To understand the role of the splice regulator muscleblind 1 (MBNL1) in the development of
RNA splice defects in
myotonic dystrophy I (DM1), we purified
RNA-independent MBNL1 complexes from normal human myoblasts and examined the behavior of these complexes in DM1 myoblasts.
Antibodies recognizing MBNL1 variants (MBNL1(CUG)), which can sequester in the toxic CUG
RNA foci that develop in DM1 nuclei, were used to purify MBNL1(CUG) complexes from normal myoblasts. In normal myoblasts, MBNL1(CUG) bind 10
proteins involved in remodeling
ribonucleoprotein complexes including
hnRNP H, H2, H3, F, A2/B1, K, L, DDX5, DDX17, and DHX9. Of these
proteins, only MBNL1(CUG) colocalizes extensively with DM1 CUG foci (>80% of foci) with its partners being present in <10% of foci. Importantly, the stoichiometry of MBNL1(CUG) complexes is altered in DM1 myoblasts, demonstrating an increase in the steady state levels of nine of its partner
proteins. These changes are recapitulated by the expression of expanded CUG repeat
RNA in Cos7 cells. Altered stoichiometry of MBNL1(CUG) complexes results from aberrant
protein synthesis or stability and is unlinked to PKCĪ± function. Modeling these changes in normal myoblasts demonstrates that increased levels of
hnRNP H, H2, H3, F, and DDX5 independently dysregulate splicing in overlapping
RNA subsets. Thus expression of expanded CUG repeats alters the stoichiometry of MBNL1(CUG) complexes to allow both the reinforcement and expansion of RNA processing defects.