By Amy Adams, MS

Reviewed by Jeremy Walson, MD

MIDD in Families

MIDD is inherited only through the mother. This is because the gene responsible for MIDD is found within tiny organs called mitochondria that are inside cells. (Mitochondria are responsible for generating energy for the cell. Although most genes are found within the nucleus of the cell, a few are contained outside the nucleus in the mitochondria.) The female egg contains many mitochondria but the sperm contains none. For this reason, people inherit all of their mitochondria — and mitochondrial genes — from their mothers. When a gene can only be inherited from the mother, it is called maternal inheritance.

	Why do Mitochondria have their own DNA?
	Scientists believe that long ago mitochondria were actually independent bacteria. These bacteria formed a close relationship with larger cells where each party provided something beneficial to the other (a relationship called symbiosis.) Over time, the bacteria transferred all but a small portion of their DNA over to the larger cells and were no longer able to live independently. It is thought that our mitochondria are the descendants of these bacteria and that our mitochondrial DNA is that last bit of DNA that the bacteria kept for themselves.

Although MIDD tends to occur at a younger age than most Type 2 diabetes, it can occur at a range of ages and in a range of severity. In fact, in one study of people with MIDD, about half the people had originally been diagnosed with Type 1 diabetes (these people developed a severe form of diabetes at a young age) and the other half had been diagnosed with Type 2 (these people developed a mild form of diabetes late in life).

The reason MIDD is so variable has to do with how we inherit mitochondria. The mother's egg contains many mitochondria, some of which have the MIDD mutation and some of which do not. As cells divide, the mitochondria are not distributed evenly. Thus, not all cells contain the same number of mutation-carrying mitochondria. If fewer pancreas cells contain the mutation in one person, they are likely to develop diabetes later than in someone whose pancreas cells contain many mitochondria with the mutation.

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The Gene

A single gene mutation called 3243 puts a person at serious risk for developing MIDD. The gene makes a transfer RNA — a molecule that helps translate a DNA sequence into protein. The mutation is called 3243 because the mutation takes place at the 3243rd position in the gene. Although researchers have discovered that the 3243 gene mutation is responsible for MIDD, they aren't sure how it causes diabetes.

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Genetic Testing

Although there is a genetic test for the MIDD mutation, the test is not yet commercially available; It is primarily used for research purposes. MIDD effects such a small part of the population that a test would not be widely used. Researchers also don't know enough about how to alter the course of MIDD, so a commercial version of the test would identify people with the MIDD mutation without altering their treatment plan or providing a way to prevent the disease.

References

Maassen, J. A., et al. (1996). The molecular basis and clinical characteristics of Maternally Inherited Diabetes and Deafness (MIDD), a recently recognized diabetic subtype. Exp Clin Endocrinol Diabetes, 104, 205-211.

van den Ouweland, et al. (1994). Maternally inherited diabetes and deafness is a distinct subtype of diabetes and associates with a single point mutation in the mitochondrial tRNA(Leu(UUR)) gene. Diabetes, 43, 746-751.

Gerbitz, K. D., et al. (1995). Mitochondrial diabetes mellitus: a review. Biochim Biophys Acta, 1271, 253-260.