Downregulation of CRX, a Group 3-specific oncogenic transcription factor, inhibits TGF-ß/activin signaling in medulloblastoma cells.

Medulloblastoma, the most common malignant brain tumor in children, consists of four molecular subgroups WNT, SHH, Group 3, and Group 4. Group 3 has the worst survival rate among the four subgroups and is characterized by the expression of retina-specific genes. CRX, the master regulator of the photoreceptor differentiation, is aberrantly expressed in Group 3 medulloblastomas. CRX expression increased the proliferation, anchorage-independent growth, invasion potential, and tumorigenicity of medulloblastoma cells indicating the oncogenic role of CRX in medulloblastoma pathogenesis. CRX knockdown resulted in the downregulation of expression of several retina-specific genes like IMPG2, PDC, RCVRN. and Group 3 specific genes like GABRA5, MYC, PROM1. Thus, CRX plays a major role not only in the expression of retina-specific genes but also in defining Group 3 identity. Increased expression of several pro-apoptotic genes upon CRX knockdown suggests that CRX could protect Group 3 medulloblastoma cells from cell death. Several negative regulators of the TGF-ß signaling pathway like SMAD7, PMEPA1, KLF2 were upregulated upon the CRX knockdown. Western blot analysis showed a decrease in the levels of (Phospho)-SMAD2, total levels of SMAD2, SMAD4, and an increase in the levels of SMAD7 indicating inhibition of the TGF-ß signaling pathway upon CRX knockdown. Copy number variations in several genes involved in the TGF-ß signaling pathway occur in a subset of Group 3 tumors. Autocrine TGF-ß/activin signaling has recently been reported to be active in a subset of Group 3 medulloblastomas. CRX knockdown resulting in the inhibition of the TGF-ß/activin signaling pathway demonstrates an interaction between the two Group 3 specific oncogenic pathways and suggests simultaneous targeting of both CRX and TGF-ß signaling as a possible therapeutic strategy.

Downregulation of ARID1B, a tumor suppressor in the WNT subgroup medulloblastoma, activates multiple oncogenic signaling pathways.

Medulloblastoma, a common pediatric malignant brain tumor, consists of four distinct molecular subgroups WNT, SHH, Group 3 and Group 4. Exome sequencing of 11 WNT subgroup medulloblastomas from an Indian cohort identified mutations in several chromatin modifier genes, including genes of the mammalian SWI/SNF complex. The genome of WNT subgroup tumors is known to be stable except for monosomy 6. Two tumors, having monosomy 6, carried a loss of function mutation in the ARID1B gene located on chromosome 6. ARID1B expression is also lower in the WNT subgroup tumors compared to other subgroups and normal cerebellar tissues that could result in haploinsufficiency. The short hairpin RNA-mediated knockdown of ARID1B expression resulted in a significant increase in the malignant potential of medulloblastoma cells. Transcriptome sequencing identified upregulation of several genes encoding cell adhesion proteins, matrix metalloproteases indicating the epithelial-mesenchymal transition. The ARID1B knockdown also upregulated ERK1/ERK2 and PI3K/AKT signaling with a decrease in the expression of several negative regulators of these pathways. The expression of negative regulators of the WNT signaling like TLE1, MDFI, GPX3, ALX4, DLC1, MEST decreased upon ARID1B knockdown resulting in the activation of the canonical WNT signaling pathway. Synthetic lethality has been reported between SWI/SNF complex mutations and EZH2 inhibition, suggesting EZH2 inhibition as a possible therapeutic modality for WNT subgroup medulloblastomas. Thus, the identification of ARID1B as a tumor suppressor and its downregulation resulting in the activation of multiple signaling pathways opens up opportunities for novel therapeutic modalities for the treatment of WNT subgroup medulloblastoma.

Autophagy inhibition impairs the invasion potential of medulloblastoma cells.

Medulloblastoma, a highly malignant pediatric brain tumor, consists of four distinct molecular subgroups called WNT, SHH, Group 3, and Group 4 that differ in their clinical characteristics with the WNT subgroup having excellent survival rate. About 1/3rd medulloblastomas have metastasis at the time of diagnosis suggesting, high invasion potential of these tumors. We have earlier reported that the tumor-suppressive role of miR-204 and miR-30a is accompanied by inhibition of autophagy in medulloblastoma cells. In the present study, we have investigated the role of autophagy in medulloblastoma biology. Autophagy was inhibited in the medulloblastoma cell lines belonging to the SHH, Group 3, and Group 4 using the shRNA mediated knockdown of ATG5, an upstream regulator of autophagy. The effect of autophagy inhibition was studied on the growth and malignant behavior of medulloblastoma cells. ATG5 knockdown resulted in the autophagy inhibition in medulloblastoma cells as judged by the reduction in the flux of LC3B, a marker for autophagy. Autophagy inhibition did not result in a significant difference in the proliferation and anchorage-independent growth of the medulloblastoma cells. On the other hand, autophagy inhibition brought about a substantial reduction in the invasion potential of all three medulloblastoma cell lines studied. The present study suggests a therapeutic potential for autophagy inhibitors in the treatment of medulloblastoma. Autophagy inhibitors could be effective in reducing the dose of craniospinal radiation, thereby leading to a significant reduction in the treatment-related side effects.

Restoration of miR-193a expression is tumor-suppressive in MYC amplified Group 3 medulloblastoma.

Medulloblastoma, a highly malignant pediatric brain tumor, consists of four molecular subgroups, namely WNT, SHH, Group 3, and Group 4. The expression of miR-193a, a WNT subgroup-specific microRNA, was found to be induced by MYC, an oncogenic target of the canonical WNT signaling. MiR-193a is not expressed in Group 3 medulloblastomas, despite MYC expression, as a result of promoter hypermethylation. Restoration of miR-193a expression in the MYC amplified Group 3 medulloblastoma cells resulted in inhibition of growth, tumorigenicity, and an increase in radiation sensitivity. MAX, STMN1, and DCAF7 were identified as novel targets of miR-193a. MiR-193a mediated downregulation of MAX could suppress MYC activity since it is an obligate hetero-dimerization partner of MYC. MYC induced expression of miR-193a, therefore, seems to act as a feedback inhibitor of MYC signaling. The expression of miR-193a resulted in widespread repression of gene expression that included not only several cell cycle regulators, WNT, NOTCH signaling genes, and those encoding DNA replication machinery, but also several chromatin modifiers like SWI/SNF family genes and histone-encoding genes. MiR-193a expression brought about a reduction in the global levels of H3K4me3, H3K27ac, the histone marks of active chromatin, and an increase in the levels of H3K27me3, a repressive chromatin mark. In cancer cells having high MYC expression, MYC brings about transcriptional amplification of all active genes apart from the induction of its target genes. MiR-193a, on the other hand, brought about global repression of gene expression. Therefore, miR-193a has therapeutic potential in the treatment of not only Group 3 medulloblastomas but possibly other MYC overexpressing aggressive cancers as well.

Downregulation of miR-204 expression defines a highly aggressive subset of Group 3/Group 4 medulloblastomas.

Genome-wide expression profiling studies have identified four core molecular subgroups of medulloblastoma: WNT, SHH, Group 3 and Group 4. Molecular markers are necessary for accurate risk stratification in the non-WNT subgroups due to the underlying heterogeneity in genetic alterations and overall survival. MiR-204 expression was evaluated in molecularly classified 260 medulloblastomas from an Indian cohort and in 763 medulloblastomas from the MAGIC cohort, SickKids, Canada. Low expression of miR-204 in the Group 3 / Group 4 tumors identify a highly aggressive subset of tumors having poor overall survival, in the two independent cohorts of medulloblastomas. Downregulation of miR-204 expression correlates with poor survival within the Group 4 as well indicating it as a valuable risk-stratification marker in the subgroup. Restoration of miR-204 expression in multiple medulloblastoma cell lines was found to inhibit their anchorage-independent growth, invasion potential and tumorigenicity. IGF2R was identified as a novel target of miR-204. MiR-204 expression resulted in downregulation of both M6PR and IGF2R that transport lysosomal proteases from the Golgi apparatus to the lysosomes. Consistent with this finding, miR-204 expression resulted in reduction in the levels of the lysosomal proteases in medulloblastoma cells. MiR-204 expression also resulted in inhibition of autophagy that is known to be dependent on the lysosomal degradation pathway and LC3B, a known miR-204 target. Treatment with HDAC inhibitors resulted in upregulation of miR-204 expression in medulloblastoma cells, suggesting therapeutic role for these inhibitors in the treatment of medulloblastomas. In summary, miR-204 is not only a valuable risk stratification marker in the combined cohort of Group 3 / Group 4 medulloblastomas as well as in the Group 4 itself, that has paucity of good prognostication markers, but also has therapeutic potential as indicated by its tumor suppressive effect on medulloblastoma cells.