Altered expression of the MYCN oncogene modulates MRP gene expression and response to cytotoxic drugs in neuroblastoma cells

We have recently shown a close correlation between expression of the Multidrug Resistance-associated Protein (MRP) gene and the MYCN oncogene and provided evidence that high MRP expression is a powerful independent predictor of poor outcome in neuroblastoma (Norris et al., New Engl. J. Med., 334, 231-238, 1996). The effect of MYCN down-regulation on MRP expression and response to cytotoxic drugs was investigated in NBL-S neuroblastoma cells transfected with MYCN antisense RNA constructs. Concomitant with MYCN down-regulation, the level of MRP expression was decreased in the NBAS-4 and NBAS-5 antisense transfectants. These cells demonstrated significantly increased sensitivity to the high affinity MRP substrates vincristine, doxorubicin, sodium arsenate and potassium antimony tartrate, but not to the poor MRP substrates, taxol or cisplatin. Similarly, transfection of full-length MYCN cDNA into SH-EP neuroblastoma cells resulted in increased MRP expression and significantly increased resistance specifically to MRP substrates. The results provide evidence for the MYCN oncogene influencing cytotoxic drug response via regulation of MRP gene expression. Our data also provide a link between the malignant and chemoresistant phenotypes of this childhood malignancy.

Lack of homozygously inactivated p73 in single-copy MYCN primary neuroblastomas and neuroblastoma cell lines

We examined 18 neuroblastoma cell lines and 32 primary single-copy MYCN tumor specimens to determine whether mutations of p73, a novel p53-related gene located in chromosome band 1p36.33, contribute to the genesis or progression of childhood neuroblastoma. By fluorescence in situ hybridization, 16 of the 18 cell lines, but only 3 of the 32 primary tumors, had evidence of a deleted p73 allele. Sequence analysis of the p73 coding region in the mRNAs expressed by these cell lines and tumors did not reveal inactivating mutations, suggesting that p73 is not homozygously inactivated in neuroblastoma. However, several novel splice forms of p73 mRNAs were identified, including one without exon 11 that predominated in multiple MYCN-amplified cell lines. Its encoded p73 protein differed from other splice forms in that the C-terminus was derived from an alternative reading frame. Further study of the functional properties of the protein encoded by this splice form of p73 will be needed to determine whether it contributes to the pathogenesis of childhood neuroblastoma with MYCN gene amplification.

Clinical impact and prognostic value of metaiodobenzylguanidine imaging in children with metastatic neuroblastoma

PURPOSE

The clinical value of metaiodobenzylguanidine (mIBG) scintigraphy in patients with disseminated neuroblastoma (NB) at the time of diagnosis and after induction chemotherapy was evaluated.

PATIENTS AND METHODS

The medical records and imaging studies of 30 patients with stage 4 NB who underwent mIBG scintigraphy and 99mTc hydroxy methylene diphosphonate bone scintigraphy at the time of diagnosis were reviewed. Scores were calculated for the mIBG and bone scintigrams, and outcome according to the initial and follow-up imaging studies was determined.

RESULTS

Discrepancies between bone scintigraphy and mIBG osteomedullary localization were seen in six patients. For the entire cohort, 2-year event-free survival did not significantly differ for the group of patients with initial mIBG or bone scintigraphy scores > or = 10 compared to those with scores < 10 (P = 0.23 and 0.61, respectively). However, for patients older than 1 year, a trend associating worse outcome with mIBG scores > or = 10 at diagnosis was seen (P = 0.08). A trend correlating abnormal mIBG scintigraphy after induction therapy and poor outcome was also observed (P = 0.09). Outcome did not correlate with the results of the bone scintigram studies performed after induction chemotherapy (P = 0.68).

CONCLUSION

Because a discordance between mIBG and bone scintigraphy results were seen in a subset of stage 4 NB patients, both imaging studies should be performed at the time of diagnosis. mIBG imaging studies performed at the time of diagnosis and after induction chemotherapy may be of prognostic value, particularly in stage 4 patients older than 1 year.

Fluorescence in situ hybridization analysis of chromosome 1p36 deletions in human MYCN amplified neuroblastoma

BACKGROUND/PURPOSE

Deletion of the short arm of chromosome 1 (1p) is one of the poor prognostic factors in human neuroblastomas. Recent studies have suggested that one or more of the neuroblastoma tumor suppressor genes reside in this region and have identified the shortest region of overlap (SRO) on 1p36. The purpose of this study was to examine deletions of 1p in human neuroblastomas by fluorescence in situ hybridization (FISH).

METHODS

Two-color FISH analysis was performed to detect chromosome 1p36 abnormalities in 42 MYCN-amplified neuroblastomas. Four different probes from the 1p36 region, the E2F2, NPPA, D1S160, and CDC2L1 loci were used for detection of 1p abnormalities. A repeat sequence probe, which is specific for the heterochromatic region of chromosome 1 (pUC1.77), was used as a control.

RESULTS

Large deletions of 1p36 were observed in 31 (73.8%) of 42 tumors, whereas the remaining 11 (26.2%) showed no deletion. In these 11 tumors, a translocation of 1p was found in one and a duplication of 1p was detected in another.

CONCLUSIONS

A strong correlation between 1p abnormalities and MYCN amplification was found in this study. MYCN-amplified neuroblastomas were found to show large deletions of 1p encompassing the SRO. FISH provided a rapid and reliable method to detect hemizygous deletions of 1p.

Prognostic significance of age, MYCN oncogene amplification, tumor cell ploidy, and histology in 110 infants with stage D(S) neuroblastoma: the pediatric oncology group experience--a pediatric oncology group study

PURPOSE

Although a high rate of spontaneous regression is observed in infants with stage D(S) neuroblastoma (NB), survival is not uniform. To determine the prognostic relevance of age at diagnosis, therapy, and tumor biology in infants with stage D(S) NB, we reviewed the Pediatric Oncology Group (POG) experience.

PATIENTS AND METHODS

A review of patients diagnosed with stage D(S) NB registered on POG protocols was performed. Survival according to age at diagnosis, treatment, and tumor biology was determined.

RESULTS

Between 1987 and 1996, 110 infants with stage D(S) NB had an estimated 3-year survival rate of 85% +/- 4%; survival rate was 71% +/- 8% for infants 2 months of age or younger, and 68% +/- 12%, 44% +/- 33%, and 33% +/- 19% for patients with diploid, MYCN-amplified, and unfavorable histology tumors, respectively. Survival rates were similar for patients who received adjuvant chemotherapy versus those who did not (82% +/- 5% v 93% +/- 6%, respectively; P = .187). Furthermore, there was no statistical difference in survival rate for patients who underwent complete resection of their primary tumor compared with those who underwent partial resection or biopsy only (90% +/- 5% v 78% +/- 7%, respectively; P = .083).

CONCLUSION

Our review confirmed that the survival of infants with stage D(S) NB is excellent. However, subsets of patients with poor prognosis can be identified by young age and unfavorable biologic factors. More effective therapy is needed for the group of stage D(S) infants who show unfavorable clinical and biologic features.

Advances in the diagnosis and treatment of neuroblastoma

Neuroblastoma, a childhood neoplasm arising from neural crest cells, is characterized by a diversity of clinical behavior, ranging from spontaneous remission to rapid tumor progression and death. To some extent, outcome can be predicted by the stage of disease and age at diagnosis. The molecular events responsible for the variability in response to treatment and rate of tumor growth, however, remain largely unknown. Over the past decade, transformation-linked genetic changes have been identified in neuroblastoma tumors that have contributed to our understanding of tumor predisposition, metastasis, treatment responsiveness, and prognosis. This review discusses the recent advances in the understanding of neuroblastoma at the cellular and molecular levels, and the role that tumor biology plays in determining appropriate risk-based treatment for patients with neuroblastoma.

Evidence that the MYCN oncogene regulates MRP gene expression in neuroblastoma

We have recently shown that expression of the multidrug resistance-associated protein (MRP) gene is a powerful prognostic indicator in childhood neuroblastoma and have suggested that the MYCN oncogene may regulate MRP gene expression. To address this hypothesis, we have examined the relationship between MYCN and MRP gene expression in neuroblastoma tumours and cell lines. MYCN and MRP gene expression were highly correlated in 60 primary untreated tumours both with (P = 0.01) and without MYCN gene amplification (P < 0.0001). Like MRP, high MYCN gene expression was significantly associated with reduced survival, both in the overall study population and in older children without MYCN gene amplification (relative hazards = 13.33 and 19.61, respectively). Inhibition of MYCN, through the introduction of MYCN antisense RNA constructs into human neuroblastoma cells in vitro, resulted in decreased MRP gene expression, determined both by RNA-PCR and Western analysis. The data are consistent with MYCN influencing neuroblastoma outcome by regulating MRP gene expression.

Treatment of poor-risk neuroblastoma patients with high-dose chemotherapy and autologous peripheral stem cell rescue

A single institutional pilot study was conducted in which 12 poor-risk neuroblastoma (NB) patients were uniformly treated with multi-agent induction chemotherapy followed by myeloablative consolidation chemotherapy and unpurged peripheral blood stem cell (PBSC) rescue. In addition to using standard criteria for evaluating response to induction chemotherapy, tumor cell contamination of the peripheral blood and/or bone marrow was analyzed in seven patients by immunocytology using a panel of five anti-NB monoclonal antibodies. Seven patients had morphologic evidence of bone marrow disease at the time of diagnosis, and two additional patients had tumor cells detected in bone marrow samples by immunocytology prior to the second cycle of chemotherapy. After three cycles of chemotherapy, two of the 12 patients continued to have evidence of bone marrow disease. Samples from 29 PBSC harvests collected from nine patients were also analyzed for the presence of contaminating tumor cells by immunocytology. In each case, the stem cells were found to be free of tumor. Eleven of the 12 patients underwent myeloablative therapy and PBSC rescue; five patients remain alive without disease progression, 28+ to 53+ months from diagnosis, and six patients have developed recurrent disease. We conclude that PBSCs can be successfully harvested from children with NB, and used for hematopoietic reconstitution following myeloablative chemotherapy. However, more effective therapy for poor-risk NB patients is still urgently needed.

Activity of a 40 kDa RNA-binding protein correlates with MYCN and c-fos mRNA stability in human neuroblastoma

Subclones of neuroblastic (N) and substrate adherent (S) cells have been established from neuroblastoma tumours cultured in vitro which differ in growth characteristics and MYCN expression. N cells derived from the NBL-W cell line (W-N) express 5-fold higher levels of MYCN mRNA and 10-fold higher levels of MYCN protein than S cells (W-S), despite having the same MYCN copy number. In an effort to identify the molecular mechanisms responsible for the disparity in steady-state MYCN levels, the rate of MYCN mRNA degradation was measured in the two subclones. The half-life of MYCN mRNA in the W-N cells was approximately 45 min compared to approximately 6 min in the W-S cells. Similarly, the half-life of another labile mRNA, c-fos, differed in W-N and W-S cells (30 min versus 15 min, respectively). The turnover of labile mRNAs is thought to be mediated by the interactions of trans-acting factors with AU-rich elements within the 3' untranslated region. RNA UV cross-linking assays using W-N cell lysate demonstrated abundant quantities of a protein, 40 kDa in size (p40), that bound specifically to AU-rich elements within the MYCN and c-fos 3' untranslated region. However, p40 was barely detectable in W-S cells. Our studies suggest that p40 may play a role in determining neuroblastoma phenotype by regulating MYCN and c-fos mRNA turnover.

Long-term neurologic outcome in children with opsoclonus-myoclonus associated with neuroblastoma: a report from the Pediatric Oncology Group

A retrospective data collection was performed on 29 children diagnosed with neuroblastoma and opsoclonus-myoclonus between 1983-1993 from Pediatric Oncology Group institutions. The aim was to describe neurologic outcome in children with neuroblastoma and opsoclonus-myoclonus. Age at diagnosis ranged from one month to 4 years (median age, 18 months). The duration of opsoclonus-myoclonus symptoms prior to the diagnosis of neuroblastoma ranged from 6 days to 17 months (median duration, 6 weeks). There was a prevalence of low stage disease according to the POG staging system: stage A (n = 18), stage B (n = 3), stage C (n = 7), stage D (n = 1). There was a predominance of paraspinal primary tumors. There was no case of Nmyc amplification (0/17), and 2/8 cases were diploid. Treatment for neuroblastoma consisted of surgery alone in 19/29 (18 stage A, 1 stage C in thorax), and surgery plus chemotherapy in 10/ 29. No patient received radiotherapy. Treatment for opsoclonus-myoclonus ranged varied. Six children received no treatment for opsoclonus-myoclonus. The following agents were used ACTH (n = 14), prednisone (n = 12), IV IgG (n = 6), immuran (n = 2), depakote (n = 1), and inderal (n = 1). Eighteen of 29 children (62%) had resolution of opsoclonus-myoclonus symptoms. The range of time for recovery was a few days to 3 years. However the majority recovered over several months. Twenty of 29 children (69%) had persistent neurologic deficits including speech delay, cognitive deficits, motor delay, and behavioral problems. Of the 9 children who had complete recovery of opsoclonus-myoclonus without neurologic sequelae, age at diagnosis and duration of symptoms were not different from the entire group. Interestingly, 6/9 children with complete recovery received chemotherapy as part of their treatment. In conclusion, persistent neurologic deficits are characteristic for children with neuroblastoma and opsoclonus-myoclonus. Treatment with chemotherapy may improve the neurologic outcome.

Genetic staging of unresectable or metastatic neuroblastoma in infants: a Pediatric Oncology Group study

BACKGROUND

Current staging systems for unresectable or metastatic neuroblastoma do not reliably predict responses to chemotherapy in infants under 1 year of age. Previous studies have indicated that the DNA content, or ploidy, of malignant neuroblasts can discriminate between good and poor responders in this group of patients, but the clinical utility of ploidy assessment has remained in question.

PURPOSE

We tested, in a prospective nonrandomized study, the hypothesis that neuroblast ploidy could be used as the sole guide for treatment selection in infants with unresectable or metastatic tumors and could differentiate between those who would respond to our previous standard regimen and those who would benefit from an immediate switch to another therapy.

METHODS

One hundred seventy-seven infants were enrolled in this trial. Five of these infants were subsequently excluded (two ineligible, two lacking ploidy information, and one protocol violation); therefore, 172 patients were included in the study. One hundred thirty infants with hyperdiploid tumors (DNA index > 1.0; better prognosis in retrospective studies) were treated with a well-tolerated regimen of cyclophosphamide (150 mg/m2 per day orally or intravenously on days 1-7) and doxorubicin (35 mg/m2 intravenously on day 8). Forty-two infants with diploid tumors (DNA index = 1.0; worse prognosis in retrospective studies) received cisplatin (90 mg/m2 intravenously on day 1) and teniposide (100 mg/ m2 intravenously on day 3) after an initial course of cyclophosphamide plus doxorubicin. Statistical end points were response and long-term survival. In addition, we assessed within each ploidy group (i.e., patients with hyperdiploid tumors and those with diploid tumors) the prognostic significance of NMYC gene copy number, tumor stage, and other variables commonly measured in this disease.

RESULTS

Of the 127 assessable infants with hyperdiploid tumors, 115 (91%) had complete responses--85 after receiving five courses of cyclophosphamide plus doxorubicin and 30 after receiving further therapy including cisplatin plus teniposide. The 3-year survival estimate for the entire hyperdiploid group was 94% (95% confidence interval [CI] = 89%-98%). Nineteen (46%) of 41 assessable infants with diploid tumors were complete responders. The overall 3-year survival estimate for this group was 55% (95% CI = 39%-70%). Prognostic factor analysis indicated that NMYC gene amplification and an elevated serum lactate dehydrogenase level were statistically significant markers of higher risk disease within the diploid group (two-sided P values of .005 and .003, respectively). Only NMYC was predictive in the hyperdiploid group (P = .003).

CONCLUSION

Use of a prognostic staging system based on tumor cell ploidy, augmented with the NMYC gene copy number and serum level of lactate dehydrogenase, would very likely improve the treatment of infants with unresectable or metastatic neuroblastoma. Patients with diploid tumors characterized by an amplified NMYC locus represent a particularly unfavorable risk group that may benefit from innovative new therapies.

Modulation of N-myc expression alters the invasiveness of neuroblastoma

N-myc oncogene expression plays a pivotal role in the biology of neuroblastoma, a common childhood tumor. High N-myc expression is associated with advanced disease stage, and in animal models, increased expression results in increased metastatic potential. In normal embryologic development, N-myc expression is associated with neuroblast migration out from the neural crest. To further define the relationship between N-myc and metastasis, an in vitro assay was adapted to measure tumor cell attachment, motility, and proteolytic ability in neuroblastoma cell lines. These parameters were examined in a non-amplified, uniformly N-myc overexpressing cell line and its anti-sense N-myc expressing clones. These lines have been characterized previously, and have a decrease in N-myc expression, growth rate, and tumorigenicity relative to the parent line and vector-only control transfectant. Decrease in N-myc expression resulted in a non-proportional increase of tumor cell attachment, and a proportional decrease in both tumor cell motility and proteolytic ability. In further experiments, assay of a N-myc-amplified overexpressing cell line with an intrinsic heterogeneous pattern of expression demonstrated that motile cells expressed higher amounts of N-myc relative to the general population. Together these relationships indicate that N-myc plays a causative role in the invasive phenotype, and suggest that metastasis may, in part, result from the disruption of a developmentally important normal process.

Binding of a 40-kDa protein to the N-myc 3'-untranslated region correlates with enhanced N-myc expression in human neuroblastoma

Subclones of neuronal (N) and non-neuronal (S) cells established from neuroblastoma tumors cultured in vitro differ in their growth characteristics and N-myc expression. N (W-N) cells derived from the NBL-W cell line express 5-fold higher levels of N-myc mRNA and 10-12-fold higher levels of protein than S cells (W-S), despite having the same N-myc copy number. This study demonstrates that the steady-state levels of N-myc are largely determined by differences in N-myc mRNA stability. The half-life of N-myc mRNA in the W-N cells is approximately 35 min compared with approximately 6 min in the W-S cells. Turnover of labile mRNAs is thought to be mediated in part by the interactions of trans-acting factors with elements within the 3'-untranslated region. RNA UV cross-linking assays using W-N cell lysate demonstrate abundant quantities of a protein complex that is 40 kDa in size (p40) that binds to the N-myc 3'-untranslated region. p40 is barely detectable in W-S cells. We have mapped two distinct regions within the 3'-UTR that specifically bind p40 (base pairs 5694-5715 and 6465-6482). Analysis of nine additional neuroblastoma cell lines shows that p40 activity correlates with enhanced expression of N-myc. p40 activity is also detected in 5 of 19 primary neuroblastomas, and activity is associated with clinically aggressive disease. In the accompanying study, we identify p40 as a member of the embryonic lethal abnormal vision (ELAV)-like family of RNA-binding proteins. Our studies suggest that ELAV-like proteins may play a role in the regulation of N-myc mRNA turnover and thereby modulate the steady-state levels of N-myc expression and tumor cell phenotype.

Differential activity of ELAV-like RNA-binding proteins in human neuroblastoma

Many short-lived mRNAs contain AU-rich instability elements within their 3'-untranslated region. Cellular factors that bind to these elements are thought to play a role in the regulation of mRNA degradation. In the accompanying paper (Chagnovich, D., and Cohn, S. L. (1996) J. Biol. Chem. 271, 33580-33586) we characterized the binding activity of a 40-kDa protein (p40) that interacts with high specificity with at least two AU-rich elements located within the 3'-untranslated region of N-myc. p40 activity correlates with N-myc mRNA stability in subclones of the NBL-W neuroblastoma cells line (W-N and W-S). In an effort to determine the identity of p40 we performed immunoblotting studies, immunoprecipitation experiments, and RNA gel mobility shift assays using antibodies that are directed against known RNA-binding proteins. In this paper we demonstrate that in W-N and W-S cells, p40 activity parallels the expression of embryonic letal abnormal vision (ELAV)-like proteins, and that antibodies directed against this family of RNA-binding proteins recognize p40. We also show that purified ELAV-like proteins (HuD and Hel-N1) bind with high specificity to the same N-myc 3'-untranslated region sequences as p40. Our data indicate that p40 is a member of the ELAV-like family, and suggest that this family of RNA-binding proteins may regulate N-myc mRNA turnover.

Physical mapping of the DDX1 gene to 340 kb 5' of MYCN

One of the most important prognostic factors in neuroblastoma is amplification of the MYCN gene, which is strongly associated with advanced stages of disease and a poor prognosis. Although the MYCN amplicon sometimes spans more than 1 Mb, no other consistently expressed sequences from the MYCN amplicon have been reported. However, DDX1, a gene encoding a DEAD box protein, was recently mapped to chromosome 2p24 and is frequently co-amplified with MYCN. Therefore, we performed genomic mapping with YACs to determine the physical relationship between DDX1 and MYCN, and whether DDX1 was contained within the core region of amplification. Based on YAC restriction mapping and content analysis, DDX1 maps 340 kb 5' of MYCN, outside the core domain of consistent amplification. Interestingly, we also determined by sequence analysis and detailed restriction mapping that G21, previously isolated as a 'neuroblastoma-specific' cDNA clone from an MYCN amplicon, is a partial cDNA of DDX1. Our data confirm that DDX1 is amplified in some but not all MYCN-amplified tumors, and that it is rearranged in other cases. This suggests that the co-amplification of DDX1 is due to its proximity to MYCN.

Analysis of possible WT1 RNA processing in primary Wilms tumors

WT1 RNA processing abnormalities have been suggested to play a role in the development of Wilms tumor by reports of editing at codon 280 in the rat WT1 transcript (codon 281 in humans) and aberrant splicing of exon 2 in WT1 transcripts from Wilms tumor xenograft cell lines. Both events result in a functionally changed WT1 protein and are potential mechanisms of altering normal protein function in the absence of WT1 DNA mutations. To determine whether either of these RNA processing events occurs in primary Wilms tumors, we analysed WT1 mRNA from 15 primary tumors. There was no evidence of WT1 RNA editing at codon 281, and only one primary tumor displayed aberrant splicing of exon 2. Sequence and Southern analysis of DNA from this tumor did not reveal any alteration in or around exon 2. These results suggest that neither RNA editing at codon 281 nor aberrant exon 2 splicing is a frequent mechanism of WT1 alteration during tumorigenesis.

Loss of DCC expression in neuroblastoma is associated with disease dissemination

DCC, a candidate tumor suppressor gene from chromosome 18q21, is most highly expressed in the developing nervous system. In vitro studies suggest a role for DCC in neuronal differentiation, and 18q allelic loss occurs in a subset of neuroblastomas. To address the hypothesis that loss of DCC function may contribute to tumorigenesis in cells of neural origin, we utilized a combination of RNase protection, immunoblotting, and immunohistochemical approaches to characterize DCC expression in 62 primary neuroblastomas and 16 neuroblastoma cell lines. The DCC protein was undetectable in 38% of the primary tumors and 56% of the cell lines. Of note, primary tumors lacking DCC expression were more likely to have been obtained from patients with disseminated or stage D disease (P = 0.01). In addition, loss of DCC expression was observed in three of six primary tumors from stage DS patients. No consistent relationship between the loss of DCC expression and N-myc amplification was observed in our studies. Our findings suggest that loss of DCC expression may contribute to the dissemination of neuroblastoma cells, perhaps through alterations in growth and differentiation pathways distinct from those regulated by N-myc.

Tumor angiogenesis correlates with metastatic disease, N-myc amplification, and poor outcome in human neuroblastoma

PURPOSE

To determine if the clinical outcome of children with neuroblastoma (NB) is correlated with the degree of tumor neovascularization and to assess the relationship of stage, N-myc copy number, and histology to angiogenesis.

MATERIALS AND METHODS

The vascularity of primary untreated NB from 50 patients diagnosed at a single institution between 1984 and 1994 was evaluated. An image processor was used to analyze the tumor tissue area for each histologic slide of tumor, and a vascular index (VI) was calculated, where VI = total number of vessels/mm2 of tissue area. Tumors were classified histologically according to the criteria of Shimada et al (J Natl Cancer Inst 73:405-416, 1984), and N-myc copy number was determined by Southern blot analysis.

RESULTS

We found that higher VI (> 4.0) in NB strongly correlated with widely disseminated disease (P = .006) and poor survival (P < .0001). VI more than 4.0 was also statistically associated with N-myc amplification (P = .02) and unfavorable histology (P = .02). Univariate analysis demonstrated that disease stage, tumor histology, and N-myc copy number were also predictive of outcome. Cox regression analysis showed that VI provided independent prognostic information.

CONCLUSION

Our studies indicate that angiogenesis may play an important role in determining the biologic behavior of NB. Antiangiogenic therapy may prove to be effective in the treatment of children with highly vascular, widely disseminated NB.

Expression of the gene for multidrug-resistance-associated protein and outcome in patients with neuroblastoma

BACKGROUND

Overexpression of the gene for the multidrug-resistance-associated protein (MRP) has been linked with resistance to chemotherapeutic agents (multidrug resistance) in vitro. The expression of MRP by neuroblastoma cells correlates with N-myc oncogene amplification, a well-established prognostic indicator in patients with neuroblastoma.

METHODS

To relate MRP gene expression to established prognostic markers and the clinical outcome of neuroblastoma, we analyzed MRP expression in specimens of primary tumors from 60 patients with neuroblastoma.

RESULTS

Levels of MRP gene expression were significantly higher in tumors with N-myc amplification than in tumors without such amplification (P < 0.001). High levels of MRP expression were strongly associated with reductions in both survival and event-free survival (P < 0.001) in the overall study population and in subgroups of patients without N-myc amplification and patients with localized disease. For the overall study population, the five-year cumulative survival rates in the groups with high and low levels of MRP expression were 57 percent (95 percent confidence interval, 37 to 78 percent) and 94 percent (95 percent confidence interval, 86 to 100 percent), respectively. In contrast, expression of the MDR1 multi-drug-resistance gene was not predictive of survival or event-free survival. After adjustment by multivariate analysis for the effects of N-myc amplification and other prognostic indicators, high levels of MRP expression retained significant prognostic value for poor survival (relative hazard, 14.9; P = 0.01) and poor event-free survival (relative hazard, 9.7; P = 0.004), whereas N-myc amplification had no prognostic value.

CONCLUSIONS

High levels of MRP gene expression in patients with neuroblastoma correlate strongly with poor outcome. The findings suggest that expression of this multidrug-resistance gene accounts for the association between N-myc amplification and reduced survival.

Up-regulation of HOXC6, HOXD1, and HOXD8 homeobox gene expression in human neuroblastoma cells following chemical induction of differentiation

An early event in the pathogenesis of neuroblastoma (NB), a tumor derived from embryonal neural crest tissue, appears to be the arrested differentiation of neuroblasts. However, NB cells can be induced to differentiate in vitro with numerous chemicals including retinoic acid (RA) and dibutyryl cyclic AMP (db-cAMP). One family of transcription factors, encoded by the homeobox (HOX) genes, plays a crucial role in Drosophila, Xenopus, and mammalian embryonic differentiation and development. We have previously identified six HOX genes (HOXC6, HOXC8, HOXD1, HOXD4, HOXD8, and HOXD9), by a sensitive PCR-based approach, in a cDNA library prepared from the human LA-N-5 NB cell line induced to differentiate with RA. In this report, we studied the regulation of these six HOX genes in a series of NB cell lines chemically induced to differentiate. Untreated NB cells express low or undetectable levels of HOX mRNA, and HOXC8 remains undetectable in the induced cells. However, a significant induction of HOXC6, HOXD1, and HOXD8 expression is seen in the RA-treated NB cell lines, albeit with different patterns and degree of up-regulation. db-cAMP treatment also induced HOXC6 and HOXD8 expression in two of the three NB cell lines analyzed. Low levels of HOXD4 and HOXD9 induction were observed in two and one RA-treated NB cell line, respectively. Up-regulation of HOXC6, HOXD1, and HOXD8 expression in human NB cells, chemically induced to differentiate, appears to be associated with maturation toward a differentiated neuronal phenotype.

Co-amplification and concomitant high levels of expression of a DEAD box gene with MYCN in human neuroblastoma

MYCN gene amplification is strongly correlated with poor prognosis in neuroblastoma (NB), the second most common solid pediatric tumor. However, increased MYCN expression seen in tumors that lack MYCN amplification does not correlate with aggressive clinical behavior. Whereas the MYCN gene spans only 7 kb, the MYCN amplicon has been shown to range in size from 350 kb to more than 1 Mb. Given the large size of the amplicon, it is possible that additional genes are co-amplified in NBs whose expression may contribute to the aggressive phenotype associated with MYCN-amplified tumors. We isolated a cDNA clone from a human NB library that is identical to DDXI, a gene recently reported to be preferentially expressed in two retinoblastoma cell lines that also express high levels of MYCN. DDXI belongs to a family of genes that encode DEAD (Asp-Glu-Ala-Asp) box proteins, putative ATP-dependent RNA helicases implicated in a number of cellular processes involving alterations of RNA secondary structure. We examined the frequency of DDXI amplification in 15 NB cell lines, 1 neuroepithelioma cell line, and 122 NB tumors by Southern blot analyses, and we found that 7 of 10 MYCN-amplified cell lines and 27 of 40 (68%) MYCN-amplified tumors also harbored multiple copies of the DDXI gene. Amplification of DDXI was associated with high levels of DDXI mRNA expression in the NB cell lines and tumors as examined by Northern analysis. Neither DDXI gene amplification nor enhanced expression was observed in tumors or cell lines that lacked MYCN amplification. Because RNA helicases play important roles in both post-transcriptional and translational gene regulation, high levels of DDXI expression consequent to genomic amplification may contribute to the malignant phenotype of a subset of NBs.

Lack of correlation of N-myc gene amplification with prognosis in localized neuroblastoma: a Pediatric Oncology Group study

Multiple copies of N-myc proto-oncogene are only rarely detected in localized neuroblastomas (NBs), and the prognostic relevance of amplification in this subset of patients is not clear. We analyzed a series of 850 children with NB admitted to a Pediatric Oncology Group NB Biology Study and identified six patients with localized NBs harboring N-myc gene amplification. Three patients whose tumors showed favorable histology by Shimada classification and low-risk histological features according to the Joshi classification have remained disease-free, whereas two of three patients with unfavorable histology tumors have developed recurrent disease. Although earlier studies have indicated that N-myc amplification is associated with diploid DNA content, flow cytometric analysis revealed that only two of the localized tumors contained stem lines with diploid DNA content. Loss of chromosome 1p was not detected by fluorescence in situ hybridization in the two tumors examined. N-myc protein was detected by immunohistochemical studies in four of the five NBs analyzed. However, N-myc protein was not visualized in one of the tumors with stroma-rich histology, and Western blot analysis revealed only low levels of N-myc protein expression in another NB with favorable histology. These studies indicate that the presence of N-myc amplification in localized NBs does not necessarily portend an adverse outcome. Furthermore, the biological features of this subset of N-myc-amplified NBs appear to differ from those of more advanced N-myc-amplified tumors.