Research And Grants

Radiation Resistance in Medulloblastoma

Medulloblastomas are childhood brain tumors occurring in the cerebellum, a region of the brain that regulates posture, coordination, and may play roles in cognition and emotion.  An important phase of cerebellar development takes place after birth, marked by very rapid division of cells destined to become cerebellar neurons.  These rapidly dividing cells are vulnerable to accumulating gene mutations which may cause medulloblastoma.  Genetically engineered mouse models for medulloblastoma have been created that closely resemble human medulloblastomas in their genetic characteristics and histological patterning.

The standard of care for children diagnosed with medulloblastoma is surgical removal of as much of the tumor as possible, chemotherapy, and radiation of the brain and spinal cord.  While this regimen yields a cure rate of ~70%, when tumors recur due to the presence of treatment-resistant tumor cells, the prognosis is bleak.   Our goal is to discover ways to improve medulloblastoma cells’ response to treatment, by investigating proteins that play roles in therapy resistance.  Here, we propose to characterize a factor we have identified in mouse and human medulloblastoma cells, named IGFBP2.  We found that IGFBP2 promotes medulloblastoma cell division and causes activity of proteins that in other cancers permit tumor cells to survive radiation and chemotherapy. 

IGFBP2 stands for Insulin-like Growth Factor Binding Protein-2.  IGFBP2 is a protein secreted by many cells in the developing nervous system.  Its main function is to interact with Insulin-like Growth Factor (IGF) and limit how much of it can attach to its receptor at any time, thus preventing over-activation of the growth signals turned on by IGF, which could lead to abnormal development.  Most IGFPB2 production ceases after development is complete.  However, in many types of cancer cells IGFBP2 comes back on, and has functions independent of regulating IGF.  In the adult brain tumor glioblastoma, IGFBP2 has been shown to increase cell division and go to the nucleus where it has effects on genes being turned on or off, at least partly by interacting with another important gene regulator in cancer known as STAT3.  In some cancers IGFBP2 promotes resistance to therapy, and in many cancers high levels of IGFBP2 are associated with reduced survival.  

IGFBP2 has not been well-studied in medulloblastoma.  We discovered that IGFBP2 is produced by medulloblastoma cells of a specific genetic subclass called SHH.  Using mouse models and human patient samples, we explored effects of interfering with IGFBP2 production or activity.  We found that IGFBP2 is necessary for medulloblastoma cell proliferation, and that it effects levels of proteins that have been linked to the ability of other types of tumor cells to evade therapy.   We also observed that IGFBP2 causes increased activity of STAT3 in medulloblastoma cells.  STAT3 has been shown to enable some types of tumor cells to survive exposure to radiation, thus allowing tumors to regrow.

Our Cure Starts Now proposal has two Specific Aims.  The first is to test the hypothesis that IGFBP2 increases the ability of medulloblastoma cells to survive radiation, through its effects on STAT3 and other proteins that can render tumor cells resistant to therapy.  Here we will use slices of mouse medulloblastoma placed in tissue culture and exposed to radiation with or without treatment with IGFBP2 blocking antibodies.  The slice cultures provide an advantage over dissociated cell cultures because the slices maintain the microenvironmental organization of the tumor, including the presence of non-tumor cells such as blood vessels and astrocytes, which play critical roles in responding to radiation.  We have found that in medulloblastomas in vivo, astrocytes produce IGFBP2 which they may use to communicate with tumor cells.  We will measure the effects of IGFBP2 blocking antibodies on markers of cell survival, cell death, DNA repair, and STAT3 activity after exposure to radiation.

Our second aim focusses on the activity of IGFBP2 in the nucleus.  Others have shown that IGFBP2’s cancer-promoting effects require its going into the tumor cell nucleus, and we have shown that IGFBP2 is found in medulloblastoma cell nuclei.  In the nucleus, IGFBP2 could be binding to DNA and affecting which genes are turned on or off, by itself or together with STAT3 as in glioblastoma.  To discover genes that are turned on or off by IGFBP2 and/or STAT3 we will use a novel technique called CUT&RUN.  With this technique, after experimental treatment DNA is purified from the cells and mixed with antibodies against IGFBP2/STAT3.  The antibody:DNA complexes are then isolated and the DNA is sequenced.  This DNA should contain genes which are being turned on or off.  We will then validate these genes using additional approaches.  We hypothesize that these genes will represent a “radiation survival signature” and may include new targets for developing therapies against, to increase medulloblastoma cells’ susceptibility to radiation.