No abstract

Haploinsufficiency of chromosome 17p and c-Myc amplification distinguish group 3 medulloblastomas which are associated with early metastasis, rapid recurrence, and swift mortality. Tumor suppressor genes on this locus have not been adequately characterized. We elucidated the role of miR-212-3p in the pathophysiology of group 3 tumors. First, we learned that miR-212-3p undergoes epigenetic silencing by histone modifications in group 3 tumors. Restoring its expression reduced cancer cell proliferation, migration, colony formation, and wound healing in vitro and attenuated tumor burden and improved survival in vivo. MiR-212-3p also triggered c-Myc destabilization and degradation, leading to elevated apoptosis. We then isolated an oncogenic target of miR-212-3p, i.e. NFIB, a nuclear transcription factor implicated in metastasis and recurrence in various cancers. Increased expression of NFIB was confirmed in group 3 tumors and associated with poor survival. NFIB silencing reduced cancer cell proliferation, migration, and invasion. Concurrently, reduced medullosphere formation and stem cell markers (Nanog, Oct4, Sox2, CD133) were noted. These results substantiate the tumor-suppressive role of miR-212-3p in group 3 MB and identify a novel oncogenic target implicated in metastasis and tumor recurrence.

Medulloblastoma (MB), the most common malignant pediatric brain tumor and a leading cause of childhood mortality, is stratified into four primary subgroups, WNT (wingless), SHH (sonic hedgehog), group 3, and group 4. Patients with group 3 tumors have the poorest prognosis. Loss of 17p13.3, which houses the tumor suppressor gene miR-1253, is a frequent high-risk feature of group 3 tumors. In this study, we show that miR-1253 levels can disrupt iron homeostasis, induce oxidative stress and lipid peroxidation, triggering an iron-mediated form of cell death called ferroptosis. In silico and in vitro analyses of group 3 tumors revealed deregulation of ABCB7, a mitochondrial iron transporter and target of miR-1253, and GPX4, a critical regulator of ferroptosis. Restoration of miR-1253 levels in group 3 cell lines resulted in downregulation of ABCB7 and GPX4, consequently increasing cytosolic and mitochondrial labile iron pools, reducing glutathione levels, in turn, resulting in mitochondrial oxidative stress and lipid peroxidation. Together, these events accelerated cancer cell death. Treating miR-1253-expressing cancer cells with cisplatin potentiated cell death by further elevating oxidative stress, depleting glutathione levels, and augmenting lipid peroxidation, with added inhibitory effects on cell viability and colony formation. Treatment with a ferroptosis inhibitor (ferrostatin-1) lead to recovery from the cytotoxic effects of this combination therapy. Together, these findings reveal a novel role for miR-1253 in enhancing ferroptosis to attenuate group 3 tumor cell growth. Our studies provide a proof-of-concept for using miR-based therapeutics to augment current chemotherapeutics in high-risk tumors. Leveraging the tumor-suppressive properties of miRNAs as adjuncts to chemotherapy may provide a promising alternative to current therapeutic strategies.

Medulloblastomas (MB), the most common malignant pediatric brain tumor and a leading cause of childhood mortality, are stratified into four primary subgroups. Deletions within chromosomal locus 17p13.3, which houses multiple tumor suppressor genes including miR-1253, characterize high-risk group 3 tumors. These aggressive tumors also enrich iron transport genes to satisfy their high proliferative need. MiR-1253 targets iron transport by inhibiting the mitochondrial Fe-S transporter, ABCB7. This study elucidated the impact of repressing ABCB7 on cisplatin cytotoxicity in group 3 MB and whether these effects were mediated by ferroptosis. In silico and in vitro analyses revealed specific enrichment of ABCB7 and GPX4, a critical regulator of ferroptosis, in group 3 MB cell lines and tumors. MiR-1253 overexpression (miR-1253OE) resulted in downregulation of both ABCB7 and GPX4, concurrently increasing mitochondrial iron overload, mitochondrial oxidative stress, and lipid peroxidation, leading to cell death and abrogation of medullosphere formation; ABCB7 knockdown (ABCB7KD) recapitulated these effects and abrogated GPX4 expression. Fractionation studies confirmed the inhibitory impact of miR-1253OE and ABCB7KD on GPX4 expression in the cytosol and mitochondria. Seahorse studies revealed that the bulk of ATP generation was occurring in the cytoplasm through glycolysis and not via oxidative phosphorylation, suggesting mitochondrial dysfunction was triggered when ABCB7 was repressed. Cisplatin, a chemotherapeutic agent used in group 3 MB treatment, induces cell death by DNA crosslinking; it also inhibits GPX4 expression. In miR-1253OE and ABCB7KD group 3 cancer cells, cisplatin IC50 was reduced 2-fold. Resultantly, cisplatin treatment augmented oxidative stress and lipid peroxidation, depleted glutathione stores, and culminated in a higher index of ferroptosis. In a mouse model of group 3 tumors, ABCB7KD potentiated cisplatin effects and dramatically prolonged survival. The current study illustrates how targeting iron transport can augment ferroptosis to potentiate cisplatin cytotoxicity in group 3 MB tumors.

Background: Medulloblastoma (MB), the most frequent malignant pediatric brain tumor, is subdivided into four primary subgroups, wingless-type (WNT), sonic hedgehog (SHH), group 3, and group 4. Haploinsufficiency of chromosome 17p13.3 and c-Myc amplification distinguish high-risk group 3 tumors associated with rapid metastasis, recurrence and early mortality. We sought to identify the role of miR-212-3p, which resides on chromosome 17p13.3, in the pathophysiology of group 3 MB. Methods: We first determined miR-212-3p expression in group 3 MB using several publicly-available datasets with confirmatory studies in vitro. We then identified epigenetic regulation by studying methylation and HDAC modifications along the promoter region. We used two systems for expression restoration, i.e. transient transfection or stable induction, to delineate miR-212-3p tumor suppressive and biochemical properties via assays assessing cancer proliferation, migration, invasion, colony formation, along with cell cycle and apoptosis analyses. We then compared MB and miR target databases to isolate a putative target whose biochemical and oncogenic properties were similarly elucidated using either transient silencing of target expression or stable induction of miR-212-3p. Results: RNA expression analyses revealed dramatically reduced miR-212-3p levels in group 3 tumors and cell lines mainly through epigenetic silencing via histone modifications. Restoring miR-212-3p expression reduced in vitro cancer cell proliferation, migration, colony formation, and wound healing. Elevated miR-212-3p levels shifted c-Myc phosphorylation (from serine-62 to threonine-58), triggering destabilization and degradation; concurrently, its pro-apoptotic binding partners, i.e., Bin-1 and P19ARF, were upregulated with subsequent elevated apoptotic signals. Using a combination of transcriptomic data and dual luciferase assay, we isolated an oncogenic target of miR-212-3p, i.e. NFIB, a nuclear transcription factor implicated in metastasis and recurrence in various cancers. Increased expression of NFIB was confirmed in group 3 tumors, with poor survival shown in high-expressing patients. Transient NFIB silencing in vitro reduced cancer cell proliferation, colony formation, migration, and invasion. Concurrently, in group 3 MB cells, reduced medullosphere formation along with decreased expression of stem cell markers (Nanog, Oct4, Sox2, CD133) were noted. Conclusion: These results substantiate the tumor-suppressive role of miR-212-3p in group 3 MB and provide a potential therapeutic oncogenic target implicated in metastasis and tumor recurrence.

Amongst the 4 subgroups of medulloblastoma (MB), tumors falling into group 3 are the most aggressive and associated with increased incidence of aberrations on chromosome 17p, c-Myc amplification, metastases at diagnosis, and rapid tumor relapse. Thus, patients with group 3 tumors suffer the worst prognosis with a 5-year survival rate of <50%. We have prior identified a novel tumor-suppressive microRNA, miR-212, silenced on chromosome 17p and its deregulated oncoprotein target, Nuclear Factor I/B (NFIB). Here, we sought to identify the role of NFIB in group 3 MB pathophysiology. NFIB is a transcription factor that regulates chromosomal gene accessibility and expression of pro-metastatic genes in various cancers. Transcriptomic interrogation of group 3 tumors revealed deregulated expression of NFIB. Kaplan-Meier survival analysis confirmed poorer survival in NFIB high-expressing patients. Using inducible silencing of NFIB in a classic group 3 MB cell line, HDMB03, we observed downregulation of key driver genes (49 genes, Log2 fold change < -0.5, p < 0.001) associated with group 3 MB pathogenesis by RNA sequencing. NFIB expression knockdown (NFIBKD) further reduced tumor cell growth and aggressiveness, as evidenced by reduced proliferation, colony formation, migration, and invasion. NFIBKD also affected group 3 MB stemness, with attenuation of medullospheres and a reduction in stem cell markers (Nanog, Oct4, Sox2, CD133). Moreover, NFIBKD destabilized c-Myc phosphorylation at serine-62, resulting in reduced total c-Myc levels and subsequent cellular apoptosis. Concurrently, NFIBKD decreased the expression of upstream activators of c-Myc such as p-Akt and p-Erk. Taken together, these results validate the oncogenic role of NFIB in group 3 medulloblastomas and provide a potential new therapeutic target.

Across the molecular spectrum of medulloblastoma (MB), group 3 (G3) tumors are the most aggressive with <50% five-year survival, the lowest of all MB subgroups. G3 MB tumors are characterized by frequent metastases at diagnosis, unique methylation profiles, MYC amplification, and i17q, but these unique molecular features have yet to be exploited for therapeutic purposes despite their contribution to the disease process. As such, we sought to address this gap in survivorship by identifying FDA-approved compounds with the potential to inhibit cellular processes critical to G3 MB tumor proliferation and metastasis, aiming to exploit the unique molecular pathogenesis of G3 tumors. Guided by analysis of RNA-sequencing data from locally obtained, patient-derived MB samples against the LINCS chemical perturbagens database, we identified nortriptyline (NT), a tricyclic antidepressant, as a candidate MB therapeutic due to: 1) its ability to revert the transcriptomic signature of G3 MB to a normal cerebellum-like state and 2) its ability to cross the blood-brain barrier. We first identified the IC50 of NT in D425 and HDMB03 cells as 28μM and 20μM, respectively. Then, we observed that NT increased apoptosis of HDMB03 cells 3-fold by flow cytometry and confirmed our observations with Western blotting of apoptotic markers. Additionally, NT treatment resulted in abrogation of colony formation, impairment of wound healing, and inhibition of cell migration and invasion in vitro in HDMB03 cells. In all, transcriptome-driven drug repurposing holds great promise, as identifying novel uses for compounds with a known safety profile can deliver effective treatments into the hands of both patients and physicians in an expedited manner when compared to traditional means.

Medulloblastoma (MB), the most frequent malignant pediatric brain tumor is subdivided into four primary subgroups, i.e. wingless-type (WNT), sonic hedgehog (SHH), group 3, and group 4. Haploinsufficiency of chromosome 17p13.3 and c-myc amplification distinguish high-risk group 3 tumors, which are associated with rapid metastasis, recurrence and early mortality. We sought to identify the role of miR-212, which resides on chromosome 17p13.3, in the pathophysiology of group 3 MB. RNA expression analyses revealed dramatically reduced levels of miR-212 in group 3 tumors and cell lines mainly through epigenetic silencing via histone modifications (deacetylation). Restoring in vitro miR-212 expression reduced tumor cell proliferation, colony formation, wound healing, migration and invasion with decreased p-AKT and p-ERK levels in group 3 MB cell lines. Interestingly, a shift in differential c-myc phosphorylation (from serine-62 to threonine-58) was also discovered with miR-212 expression, resulting in reduced total c-myc levels, concurrent with elevated cellular apoptosis. In turn, pro-apoptotic binding partners of c-myc, i.e. Bin-1 and P19ARF, were upregulated in these cells. These findings were recapitulated in stable inducible miR-212 expressing tumor cells. Using a combination of transcriptomic data and a dual luciferase assay, we isolated an important oncogenic target of miR-212, i.e, NFIB, a nuclear transcription factor implicated in metastasis and recurrence. Increased expression of NFIB was confirmed in group 3 tumors, with poor survival shown in high NFIB-expressing patients. As prior, transient NFIB silencing in vitro reduced not only tumor cell proliferation, colony formation, wound healing, migration and invasion, but also medullosphere formation along with decreased expression of stem cell markers (Nanog, Oct4, Sox2, CD133), confirming its role in tumor recurrence possibly via augmenting tumor stemness. Taken together, these results substantiate the tumor suppressive role of miR-212 in group 3 MB and provide a potential new oncogenic target implicated in tumor recurrence, NFIB.