ALTERED PHOSPHORYLATION AND INTRACELLULAR-DISTRIBUTION OF A (CUG)(N) TRIPLET REPEAT RNA BINDING-PROTEIN IN PATIENTS WITH MYOTONIC-DYSTROPHYAND IN MYOTONIN PROTEIN-KINASE KNOCKOUT MICE

Myotonic dystrophy (DM) is associated with expansion of CTG repeats in the 3'-untranslated region of the myotonin protein kinase (DMPK) gene . The molecular mechanism whereby expansion of the (CUG)(n) repeats in the 3'-untranslated region of DMPK gene induces DM is unknown. We pre viously isolated a protein with specific binding to CUG repeat sequenc es (CUG-BP/hNab50) that possibly plays a role in mRNA processing and/o r transport. Here we present evidence that the phosphorylation status and intracellular distribution of the RNA CUG-binding protein, identic al to hNab50 protein (CUG-BP/hNab50), are altered in homozygous DM pat ient and that CUG-BP/hNab50 is a substrate for DMPK both in vivo and i n vitro. Data from two biological systems with reduced levels of DMPK, homozygous DM patient and DMPK knockout mice, show that DMPK regulate s both phosphorylation and intracellular localization of the CUG-BP/hN ab50 protein. Decreased levels of DMPK observed in DM patients and DMP K knockout mice led to the elevation of the hypophosphorylated form of CUG-BP/hNab50. Nuclear concentration of the hypophosphorylated CUG-BP /hNab50 isoform is increased in DMPK knockout mice and in homozygous D M patient. DMPK also interacts with and phosphorylates CUG-BP/hNab50 p rotein in vitro. DMPK-mediated phosphorylation of CUG-BP/hNab50 result s in dramatic reduction of the CUG-BP2, hypophosphorylated isoform, ac cumulation of which was observed in the nuclei of DMPK knockout mice. These data suggest a feedback mechanism whereby decreased levels of DM PK could alter phosphorylation status of CUG-BP/hNab50, thus facilitat ing nuclear localization of CUG-BP/hNab50. Our results suggest that DM pathophysiology could be, in part, a result of sequestration of CUG-B P/hNab50 and, in part, of lowered DMPK levels, which, in turn, affect processing and transport of specific subclass of mRNAs.