Cytogenetic and histopathologic studies of congenital supratentorial primitive neuroectodermal tumors: a case report

Primitive neuroectodermal tumors (PNET) represent about 25% of primary central nervous system tumors in childhood, but congenital PNETs are rare. Cytogenetic studies and studies on molecular pathology have identified several genetic alterations in medulloblastoma, but molecular investigations on supratentorial PNETs are infrequent. We present a male newborn with a large congenital PNET of the right cerebral hemisphere and the molecular analysis of the tumor. Tumor tissue was investigated by routine histology and immunohistochemistry. Fluorescence in-situ hybridization was carried out on native tumor tissue to investigate deletions on chromosome 17p and to analyze c-Myc or N-Myc amplifications. Histologic examination revealed a primitive neuroectodermal tumor with massive extension covering almost the entire right hemisphere. Genetic analysis of the native tumor tissue of our patient excluded a deletion of chromosome 17p. An amplification of the c-Myc or N-Myc oncogene was absent using fluorescence in-situ hybridization. Despite unremarkable genetic analysis in our case prognosis was poor, suggesting that there are additional, yet unknown constitutional genetic aberrations in the pathogenesis of congenital supratentorial PNET.

Mapping of the breakpoints on the short arm of chromosome 17 in neoplasms with an i(17q)

Isochromosomes are monocentric or dicentric chromosomes with homologous arms that are attached in a reverse configuration as mirror images. With an incidence of 3-4%, the i(17q) represents the most frequent isochromosome in human cancer. It is found in a variety of tumors, particularly in blast crisis of chronic myeloid leukemia (CML-BC), acute myeloid leukemia (AML), non-Hodgkin's lymphoma (NHL), and medulloblastoma (MB), and indicates a poor prognosis. To determine the breakpoints on the molecular genetic level, we analyzed 18 neoplasms (six CML, four AML, one NHL, and seven MB) with an i(17q) and two MB with a pure del(17p) applying fluorescence in situ hybridization (FISH) with yeast artificial chromosome (YAC) clones, P1-artificial chromosome (PAC) clones, and cosmids from a well-characterized contig covering more than 6 Mb of genomic DNA. We identified four different breakpoint cluster regions. One is located close to or within the centromere of chromosome 17 and a second in the Charcot-Marie-Tooth (CMT1A) region at 17(p11.2). A third breakpoint was found telomeric to the CMT1A region. The fourth, most common breakpoint was detected in MB, AML, and in CML-BC specimens and was bordered by two adjacent cosmid clones (clones D14149 and M0140) within the Smith-Magenis syndrome (SMS) region. These results indicate that the low copy number repeat gene clusters which are present in the CMT and SMS regions may be one of the factors for the increased instability that may trigger the formation of an i(17q).

Molecular analysis of childhood primitive neuroectodermal tumors defines markers associated with poor outcome

PURPOSE

The diagnostic and prognostic significance of well-defined molecular markers was investigated in childhood primitive neuroectodermal tumors (PNET).

MATERIALS AND METHODS

Using microsatellite analysis, Southern blot analysis, and fluorescence in situ hybridization (FISH), 30 primary tumors and six CSF metastasis specimens were analyzed for loss of heterozygosity (LOH) of chromosomes 1q31, 6q, 9q22, 10q, 11, 16q22, and 17p13.1 and/or high-level amplification of the c-myc gene. Experimental data were compared with clinical stage and outcome.

RESULTS

LOH of chromosome 17p13.1 was found most frequently (14 of 30 tumors, six of six CSF metastasis specimens); LOH of chromosomes 10q, 16q22, 11, 6, 9q22, and 1q31 was observed in 20.6%, 20%, 14.3%, 12%, 10%, and 0%, respectively. Eight of 32 tumors and CSF specimens showed amplification of c-myc. All tumors with amplification of c-myc were resistant to therapy and had a fatal outcome (mean survival time, 9.3 months). Tumors that displayed LOH of chromosome 17p were associated with metastatic disease. The prognosis of these tumors was worse only when associated with amplification of c-myc. Three of three patients with LOH of 9q22 relapsed.

CONCLUSION

In our study, amplification of c-myc was a poor-prognosis marker in PNET. LOH of chromosome 17p was associated with metastatic disease. Molecular analysis of primary tumors using these markers may be useful for stratification of children with PNET in future prospective studies. The other aberrations investigated were not of significant prognostic value, but may provide an entry point for future large-scale molecular studies.

High-resolution deletion mapping of chromosome arm 17p in childhood primitive neuroectodermal tumors reveals a common chromosomal disruption within the Smith-Magenis region, an unstable region in chromosome band 17p11.2

Loss of heterozygosity (LOH) on chromosome arm 17p is the most common genetic aberration in childhood primitive neuroectodermal tumors (PNETs). To determine the frequency and extent of 17p deletions, 29 loci on 17p were investigated in 24 tumors by using restriction fragment length polymorphism (RFLP) and microsatellite analysis. LOH on 17p was found in 9 of 24 tumors. In all tumors with LOH, a continuous stretch from the telomere to chromosome band 17p11.2 was completely deleted, and no interstitial or terminal small-scale deletions were detected in the remaining 15 tumors. In four tumors with LOH on 17p, the chromosomal breakpoint was located between D17S953 and D17S805. To identify this deletion breakpoint on the cytogenetic map of chromosome 17 and to exclude uniparental disomy, we verified our data by using fluorescence in situ hybridization (FISH) analyses. By using two yeast artificial chromosome (YAC) clones that were positive for D17S689 and D17S953, the same breakpoint was confirmed in two specimens of cerebrospinal fluid (CSF) metastases by using FISH on interphase preparations. We demonstrate that, in most childhood PNETs with LOH on 17p, the breakpoint is close to, but not within, the centromere. It varies, and it occurs predominantly between the two markers D17S689 and D17S953, which is an unstable chromosomal region that is deleted or duplicated in the Smith-Magenis syndrome. Because LOH of 17p is associated with the formation of isochromosome 17q in the majority of PNETs, this study provides entry points to determine the molecular nature of this phenomenon.

No preferential loss of one parental allele of chromosome 17p13.3 in childhood medulloblastoma

Medulloblastomas are the most frequent malignant tumors of the central nervous system in childhood. Loss of heterozygosity of the telomeric part of chromosome 17p has been described frequently in these tumors, suggesting a possible second tumor-suppressor gene in this region apart from the p53 gene. Using restriction-fragment-length polymorphism and microsatellite polymorphism analysis, we screened 16 medulloblastomas for 17p13.3 deletions and compared the DNA patterns of patients' tumors and normal tissues with those of one or both parents. Loss of heterozygosity of chromosome 17p13.3 could be detected in 6 out of 16 tumors (37.5%). In 4 of these 6 tumors the maternal allele was deleted, in 2 tumors the paternal allele. Although the number of tumors investigated is limited, we find no evidence that genomic imprinting of the chromosomal region most frequently deleted in medulloblastoma plays a role in tumorigenesis.

Analysis of the GAP-related domain of the neurofibromatosis type 1 (NF1) gene in childhood brain tumors

The neurofibromatosis type-1 (NF1) gene contains a 360-bp region with significant homology to the catalytic domain of mammalian GTPase-activating protein. This particular GAP-related domain of the NF1 gene (NF1-GRD) stimulates ras GTPase and inactivates ras protein p21ras. Therefore, it has been suggested that the NF1 gene represents another tumor-suppressor gene. In the search for molecular markers of possible diagnostic relevance, childhood brain-tumor specimens of different histologic diagnoses were tested for mutations of the so-called FLR-exon within the NF1-GRD. This part of the NF1-GRD has been shown to be most crucial for the GAP-like function. Using a highly sensitive PCR-SSCP technique, we tested 51 tumor specimens were tested, but found no mutations. We conclude that inactivation of this putative tumor-suppressor gene by mutations does not play a significant role in tumorigenesis of childhood brain tumors. Next, we compared the splice variants of the NF1-GRD in 33 brain tumors and 8 extraneural embryonal tumors. Primitive neuroectodermal tumors (PNET) (n = 10) and one intracranial teratoma were the only tumors that predominantly expressed a splice pattern that can be observed in the immature developing brain. In contrast to other embryonal neuronal tumors, this NF1-GRD splicing pattern could not be modified in a newly established medulloblastoma cell line by retinoic acid treatment. Since this particular splice variant suppresses p21ras more effectively than other NF1-GRD transcripts, its predominant expression may interfere with the physiological signal transduction of p21ras during differentiation of neurons. There may be a neurofibromin-induced and p21ras-mediated differentiation pathway of neuronal stem cells that is blocked in PNET. Such an arrest of a p21ras-dependent differentiation pathway may explain the persistence of primitive pluripotent neuronal cells in PNET.