Genetics may play a role in the development of diabetes for many patients, along with environment and behavior. Scientists have linked several specific genes to the various forms of diabetes.
The risks of developing type 1 diabetes, an autoimmune disease, are greater when a parent also has the disease. This factor indicates a probable genetic factor. A single gene that is responsible for type 1 diabetes has not been identified. However, 19 sections of the human genome (complete genetic material) are believed to be involved.
Type 2 diabetes is also highly complex developmentally. It is the most common type of the disease and has the clearest association with genetic inheritance. It may involve multiple genes that cause insulin resistance or that result in loss of insulin being produced by the pancreas. Researchers have found that exercise and weight loss may often prevent type 2 diabetes in people with genetic risk factors.
For other types of diabetes, such as maturity-onset diabetes of the young (MODY), a genetic link is clearly established. There are six types of MODY, each associated with a single genetic defect inherited from a parent. Other diabetic conditions with identified genetic causes include Wolfram syndrome, type A insulin resistance and Rabson-Mendenhall syndrome, which also causes insulin resistance.
It is easier to understand genetics and the link to diabetes by learning some of the basic terms used to study the inheritance of biological characteristics. These include:
- Chromosomes. Rod-shaped element in the nucleus (center) of each cell. Chromosomes contain the hereditary information that guides all cell actions. A person typically has 46 chromosomes, and one set of 23 chromosomes is inherited from each parent. The sex-determining chromosomes X and Y are two of the 46 chromosomes.
- Genes. The basic unit of hereditary. Each gene is located in a specific site on a chromosome. Each human cell has about 20,000 to 25,000 genes. A person inherits two copies of each gene, one from each parent. They help determine physical characteristics such as eye color and skin color. Genes also contribute to other characteristics in which environment and behavior play a role, such as body weight, intelligence and the risk of diabetes and other diseases. How genes are involved in diabetes is not clearly understood.
- Genome. All of the genetic material in an organism’s chromosomes.
- DNA (deoxyribonucleic acid). A complex substance in genes that contains genetic information needed to make the body’s proteins. These proteins allow the body to function and grow.
- Allele. Variations of a single gene. An individual’s genes have two alleles, one inherited from the mother and one from the father. Differences in alleles are one factor involved in differences among people in physical characteristics and susceptibility to disease.
Genetics and type 1 diabetes
Type 1 diabetes is an autoimmune disease, in which the immune system destroys the body’s capacity to produce insulin. Insulin is a hormone that helps cells use glucose (blood sugar) for energy.
Researchers have found many factors that appear to be linked to type 1 diabetes, but there does not seem to be any single cause. Some of the factors include viruses, certain diets, chemicals and other environmental factors. Genetics is also believed to play a role, which is indicated by the disease occurring multiple times in a family.
A family history of type 1 diabetes is a major indicator that a person may also develop the disorder. The risk of a child developing type 1 diabetes in association with a parent with the disease is the following:
- 1 in 17 when the father has diabetes
- 1 in 25 when the mother has diabetes and gave birth to the child before age 25
- 1 in 100 when the mother has diabetes and gave birth to the child after age 25
- 1 in 4 to 1 in 10 when both parents have diabetes
For each of these figures, the risk of diabetes doubles if the parent developed the disease before age 11.
The high rate of occurrence of diabetes within families helped encourage research into genetic links to the disease. Scientists are trying to determine which genes are involved in individuals with type 1 diabetes and how genetic mutations interfere with the production of insulin. To date, they have not found a single gene responsible for type 1 diabetes. However, there are about 19 sections of the human genome that appear to be related to a susceptibility to type 1 diabetes.
One section of the genome contains several genes involved in building human leukocyte antigens (HLA). The proteins formed by these genes help the immune system recognize the body’s own cells. When HLA genes malfunction, an autoimmune disorder may occur.
There are many types (alleles) of HLA genes. According to the American Diabetes Association, 95 percent of people with type 1 diabetes have HLA-DR3, HLA-DR4 or both alleles. The HLA genes involved in diabetes also vary by ethnicity and race. HLA-DR3 and HLA-DR4 are most common in white diabetic patients, but HLA-DR7 is common in patients of African descent and HLA-DR9 is common in patients of Japanese descent.
The inherited HLA-DR gene makes individuals more susceptible to type 1 diabetes, but it is not the only factor. Other genes, including other HLA genes (HLA-DQ) and IDDM2 (the insulin gene), are also likely to be involved. Researchers are continuing to study additional gene clusters and chromosomes to determine their role in type 1 diabetes.
The genetic factor is a complicated issue in the development of type 1 diabetes. Most people with the disease do not have a parent or sibling with it. Also, identical twins have identical genes, but there is only a 50 percent chance of both twins developing the disease. This means that factors in addition to genetics play a role in the development of the disease.
A form of type 1 diabetes that can develop in adults is known as latent autoimmune diabetes of adulthood (LADA). It is not known how the genetic risk factors for LADA may differ from those of standard type 1 diabetes.
Genetics and type 2 diabetes
Type 2 diabetes is a condition in which the body resists the insulin produced by the pancreas and may fail to make enough insulin to maintain normal glucose (blood sugar) levels. It is by far the most common form of diabetes.
A genetic link to type 2 diabetes appears much stronger than for type 1. Identical twins share the same genetic code, but should one identical twin develop type 2 diabetes, the risk for the other twin is 60 to 75 percent, according to the American Diabetes Association.
Evidence for the genetic basis of the disease is also provided by its frequency among racial and ethnic groups. In the United States, type 2 diabetes occurs most frequently among African Americans, Hispanics/Latinos, Native Americans, Alaska Natives, Asian Americans and Pacific Islanders.
Some researchers believe that this prevalence is due to the presence of a “thrifty” gene or set of genes inherited amongst these ethnic groups. In theory, the thrifty gene originally altered the use of insulin to enable people to store energy in the body more efficiently to survive periods of food scarcity. Surviving food scarcity periods is generally no longer an issue in modern Western societies, and the thrifty gene has shifted from being a survival mechanism to being a risk factor for diabetes.
Research indicates that in most cases of type 2 diabetes there is more than one gene involved and that the gene combinations may differ between families. In addition, the genes may have only slight variations, and it is possible that the variations are common in the human population. These factors make research into the genetics of type 2 diabetes difficult.
These genes appear to affect the way insulin acts on tissues, creating insulin resistance. This condition impairs the ability of cells to use the insulin that is present, and it accumulates in the bloodstream, causing hyperinsulinemia. In addition, other genes may affect the insulin-making beta cells of the pancreas, limiting their ability to produce enough insulin to overcome the resistance.
Scientists in recent years have linked several genes to development of type 2 diabetes. For example:
- A gene variant called TCF7L2 could predispose close to 38 percent of Northern European populations and many black Americans to type 2 diabetes. It might increase their risk of diabetes by 80 percent. However, these researchers found that moderate exercise and weight loss – the same preventive treatments touted in the landmark Diabetes Prevention Program – could virtually erase the additional risk in people with this genetic makeup.
- A gene called ARNT (aryl hydrocarbon receptor nuclear translocator) has been found to be expressed abnormally in the beta cells of people with type 2 diabetes. The discovery provides new insight into the development of the disease, and potentially a treatment.
The identification of some genes involved in type 2 diabetes has led to the development of genetic tests for the disease. These tests examine samples of blood or cells from the inside of the cheek for the responsible gene. Interpreting the results is complicated, and genetic counseling is recommended to patients interested in undergoing genetic screening.
Genetics can also contribute to factors that may lead to type 2 diabetes. Obesity, which is caused by both genetic and environmental factors, increases insulin resistance and the risk of type 2. Other risk factors for type 2 diabetes that may involve a genetic component include:
- Unhealthy levels of cholesterol and triglycerides (hyperlipidemia)
- Metabolic syndrome
- High blood pressure
Though a family history of type 2 diabetes is one of the strongest risk factors for developing the disease, this genetic predisposition is far more significant for those who consume an unhealthy, high-calorie diet and get little exercise.
Gestational diabetes, the form that affects some women during pregnancy, is also a metabolic disorder and may have a similar genetic composition to type 2 diabetes.
Genetics and other diabetic conditions
Several genes have been linked to some of the less common forms of diabetes and associated conditions. Researchers have identified certain genes that may be a factor in syndromes of extreme insulin resistance, diabetes-deafness syndromes and maturity-onset diabetes of the young (MODY).
MODY is a rare form of diabetes that is caused by inherited genetic defects. It accounts for less than 5 percent of diabetes cases, according to the American Diabetes Association. In most cases, MODY occurs in three or more generations of a family and often develops in the teen years or early 20s. Children of patients with MODY have a 50 percent risk of developing the condition. The disorder is sometimes considered a variation of type 2 diabetes but can be misdiagnosed as type 1.
There are several types of MODY. Each appears to be caused by a single-gene mutation. The genetic defects primarily affect the function of beta cells (pancreas cells that produce insulin), causing the development of this disease. The six forms of MODY and the associated genetic mutation are:
- MODY1 (hepatic nuclear factor-4alpha or HNF-4alpha)
- MODY2 (pancreatic glucokinase or GCK)
- MODY3 (HNF-1alpha)
- MODY4 (insulin promoter factor-1 or IPF-1)
- MODY5 (HNF-1beta)
- MODY6 (neurogenic differentiation factor-1 or neuroD1)
Other rare forms of diabetes can also be caused by single genes. The action of insulin and cellular sensitivity to insulin may be affected by the insulin receptor gene. A genetic mutation can also affect a molecule called PPAR-gamma, which affects insulin action and is a target for some antidiabetic agents, medications for type 2 diabetes.
Certain genetic defects of the beta cell can lead to Wolfram syndrome. This disorder is also known by the acronym DIDMOAD (diabetes insipidus, diabetes mellitus, optic atrophy and deafness), which refers to many of the conditions that occur in conjunction with it. The gene responsible, WSF1, is active in the pancreas and nerve cells.
Several diabetic disorders involve defects in the insulin receptor gene. Indicators are hyperinsulinemia and the skin condition acanthosis nigricans. These disorders include:
- Type A insulin resistance. A condition in which the body’s cells lose their ability to use insulin. Insulin resistance can cause an increased production of male hormones in girls, leading to polycystic ovarian syndrome.
- Leprechaunism (also called Donohue syndrome). A rare fatal form of mental and physical retardation that often causes death within the first year of life.
- Rabson-Mendenhall syndrome. Another rare congenital disorder involving severe insulin resistance and poor prognosis.
There are also numerous genetic syndromes that can lead to secondary diabetes or prediabetes. Although many subcategories exist, the main classifications include:
- Pancreatic deficiencies
- Mutant insulin syndromes
- Glucokinase gene mutations
- Mitochondrial DNA gene mutation
- Chromosomal defects
- Hereditary neuromuscular disorders
Ongoing research into genetics and diabetes
The goal of understanding the genetics of diabetes is to help identify individuals at high risk so that preventive strategies can be initiated, such as exercise and diet or possibly antibody therapies. Scientists can also use genetic information to produce drugs targeted at the specific proteins manufactured by aberrant genes. Helping those at high risk for type 2 diabetes may also include targeting the genetic causes of obesity.
Researchers have identified regions of genes that influence the immune system, which is related to type 1 diabetes. This information, combined with the ability to detect the autoimmune process that leads to type 1 diabetes in advance of clinical symptoms, has led to the development of several clinical trials that propose to study the influence of environmental factors on the development of diabetes. Through these studies it is hoped that scientists will find ways to prevent some of these factors.
- SEARCH for Diabetes in Youth, involving more than 5 million young Americans, whose goals include noting the causes and characteristics of the various forms of diabetes
- The National Children’s Study, which is to follow at least 100,000 American children from birth through adulthood in an effort to find the causes of diseases ranging from diabetes to autism
- The Environmental Determinants of Diabetes in the Young (TEDDY), testing more than 200,000 babies for several genes known to increase the risk of type 1 diabetes, is expected to identify 13,000 high-risk babies, half of whom will be followed for 15 years in an effort to determine which environmental factors also contribute to type 1 diabetes
- Prospective Assessment in Newborns for Diabetes Autoimmunity (PANDA), using a blood sample to identify infants at high genetic risk of type 1 diabetes and following them for five years
- Diabetes Autoimmunity Study in the Young (DAISY) at the University of Colorado School of Medicine, which is investigating genetic predisposition to type 1 diabetes
- Trial to Reduce Insulin-Dependent Diabetes in the Genetically at Risk (TRIGR), which is looking at environmental triggers of food proteins and the affect on diabetes
- Genetics of Kidneys in Diabetes (GoKinD), which is examining the genetic bases of diabetic nephropathy
Research also continues in developing genetic tests that may help patients understand their risk of developing diabetes and other inherited diseases.
Questions for your doctor
Preparing questions in advance can help patients have more meaningful discussions with their healthcare providers regarding their conditions. Patients may wish to ask the following questions about genetics and diabetes:
- What are genes, and how are they involved in disease?
- How is diabetes associated with genetics?
- If I have a family member with diabetes, what are the chances I will also get it?
- How likely is it that I could pass diabetes to my children?
- In addition to genetics, what other factors may help determine if I develop diabetes?
- How is diabetes inherited?
- How does my family history of diabetes tie in with the genetic causes of diabetes?
- Are there ways to test for genes that may cause diabetes? Should my family or I have genetic testing?
- What can I do to prevent diabetes if I am genetically predisposed to the disease?
- What other genetic conditions can lead to diabetes? Do I have or am I at risk for any of these conditions?