrix

Existing SNP-heritability estimators that leverage summary statistics from genome-wide association studies (GWAS) are much less efficient (i.e., have larger standard errors) than the restricted maximum likelihood (REML) estimators which require access to individual-level data. We introduce a new method for local heritability estimation-Heritability Estimation with high Efficiency using LD and association Summary Statistics (HEELS)-that significantly improves the statistical efficiency of summary-statistics-based heritability estimator and attains comparable statistical efficiency as REML (with a relative statistical efficiency >92%). Moreover, we propose representing the empirical LD matrix as the sum of a low-rank matrix and a banded matrix. We show that this way of modeling the LD can not only reduce the storage and memory cost, but also improve the computational efficiency of heritability estimation. We demonstrate the statistical efficiency of HEELS and the advantages of our proposed LD approximation strategies both in simulations and through empirical analyses of the UK Biobank data.

Sequence Variant Analysis of the APOCII Locus among an Arab Cohort

Apolipoprotein CII (ApocII) plays a key role in regulating lipoprotein lipase (LPL) in lipid metabolism and transport. Numerous polymorphisms within APOCII are reportedly associated with type 2 diabetes mellitus (T2DM), dyslipidemia, and aberrant plasma lipid levels. Few studies have investigated sequence variants at APOCII loci and their association with metabolic disorders. This study aimed to identify and characterize genetic variants by sequencing the full APOCII locus and its flanking sequences in a sample of the Kuwaiti Arab population, including patients with T2DM, hypertriglyceridemia, non-Arab patients with T2DM, and healthy Arab controls. A total of 52 variants were identified in the noncoding sequences: 45 single nucleotide polymorphisms, wherein five were novel, and seven insertion deletions. The minor allele frequency (MAF) of the 47 previously reported variants was similar to the global MAF and to that reported in major populations. Sequence variant analysis predicted a conserved role for APOCII with a potential role for rs5120 in T2DM and rs7133873 as an informative ethnicity marker. This study adds to the ongoing research that attempts to identify ethnicity-specific variants in the apolipoprotein gene loci and associated LPL genes to elucidate the molecular mechanisms of metabolic disorders.

Apolipoprotein E (APOE) Haplotypes in Healthy Subjects from Worldwide Macroareas: A Population Genetics Perspective for Cardiovascular Disease, Neurodegeneration, and Dementia

Human APOE is a 299-amino acid long protein expressed and secreted in several tissues and body districts, where it exerts different functions mainly related to lipid metabolism, with specific activities around cholesterol transport and absorption/elimination. It has three main isoforms, determined by the pair of mutations rs7412-C/T and rs429358-C/T, which gives rise to the functionally different APOE variants ε2, ε3, and ε4. These have a distinct impact on lipid metabolism and are differentially implicated in Alzheimer’s disease and neurodegeneration, cardiovascular disease, and dyslipidemia. A plethora of other single nucleotide variants along the sequence of the APOE gene have been studied in cohorts of affected individuals, where they also modulate the influence of the three main isoforms to determine the risk of developing the disease. However, no contextual analysis of gene-long haplotypes has been carried out so far, and never extensively in cohorts of healthy individuals from different worldwide populations. Leveraging a rich population genomics dataset, this study elucidates the distribution of APOE variants and haplotypes that are shared across populations and to specific macroareas, revealing a variety of risk-allele associations that distinguish specific ancestral backgrounds and can be leveraged for specific ancestry-informed screenings in medicine and public health.

Accurate and Efficient Estimation of Local Heritability using Summary Statistics and LD Matrix

Existing SNP-heritability estimation methods that leverage GWAS summary statistics produce estimators that are less efficient than the restricted maximum likelihood (REML) estimator using individual-level data under linear mixed models (LMMs). Improving statistical efficiency, i.e., increasing the precision and reducing the variance of a heritability estimator, is particularly important for regional analyses, as local genetic variances tend to be small. We introduce a new estimator for local heritability, "HEELS", which attains comparable statistical efficiency as REML ( i.e . relative efficiency greater than 92%) but only requires summary-level statistics - Z-scores from marginal association tests and the empirical LD. HEELS significantly improves the statistical efficiency of the existing summary-statistics-based heritability estimators, such as GRE and LDSC, by reducing the variance of the heritability estimator by more than 3-fold and 7-fold, for GRE and LDSC respectively. Moreover, HEELS remains statistically unbiased and efficient under model mis-specification. We also introduce a unified framework to evaluate and compare the performance of different LD approximations. We propose representing the empirical LD as the sum of a low-rank matrix and a banded matrix. This approximation not only reduces the storage cost and thus improves the portability of the LD matrix, but also increases the computational efficiency of the HEELS estimation. We demonstrate the statistical efficiency of HEELS and the advantages of our proposed LD approximation strategies both in simulations and through empirical analyses of the UK Biobank data.

Lipoprotein Metabolism, Protein Aggregation, and Alzheimer's Disease: A Literature Review

Alzheimer's disease (AD) is the most common form of dementia. The physiopathology of AD is well described by the presence of two neuropathological features: amyloid plaques and tau neurofibrillary tangles. In the last decade, neuroinflammation and cellular stress have gained importance as key factors in the development and pathology of AD. Chronic cellular stress occurs in degenerating neurons. Stress Granules (SGs) are nonmembranous organelles formed as a response to stress, with a protective role; however, SGs have been noted to turn into pathological and neurotoxic features when stress is chronic, and they are related to an increased tau aggregation. On the other hand, correct lipid metabolism is essential to good function of the brain; apolipoproteins are highly associated with risk of AD, and impaired cholesterol efflux and lipid transport are associated with an increased risk of AD. In this review, we provide an insight into the relationship between cellular stress, SGs, protein aggregation, and lipid metabolism in AD.

Infertility and treatments used have minimal effects on first-trimester placental DNA methylation and gene expression

OBJECTIVE

To determine whether deoxyribonucleic acid (DNA) methylation alterations exist in the first-trimester human placenta between conceptions using fertility treatments and those that do not and, if so, whether they are the result of underlying infertility or fertility treatments. We also assessed whether significant alterations led to changes in gene expression.

DESIGN

We compared DNA methylation of the first-trimester placenta from singleton pregnancies that resulted in live births from unassisted, in vitro fertilization (IVF), and non-IVF fertility treatment (NIFT) conceptions using the Infinium MethylationEPIC BeadChip array. Significant CpG sites were compared with corresponding ribonucleic acid sequencing analysis in similar cohorts to determine whether methylation alterations lead to differences in gene expression.

SETTING

Academic medical center.

PATIENT(S)

A total of 138 singleton pregnancies undergoing chorionic villus sampling resulting in a live birth were recruited for methylation analysis (56 unassisted, 38 NIFT, and 44 IVF conceptions). Ribonucleic acid-sequencing data consisted of 141 subjects (74 unassisted, 33 NIFT, and 34 IVF conceptions) of which 116 overlapped with the methylation cohort.

INTERVENTION(S)

In vitro fertilization-conceived pregnancy or pregnancy conceived via NIFT, such as ovulation induction and intrauterine insemination.

MAIN OUTCOME MEASURE(S)

Significant methylation changes at CpG sites after adjustment for multiple comparisons. The secondary outcome was gene expression changes of significant CpG sites.

RESULT(S)

Of the 741,145 probes analyzed in the placenta, few were significant at Bonferroni <0.05: 185 CpG sites (0.025%) significant in pregnancies conceived with the fertility treatments (NIFT + IVF) vs. unassisted conceptions; 28 in NIFT vs. unassisted; 195 in IVF vs. unassisted; and only 13 (0.0018%) in IVF vs. NIFT conceptions. Of all significant CpG sites combined, 10% (35) were located in genes with suggestive gene expression changes, but none were significant after adjustment for multiple comparisons (ribonucleic acid sequencing false discovery rate <0.05). None of the 13 differentially methylated probes in the IVF vs. NIFT placenta were located in genes with suggestive IVF vs. NIFT gene expression differences.

CONCLUSION(S)

Underlying infertility is the most significant contributor to the minimal differences in first-trimester placental methylation, and not the specific fertility treatment used, such as IVF.

Plasma proteomics reveals crosstalk between lipid metabolism and immunity in dairy cows receiving essential fatty acids and conjugated linoleic acid

Essential fatty acids (EFA) and conjugated linoleic acids (CLA) are unsaturated fatty acids with immune-modulatory effects, yet their synergistic effect is poorly understood in dairy cows. This study aimed at identifying differentially abundant proteins (DAP) and their associated pathways in dairy cows supplied with a combination of EFA and CLA during the transition from antepartum (AP) to early postpartum (PP). Sixteen Holstein cows were abomasally infused with coconut oil as a control (CTRL) or a mixture of EFA (linseed + safflower oil) and CLA (Lutalin, BASF) (EFA + CLA) from − 63 to + 63 days relative to parturition. Label-free quantitative proteomics was performed on plasma samples collected at days − 21, + 1, + 28, and + 63. During the transition time, DAP, consisting of a cluster of apolipoproteins (APO), including APOE, APOH, and APOB, along with a cluster of immune-related proteins, were related to complement and coagulation cascades, inflammatory response, and cholesterol metabolism. In response to EFA + CLA, specific APO comprising APOC3, APOA1, APOA4, and APOC4 were increased in a time-dependent manner; they were linked to triglyceride-enriched lipoprotein metabolisms and immune function. Altogether, these results provide new insights into metabolic and immune adaptation and crosstalk between them in transition dairy cows divergent in EFA + CLA status.

Apolipoprotein E and Atherosclerosis

PURPOSE OF REVIEW

The functions, genetic variations and impact of apolipoprotein E on lipoprotein metabolism in general are placed in the context of clinical practice dealing with moderate dyslipidaemia as well as dysbetalipoproteinemia, a highly atherogenic disorder and lipoprotein glomerulopathy.

RECENT FINDINGS

Additional variants of apolipoprotein E and participation of apolipoprotein E in inflammation are of interest. The mostly favourable effects of apolipoprotein E2 as well as the atherogenic nature of apolipoproteinE4, which has an association with cognitive impairment, are confirmed. The contribution of remnant lipoproteins of triglyceride-rich lipoproteins, of which dysbetalipoproteinemia represents an extreme, is explored in atherosclerosis. Mimetic peptides may present new therapeutic approaches. Apolipoprotein E is an important determinant of the lipid profile and cardiovascular health in the population at large and can precipitate dysbetalipoproteinemia and glomerulopathy. Awareness of apolipoprotein E polymorphisms should improve medical care.

Hepatic lipase (LIPC) sequencing in individuals with extremely high and low high-density lipoprotein cholesterol levels

Common variants in the hepatic lipase (LIPC) gene have been shown to be associated with plasma lipid levels; however, the distribution and functional features of rare and regulatory LIPC variants contributing to the extreme lipid phenotypes are not well known. This study was aimed to catalogue LIPC variants by resequencing the entire LIPC gene in 95 non-Hispanic Whites (NHWs) and 95 African blacks (ABs) with extreme HDL-C levels followed by in silico functional analyses. A total of 412 variants, including 43 novel variants were identified; 56 were unique to NHWs and 234 were unique to ABs. Seventy-eight variants in NHWs and 89 variants in ABs were present either in high HDL-C group or low HDL-C group. Two non-synonymous variants (p.S289F, p.T405M), found in NHWs with high HDL-C group were predicted to have damaging effect on LIPC protein by SIFT, MT2 and PP2. We also found several non-coding variants that possibly reside in the circRNA and lncRNA binding sites and may have regulatory potential, as identified in rSNPbase and RegulomeDB databases. Our results shed light on the regulatory nature of rare and non-coding LIPC variants as well as suggest their important contributions in affecting the extreme HDL-C phenotypes.

High density lipoprotein cholesterol and apolipoprotein A-I are associated with greater cerebral perfusion in multiple sclerosis

BACKGROUND

The pathophysiological mechanisms underlying the associations of multiple sclerosis (MS) neurodegeneration serum cholesterol profiles is currently unknown.

OBJECTIVE

To determine associations between lipid profile measures and cerebral perfusion-based indices in MS patients.

METHODS

Seventy-seven MS patients underwent 3 T MRI. Cerebral blood volume (CBV), time-to-peak (TTP) and mean transit time (MTT) measures were computed from dynamic susceptibility contrast (DSC) perfusion-weighted imaging (PWI) for normal-appearing brain tissue (NABT), GM, cortex, deep gray matter (DGM) and thalamus. Total cholesterol, low and high-density lipoprotein cholesterol (LDL-C and HDL-C) and the apolipoproteins (Apo), ApoA-I, ApoA-II, ApoB, ApoC-II and ApoE levels were measured in plasma. Age and body mass index (BMI)-adjusted correlations were used to assess the associations between PWI and lipid profile measures.

RESULTS

Higher HDL-C levels were associated with shorter MTT, which are indicative of greater perfusion, in NABT (p = 0.012), NAWM (p = 0.021), GM (p = 0.009), cortex (p = 0.014), DGM p = 0.015; and thalamus p = 0.015). The HDL-C-associated apolipoproteins, ApoA-I and ApoA-II, were associated with shorter MTT of the same brain regions (all p < 0.028). HDL-C and ApoA-I levels were also associated with shorter TTP, indicative of faster cerebral blood delivery. ApoC-II was associated with lower nCBV of the GM and cortex (p = 0.035 and p = 0.014, respectively).

CONCLUSION

The HDL pathway is associated with better global brain perfusion and faster cerebral blood delivery as measured by shorter MTT and TTP, respectively. ApoC-II may be associated with lower cortical and DGM perfusion.

A minor allele of the haplotype located in the 19q13 loci is associated with a decreased risk of hyper-LDL-cholesterolemia, and a balanced diet and high protein intake can reduce the risk

BACKGROUND

Although the human chromosome 19q13 loci are reported to be associated with hyper-LDL-cholesterolemia, the haplotype of single nucleotide polymorphism (SNP) has not been studied. Therefore, the association of the haplotype in 19q13 loci with hyper-LDL-cholesterolemia was determined and their interactions with lifestyles and nutrient intakes were evaluated in 28,445 Koreans aged > 40 years.

METHODS

SNPs were selected from 19q13 loci that had an association with hyper-LDL-cholesterolemia with the adjustment of confounders (age, gender, area of residence, and body mass index). Haplotype was constructed from the selected SNPs. An adjusted odds ratio of the haplotype for hyper-LDL-cholesterolemia and the interaction between haplotype and lifestyles was analyzed after adjusting for covariates.

RESULTS

Hyper-LDL-cholesterolemia had an association with apolipoprotein E (APOE)_ rs7259620, translocase of outer mitochondrial membrane 40(TOMM40)_rs157581, poliovirus receptor-related 2(PVRL2)_rs403155, exocyst complex component 3-like 2(EXOC3L2)_ rs10406604 and CD3e molecule-associated protein (CD3EAP)_rs3212986 in 19q13. The haplotype of these SNPs had a negative association with hyper-total-cholesterolemia and hyper-LDL-cholesterolemia by 0.669 and 0.684 times, respectively, after adjusting for covariates. The incidence of cardiovascular diseases, especially myocardial infarction, had a negative association with the minor alleles. The balanced diet pattern (BD) and protein intake had a significant interaction with the haplotype: the major-allele of the haplotype exhibited a positive association with hyper-LDL-cholesterolemia, compared to the minor allele, only when combined with a high intake of BD. The participants with the minor allele exhibited a lower hyper-LDL-cholesterolemia risk compared to those with the major allele only with high protein intake.

CONCLUSION

The minor allele of haplotype located in 19q13 loci protected against hyper-LDL-cholesterolemia, especially with BD and high protein intake. The minor allele also had a negative association with myocardial infarction events.

An adaptive test for meta-analysis of rare variant association studies

Single genome-wide studies may be underpowered to detect trait-associated rare variants with moderate or weak effect sizes. As a viable alternative, meta-analysis is widely used to increase power by combining different studies. The power of meta-analysis critically depends on the underlying association patterns and heterogeneity levels, which are unknown and vary from locus to locus. However, existing methods mainly focus on one or only a few combinations of the association pattern and heterogeneity level, thus may lose power in many situations. To address this issue, we propose a general and unified framework by combining a class of tests including and beyond some existing ones, leading to high power across a wide range of scenarios. We demonstrate that the proposed test is more powerful than some existing methods in simulation studies, then show their performance with the NHLBI Exome-Sequencing Project (ESP) data. One gene (B4GALNT2) was found by our proposed test, but not by others, to be statistically significantly associated with plasma triglyceride. The signal was driven by African-ancestry subjects but it was previously reported to be associated with coronary artery disease among European-ancestry subjects. We implemented our method in an R package aSPUmeta, publicly available at https://github.com/ytzhong/metaRV and will be on CRAN soon.

A phenome-wide association study (PheWAS) in the Population Architecture using Genomics and Epidemiology (PAGE) study reveals potential pleiotropy in African Americans

We performed a hypothesis-generating phenome-wide association study (PheWAS) to identify and characterize cross-phenotype associations, where one SNP is associated with two or more phenotypes, between thousands of genetic variants assayed on the Metabochip and hundreds of phenotypes in 5,897 African Americans as part of the Population Architecture using Genomics and Epidemiology (PAGE) I study. The PAGE I study was a National Human Genome Research Institute-funded collaboration of four study sites accessing diverse epidemiologic studies genotyped on the Metabochip, a custom genotyping chip that has dense coverage of regions in the genome previously associated with cardio-metabolic traits and outcomes in mostly European-descent populations. Here we focus on identifying novel phenome-genome relationships, where SNPs are associated with more than one phenotype. To do this, we performed a PheWAS, testing each SNP on the Metabochip for an association with up to 273 phenotypes in the participating PAGE I study sites. We identified 133 putative pleiotropic variants, defined as SNPs associated at an empirically derived p-value threshold of p<0.01 in two or more PAGE study sites for two or more phenotype classes. We further annotated these PheWAS-identified variants using publicly available functional data and local genetic ancestry. Amongst our novel findings is SPARC rs4958487, associated with increased glucose levels and hypertension. SPARC has been implicated in the pathogenesis of diabetes and is also known to have a potential role in fibrosis, a common consequence of multiple conditions including hypertension. The SPARC example and others highlight the potential that PheWAS approaches have in improving our understanding of complex disease architecture by identifying novel relationships between genetic variants and an array of common human phenotypes.

Apolipoprotein C1: Its Pleiotropic Effects in Lipid Metabolism and Beyond

Apolipoprotein C1 (apoC1), the smallest of all apolipoproteins, participates in lipid transport and metabolism. In humans, APOC1 gene is in linkage disequilibrium with APOE gene on chromosome 19, a proximity that spurred its investigation. Apolipoprotein C1 associates with triglyceride-rich lipoproteins and HDL and exchanges between lipoprotein classes. These interactions occur via amphipathic helix motifs, as demonstrated by biophysical studies on the wild-type polypeptide and representative mutants. Apolipoprotein C1 acts on lipoprotein receptors by inhibiting binding mediated by apolipoprotein E, and modulating the activities of several enzymes. Thus, apoC1 downregulates lipoprotein lipase, hepatic lipase, phospholipase A2, cholesterylester transfer protein, and activates lecithin-cholesterol acyl transferase. By controlling the plasma levels of lipids, apoC1 relates directly to cardiovascular physiology, but its activity extends beyond, to inflammation and immunity, sepsis, diabetes, cancer, viral infectivity, and-not last-to cognition. Such correlations were established based on studies using transgenic mice, associated in the recent years with GWAS, transcriptomic and proteomic analyses. The presence of a duplicate gene, pseudogene APOC1P, stimulated evolutionary studies and more recently, the regulatory properties of the corresponding non-coding RNA are steadily emerging. Nonetheless, this prototypical apolipoprotein is still underexplored and deserves further research for understanding its physiology and exploiting its therapeutic potential.