Myotonic dystrophy (DM1) is an autosomal dominant neuromuscular disorder associated with a (CTG) n expansion in the 3¢-untranslated region of the DM1 protein kinase (DMPK) gene. To explain disease pathogenesis, the RNA dominance model proposes that the DM1 mutation produces a gain-of-function at the RNA level in which CUG repeats form RNA hairpins that sequester nuclear factors required for proper muscle development and maintenance. Here, we identify the triplet repeat expansion (EXP) RNAbinding proteins as candidate sequestered factors. As predicted by the RNA dominance model, binding of the EXP proteins is speci®c for dsCUG RNAs and proportional to the size of the triplet repeat expansion. Remarkably, the EXP proteins are homologous to the Drosophila muscleblind proteins required for terminal differentiation of muscle and photoreceptor cells. EXP expression is also activated during mammalian myoblast differentiation, but the EXP proteins accumulate in nuclear foci in DM1 cells. We propose that DM1 disease is caused by aberrant recruitment of the EXP proteins to the DMPK transcript (CUG) n expansion.

Many RNA-binding proteins of the nucleus and cytoplasm, including pre-mRNA-, mRNA-, snRNA-, and pre-rRNA-binding proteins, contain a putative RNAbinding domain of approximately 90 amino acids whose amino acid sequence is conserved from yeast to man. The most highly conserved motif within this RNAbinding domain is an octapeptide, termed the ribonucleoprotein consensus sequence {RNP-CSI, which is an identifying feature of this group of proteins. Frequently, these proteins contain several similar, but nonidentical, RNP-CS-type RNA-binding domains. All of these proteins also contain at least one auxiliary domain that is unique to each type of protein and most likely functions in protein-protein interactions. Many, if not all, of the RNP-CS-type proteins display binding preferences for specific RNA sequences, and several have been shown to interact with pre-mRNA sequences important for premRNA processing. Recent work has shown that the proteins encoded by several developmental loci in Drosophila and maize contain RNP-CS and, therefore, are most likely RNA-binding proteins. Here we provide an overview of the structural characteristics of these proteins and speculate on how the modular structure of RNP-CS-type RNA-binding proteins may facilitate their participation in pathways that regulate development at the post-transcriptional level.

The phenotypes in myotonic dystrophy types 1 and 2 (DM1 and DM2) are similar, suggesting a shared pathophysiologic mechanism. DM1 is caused by expansion of a CTG repeat in the DMPK gene. Pathogenic effects of this mutation are likely to be mediated, at least in part, by the expanded CUG repeat in mutant mRNA. The mutant transcripts are retained in the nucleus in multiple discrete foci. We investigated the possibility that DM2 is also caused by expansion of a CTG repeat or related sequence. Analysis of DNA by repeat expansion detection methods, and RNA by ribonuclease protection, did not show an expanded CTG or CUG repeat in DM2. However, hybridization of muscle sections with fluorescence-labeled CAGrepeat oligonucleotides showed nuclear foci in DM2 similar to those seen in DM1. Nuclear foci were present in all patients with symptomatic DM1 (n = 9) or DM2 (n = 9) but not in any disease controls or healthy subjects (n = 23). The foci were not seen with CUG-or GUC-repeat probes. Foci in DM2 were distinguished from DM1 by lower stability of the probe-target duplex, suggesting that a sequence related to the DM1 CUG expansion accumulates in the DM2 nucleus. Muscleblind proteins, which interact with expanded CUG repeats in vitro, localized to the nuclear foci in both DM1 and DM2. These results support the idea that nuclear accumulation of mutant RNA is pathogenic in DM1, suggest that a similar disease process occurs in DM2, and point to a role for muscleblind in the pathogenesis of both disorders.

Several proteins of heterogeneous nuclear ribonucleoprotein (hnRNP) particles display very high binding affinities for different ribonucleotide homopolymers. The specificity of some of these proteins at high salt concentrations and in the presence of heparin allows for their rapid one-step purification from HeLa nucleoplasm. We show that the hnRNP C proteins are poly(U)-binding proteins and compare their specificity to that of the previously described cytoplasmic poly(A)-binding protein. These findings provide a useful tool for the classification and purification of hnRNP proteins from various tissues and organisms and indicate that different hnRNP proteins have different RNA-binding specificities.