This page will try and summarize information on myotonic dystrophy and diabetes.
Pathogenesis of Diabetics in Myotonic Dystrophy.
Myotonic dystrophy (MD) is the most common adult form of muscular dystrophy with an estimated prevalence of 1 in 8000 and is often complicated by diabetes. MD is dominantly inherited and is due to heterozygosity for a tri-nucleotide repeat expansion mutation in a protein kinase gene and it was suggested that this induces derangement of RNA metabolism also able to reduce insulin receptor expression. To test whether the abnormal RNA metabolism or a specific malfunction of protein kinase gene may induce insulin resistance prior to the onset of diabetes, we studied 5 glucose-tolerant MD patients (3F/2M, 41 [+ or -] 8yrs, 59 [+ or -] 7 kg, BMI21 [+ or -] 2 kg/[m.sup.2]) and 5 matched healthy subjects, by means of a) dual x-ray energy absorption b) euglycemic-hyperinsulinemic clamp (1 mU/kg/min) c) primed-continuous infusion of 6,6-[d.sup.2]-glucose and I-[sup.13]C-leucine d) indirect calorimetry. Fasting plasma insulin were similar, but proinsulin concentrations were increased in M!
D patients (p=0.01) and the ratio intact proinsulin/insulin (20 [+ or -] 4% vs 5 [+ or -] 1%; p=0.01) was 4-fold higher in MD. MD showed increased body fat mass (35 [+ or -] 5 vs 26 [+ or -] 2%; p=0.05) but lipid oxidation and FFA concentration in the post absorptive and clamp conditions were comparable between the two groups. Glucose metabolism (oxidative and non-oxidative) during insulin stimulation was comparable to normals (6.9 [+ or -] 1.4 vs 8.2 [+ or -] 1.1 mg/]kg FFM.min]; p=0.49). Leucine flux in the post absorptive condition was slightly increased and its sensitivity to insulin was impaired in MD (suppression =8[+ or -]2 vs 19 [+ or -] 2%; p=0.05); also suppression of plasma glutamine (8 [+ or -] 5%) and phenylalanine (8 [+ or -] 2%) concentrations during the clamp were similar than in normals (33 [+ or -] 7 and 15 [+ or] 3% respectively; p=0.05). In summary, MD showed alterations of protein metabolism in both post absorptive and insulin stimulated conditions resulti!
ng in increased proteolysis and muscle wasting. Insulin dependent glucose metabolism is preserved; therefore insulin resistance for glucose is not a major factor in the pathogenesis of diabetes in MD. On the contrary, abnormal insulin cleavage leading to increased proinsulin levels, probably related to specific protein kinase gene malfunction, represents a marker of secretory dysfunction capable to induce diabetes mellitus.
GIANLUCA PERSEGHIN, MAURO COMOLA, CINZIA ARCELLONI, EMANUELA PAGLIATO, ROBERTO LANZI, ALBERTO BATTEZZATI(*), LIVIO LUZI(*), Milan, Italy
(*) ADA Professional Section Member. See Duality of Interest Information.