Targeting Glucocorticoid Atrophy Signaling to Treat Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by loss-of-function mutations in the dystrophin gene and results in skeletal muscle deterioration and bone loss. The current standard of care for DMD is chronic administration of glucocorticoids, which initially slow the disease. However, because of the direct negative effects of glucocorticoids on muscle and bone cells, chronic treatment of DMD patients with glucocorticoids leads to even lower bone mineral density than untreated patients. Thus, there is a need to identify new therapeutic targets to improve clinical outcomes in DMD. The exact cellular and molecular mechanisms underlying pathologic bone loss in DMD and the impact of glucocorticoids on bone cell populations in DMD remain unclear. We have shown that glucocorticoids increase the expression of Atrogin1 and MuRF1, two atrophy signaling genes (atrogenes) that mark proteins for proteasomal degradation, in muscle and bone cells. Genetic inhibition of MuRF1 blunted glucocorticoid-induced muscle loss and suppression of mineral production by osteoblasts in vitro. Further, the proteasomal inhibitor Carfilzomib improved muscle function and mitigated bone loss induced by glucocorticoids in mice. These results suggest that Atrogin1, MuRF1, and proteasomal degradation contribute to the harmful effects of glucocorticoids in bone and muscle. However, whether inhibition of atrogenes or proteolysis improves muscle and bone in DMD by interfering with the effects of glucocorticoids is unknown. We have also shown that glucocorticoids increase the production of Sclerostin, a suppressor of bone formation, and that genetic inhibition of Sclerostin protects bone from glucocorticoids in mice. DMD mice have increased Sclerostin levels. Yet, whether pharmacologic inhibition of Sclerostin can improve bone health in the context of DMD and glucocorticoid therapy has not been determined. Based on this, we hypothesize that interference with atrophy signals induced by glucocorticoids will preserve bone and muscle in DMD. To test this, we propose in Aim 1 to identify molecular changes in osteoblast-lineage cells caused by DMD and glucocorticoids. To do this, we will perform single-cell RNA sequencing of osteoblastic cells using the 10X Chromium platform. Aim 2 will determine whether inhibition of Atrogin1 and MuRF1 blunts musculoskeletal atrophy caused by glucocorticoids in DMD mice by using CRISPR interference to simultaneously suppress Atrogin1 and MurF1 in osteoblasts or muscle. Aim 3 will determine if co-administration of glucocorticoids and agents interfering with proteolysis and Sclerostin are effective treatments for DMD that preserve muscle and bone mass. Successful completion of these studies will advance knowledge of the effects of glucocorticoids on DMD and guide the development of new therapeutic approaches to improve outcomes in patients with muscular dystrophy.