Search

The sonic hedgehog (Shh) pathway is activated in ∼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. [Cancer Res 2007;67(8):3871–7]

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.

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.

X-chromosome inactivation is the process by which female mammals (with two X chromosomes) achieve expression of X-chromosomal genes equivalent to that of males (one X and one Y chromosome)1,2. This results in the transcriptional silencing of virtually all genes on one of the X chromosomes in female somatic cells. X-chromosome inactivation has been shown to act in cis and to initiate and spread from a single site on the X chromosome known as the X-inactivation centre (Xic)2,3. The Xic has been localized to a 450-kilobase region of the mouse X chromosome4. The Xist gene also maps to this region and is expressed exclusively from the inactive X chromosome3–7. Xist is unusual in that it appears not to code for a protein but produces a nuclear RNA which colocalizes with the inactive X chromosome4,8. The creation of a null allele of Xist in embryonic stem cells has demonstrated that this gene is required for X inactivation to occur in cis9. Here we show that Xist, introduced onto an autosome, is sufficient by itself for inactivation in cis and that Xist RNA becomes localized close to the autosome into which the gene is integrated. In addition, the presence of autosomal Xist copies leads to activation of the endogeneous Xist gene in some cells, suggesting that elements required for some aspects of chromosome counting are contained within the construct. Thus the Xist gene exhibits properties of the X-inactivation centre.

Gene expression profiling indicates that the Sonic Hedgehog (Shh) pathway is active in ∼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. (Cancer Res 2006; 66(8): 4215-22)

Medulloblastoma is the most common malignant pediatric brain tumor for which no satisfactory treatments exist. The Sonic Hedgehog signaling pathway seems to play an important role in the pathology of this disease. Here we review our recent demonstration that a small-molecule inhibitor of this pathway can regress tumors that arise in a transgenic mouse model of medulloblastoma. These and other findings suggest that inhibitors of Sonic Hedgehog signaling may offer an effective way to target some malignancies.

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.

Opitz G/BBB syndrome (OS) is a genetically heterogeneous disorder with an X-linked locus and an autosomal locus linked to 22q11.2. OS affects multiple organ systems with often variable severity even between siblings. The clinical features, which include hypertelorism, cleft lip and palate, defects of cardiac septation, hypospadias, and anorectal anomalies, indicate an underlying disturbance of the developing ventral midline of the embryo. The gene responsible for X-linked OS, FXY/MID1, is located on the short arm of the human X chromosome within Xp22.3 and encodes a protein with both an RBCC (RING finger, B-box, coiled coil) and a B30.2 domain. The Fxy gene in mice is also located on the X chromosome but spans the pseudoautosomal boundary in this species. Here we describe a gene closely related to FXY/MID1, called FXY2, which also maps to the X chromosome within Xq22. The mouse Fxy2 gene is located on the distal part of the mouse X chromosome within a region syntenic to Xq22. Analysis of genes flanking both FXY/MID1 and FXY2 (as well as their counterparts in mouse) suggests that these regions may have arisen as a result of an intrachromosomal duplication on an ancestral X chromosome. We have also identified in both FXY2 and FXY/MID1 proteins a conserved fibronectin type III domain located between the RBCC and B30.2 domains that has implications for understanding protein function. The FXY/MID1 protein has previously been shown to colocalize with microtubules, and here we show that the FXY2 protein similarly associates with microtubules in a manner that is dependent on the carboxy-terminal B30.2 domain.