Myotonic dystrophy type I (DM1) is a disabling neuromuscular disease with no causal treatment available. This disease is caused by expanded CTG trinucleotide repeats in the 3 UTR of the dystrophia myotonica protein kinase gene. On the RNA level, expanded (CUG) n repeats form hairpin structures that sequester splicing factors such as muscleblind-like 1 (MBNL1). Lack of available MBNL1 leads to misregulated alternative splicing of many target pre-mRNAs, leading to the multisystemic symptoms in DM1. Many studies aiming to identify small molecules that target the (CUG) n -MBNL1 complex focused on synthetic molecules. In an effort to identify new small molecules that liberate sequestered MBNL1 from (CUG) n RNA, we focused specifically on small molecules of natural origin. Natural products remain an important source for drugs and play a significant role in providing novel leads and pharmacophores for medicinal chemistry. In a new DM1 mechanism-based biochemical assay, we screened a collection of isolated natural compounds and a library of over 2100 extracts from plants and fungal strains. HPLC-based activity profiling in combination with spectroscopic methods were used to identify the active principles in the extracts. The bioactivity of the identified compounds was investigated in a human cell model and in a mouse model of DM1. We identified several alkaloids, including the -carboline harmine and the isoquinoline berberine, that ameliorated certain aspects of the DM1 pathology in these models. Alkaloids as a compound class may have potential for drug discovery in other RNA-mediated diseases.
Myotonic dystrophy type I (DM1)4 is the most common muscular dystrophy in the adult population, with a relatively high prevalence of about 1:8000 (1). This autosomal dominantly inherited disease affects multiple organs, most prominently the skeletal muscle, with wasting, weakness, and an inability to relax (myotonia) (1). Currently, there is no effective treatment for this disabling disease. The pathomechanism of DM1 is linked to a CTG n expansion in the 3Ј UTR of the dystrophia myotonica protein kinase (DMPK) gene (2, 3), leading to a toxic gain-of-function RNA (4, 5). The mutant DMPK transcript is entrapped within nuclei of affected cells, where it forms aggregates (foci) with splicing factors such as muscleblind-like 1 (MBNL1) (6, 7). Bound to mutant DMPK (CUG) n RNA, MBNL1 is no longer available for correct splicing of its target pre-mRNAs (8, 9). Thus, the splicing of a multitude of pre-mRNAs is misregulated, including the skeletal muscle chloride channel (CLCN1), the insulin receptor (INSR), sarcoplasmic/endoplasmic reticulum Ca 2ϩ ATPase 1 (SERCA1), and cardiac troponin T type 2 (TNNT2) pre-mRNA (10 -16). Interestingly, the missplicing of some pre-mRNAs can be linked directly to a certain disease symptom, e.g. myotonia in the case of the CLCN1 pre-mRNA. MBNL1 sequestration by (CUG) n RNA causes inclusion of alternative exon 7a, leading to a shift in the open reading frame and to premature termination of translation (12, 1...