S1. Nutlin-3a treatment caused apoptosis and cell cycle arrest. S2. Whole blots of MDM2 and p-MDM2S166 in SmoA1 tumors. S3. Normalized expression levels of I2PP2A mRNA in medulloblastoma patients showed a small difference between SHH medulloblastoma samples and cerebellar controls. S4. p-MDM2 is regulated by both PP2A and p-Akt. S5. Lentivirus and retrovirus infection on SmoA1 MBCs are successful. S6. I2PP2A knockdown affected p53 targets and SHH signaling only in ONS76 cells.
ARTICLE ABSTRACT
Medulloblastomas, the most common malignant pediatric brain tumors, have been genetically defined into four subclasses, namely WNT-activated, Sonic Hedgehog (SHH)-activated, Group 3, and Group 4. Approximately 30% of medulloblastomas have aberrant SHH signaling and thus are referred to as SHH-activated medulloblastoma. The tumor suppressor gene TP53 has been recently recognized as a prognostic marker for patients with SHH-activated medulloblastoma; patients with mutant TP53 have a significantly worse outcome than those with wild-type TP53. It remains unknown whether p53 activity is impaired in SHH-activated, wild-type TP53 medulloblastoma, which is about 80% of the SHH-activated medulloblastomas. Utilizing the homozygous NeuroD2:SmoA1 mouse model with wild-type Trp53, which recapitulates human SHH-activated medulloblastoma, it was discovered that the endogenous Inhibitor 2 of Protein Phosphatase 2A (SET/I2PP2A) suppresses p53 function by promoting accumulation of phospho-MDM2 (S166), an active form of MDM2 that negatively regulates p53. Knockdown of I2PP2A in SmoA1 primary medulloblastoma cells reduced viability and proliferation in a p53-dependent manner, indicating the oncogenic role of I2PP2A. Importantly, this mechanism is conserved in the human medulloblastoma cell line ONS76 with wild-type TP53. Taken together, these findings indicate that p53 activity is inhibited by I2PP2A upstream of PP2A in SHH-activated and TP53-wildtype medulloblastomas.
This study suggests that I2PP2A represents a novel therapeutic option and its targeting could improve the effectiveness of current therapeutic regimens for SHH-activated or other subclasses of medulloblastoma with wild-type TP53.
