Myotonic dystrophy type 1 (DM1) is caused by a CTG nucleotide repeat expansion within the 3'-untranslated region (3'-UTR) of the Dystrophia Myotonica protein kinase gene. In this study, we explored therapeutic genome editing using CRISPR/Cas9 via targeted-deletion of expanded CTG repeats and targeted insertion of polyadenylation signals in the 3'-UTR upstream of the CTG repeats to eliminate toxic RNA CUG repeats. We found paired SpCas9 or SaCas9 guide RNA induced deletion of expanded CTG repeats. However, this approach incurred frequent inversion in both the mutant and normal alleles. In contrast, the insertion of polyadenylation signals in the 3'-UTR upstream of the CTG repeats eliminated toxic RNA CUG repeats, which led to phenotype reversal in differentiated neural stem cells, forebrain neurons, cardiomyocytes and skeletal muscle myofibers. We concluded that targeted insertion of polyadenylation signals in the 3'-UTR is a viable approach to develop therapeutic genome editing for DM1.