Loss of p53 but not ARF accelerates medulloblastoma in mice heterozygous for patched.

Brain malignancies represent the most common solid tumors in children, and they are responsible for significant mortality and morbidity. The molecular basis of the most common malignant pediatric brain tumor, medulloblastoma, is poorly understood. Mutations in several genes including the human homologue of the Drosophila segment polarity gene, patched (PTCH), the adenomatous polyposis coli gene (APC), beta-catenin, and p53 have been reported in subsets of hereditary and sporadic medulloblastoma. Inactivation of one Ptc allele in mice results in a 14% incidence of medulloblastoma. Here, we report a dramatic increase in the incidence (>95%) and accelerated development (prior to 12 weeks of age) of medulloblastoma in mice heterozygous for Ptc that lack p53. The acceleration of tumorigenesis in Ptc+/- mice is specific for loss of p53, because no change in tumor incidence was observed in Ptc+/- mice carrying a mutation in APC (Min+/-) or in Ptc+/- mice deficient in p19ARF. Thus, there is a specific interaction between p53 loss and heterozygosity of Ptc that results in medulloblastoma. This may be a consequence of increased genomic instability associated with loss of p53 function that may enhance the rate of acquisition of secondary mutations. Ptc+/- p53-/- mice provide a useful model for investigation of the molecular bases of medulloblastoma and for evaluation of the efficacy of therapeutic intervention strategies in a spontaneously arising endogenous brain tumor.

The normal patched allele is expressed in medulloblastomas from mice with heterozygous germ-line mutation of patched.

Defects in a developmental signaling pathway involving mammalian homologues of the Drosophila segment polarity gene, patched (ptc) and its ligand, sonic hedgehog (shh), contribute to tumor formation in several tissues. Recently, a subset of medulloblastoma, the most common malignant brain tumor in children, was found to contain somatic mutations in the human ptc gene. In addition, basal cell nevus syndrome (BCNS), or Gorlin syndrome, which is characterized by developmental anomalies and a predisposition to skin and nervous system malignancies, is associated with germ-line mutation of ptc. Targeted disruption of both alleles of ptc in mice results in embryonic lethality. However, ptc+/- mice survive and develop spontaneous cerebellar brain tumors, suggesting that ptc may function as a tumor suppressor gene. Therefore, we investigated ptc+/-mice as a model for human medulloblastoma. We report that 14% of ptc+/- mice develop central nervous system tumors in the posterior fossa by 10 months of age, with peak tumor incidence occurring between 16 and 24 weeks of age. The tumors exhibited several characteristics of human medulloblastoma, including expression of intermediate filament proteins specific for neurons and glia. Full-length ptc mRNA was present in all tumors analyzed, indicating that there was no loss of heterozygosity at the ptc locus. Nucleotide sequence of ptc mRNA from four tumors failed to identify any mutations. However, a comparison of the normal ptc sequence from C57BL/6 and 129Sv mice did reveal several polymorphisms. High levels of glil mRNA and protein were detected in the tumors, suggesting that the shh/ptc pathway was activated despite the persistence of ptc expression. These data indicate that haploinsufficiency of ptc is sufficient to promote oncogenesis in the central nervous system.

Fragile X mental retardation protein: nucleocytoplasmic shuttling and association with somatodendritic ribosomes.

Fragile X syndrome, a leading cause of inherited mental retardation, is attributable to the unstable expansion of a CGG-repeat within the FMR1 gene that results in the absence of the encoded protein. The fragile X mental retardation protein (FMRP) is a ribosome-associated RNA-binding protein of uncertain function that contains nuclear localization and export signals. We show here detailed cellular localization studies using both biochemical and immunocytochemical approaches. FMRP was highly expressed in neurons but not glia throughout the rat brain, as detected by light microscopy. Although certain structures, such as hippocampus, revealed a strong signal, the regional variation in staining intensity appeared to be related to neuron size and density. In human cell lines and mouse brain, FMRP co-fractionated primarily with polysomes and rough endoplasmic reticulum. Ultrastructural studies in rat brain revealed high levels of FMRP immunoreactivity in neuronal perikarya, where it is concentrated in regions rich in ribosomes, particularly near or between rough endoplasmic reticulum cisternae. Immunogold studies also provided evidence of nucleocytoplasmic shuttling of FMRP, which was localized in neuronal nucleoplasm and within nuclear pores. Moreover, labeling was observed in large- and small-caliber dendrites, in dendritic branch points, at the origins of spine necks, and in spine heads, all known locations of neuronal polysomes. Dendritic localization, which was confirmed by co-fractionation of FMRP with synaptosomal ribosomes, suggests a possible role of FMRP in the translation of proteins involved in dendritic structure or function and relevant for the mental retardation occurring in fragile X syndrome.

FMRP associates with polyribosomes as an mRNP, and the I304N mutation of severe fragile X syndrome abolishes this association.

Fragile X mental retardation is caused by the lack of FMRP, a selective RNA-binding protein associated with ribosomes. A missense mutation, I304N, has been found to result in an unusually severe phenotype. We show here that normal FMRP associates with elongating polyribosomes via large mRNP particles. Despite normal expression and cytoplasmic mRNA association, the I304N FMRP is incorporated into abnormal mRNP particles that are not associated with polyribosomes. These data indicate that association of FMRP with polyribosomes must be functionally important and imply that the mechanism of the severe phenotype in the I304N patient lies in the sequestration of bound mRNAs in nontranslatable mRNP particles. In the absence of FMRP, these same mRNAs may be partially translated via alternative mRNPs, although perhaps abnormally localized or regulated, resulting in typical fragile X syndrome.

Nuclease sensitivity of permeabilized cells confirms altered chromatin formation at the fragile X locus.

Fragile X syndrome is caused by the expansion and concomitant methylation of a CGG repeat in the 5' untranslated region of the FMR1 gene which results in the transcriptional silencing of the FMR1 gene, delayed replication of the FMR1 locus, and the formation of a folate sensitive fragile site (FRAXA) at Xq27.3. The mechanism by which repeat expansion and methylation causes these changes is unknown. An in vivo system in which cells were permeabilized with lysophosphatidylcholine followed by digestion with MspI endonuclease was utilized to assess the chromatin conformation at the fragile X locus. The FMR1 gene was inaccessible to MspI digestion in fragile X patients, but not in normal or carrier individuals, confirming that altered chromatin conformation results from the repeat expansion and methylation seen in fragile X syndrome.

The fragile X mental retardation protein is a ribonucleoprotein containing both nuclear localization and nuclear export signals.

Fragile X syndrome is a frequent cause of mental retardation resulting from the absence of FMRP, the protein encoded by the FMR1 gene. FMRP is an RNA-binding protein of unknown function which is associated with ribosomes. To gain insight into FMRP function, we performed immunolocalization analysis of FMRP truncation and fusion constructs which revealed a nuclear localization signal (NLS) in the amino terminus of FMRP as well as a nuclear export signal (NES) encoded by exon 14. A 17 amino acid peptide containing the FMRP NES, which closely resembles the NES motifs recently described for HIV-1 Rev and PKI, is sufficient to direct nuclear export of a microinjected protein conjugate. Sucrose gradient analysis shows that FMRP ribosome association is RNA-dependent and FMRP is found in ribonucleoprotein (RNP) particles following EDTA treatment. These data are consistent with nascent FMRP entering the nucleus to assemble into mRNP particles prior to export back into the cytoplasm and suggests that fragile X syndrome may result from altered translation of transcripts which normally bind to FMRP.

Assessing the effects of the naked neck gene on chronic heat stress resistance in two genetic populations.

The effect of the naked neck (Na) gene on resistance to chronic heat stress was studied in the F2 generation of two populations of chickens genetically differing in growth. The Na gene was introduced into the Athens-Canadian randombred (ACRB), a small BW population, and into a large BW commercial broiler population. Naked neck and normally feathered birds were maintained in either a chronic heat stress (32 C) environment or a control (21 C) environment from 4 to 8 wk of age. Body weight, BW gain, feed consumption, and feed efficiency were calculated at 4, 6, and 8 wk. The 32 C environment significantly reduced gain and feed consumption at all age intervals in both populations. Feed conversion ratio (FCR) was lower in the 32 C environment in the ACRB population, but in the broiler population FCR either was the same or increased when compared with the 21 C environment. In the F2 generation of both populations the naked neck birds were significantly larger than the normally feathered birds although the two types segregated from the same parents. In the ACRB population the relative growth response was not significantly different between naked neck and normal feathered birds, indicating that the Na gene did not confer resistance to chronic heat stress in the small BW population. In the broiler population, an assessment of the relative growth response in the 32 C environment showed that the naked neck birds had a smaller reduction in BW gain and better feed efficiency than the normally feathered birds, indicating that in the large BW broiler population the Na gene did confer resistance to chronic heat stress.