Obesity: Common Symptom of Diverse Gene-Based Metabolic Dysregulations

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1 Obesity: Common Symptom of Diverse Gene-Based Metabolic Dysregulations The Genetics of Human Noninsulin-Dependent (Type 2) Diabetes Mellitus 1 Steven C. Elbein Division of Endocrinology and Metabolism, Veterans Affairs Medical Center and University of Arkansas for Medical Sciences, Little Rock, AR ABSTRACT Familial aggregation and concordance in monozygotic and dizygotic twins argue strongly for a genetic etiology to noninsulin-dependent diabetes (NIDDM). Nonetheless, studies of pathways implicated by the known physiology have failed to identify gene defects that can explain the genetic susceptibility. In contrast, studies of early onset dominant diabetes have revealed three major loci resulting in diminished insulin secretion. Recently, studies have taken a new approach to map the genes causing typical NIDDM using large numbers of families or sibling pairs. The first reports of these studies have suggested possible loci on chromosomes 1, 2 and 12, but no report has been confirmed. Other studies have examined the quantitative defects that may be precursors of clinical NIDDM such as hyperinsulinemia, hyperglycemia, insulin response to glucose and obesity. These studies have suggested additional loci that may contribute to NIDDM susceptibility, but the genes responsible for most of these loci remain unknown. Studies of NIDDM susceptibility and the role of obesity genes in that susceptibility have entered an exciting new phase, but the challenges of complex disease genetics in humans will have to be conquered to translate this research into preventive or therapeutic benefits. J. Nutr. 127: 1891S 1896S, 1997 KEY WORDS: diabetes obesity genetics Noninsulin-dependent diabetes mellitus (NIDDM) 2 is strongly defining genetic susceptibility to NIDDM is referred to the inherited, as evidenced by a high concordance in identical many excellent reviews (Elbein et al. 1994a). This review twins and strong familial aggregation. In unbiased studies, the will define the current state of the search for human NIDDM concordance in identical twins is Ç70%, whereas the lifetime susceptibility genes and how obesity might relate both to the risk to siblings is only half this rate (Rewers and Hamman genetic susceptibility and to the phenotypic expression of inde- 1995). A history of NIDDM in a first-degree relative doubles pendent NIDDM susceptibility genes. the risk of diabetes. Offspring of two diabetic parents have an The physiology of NIDDM has provided some clues to pos- 80% lifetime risk of diabetes (Kenny et al. 1995, Rewers and sible defects. Insulin resistance is nearly ubiquitous. The off- Hamman 1995). Further evidence for a genetic role is sug- spring of two diabetic parents, who historically are at particugested by the wide variation in incidence and prevalence larly high risk for future NIDDM, are insulin resistant many among different ethnic groups. Thus, Pima Indians have a years before NIDDM development (Warram et al. 1990). In nearly 50% prevalence that is marked by a degree of insulin Pima Indians, San Antonio Hispanics and Utah Caucasians, resistance not seen in Caucasians, and both Hispanics and insulin resistance is inherited in an autosomal fashion, sug- African-Americans are also at high risk (Kenny et al. 1995). gesting that a single gene might influence this intermediate These facts and the development of new molecular genetic phenotype (Bogardus et al. 1989, Schumacher et al. 1992, methodologies have made the etiology of NIDDM a focus for Stern et al. 1996). Studies of patients with NIDDM and insulin geneticists and diabetologists. The reader desiring a detailed resistance have demonstrated a primary defect in insulin-medianalysis of the challenges and progress toward the goals of ated glucose uptake into muscle, and specifically nonoxidative uptake into glycogen (DeFronzo et al. 1992, Rothman et al. 1995, Vaag et al ). Insulin sensitivity is reduced 50% 1 Presented as part of a symposium Obesity: Common Symptom of Diverse Gene-Based Metabolic Dysregulations, Little Rock, Arkansas, March 4, This on average in NIDDM, and glycogen synthesis and storage conference was co-sponsored by the National Center for Toxicological Research/ are reduced by 50 70% (DeFronzo et al. 1992). Like insulin Food and Drug Administration and the University of Arkansas for Medical Sciresistance in general, this specific defect in glycogen synthesis ences. It was supported by generous grants from The Jane B. Mendel Family Trust, Amgen, Wyeth-Ayerst Laboratories Division of American Home Products is present in first-degree family members of an NIDDM proand The Governor Winthrop Rockefeller Memorial Lecture Series-University of band (Vaag et al. 1992). Arkansas. Guest editor for this symposium was George L. Wolff, Division of Bio- Individuals at risk for NIDDM also show various forms of chemical Toxicology, National Center for Toxicological Research/FDA, Jefferson, AR pancreatic b-cell dysfunction (Polonsky et al. 1996). Those 2 Abbreviations used: ASP, affected sibling pair; BMI, body mass index; FABP, with both impaired glucose tolerance (IGT) and NIDDM demfatty acid binding protein; FFA, free fatty acids; HNF, hepatocyte nuclear tranonstrate loss of first-phase insulin release in response to an IV scription factor; IGT, impaired glucose tolerance; IRS-1, insulin receptor substrate 1; MODY, maturity onset diabetes of the young; NIDDM, noninsulin-dependent glucose load (S. C. Elbein, unpublished data). Although this diabetes mellitus; QTL, quantitative trait locus; S I, sensitivity index. defect may result from glucotoxicity (Rossetti et al. 1990), /97 $ American Society for Nutritional Sciences. 1891S

2 1892S SUPPLEMENT more subtle defects of insulin secretion are present in family ods. Until recently, most studies compared the frequency of a members at risk. Thus, the normal cyclic patterns of insulin polymorphic marker near or within a gene of potential interest release are lost in both NIDDM subjects and in relatives at in diabetic and nondiabetic individuals. These studies have risk (Polonsky et al. 1996). An increased proinsulin:insulin implicated several loci, including the muscle glycogen synthase ratio is present in NIDDM, the intermediate and high risk gene in Finnish diabetics (Groop et al. 1993), an amino acid stage of IGT, and in euglycemic relatives at risk (Kahn et al. variant of the glucagon receptor gene in French and Sardinian 1995). Among Pima Indians, the risk of future NIDDM is diabetics (Hager et al. 1995), and the insulin receptor substrate highest among those with both insulin resistance and a defect 1 (IRS-1) (Elbein et al. 1994a). Similar studies were used to in insulin secretion (Lillioja et al. 1993). Defects of both insu- suggest an association of the b-3 adrenergic receptor gene with lin secretion and insulin sensitivity are apparent in offspring obesity and earlier onset of NIDDM (Clement et al. 1995, of NIDDM parents in Utah Caucasian pedigrees ascertained Walston et al. 1995, Widen et al. 1995). Recent studies from for multiple NIDDM siblings when insulin secretion is appropriately our laboratory, in collaboration with the laboratories of Per- normalized for the degree of insulin resistance (SCE, mutt and Turner, found an association of a silent change in unpublished data). Thus inherited defects of both insulin sensitivity the sulfonylurea receptor gene and NIDDM (Inoue et al. and b-cell function are likely to contribute to NIDDM 1996). The well-described artifacts of association due to popu- susceptibility. lation stratification constitute a potential weakness of associa- In contrast to defects that are at least partially inherited, tion studies. A second problem with published studies is the NIDDM is also characterized by increased hepatic glucose production tendency to search for multiple associations from multiple lab- from both glycogenolysis and gluconeogenesis (De- oratories without appropriate correction in reporting signifi- Fronzo et al. 1992). This defect appears relatively late in the cance levels. In general, these studies have at best defined course of NIDDM development in most studies and thus is weak susceptibility loci that have been difficult to confirm. less likely to result directly from genetic susceptibility. On the Furthermore, in the case of glycogen synthase (Orho et al. other hand, like the defect in insulin sensitivity, this defect 1995) and the sulfonylurea receptor gene (Inoue et al. 1997), may be closely related to obesity. Both peripheral insulin resistance among others, the molecular defect that might explain the and the increased hepatic glucose output may result from association has not been found. high levels of circulating free (nonesterified) fatty acids (FFA). Because studies of candidate genes for NIDDM have been These high levels of FFA in turn appear to relate to the amount largely negative (see below), investigators have begun using of visceral fat (Boden 1997). Thus, inherited susceptibility to the human linkage map to search for unknown susceptibility obesity and particularly visceral obesity may contribute to both genes. Several statistical and design methods have been applied the hepatic and peripheral defects seen in NIDDM. Finally, to families with multiple diabetic siblings. All use highly both individuals with NIDDM and offspring of two NIDDM polymorphic markers that do not themselves cause disease parents have reduced glucose effectiveness, i.e., the ability of and that are not usually within genes. The sharing of alleles glucose to mediate its own uptake independent of insulin. designated by these markers is examined among affected individuals, The potentially complex model of environmental and genetic and significant deviation from chance sharing is inter- susceptibility to NIDDM is summarized in Figure 1. preted as evidence for a susceptibility gene in close proximity The role of obesity in the etiology of NIDDM is complex. to the genetic marker. Although most studies of NIDDM have In epidemiologic studies, obesity alone increases the risk of applied this method to sharing between affected siblings (affected NIDDM twofold but interacts with a family history of NIDDM sibling pair analysis, or ASP), we and others have ap- to increase the risk for obese patients with a family history of plied this method to extended families using multiple singlegene NIDDM fourfold ( Kenny et al. 1995). Although obesity appears models that might approximate true NIDDM inheritance. to exert its effects by reducing insulin sensitivity, this Another method of study is to examine quantitative measures relationship is complex. When obesity is measured as body of NIDDM risk among individuals. In quantitative trait locus mass index (BMI), a nonlinear relationship is seen between (QTL) analysis of humans, as in animals, a locus is implicated BMI and the insulin sensitivity index (S I ), a measure of insulin by the significant correlation of alleles shared at a locus beaction. As BMI increases, S I decreases in a nonlinear fashion, tween siblings with the square of the difference in the trait but variance also decreases (Kahn et al. 1993). Furthermore, as values between those siblings. Each of these methods has been stated in S. C. Elbein s unpublished data, the insulin secretory applied to candidate genes, i.e., those genes that are in pathways response when normalized for S I was low in obese (BMI ú 30 suspected to influence glucose homeostasis. However, kg/m 2 ) members of families ascertained for two NIDDM sib- greater success has derived from studies of anonymous markers lings. Thus, obesity as measured by BMI may contribute to placed at regular intervals throughout the human gene map. both the insulin resistance and the insulin secretory effects These studies, known as the genome-wide scan, test markers seen in NIDDM. The role of visceral fat in these measures is at set (usually 10- to 15-cM) intervals on all human autosomes. less well studied. All current studies use markers with multiple alleles determined Although it is clear that obesity affects NIDDM susceptibility, by short DNA sequence variations and detected by the interaction between NIDDM susceptibility loci and polymerase chain reaction. Most recently, statistical methods obesity is uncertain. Obesity may mark a unique subset of and computer programs have permitted these adjacent markers NIDDM patients with more profound insulin resistance and a to be considered simultaneously (multipoint analysis) with the relatively mild pancreatic b-cell defect. Alternatively, obesity potential for considerably increased power to find susceptibility may act equally in all susceptible individuals to increase the loci (Kruglyak and Lander 1995, Kruglyak et al. 1996). penetrance of genetic loci. The latter appears to be the case Most forms of NIDDM do not follow simple Mendelian in some single-gene disorders such as glucokinase mutations. inheritance, and thus are considered among the complex genetic Finally, obesity may decrease the age of onset of NIDDM in diseases. In contrast, NIDDM with onset before age 25 susceptible family members in a manner analogous to preg- is unique in that many families show autosomal dominant nancy and gestational diabetes. Evidence exists for all three inheritance. Unlike most NIDDM, these individuals are gener- roles. ally not obese and they have predominant defects in insulin The genetics of NIDDM has been studied by several meth- secretion with normal insulin sensitivity. Three loci have been

3 GENETICS OF NIDDM 1893S FIGURE 1 The figure shows a model of diabetes pathogenesis and the interaction of multiple susceptibility loci and environmental factors. The model presumes a b cell defect as the final stage, which is most consistent with current hypotheses although not universally accepted. defined. MODY1, localized to chromosome 20 by a genome- clear, but late onset of NIDDM has been noted in some family wide search in a single large family, has now been traced to a members who carry the mutation. Furthermore, HNF1a muta- nonsense mutation (Q268X) in the hepatocyte nuclear tran- tions may be present in individuals with early onset (before scription factor 4 a (NF4a) (Yamagata et al. 1996a). This rare age 40) NIDDM who do not meet the usual criteria for MODY mutation in a member of the steroid/thyroid hormone receptor (Dussoix et al. 1997, Kaisaki et al. 1997). superfamily causes severe diabetes with all of the complications Studies of typical NIDDM have focused heavily on candi- of typical NIDDM. Presumably the HNF4a acts through the date genes implicated from the known physiology of NIDDM. regulation of a downstream transcription factor, hepatocyte These can be divided into studies of genes for insulin action nuclear transcription factor 1 alpha (HNF1a) (see below), but and genes that control pathways of insulin secretion. As discussed the precise mechanism is unknown. MODY2 was identified above, candidate genes have been studied by association by linkage studies of a candidate gene, glucokinase (GCK). and linkage studies. Many candidate genes also have been Glucokinase mutations account for 50% of maturity onset diabetes examined directly by molecular screening techniques. Alin of the young (MODY) in France and have been found though a detailed discussion is beyond the intent of this review, English families (Froguel et al 1993). They are relatively a nonexhaustive list of genes examined is shown in Table 1. rare elsewhere and do not contribute to late onset NIDDM In general, no single locus that can explain the genetic propensity (Elbein et al and 1994b). Mutations are found throughout to either NIDDM or insulin resistance has been identified the gene and appear to alter the glucose set point for among known candidate genes. Although there is some controversy insulin secretion. Insulin secretin rates in response to glucose regarding the roles of IRS1 and fatty acid binding infusion are diminished (Polonsky et al. 1996). Clinically, protein 2 (FABP2; see below), there is no consistent evidence patients have mild fasting hyperglycemia and little risk of complications, to implicate these loci in NIDDM pathogenesis when all stud- although diabetes may be more severe in the pres- ies are considered. There is more evidence for a role of genes ence of obesity. for insulin secretion (Table 2); however, no locus has been MODY3 may be the most important single-gene cause of definitely demonstrated to be an important cause of typical NIDDM. The locus was initially mapped to chromosome 12q late onset NIDDM, although the role of MODY3 mutations by linkage analysis of nonglucokinase-linked French MODY in some subsets of NIDDM is uncertain (Mahtani et al. 1996). families (Vaxillaire et al. 1995). It accounts for 25% of French The sulfonylurea receptor has been implicated in association MODY diabetes. Recently Bell and colleagues identified the studies in Caucasian populations from Utah and the United locus as HNF1a (Yamagata et al. 1996b). A large number of Kingdom (Inoue et al. 1996), but is not linked to NIDDM in mutations have been identified in English, French, German, several family studies (Elbein and Hoffman 1996, Stirling et Finnish and American families. Unlike glucokinase, the same al. 1995), and no disease-causing mutation has been identified mutations have occurred in several unrelated families, and in either the sulfonylurea receptor or the contiguous potassium mutations appear to be clustered in several exons (Froguel channel gene (Inoue et al and 1997). 1996). The role of this locus in more typical NIDDM is un- Although success has been limited in finding a role for

4 1894S SUPPLEMENT TABLE 1 Candidate genes for insulin action Genes largely eliminated by linkage or molecular screening Insulin receptor Muscle and liver glycogen synthase Hexokinase 2 Insulin responsive glucose transporter (GLUT4) Ras-like protein associated with diabetes (rad) Loci associated with lipid metabolism Apolipoprotein A1 cluster Apolipoprotein CIII cluster Apolipoprotein A2 Apolipoprotein B Hepatic lipase Lipoprotein lipase Cholesterol-ester transport protein (CETP) LDL receptor Genes unlikely to play a major role (controversial evidence) Intestinal fatty acid binding protein (FABP2) Insulin receptor substrate 1 (IRS1) known loci in NIDDM susceptibility, genetic subgroups of NIDDM have been defined. Although rare, insulin gene muta- tions causing uncleaved proinsulin or fully cleaved insulin with reduced activity have been described in a small number of families (Steiner et al. 1990). Interestingly, diabetes is a vari- able feature of these autosomal dominant mutations, suggesting a relatively low penetrance of NIDDM in homozygotes. Both autosomal dominant and recessive forms of insulin receptor mutations have contributed to diabetes in some families, al- though again the dominant feature is hyperinsulinemia (Taylor et al. 1990). Mutations causing decreased insulin binding to the receptor or diminished tyrosine kinase activity are an un- usual cause of insulin-resistant diabetes. Mitochondrial DNA mutations result in insulin-deficient diabetes in up to 1% of NIDDM in some populations (Elbein and Hoffman 1996, Elbein et al. 1994a). This disorder, usually resulting from a muta- tion of mitochondrial leucine trna, has been associated with maternal transmission and sensorineural hearing loss. Genome-wide screens of families with multiple NIDDM siblings are in progress in several laboratories, including ours. Hanis and colleagues initially reported linkage to a marker near the telomere of chromosome 2q with NIDDM (NIDDM1) in Hispanics from Starr County, Texas (Hanis et al. 1996). Other published studies (Mahtani et al. 1996, Stern et al. 1996) and unpublished work in Pima Indians and in our laboratory have failed to confirm evidence for linkage, however. Subsequently, Mahtani et al. (1996) reported a second locus at or near MODY3 that they identified in a small subgroup of Botnian Finnish families in the lowest quartile for insulin secretion. They found no evidence for a major NIDDM locus in the remaining 75% of 26 Botnian families. This analysis has not been repeated, although most groups find no evidence for linkage when families are not stratified by insulin secretion. A study of 32 Hispanic families in San Antonio (Stern et al. 1996) used somewhat different methods based on glucose as a quantitative trait. They reported suggestive linkage to regions of chromosome 6 and 11, but no linkage to NIDDM1 or NIDDM2. Our laboratory has completed studies of 19 Caucasian families (469 family members) from Utah. Our families were selected for at least two NIDDM siblings with onset before age 65. All available siblings and offspring of diabetic individuals were screened by a 75-g oral glucose tolerance test if not known to be diabetic. We have estimated marker allele frequencies from 100 unrelated spouses. Regions of possible linkage were examined in a total of 42 families; more recently we have entered an additional 20 families into the study. We also found no linkage to NIDDM1 or NIDDM2, and no region has met current genome-wide proposals for significant linkage. We have analyzed our linkage data using both sibling pair and parametric ( lod score ) linkage methods, and both two-point and multipoint analysis. To date, our highest two-point significance level (lod score) is for linkage to a region of chromosome 7 also reported to be linked to NIDDM in Pima Indians, but our recent analyses suggest that this locus, if real, is a relatively weak susceptibility locus. Our multipoint analysis provided suggestive evidence for linkage near the apolipoprotein A2 region of chromosome 1. These results have not been reported in other studies, however. Work is in progress in our laboratory to extend these findings. Because NIDDM may represent the final outcome of several converging intermediate traits, many groups have attempted to study the intermediate traits directly as QTL. Among the traits studies are insulin sensitivity, insulin levels, fasting and postchallenge glucose levels and measures of obesity. In Pima Indians, insulin resistance as measured by insulin clamp studies was linked to the intestinal fatty acid binding protein locus (FABP2) on chromosome 4 (Prochazka et al. 1993). Subsequently, this group identified an amino acid variant of FABP2 that may partially account for insulin resistance in this popula- tion (Baier et al. 1995). The same group mapped a locus for acute insulin response to glucose to a marker on chromosome 1p (D1S198; Thompson et al. 1995), although these findings are without published confirmation. In preliminary work from our laboratory, we were unable to identify a locus for fasting or postchallenge insulin levels despite evidence for Mendelian inheritance of both fasting and 1-h postchallenge insulin in Utah families (Schumacher et al. 1992). However,we did map fasting glucose to a locus on chromosome 9 using a quantitative multipoint approach. This locus is not linked to either diabetes or postchallenge glucose. A number of laboratories have examined the role of obesity and obesity genes in NIDDM. We and others have tested the human homologs of mouse obesity genes for linkage to NIDDM in humans (Table 3), but these studies have been uniformly negative. Among diabetes-prone populations, there has been evidence for linkage of tumor necrosis factor a (TNFa) to obesity in Pima Indians (Norman et al. 1995) but not in our population. Recently, both leptin levels and fat TABLE 2 Candidate genes for insulin secretion Glucose sensing Glucokinase (GCK) GLUT2 glucose transporter (GLUT2) b-cell potassium channel (SUR/K ATP ) Insulin gene transcription factors CCKB CDX3 ISL1 MODY1 (HNF4a) MODY3 (HNF1a) Possible insulin secretion modulators Glucagon (GLP-1) Glucagon-like peptide receptor (GLP1R) Insulin (INS) IAPP (amylin) Glucokinase regulatory protein (GCKR) D1S198 (acute insulin response in Pima Indians) Insulin-processing enzymes (prohormone converase 2 and 3; carboxypeptidase E)

5 GENETICS OF NIDDM 1895S TABLE 3 Mouse and human obesity loci not involved in NIDDM by linkage or molecular screening Leptin (OB) Leptin receptor (OBR) Agouti homolog (chromosome 20) Lipoprotein lipase (LPL) Tumor necrosis factor (TNFa) Prader-Willi region (chromosome 15) Carboxypeptidase E (CPE) cited from S. C. Elbein s laboratory was supported by NIH grant DK39311, a family acquisition grant from the American Diabetes Association and the Research Service of the Department of Veterans Affairs. The assistance of Sandra Hasstedt and Mark Leppert of the University of Utah and the staff of the General Clinical Research Center of the University of Utah (PHS grant M01-RR00064) are gratefully acknowledged for their collaborative contributions to work cited from our laboratory. REFERENCES Baier, L. J., Sacchettini, J. C., Knowler, W. C., Eads, J., Paolisso, G., Tataranni, P. A., Mochizuki, H., Bennett, P. H., Bogardus, C. & Prochazka, M. (1995) An amino acid substitution in the human intestinal fatty acid binding protein is associated with increased fatty acid binding, increased fat oxidation, and insulin resistance. J. Clin.Invest. 95: mass were reported to be linked to a marker on chromosome 2p in 10 large Hispanic families from San Antonio (Comuzzie Boden, G. (1997) Role of fatty acids in the pathogenesis of insulin resistance et al. 1997), which explained 47% of the leptin variance. The and NIDDM. Diabetes 45: nature of this locus is unknown. In 283 Pima Indian sibling Bogardus, C., Lillioja, S., Nyomba, B. L., Zurlo, F., Swinburn, B., Puente, E., Knowler, W. C., Ravussin, E., Mott, D. M. & Bennett, P. H. (1989) Distribupairs, percentage of body fat was linked using similar QTL tion of in vivo insulin action in Pima Indians as a mixture of three normal methods to chromosome 11 (D11q21 22), although this study distributions. Diabetes 38: did not meet genome-wide significance levels for proof of link- Clement, K., Vaisse, C., Manning, B. S., Basdevant, A., Guy Grand, B., Ruiz, J., age (Norman et al. 1997). Neither study has localized obesity Silver, K. D., Shuldiner, A. R., Froguel, P. & Strosberg, A. D. (1995) Genetic variation in the beta 3-adrenergic receptor and an increased capacity to gain loci to known candidate genes. Despite initial reports for an weight in patients with morbid obesity. N. Engl. J. Med. 333: association of a common amino acid polymorphism of the b- Comuzzie, A. G., Hixson, J. E., Almasy, L., Mitchell, B. D., Mahaney, M. C., Dyer, 3 adrenergic receptor gene with insulin resistance in Finns T. D., Stern, M. P., MacCluer, J. W. & Blangero, J. (1997) A major quantitative trait locus determining serum leptin levels and fat mass is located on (Widen et al. 1995), earlier onset NIDDM in Pima Indians human chromosome 2. Nat. Genet. 15: (Walston et al. 1995) and increased BMI in France (Clement DeFronzo, R. A., Bonadonna, R. C. & Ferrannini, E. (1992) Pathogenesis of et al. 1995), these associations have been difficult to confirm. NIDDM. A balanced overview. Diabetes Care 15: Dussoix, P., Vaxillaire, M., Iynedjian, P. B., Tiercy, J.-M., Ruiz, J., Sinas, G. A., We examined family members of NIDDM probands and were Berger, W., Zahnd, G., Froguel, P. & Philippe, J. (1997) Diagnostic heterounable to document any role for this variant in the context geneity of diabetes in lean young adults: classification based on immunological and genetic parameters. Diabetes 46: of high risk families (Elbein et al. 1996). Elbein, S. C. & Hoffman, M. D. (1996) Role of mitochondrial DNA trna leucine In collaboration with Sandy Hasstedt (Human Genetics, and glucagon receptor missense mutations in Utah white diabetic patients. University of Utah), we have examined the role of obesity in Diabetes Care 19: our Caucasian families ascertained for at least two NIDDM Elbein, S. C., Hoffman, M., Barrett, K., Wegner, K., Miles, C., Bachman, K., Ber- kowitz, D., Shuldiner, A. R., Leppert, M. F. & Hasstedt, S. (1996) Role of the siblings. We initially tested for segregation of BMI in 616 b3-adrenergic receptor locus in obesity and noninsulin-dependent diabetes members of 42 families. We found evidence for two recessive among members of Caucasian families with a diabetic sibling pair. J. Clin. obesity loci, one for extreme obesity with mean BMI 39 and Endocrinol. Metab. 81: a gene frequency of 0.42, and a second moderate obesity locus Elbein, S. C., Hoffman, M. D., Bragg, K. L. & Mayorga, R. A. (1994a) The genet- ics of NIDDM. An update. Diabetes Care 17: with mean BMI 32 and a gene frequency of Together, Elbein, S. C., Hoffman, M., Chiu, K., Tanizawa, Y. & Permutt, M. A. (1993) Linkage analysis of the glucokinase locus in familial type 2 (non-insulin-dependent) these loci account for 68% of the variance in BMI. Homozygosdiabetic pedigrees. Diabetologia 36: ity at both loci resulted in a mean BMI of 47. Both loci Elbein, S. C., Hoffman, M., Qin, H., Chiu, K., Tanizawa, Y. & Permutt, M. A. raised the fasting insulin and postchallenge glucose levels and (1994b) Molecular screening of the glucokinase gene in familial type 2 (nonresulted in earlier onset of NIDDM. Thus, the obesity loci insulin-dependent) diabetes mellitus. Diabetologia 37: appear to modulate the expression of the underlying NIDDM Elbein, S. C., Wegner, K., Miles, C., Kahn, S. E. (1997) Insulin sensitivity and secretion measures in relatives of an NIDDM sibling pair. Diabetes 46 (suppl. susceptibility loci. We used this model to test for linkage to a 1): 172 (abs.). large number of candidate genes for obesity, including leptin, Froguel, P., Zouali, H., Vionnet, N., Velho, G., Vaxillaire, M., Sun, F., Lesage, S., the leptin receptor, lipoprotein lipase, hepatic lipase, TNFa, Stoffel, M., Takeda, J., Passa, P., Permutt, M. A., Beckmann, J. S., Bell, G. I. & Cohen, D. (1993) Familial hyperglycemia due to mutations in glucokinase. glycogen synthase, the agouti region, the Prader-Willi region, Definition of a subtype of diabetes mellitus. N. Engl. J. Med. 328: the b-3 adrenergic receptor locus and the apolipoprotein Froguel, P. (1996) Glucokinase and MODY: from the gene to the disease. Diabetic Med. 13 (Suppl. 6): S96 S97. genes. We found no evidence for linkage of these candidate Groop, L. C., Kankuri, M., Schalin Jantti, C., Ekstrand, A., Nikula Ijas, P., Widen, genes to obesity under the segregation model or with the use E., Kuismanen, E., Eriksson, J., Franssila Kallunki, A., Saloranta, C. & Koskiof nonmodel-based methods. mies, S. (1993) Association between polymorphism of the glycogen synthase gene and non-insulin-dependent diabetes mellitus. N. Engl. J. Med. In summary, many laboratories are currently applying genetic tools to the search for NIDDM susceptibility loci in 328: Hager, J., Hansen, L. & Vaisse, C. (1995) A missense mutation in the glucagon humans. To date, success in identifying a cause for the bulk of receptor gene is associated with non-insulin-dependent diabetes mellitus. NIDDM is limited. Initial studies from multiple ethnic groups Nat. Genet. 9: suggest that NIDDM will result from a complex interaction of Hanis, C. L., Boerwinkle, E., Chakraborty, R., Ellsworth, D. L., Concannon, P., Stirling, B., Morrison, V. A., Wapelhorst, B., Spielman, R. S., Gogolin-Ewens, several susceptibility genes and the environment. Obesity, K. J., Shephard, J. M., Williams, S. R., Risch, N., Hinds, D., Iwasaki, N., Ogata, both genetic and environmental, will play an important role M., Ommoi, Y., Petzold, C., Rietzsch, H., Schroeder, H.-E., Schulze, J., Cox, in modulating the expression of these loci. However, it seems N. J., Menzel, S., Boriraj, V. V., Chen, X., Lim, L. R., Linder, T., Mereu, L. E., Wang, Y.-Q., Xiang, K., Yamagata, K., Yang, Y. & Bell, G. I. (1996) A geless likely that obesity loci will define a unique subgroup of nome-wide search for human non-insulin-dependent (type 2) diabetes genes NIDDM or that unique obesity loci will be found among reveals a major susceptibility locus on chromosome 2. Nat. Genet. 13: 161 NIDDM families ACKNOWLEDGMENTS Inoue, H., Ferrer, J., Elbein, S. C., Warren-Perry, M., Zhang, Y., Millns, H., Turner, R., Suarez, B., Seino, S. & Permutt, M. A. 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