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MODY 2
Classification and external resources
OMIM 125851

MODY 2 is a form of maturity onset diabetes of the young.

MODY 2 is due to any of several mutations in the GCK gene on chromosome 7 for glucokinase. Glucokinase serves as the glucose sensor for the beta cell. Normal glucokinase triggers insulin secretion as the glucose exceeds about 90 mg/dl (5 mM). These loss-of-function mutations result in a glucokinase molecule that is less sensitive or less responsive to rising levels of glucose. The beta cells in MODY 2 have a normal ability to make and secrete insulin, but do so only above an abnormally high threshold (e.g., 126-144 mg/dl, or 7-8 mM). This produces chronic, mild hyperglycemia which is usually asymptomatic. It is usually detected by accidental discovery of mild hyperglycemia (e.g., during pregnancy screening). An oral glucose tolerance test is much less abnormal than would be expected from the impaired (elevated) fasting glucose, since insulin secretion is usually normal once the glucose has exceeded the threshold for that specific variant of the glucokinase enzyme. It can usually be controlled by dietary measures (primarily avoiding large amounts of carbohydrate). The degree of hyperglycemia does not usually worsen with age and long-term diabetic complications are rare.

This type of MODY demonstrates the common circulation but complex interplay between maternal and fetal metabolism and hormone signals in the determination of fetal size. Because MODY2 is an autosomal dominant condition, an affected mother will pass it to 50% of her children. A small number of infants will have a new mutation not present in their mothers. If the mother is affected and the fetus is not, the maternal glucose will be somewhat high and the normal pancreas of the fetus will make lots of insulin, resulting in a large infant. If the fetus is affected but mother is not, glucoses will be normal and fetal insulin production will be low, resulting in intrauterine growth retardation. Finally, if both mother and fetus have the disease, the two defects will offset each other and fetal size will be unaffected.

When both GCK genes are affected the diabetes appears earlier and the hyperglycemia is more severe. A form of permanent neonatal diabetes has been caused by homozygous mutations in the GCK gene.

References

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