Abstract #5686: Clonal selection and parallel evolution define the hierachical structure of metastatic medulloblastoma
Authors: Xiaochong Wu, Lara Collier, Jessica Mcleod, Paul Northcott, Sidney Croul, Adam Dupuy, Jujnun Zhang, Steve Scherer, David Largaespada, Michael Taylor
Journal: Cancer Research (2009)

AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO

Dissemination of medulloblastoma through the CSF is a grave prognostic marker. Treatment involves irradiation of the entire craniospinal axis, resulting in severe complications.Very little is known of the cellular biology or genetics of medulloblastoma dissemination. The Sleeping Beauty (SB) system is a novel functional genomic tool for cancer gene discovery. In a modified SB system we use a Math1 enhancer to express the SB11 transposase in the murine cerebellar EGL, a putative cell of origin for medulloblastoma. Neither Math1-SB11 transgenic mice, or transposon donor mice, or mice carrying both transposon and transposase develop cancer. However, by 10 weeks of age, nearly 100% of Math1-SB11, SB transposon donor, ptch +/- mice develop medulloblastoma with leptomeningeal metastases, where-as 10-20% of Ptch +/- mice develop localized medulloblastoma by 8 months. SB transposition sites in medulloblastoma and its metastases can be rapidly determined through linker mediated-PCR followed by next generation 454 sequencing. We have sequenced over 158,000 insertion sites from a series of >140 SB-induced primary medulloblastomas, as well as matched spinal and frontal lobe leptomeningeal mets. Statistical analysis identified 29 common insertion sites (CISs) in the primary tumors including well known medulloblastoma cancer genes such as CREBBP , PTEN , and MXI1 , as well as other known cancer genes such as FHIT , BCL9 , CACNA2D1 , and ERAS . CISs also frequently targeted genes involved in neural stem cell biology such as MSI1 , NFIA , NFIB , and CHRDL1 . Strikingly, less than 50% of CISs were found in both the primary and matched mets. While each primary/met pair is descended from a common ancestor cell as shown by common clonal insertions, both primary and matched mets shared unique clonal events. PCR for specific insertion sites revealed that clonal events present in the mets were present in a very small subclone (<1%) of the primary. This highly supports the clonal evolution theory of metastasis, and suggests that these insertions are acting as metastasis virulence genes. In each case examined, both spinal and frontal lobe metastases shared a common, clonal insertion that was highly subclonal in the primary tumor, strongly suggesting that leptomeningeal dissemination arises only once, or from one subclonal population in the primary. We also observe highly clonal insertions in the primary that are not present in the matched mets. This is consistent with the metastatic cells having left the primary site early, with the subsequent selection of a \#8216;post dispersion event\#8217; in the primary, highly supporting the paralell evolution model of metastasis. Our results suggest reasons for the failure of many targeted therapies based on studies limited to the primary tumor, have important implications for the future of medulloblastoma clinical trials, and provide further insights into the mechansims of cancer metastasis.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5686.