Patients with type 2 diabetes mellitus represent 90 – 95% of patients with diabetes mellitus. Type 2 diabetes mellitus is, therefore, the most common form of diabetes mellitus. Patients with T2DM develop insulin resistance or relative insulin deficiency, which generally do not require insulin treatment for survival at onset but may require insulin later when metabolic control by diet and other medications declines. Ketoacidosis is not common in type 2 diabetes and usually
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caused by illness-triggered stress such as infections. The cause of type 2 diabetes is heterogeneous. Age, obesity, lack of exercise and genetic predisposition are known risk factors for the development of type 2 diabetes, although specific aetiology remains unclear (ADA, 2011).
Genetic and environmental risk factors
Genetic predisposition has a markedly higher influence on risk of T2DM than for T1DM. Monozygotic twins are found 15 – 25% concordant for T1DM while 69 – 90% concordant for T2DM and the concordance rate increases with age in T2DM (Lo et al., 1991, Fonseca and John-Kalarickal, 2010). A family history increases the risk of T2DM by 2 – 4 fold (Stumvoll et al., 2005). The prevalence of T2DM among the Pima Indians in the US is ca. 35%. The prevalence of obesity in adult Pimas is 75%. A family history of T2DM is a stronger risk factor for developing T2DM than other factors combined – obesity, gender and physical fitness (Fonseca and John-Kalarickal, 2010). Genome-wide association studies have identified multiple susceptibility gene loci for T2DM – Table 4. Environmental factors such as obesity, sedentary lifestyle and unhealthy diet may increase the risk of T2DM in predisposed populations, by disturbing insulin action and/or insulin secretion. Obesity alone is a strong independent risk factor for T2DM. Central obesity reflected by the waist-hip ratio is closely associated with insulin resistance and has been shown as predictor of T2DM in several studies (Narayan et al., 2007, Grundy, 2000, Groop, 1999).
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Table 4: Candidate genes associated with T2DM Gene locus Encoded protein Function of
protein
Probable mechanism
PPAR-γ Peroxisome proliferator- activated receptor gamma
Nuclear receptor (transcription factor)
Insulin resistance
GYS1 Glycogen synthase Enzyme Alteration of
glycogen storage IRS1 Insulin receptor substrate 1 Docking protein
(insulin signaling)
Probably β-cell dysfunction
INS Proinsulin Hormone β-cell dysfunction
KCJN11 Potassium inwardly rectifying channel, subfamily J, member 11
Potassium channel β-cell or α-cell dysfunction ABCC8 Sulfonylurea receptor 1 Potassium channel
(subunit)
Probably β-cell dysfunction SLC2A1 Glucose transporter 1 Facilitated
transport
Unclear
SLC30A8 Zinc transporter Zinc transport β-cell dysfunction PPARGC1 PPAR-γ-coactivator-1 Transcriptional
cofactor
Unclear, possibly pleiotropic CAPN10 Calpain-10 Cysteine protease Unclear, possibly
pleiotropic TCF7L2 Transcription factor 7-like 2 Transcription
factor
β-cell dysfunction FTO Fat mass and obesity-
associated protein
Enzyme Altered BMI
HHEX Haematopoietically expressed homeobox protein Transcription factor β-cell dysfunction
IDE Insulin degrading enzyme Enzyme β-cell dysfunction CDKAL1 CDK5 regulatory subunit-
associated protein 1-like 1
β-cell dysfunction CDKN2A/ 2B Cyclin-dependent kinase inhibitor Cell growth regulator β-cell dysfunction IGF2BP2 Insulin-like growth factor 2
mRNA binding protein 2
Regulation of IGF2 translation
β-cell dysfunction
11 NF1B Hepatocyte nuclear factor 1
homeobox B
Transcription factor
β-cell dysfunction WFS1 Wolfram syndrome 1 Transmembrane
protein
Unknown JAZF1 Nuclear protein with three
C2H2-type zinc fingers
Transcriptional repressor
β-cell dysfunction CDC123/
CAMK1D
Cell division cycle protein 123 homolog
Calcium/calmodulin-
dependent protein kinase 1D
Enzyme Unknown TSPAN8/ LGR5 Tetraspanin 8 Leucine-rich repeat containing G protein- coupled receptor 5 Transmembrane protein Unknown
THADA Thyroid adenoma associated protein
Unknown ADAMTS9 ADAM metallopeptidase
with thrombospondin type 1 motif 9
Unknown
NOTCH2 Notch homologue 2 Unknown
KCNQ1 Potassium voltage-gated channel, KQT-like, member 1
Potassium channel β-cell dysfunction
Adapted from Stumvoll et al. (2005) and Prokopenko et al. (2008)
Natural history of Type 2 diabetes mellitus
It is not clear whether abnormalities in insulin action or secretion occur as the primary defect, or both are involved in the primary pathogenesis of T2DM. However, epidemiological and clinical studies suggest that the development of T2DM is mostly commonly initiated with insulin resistance in most population. In the early stage preceding T2DM, fasting glucose and glucose tolerance remain normal with increased response of fasting insulin and glucose-stimulated insulin. The latter compensates for insulin resistance. IGT then develops where increased fasting and postprandial insulin levels attempt to compensate for insulin resistance. β-cell function begins to decline. It may be attributed to impaired metabolism and/or sensing of glucose, defective stimulatory molecules, amyloid deposition
12 and reduction in β-cell mass.
Investigation of the mechanisms of insulin resistance has focused on several targets. For example, the insulin receptor substrates IRS-1 and IRS-2 are important mediators in insulin signalling. Impaired IRS-1 phosphorylation in response to insulin was found in muscle and adipocytes in T2DM. PI-3-kinase, involved in the insulin-stimulated translocation of a glucose transporter GLUT-4 and glycogen synthase activation, is activated by binding to tyrosine phosphorylated IRS-1 and IRS-2. Inhibited activation of this essential protein was found in muscle of type 2 patients (Thies et al., 1990, Cusi et al., 2000). Another possible mechanism of insulin resistance is via the glucose transport system. Insulin stimulates glucose transport in tissues such as skeletal muscle, adipocytes and cardiac muscle by activating the trafficking of GLUT-4 from intracellular location to plasma membrane. Despite that the gene coding and muscle protein level of GLUT-4 is normal, impaired translocation of GLUT-4 to plasma membrane was found in T2DM (Kelley et al., 1996, Kruszynska and Olefsky, 1996), probably due to dysfunctional proteins in the translocation signalling pathway, or defective insulin signalling.
A progressive loss of β-cell function will eventually leads to hyperglycaemia, often accompanied by a series of metabolic abnormalities including elevated levels of gluconeogenic precursors, lactate, alanine, pyruvate and glycerol, and increased lipolysis. Elevated levels of fasting and postprandial plasma free fatty acid (FFA), and hepatic VLDL can be observed as a result of increased lipolysis. Once the disease is established, insulin resistance can be aggravated by increased plasma FFA and hyperglycaemia. The major insulin- targeting tissues are skeletal muscle, liver and adipose tissue. Metabolic disturbances in T2DM are caused by defective insulin action and/or secretion on these tissues including decreased insulin-stimulated skeletal muscle glucose uptake, overproduction of hepatic glucose, and impaired suppression of plasma glucagon and adipocyte lipolysis, which contribute to fasting and postprandial hyperglycaemia. Generally it takes longer to develop the clinical symptoms in T2DM than T1DM due to the pathogenesis of the disease. It is estimated that at least 30% of the T2DM population is undiagnosed. Many patients have already
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developed vascular complications upon diagnosis due to tissue damages by significant hyperglycaemia for 5 – 10 years before diagnosis (Fonseca and John- Kalarickal, 2010).