Medulloblastomas derived from Cxcr6 mutant mice respond to treatment with a smoothened inhibitor.

The sonic hedgehog (Shh) pathway is activated in approximately 30% of human medulloblastoma resulting in increased expression of downstream target genes. In about half of these cases, this has been shown to be a consequence of mutations in regulatory genes within the pathway, including Ptc1, Smo, and Sufu. However, for some tumors, no mutations have been detected in known pathway genes. This suggests that either mutations in other genes promote tumorigenesis or that epigenetic alterations increase pathway activity in these tumors. Here, we report that 3% to 4% of mice lacking either one or both functional copies of Cxcr6 develop medulloblastoma. Although CXCR6 is not known to be involved in Shh signaling, tumors derived from Cxcr6 mutant mice expressed Shh pathway target genes including Gli1, Gli2, Ptc2, and Sfrp1, indicating elevated pathway activity. Interestingly, the level of Ptc1 expression was decreased in tumor cells although two normal copies of Ptc1 were retained. This implies that reduced CXCR6 function leads to suppression of Ptc1 thereby increasing Smoothened function and promoting tumorigenesis. We used a direct transplant model to test the sensitivity of medulloblastoma arising in Cxcr6 mutant mice to a small-molecule inhibitor of Smoothened (HhAntag). We found that transplanted tumors were dramatically inhibited in mice treated for only 4 days with HhAntag. These findings suggest that HhAntag may be effective against tumors lacking mutations in known Shh pathway genes.

The hedgehog pathway and neurological disorders.

The hedgehog pathway is a major regulator of embryonic development, and mutations that decrease its activity are known to be associated with severe defects in nervous system development. Recent evidence suggests hedgehog continues to function in adult tissue, normal as well as diseased, by regulating both cell proliferation and the production of growth and angiogenic factors. In the adult nervous system, this dual ability is especially important in regulating the behavior of neural stem and progenitor cells. This review summarizes information connecting hedgehog signaling and neural diseases, including neurodegenerative disorders and brain tumors, particularly medulloblastoma. We also describe the discovery and utility of small molecule agonists and antagonists of this pathway and their potential as novel types of therapeutics.

Shh pathway activity is down-regulated in cultured medulloblastoma cells: implications for preclinical studies.

Gene expression profiling indicates that the Sonic Hedgehog (Shh) pathway is active in approximately 30% of human medulloblastomas, suggesting that it could provide a useful therapeutic target. Previously, we showed that spontaneous medulloblastomas in Ptc1(+/-)p53-/- mice could be eradicated by treatment with a small-molecule inhibitor (HhAntag) of Smoothened (Smo). Here, we compared the responses of mouse medulloblastoma cells propagated in flank allografts, either directly or after culture in vitro, to HhAntag. We found that Shh pathway activity was suppressed in medulloblastoma cells cultured in vitro and it was not restored when these cells were transplanted into the flank of nude mice. The growth of these transplanted tumor cells was not inhibited by treatment of mice with doses of HhAntag that completely suppressed Smo activity. Interestingly, tumor cells transplanted directly into the flank maintained Smo activity and were sensitive to treatment with HhAntag. These findings indicate that propagation of tumor cells in culture inhibits Smo activity in a way that cannot be reversed by transplantation in vivo, and they raise concerns about the use of cultured tumor cells to test the efficacy of Shh pathway inhibitors as anticancer therapies.

Suppression of the Shh pathway using a small molecule inhibitor eliminates medulloblastoma in Ptc1(+/-)p53(-/-) mice.

Medulloblastoma is the most common malignant pediatric brain tumor. Current treatment is associated with major long-term side effects; therefore, new nontoxic therapies, targeting specific molecular defects in this cancer, need to be developed. We use a mouse model of medulloblastoma to show that inhibition of the Sonic Hedgehog (Shh) pathway provides a novel therapy for medulloblastoma. A small molecule inhibitor of the Shh pathway, HhAntag, blocked the function of Smoothened in mice with medulloblastoma. This resulted in suppression of several genes highly expressed in medulloblastoma, inhibition of cell proliferation, increase in cell death and, at the highest dose, complete eradication of tumors. Long-term treatment with HhAntag prolonged medulloblastoma-free survival. These findings support the development of Shh antagonists for the treatment of medulloblastoma.

Medulloblastoma and retinoblastoma: oncology recapitulates ontogeny.

One major factor hindering progress of pediatric cancers of the nervous system has been the lack of satisfactory model systems for testing novel therapies. A mouse strain, mutant for the Rb1 gene was generated 12 years ago in the hope of producing a model in which to study retinoblastoma. Surprisingly, the Rb(+/-) mice never developed retinoblastoma. Now, Zhang, Schweers and Dyer produce triply deficient Rb, p107 and p53 mutant retinal progenitor cells. All such mice develop intraocular retinoblastoma with invasion of the tumor into the anterior chamber of the eye. This dramatic finding represents the first description of a heritable mouse model of retinoblastoma, which has eluded investigators for the last 12 years. Such models provide an unprecedented opportunity to advance knowledge of tumorigenesis and to develop non-toxic intervention strategies which eradicate disease.