Diabetes Health Article

NIDDM

The genetic or heritability component of non-insulin dependent diabetes is thought to be greater than in IDDM. Studies estimate a concordance of up to 100%, with most studies estimating greater than 70%. Most experts interpret this relatively high concordance to reflect a somewhat high heritability. High concordance may also partly reflect the fact that the environment of all those studied—for example, in the United States—is highly uniform. Therefore, all those who have genetic susceptibility can be considered to be exposed to a sufficient number environmental risk factors to trigger NIDDM.

GENES ASSOCIATED WITH NIDDM SUSCEPTIBILITY Genetic susceptibility in NIDDM is highly heterogeneous—meaning that variations in many different genes contribute to disease susceptibility. Although multiple susceptibility genes have been established or are suspected, major genes that confer a clearly high susceptibility do not play a major role—such as that seen with DR3 and DR4 in IDDM. NIDDM-associated genes include, but are not limited to, those found in the table.

NIDDM due to insulin gene mutations

In some families, NIDDM appears to be inherited in an autosomal dominant fashion. Late onset of the disease may occur together with characteristic lab values. Some such families have been shown to carry specific mutations in the insulin gene. Three mutations are known and produce altered forms of the insulin protein that apparently do not function as well as the usual type of insulin. These include Insulin Los Angeles, Insulin Wakayama, and Insulin Chicago. Although only present in about 0.5% of people with NIDDM, these mutations can lead to a dominant form of the disease. Other alterations in the insulin gene—including other point mutations and a variation outside the gene called the 5' VNTR—may also contribute to NIDDM development or susceptibility in some populations.

Syndromes with NIDDM as a feature

As seen in IDDM, there are several distinct syndromes that have NIDDM as a potential feature. Aside from NIDDM, other characteristic features are present in each of these syndromes. The genetic basis for many of these conditions is known or suspected. These include syndromes with pancreatic disease (i.e. hemochromatosis and thalassemia) and syndromes due to mutations of the DNA of mitochondria—the cellular organelles that create energy. The latter includes MELAS syndrome, as well as a large deletion of mitochondrial DNA associated with diabetes and deafness. There are multiple syndromes with glucose intolerance resulting from or in association with a variety of other conditions. These include obesity, chromosomal imbalances, diseases of the endocrine system, or diseases of metabolism. As might be expected, mutations in the insulin receptor gene account for an increased risk for NIDDM. About 0.1% to 1% of the population carries such mutations, which leads to insulin resistance in some instances. Individuals who inherit two mutated copies of the insulin receptor gene may have extreme insulin resistance or diabetes. Some such individuals may have one of two rare syndromes. Donohue syndrome usually leads to death in the newborn period due to many serious complications resulting from very extreme insulin resistance. Rabson-Mendenhall syndrome is another very rare syndrome that affects multiple body systems and has been associated with the insulin receptor gene. Finally, mutations in this gene can lead to an inherited form of diabetes with acanthosis nigricans, a highly pigmented skin condition.

Demographics

Worldwide, diabetes mellitus represents a large proportion of the common, chronic diseases caused by multiple factors. Between 5% and 10% of adults in the Western world are affected by some form of diabetes. About 1/10,000 people have IDDM. The incidence of NIDDM is about three-fold that of IDDM—up to 5% of the U.S. population age 20-74. Up to an additional 11% have impaired glucose tolerance (IGT), which can represent an early stage of NIDDM.

Incidence rates of all types of diabetes vary among ethnic groups—a result of differing genetic and environmental backgrounds. For IDDM, incidence rates range from less than 1/100,000 among Japanese to greater than 25/100,000 among Scandinavians. Ethnic variation follows a different pattern for diabetes overall, which consists primarily of those with NIDDM and IGT. While NIDDM rates are very low among the Eskimo, IGT is very common.

Population studies suggest that there may be one or more major genes that influence diabetes susceptibility, particularly NIDDM susceptibility, in certain populations with high to very high incidence rates of clinical diabetes. These include Mexican Americans, Pima Indians, Oklahoma Seminoles, and several populations in the South Pacific including the Nauruans.

In other populations, increased incidences of NIDDM suggest the role of environmental factors in the disease's development. Changes in diet and lifestyle are implicated as contributing factors in the increased incidence seen by members of ethnic groups who have experienced Westernization due to immigration patterns or other cultural changes. Such factors may play a role in the increased incidence of NIDDM seen in African Americans, Japanese Americans, certain Native American groups, South Pacific Nauruans, and recently Westernized aboriginal Australians. Differences in incidence rates among various populations is a reflection of the multiple underlying genetic and environmental factors that contribute to the development of all types of diabetes mellitus.