Towards a Better Preclinical Model for DM1

Optimizing preclinical and clinical tools and measures to assess the efficacy and safety of candidate therapeutics is an essential activity for any rare disease. Although the search for the holy grail of the ideal mouse efficacy model has been overdone for some diseases (there is no perfect mouse model—that’s why they are called “models”), it’s clear that the DM field can still benefit from new models and assessment technology.

In a new publication in Nature Communications (Hu et al., 2018), Dr. Thurman Wheeler (Massachusetts General Hospital) and colleagues report on development of a new mouse model allowing the non-invasive assessment of alternative splicing when testing candidate therapeutics for DM1. As a proof of concept, they showed the value of the model in assessing a new candidate antisense oligonucleotide to degrade toxic RNA.

This work was made possible by development of a GFP/DsRed bichromatic reporter construct previously used to monitor the reading frame of Tnn2—in this system, only one alternative reading frame leads to production of the flurochrome, DsRed (Orengo et al., 2006).

The research team targeted the bichromatic reporter using a human ATP2A1 minigene, chosen because of its skeletal muscle expression, missplicing in DM1 muscle, and responsiveness to expanded repeat knockdown using antisense oligonucleotides. The reporter construct was targeted to skeletal muscle using an HSA promoter. Transgenic mice generated with this construct were first evaluated in muscle regeneration models to ensure performance of the reporter construct and then crossed with the HSA^LR model of DM1.

A Non-Invasive Biomarker for Preclinical Studies in DM1

Splicing events could be monitored using in vivo fluorescence imaging of skeletal muscles in single transgenic (GFP/DsRed) and bi-transgenic (GFP/DsRed and HSA^LR) mice. The bi-transgenic mice showed a low DsRed/GFP ratio, indicative of the ATP2A1 splicing switch characteristic of DM1—with in vivo imaging results confirmed by RT-PCR. The research team subsequently evaluated the performance of the bichromatic reporter in bi-transgenic mice treated with antisense oligonucleotides designed to knockdown the expanded repeat transcript and restore MBNL levels and normalize splicing patterns. This approach increased the DsRed/GFP ratio, indicating restoration of the normal ATP2A1 splicing pattern—the splicing switch was seen within 3 days of treatment and persisted for several weeks thereafter. Moreover, the researchers were able to demonstrate the splicing correction with non-invasive, in vivo spectroscopy, indicating the potential of the mouse model to serve as a biomarker system for intermittent monitoring of splicing correction in evaluation of candidate therapeutics. The model also was used to show superiority of a novel ligand-conjugated antisense oligo (LICA) in comparison to gapmer oligo chemistry previously tested in preclinical models and clinical trials for DM1.

Synthesis

This new study demonstrates the development of a mouse model that allows non-invasive, in vivo assessment of the splicopathy that characterizes DM1 and of its correction using candidate therapies. Such a non-invasive, pharmacodynamic biomarker facilitates simultaneous determination of biodistribution, target engagement, time course of activity, dosing optimization, and level of effect assessment of candidate therapies, potentially speeding early go/no go decision making for therapy development programs targeted to primary disease mechanisms in DM1.

References:

Non-invasive monitoring of alternative splicing outcomes to identify candidate therapies for myotonic dystrophy type 1.
Hu N, Antoury L, Baran TM, Mitra S, Bennett CF, Rigo F, Foster TH, Wheeler TM.
Nat Commun. 2018 Dec 7;9(1):5227. doi: 10.1038/s41467-018-07517-y.

A bichromatic fluorescent reporter for cell-based screens of alternative splicing.
Orengo JP, Bundman D, Cooper TA.
Nucleic Acids Res. 2006;34(22):e148. Epub 2006 Nov 16.