Absence of chromosome fragility at 19q13.3 in patients with myotonic dystrophy

Myotonic dystrophy (DM) is an autosomal dominant disorder characterized by a trinucleotide (AGC) amplification at 19q13.3. The degree of trinucleotide amplification may increase in successive generations and generally correlates with severity of the disorder. Because amplification of a trinucleotide repeat is also associated with the observation of a fra(X)(q27.3) in the fragile X syndrome, we investigated whether chromosome fragility at 19q13.3 might be inducible in patients with DM. Using 3 different culture stress systems (medium 199, RPMI 1640 with excess TdR, and RPMI 1640 with FudR) and high resolution chromosome analyses, we studied 6 individuals with DM and 5 unaffected relatives representing two unrelated families. Molecular studies were done on two of the most affected patients and showed AGC repeat sequences of 970 and 1,260 at 19q13.3. The normal range of AGC repeat is 5 to 30. We found no indication of fragility at 19q13.3 in any of these individuals.