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#apaperaday: Micro-dystrophin gene therapy demonstrates long-term cardiac efficacy in a severe Duchenne muscular dystrophy model

The pick is from Molecular Therapy Methods & clinical development by Piepho et al on assessment of cardiac pathology after micro-dystrophin delivery. DOI: 10.1016/j.omtm.2023.02.001.

Authors that micro-dystrophin delivery with adeno associated viral vectors (AAV) to skeletal muscle has been confirmed in Duchenne patients. However, effects in heart are speculated: no heart biopsies are obtained (obviously) in humans, while we know it is expressed in animals.

The functional effects in hearts of treated patients will not become apparent for years as most treated patients are young and have only limited pathology and it is difficult to assess whether this deviates from natural history until patients are older.

Here authors wanted to study effects in heart in more detail in a mouse model, the fiona mdx mouse. This is a mouse that expresses utrophin highly in skeletal muscle, but not in heart. Therefore, muscle pathology is limited, while heart pathology is apparent from 6 months on.

Authors used AAV 6 and a micro-dystrophin with the nNOS binding site (similar/identical to the one used by Solid). I do not understand why authors use AAV6 as in Duchenne trials AAV9 is used for this micro-dystrophin and the Pfizer one (AAV7.4 by Sarepta and AAV8 by Genethon)

Mice were injected at 4 weeks of age with saline or with 2.10(12) viral genomes (= 1.10(14) vg/kg). Heart function was assessed with ECG in a blinded fashion (hooray) at 6, 9, 12, 15 and 18 months. No wild types were included in the study.

Ejection fraction (EF) was >55% (normal) for treated mice until 18 months. Untreated mice had a reduced EF until 12m, but then improved unexpectedly. Having normal reference mice in the study would have been useful to see whether they really improved or relatively to treated mice.

The untreated mice did show other pathology also at later timepoints confirming the pathology had not disappeared. Authors looked for an explanation for the improved heart function and checked whether utrophin expression in heart was increased at 18 months: it did not.

Authors confirmed micro-dystrophin expression in the treated animals, while it was absent in non-treated animals. Histology analysis showed more fibronectin in untreated animals, however other fibrosis markers were similar for treated and untreated or only mildly reduced in treated mice.

Authors discuss their study shows there is therapeutic effect to be expected for the heart as well – in mice. How this extrapolates to humans remains to be seen as a different AAV serotype was used. Authors conclude that more work is needed. Agreed – with wild types this time!

Authors also discuss the unexpected improvement of the untreated mice and suggest that this might have been due to the pandemic, when mice had less stress and were handled less than normal. I think this may actually be the case. Mdx mice are very sensitive to stress.

Frequent handling can exacerbate the heart pathology, so it makes sense that less handling can be protective. This showcases how the pandemic has had an influence on research, especially the longer term experiments initiated before the pandemic.