Wnt signaling and the control of human stem cell fate.

Wnt signaling determines major developmental processes in the embryonic state and regulates maintenance, self-renewal and differentiation of adult mammalian tissue stem cells. Both ß-catenin dependent and independent Wnt pathways exist, and both affect stem cell fate in developing and adult tissues. In this review, we debate the response to Wnt signal activation in embryonic stem cells and human, adult stem cells of mesenchymal, hematopoetic, intestinal, gastric, epidermal, mammary and neural lineages, and discuss the need for Wnt signaling in these cell types. Due to the vital actions of Wnt signaling in developmental and maintenance processes, deregulation of the pathway can culminate into a broad spectrum of developmental and genetic diseases, including cancer. The way in which Wnt signals can feed tumors and maintain cancer stem stells is discussed as well. Manipulation of Wnt signals both in vivo and in vitro thus carries potential for therapeutic approaches such as tissue engineering for regenerative medicine and anti-cancer treatment. Although many questions remain regarding the complete Wnt signal cell-type specific response and interplay of Wnt signaling with pathways such as BMP, Hedgehog and Notch, we hereby provide an overview of current knowledge on Wnt signaling and its control over human stem cell fate.

Screening for genetic variants in BDNF that contribute to childhood obesity.

UNLABELLED: What is already known about this subject BDNF is involved in the regulation of food intake and body weight. BDNF deficient animal models are obese. Chromosomal abnormalities cause obesity in humans. What this study adds Evaluation of point mutations in BDNF. Identification of BDNF mutations in obese children. Point mutations in BDNF are not a common cause of childhood obesity. INTRODUCTION: There is ample evidence that BDNF has a role in the regulation of food intake and body weight. Study of various mouse models gave a clear indication that BDNF deficiency leads to the development of obesity. Functional loss of one copy of the BDNF gene, due to chromosomal rearrangements or microdeletions, can cause an obesity phenotype in humans. Therefore, we wanted to investigate whether point mutations in the gene also result in a comparable phenotype. METHODS: We screened 554 severely overweight and obese children and adolescents and 565 lean adults for mutations in the coding region of BDNF. Mutation screening was performed by high-resolution melting curve analysis and direct sequencing. RESULTS: Screening of obese patients led to the identification of two synonymous variations (V37V and H65H) and two non-synonymous coding mutations (T2I and V46M) in the BDNF gene. When we subsequently screened our control population, we found T2I with comparable frequency and confirmed that this is a rare and non-pathogenic variant. In addition, we found another non-synonymous mutation (N187S) in the control population. CONCLUSIONS: In silico analysis of the V46M variant did not support a clear disease-causing effect and no family data were available in order to determine whether the mutation segregates with obesity. However, we cannot rule out a possible pathogenic effect for this variant. In general, we tend to conclude that mutations in the coding region of BDNF are uncommon in obese patients and are therefore not likely to play an essential role in the pathogenesis of childhood obesity.

Common genetic variation in sFRP5 is associated with fat distribution in men.

Considering the role of sFRP5 in Wnt signalling, an important group of pathways regulating adipogenesis and inflammation, we performed a genetic association study on sFRP5 polymorphisms in a population of obese and lean individuals. Using information from the HapMap, two tagSNPs were identified in the sFRP5 gene region and genotyped on a population of 1,014 obese, non-diabetic individuals and 606 lean controls. We performed logistic and linear regression analysis including a wide variety of obesity parameters (BMI, waist circumference, height, WHR, fat mass, fat mass percentage and visceral, subcutaneous and total abdominal fat), in addition to OGTT and HOMA-IR values. We were able to show a significant association of sFRP5 with both total abdominal and subcutaneous fat. The association signal was only seen in obese males, and in this population, the minor allele of rs7072751 explains 1.8 % of variance in total abdominal fat. In addition, we saw a trend towards an association of rs10748709 with glucose metabolism. Although further research is necessary, we can conclude that sFRP5 is a significant regulator of fat development and distribution in obese males. We postulate that altered transcription factor binding on the rs7072751 surrounding sequence might play a role in the associations we found with both total abdominal and subcutaneous fat. In addition, although no conclusive evidence was found, our results indicate that sFRP5 genetic variation may affect glucose metabolism and it would be interesting to investigate this further.

Mutation screen of the SIM1 gene in pediatric patients with early-onset obesity.

BACKGROUND: The transcription factor SIM1 (Single-minded 1) is involved in the control of food intake and in the pathogenesis of obesity. In mice, Sim1 is involved in the development of the paraventricular nucleus, and Sim1 deficiency leads to severe obesity and hyperphagia. In humans, chromosomal abnormalities in the SIM1 gene region have been reported in obese individuals. Furthermore, recent data also suggest that loss-of-function point mutations in SIM1 are responsible for SIM1 haplo-insufficiency that is involved in causing human obesity. In this study, we therefore wanted to expand the evidence regarding the involvement of SIM1 mutations in the pathogenesis of severe early-onset obesity. METHODS: We screened 561 severely overweight and obese children and adolescents and 453 lean adults for mutations in the coding region of the SIM1 gene. Mutation screening in all patients and lean individuals was performed by high-resolution melting curve analysis combined with direct sequencing. To evaluate the effect of the mutations on SIM1 transcriptional activity, luciferase reporter assays were performed. RESULTS: Mutation analysis identified four novel nonsynonymous coding variants in SIM1 in four unrelated obese individuals: p.L242V, p.T481K, p.A517V and p.D590E. Five synonymous variants, p.P57P, p.F93F, p.I183I, p.V208V and p.T653T, were also identified. Screening of the lean control population revealed the occurrence of four other rare SIM1 variants: p.G408R, p.R471P, p.S492P and p.S622F. For variants p.T481K and p.A517V, which were found in obese individuals, a decrease in SIM1 transcriptional activity was observed, whereas the transcriptional activity of all variants found in lean individuals resembled wild type. CONCLUSIONS: In this study, we have demonstrated the presence of rare SIM1 variants in both an obese pediatric population and a population of lean adult controls. Further, we have shown that functional in vitro analysis of SIM1 variants may help in distinguishing benign variants of no pathogenic significance from variants which contribute to the obesity phenotype.

No conclusive evidence for association of polymorphisms in the adiponectin receptor 1 gene, AdipoR1, with common obesity.

AdipoR1 is one of the adiponectin receptors which are important for adiponectin signaling. Because adiponectin is a candidate gene for common obesity, it is also hypothesized that variations in AdipoR1 may be involved in the development of complex obesity. Therefore, we designed an association study for the AdipoR1 gene. We performed a case-control association study including 1,021 obese subjects (mean age 42 ± 12 years; mean BMI 38.2 ± 6.2 kg/m²) and 226 lean, healthy individuals (mean age 36 ± 7 years; mean BMI 22.1 ± 1.7 kg/m²). Nine tagSNPs were selected to cover the entire AdipoR1 gene and surrounding 7 kb region (based on HapMap data). TagSNPs were genotyped using AcycloPrime-Fluorescence Polarization (FP) SNP Detection kits and TaqMan Pre-Designed SNP Genotyping assays according to manufacturer's protocols. We found that the rs1075399 non-reference allele decreases obesity risk by 45 % in men only [odds ratio (OR) = 0.55, 95 % CI 0.35-0.87, nominal P = 0.010]. However, after Bonferroni correction for multiple testing, this association is lost. None of the other tagSNPs were associated with obesity when studying the entire population, nor when looking at men and women separately. Quantitative analysis of the effect of each SNP on height, weight, and BMI revealed that none of the tagSNPs are associated with weight or BMI. We report here that we found no decisive evidence for association between AdipoR1 tagSNPs and complex obesity in our Belgian Caucasian population.

Genetic association between WNT10B polymorphisms and obesity in a Belgian case-control population is restricted to males.

The Wnt pathway has been shown to play an important role in maintenance of stem cells and cell fate decisions in embryonic and adult stem cell populations. Activation of the Wnt pathway in mesenchymal stem cells and 3 T3-L1 cells inhibits adipogenesis and can lead to osteoblastogenesis. To evaluate the role of the Wnt pathway in adipogenesis and obesity further, we analysed the genetic association between polymorphisms in WNT10B, an activator of the Wnt pathway, and various obesity parameters in a Belgian population. Four tagSNPs that captured variation of ten SNPs (MAF>5%) in a 15.2 kb region spanning the WNT10B gene and its 3' and 5' flanking regions were genotyped. Our population consisted of 1013 obese patients (BMI≥30 kg/m(2); 468 males) and 531 lean healthy individuals (18.5 kg/m(2)≤BMI≤24.9 kg/m(2); 194 males). We found a significant association with body mass index (BMI) for three of the genotyped tagSNPs (rs4018511, rs10875902, rs833841) in the male population as analysed by logistic regression. Allelic heterogeneity testing demonstrated that these associations all represent the same significant signal. Two of the three significant SNPs were also found to be associated with BMI and weight in the male population as analysed by linear regression. In conclusion, common variation in WNT10B was shown to be associated with BMI and weight in a case-control population of Belgian males. Nonetheless, replication of this result and elucidation of the molecular actions of WNT10B remain necessary.