In today’s #apaperaday, Prof. Aartsma-Rus reads and comments on the paper titled: Human dystrophin expressing chimeric (DEC) cell therapy ameliorates cardiac, respiratory, and skeletal muscle’s function in Duchenne muscular dystrophy.
Today’s paper a day is by Siemionow et al in Stem Cells Translational Medicine on stem cell transplantation of chimeric human myoblasts IN a MOUSE model. Doi 10.1002/sctm.21-0054.
Duchenne is caused by lack of dystrophin. This affects amongst others skeletal muscles, diaphragm and heart. With optimal care, most patients pass away due to respiratory or heart failure in the 2nd-4th decade of life, without care patients used to die before age 20.
Strategies to restore dystrophin expression are under study. Here authors focus on stem cell treatment using myoblasts (skeletal muscle stem cells). Normally transplanting a stem cell from a healthy donor to a patient requires immune suppression (just like with organ transplants)
Here authors want to avoid an immune response by generating chimeric myoblasts, i.e. fusing myoblasts of patients with donors. That way the myoblast expresses dystrophin but has the patient characteristics and is not rejected (is the idea).
Delivery of myoblasts is challenging. Injections into the circulation do not lead to homing of myoblasts to muscles. Here authors transplant myoblasts into large bones. Afterwards myoblasts migrate to skeletal muscles – this was already confirmed for mouse chimeric myoblasts.
Here authors want to test whether human chimeric myoblasts can also restore dystrophin in mdx mice. Because the mice will see the human cells as ‘foreign’ they need mdx/scid mice (mice without a functioning immune system so there is no rejection possible).
Authors generated chimeric human myoblasts through fusion in a dish, expansion in a dish and then transplantation (0.5 or 1 million) into the femur of mdx/scid male mice of 6-8 weeks old. Analyses were done 30 and 90 days post injection.
Authors claim ~15% dystrophin restoration in skeletal muscle, diaphragm & heart. They mean % dystrophin positive fibers. We know from exon skipping that % of dystrophin amount is likely lower than % of positive fibers. Authors claim dose dependent increase that I do not see.
Authors claim functional effects (see image) were seen in the heart after 90 days (ejection fraction & shortening unchanged from baseline) But this lack of change is due to lower baseline for high dose, not due to improvement. Also wild types are missing so uncertain if things are improving.
Treatment also reduced inflammation and fibrosis and number of centrally located nuclei in diaphragm and gastrocnemius. Respiratory function differed in treated mice compared to vehicle treated groups but again lack of wild types makes drawing conclusions difficult.
For gastrocnemius strength increased and muscles were more protected against fatigue. However, as wild types are lacking it is not possible to see the effect size. For inflammatory levels and fibrosis wild types were included and their reductions were minor.
Authors discuss that using scid/mdx mice meant they cannot assess whether there is an immune response to the chimeric human myoblasts. Transplanted mouse chimeric myoblasts into mdx (no scid) were not rejected. However, these were transplanted into genetically identical animals.
As yet I am not convinced rejection will not occur for chimeric cells. They will also have characteristics of foreign (donor) cells. Another thing to consider is that mouse muscle will be of relatively good quality compared to human situation, so effect size likely overestimated
Finally, we know that transplanted cells also produce growth factors that are protective to heart and muscle (cardiosphere approach). In this study it is not known how much of the effect is due to these factors and how much is due to the (limited) dystrophin restoring facts.
The effect of growth factors is expected to last for ~90 days. Ideally longer term studies are done in mouse models to assess if functional effects last longer. Also important for assessing heart function effects as heart pathology is not prominent yet in 5 month old mice.
In summary, I like that the authors look at diaphragm, heart and skeletal muscle and that they look at histology and function. However, I do not like the lack of wild type references for functional studies and the study design could be more optimal (e.g. older mice for heart function)
Note that this approach is currently tested in a clinical trial. Early results appear promising (as presented in the press release) – however, they are very premature. There is no placebo group and results are very short term for now.
Pictures by Annemieke, used with permission.