20.109(F11): The Role of Brca2 in Medulloblastoma

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The Role of Brca2 in Medulloblastoma

Introduction

Medulloblastoma is a cerebellar tumor affecting children and young adults. It accounts for about twenty percent of pediatric brain tumors. Despite current treatment, which includes surgery, radiotherapy, and chemotherapy, recurrence of the tumor is frequent and mortality remains relatively high.


Past studies have shown that defects in DNA repair play an important role in the pathogenesis of medulloblastoma. The developing nervous system is particularly sensitive to DNA double stranded breaks (DSBs), which trigger a signaling cascading leading the two main repair processes, homologous recombination (HR) and/or nonhomologous end joining (NHEJ), or to apoptosis that eliminates the damaged cells. Inactivation of DNA DSB repair can lead to medulloblastoma.


A study from 2007 demonstrated that the BRCA2 gene plays a key role in suppressing the onset of medulloblastoma. BRCA2 normally interacts with RAD51, a protein essential for HR, and is responsible for the translocation of RAD51 to the sites of DNA damage processing. (BRCA2 is also necessary for the prevention of Fanconi Anemia, a rare recessive genetic syndrome characterized by congenital abnormalities, bone marrow failture, and cancer susceptibility.)


Furthermore, BRCA2 is important for neural development. Inactivation of BRCA2 leads to microephaly and cerebellar defects. However, though BRCA2 loss has a profound effect on early development, a substantial amount of neural development still occurs.

Possible Research Goal

1. Building off of the BRCA2/neural development study, we hypothesize that repair pathways, such as NHEJ, may come into play during neural development when BRCA2 is lost. Furthermore, we hypothesize that BRCA2 loss may have different effects at different stages of development. BRCA2 may have a more severe effect at early stages than later stages of development. If true, this has important implications in stem cell development. BRCA2 may have an important role in maintaining genome stability in stem cells.

2. We also think it would be interesting to look at the effects of BRCA2 loss at various time points in tumorigenesis in medulloblastoma. We can also study the inter-relationship between the various other DNA repair pathways and BRCA2 at specific time points in tumorigenesis. Knowledge of this may lead to temporally-specific, targeted treatment for medulloblastoma. For instance, if BRCA2 loss is found to be important early on in tumorgenesis, the next step may be to develop biological markers that can detect, in vivo, the loss of BRCA2.

Experimental Design

1. We can generate mice with BRCA2 deleted only from the nervous system. We want to do this deletion at specific time points in development. This is to be followed by karyotyping experiments so we can determine the aneuploid response of cells. We can also knockout various DNA repair mechanisms at different time points to study the importance of various DNA repair mechanisms at different time points in development. We will inactivate the proteins involved in NHEJ (Lig4, Xrcc4, and Ku80) and HR (Xrcc2 or Brca2) at different time points in development.

2.We will use conditional gene inactivation to only knockout BRCA2 in the nervous system. We want to do this at specific time points in tumorigenesis. We will look at the ensuing development/lack of development of tumor.

References

This review talks about the methods of inflicting HR deficient, cancerous cells with a synthetically lethal phenotype. When cells are unable to undergo HR, they rely on PARP-1 for DNA replication. If these deficient cells can be given a PARP-1 inhibitor, they will be unable to replicate DNA and undergo apoptosis.

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  • Daniel RA, Rozanska AL, Mulligan EA, Drew Y, Thomas HD, Castelbuono DJ, Hostomsky Z, Plummer ER, Tweddle DA, Boddy AV, Clifford SC, Curtin NJ. (2010). Central nervous system penetration and enhancement of temozolomide activity in childhood medulloblastoma models by poly(ADP-ribose) polymerase inhibitor AG-014699. British Journal of Cancer. 103(10), 1588-1596. http://www.nature.com/bjc/journal/v103/n10/full/6605946a.html.

Temozolomide is an alkylating agent that acts against medulloblastoma. PARP-1 has been shown to enhance this agent's activity. This paper assesses the effects of a specific PARP inhibitor on temozolomide-inhibited medulloblastoma.

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  • Tanori, M., Mancuso, M., Pasquali1, E., Leonardi, S., Rebessi, S., Majo, V.D., Guilly, M.N., Giangaspero, F., Covelli, V., Pazzaglia, S., Saran, A. (2008). PARP-1 cooperates with Ptc1 to suppress medulloblastoma and basal cell carcinoma. Carcinogenesis, 29(10), 1911–1919. http://carcin.oxfordjournals.org/content/29/10/1911.full.pdf.

This paper discusses PARP-1's effects on the "sonic hedgehog" development pathway. The patch protein (Ptc1) is a negative regulator for the development pathway and developmental defects occur when it is altered. To test for the interaction between PARP-1 and Ptc1, PARP-1 null mice were crossed with Ptc1 heterozygous ones. It was shown that PARP-1 inactivation makes brain cells more sensitive to radiation.

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Frappart et al discuss the role of BRCA2 in brain development. BRCA2 loss affects neural development, especially during embryonic and postnatal development. As such, BRCA2 can be considered a potent medulloblastoma inhibitor.

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This paper discusses a new Medulloblastoma model in which tumors arise in p53 and PARP deficient mice. Medulloblastoma is strongly inhibited by drugs blocking the hedgehog pathway. The paper also discusses some of the genetics behind the cancer.

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