Type 1 diabetes is an autoimmune disease in which the immune cells of the body attack and destroy the cells in the pancreas that secrete insulin. Hereditary factors, diet and viral infection contribute to the risk of developing type 1 diabetes. The exact environmental triggers for type 1 diabetes are unknown, but interestingly are more prevalent in white industrialized populations. Type 2 diabetes is also a metabolic disease but in this disease cells stop responding to insulin. There are genetic factors that increase the risk of developing type 2 diabetes, but its incidence is significantly related to obesity and a sedentary lifestyle. Both type 1 and type 2 diabetes are called polygenic diseases, meaning that many genes contribute to their development.
Type 1 Diabetes
Dr. Warren at the Joslin Diabetes Center has concluded that type 1 diabetics do have genes that predispose them to the disease, but unknown environmental factors trigger the disease. The evidence for a genetic contribution to the development of type 1 diabetes is found in identical twin studies. If an identical twin has the disease the other twin, who carries identical DNA, has a 50 to 60 percent chance of developing the disease. If an immediate family member has type 1 diabetes, another person of the same family has a 10 to 20 times increased risk of developing the disease in comparison to the general population. However, even this perceived risk from certain genetic factors is not entirely genetic as family members do not only share similar DNA but also typically live in the same environment.
From analyzing the DNA of individuals with type 1 diabetes, 18 regions of the genome have been linked to an increased risk of type 1 diabetes, according to the National Center for Biotechnical Information. These regions of DNA contain multiple genes and are not all well defined. The most characterized region contains the human leukocyte antigen genes which encode for proteins that act in immune response. These proteins are involved in self-recognition and play a role in the autoimmune destruction of insulin-secreting cells in type 1 diabetes. Having certain human leukocyte antigen genes predisposes an individual to type 1 diabetes but this alone does not assure development of this disease. In families with an increased prevalence of type 1 diabetes, genetic screening may be used to identify individuals who may have an increased risk for the disease.
Type 2 Diabetes
The best evidence suggesting hereditary factors contribute to type 2 diabetes is also from identical twin studies. If one twin develops type 2 diabetes then the other is 50 to 70 percent likely to also develop type 2diabetes, according to the Joslin Diabetes Center, an institution affiliated with the Harvard Medical School. However, while about a third of type 2 diabetics have a strong genetic basis for the disease, in the other 70 percent of type 2 diabetics the development of the disease is more dependent on environmental and behavioral factors. One of the difficulties in determining the genetic contribution to type 2 diabetes is that families do not only share genes, but also diet and exercise habits.
Genes that increase the susceptibility of a person to type 2 diabetes either encode for proteins that influence the function of insulin-secreting cells or the function of cells targeted by insulin. To date, research has only defined two genes as type 2 diabetes susceptibility genes. They are calpain 10 and hepatocyte nuclear factor 4 alpha). Many other candidate genes are being investigated. A complete review of this research is found in "The Genetic Landscape of Diabetes," a publication of the National Center for Biotechnical Information.
Other Diabetes
Maturity onset diabetes in the young, also known as MODY, is a heritable form of diabetes that occurs in young children. If a parent has MODY the child has a 1 in 2 chance of developing the disease, according to the American Diabetes Association. It is caused by a mutation in one gene and is therefore called a monogenic disease in contrast to the polygenic type 1 and type 2 diabetes. The mutated gene is in involved in insulin production. This disease is similar to a milder form of type 1 diabetes in that insulin sensitivity is not affected but the cells that secrete insulin do not function as well and less insulin is secreted in response to increasing blood glucose levels.
References
- Joslin Diabetes Center: Genetics and Diabetes: What's your Risk?
- American Diabetes Association:Genetics of Diabetes.
- National Center for Biotechnical Information: The Genetic Landscape of Diabetes.
- "Endocrine Reviews": The Genetic Basis of Type 2 Diabetes Mellitus: Impaired Insulin Secretion versus Impaired Insulin Sensitivity; John E. Gerich; 1998
