Patient-derived 3D engineered human muscle model recapitulates CLCN1 mis-splicing and myotonia in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) lacks human in vitro models that directly link RNA toxicity to mature skeletal muscle function, particularly myotonia. Here, we engineer contractile 3D human skeletal muscle tissues from immortalized myoblasts derived from three DM1 patients representing juvenile, adult, and late-onset subtypes. These tissues reproduce key molecular features of DM1, including nuclear RNA foci, MBNL1 sequestration, and widespread mis-splicing. Functionally, DM1 tissues exhibit impaired calcium handling, subtype-dependent weakness, rapid fatigue, and a fiber-type distribution characterized by increased slow type I fibers and pathological MyHC-I/IIx hybrids. Notably, the 3D environment enables expression and complete pathogenic mis-splicing of CLCN1--undetectable in matched 2D cultures--accompanied by myotonia-like delayed relaxation. Using this model, we assessed therapeutic responses of candidate small-molecule modulators. Phenylbutazone reduced RNA foci and MBNL1 sequestration but failed to rescue spliceopathy or function. In contrast, calcitriol induced coordinated transcriptomic remodeling and robustly rescued myotonia-like relaxation despite persistent CLCN1 mis-splicing. These findings establish a functionally mature human DM1 muscle model and highlight compensatory network activation as a strategy to improve muscle function in DM1.