Aggravating the phenotype of dystrophic mice for improving preclinical research and clinical translation for Duchenne muscular dystrophy

Dr. Olivier Dorchies, University of Geneva

Abstract (lay summary see below)

Although the mdx/mdx5Cv mice and dko mice have been widely used for decades, they do not properly replicate the disease severity and progression experienced by Duchenne muscular dystrophy (DMD) patients. The D2-mdx mouse (DBA/2J background) that recently focused interest also has pros and cons. The need for mouse models that would be better phenocopies of DMD is largely acknowledged in the Duchenne community. However, the development, characterization and validation of better DMD models is a long term, costly and risky process.
We have generated several new mouse lines designed to match more closely the pathogenic features, disease progression, and overall severity of DMD than do existing lines of dystrophic mice. After a preliminary phenotyping, we selected for further development: (i) 5Cv/Uflox-MC, an easy-to-maintain line bearing floxed utrophin (Utrn) alleles that allow downregulating utrophin, in skeletal muscle only, upon administration of doxycycline; (ii) 5Cv/L, a line in which the Ltbp4DEL allele was transferred free of the co-segregating factor Abcc6MUT, from DBA/2J mice to mdx5Cv mice, for enhancing fibrosis, as in a subset of severely affected DMD patients.
As a first aim, we will extend the characterization of 5Cv/Uflox-MC for correlating utrophin loss with disease severity, and that of 5Cv/L mice with focus on fibrosis and atrophy. This part of the work will allow examining further shortcomings observed during preliminary phenotyping. (i) 5Cv/Uflox-MC mice seem not to respond to doxycycline in all developmental stages; (ii) the ability of Ltbp4DEL and Abcc6MUT at worsening the condition would be much less in the C57BL/6J background (5Cv/L derived from mdx5Cv) than in the original DBA/2J background.
As a second aim, we will (i) perform a functional analysis of the murine Utrn promoter towards new dystrophic lines in which regulatory elements that control muscle-specific utrophin expression would be genetically altered; (ii) correct Ltbp4DEL and/or Abcc6MUT to their normal versions in dystrophic mice in the DBA/2J background. This will allow dissecting the contribution of Ltbp4DEL and/or Abcc6MUT vs other genetic factors to murine disease severity.
Some of these new lines will likely be superior to existing ones for modelling DMD. Ultimately, such lines would produce more reliable pre-clinical data, facilitating selection of therapeutic candidates, reducing failure at the clinical stage and accelerating translation to patients.

Lay summary

La dystrophie musculaire de Duchenne (DMD) touche les garçons vers 3 ans. Ils décèdent généralement avant 30 ans de paralysie musculaire progressive. Les souris dystrophiques utilisées comme modèles de DMD ne reproduisent pas assez fidèlement les caractéristiques de cette myopathie : certaines découvertes ne sont pas pertinentes pour comprendre la maladie et de nombreux traitements prometteurs en laboratoire échouent lors des essais cliniques. Nous avons créé plusieurs nouvelles lignées de souris dystrophiques. Dans la première, nous diminuons l’expression de l’utrophine, une protéine que les souris myopathes produisent en abondance pour compenser la maladie. Dans une autre, nous avons introduit une version mutée du gène Ltbp4 pour aggraver, comme chez les patients, la fonte musculaire et la fibrose. Ces nouvelles souris, et d’autres dans notre projet, pourraient améliorer la recherche préclinique pour sélectionner les meilleurs candidats médicaments vers les essais cliniques.