Maturity onset diabetes of young, or MODY, primarily results from a variety of monogenic defects that affect the functioning of pancreatic islet β-cells and reduce their production of insulin. This in turn leads to hyperglycemia or high blood sugar which causes the body to become resistant to insulin. The impaired secretion of insulin seen in MODY is similar to the deficiency found in Type-I diabetes. Yet, unlike Type-I diabetes, MODY develops slowly and does not completely destroy the ability of the pancreas to secrete insulin. Rather, it impairs insulin secretion so that the body cannot adequately control blood glucose levels from one moment to the next.Molecular defects in over twelve genes have been identified in MODY patients. All of these genes encode proteins involved in glucose homeostasis of the pancreatic β-cell. The glucokinase gene (associated with MODY2) encodes an enzyme of the glycolytic pathway which modulates insulin secretion in response to glycemic variations. Five genes encode nuclear proteins that control the appropriate expression of β-cell genes. HNF-1α (MODY3) and HNF-1β (MODY5) are atypical homeodomain proteins. Insulin Promoter Factor-1/MODY4 (PDX1) encodes a homeodomain containing protein, HNF-4α is a steroid nuclear receptor family member and Neurogenic Differentiation-1, or β-cell E-box Transactivator-2 (MODY6) is a basic helix-loop-helix transcription factor. HNF-4&Alpha: activates the expression of genes involved in the transport and metabolism of many nutrients including lipids and glucose. HNF-4α expression is in turn controlled by HNF-3α and HNF-3β. It is required for normal hepatic function, directly activates the insulin gene promoter and is required for glucose induced insulin secretion. The glycolytic enzyme glucokinase which is expressed at highest levels in the pancreatic β-cell catalyzes the transfer of phosphate from ATP to glucose to form Glucose-6-phosphate. Glucose is transported into the β-cell by a specific GLUT2 on the cell surface. The generation of ATP by glycolysis and the Krebs cycle leads to inhibition and closure of the ATP-sensitive K+ channels (the target of sulfonylurea drugs), depolarization of the plasma membrane, opening of the voltage dependent Ca2+ channels, influx of extracellular Ca2+ and mobilization of Ca2+ from intracellular stores. These activities lead to the fusion of insulin-containing secretory granules with the plasma membrane and the release of insulin into the circulation. A mutation in one of the alleles of the genes encoding HNF-4α, glucokinase, HNF-1α, PDX1, HNF-1β and NeuroD1 leads to a reduction in β-cell glucokinase activity resulting in decreased glucose phosphorylation in the β-cell and glucose stimulated insulin release at any blood glucose concentration. MODY4 and MODY6 are also caused by mutations in genes encoding PDX1 which is activated by inductive signal that acts via the insulin receptor and/or the IGF1R.
The most common clinical presentation of MODY is mild, asymptomatic hyperglycemia in non-obese children, adolescents, and young adults who have a prominent family history of diabetes, often in successive generations. Non-genetic factors that affect insulin sensitivity such as infection, puberty, pregnancy and rarely, obesity may trigger the onset of diabetes and also affect the severity of hyperglycemia in MODY. According to current estimates, MODY may account for 1 to 5 percent of all cases of diabetes in the United States and other industrialized countries. MODY is treated very much like Type-II diabetes with diet and exercise, often in combination with oral therapies, insulin, or both. (Upgraded 03/2020)