Recessive inheritance of obesity in familial non-insulin-dependent diabetes mellitus, and lack of linkage to nine candidate genes

Segregation of a latent high adiposity phenotype in families with a history of type 2 diabetes mellitus implicates rare obesity-susceptibility genetic variants with large effects in diabetes-related obesity

Background

We recently reported significantly greater weight gain in non-diabetic healthy subjects with a 1^st degree family history (FH+) of type 2 diabetes mellitus (T2DM) than in a matched control group without such history (FH−) during voluntary overfeeding, implying co-inheritance of susceptibilities to T2DM and obesity. We have estimated the extent and mode of inheritance of susceptibility to increased adiposity in FH+.

Methods

Normoglycaemic participants were categorised either FH+ (≥1 1^st degree relative with T2DM, 50F/30M, age 45±14 (SD) yr) or FH− (71F/51M, age 43±14 yr). Log-transformed anthropometric measurements (height, hip and waist circumferences) and lean, bone and fat mass (Dual Energy X-ray Absorptiometry) data were analysed by rotated Factor Analysis. The age- and gender-adjusted distributions of indices of adiposity in FH+ were assessed by fits to a bimodal model and by relative risk ratios (RR, FH+/FH−) and interpreted in a purely genetic model of FH effects.

Results

The two orthogonal factors extracted, interpretable as Frame and Adiposity accounted for 80% of the variance in the input data. FH+ was associated with significantly higher Adiposity scores (p<0.01) without affecting Frame scores. Adiposity scores in FH+ conformed to a bimodal normal distribution, consistent with dominant expression of major susceptibility genes with 59% (95% CI 40%, 74%) of individuals under the higher mode. Calculated risk allele frequencies were 0.09 (0.02, 0.23) in FH−, 0.36 (0.22, 0.48) in FH+ and 0.62 (0.36, 0.88) in unobserved T2DM-affected family members.

Conclusions

The segregation of Adiposity in T2DM-affected families is consistent with dominant expression of rare risk variants with major effects, which are expressed in over half of FH+ and which can account for most T2DM-associated obesity in our population. The calculated risk allele frequency in FH− suggests that rare genetic variants could also account for a substantial fraction of the prevalent obesity in this society.

Genetic association study of selected candidate genes (ApoB, LPL, Leptin) and telomere length in obese and hypertensive individuals

Background

A genetic study was carried out among obese and hypertensive individuals from India to assess allelic association, if any, at three candidate loci: Apolipoprotein B (ApoB) minisatellite and two tetranucleotide repeat loci; LPL (Lipoprotein lipase) and Leptin. Attempt has also been made to find out whether telomere length attrition is associated with hypertension and obese individuals.

Methods

Venous blood samples were collected from 37 normal, 35 obese and 47 hypertensive individuals. Genomic DNA was extracted from peripheral blood mononuclear cells (PBMC) and PCR amplifications were achieved using locus specific primers. Genotyping of ApoB minisatellite was performed using 4% polyacrylamide gel electrophoresis (PAGE) followed by silver staining, whereas LPL and Leptin loci were genotyped using ALF Express™ DNA sequencer. Telomere length was determined using a recently developed real time based quantitative PCR, where the relative telomere length was determined by calculating the relative ratio of telomere (T) and single copy gene (S) PCR products which is expressed as T/S ratio.

Results

All the three loci are highly polymorphic, display high heterozygosity and conform to Hardy-Weinberg's equilibrium expectations. ApoB minisatellite displayed 14 alleles, whereas LPL and Leptin tetranucleotide loci were having 9 and 17 alleles, respectively. Interestingly two new alleles (9 and 11 repeats) were detected at ApoB locus for the first time. The alleles at Leptin locus were classified as Class I (lower alleles: 149-200 bp) and Class II alleles (higher alleles: >217 bp). Higher alleles at ApoB (>39 repeats), predominant allele 9 at LPL and alleles 164 bp and 224 bp at Leptin loci have shown allelic association with hypertensive individuals. After adjusting the influence of age and gender, the analysis of co-variance (ANCOVA) revealed the relative telomere length (T/S ratio) in hypertensive individuals to be (1.01 ± 0.021), which was significantly different (P < 0.001) from obese (1.20 ± 0.023) and normal (1.22 ± 0.014) individuals. However, no significant difference in the relative telomere length was observed among male and female individuals, although age related decrease in telomere length was observed in these limited sample size.

Conclusion

The present study revealed that allelic association at ApoB, LPL, Leptin loci and loss of telomere length may have strong genetic association with hypertensive individuals. However, further study on larger sample size is needed to draw firm conclusions.

The polymorphisms of UCP1 genes associated with fat metabolism, obesity and diabetes

Uncoupling protein 1 (UCP1), a 32-kDa protein located in the inner mitochondrial membrane, is abundant in brown adipose tissue, as a proton transporter in mitochondria inner membrane which uncouples oxidative metabolism from ATP synthesis and dissipates energy through the heat. UCP1 has been reported to play important roles for energy homeostasis in rodents and neonate of larger mammals including human. Recently, numerous candidate genes were searched to determine the genetic factors implicated in the pathogenesis of obesity, related metabolic disorders and diabetes. UCP-1, which plays a major role in thermogenesis, was suggested to be one of the candidates. This review summarizes data supporting the existence of brown adipocytes and the role of UCP1 in energy dissipation in adult humans, and the genetic variety association with the fat metabolism, obesity and diabetes.

New insights into the genetics of body weight

PURPOSE OF REVIEW

These last months, the wave of genome-wide association scans finally reached the shores of body weight and obesity complex trait. In parallel, thanks to the increasing sequencing and genotyping capacities, large studies on rare mutations can now be carried out.

RECENT FINDINGS

In this review, I tried to cover the most recent findings in genome-wide association analyses, the outcome of conclusions subsequently not replicated, and the weight of rare mutations with strong effects on common obesity. The strongest predictor of obesity, FTO, is responsible for 1% of the total heritability, and results from other genome-wide scans do not provide, so far, any clue of other variants of this effect size. Thus, monogenic obesity studies might well reinstall the importance of rare nonsynonymous mutations of already known genes, especially melanocortin-4 receptor gene, in the general population. Nevertheless, additional genome-wide association analyses and replication are expected to confirm these first intuitions.

SUMMARY

Initial results both support the common variant-common disease hypothesis because at least one such variant exists in FTO, and also tone down its importance because such variants may be fewer than expected. Moreover, having a polymorphism associated with body weight is clearly not the end but rather the beginning of a long search for the gene function and pathway.

A locus on mouse Chromosome 9 (Adip5) affects the relative weight of the gonadal but not retroperitoneal adipose depot

To identify the gene or genes on mouse Chromosome 9 that contribute to strain differences in fatness, we conducted an expanded mapping analysis to better define the region where suggestive linkage was found, using the F(2 )generation of an intercross between the C57BL/6ByJ and 129P3/J mouse strains. Six traits were studied: the summed weight of two adipose depots, the weight of each depot, analyzed individually (the gonadal and retroperitoneal depot), and the weight of each depot (summed and individual) relative to body size. We found significant linkage (LOD = 4.6) that accounted for the relative weight of the summed adipose depots, and another for the relative weight of the gonadal (LOD = 5.3) but not retroperitoneal (LOD = 0.9) adipose depot. This linkage is near marker rs30280752 (61.1 Mb, Build 34) and probably is equivalent to the quantitative trait locus (QTL) Adip5. Because the causal gene is unknown, we identified and evaluated several candidates within the confidence interval with functional significance to the body fatness phenotype (Il18, Acat1, Cyp19a1, Crabp1, Man2c1, Neil1, Mpi1, Csk, Lsm16, Adpgk, Bbs4, Hexa, Thsd4, Dpp8, Anxa2, and Lipc). We conclude that the Adip5 locus is specific to the gonadal adipose depot and that a gene or genes near the linkage peak may account for this QTL.

Loci contributing to adult height and body mass index in African American families ascertained for type 2 diabetes

Height and body mass index (BMI) have high heritability in most studies. High BMI and reduced height are well-recognized as important risk factors for a number of cardiovascular diseases. We investigated these phenotypes in African American families originally ascertained for studies of linkage with type 2 diabetes using self-reported height and weight. We conducted a genome wide scan in 221 families containing 580 individuals and 672 relative pairs of African American descent. Estimates of heritability and support for linkage were assessed by genetic variance component analyses using SOLAR software. The estimated heritabilities for height and BMI were 0.43 and 0.64, respectively. We have identified major loci contributing to variation in height on chromosomes 15 (LOD = 2.61 at 35 cM, p = 0.0004), 3 (LOD = 1.82 at 84 cM, p = 0.0029), 8 (LOD = 1.92 at 135 cM, p = 0.0024) and 17 (LOD = 1.70 at 110 cM, p = 0.0044). A broad region on chromosome 4 supported evidence of linkage to variation in BMI, with the highest LOD = 2.66 at 168 cM (p = 0.0005). Two height loci and two BMI loci appear to confirm the existence of quantitative trait loci previously identified by other studies, providing important replicative data to allow further resolution of linkage regions suitable for positional cloning of these cardiovascular disease risk loci.

Linkage exclusion analysis of two candidate regions on chromosomes 7 and 11: Leptin and UCP2/UCP3 are not QTLs for obesity in US Caucasians

Leptin (LEP) and the uncoupling proteins 2 and 3 (UCP2/UCP3) are key molecules involved in the regulation of food intake and energy expenditure. However, their contribution to variation of obesity phenotypes in the general population remains controversial. The present study is to investigate whether chromosomal regions 7q and 11q, which contain LEP and UCP2/UCP3, respectively, can be excluded for linkage with obesity phenotypes. The obesity phenotypes include body mass index (BMI), fat mass, and percentage fat mass (PFM), with the latter two measured by dual-energy X-ray absorptiometry. We conducted exclusion linkage analyses using a variance component approach in a sample of 1816 individuals coming from 79 extended Caucasian pedigrees. In this study, we were able to exclude chromosomal region 7q containing LEP as having an effect on fat mass and PFM at effect sizes of 5% or greater, and on BMI at effect sizes of 10% or greater. We were able to exclude chromosomal region 11q containing UCP2/UCP3 as having an effect on fat mass and PFM at effect sizes of 10% or greater, and on BMI at effect sizes of 5% or greater. Our results suggest that the LEP and UCP2/UCP3 genes are unlikely to have a substantial effect on variation in obesity phenotypes in this particular US Caucasian population.

Lack of evidence for a major gene in the Mendelian transmission of BMI in Chinese

OBJECTIVES

To determine the heritability of BMI and to examine the mode of inheritance of BMI variation in Chinese.

RESEARCH METHODS AND PROCEDURES

Familial correlation and complex segregation analyses for BMI were undertaken in a Chinese sample composed of 392 nuclear families, with 1190 total individuals.

RESULTS

A moderate heritability was found for BMI (h2 = 0.419-0.492). The obtained results do not support a major gene for BMI in our samples. BMI may be inherited in a complex and non-Mendelian manner in Chinese.

DISCUSSION

The findings of this study suggest that identification of specific genes for BMI in Chinese, at least within the same data set, is a serious challenge because of the lack of evidence of a major gene for BMI in our Chinese sample.

Model-fitting and linkage analysis of sodium–lithium countertransport

Increased sodium-lithium countertransport activity (SLC) associates with hypertension and is highly heritable, yet the underlying genes remain unknown. SLC, measured on 1113 and remeasured 2-3 years later on 675 adult members of 48 Utah pedigrees, was tested for candidate gene association, major locus inheritance, and linkage to genome scan markers using a bivariate model with genotype-specific effects of age, body mass index (BMI), and triglycerides level (TG). No effect of the alpha-adducin Gly460Trp polymorphism on SLC was found. In contrast, SLC increased with age in carriers of apolipoproteinE varepsilon2 (85 individuals; 8.7% of the sample) and decreased in noncarriers. Model-fitting analyses inferred two additional loci with genotype-specific responses to BMI and TG. Using the inferred model, lod scores >2 were obtained for D3S3038, D11S4464, and D10S677 for the BMI-responsive locus, and for D8S1048 for the TG-responsive locus.

Aspectos genéticos da obesidade

A obesidade definida como a acumulação excessiva de gordura corporal deriva de um desequilíbrio crônico entre a energia ingerida e a energia gasta. Neste desequilíbrio podem estar implicados diversos fatores relacionados com o estilo de vida (dieta e exercício físico), alterações neuro-endócrinas, juntamente com um componente hereditário. O componente genético constitui um fator determinante de algumas doenças congênitas e um elemento de risco para diversas doenças crônicas como diabetes, osteoporose, hipertensão, câncer, obesidade, entre outras. O aumento da prevalência da obesidade em quase todos os países durante os últimos anos, parece indicar que existe uma predisposição ou susceptibilidade genética para a obesidade, sobre a qual atuam os fatores ambientais relacionados com os estilos de vida, em que se incluem principalmente os hábitos alimentares e a atividade física. A utilização de modelos animais de obesidade, a transferência génica e os estudos de associação e ligamento, permitiram a identificação de vários genes implicados na obesidade.

Uncoupling protein-2 polymorphisms in type 2 diabetes, obesity, and insulin secretion

The onset of type 2 diabetes (T2DM) is preceded by obesity, insulin resistance, and impaired beta-cell function. Uncoupling protein-2 (UCP2) is a widely expressed inner mitochondrial membrane protein. Common polymorphisms of the UCP2 gene have been implicated in diabetes, in obesity, and with changes in UCP2 mRNA levels. We tested the hypothesis that common UCP2 variants influence T2DM susceptibility in four parallel studies of separate populations. We typed the -866 promoter (G/A) variant, a nonsynonymous (Ala55Val or A55V) single-nucleotide polymorphism in exon 4, and a 45-nt insertion in the 3'-untranslated (3'UTR) region. Study populations included a case-control population study, a family-based association study, and a metabolic study of individuals who had been characterized for insulin sensitivity and secretion. To evaluate UCP2 mRNA levels, we examined a fourth population of subjects, who had undergone subcutaneous fat biopsy. All three variants showed a trend to an association with T2DM (P = 0.05 to 0.07) in the population but not the family-based association study. The 3' insertion/deletion (3'UTR I/D) variant was associated with body mass index (BMI, P = 0.035) among nondiabetic family members. Haplotype combinations were significantly associated with BMI (P = 0.028), triglyceride levels (P = 0.026), and fasting insulin (P = 0.029); highest values for the three traits were observed in individuals with the heterozygous combination GVI/AVD. In the metabolic study, all three variants were associated with an index of beta-cell compensation for insulin sensitivity (disposition index), particularly in interaction with family membership (P < 0.000001). Individuals homozygous for the -866 A allele had decreased adipose mRNA levels relative to GG homozygous individuals (P = 0.009), but the 3'UTR I/D variant had no impact on mRNA levels. We confirm modest effects of UCP2 variants on BMI and T2DM and show significant effects on insulin secretion in interaction with family-specific factors. However, the associated allele and the effects on gene expression are opposite to those reported previously.

Comparison between two analytic strategies to detect linkage to obesity with genetically determined age of onset: the Framingham Heart Study

Background

Genes have been found to influence the age of onset of several diseases and traits. The occurrence of many chronic diseases, obesity included, appears to be strongly age-dependent. However, an analysis of potential age of onset genes for obesity has yet to be reported. There are at least two analytic methods for determining an age of onset gene. The first is to consider a person affected if they possess the trait before a certain age (an early age of onset phenotype). The second is to define the phenotype based on the residual from a survival analysis.

Results

No regions provided evidence for linkage at the more stringent level of p < 0.001. However, five regions showed consistent suggestive evidence for linkage (one marker with p < 0.01 and a second contiguous marker at p < 0.05). These regions were chromosome 1 (280–294 cM) and chromosome 16 (56–64 cM) for overweight using the survival analysis residual method and chromosome 13 (102–122 cM), chromosome 17 (127–138 cM), and chromosome 19 (23–47 cM) for obese before age 35.

Conclusion

Only one region (chromosome 19 at 23–47 cM) showed somewhat consistent results between the two analytic methods. Potential reasons for inconsistent results between the two methods, as well as their strengths and weaknesses, are discussed. The use of both methods together to explore the genetics of the age of onset of a trait may prove to be beneficial in determining a gene that is linked only to an early age of onset phenotype versus one that determines age of onset through all age groups.

Role of leptin in the regulation of glucagon-like peptide-1 secretion

Glucagon-like peptide-1 (GLP-1), released from intestinal endocrine L cells, is a potent insulinotropic hormone. GLP-1 secretion is diminished in obese patients. Because obesity is linked to abnormal leptin signaling, we hypothesized that leptin may modulate GLP-1 secretion. Leptin significantly stimulated GLP-1 secretion (by up to 250% of control) from fetal rat intestinal cells, a mouse L cell line (GLUTag), and a human L cell line (NCI-H716) in a dose-dependent manner (P < 0.05-0.001). The long form of the leptin receptor was shown to be expressed, and leptin induced the phosphorylation of STAT3 in the three cell types. The leptin receptor was also expressed by rodent and human intestinal L cells, and leptin (1 mg/kg i.p.) significantly stimulated GLP-1 secretion in rats and ob/ob mice. To determine the effect of leptin resistance on GLP-1 secretion, C57BL/6 mice were fed a high-fat (45%) or low-fat (10%) diet for 8 weeks. Mice on the high-fat diet became obese; developed glucose intolerance, hyperinsulinemia, and hyperleptinemia; and were leptin resistant. Mice on the high-fat diet also had twofold lower basal plasma GLP-1 and a diminished GLP-1 response to oral glucose, by 28.5 +/- 5.0% (P < 0.05). These results show for the first time that leptin stimulates GLP-1 secretion from rodent and human intestinal L cells, and they suggest that leptin resistance may account for the decreased levels of GLP-1 found in obese humans.

Human leptin locus (LEP) alleles and BMI in Samoans

OBJECTIVES

Because of their location in known candidate gene regions for obesity the associations between six microsatellite markers (D2S2170, D2S144, D2S1268, D2S1788, D2S1348 and a tetranucleotide repeat in the 3' UTR of the LEP locus) and body mass index (BMI) were studied in adult Samoans.

DESIGN

The study was designed to detect differences in the proportion of alleles at the six microsatellite markers between two groups of adult Samoans at the extremes of the longitudinal BMI distribution.

SUBJECTS AND MEASUREMENTS

The 181 unrelated Samoan participants were 25-55 y of age, reported that all four grandparents were Samoan, resided in American Samoa (AS) or Samoa (S) and were without diagnosed hypertension or type 2 diabetes. Initial statistical analysis was based on chi(2) tests of independence between marker allele frequencies and BMI status at each marker. The association of individual alleles with BMI status was tested by aggregating a marker's allelic data into a two-by-two contingency table and applying a two-tailed version of Fisher's exact test, with a Bonferroni correction to account for the multiple testing implicit in the procedure.

RESULTS

There were no significant differences in allele frequencies at any of the markers between AS and S, as expected from our prior population genetic analyses. Only the LEP gene 3'-tetranucelotide repeat was associated (P<0.006) with BMI status. The distribution of the marker alleles at the LEP locus was significantly associated with the BMI groups (P<0.01), due to the low frequency of allele 226 in the high BMI group. The same pattern of association was found in sub-group analyses with low BMI individuals from AS and high BMI individuals from S.

CONCLUSION

These findings indicate that the leptin 3'-tetranucleotide repeat is associated with high BMI in adult Samoans, with allele 226 having a low frequency in the high BMI group.

A meta-analytic investigation of linkage and association of common leptin receptor (LEPR) polymorphisms with body mass index and waist circumference

METHODS

We analyzed data pooled from nine studies on the human leptin receptor (LEPR) gene for the association of three alleles (K109R, Q223R and K656N) of LEPR with body mass index (BMI; kg/m(2)) and waist circumference (WC). A total of 3263 related and unrelated subjects from diverse ethnic backgrounds including African-American, Caucasian, Danish, Finnish, French Canadian and Nigerian were studied. We tested effects of individual alleles, joint effects of alleles at multiple loci, epistatic effects among alleles at different loci, effect modification by age, sex, diabetes and ethnicity, and pleiotropic genotype effects on BMI and WC.

RESULTS

We found that none of the effects were significant at the 0.05 level. Heterogeneity tests showed that the variations of the non-significant effects are within the range of sampling variation.

CONCLUSIONS

We conclude that, although certain genotypic effects could be population-specific, there was no statistically compelling evidence that any of the three LEPR alleles is associated with BMI or WC in the overall population.

A combined analysis of genomewide linkage scans for body mass index from the National Heart, Lung, and Blood Institute Family Blood Pressure Program

A combined analysis of genome scans for obesity was undertaken using the interim results from the National Heart, Lung, and Blood Institute Family Blood Pressure Program. In this research project, four multicenter networks of investigators conducted eight individual studies. Data were available on 6,849 individuals from four ethnic groups (white, black, Mexican American, and Asian). The sample represents the largest single collection of genomewide scan data that has been analyzed for obesity and provides a test of the reproducibility of linkage analysis for a complex phenotype. Body mass index (BMI) was used as the measure of adiposity. Genomewide linkage analyses were first performed separately in each of the eight ethnic groups in the four networks, through use of the variance-component method. Only one region in the analyses of the individual studies showed significant linkage with BMI: 3q22.1 (LOD 3.45, for the GENOA network black sample). Six additional regions were found with an associated LOD >2, including 3p24.1, 7p15.2, 7q22.3, 14q24.3, 16q12.2, and 17p11.2. Among these findings, the linkage at 7p15.2, 7q22.3, and 17p11.2 has been reported elsewhere. A modified Fisher's omnibus procedure was then used to combine the P values from each of the eight genome scans. A complimentary approach to the meta-analysis was undertaken, combining the average allele-sharing identity by descent (pi) for whites, blacks, and Mexican Americans. Using this approach, we found strong linkage evidence for a quantitative-trait locus at 3q27 (marker D3S2427; LOD 3.40, P=.03). The same location has been shown to be linked with obesity-related traits and diabetes in at least two other studies. These results (1) confirm the previously reported obesity-susceptibility locus on chromosomes 3, 7, and 17 and (2) demonstrate that combining samples from different studies can increase the power to detect common genes with a small-to-moderate effect, so long as the same gene has an effect in all samples considered.

Quantitative-trait loci influencing body-mass index reside on chromosomes 7 and 13: the National Heart, Lung, and Blood Institute Family Heart Study

Obesity is a risk factor for many chronic diseases, including glucose intolerance, lipid disorders, hypertension, and coronary heart disease. Even though the body-mass index (BMI) is a heterogeneous phenotype reflecting the amount of fat, lean mass, and body build, several studies have provided evidence of one or two major loci contributing to the variation in this complex trait. We sought to identify loci with potential influence on BMI in the data obtained from National Heart, Lung, and Blood Institute Family Heart Study. Two complementary samples were studied: (a) 1,184 subjects in 317 sibships, with 243 markers typed by the Utah Molecular Genetics Laboratory (UMGL) and (b) 3,027 subjects distributed among 401 three-generation families, with 404 markers typed by the Mammalian Genotyping Service (MGS). A genome scan using a variance-components-based linkage approach was performed for each sample, as well as for the combined sample, in which the markers from each analysis were placed on a common genetic map. There was strong evidence for linkage on chromosome 7q32.3 in each sample: the maximum multipoint LOD scores were 4.7 (P<10-5) at marker GATA43C11 and 3.2 (P=.00007) at marker D7S1804, for the MGS and UMGL samples, respectively. The linkage result is replicated by the consistent evidence from these two complementary subsets. Furthermore, the evidence for linkage was maintained in the combined sample, with a LOD score of 4.9 (P<10-5) for both markers, which map to the same location. This signal is very near the published location for the leptin gene, which is the most prominent candidate gene in this region. For the combined-sample analysis, evidence of linkage was also found on chromosome 13q14, with D13S257 (LOD score 3.2, P=.00006), and other, weaker signals (LOD scores 1.5-1.9) were found on chromosomes 1, 2, 3, 5, 6, 14, and 15.

O futuro da epidemiologia genética de características complexas

A epidemiologia genética evoluiu de um enfoque em estudos sobre doenças mendelianas raras para a análise genética de características complexas. Com o advento de informações sobre a completa seqüência de genes ao longo do genoma humano e de outros organismos, o interesse da epidemiologia genética em desvendar a natureza dos fatores que influenciam essas características se tornou primordial. São apresentados os principais métodos empregados no estudo de doenças complexas bem como suas principais vantagens e desvantagens. Discute-se a importância na determinação da amostra e o uso de fenótipos e marcadores genéticos apropriados. Como exemplo das estratégias citadas tomamos o estudo de índice de massa corporal (BMI) para ilustrar um fator genético principal localizado no cromossomo 7. Em uma discussão sobre tendências no estudo de ligação, embora reconhecendo que famílias e genealogias continuarão sendo o foco principal das amostras, discute-se alguns novos e eficientes tipos de amostragem (como por exemplo, controles não-relacionados) em que amostras de conjunto de DNA serão universalmente empregadas. O reconhecimento da heterogeneidade genética entre estudos e sua interpretação será uma das mais importantes características no futuro das análises de características complexas.

Genetic and environmental correlations between various anthropometric and blood pressure traits among adult Samoans

Shared polygenic effects (i.e., pleiotropy) are assumed to exist for such obesity-related phenotypes as blood pressure and adiposity. It is possible to identify these shared genetic effects through bivariate genetic analyses. This analysis of 1,342 adult Samoans, across 801 pedigrees, indicates that significant heritable components (P < 0.05) ranging from 29-58% exist for weight, height, systolic blood pressure, diastolic blood pressure, triceps skinfold, subscapular skinfold, body mass index, and sum of skinfolds. In general, the anthropometric measurements share additive genetic effects, as do the anthropometric measures, with blood pressure. Heritabilities for central fat distribution are not significant in this population, which could be due to a lack of power. On the other hand, heritabilities have been found in Hispanics; hence the genes responsible for central fat distribution may not be evenly distributed among populations.

Genome scan for adiposity in Dutch dyslipidemic families reveals novel quantitative trait loci for leptin, body mass index and soluble tumor necrosis factor receptor superfamily 1A

OBJECTIVE

To search for novel genes contributing to adiposity in familial combined hyperlipidemia (FCH), a disorder characterized by abdominal obesity, hyperlipidemia and insulin resistance, using a 10cM genome-wide scan.

DESIGN

Plasma leptin and soluble tumor necrosis factor receptor superfamily members 1A and 1B (sTNFRSF1A and sTNFRSF1B, also known as sTNFR1 and sTNFR2) were analyzed as unadjusted and adjusted quantitative phenotypes of adiposity, in addition to body mass index (BMI), in multipoint and single-point analyses. In the second stage of analysis, an important chromosome 1 positional candidate gene, the leptin receptor (LEPR), was studied.

SUBJECTS

Eighteen Dutch pedigrees with familial combined hyperlipidemia (FCH) (n= 198) were analyzed to search for chromosomal regions harboring genes contributing to adiposity.

RESULTS

Multipoint analysis of the genome scan data identified linkage (log of odds, LOD, 3.4) of leptin levels to a chromosomal region defined by D1S3728 and D1S1665, flanking the leptin receptor (LEPR) gene by approximately 9 and 3 cM, respectively. The LOD score decreased to 1.8 with age- and gender-adjusted leptin levels. Notably, BMI also mapped to this region with an LOD score of 1.2 (adjusted BMI: LOD 0.5). Two polymorphic DNA markers in LEPR and their haplotypes revealed linkage to unadjusted and adjusted BMI and leptin, and an association with leptin levels was found as well. In addition, the marker D8S1110 showed linkage (LOD 2.8) with unadjusted plasma concentrations of soluble TNFRSF1A. BMI gave a LOD score of 0.6. Moreover, a chromosome 10 q-ter locus, AFM198ZB, showed linkage with adjusted BMI (LOD 3.3).

CONCLUSION

These data provide evidence that a human chromosome 1 locus, harboring the LEPR gene, contributes to plasma leptin concentrations, adiposity and body weight in humans affected with this insulin resistant dyslipidemic syndrome. Novel loci on chromosome 8 and 10 qter need further study.

Genetic inheritance of body mass index in African-American and African families

Numerous studies have shown recessive major gene inheritance of body mass index (BMI) in white populations; few have examined the inheritance of BMI in the African-American population where obesity is more prevalent, nor in African populations where obesity is comparatively rare. To evaluate the inheritance of obesity in two different populations of African origin, we used segregation analysis to determine the transmission of BMI in 95 African-American families and 400 Nigerian families. Probands were selected from participants in the population-based International Collaborative Study on Hypertension in Blacks. Using class D regressive models, results from the segregation analysis of the African-American data showed evidence of a major gene effect on BMI. The Nigerian results were strikingly similar, with comparable estimates for the genotype frequencies and means and strong evidence for a major effect in the transmission of BMI. The high BMI allele frequency estimate of 24% is consistent with estimates in other studies, but the mode of transmission appeared codominant, which differs from studies involving predominantly white populations. In the Nigerian analysis, however, the probability of a high BMI homozygote parent transmitting a low BMI allele to his/her offspring was significantly different from the Mendelian expectation of zero (estimated tau(BB) = 0.45), suggesting that additional complexities exist in the major gene inheritance of BMI in this population. The strong similarity of the genotype frequencies and means obtained from the African-American and Nigerian samples suggests that a common codominant major gene effect may contribute to the variation in BMI in both populations.

Genetic and environmental influences on human cord blood leptin concentration

OBJECTIVE

To examine in a population sample of cord blood the time structure (chronome) of leptin, an adipocyte-derived hormone, and to assess any effect of a familial history of noninsulin-dependent diabetes mellitus and obesity, separately, on both the maternal and the paternal side.

SUBJECTS AND METHODS

Leptin concentration was determined in cord blood from 93 infants. Effects of gender, gestational age, birth weight, maternal weight, familial antecedents of obesity and noninsulin-dependent diabetes mellitus, and circadian and about-yearly stage were assessed by linear regression and ANOVA.

RESULTS

Cord blood leptin concentration is elevated in the presence of a family history of obesity on the paternal side, but not on the maternal side. Leptin concentrations are higher in spring and summer than in fall and are higher in infants born before noon. In keeping with earlier work, leptin concentration in cord blood correlates positively with birth weight and height and is higher in infants who are appropriate for or large for gestational age than in infants who are small for gestational age or born prematurely.

DISCUSSION

Changes along the scales of the day and the seasons point to synchronizing environmental as well as genetic influence. An association of cord blood leptin concentration with obesity on the paternal side may help clarify the role of leptin in parental contributions to human obesity and may prompt focus on cholesterol metabolism.

Genome scan for human obesity and linkage to markers in 20q13

Obesity is a highly prevalent, multigenic trait that predicts increased morbidity and mortality. Here we report results from a genome scan based on 354 markers in 513 members of 92 nuclear families ascertained through extreme obesity and normal body weight. The average marker interval was approximately 10 cM. We examined four correlated obesity phenotypes, including the body-mass index (BMI) (both as a quantitative trait and as a discrete trait with a threshold of BMI > or /=30 kg/m2) and percentage of fat (both as a quantitative trait and as a discrete trait with a threshold of 40%) as assessed by bioelectrical impedance. In the initial stage of the genome scan, four markers in 20q gave positive evidence for linkage, which was consistent across most obesity phenotypes and analytic methods. After saturating 20q with additional markers (25 markers total) in an augmented sample of 713 members from 124 families, we found linkage to several markers in a region, 20q13, previously implicated in both human and animal studies. Three markers (D20S107, D20S211, and D20S149) in 20q13 had empirical P values (based on Monte Carlo simulations, which controlled for multiple testing) < or /=. 01 for single-point analysis. In addition, the parametric, affecteds-only analysis for D20S476 yielded a LOD score of 3.06 (P=. 00009), and the affected-sib-pair test yielded a LOD score of 3.17 (P=.000067). Multipoint analyses further strengthened and localized these findings. This region includes several plausible candidate genes for obesity. Our results suggest that one or more genes affecting obesity are located in 20q13.

Evidence for at least two major loci influencing human fatness

The genetics of human fatness has been the subject of many recent studies, motivated by the increased morbidity and mortality associated with obesity, as well as the increasing prevalence of overweight and obesity. The body-mass index (BMI) and fat mass (FM), measured by underwater weighing, were assessed for 1,630 individuals from approximately 300 families from phase 1 of the Quebec Family Study. The two phenotypes are highly correlated ( approximately .8) in adults, and previous segregation analysis revealed evidence for a recessive major gene for each trait. In our study, we utilized bivariate segregation analysis to determine the source(s) of phenotypic correlation-namely, a pleiotropic major gene, shared familial factors/polygenes, or shared nontransmitted environmental factors. Analysis was performed by use of the Pedigree Analysis Package, with extensions to the bivariate case. Tests of hypotheses provided evidence for two pleiotropic recessive loci, together accounting for 64% and 47% of the variance in BMI and FM, respectively. Under the model, all sources of phenotypic correlation were significant: 73% of the covariance was attributed to the pleiotropic major loci, 8% to residual familial effects, and 19% to nontransmitted environmental factors. The high degree of genetic identity between the two traits is not surprising, since the BMI often is used as a surrogate for FM; however, simultaneous analysis of both phenotypes enabled the detection of a second major locus, which apparently does not affect extreme overweight (as does the primary major locus) but which affects variation in the "normal" range.

Neuropeptides, the hypothalamus and obesity: insights into the central control of body weight

Body weight tends to remain relatively stable for long periods over an adult's lifespan. Dieting can reduce weight by 5-10%, but in most individuals attempts to lose larger amounts of weight are counteracted by a reduction in energy expenditure and an increase in hunger. The fact that body weight appears to be actively defended in this manner suggests that it is homeostatically regulated at a certain "set-point". Such a mechanism is likely to be centrally controlled by the brain since the hypothalamus can sense the amount of adipose tissue stored in the body and can alter both energy intake and expenditure. Over the past four years a number of major advances have reinforced the critical role the brain may play in controlling body weight, and these have greatly enhanced our understanding of this area. Advances have included the identification of several genetic mutations that cause obesity in animal models, examination of the metabolic consequences of such mutations and the development of mice with genetically engineered altered neuropeptide levels. This review summarises what has been recently discovered about the regulation of body weight by the brain and how this may be disrupted in obesity.

Leptin: a molecule integrating somatic energy stores, energy expenditure and fertility

The signaling of fat mass to central nervous system (CNS) regulators of food intake, energy expenditure and fertility has been inferred by experimental physiologists for over 75 years. The ability to modify such phenotypes based upon the status of body energy stores (fat) has critical survival value and, therefore, has been the object of potent selection pressure in evolution. The recent molecular cloning of the mouse ob mutation and the subsequent elucidation of the fundamentals of its regulatory physiology has identified a protein secreted by adipocytes, leptin, as a plausible candidate for a humoral signal with the requisite endocrinology and neurobiology.

Evidence for multiple determinants of the body mass index: the National Heart, Lung, and Blood Institute Family Heart Study

The body mass index (BMI) is a complex phenotype representing the amount of fat mass, lean mass, body build and proportions, and it is likely to be affected by various metabolic processes, hormonal effects, energy intake and expenditure, and interactions within and among these broad categories of etiologic factors. Nonetheless, several previous studies have reported evidence for major gene segregation for the BMI in various populations. Data on a random sample of Caucasian families participating in the National Heart, Lung, and Blood Institute (NHLBI) Family Heart Study were analyzed to document the extent of familial resemblance and to investigate whether a similar monogenic inheritance pattern could be detected. Genetic analysis was carried out on age- and sex-adjusted BMI values. Familial correlations were significant implying a maximal heritability, including all genetic and environmentally inherited additive factors, of 41% to 59%. Segregation analysis revealed the presence of two maximum likelihood solutions, one characterized as a recessive Mendelian gene and the other as a major effect with an ambiguous transmission pattern. The presence of two such solutions is consistent with detection of two separate factors, each influencing the BMI distribution in a substantive manner. The evidence also supports a multifactorial background for BMI and suggests that the frequencies of these two factors, one of which appears to be a gene, may vary among diverse populations in the United States.

Meta-analysis of linkage data under worst-case conditions: a demonstration using the human OB region

To date, few methods have been developed explicitly for meta-analysis of linkage analyses. Moreover, the methods that have been developed or suggested generally depend on certain ideal situations and have not been widely applied. In this article, we apply standard statistical theory and meta-analytic techniques in novel ways to five published papers discussing the evidence of linkage of body mass index (BMI) to the region of the human genome containing the OB gene. These methods are "inference based," meaning that they allow one to make statements about the statistical significance of the entire body of evidence. As currently developed, they do not allow specific statements to be made about the amount of variance explained by any putative locus or allow precise confidence intervals to be placed around the putative location of a linked locus. By applying these techniques to the literature on linkage in the human OB gene region, we are able to show that the evidence for linkage somewhere in the region is extremely strong (P = 1.5 x 10[-5]).