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.

Restoration of miR-30a expression inhibits growth, tumorigenicity of medulloblastoma cells accompanied by autophagy inhibition.

Medulloblastoma is a highly malignant pediatric brain tumor. About 30% patients have metastasis at diagnosis and respond poorly to treatment. Those that survive, suffer long term neurocognitive, endocrine and developmental defects due to the cytotoxic treatment to developing child brain. It is therefore necessary to develop targeted treatment strategies based on underlying biology for effective treatment of medulloblastoma with minimal side effects. Medulloblastomas are believed to be the result of deregulated nervous system development as evident from the role of WNT and SHH developmental signaling pathways in pathogenesis of medulloblastomas. MicroRNAs are known to play vital roles in nervous system development as well as in cancer. MicroRNA profiling of medulloblastomas identified miR-30 family members' expression to be downregulated in medulloblastomas belonging to the four known molecular subgroups viz. WNT, SHH, Group 3 and Group 4 as compared to that in normal brain tissues. Furthermore, established medulloblastoma cell lines Daoy, D283 and D425 were also found to underexpress miR-30a. Restoration of miR-30a expression using inducible lentiviral vector inhibited proliferation, clonogenic potential and tumorigenicity of medulloblastoma cells. MiR-30a is known to target Beclin1, a mediator of autophagy. MiR-30a expression was found to downregulate Beclin1 expression and inhibit autophagy in the medulloblastoma cell lines as judged by downregulation of LC3B expression and its turnover upon chloroquine treatment and starvation induced autophagy induction. MiR-30a therefore could serve as a novel therapeutic agent for the effective treatment of medulloblastoma by inhibiting autophagy that is known to play important role in cancer cell growth, survival and malignant behavior.