Medulloblastoma: the biology of radiation-sensitive cancer. Timothy R Gershon, MD, PhD Department of Neurology UNC Chapel Hill

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1 Medulloblastoma: the biology of radiation-sensitive cancer Timothy R Gershon, MD, PhD Department of Neurology UNC Chapel Hill

2 Medulloblastoma: the treatable brain tumor most common malignant brain tumor in children Distinguished from gliomas: Cushing and Bailey 1925 cerebellar location occurrence in childhood Progenitor histology

3 Medulloblastoma: potentially curable Until 1950 s medulloblastoma like glioblastoma was 100% fatal Edith Paterson, MD, 1953: 1 st use of craniospinal radiation Increased 3 year survival to 57% Current therapy: 80% survival rate

4 Medulloblastoma is unique in response to conventional therapy EFS and OS rates children with medulloblastoma in COG9961 (RT+ chemotherapy) EFS and OS rates children with high grade glioma in ACNS0126 (RT+ temozolomide) Packer R J et al. JCO 2006;24: Cohen K J et al. Neuro Oncol 2011;13: Why does medulloblastoma respond to treatment? Why does treatment fail 20% of patients?

5 What about medulloblastoma makes it treatment responsive? If we could answer we might Predict which patients will not respond Spare them useless morbidity Offer experimental therapy Target mechanism of response more precisely Unique developmental biology may hold the key Animal model of tumor treatment

6 Postnatal neurogenesis and medulloblastoma Cerebellum is a site of postnatal neurogenesis, Cerebellar granule neuron progenitors (CGNPs) proliferate in response to Sonic Hedgehog (Shh) P7, sagittal section

7 Postnatal neurogenesis in the cerebellum GGNPs proliferate in the external granule cell layer (EGL) EGL contains proliferating and differentiation layers CGNP proliferation is time limited ends by P15 in mice, 1 year humans PCNA/p27 PCNA/p27

8 Shh links postnatal neurogenesis and medulloblastoma Cerebellum is a site of postnatal neurogenesis, Cerebellar granule neuron progenitors (CGNPs) proliferate in response to Sonic Hedgehog (Shh) P7, sagittal section Activating mutations in the Shh pathway in patients cause medulloblastoma

9 Shh links postnatal neurogenesis and medulloblastoma Cerebellum is a site of postnatal neurogenesis, Cerebellar granule neuron progenitors (CGNPs) proliferate in response to Sonic Hedgehog (Shh) P7, sagittal section Activating mutations in the Shh pathway in patients cause medulloblastoma Tumors recapitulated in mice with activating Shh pathway mutations

10 Modeling medulloblastoma in vivo Smoothened is a protein that transduces Shh signaling SmoM2 transgenic mice (Andrew McMahon, Harvard University): constitutively active allele of Smo Cre-activated Math1-cre mice (David Rowitch, UCSF) Express cre in neural progenitors Math1-cre mouse X SmoM2 mouse Cre LOX STOP LOX SmoM2 100% medulloblastoma by P12, fatal by P45 P17 Wild type P17 Math1-cre;SmoM2 SmoM2 tumor

11 Modeling radiation in vivo 10Gy X-rays focused on the posterior fossa Cause DNA damage detectable by ph2ax No Radiation 1 hr after Radiation

12 Neural progenitors die after RT Cleaved caspase 3/hematoxylin no apoptosis right after RT 4 hours after RT: wave of progenitor cell death neurons resist apoptosis

13 Cerebellar progenitors are highly vulnerable to RT Cell death is restricted to dividing cells located in the EGL PCNA+ layer within the EGL

14 Progenitors do not die simply because they are dividing Cleaved caspase 3/hematoxylin Progenitor death after RT requires pro-apoptotic Bax Genetic deletion of Bax eliminates apoptosis after RT

15 Progenitors do not die simply because they are dividing Cleaved caspase 3/hematoxylin Progenitor death after RT requires pro-apoptotic Bax Genetic deletion of Bax eliminates apoptosis after RT Progenitors die through an active process

16 Medulloblastomas respond like progenitors to RT Cleaved caspase 3/hematoxylin Minimal cell death 1 hr after RT Wave of cell death starts at 4 hrs

17 Tumor regression 5 days after RT

18 Percent survival Effect of RT on survival 100% 80% p=0.001 No RT 10Gy single dose 60% 40% 20% 0% Time (days) Math1-cre;SmoM2 models radiation-sensitive medulloblastoma

19 What happens to treatment response when apoptosis is blocked? No cell death in Bax KO tumors after RT Typical treatment response Treatment response when Bax deleted

20 Percent survival Bax-deleted medulloblastoma is RT resistant 100% 80% 60% p=0.001 Bax WT 10Gy Bax WT Bax f/f 10Gy Bax f/f 40% 20% RT only benefits mice intact Bax Bax-deficient mice get no RT benefit 0% Time (days) Medulloblastoma shares the RT sensitivity of neural progenitors

21 Does sensitivity to RT depend on cell of origin? SmoM2: a versatile model Activation targeted by cre strain: Math1-cre -> progenitors hgfap-cre -> brain stem cells either causes only medulloblastoma Math1-cre;SmoM2: 100% mice die by P40 hgfap-cre;smom2: 100% mice die by P20

22 Sensitivity to RT depends on cell of origin Math1-cre tumors are sensitive to RT ( vs ) hgfap-cre tumors are resistant ( vs. )

23 Activating SmoM2 in stem cells produces resistant tumor stem cell originated tumors: Patchy cell death after RT Many more resistant cells Sensitivity derives from cell of origin

24 Conclusions Radiation sensitivity is an active process in medulloblastoma: DNA damage triggers an intrinsic apoptotic program Cell death enforces fidelity in neural progenitors Progenitor-derived tumors access this same program Cell of origin can determine response after RT Blocking this process is sufficient make cancer resistant to RT Mutations in the death pathway Cell of origin effects: chromatin-mediated changes in repertoire Targeting these blocks may render more cancers treatable.

25 Acknowledgements Gershon Lab Andrew Crowther Kat Tech Hedi Liu Alyssa Stewart Jessica Meidinger Mentor Mohanish Deshmukh Key Collaborators Hong Yuan, BRIC Sha Chang, Radiation Oncology Funding NIH, St Baldrick s Foundation AICR Morgan Adams Foundation Matthew Larson Brain Tumor Fnd