The predicted protein kinase activity of the cloned gene product of the human myotonic dystrophy locus has been experimentally verified. Affinity-purified recombinant DM protein kinase became phosphorylated itself and transphosphorylated histone H1. These activities were not present in the bacterial host cells and were exhibited by DMPK and DMPKH, recombinant proteins which contain the protein kinase domain but exhibit distinct sizes, 43 and 66 kDa, respectively. DMPKH was further purified by velocity sedimentation on sucrose gradients; both activities migrated with the recombinant protein at 41 S, consistent with discrete multimeric particles. Phosphoamino acid analysis showed that threonine (predominantly) and serine were phosphorylated in both DMPKH and histone H1. Although PKA and PKC are the known types of protein kinase with closest sequence homology to the DM protein kinase domain, purified DMPKH was inhibited by 4 mM but not 0.04-0.4 mM H7 and H8, which inhibit PKA and PKC with Ki's of 0.4-15 microM. Specific inhibitors of other classes of multifunctional serine/threonine protein kinases such as casein kinases I (CKI-7) and II (heparin) and calcium/calmodulin-dependent protein kinase II (KN-62) did not inhibit DMPKH. DMPKH did not phosphorylate membrane-associated phosphoproteins such as acetylcholine receptor or spectrin which are known to be substrates for PKA, PKC, and CKI and -II, respectively. These experimental results suggest that the active center of the recombinant human myotonic dystrophy protein kinase may have properties distinct from the well-studied classes of serine/threonine protein kinases, in contrast to predictions based upon primary structure alone.
We studied the expansion of the GCT repeats within the myotonic dystrophy protein kinase gene in nine myotonic dystrophy (DM) kindreds. Southern blot and polymerase chain reaction analyses of the repeat region demonstrated the expansion in all 62 patients with the diagnosis of DM. Among 43 DM parent-child pairs, age of onset in the child was earlier than in the parent in 36 pairs, in the same decade as the parent in five, and undetermined in two. The clinical anticipation observed in the 36 pairs accompanied an increase in the fragment size in 32, a decrease in two, and no apparent change in two pairs. In the remaining pairs without documented clinical anticipation, the fragment size increased in four, decreased in two, and was apparently unchanged in one. Overall, the size of expansion showed an inverse correlation with the age of onset (p < 0.001). In all seven pairs in which the fragment did not increase in size, the affected parent was male. Two congenital DM children born to affected mothers had expanded DNA greater than 4.5 kb. The differences between parent and child in age of onset significantly correlated with the differences in the expansion size among father-child pairs (p < 0.001) but not mother-child pairs (p > 0.5). Our data suggest that the expansion of the GCT repeats plays an important role in anticipation although other factors, including the sex of the affected parent, may have significant effects on molecular mechanisms of anticipation.
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