**7. Genes in type 2 Diabetes Mellitus**

T2DM is a complex diseases arise from interactions of multiple genes with environmental factors. These genes are variables in strength, site of interaction and are different in general population according to race and ethnicity. The diabetes genes are involved in insulin signaling, insulin secretion, IR, glucose metabolism and obesity. In addition, T2DM could be a component of many syndromes of identified inherited genes [72]. The genetic risk for T2DM changes as humans first began migrating around the world, implying a strong environmental component has affected the genetic-basis of the disease [73, 74]. Immigrants to Western developed countries, for instance, may be more prone to diabetes as compared to its lower incidence in their countries of origins [75]. In meta-analysis of different ethnicities was included in a cross –sectional data from 16 cohorts from the DECODA (Diabetes Epidemiology: Collaborative Analysis of Diagnostic Criteria in Asia) demonstrated; the waist-to-stature ratio in Asian was stronger than BMI is associated with DM in adjusted age group in both genders [76]. In Japanese-American the prevalence of DM was reduced among the men who had retained a more Japanese lifestyle through higher levels of physical activity and consumed more carbohydrates with less fat and animal protein in their diet. An inverse association between DM and being bom in Japan was observed independent of age, body mass index, physical activity, and percentages of calories from fat or carbohydrates [77]. In Fiji population the disease pattern is determined by the modern lifestyle as in many industrial counties. The increase in mortality in this population was contributing to 6% and 30% due to DM and CVD respectively [78]. In USA, the large burden of T2DM in relative to non-Latino Whites, Latinos —those of Mexican origin in particular is related to mixed acculturation to US lifestyles and with greater time living in the States [79]. This was supported by other two studies. The first showed IR, hyperinsulineamia and T2DM with secondary lipid disturbances are the possible mechanism of higher morbidity and mortality from CHD in UK immigrants from the Indian subcontinent (South Asian) than other general population independently of the dietary fat intakes, smoking, blood pressure, or plasma lipids [80]. The second study also observed IR is the common pathogenic mechanism and is a risk factor of increasing the prevalence of CHD, T2DM, low HDL and hypertriglyceridemia in first generation immigrant Asian Indian to the USA compared with native Caucasian population [81]. The two major environmental risk factors for T2DM are obesity (≥ 120% ideal body weight or a body mass index > 30 k/m2 ) and the sedentary lifestyle [82, 83]. Thus, the tremendous increase in the rates of T2DM in recent years has been attributed, primarily, to the dramatic rise in obesity worldwide [84]. Obesity is a progressive disease and up to 80% of patients with T2DM and PCOS patients can be attributed to obesity. It is a progressive condition leads to medical co-morbidities which are more prevalent with age and higher BMI, particularly in those with a central obesity [85]. In Pima Indians adolescents, the prevalence of T2DM was 50.9 per 1000 in 1967-1976 increased to 6 folds in 1987-1996. In American Indian and first nation youth, T2DM was more prevalent in 10-19 years old young group. Mainly in obese subjects with acanthosis nigiricans and had family history of the disease [86]. Sinha R, reported 25% of obese children (4 to 10 years of age) and 21%t of obese adolescents (11 to 18 years of age) had IGT and 4% of the obese adolescent has silent T2DM [87]. The increase in physical activity plays an important role in reducing risk of obesity and T2DM. The prolonged TV watching is associated with a significantly increased risk of obesity and T2DM, independent of diet and exercise. Men watching TV more than 40 hours per week have threefold of increase risk of developing T2DM compared with those less than I hour TV watching. Therefore, the public health campaign of decrease sedentary life should accompany the increase in physical activity [88].

transcriptional events mediated by NF-κB. Inhibition occurs due to activation of several serine/threoning kinases (JNK, IKK, and p38 MAPK) by excess in ROS. The later generation increased by mitochondrial β- oxidation triggered by ex‐ cess fat accumulation. (*C*) IR exacerbated by mitochondrial dysfunction through increasing intracellular lipid accumu‐ lation. (*D*) Insulin signaling suppressed by activation of JNK or through a potential increase in ROS production and both activated by cellular ER stresses responses. (*E*) The cell-extrinsic modulators such as endocrine and inflammatory

WAT: White adipose tissue, AT: adipose tissue, T2DM: type 2 diabetes mellitus, IR: insulin resistance, IS: insulin sensitivity, IGT: impaired glucose tolerance, TNF-α: tumor necrosis alpha, IL: interleukin, RBP4: Retinol binding protein, SC: subcutaneous, MCP-1: monocyte chemotactic protein 1, PAI: plasminogen activator inhibitor, CXCL5: chemokines molecules

Glucose transporters (GLUT) are integral membrane proteins that mediate the transport of

Sugar transport catalyzed by 11 out of 14 members of the human GLUT family. There are specific characteristics of each isotypes. They are different in expression profile, substrate specificity and kinetic characteristics. Therefore, the tissue adaptation of the glucose uptake will be determined and regulated by specific tissue gene expression. GLUT4 malfunction in expression or regulation contributes to IR while GLU2 plays a role in hormonal and neuronal control by acting a glucose sensor in β-cells of the pancreas and neuronal cells [68]. GLUT1 is ubiquitously expressed with particularly high levels in human erythrocytes and in the endothelial cells lining the blood vessels of the brain. GLUT3 is expressed primarily in neurons and, together, GLUT1 and GLUT3 allow glucose to cross the blood-brain barrier and enter

The GLUTI, GLUT2, and GLUU3 are the major glucose transporters isoforms present in these cells and they are constitutively localized to the plasma membrane. The glucose flux across the membrane is largely dependent upon the circulating blood glucose level or concentration. In acute state, the glucose regulated the transport system in the muscles and fat cells responded

GLUT4 is the primary hormonally-responsive transporter and it is the major insulin-respon‐ sive transporter [69]. GLUT4 expression is also reduced by low insulin states, such as in muscle during fasting, and in IR adipose tissue [71]. The malfunction of glucose transporter expression

T2DM is a complex diseases arise from interactions of multiple genes with environmental factors. These genes are variables in strength, site of interaction and are different in general

or regulation (GLUT4) appears to contribute to the IR syndrome [68].

glucose and structurally-related substances across the cellular membranes [67].

signals can intensified IR [57].

72 Treatment of Type 2 Diabetes

**6.1. Glucose transporters**

**6. Others**

neurons [69].

within minutes to insulin [70].

**7. Genes in type 2 Diabetes Mellitus**

(CXCL5 ligand 5), TGFβ: transforming growth factor β.

Physical activity has also been inversely related to body mass index and IGT. Interventional studies in China, Finland and the US demonstrated the decrease in the risk of incidence and in the progression from pre-diabetes stage (IGT) to T2DM by intensive lifestyle interventions targeting diet and exercise. The Finnish Diabetes Prevention Study showed a reduction by 58% and in the china study was 31% with diet and 42% with diet and exercise. This was supported by USA study showed the same risk reduction with intensive lifestyle as in the Finnish study and also a reduction of 31% with metformin indicating that lifestyle intervention is better than drugs in T2DM prevention [89, 90, 91].

Xue Sun [92] reviewed a large-scale of association studies, and the genome-wide association studies (GWAS). Both have successfully identified multiple genes that contribute to T2DM susceptibility. Linkage analysis, candidate gene approach, large-scale association studies, and GWAS have identified approximately 70 loci conferring susceptibility to T2DM. Among them, 45 loci were identified in European populations, and the other 29 loci were identified in Asian populations, especially in East and South Asians. The immediate benefit derived from these findings was the better understanding of the pathophysiology of T2DM. A great number of studies have suggested that genetic variants in or near KCNJ11, TCF7L2, WFS1, HNF1B, IGF2BP2, CDKN2ACDKN2B, CDKAL1, SLC30A8, HHEX/IDE, KCNQ1, THADA, TSPAN8/ LGR5, DC123/CAMK1D, JAZF1, MTNR1B, DGKB/TMEM195, GCK, PROX1, ADCY5, SRR, CENTD2, ST6GAL1, HNF4A, KCNK16, FITM2-R3HDML-HNF4A, GLIS3, GRB14, ANK1, BCAR1, RASGRP1, and TMEM163 may confer T2DM risk through impaired -β-cell function [93-97] whereas PPARγ, ADAMTS9, IRS1, GCKR, RBMS1/ITGB6, PTPRD, DUSP9, HMGA2, KLF14, GRB14, ANKRD55, and GRK5 have an impact on insulin action [94,98-100]. FTO and MC4R, previously identified genes associated with obesity, appear to confer T2DM risk through their primary effects on BMI, but recent GWAS have shown that their effects on T2DM were independent of BMI, though FTO may have a small but detectable influence on T2DM risk through insulin action [101,102].

Barnet AH; concluded the higher concordance rate in identical twins than in non-identical twins regardless the age of the onset of the disease and in identical twins, the concordant for DM is usually in older age group compared with the younger [103]. In Japan, The Concordance between monozygotic twins was 83% for T2DM and was 40% between dizygotic twins for T2DM. This concordance was significantly greater in monozygotic than in dizygotic twins and among twins with later onset of DM (after the age of 20) than early onset. It was also observed, the loss of the early-phase insulin response for OGTT in T2DM co-twins [104]. The role of obesity or BMI in twins as a risk of DM varies in heterogeneous genetic background. The index twins of concordant pairs had been less obese than discordant pairs. This is suggestive obesity has a role in pathogenesis of DM in those with weaker genetic susceptibility for the disease [5].

Nonetheless, non diabetes first-degree relatives of T2DM patients have an almost three fold increased lifetime risk of T2DM in comparison to the background population [106]. In other study 40% of first-degree relatives of T2DM patients develop diabetes as compared to 6% in the general population [106]. IR is an early metabolic feature of nondiabetic first-degree relatives of T2DM patients [107,108] and also shows familial clustering in keeping with an underlying genetic predisposition [109,110]. The defects of insulin action are retained in cultured skeletal muscle cells from IR subjects and T2DM patients [111,112] suggesting that genetic variation contributes to decreased insulin action. While IR is a common feature of T2DM, the severity and clinical importance varies considerably across the T2DM population [113].

#### **7.1. Inherited DM: Maturity onset diabetes of the young or MODY**

Physical activity has also been inversely related to body mass index and IGT. Interventional studies in China, Finland and the US demonstrated the decrease in the risk of incidence and in the progression from pre-diabetes stage (IGT) to T2DM by intensive lifestyle interventions targeting diet and exercise. The Finnish Diabetes Prevention Study showed a reduction by 58% and in the china study was 31% with diet and 42% with diet and exercise. This was supported by USA study showed the same risk reduction with intensive lifestyle as in the Finnish study and also a reduction of 31% with metformin indicating that lifestyle intervention is better than

Xue Sun [92] reviewed a large-scale of association studies, and the genome-wide association studies (GWAS). Both have successfully identified multiple genes that contribute to T2DM susceptibility. Linkage analysis, candidate gene approach, large-scale association studies, and GWAS have identified approximately 70 loci conferring susceptibility to T2DM. Among them, 45 loci were identified in European populations, and the other 29 loci were identified in Asian populations, especially in East and South Asians. The immediate benefit derived from these findings was the better understanding of the pathophysiology of T2DM. A great number of studies have suggested that genetic variants in or near KCNJ11, TCF7L2, WFS1, HNF1B, IGF2BP2, CDKN2ACDKN2B, CDKAL1, SLC30A8, HHEX/IDE, KCNQ1, THADA, TSPAN8/ LGR5, DC123/CAMK1D, JAZF1, MTNR1B, DGKB/TMEM195, GCK, PROX1, ADCY5, SRR, CENTD2, ST6GAL1, HNF4A, KCNK16, FITM2-R3HDML-HNF4A, GLIS3, GRB14, ANK1, BCAR1, RASGRP1, and TMEM163 may confer T2DM risk through impaired -β-cell function [93-97] whereas PPARγ, ADAMTS9, IRS1, GCKR, RBMS1/ITGB6, PTPRD, DUSP9, HMGA2, KLF14, GRB14, ANKRD55, and GRK5 have an impact on insulin action [94,98-100]. FTO and MC4R, previously identified genes associated with obesity, appear to confer T2DM risk through their primary effects on BMI, but recent GWAS have shown that their effects on T2DM were independent of BMI, though FTO may have a small but detectable influence on T2DM

Barnet AH; concluded the higher concordance rate in identical twins than in non-identical twins regardless the age of the onset of the disease and in identical twins, the concordant for DM is usually in older age group compared with the younger [103]. In Japan, The Concordance between monozygotic twins was 83% for T2DM and was 40% between dizygotic twins for T2DM. This concordance was significantly greater in monozygotic than in dizygotic twins and among twins with later onset of DM (after the age of 20) than early onset. It was also observed, the loss of the early-phase insulin response for OGTT in T2DM co-twins [104]. The role of obesity or BMI in twins as a risk of DM varies in heterogeneous genetic background. The index twins of concordant pairs had been less obese than discordant pairs. This is suggestive obesity has a role in pathogenesis of DM in those with weaker genetic susceptibility for the disease [5].

Nonetheless, non diabetes first-degree relatives of T2DM patients have an almost three fold increased lifetime risk of T2DM in comparison to the background population [106]. In other study 40% of first-degree relatives of T2DM patients develop diabetes as compared to 6% in the general population [106]. IR is an early metabolic feature of nondiabetic first-degree relatives of T2DM patients [107,108] and also shows familial clustering in keeping with an underlying genetic predisposition [109,110]. The defects of insulin action are retained in cultured skeletal

drugs in T2DM prevention [89, 90, 91].

74 Treatment of Type 2 Diabetes

risk through insulin action [101,102].

The transcription factor genes play a crucial role in the normal development and function of the β-cell. MODY is a distinct type of heterogeneous group of disorders caused by mutations in β-cell transcription factors. MODY is an autosomal dominant mode of inheritance β- cell dysfunction in young age group (usually before 25 years). It is difficult to distinguish between MODY and other forms of DM because of primary defect is pancreatic β-cell dysfunction in patients with MODY. There are at least nine mutations of different gens that result in MODY phenotypes which account in about 1-2% or 2-5% of patients diagnosed with DM (approxi‐ mately 20000 in UK). The transcription factors hepatocyte nuclear factor HNF - α1, HNF-4α, insulin promoter factor (IPF)-1, HNF-1β, and NeuroD1 are the main identified genes in MODY. The Glucokinase (GCK) is an enzyme responsible for glucose phosphorylation, whereas HNF-1α, HNF-4α, IPF1, HNF-1β, and NEUROD1 are all transcription factors that modulate the expression of several genes involved in differentiation and function of β-cells. This enzyme is a key in blood glucose homeostasis and the defective in kinase gene metabolism implicated in the pathogenesis of DM. There is a linkage between the GCK locus on chromosome 7p and diabetes in 16 French families with MODY and the same linkage was demonstrated in a large 5-generation pedigree (BX) with 15 members with DM. Mutations in the GCK gene cause a mild, asymptomatic and non-progressive fasting hyperglycaemia from birth usually requiring no treatment. The gene on chromosome 7 (MODY2) encodes the glycolytic enzyme glucoki‐ nases plays a key role in generating the metabolic signal for insulin secretion and in integrating hepatic glucose uptake. Other linkage studies have localized other genes mutation in MODY on human chromosomes 20 (MODY1) and 12 (MODY3), with MODY2 and MODY3 being allelic with the genes encoding glucokinase. MODY1 is an encoding (hepatocyte nuclear factor) HNF-4α (gene symbol, TCF14), a member of the steroid/thyroid hormone receptor superfamily and an upstream regulator of HNF-1 α expression. This is a transcription factor involved in tissue-specific regulation of liver genes but also expressed in pancreatic islets, insulinoma cells and other tissues. Mutations in the GCK and HNF1- α/4-α genes account for up to 80% of all MODY cases, these mutations appear in different transcription factor genes result in different clinical presentations. Mutations in the genes encoding the transcription factors HNF-1α and HNF-4α cause a progressive insulin secretory defect and hyperglycemia while mutations in the GCK gene cause a mild and non-progressive disease. MODY3 form has also mutations in the gene encoding hepatocyte nuclear factor-1 α (HNF-1α, which is encoded by the gene TCF1). Mutations in HNF-1α are highly penetrant with 63%of mutation carriers having diabetes by the age of 25 years, 78.6% by 35 years, and 95.5% by 55 years resulted in progressive β- cell dysfunction with increasing treatment requirements, greater risk of complications with age and appear to be renal dysfunction, which is often diagnosed before diabetes. Mutations in HNF-4α result in same mode of β- cell deterioration in β-cells function but the diagnosis in the later age. Mutations in IPF-1 (PDX-1) are not a common cause of MODY. MODY1 form has mutations in the gene encoding hepatonuclear factor HNF-4 α (gene symbol, TCF14) is a member of the steroid/thyroid hormone receptor superfamily and an upstream regulator of HNF-1α expression. The deficient binding of NEUROD1 or binding of a transcriptionally inactive NEUROD1 polypeptide to target promoters in pancreatic islets leads to the develop‐ ment of T2DM in humans. In mice, two mutations in NEUROD1 in the heterozygous state were described in development of T2DM. The truncated polypeptide lacking the carboxyterminal trans-activation domain, a region that associates with the co-activators CBP and p300 is more severe clinically than mutation at Arg 111 in the DNA-binding domain, abolishes Ebox binding activity of NEUROD1 [114-125]. Other rare forms of MODY are CEL-MODY, ABCC8, KCNJ11 and UPD6 [126]. There are many factors determined the clinical presentation of various subtype of MODY including, the severity, the course of insulin secretion defect, the risk of microvascular complications and the presence of other abnormalities or defects in diabetes patients [127].

#### **7.2. Mitochondrial diabetes: A monogenic Diabetes Mellitus**

Maternallyinheriteddiabetes anddeafness (MIDD)is anewsub-typeofdiabeteswithmutation in mitochondrial DNA. The mitochondrial genome is passed and inherited exclusively by maternalline.InpatientswithT2DMtherecouldbeincreaseintransmissionofinheriteddiseases by maternal line more than paternal. However, only 0.5-2.8% of patients with DM demonstrat‐ ed the frequency of adenine to guanine mutation. The adenine to guanine transition mutation at position 3243 in the dihydrouridine loop of mitochondrial tRNALeu(UUR) gene is specific to patients withaneurological syndromeMELAS (myopathy, encephalopathy,lactic acidosis and stroke like disease). There is also 3243 mutation is associated with the phenotypical distinct of MIDD sub-type. In a study of the genotype of patients with T2DM of North European extrac‐ tion with one or more features of MIDD and/or MELAS, Two patients were identified with the mutation giving a prevalence rate of 0.13% for the whole study population, and 0.45% for the sample with phenotypic features of MIDD. Other mitochondrial gene defects and mutation of 3234 are linked to the development of T2DM [128-133].
