Obesity risk gene TMEM18 encodes a sequence-specific DNA-binding protein

Expression of obesity-related genes in human spermatozoa affects the outcomes of reproductive treatments

OBJECTIVE

To study the abundance of obesity-related gene (ORG) mRNA in human spermatozoa and its association with sperm quality parameters, embryonic development, and pregnancy rates after assisted reproduction treatment (ART).

DESIGN

Cross-sectional study of spermatozoa ORG mRNA expression, and sperm and embryonic development parameters of infertile couples attending a single ART center.

SETTING

University, in collaboration with a medically assisted reproduction center.

PATIENT(S)

One hundred six couples seeking fertility treatment and receiving ART.

INTERVENTION(S)

Expression of spermatozoa ORG mRNA was assessed by quantitative reverse transcription-polymerase chain reaction. Sperm and embryonic development parameters were measured by board-certified embryologists. Serum β-human chorionic gonadotropin levels and fetal heartbeat detection on ultrasound were used to document biochemical and clinical pregnancy, respectively.

MAIN OUTCOME MEASURE(S)

Correlations between the abundance of ORG transcripts in spermatozoa and sperm quality, embryonic development, and achievement of pregnancy.

RESULTS

The abundance of spermatozoa FTO mRNA was positively correlated with total sperm count (r = 0.5030), fertilization rate (r = 0.4854), embryo cleavage rate (r = 0.5705), and high-quality embryo rate (r = 0.6982). The abundance of spermatozoa MC4R transcript was negatively correlated with sperm viability (r = -0.3111) and positively correlated with biochemical pregnancy (r = 0.4420). The abundance of MC4R and GNPDA2 transcripts was higher in spermatozoa of men with asthenozoospermia and teratozoospermia than in those with normozoospermia.

CONCLUSION

To our knowledge, this is the first report showing that the abundance of MC4R and FTO transcripts in spermatozoa is associated with sperm and embryo quality parameters, as well as pregnancy rates. Overall, these results further support the view that male factors beyond classic sperm quality parameters, namely the abundance of ORG transcripts, also affect the outcome of ART.

Obesity-related genes are expressed in human Sertoli cells and modulated by energy homeostasis regulating hormones

The strong hormonal dysregulation associated with obesity is responsible for the disruption of several reproductive events. Sertoli cells (SCs) function is dependent on energetic homeostasis and thus, directly associated with energy homeostasis regulating hormones. To further understand the influence of those hormones with SCs function and obesity, we hypothesize that human SCs express obesity-related genes (ORG; MC4R, GNPDA2, TMEM18, and FTO) and that they respond to energy homeostasis regulating hormones (leptin, ghrelin, and glucagon-like protein 1 [GLP-1]) stimuli. To test our hypothesis, SCs were cultured with increasing doses of leptin (0, 5, 25, or 50 ng/ml, for 24 h), ghrelin (0, 20, 100, and 500 pM, for 24 h), and GLP-1 (10, 1000, or 1 × 105 pM, for 6 h). The presence and abundance of ORG transcripts and proteins in SCs were accessed by polymerase chain reaction techniques, Western blot analysis, and immunofluorescence staining. Our results show that human SCs express MC4R, GNPDA2, TMEM18, and FTO in specific cellular locations. MC4R and FTO expression in human SCs was not responsive to the treatments. However, GNPDA2 and TMEM18 expression increased after exposure to the highest concentration of leptin and ghrelin, respectively. We highlight for the first time that human SCs express ORG and that these are responsive to energy homeostasis hormonal stimuli. GNPDA2 and TMEM18 expression respond in opposite directions according to overall energy status, mediated by energy homeostasis regulating hormones. Leptin and ghrelin control of ORG expression by human SCs can be associated with overweight-related infertility and subfertility in males.

The Obesity-Susceptibility Gene TMEM18 Promotes Adipogenesis through Activation of PPARG

TMEM18 is the strongest candidate for childhood obesity identified from GWASs, yet as for most GWAS-derived obesity-susceptibility genes, the functional mechanism remains elusive. We here investigate the relevance of TMEM18 for adipose tissue development and obesity. We demonstrate that adipocyte TMEM18 expression is downregulated in children with obesity. Functionally, downregulation of TMEM18 impairs adipocyte formation in zebrafish and in human preadipocytes, indicating that TMEM18 is important for adipocyte differentiation in vivo and in vitro. On the molecular level, TMEM18 activates PPARG, particularly upregulating PPARG1 promoter activity, and this activation is repressed by inflammatory stimuli. The relationship between TMEM18 and PPARG1 is also evident in adipocytes of children and is clinically associated with obesity and adipocyte hypertrophy, inflammation, and insulin resistance. Our findings indicate a role of TMEM18 as an upstream regulator of PPARG signaling driving healthy adipogenesis, which is dysregulated with adipose tissue dysfunction and obesity.

Obesity-Related Genetic Determinants of Heart Failure Prognosis

PURPOSE

Recent advances in genomics offer a smart option for predicting future risk of disease and prognosis. The objective of this study was to examine the prognostic value in heart failure (HF) patients, of a series of single nucleotide polymorphisms (SNPs).

METHODS

A selection of 192 SNPs found to be related with obesity, body mass index, circulating lipids or cardiovascular diseases were genotyped in 191 patients with HF. Anthropometrical and clinical variables were collected for each patient, and death and readmission by HF were registered as the primary endpoint.

RESULTS

A total of 53 events were registered during a follow-up period of 438 (263-1077) days (median (IQR)). Eight SNPs strongly related to obesity and HF prognosis were selected as possible prognostic variables. From these, rs10189761 and rs737337 variants were independently associated with HF prognosis (HR 2.295 (1.287-4.089, 95% CI); p = 0.005), whereas rs10423928, rs1800437, rs737337 and rs9351814 were related with bad prognosis only in obese patients (HR 2.142 (1.438-3.192, 95% CI); p = 0.00018). Combined scores of the genomic variants were highly predictive of poor prognosis.

CONCLUSIONS

SNPs rs10189761 and rs737337 were identified, for the first time, as independent predictors of major clinical outcomes in patients with HF. The data suggests an additive predictive value of these SNPs for a HF prognosis. In particular for obese patients, SNPs rs10423928, rs1800437, rs737337 and rs9351814 were related with a bad prognosis. Combined scores weighting the risk of each genomic variant could effect interesting new tools to stratify the prognostic risk of HF patients.

The role and possible mechanism of lncRNA AC092159.2 in modulating adipocyte differentiation

Obesity is a major risk factor for metabolic diseases, while adipocyte differentiation is closely related to obesity occurrence. Long noncoding RNAs (lncRNAs) are a unique class of transcripts in regulation of various biological processes. Using lncRNA microarray, we found lncRNA AC092159.2 was highly expressed in differentiated HPA-v and located ~247 bp upstream of the TMEM18, which was associated with BMI and obesity. We aimed to explore the role of AC092159.2 in adipogenesis and the underlying mechanisms. The effects of AC092159.2 gain- and loss-of-function on HPA-v adipogenesis were determined with lentivirus and siRNA-mediated cell transduction, respectively. Lipid accumulation was evaluated by oil red O staining; the expression of AC092159.2, TMEM18 and several adipogenesis makers in HPA-v were analyzed by qPCR/Western blot. We found that the expression of AC092159.2 gradually increased during HPA-v differentiation, and its expression in omental adipose tissue was positively related with BMI among 48 human subjects. Overexpression of AC092159.2 promoted adipocytes differentiation while knockdown of it led to an adipogenic defect. Moreover, the expression of AC092159.2 and TMEM18 were positively correlated during adipogenic differentiation. AC092159.2 overexpression boosted TMEM18 expression while AC092159.2 knockdown restrained TMEM18 expression. Further rescue experiments showed that TMEM18 knockdown partially restrained adipogenic differentiation in AC092159.2 overexpressed HPA-v and adipogenic defect caused by AC092159.2 knockdown could be rescued by TMEM18 overexpression. Luciferase reporter assays revealed that AC092159.2 had a transcriptional activation effect on TMEM18. We concluded that lncRNA AC092159.2 promoted human adipocytes differentiation possibly by regulating TMEM18.

TMEM18: A Novel Prognostic Marker in Acute Myeloid Leukemia

BACKGROUND

Certain nuclear envelope proteins are associated with important cancer cell characteristics, including migration and proliferation. Abnormal expression of and genetic changes in nuclear envelope proteins have been reported in acute myeloid leukemia (AML) patients. Transmembrane protein 18 (TMEM18), a nuclear envelope protein, is involved in neural stem cell migration and tumorigenicity.

METHODS

To examine the prognostic significance of TMEM18 in AML patients, we analyzed an AML cohort from The Cancer Genome Atlas (TCGA, n = 142).

RESULTS

Kaplan-Meier survival analysis revealed that TMEM18 overexpression was associated with a better AML prognosis with good discrimination (p = 0.019). Interestingly, this ability to predict the prognosis was significant in male AML patients, but not in female ones. C-index and area-under-the-curve analyses further supported this discriminative ability and multivariate analysis confirmed its prognostic significance (p = 0.00347). Correlation analysis revealed that TMEM18 had a statistically significant positive correlation with nuclear envelop protein 133 (NUP133), NUP35, NUP54, NUP62, and NUP88.

CONCLUSION

Because the current AML prognostic factors do not take mRNA expression into consideration unlike other cancers, the development of mRNA-based prognostic factors would be beneficial for accurate prediction of the survival of AML patients. Therefore, TMEM18 gene is a potential biomarker for AML.

Obesity-associated gene TMEM18 has a role in the central control of appetite and body weight regulation

An intergenic region of human chromosome 2 (2p25.3) harbors genetic variants which are among those most strongly and reproducibly associated with obesity. The gene closest to these variants is TMEM18, although the molecular mechanisms mediating these effects remain entirely unknown. Tmem18 expression in the murine hypothalamic paraventricular nucleus (PVN) was altered by changes in nutritional state. Germline loss of Tmem18 in mice resulted in increased body weight, which was exacerbated by high fat diet and driven by increased food intake. Selective overexpression of Tmem18 in the PVN of wild-type mice reduced food intake and also increased energy expenditure. We provide evidence that TMEM18 has four, not three, transmembrane domains and that it physically interacts with key components of the nuclear pore complex. Our data support the hypothesis that TMEM18 itself, acting within the central nervous system, is a plausible mediator of the impact of adjacent genetic variation on human adiposity.

AtTMEM18 plays important roles in pollen tube and vegetative growth in Arabidopsis

In flowering plants, pollen tube growth is essential for delivery of male gametes into the female gametophyte or embryo sac for double fertilization. Although many genes have been identified as being involved in the process, the molecular mechanisms of pollen tube growth remains poorly understood. In this study, we identified that the Arabidopsis Transmembrane Protein 18 (AtTMEM18) gene played important roles in pollen tube growth. The AtTMEM18 shares a high similarity with the Transmembrane 18 proteins (TMEM18s) that are conserved in most eukaryotes and may play important roles in obesity in humans. Mutation in the AtTMEM18 by a Ds insertion caused abnormal callose deposition in the pollen grains and had a significant impact on pollen germination and pollen tube growth. AtTMEM18 is expressed in pollen grains, pollen tubes, root tips and other vegetative tissues. The pollen-rescued assays showed that the mutation in AtTMEM18 also caused defects in roots, stems, leaves and transmitting tracts. AtTMEM18-GFP was located around the nuclei. Genetic assays demonstrated that the localization of AtTMEM18 around the nuclei in the generative cells of pollen grains was essential for the male fertility. Furthermore, expression of the rice TMEM18-homologous protein (OsTMEM18) driven by LAT52 promoter could recover the fertility of the Arabidopsis attmem18 mutant. These results suggested that the TMEM18 is important for plant growth in Arabidopsis.

The Drosophila ortholog of TMEM18 regulates insulin and glucagon-like signaling

Transmembrane protein 18 (TMEM18) is an ill-described, obesity-related gene, but few studies have explored its molecular function. We found single-nucleotide polymorphism data, suggesting that TMEM18 may be involved in the regulation/physiology of metabolic syndrome based on associations with insulin, homeostatic model assessment-β (HOMAβ), triglycerides, and blood sugar. We then found an ortholog in the Drosophila genome, knocked down Drosophila Tmem18 specifically in insulin-producing cells, and tested for its effects on metabolic function. Our results suggest that TMEM18 affects substrate levels through insulin and glucagon signaling, and its downregulation induces a metabolic state resembling type 2 diabetes. This work is the first to experimentally describe the metabolic consequences of TMEM18 knockdown, and further supports its association with obesity.

Analysis of Genes Involved in Body Weight Regulation by Targeted Re-Sequencing

Introduction

Genes involved in body weight regulation that were previously investigated in genome-wide association studies (GWAS) and in animal models were target-enriched followed by massive parallel next generation sequencing.

Methods

We enriched and re-sequenced continuous genomic regions comprising FTO, MC4R, TMEM18, SDCCAG8, TKNS, MSRA and TBC1D1 in a screening sample of 196 extremely obese children and adolescents with age and sex specific body mass index (BMI) ≥ 99^th percentile and 176 lean adults (BMI ≤ 15^th percentile). 22 variants were confirmed by Sanger sequencing. Genotyping was performed in up to 705 independent obesity trios (extremely obese child and both parents), 243 extremely obese cases and 261 lean adults.

Results and Conclusion

We detected 20 different non-synonymous variants, one frame shift and one nonsense mutation in the 7 continuous genomic regions in study groups of different weight extremes. For SNP Arg695Cys (rs58983546) in TBC1D1 we detected nominal association with obesity (p_TDT = 0.03 in 705 trios). Eleven of the variants were rare, thus were only detected heterozygously in up to ten individual(s) of the complete screening sample of 372 individuals. Two of them (in FTO and MSRA) were found in lean individuals, nine in extremely obese. In silico analyses of the 11 variants did not reveal functional implications for the mutations. Concordant with our hypothesis we detected a rare variant that potentially leads to loss of FTO function in a lean individual. For TBC1D1, in contrary to our hypothesis, the loss of function variant (Arg443Stop) was found in an obese individual. Functional in vitro studies are warranted.

Determination of obesity associated gene variants related to TMEM18 through ultra-deep targeted re-sequencing in a case-control cohort for pediatric obesity

Genome-wide association studies (GWAS) have revealed association of a locus approximately 25b downstream of the TMEM18 gene with body mass and obesity. We utilized targeted re-sequencing of the body mass associated locus in proximity of TMEM18 in a case-control population of severely obese children and adolescents from the Stockholm area. We expanded our study to include the TMEM18 gene itself, with the aim of identifying body mass associated genetic variants. Sequencing was performed on the SOLiD platform, on long-range PCR fragments generated through targeted amplification of the regions of interest. Candidate single nucleotide polymorphisms (SNPs) were validated by TaqMan genotyping. We were able to observe 131 SNPs across the re-sequenced regions. Chi squared tests comparing the allele frequencies between cases and controls revealed 57 SNPs as candidates for association with obesity. Validation and replication genotyping revealed robust associations for SNPs within the haplotype block region located downstream from the TMEM18 gene. This study provides a high resolution map of the genetic variation pattern in the TMEM18 gene, as well as the associated haplotype block, and further strengthens the association of variants within the proximal haplotype block with obesity and body mass.

Discussing the putative role of obesity-associated genes in the etiopathogenesis of eating disorders

In addition to the identification of mutations clearly related to Mendelian forms of obesity; genome-wide association studies and follow-up studies have in the last years pinpointed several loci associated with BMI. These genetic alterations are located in or near genes expressed in the hypothalamus that are involved in the regulation of eating behavior. Accordingly, it seems plausible that these SNPs, or others located in related genes, could also help develop aberrant conduct patterns that favor the establishment of eating disorders should other susceptibility factors or personality dimensions be present. However, and somewhat surprisingly, with few exceptions such as BDNF, the great majority of the genes governing these pathways remain untested in patients with anorexia nervosa, bulimia nervosa or binge-eating disorder. In the present work, we review the few existing studies, but also indications and biological concepts that point to these genes in the CNS as good candidates for association studies with eating disorder patients.

Association study of common polymorphisms in MSRA, TFAP2B, MC4R, NRXN3, PPARGC1A, TMEM18, SEC16B, HOXB5 and OLFM4 genes with obesity-related traits among Portuguese children

At least 52 genetic loci were associated with obesity-related traits. However, little is known about the genetic basis of obesity among children. This study aims to test whether 10 polymorphisms in obesity-related genes methionine sulfoxide reductase A (MSRA), transcription factor AP-2 beta (TFAP2B), melanocortin 4 receptor (MC4R), neurexin 3 (NRXN3), peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PPARGC1A), transmembrane protein 18 (TMEM18), homolog of S. cerevisiae Sec16 (SEC16B), homeobox B5 (HOXB5) and olfactomedin 4 (OLFM4) are associated with the risk of obesity in Portuguese children. A total of 730 children aging from 6 to 12 years old, recruited randomly from public schools in Portugal, were analysed. Anthropometric measurements were obtained and children were classified into three phenotypic groups, normal weight (n=256), overweight (n=320) and obese (n=154), according to the International Obesity Task Force cutoffs. Polymorphisms were genotyped by allelic discrimination TaqMan assays. The MC4R rs12970134 polymorphism was nominally associated with body mass index (BMI) (P=0.035), BMI Z-score (P=0.043) and waist circumference (P=0.020), and borderline associated with weight (P=0.053). Near nominal associations were also found for the PPARGC1A rs8192678 polymorphism with weight (P=0.061), and for the MSRA rs545854 polymorphism with BMI (P=0.055) and BMI Z-score (P=0.056). Furthermore, logistic regression showed that MC4R rs12970134 and TFAP2B rs987237 were nominally, respectively, associated (P=0.029) and borderline associated (P=0.056) with the obese phenotype. This study highlighted the possible association of MC4R, PPARGC1A, MSRA and TFAP2B polymorphisms with several obesity-related traits in a sample of Portuguese children. The two significant associated TFAP2B rs987237 and MC4R rs12970134 polymorphisms showed an opposite direction of effect to that in the original reports.

Functional analysis of seven genes linked to body mass index and adiposity by genome-wide association studies: a review

Genome-wide association studies (GWAS) have identified a total of about 40 single nucleotide polymorphisms (SNPs) that show significant linkage to body mass index, a widely utilised surrogate measure of adiposity. However, only 8 of these associations have been confirmed by follow-up GWAS using more sophisticated measures of adiposity (computed tomography). Among these 8, there is a SNP close to the gene FTO which has been the subject of considerable work to diagnose its function. The remaining 7 SNPs are adjacent to, or within, the genes NEGR1, TMEM18, ETV5, FLJ35779, LINGO2, SH2B1 and GIPR, most of which are less well studied than FTO, particularly in the context of obesity. This article reviews the available data on the functions of these genes, including information gleaned from studies in humans and animal models. At present, we have virtually no information on the putative mechanism associating the genes FLJ35779 and LINGO2 to obesity. All of these genes are expressed in the brain, and for 2 of them (SH2B1 and GIPR), a direct link to the appetite regulation system is known. SH2B1 is an enhancer of intracellular signalling in the JAK-STAT pathway, and GIPR is the receptor for an appetite-linked hormone (GIP) produced by the alimentary tract. NEGR1, ETV5 and SH2B1 all have suggested roles in neurite outgrowth, and hence SNPs adjacent to these genes may affect development of the energy balance circuitry. Although the genes have central patterns of gene expression, implying a central neuronal connection to energy balance, for at least 4 of them (NEGR1, TMEM18, SH2B1 and GIPR), there are also significant peripheral functions related to adipose tissue biology. These functions may contribute to their effects on the obese phenotype.

Pollen-expressed transcription factor 2 encodes a novel plant-specific TFIIB-related protein that is required for pollen germination and embryogenesis in Arabidopsis

Pollen germination and embryogenesis are important to sexual plant reproduction. The processes require a large number of genes to be expressed. Transcription of eukaryotic nuclear genes is accomplished by three conserved RNA polymerases acting in association with a set of auxiliary general transcription factors (GTFs), including B-type GTFs. The roles of B-type GTFs in plant reproduction remain poorly understood. Here we report functional characterization of a novel plant-specific TFIIB-related gene PTF2 in Arabidopsis. Mutation in PTF2 caused failure of pollen germination. Pollen-rescue revealed that the mutation also disrupted embryogenesis and resulted in seed abortion. PTF2 is expressed prolifically in developing pollen and the other tissues with active cell division and differentiation, including embryo and shoot apical meristem. The PTF2 protein shares a lower amino acid sequence similarity with other known TFIIB and TFIIB-related proteins in Arabidopsis. It can interact with TATA-box binding protein 2 (TBP2) and bind to the double-stranded DNA (dsDNA) as the other known TFIIB and TFIIB-related proteins do. In addition, PTF2 can form a homodimer and interact with the subunits of RNA polymerases (RNAPs), implying that it may be involved in the RNAPs transcription. These results suggest that PTF2 plays crucial roles in pollen germination and embryogenesis in Arabidopsis, possibly by regulating gene expression through interaction with TBP2 and the subunits of RNAPs.

Genetic variation and reproductive timing: African American women from the Population Architecture using Genomics and Epidemiology (PAGE) Study

Age at menarche (AM) and age at natural menopause (ANM) define the boundaries of the reproductive lifespan in women. Their timing is associated with various diseases, including cancer and cardiovascular disease. Genome-wide association studies have identified several genetic variants associated with either AM or ANM in populations of largely European or Asian descent women. The extent to which these associations generalize to diverse populations remains unknown. Therefore, we sought to replicate previously reported AM and ANM findings and to identify novel AM and ANM variants using the Metabochip (n = 161,098 SNPs) in 4,159 and 1,860 African American women, respectively, in the Women's Health Initiative (WHI) and Atherosclerosis Risk in Communities (ARIC) studies, as part of the Population Architecture using Genomics and Epidemiology (PAGE) Study. We replicated or generalized one previously identified variant for AM, rs1361108/CENPW, and two variants for ANM, rs897798/BRSK1 and rs769450/APOE, to our African American cohort. Overall, generalization of the majority of previously-identified variants for AM and ANM, including LIN28B and MCM8, was not observed in this African American sample. We identified three novel loci associated with ANM that reached significance after multiple testing correction (LDLR rs189596789, p = 5×10⁻⁰⁸; KCNQ1 rs79972789, p = 1.9×10⁻⁰⁷; COL4A3BP rs181686584, p = 2.9×10⁻⁰⁷). Our most significant AM association was upstream of RSF1, a gene implicated in ovarian and breast cancers (rs11604207, p = 1.6×10⁻⁰⁶). While most associations were identified in either AM or ANM, we did identify genes suggestively associated with both: PHACTR1 and ARHGAP42. The lack of generalization coupled with the potentially novel associations identified here emphasize the need for additional genetic discovery efforts for AM and ANM in diverse populations.

Energization of vacuolar transport in plant cells and its significance under stress

The plant vacuole is of prime importance in buffering environmental perturbations and in coping with abiotic stress caused by, for example, drought, salinity, cold, or UV. The large volume, the efficient integration in anterograde and retrograde vesicular trafficking, and the dynamic equipment with tonoplast transporters enable the vacuole to fulfill indispensible functions in cell biology, for example, transient and permanent storage, detoxification, recycling, pH and redox homeostasis, cell expansion, biotic defence, and cell death. This review first focuses on endomembrane dynamics and then summarizes the functions, assembly, and regulation of secretory and vacuolar proton pumps: (i) the vacuolar H(+)-ATPase (V-ATPase) which represents a multimeric complex of approximately 800 kDa, (ii) the vacuolar H(+)-pyrophosphatase, and (iii) the plasma membrane H(+)-ATPase. These primary proton pumps regulate the cytosolic pH and provide the driving force for secondary active transport. Carriers and ion channels modulate the proton motif force and catalyze uptake and vacuolar compartmentation of solutes and deposition of xenobiotics or secondary compounds such as flavonoids. ABC-type transporters directly energized by MgATP complement the transport portfolio that realizes the multiple functions in stress tolerance of plants.

Unraveling the brain regulation of appetite: lessons from genetics

Over the past 20 years, genetic studies have illuminated critical pathways in the hypothalamus and brainstem mediating energy homeostasis, such as the melanocortin, leptin, 5-hydroxytryptamine and brain-derived neurotrophic factor signaling axes. The identification of these pathways necessary for appropriate appetitive responses to energy state has yielded insight into normal homeostatic processes. Although monogenic alterations in each of these axes result in severe obesity, such cases remain rare. The major burden of disease is carried by those with common obesity, which has so far resisted yielding meaningful biological insights. Recent progress into the etiology of common obesity has been made with genome-wide association studies. Such studies now reveal more than 32 different candidate obesity genes, most of which are highly expressed or known to act in the CNS, emphasizing, as in rare monogenic forms of obesity, the role of the brain in predisposition to obesity.