![]() A mother with a small CTG repeat expansion and few or no noticeable symptoms can give birth to a baby with a large CTG expansion and the congenital-onset form of DM1.Īt first, it was believed that the main effect of the expanded DNA in the DMPK gene was a decrease in the amount of available DMPK protein in cells. The congenital-onset form of DM1 appears to occur mostly when the DMPK gene flaw comes from the mother. When the DMPK gene expansion is transmitted from parent to child, it often expands, causing the disease to manifest earlier with each generation in a family. This phenomenon results in expansion of CTG repeats in the DNA due to abnormal DNA repair throughout life. ![]() With CTG repeat lengths greater than 1,000, DM1 may manifest as congenital MD.Ī phenomenon known as somatic mosaicism was observed in DM1 patients. CTG repeat lengths greater than 800 may manifest as childhood DM1. ![]() Repeats in the range of 50 to 1,000 are seen in individuals with classic DM1. A mutation of 50 to approximately 150 CTG repeats can manifest as a mild DM1 type. Individuals with a CTG repeat size between 38 and 49, designated premutation status or mutable normal, are asymptomatic. It is important to remember that these correlations are by no means perfect and should not be taken as absolute predictors of the course of the disease. However, in DM2 there is no definite correlation between repeat length and the severity of disease. In DM1, the number of repeats correlates with the age of onset and the severity of the disorder. In DM1, there can be hundreds or even thousands of CTG repeats in the DMPK gene. The normal number of "CTG repeats" in the DMPK gene is fewer than 35 repeats. ![]() The DNA building blocks cytosine, thymine, and guanine (abbreviated as CTG) are repeated many more times than average in this disorder. The defect was identified in 1992 as the cause of DM1. In DM1, the abnormal DNA expansion is in the DMPK (dystrophia myotonica protein kinase) gene on chromosome 19 q 13.3. Also, it is known that the repeat expansions exert a dominant toxic effect on other genes not localized to either the DM1 or DM2 genes, which is known as a “trans” effect. DM provides an example of mechanism of disease called RNA toxicity, which results from the expanded repeats in the flawed gene transcripts. The expansions occur in two different genes but appear to have similar effects on various cells, particularly the cells of the voluntary and involuntary muscles, including the heart and nervous system. Type 1 myotonic dystrophy (DM1) and type 2 myotonic dystrophy (DM2) are both caused by abnormally expanded stretches of DNA. ![]()
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