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Candidate genes for insulin resistance: what's new?
International Congress Series 1253 (2003) 55 – 61
Candidate genes for insulin resistance: what’s new? Markku Laakso *, Agata Kubaszek Department of Medicine, University of Kuopio, 70210 Kuopio, Finland Received 23 October 2002; accepted 7 February 2003
Abstract Type 2 diabetes is genetically heterogenous, and the probable contribution of several genes is needed to explain the whole spectrum of various phenotypes. A candidate gene approach can be applied to investigate the effect of variants in the genes of interest on the risk of type 2 diabetes. Candidate genes are selected on the basis of knowledge about their role in the pathophysiology of this disease. Candidate genes for insulin resistance can include genes regulating several steps in insulin, glucose and lipid metabolism. In order to investigate properly the genetic basis of type 2 diabetes, the following requirements must be fulfilled: (1) a prospective study design, (2) a large number of individuals at high risk of developing type 2 diabetes, 3) availability of functional gene variants having known mechanism(s) to cause type 2 diabetes, and (4) a possibility to evaluate gene – environment interaction. These requirements are discussed in the context of screening of candidate gene in the Finnish Diabetes Prevention Study (DPS). D 2003 Elsevier Science B.V. All rights reserved. Keywords: Type 2 diabetes; Insulin resistance; Candidate genes; Lifestyle
1. Introduction Type 2 diabetes is a common metabolic disease having a prevalence of 3 –5% in western countries. It is characterized by varying degrees of insulin resistance and beta cell dysfunction. Insulin resistance can be defined as a state where normal concentration of insulin produces a less than normal biological response in insulin-sensitive tissues [1]. Although type 2 diabetes is clearly a familial disease, the genetics of this disease is complex due to non-Mendelian inheritance. Type 2 diabetes is genetically heterogenous, and the probable contribution of several genes is needed to explain the whole spectrum of * Corresponding author. Tel.: +358-17-172151; fax: +358-17-173993. E-mail addresses: [email protected] (M. Laakso), [email protected] (A. Kubaszek). 0531-5131/03 D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0531-5131(03)00139-0
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M. Laakso, A. Kubaszek / International Congress Series 1253 (2003) 55–61
various phenotypes. Because the number of type 2 diabetic patients is rapidly increasing almost in all countries worldwide, the interaction between genes and environmental factors, particularly obesity and secondary lifestyle, has to be of substantial importance for this ‘epidemic’.
2. Dissecting out the genetic basis of type 2 diabetes Susceptibility genes for type 2 diabetes may involve either diabetogenes, genes that can initiate the disease process, or diabetes-related genes that contribute to the risk of the disease but are not strictly specific for type 2 diabetes [2]. The mode of inheritance of type 2 diabetes is unknown except for dominantly or maternally inherited subtypes of the disease. Different strategies can be applied for studies on the genetics of type 2 diabetes. A candidate gene approach can be applied to investigate the effect of variants in the genes of interest on the risk of type 2 diabetes [3]. Candidate genes are selected on the basis of knowledge about their role in the pathophysiology of this disease. In applying candidate gene approach, we cannot, however, identify genes whose structures or roles in the pathophysiology of type 2 diabetes are unknown. To identify chromosomal regions that are linked to a disease in the whole genome, a genome-wide random search approach can be applied [4]. When applying this approach, the whole genome is screened with highly polymorphic markers, e.g. short tandem repeat polymorphisms, with a marker density less than 10 cM. Susceptibility region is verified with a denser marker map and, finally, the gene is identified by positional cloning [5].
3. Candidate genes for insulin resistance Candidate genes for insulin resistance can include genes regulating several steps in insulin, glucose and lipid metabolism. As shown in Table 1, a variety of genes could be Table 1 Potential candidate genes for insulin resistance Metabolic pathway
Examples of genes
Receptor mechanisms Insulin signaling Glucose transport Glucose metabolism Cytokines Free fatty acid and lipid metabolism Adipose tissue metabolism Energy metabolism
Insulin IRS-1, IRS-2, P-I-3-kinase GLUT4 HKII, GYS1 TNF-a, IL-6 LPL, HL Leptin, PPARg h-adrenergic receptor, UCP-2
IRS = insulin receptor substrate; P-I-3-kinase = phosphatidyl-inositol-3-kinase; GLUT4 = insulin-sensitive glucose transporter; HKII = hexokinase II; CYS1 = glycogen synthase; TNF-a = tumor necrosis factor a; IL-6 = interleukin 6; LPL = lipoprotein lipase; HL = hepatic lipase; PPARg = peroxisome proliferator-activated receptor g; UCP = uncoupling protein.
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screened as candidate genes for insulin resistance and type 2 diabetes. The findings published so far have been mostly negative, and no major candidate genes for type 2 diabetes have been identified. Perhaps the most promising candidate gene for insulin resistance at this moment is the PPARg2 gene, but even for this gene, both positive [6,7] and negative findings [8,9] have been published. The limitations of candidate gene approach are obvious and listed in Table 2. This approach can be applied only for known genes. For example, the identification of hepatic nuclear factor (HNF) genes for MODY would have been quite unlikely without a genome-wide random search. Furthermore, the identification of variants in genes for type 2 diabetes having a low frequency in population ( < 5%) is very difficult because very often variants associated with type 2 diabetes can also occur in nondiabetic subjects (the penetrance of the disease is most often < 100%). Particularly problematic are studies aiming to investigate gene– gene interactions. If variants of two genes have a prevalence of 10%, only 10 of 1000 subjects have both variants. Most of the studies applying the candidate approach have a substantially lower number of participants. The ideal situation for candidate gene approach is to have a gene having a functional variant demonstrated in in vitro studies. In this situation, a higher frequency of this variant among type 2 diabetic patients than among nondiabetic individuals gives evidence that the candidate gene could be causally linked to type 2 diabetes. However, even in this situation, we cannot always exclude the possibility that the variant is in linkage disequilibrium with some other genes, which is finally responsible for the association found. For nonfunctional variants this is an even more severe limitation. For example, in the calpain-10 gene, which was originally found by applying the genome-wide random search and positional cloning, nonfunctional variants have been linked with type 2 diabetes in some populations [10 –13]. Whether or not these variants or other variants in the calpain-10 gene or variants in another gene explain this association is not clear at this moment. Defining proper control subjects for studies on the genetics of type 2 diabetes is extremely difficult. Diabetes is defined on the basis of glucose levels only, and because a majority of subjects who develop type 2 diabetes will get this disease after 60 years of age, we cannot exclude the possibility that a person having a completely normal glucose tolerance will develop diabetes later on. Finally, selective mortality, particularly due to cardiovascular disease, makes it difficult to draw definite conclusions on gene variants causing type 2 diabetes at old age. Table 2 Limitations of the candidate gene approach Suitable only for known genes Suitable only for genes regulating known metabolic pathways Suitable only for relatively common variants Nonfunctional variants Linkage disequilibrium Definition of nondiabetic control subjects Selective mortality
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4. ‘‘Ideal setting’’ for candidate gene studies In order to investigate properly the genetic basis of type 2 diabetes, the following requirements must be fulfilled: (1) a prospective study design, (2) a large number of individuals at high risk of developing type 2 diabetes, (3) availability of functional gene variants having known mechanism(s) to cause type 2 diabetes, and (4) a possibility to evaluate gene –environment interaction (Fig. 1). To investigate the role of genetic and environmental factors on the conversion to type 2 diabetes in high risk individuals, we need to have a two-arm trial setting: (a) one study group with a lifestyle intervention, and (b) another study group without a lifestyle intervention (a control group). The follow-up of the control group gives essential information about gene variants determining the risk of diabetes in general. If the sample size is large enough it is also possible to investigate a gene –gene interaction in these individuals. If lifestyle modification is successful in the intervention group, this allows us to evaluate gene – environment interactions which are of crucial importance for the understanding of diabetes epidemic currently happening worldwide. Fig. 2 illustrates different effects of intervention on the incidence of type 2 diabetes. As shown by Fig. 2A, none of the genotypes is a risk factor for type 2 diabetes because the incidence of type 2 diabetes is similar in the control group independently of the genotypes, and because lifestyle intervention (weight loss, physical exercise, diet) reduces the risk of new cases of diabetes also independently of the genotypes. Fig. 2B shows that the A allele is a risk factor for type 2 diabetes in the control group (incidence of diabetes, 20% vs. 10%), and that intervention has a significant effect on the incidence of diabetes. However, only subjects with the BB genotype respond significantly to lifestyle intervention. Thus, the A allele is a risk factor for type 2 diabetes, because it increases the risk of diabetes more than the BB genotype in the control group, and is more ‘resistant’ to lifestyle intervention. The situation in Fig. 2C is different. Again, the A allele is a risk factor for type 2 diabetes, but the effect of lifestyle intervention seems to be paradoxical. In fact, the risk A allele becomes a ‘protective’ allele with respect to the risk of diabetes.
Fig. 1. Progression to type 2 diabetes due to genetic and environmental factors.
M. Laakso, A. Kubaszek / International Congress Series 1253 (2003) 55–61
Fig. 2. Possible effects of intervention on the incidence of type 2 diabetes.
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How relevant are considerations given in Fig. 2 for the understanding of the risk of type 2 diabetes? We have been carrying out a study aiming to determine the role of different candidate genes and their interaction with lifestyle modification in the Finnish Diabetes Prevention Study (DPS) [14]. The DPS recruited 522 obese subjects with impaired glucose tolerance. These subjects were randomized into two subgroups, one undergoing an intensive lifestyle modification program (weight loss, physical exercise, changes in diet), and another group receiving only a general information on beneficial lifestyle changes. After a mean follow-up of 3.5 years, the intervention group had a 58% reduction in the incidence of type 2 diabetes compared to the incidence in the control group. So far, we have screened variants in 10 different genes regulating insulin signaling, cytokine response, nuclear receptors, and lipid and lipoprotein metabolism. Majority of variants did not predict the conversion to diabetes. However, almost all of the variants predicting the conversion to diabetes followed the pattern presented in Fig. 2B, indicating that risk gene variants were resistant to lifestyle changes. We also have an example of a gene variant described in Fig. 2C. We studied the effect of the Pro12Ala polymorphism in exon B of the PPARg2 gene on the risk of diabetes incidence in the Finnish DPS [13]. In the control group, the 12Ala allele was predicting the conversion from impaired glucose tolerance to diabetes. However, in the intervention group, the Ala12Ala genotype was protective of developing diabetes. Therefore, in the case of the PPARg2 gene, the same genotype was a risk factor or a protective factor depending on lifestyle changes. These results indicate how important gene– lifestyle interactions are in determining the risk of type 2 diabetes. 5. Concluding remarks So far, the candidate gene approach has not been very successful in the identification of genes responsible for type 2 diabetes due to several reasons listed in Table 2. Also, the genome-wide scanning has not fulfilled the expectations in the solving of the genetic basis of type 2 diabetes. Candidate gene approach is still valuable but it should be applied more in studies having a prospective study design and a large number of subjects, and particularly in the setting of clinical trials aiming to prevent type 2 diabetes either by lifestyle changes or by drug treatment. Acknowledgements This work was partly supported by grants from the Academy of Finland, The Finnish Diabetes Research Foundation, and the European Union (QLG1-CT-1999-00674). References [1] C.R. Kahn, D. Vicent, A. Doria, Genetics of non-insulin-dependent (type-II) diabetes mellitus, Annu. Rev. Med. 47 (1996) 509 – 531. [2] M.I. McCarthy, P. Froguel, G.A. Hitman, The genetics of non-insulin-dependent diabetes mellitus: tools and aims, Diabetologia 37 (1994) 959 – 968.
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[3] C.L. Hanis, E. Boerwinkle, R. Chakrabarty, et al., A genome-wide search for human non-insulin-dependent (type 2) diabetes genes reveals a major susceptibility locus on chromosome 2, Nat. Genet. 13 (1996) 161 – 166. [4] E. Lander, L. Kruglyak, Genetic dissection of complex traits: guidelines for interpreting and reporting linkage results, Nat. Genet. 11 (1995) 241 – 247. [5] F.S. Collins, Positional cloning: let’s not call it reverse anymore, Nat. Genet. 1 (1992) 3 – 6. [6] D. Altshuler, J.N. Hirschhorn, M. Klannemark, et al., The common PPAR gamma Pro12Ala polymorphism is associated with decreased risk of type 2 diabetes, Nat. Genet. 26 (2000) 76 – 80. [7] S.S. Deeb, L. Fajas, M. Nemoto, et al., Pro12Ala substitution in PPARgamma2 associated with decreased receptor activity, lower body mass index and improved insulin sensitivity, Nat. Genet. 20 (1998) 284 – 287. [8] F.P. Mancini, O. Vaccaro, L. Sabatino, et al., Pro12Ala substitution in the peroxisome proliferator-activated receptor gamma2 is not associated with type 2 diabetes, Diabetes 48 (1999) 1466 – 1468. [9] A. Meirhaerge, L. Fajas, N. Helbecque, et al., Impact of the peroxisome proliferative activated receptor gamma2 Pro12Ala polymorphism on adiposity, lipids and non-dependent diabetes mellitus, Int. J. Obes. Relat. Metab. Disord. 24 (2000) 195 – 199. [10] P.G. Cassell, A.E. Jackson, B.V. North, et al., Haplotype combinations of calpain 10 gene polymorphisms associate with increased risk of impaired glucose tolerance and type 2 diabetes in South Indians, Diabetes 51 (2002) 1622 – 1628. [11] M.J. Garant, W.H. Kao, F. Brancati, et al., SNP43 of CAPN10 and the risk of type 2 diabetes in AfricanAmericans: the Atherosclerosis Risk in Communities Study, Diabetes 51 (2002) 231 – 237. [12] Y. Horikawa, N. Oda, N.J. Cox, et al., Genetic variation in the gene encoding calpain-10 is associated with type 2 diabetes mellitus, Nat. Genet. 26 (2000) 163 – 175. [13] J. Tuomilehto, J. Lindstro¨m, J.G. Eriksson, et al., Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose tolerance, N. Engl. J. Med. 344 (2001) 1343 – 1350. [14] V.I. Lindi, M.I. Uusitupa, J. Lindstro¨m, et al., Association of the Pro12Ala polymorphism in the PPARgamma2 gene with 3-year incidence of type 2 diabetes and body weight change in the Finnish Diabetes Prevention Study, Diabetes 51 (2002) 2581 – 2586.
Candidate genes for insulin resistance: what’s new? Markku Laakso *, Agata Kubaszek Department of Medicine, University of Kuopio, 70210 Kuopio, Finland Received 23 October 2002; accepted 7 February 2003
Abstract Type 2 diabetes is genetically heterogenous, and the probable contribution of several genes is needed to explain the whole spectrum of various phenotypes. A candidate gene approach can be applied to investigate the effect of variants in the genes of interest on the risk of type 2 diabetes. Candidate genes are selected on the basis of knowledge about their role in the pathophysiology of this disease. Candidate genes for insulin resistance can include genes regulating several steps in insulin, glucose and lipid metabolism. In order to investigate properly the genetic basis of type 2 diabetes, the following requirements must be fulfilled: (1) a prospective study design, (2) a large number of individuals at high risk of developing type 2 diabetes, 3) availability of functional gene variants having known mechanism(s) to cause type 2 diabetes, and (4) a possibility to evaluate gene – environment interaction. These requirements are discussed in the context of screening of candidate gene in the Finnish Diabetes Prevention Study (DPS). D 2003 Elsevier Science B.V. All rights reserved. Keywords: Type 2 diabetes; Insulin resistance; Candidate genes; Lifestyle
1. Introduction Type 2 diabetes is a common metabolic disease having a prevalence of 3 –5% in western countries. It is characterized by varying degrees of insulin resistance and beta cell dysfunction. Insulin resistance can be defined as a state where normal concentration of insulin produces a less than normal biological response in insulin-sensitive tissues [1]. Although type 2 diabetes is clearly a familial disease, the genetics of this disease is complex due to non-Mendelian inheritance. Type 2 diabetes is genetically heterogenous, and the probable contribution of several genes is needed to explain the whole spectrum of * Corresponding author. Tel.: +358-17-172151; fax: +358-17-173993. E-mail addresses: [email protected] (M. Laakso), [email protected] (A. Kubaszek). 0531-5131/03 D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0531-5131(03)00139-0
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M. Laakso, A. Kubaszek / International Congress Series 1253 (2003) 55–61
various phenotypes. Because the number of type 2 diabetic patients is rapidly increasing almost in all countries worldwide, the interaction between genes and environmental factors, particularly obesity and secondary lifestyle, has to be of substantial importance for this ‘epidemic’.
2. Dissecting out the genetic basis of type 2 diabetes Susceptibility genes for type 2 diabetes may involve either diabetogenes, genes that can initiate the disease process, or diabetes-related genes that contribute to the risk of the disease but are not strictly specific for type 2 diabetes [2]. The mode of inheritance of type 2 diabetes is unknown except for dominantly or maternally inherited subtypes of the disease. Different strategies can be applied for studies on the genetics of type 2 diabetes. A candidate gene approach can be applied to investigate the effect of variants in the genes of interest on the risk of type 2 diabetes [3]. Candidate genes are selected on the basis of knowledge about their role in the pathophysiology of this disease. In applying candidate gene approach, we cannot, however, identify genes whose structures or roles in the pathophysiology of type 2 diabetes are unknown. To identify chromosomal regions that are linked to a disease in the whole genome, a genome-wide random search approach can be applied [4]. When applying this approach, the whole genome is screened with highly polymorphic markers, e.g. short tandem repeat polymorphisms, with a marker density less than 10 cM. Susceptibility region is verified with a denser marker map and, finally, the gene is identified by positional cloning [5].
3. Candidate genes for insulin resistance Candidate genes for insulin resistance can include genes regulating several steps in insulin, glucose and lipid metabolism. As shown in Table 1, a variety of genes could be Table 1 Potential candidate genes for insulin resistance Metabolic pathway
Examples of genes
Receptor mechanisms Insulin signaling Glucose transport Glucose metabolism Cytokines Free fatty acid and lipid metabolism Adipose tissue metabolism Energy metabolism
Insulin IRS-1, IRS-2, P-I-3-kinase GLUT4 HKII, GYS1 TNF-a, IL-6 LPL, HL Leptin, PPARg h-adrenergic receptor, UCP-2
IRS = insulin receptor substrate; P-I-3-kinase = phosphatidyl-inositol-3-kinase; GLUT4 = insulin-sensitive glucose transporter; HKII = hexokinase II; CYS1 = glycogen synthase; TNF-a = tumor necrosis factor a; IL-6 = interleukin 6; LPL = lipoprotein lipase; HL = hepatic lipase; PPARg = peroxisome proliferator-activated receptor g; UCP = uncoupling protein.
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screened as candidate genes for insulin resistance and type 2 diabetes. The findings published so far have been mostly negative, and no major candidate genes for type 2 diabetes have been identified. Perhaps the most promising candidate gene for insulin resistance at this moment is the PPARg2 gene, but even for this gene, both positive [6,7] and negative findings [8,9] have been published. The limitations of candidate gene approach are obvious and listed in Table 2. This approach can be applied only for known genes. For example, the identification of hepatic nuclear factor (HNF) genes for MODY would have been quite unlikely without a genome-wide random search. Furthermore, the identification of variants in genes for type 2 diabetes having a low frequency in population ( < 5%) is very difficult because very often variants associated with type 2 diabetes can also occur in nondiabetic subjects (the penetrance of the disease is most often < 100%). Particularly problematic are studies aiming to investigate gene– gene interactions. If variants of two genes have a prevalence of 10%, only 10 of 1000 subjects have both variants. Most of the studies applying the candidate approach have a substantially lower number of participants. The ideal situation for candidate gene approach is to have a gene having a functional variant demonstrated in in vitro studies. In this situation, a higher frequency of this variant among type 2 diabetic patients than among nondiabetic individuals gives evidence that the candidate gene could be causally linked to type 2 diabetes. However, even in this situation, we cannot always exclude the possibility that the variant is in linkage disequilibrium with some other genes, which is finally responsible for the association found. For nonfunctional variants this is an even more severe limitation. For example, in the calpain-10 gene, which was originally found by applying the genome-wide random search and positional cloning, nonfunctional variants have been linked with type 2 diabetes in some populations [10 –13]. Whether or not these variants or other variants in the calpain-10 gene or variants in another gene explain this association is not clear at this moment. Defining proper control subjects for studies on the genetics of type 2 diabetes is extremely difficult. Diabetes is defined on the basis of glucose levels only, and because a majority of subjects who develop type 2 diabetes will get this disease after 60 years of age, we cannot exclude the possibility that a person having a completely normal glucose tolerance will develop diabetes later on. Finally, selective mortality, particularly due to cardiovascular disease, makes it difficult to draw definite conclusions on gene variants causing type 2 diabetes at old age. Table 2 Limitations of the candidate gene approach Suitable only for known genes Suitable only for genes regulating known metabolic pathways Suitable only for relatively common variants Nonfunctional variants Linkage disequilibrium Definition of nondiabetic control subjects Selective mortality
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4. ‘‘Ideal setting’’ for candidate gene studies In order to investigate properly the genetic basis of type 2 diabetes, the following requirements must be fulfilled: (1) a prospective study design, (2) a large number of individuals at high risk of developing type 2 diabetes, (3) availability of functional gene variants having known mechanism(s) to cause type 2 diabetes, and (4) a possibility to evaluate gene –environment interaction (Fig. 1). To investigate the role of genetic and environmental factors on the conversion to type 2 diabetes in high risk individuals, we need to have a two-arm trial setting: (a) one study group with a lifestyle intervention, and (b) another study group without a lifestyle intervention (a control group). The follow-up of the control group gives essential information about gene variants determining the risk of diabetes in general. If the sample size is large enough it is also possible to investigate a gene –gene interaction in these individuals. If lifestyle modification is successful in the intervention group, this allows us to evaluate gene – environment interactions which are of crucial importance for the understanding of diabetes epidemic currently happening worldwide. Fig. 2 illustrates different effects of intervention on the incidence of type 2 diabetes. As shown by Fig. 2A, none of the genotypes is a risk factor for type 2 diabetes because the incidence of type 2 diabetes is similar in the control group independently of the genotypes, and because lifestyle intervention (weight loss, physical exercise, diet) reduces the risk of new cases of diabetes also independently of the genotypes. Fig. 2B shows that the A allele is a risk factor for type 2 diabetes in the control group (incidence of diabetes, 20% vs. 10%), and that intervention has a significant effect on the incidence of diabetes. However, only subjects with the BB genotype respond significantly to lifestyle intervention. Thus, the A allele is a risk factor for type 2 diabetes, because it increases the risk of diabetes more than the BB genotype in the control group, and is more ‘resistant’ to lifestyle intervention. The situation in Fig. 2C is different. Again, the A allele is a risk factor for type 2 diabetes, but the effect of lifestyle intervention seems to be paradoxical. In fact, the risk A allele becomes a ‘protective’ allele with respect to the risk of diabetes.
Fig. 1. Progression to type 2 diabetes due to genetic and environmental factors.
M. Laakso, A. Kubaszek / International Congress Series 1253 (2003) 55–61
Fig. 2. Possible effects of intervention on the incidence of type 2 diabetes.
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M. Laakso, A. Kubaszek / International Congress Series 1253 (2003) 55–61
How relevant are considerations given in Fig. 2 for the understanding of the risk of type 2 diabetes? We have been carrying out a study aiming to determine the role of different candidate genes and their interaction with lifestyle modification in the Finnish Diabetes Prevention Study (DPS) [14]. The DPS recruited 522 obese subjects with impaired glucose tolerance. These subjects were randomized into two subgroups, one undergoing an intensive lifestyle modification program (weight loss, physical exercise, changes in diet), and another group receiving only a general information on beneficial lifestyle changes. After a mean follow-up of 3.5 years, the intervention group had a 58% reduction in the incidence of type 2 diabetes compared to the incidence in the control group. So far, we have screened variants in 10 different genes regulating insulin signaling, cytokine response, nuclear receptors, and lipid and lipoprotein metabolism. Majority of variants did not predict the conversion to diabetes. However, almost all of the variants predicting the conversion to diabetes followed the pattern presented in Fig. 2B, indicating that risk gene variants were resistant to lifestyle changes. We also have an example of a gene variant described in Fig. 2C. We studied the effect of the Pro12Ala polymorphism in exon B of the PPARg2 gene on the risk of diabetes incidence in the Finnish DPS [13]. In the control group, the 12Ala allele was predicting the conversion from impaired glucose tolerance to diabetes. However, in the intervention group, the Ala12Ala genotype was protective of developing diabetes. Therefore, in the case of the PPARg2 gene, the same genotype was a risk factor or a protective factor depending on lifestyle changes. These results indicate how important gene– lifestyle interactions are in determining the risk of type 2 diabetes. 5. Concluding remarks So far, the candidate gene approach has not been very successful in the identification of genes responsible for type 2 diabetes due to several reasons listed in Table 2. Also, the genome-wide scanning has not fulfilled the expectations in the solving of the genetic basis of type 2 diabetes. Candidate gene approach is still valuable but it should be applied more in studies having a prospective study design and a large number of subjects, and particularly in the setting of clinical trials aiming to prevent type 2 diabetes either by lifestyle changes or by drug treatment. Acknowledgements This work was partly supported by grants from the Academy of Finland, The Finnish Diabetes Research Foundation, and the European Union (QLG1-CT-1999-00674). References [1] C.R. Kahn, D. Vicent, A. Doria, Genetics of non-insulin-dependent (type-II) diabetes mellitus, Annu. Rev. Med. 47 (1996) 509 – 531. [2] M.I. McCarthy, P. Froguel, G.A. Hitman, The genetics of non-insulin-dependent diabetes mellitus: tools and aims, Diabetologia 37 (1994) 959 – 968.
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[3] C.L. Hanis, E. Boerwinkle, R. Chakrabarty, et al., A genome-wide search for human non-insulin-dependent (type 2) diabetes genes reveals a major susceptibility locus on chromosome 2, Nat. Genet. 13 (1996) 161 – 166. [4] E. Lander, L. Kruglyak, Genetic dissection of complex traits: guidelines for interpreting and reporting linkage results, Nat. Genet. 11 (1995) 241 – 247. [5] F.S. Collins, Positional cloning: let’s not call it reverse anymore, Nat. Genet. 1 (1992) 3 – 6. [6] D. Altshuler, J.N. Hirschhorn, M. Klannemark, et al., The common PPAR gamma Pro12Ala polymorphism is associated with decreased risk of type 2 diabetes, Nat. Genet. 26 (2000) 76 – 80. [7] S.S. Deeb, L. Fajas, M. Nemoto, et al., Pro12Ala substitution in PPARgamma2 associated with decreased receptor activity, lower body mass index and improved insulin sensitivity, Nat. Genet. 20 (1998) 284 – 287. [8] F.P. Mancini, O. Vaccaro, L. Sabatino, et al., Pro12Ala substitution in the peroxisome proliferator-activated receptor gamma2 is not associated with type 2 diabetes, Diabetes 48 (1999) 1466 – 1468. [9] A. Meirhaerge, L. Fajas, N. Helbecque, et al., Impact of the peroxisome proliferative activated receptor gamma2 Pro12Ala polymorphism on adiposity, lipids and non-dependent diabetes mellitus, Int. J. Obes. Relat. Metab. Disord. 24 (2000) 195 – 199. [10] P.G. Cassell, A.E. Jackson, B.V. North, et al., Haplotype combinations of calpain 10 gene polymorphisms associate with increased risk of impaired glucose tolerance and type 2 diabetes in South Indians, Diabetes 51 (2002) 1622 – 1628. [11] M.J. Garant, W.H. Kao, F. Brancati, et al., SNP43 of CAPN10 and the risk of type 2 diabetes in AfricanAmericans: the Atherosclerosis Risk in Communities Study, Diabetes 51 (2002) 231 – 237. [12] Y. Horikawa, N. Oda, N.J. Cox, et al., Genetic variation in the gene encoding calpain-10 is associated with type 2 diabetes mellitus, Nat. Genet. 26 (2000) 163 – 175. [13] J. Tuomilehto, J. Lindstro¨m, J.G. Eriksson, et al., Prevention of type 2 diabetes mellitus by changes in lifestyle among subjects with impaired glucose tolerance, N. Engl. J. Med. 344 (2001) 1343 – 1350. [14] V.I. Lindi, M.I. Uusitupa, J. Lindstro¨m, et al., Association of the Pro12Ala polymorphism in the PPARgamma2 gene with 3-year incidence of type 2 diabetes and body weight change in the Finnish Diabetes Prevention Study, Diabetes 51 (2002) 2581 – 2586.