Absence of germline and somatic p53 alterations in children with sporadic brain tumors.

Cancers of the central nervous system are the most common solid tumors of childhood. Although somatic alterations of the p53 tumor suppressor gene have been implicated in brain tumorigenesis, the role of germline p53 mutations in the development of childhood brain tumors has not been well defined. As a component of an ongoing extensive study of the epidemiology of childhood brain tumors, we prospectively examined the germline and tumor p53 gene status in 85 children without a family history of cancer who were diagnosed with a sporadic malignant central nervous system tumor. Using PCR/single-strand conformational polymorphism analysis and direct DNA sequencing, 85 children were screened for the presence of constitutional p53 sequence alterations in exons 2 and 4 through 11. No mutations were identified. Commonly reported sequence polymorphisms were observed at codon 72, as well as in 2 other previously described nucleotide residues. Forty-four brain tumor samples were available for analysis and of these 40 were paired with peripheral blood. Once again, no p53 mutations were found. Of the 5 germline samples with the 2 common polymorphisms, only one had a paired tumor sample for comparison and the tumor contained the same alteration as the germline. Of note, one tumor, a PNET of the cerebellum (medulloblastoma), showed loss of heterozygosity at codon 72. We can conclude that the frequency of germline and somatic p53 mutations in sporadic childhood brain tumors is very low, probably less than 1%, and there is no need to screen these patients routinely for their germline p53 status. However, the potential significance of LOH at codon 72 remains to be elucidated.

Tissue-specific expression of SV40 in tumors associated with the Li-Fraumeni syndrome.

Inactivation of wild-type p53 tumor suppressor function is the primary mechanism of tumor initiation in Li-Fraumeni syndrome (LFS) individuals with germline p53 mutations. Tumors derived from LFS patients frequently retain the normal p53 allele, suggesting that alternative mechanisms in addition to gene deletion must be involved in inactivating wild-type p53 protein. DNA tumor viruses, such as SV40, target p53 for inactivation through the action of viral oncoproteins. We studied the probands from two unrelated LFS families, each of whom presented with multiple malignant neoplasms. Patient 1 developed an embryonal rhabdomyosarcoma (RMS) and a choroid plexus carcinoma (CPC), while patient 2 developed a CPC and subsequently presented with both an osteosarcoma (OS) and renal cell carcinoma (RCC). We utilized DNA sequence analysis and immunohistochemistry to determine p53 gene status in the germline and tumors, as well as evidence for SV40 T-antigen oncoprotein expression. Each patient harbored a heterozygous germline p53 mutation at codons 175 and 273, respectively. In patient 1, the normal p53 gene was lost while the mutant p53 allele was reduced to homozygosity in the RMS. Both normal and mutant genes were maintained in the CPC. In patient 2, normal and mutant p53 alleles were retained in both the CPC and RCC. Both specific PCR and immunostaining detected SV40 T-antigen in both CPCs and the RCC. In addition to chromosomal alterations, epigenetic mechanisms may disrupt p53 function during tumorigenesis. In two LFS patients, we found SV40 DNA sequences and viral T-antigen expression that could account for inactivation of the normal p53 protein. Inactivation of p53 or other tumor suppressors by viral proteins may contribute to tumor formation in specific tissues of genetically susceptible individuals.

Identification of MYCN Copy Number Heterogeneity by Direct FISH Analysis of Neuroblastoma Preparations.

Background: Amplification of MYCN and deletion of 1p in neuroblastoma are associated wtih a poor prognosis. MYCN copy numbers of 10 or greater are considered to be indicative of poor outcome. However, most molecular genetic methods for estimating the number of MYCN genes do not directly address copy number heterogeneity at the cellular level. Methods and Results: MYCN copy number was assessed by fluorescence in situ hybridization (FISH), differential polymerase chain reaction (PCR), and Southern blot analysis, using 29 patient tumors. Copy number estimates by PCR or Southern blot analyses identified MYCN amplification in 11 tumors. There was complete concordance between FISH MYCN results in all 11 tumors with amplification. FISH identified 5 tumors with marked heterogeneity for MYCN copy number. Two tumors in which a small percentage of cells within the specimen were amplified would have gone undetected by molecular genetic methods alone. Conclusions: FISH offers the advantage over the other methods of detecting heterogeneity, thereby revealing tumors with small numbers of amplified cells that would otherwise be missed and, in cases of low (3-10) copies of MYCN, distinguishing small numbers of amplified cells (poor prognosis) from triploid tumors (good prognosis). FISH also allows detection of 1p deletion using the same preparations, which represents another advantage. A combination of FISH and conventional molecular methods provides an accurate definition of sample heterogeneity, and should be routinely applied to all neuroblastomas with low levels of MYCN copy number.

MYCN gene amplification in rhabdomyosarcoma.

BACKGROUND: Amplification of the MYCN oncogene, formerly known as N-myc, has been seen in several malignant tumors, particularly neuroblastoma, where its association with a poor clinical outcome is the clearest example of a clinically relevant oncogene mutation in any human cancer. METHODS: The incidence and clinical significance of MYCN amplification in rhabdomyosarcoma (RMS) was assessed by Southern blot analysis in this retrospective study of seven alveolar RMS and six embryonal RMS. RESULTS: MYCN amplification (4- to 13-fold) was present in three of seven alveolar RMS (42.9%) but in none of the embryonal RMS. There was no significant difference between the clinical behavior of the MYCN-amplified and unamplified tumors, and no correlation was found with the light microscopic appearances of the tumors or with desmin immunoreactivity. CONCLUSIONS: The findings are compatible with previous studies that demonstrated cytogenetic evidence of gene amplification in RMS, and help to clarify conflicting reports in the literature about MYCN amplification in alveolar and embryonal RMS. The results raise the possibility of important biologic differences between these subtypes of RMS, differences that warrant further investigation.

Loss of heterozygosity mapping in Wilms tumor indicates the involvement of three distinct regions and a limited role for nondisjunction or mitotic recombination.

Loss of heterozygosity (LOH) for polymorphic markers is a frequently occurring event in some tumors, reflecting the role of allele loss in the development of these tumors. We have determined LOH in 38 cases of Wilms tumor for the 2 known loci on chromosome arm 11p and for a newly detected locus on chromosome arm 16q. Only 7 of the 38 tumors studied showed reduction to homozygosity of 11p13 markers. In 4 of these tumors, reduced expression of WT1 and WIT1, genes located at 11p13 and implicated in Wilms tumorigenesis, was noted. However, this was also found in 2 of 7 tumors showing LOH exclusively of 11p15 markers and in 15 of the remaining 24 tumors in which there was no LOH for 11p markers. This suggests that events not involving mitotic recombination or chromosome nondisjunction are the most common mechanisms for mutations at the 11p Wilms tumor locus. We also noted that mitotic recombination involving 11p15 loci occurred in addition to reduced expression of the 11p13 locus genes in 2 tumors, suggesting a possible interaction between these 2 loci. In addition, LOH for 16q markers was observed in 6 tumors. In one case this was coincident with reduction of WT1 and WIT1 gene expression, and in 3 other cases it occurred in addition to 11p LOH. This indicates that an additional locus on 16q is likely to be involved in Wilms tumorigenesis.

Tissue, developmental, and tumor-specific expression of divergent transcripts in Wilms tumor.

The Wilms tumor locus on chromosome 11p13 has been mapped to a region defined by overlapping, tumor-specific deletions. Complementary DNA clones representing transcripts of 2.5 (WIT-1) and 3.5 kb (WIT-2) mapping to this region were isolated from a kidney complementary DNA library. Expression of WIT-1 and WIT-2 was restricted to kidney and spleen. RNase protection revealed divergent transcription of WIT-1 and WIT-2, originating from a DNA region of less than 600 bp. Both transcripts were present at high concentrations in fetal kidney and at much reduced amounts in 5-year-old and adult kidneys. Eleven of 12 Wilms tumors classified as histopathologically heterogeneous exhibited absent or reduced expression of WIT-2, whereas only 4 of 14 histopathologically homogeneous tumors showed reduced expression. These data demonstrate a molecular basis for the pathogenetic heterogeneity in Wilms tumorigenesis.