Myotonic Dystrophy 1 (DM1) Cell Models for Research

Myotonic Dystrophy type 1 (DM1) is the most common adult-onset muscular dystrophy, with an estimated global prevalence of 1 in 8,000 individuals (WHO, 2023). The disease is caused by an expansion of a CTG repeat in the DMPK gene, leading to multisystemic symptoms including myotonia, muscle weakness, cardiac conduction defects, cataracts, and cognitive impairment. The severity and age of onset correlate with repeat length, ranging from mild late-onset forms to severe congenital DM1. There is no cure, and current management is symptomatic, highlighting the urgent need for research models to understand pathogenesis and develop therapies.

DM1 is an ideal model for studying RNA toxicity and repeat expansion disorders. The molecular mechanism involves the sequestration of RNA-binding proteins (e.g., MBNL1) by expanded CUG repeats, leading to alternative splicing dysregulation. This provides a clear path from genetic mutation to cellular phenotype. Public datasets, such as those from the Myotonic Dystrophy Clinical Research Consortium, offer extensive clinical and molecular data. Open questions include the role of repeat instability, tissue-specific vulnerability, and the development of biomarkers for disease progression.

The primary pathogenic mechanism in DM1 is RNA toxicity. The expanded CUG repeat in the DMPK mRNA forms hairpin structures that sequester MBNL proteins and upregulate CELF proteins, leading to aberrant alternative splicing of numerous genes. Key pathways affected include:

• • Splicing dysregulation: Mis-splicing of genes such as CLCN1 (chloride channel) causes myotonia; INSR (insulin receptor) contributes to insulin resistance; and TNNT2 (cardiac troponin T) affects cardiac function.
• • RNA metabolism: Sequestration of MBNL1/2 disrupts their normal role in splicing, polyadenylation, and mRNA stability.
• • Protein translation: Expanded CUG repeats can also trigger repeat-associated non-ATG (RAN) translation, producing toxic homopolymeric proteins.
• • Autophagy and proteostasis: Impaired autophagy and accumulation of toxic proteins contribute to cellular stress.

Data from TCGA and COSMIC are not directly applicable as DM1 is not a cancer; however, ClinVar lists DMPK expansions as pathogenic.

The RNA toxicity in DM1 affects multiple signaling networks:

• • Insulin/IGF-1 signaling: Mis-splicing of INSR leads to reduced insulin receptor activity, contributing to insulin resistance and metabolic dysfunction.
• • Calcium signaling: Mis-splicing of calcium channels (e.g., CACNA1S) affects excitation-contraction coupling in muscle.
• • MAPK/ERK pathway: Altered splicing of MAPK components may contribute to muscle atrophy.
• • mTOR pathway: Impaired autophagy and mTOR signaling are observed, affecting protein homeostasis.
• • Wnt signaling: Dysregulation of Wnt pathway components has been implicated in muscle regeneration defects.

Organoids, particularly muscle organoids derived from DM1 iPSCs, offer a three-dimensional model that recapitulates muscle development and can be used for drug testing.

• • DMSXL mouse: Transgenic mouse carrying >1000 CTG repeats, recapitulating severe DM1 features.
• • HSALR mouse: Expresses expanded CUG repeats in skeletal muscle, showing myotonia and splicing defects.
• • Mbnl1 knockout mouse: Models splicing dysregulation but lacks repeat expansion.
• • Patient-derived xenograft (PDX): Not commonly used for DM1 as it is not a cancer; however, muscle xenografts have been explored.
• • Induced models: CRISPR-engineered mice with expanded repeats in the Dmpk gene are being developed.

CRISPR-based gene editing enables the creation of isogenic cell lines with precise modifications in the DMPK gene or other relevant genes. For example:

• • DMPK knockout cell lines: Generated by introducing frameshift mutations to ablate DMPK protein expression, useful for studying loss-of-function effects.
• • DMPK repeat expansion knock-in lines: Using CRISPR to insert expanded CTG repeats into the endogenous DMPK locus, creating models that mimic the disease mutation.
• • MBNL1 knockout lines: To study the downstream effects of MBNL1 loss.
• • Reporter lines: Engineered to express fluorescent proteins under the control of splicing-sensitive reporters, enabling high-throughput screening.

These sequence-verified, commercially available models accelerate research by providing consistent and reproducible systems for drug discovery and functional genomics.

Gene-edited cell lines are used to validate the role of specific genes in DM1 pathogenesis. For example:

• • DMPK knockout lines help determine the contribution of DMPK protein loss versus RNA toxicity.
• • MBNL1 knockout lines allow dissection of MBNL1-dependent splicing changes.
• • CRISPR screens using libraries targeting RNA-binding proteins can identify modifiers of toxicity.

Isogenic pairs (e.g., DMPK expansion vs. corrected) are used to screen for compounds that reduce CUG RNA toxicity or correct splicing defects. These models allow high-throughput screening of small molecules, antisense oligonucleotides, and gene therapies. Resistance mechanisms can be studied by exposing cells to drugs and selecting for resistant clones.

CRISPR synthetic lethality screens can identify genes that, when knocked out, are lethal only in DM1 cells, revealing potential therapeutic targets. Additionally, gene-edited cells can be used to discover splicing biomarkers that correlate with disease severity.