Problems noted: Functional limitation, sarcopenia, sarcopenic obesity, and cachexia in the metastatic setting are common across many cancers. Functional limitation is mechanistically concomitant to the paracrine effects of cancer and is likely due to skeletal muscle dysfunction including aberrant stem-progenitor-differentiated cell myogenesis hierarchy. Although cachexia is rare in breast cancer, cancer- and/or treatment-induced skeletal muscle dysfunction and sarcopenia are common in breast cancer patients. However, it is unknown whether tumor subtypes with distinct genomic aberrations, and consequently different paracrine signaling features, differentially affect the myogenesis hierarchy. It is also unknown whether cancer-induced skeletal muscle defects are gender-specific, and, if so, how gender-enriched hormones influence myogenesis. Relevant findings from the current funding: Mammary tumors in MMTV-PyMT mice, a model for luminal B breast cancer subtype, had distinct effect on skeletal muscle compared to mammary tumors in MMTV-Neu mice. Compared to control animals, both models demonstrated reduced expression of skeletal muscle stem cell (MuSC)-associated transcription factor Hoxa9, reduced levels of myogenic microRNA miR-486 in circulation and in skeletal muscle, increased extracellular matrix deposition, and lower grip strength and rotarod performance. However, only the MMTV-PyMT model demonstrated reduced expression of Pax7, another MuSC transcription factor, and mitochondrial dysfunction. By contrast, only the MMTV-Neu's skeletal muscle phenotype resembled that of Duchenne muscular dystrophy (DMD) models. Furthermore, as with DMD models, skeletal muscle defects in MMTV-Neu could be rectified through muscle-specific overexpression of miR-486. These differences in skeletal muscle phenotype correlated with differences in circulating cytokine profiles between the two models. To further develop circulating miR-486 as a biomarker of cancer-associated skeletal muscle defects, we analyzed plasma samples of bladder, lung and pancreatic cancer patients. Intriguingly, striking reduction of circulating miR-486 in men but not women was observed in these cancers. In vitro studies showed that estradiol (E2) or toremifene, a clinically used selective estrogen receptor modulator (SERM), increased miR- 486 in myogenic cell lines and both E2 and toremifene reduced the levels of smad2, a miR-486 target. Smad2 is an integral part of myostatin/activin A/B-induced signaling that mediates muscle loss in cancer. Thus, E2 or SERMs can potentially be used to reduce skeletal muscle defect and improve quality of life for men with various cancers. Additionally, discontinuation of anti-estrogen aromatase inhibitor therapy by breast cancer patients secondary to treatment-induced muscle weakness could be due to impaired E2-mR-486 signaling. Hypothesis: Breast cancer patients experience DMD-like skeletal muscle phenotype depending on genomic aberrations in cancer, and gend...