The Alternative Splicing Factor, MBNL1, Inhibits Glioblastoma Tumor Initiation and Progression by Reducing Hypoxia-Induced Stemness.

Muscleblind-like proteins (MBNL) belong to a family of tissue-specific regulators of RNA metabolism that control premessenger RNA splicing. Inactivation of MBNL causes an adult-to-fetal alternative splicing transition, resulting in the development of myotonic dystrophy. We have previously shown that the aggressive brain cancer, glioblastoma (GBM), maintains stem-like features (glioma stem cell, GSC) through hypoxia-induced responses. Accordingly, we hypothesize here that hypoxia-induced responses in GBM might also include MBNL-based alternative splicing to promote tumor progression. When cultured in hypoxia condition, GSCs rapidly exported muscleblind-like-1 (MBNL1) out of the nucleus, resulting in significant inhibition of MBNL1 activity. Notably, hypoxia-regulated inhibition of MBNL1 also resulted in evidence of adult-to-fetal alternative splicing transitions. Forced expression of a constitutively active isoform of MBNL1 inhibited GSC self-renewal and tumor initiation in orthotopic transplantation models. Induced expression of MBNL1 in established orthotopic tumors dramatically inhibited tumor progression, resulting in significantly prolonged survival. This study reveals that MBNL1 plays an essential role in GBM stemness and tumor progression, where hypoxic responses within the tumor inhibit MBNL1 activity, promoting stem-like phenotypes and tumor growth. Reversing these effects on MBNL1 may therefore, yield potent tumor suppressor activities, uncovering new therapeutic opportunities to counter this disease. SIGNIFICANCE: This study describes an unexpected mechanism by which RNA-binding protein, MBNL1, activity is inhibited in hypoxia by a simple isoform switch to regulate glioma stem cell self-renewal, tumorigenicity, and progression.