(American Journal of Pathology. 1999;155:17-21.)
© 1999 American Society for Investigative Pathology
Use of Tumor-Specific Gene Expression for the Differential Diagnosis of Neuroblastoma from Other Pediatric Small Round-Cell Malignancies
Jayne Gilbert,
Michelle Haber,
Sharon B. Bordow,
Glenn M. Marshall and
Murray D. Norris
From the Children's Cancer Institute Australia for Medical
Research, Sydney, Australia
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Abstract
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The differential diagnosis of neuroblastoma from other small
round-cell tumors of childhood, although clinically of great
importance, is sometimes difficult due to the almost
indistinguishable appearance of such tumors by conventional microscopy.
Because neuroblastomas are characterized by the synthesis of
catecholamines, we investigated the possibility that expression
of genes involved in this pathway could serve as a molecular marker for
this disease. A reverse transcriptase polymerase chain reaction assay
was used to analyze expression of tyrosine hydroxylase and dopa
decarboxylase in 84 pediatric malignancies including 55
neuroblastomas, 6 Ewing's sarcomas/primitive neuroectodermal
tumors, 7 lymphomas, 6 leukemias, 2
rhabdomyosarcomas, 6 osteosarcomas, and 2
phaeochromocytomas. Of the 55 neuroblastoma samples analyzed,
54 expressed clearly detectable levels of both genes. The one sample
that did not express either of the genes was rediagnosed both
clinically and by molecular genetic analysis as a Ewing's sarcoma. Of
the 29 non-neuroblastoma tumor samples examined, the only tumor
samples that expressed clearly detectable levels of both tyrosine
hydroxylase and dopa decarboxylase were phaeochromocytomas. Like
neuroblastomas, these tumors are characterized by high levels
of catecholamines. These findings suggest that expression of
genes involved in catecholamine biosynthesis may be useful for
differentiating neuroblastoma from other small round-cell tumors of
childhood.
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Introduction
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Neuroblastoma is the most common solid tumor of early childhood.
Although patients with localized disease have a favorable prognosis,
the majority of children with neuroblastoma present with metastases and
have a poor outcome despite intensive multimodal therapy.1
The accurate diagnosis of this disease and other pediatric malignancies
has become increasingly important with
the continued development of treatments tailored to specific tumor
types, and the resultant improvement in survival rates.2
Neuroblastoma, together with lymphoma, osteosarcoma, Ewing's family of
tumors, rhabdomyosarcoma, and lymphoblastic leukemia, all belong to a
group of undifferentiated pediatric malignancies known as the small
round-cell tumors of childhood. In some instances, the differential
diagnosis of this group of tumors can prove difficult,2
due to the fact that they share morphological similarities that can
make them indistinguishable by conventional light microscopy.
The accurate diagnosis of small round-cell tumors can in some cases be
facilitated by cytogenetic and, more recently, by molecular biological
analysis. Thus, for example, the Ewing's family of tumors, consisting
of Ewing's sarcoma and primitive neuroectodermal tumors (PNET), is
characterized by the genetic abnormality of a chromosomal translocation
at t(11;22) in the majority of cases and the less common t(21;22) in a
small number of cases.3,4
Recent molecular advances have
allowed for the PCR-based detection of such
translocations.3,5
However, many of the small round-cell
tumors of childhood, including neuroblastoma, do not have consistent
molecular genetic abnormalities amenable to either cytogenetic or DNA
analysis. Because neuroblastomas are characterized by the secretion of
catecholamines, we have investigated the possibility of employing
expression of genes involved in the catecholamine biosynthetic pathway
as potential molecular markers for this disease. The results
demonstrated that coexpression of two genes, tyrosine hydroxylase and
dopa decarboxylase, appears to be highly specific for neuroblastoma and
suggest that these markers may aid in distinguishing neuroblastoma from
other small round-cell tumors of childhood.
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Materials and Methods
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Tumor Samples
Samples of 55 primary neuroblastoma tumors from untreated
patients, obtained either from the Neuroblastoma Tumor Bank of the
U. S. Pediatric Oncology Group (Memphis, TN), or from the
Sydney Children's Hospital, Sydney, Australia, and representing all
clinical stages, have been described previously.6
The 29
non-neuroblastoma tumor samples were obtained at diagnosis from
patients presenting at the Sydney Children's Hospital included 2
phaeochromocytomas, 6 Ewing's sarcomas/PNETs, 7 lymphomas, 6
leukemias, 2 rhabdomyosarcomas, and 6 osteosarcomas. All samples were
taken during the course of the patients' routine management.
Analysis of Gene Expression by Polymerase Chain Reaction
Total cellular RNA was isolated from frozen tumor tissue as
previously described.7
High quality intact RNA was
routinely obtained from over 95% of tumors processed. Complementary
DNA (cDNA) was synthesized from 2-µg aliquots of RNA with random
hexanucleotide primers and Moloney murine virus reverse
transcriptase.8
Aliquots of cDNA corresponding to 50 ng of
RNA were amplified in a well-established reverse transcriptase
polymerase chain reaction (RT-PCR) assay,9
which involved
co-amplification of the target gene sequence (tyrosine hydroxylase or
dopa decarboxylase, respectively) with a control sequence
(ß2-microglobulin), for 30 cycles
using gene-specific oligonucleotide primers. For amplification of dopa
decarboxylase, the forward and reverse primers, respectively, were
5'-GGGGACCACAACATGCTGCTC-3' and 5'-CCACTCCATTCAGAAGGTGCC-3'.
The primers for ß2-microglobulin
have previously been described.8
Following an initial
denaturation step of 3 minutes at 94°C, each cycle consisted of
denaturation at 94°C for 45 seconds, primer annealing at 55°C for
45 seconds, and primer extension at 72°C for 90 seconds. Where
indicated, PCR amplification of dopa decarboxylase was performed in the
absence of ß2-microglobulin primers for 35
cycles under the same PCR conditions. Due to the high guanine and
cytosine content of the tyrosine hydroxylase gene, an annealing
temperature of 72°C was used for PCR reactions amplifying this gene.
The forward and reverse primers for tyrosine hydroxylase
were 5'-CCCTGACCTGGACTTGGACCACCC-3' and
5'-TCTCCTCGGCGGTGTACTCCACAC-3', respectively.
ß2-microglobulin gene-specific primers suitable
for amplification at this elevated annealing temperature were
5'-ATGTCTCGCTCCGTGGCCTTAGCTG-3' and
5'-TCCATTCTCTGCTGGATGACGTGAG-3' for the forward and reverse
primers, respectively. Following electrophoresis on 12% polyacrylamide
gels and staining with ethidium bromide, PCR products were visualized
and photographed under ultraviolet transillumination. In all cases,
analyses were performed in triplicate.
RT-PCR analysis for the t(11;22) translocation specific to the Ewing's
family of tumors was performed essentially as described.10
Briefly, cDNA was synthesized from 1 µg of total RNA using random
hexanucleotide primers and the entire mixture was subjected to PCR
amplification with t(11;22) translocation-specific
primers.3
Following an initial denaturation step of 10
minutes at 96°C, each cycle consisted of denaturation at 96°C for
30 seconds, primer annealing at 65°C for 1 minute, and primer
extension at 75°C for 1 minute for a total of 40 cycles. PCR products
were subjected to agarose gel electrophoresis and photographed.
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Results
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To determine the frequency and specificity of tyrosine hydroxylase
and dopa decarboxylase gene transcripts in neuroblastoma and other
pediatric malignancies, expression of these target genes was evaluated
in 84 tumor samples using a competitive RT-PCR assay. Included were 55
neuroblastomas, 6 Ewing's sarcomas/PNETs, 7 lymphomas, 6 leukemias, 6
osteosarcomas, 2 rhabdomyosarcomas, and 2 phaeochromocytomas. The
phaeochromocytomas were included as positive controls, because this
tumor type is also characterized by high levels of catecholamine
secretion.11
After RT-PCR, clearly detectable tyrosine
hydroxylase gene expression was evident in 54 of 55 neuroblastoma
specimens, as shown by a representative gel in Figure 1A
. For the remaining tumor sample (Panel
A, lane 15; tumor no. AG), however, no expression of this gene could be
discerned. In contrast to this result, tyrosine hydroxylase gene
expression was undetectable in 22 of the 29 non-neuroblastoma tumor
specimens (Figure 1B)
. Of the seven specimens in which any tyrosine
hydroxylase expression was evident, only the phaeochromocytoma positive
controls displayed levels of expression comparable to those of
neuroblastoma. The remaining five specimens, comprising one Ewing's
tumor, one leukemia, two osteosarcomas, and one rhabdomyosarcoma,
displayed very low levels of tyrosine hydroxylase gene expression, as
evidenced by the tyrosine hydroxylase PCR product being considerably
weaker than the control ß2-microglobulin
product in each case (Figure 1B)
. No tyrosine hydroxylase expression
could be detected in any of the lymphomas.

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Figure 1. Expression of tyrosine hydroxylase
(TH) in pediatric tumors
following competitive RT-PCR analysis as described in the Materials and
Methods. A: Tyrosine hydroxylase gene expression in primary
neuroblastoma tumors of stage I (lanes 12), stage II
(lanes 35), stage III (lanes 69), stage IV
(lanes 1013), stage IVs (lane 14) and tumor no. AG
(lane 15). B: Tyrosine hydroxylase gene expression in
phaeochromocytomas (lanes 12), Ewing's family of tumors
(lanes 38), lymphomas (lanes 915), leukemias
(lanes 1621), osteosarcomas (lanes 2227) and
rhabdomyosarcomas (lanes 2829). W, water control;
ß2M, ß2-microglobulin.
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Variable but clearly detectable expression of the dopa decarboxylase
gene was similarly evident in the 54 neuroblastoma specimens which had
been positive for tyrosine hydroxylase expression (Figure 2A)
. The one sample which had failed to
express tyrosine hydroxylase (tumor no. AG) also showed no dopa
decarboxylase expression (Panel A, lane 15). The complete absence of
dopa decarboxylase expression from this sample was confirmed by
conducting PCR amplification of the target gene in the absence of
competing ß2-microglobulin gene primers. Even
under these more stringent conditions, no PCR product for dopa
decarboxylase was evident (data not shown). In contrast, apart from the
two phaeochromocytoma controls, expression of dopa decarboxylase was
not observed in any of the other 27 non-neuroblastoma tumor samples
examined (Figure 2B)
.

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Figure 2. Expression of dopa decarboxylase
(DDC) in pediatric tumors
following competitive RT-PCR analysis. The legend to this figure is the
same as Figure 1
.
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Further investigation of tumor no. AG, which had failed to express
either tyrosine hydroxylase or dopa decarboxylase, revealed that the
patient had independently been clinically rediagnosed as having
Ewing's sarcoma. To confirm this clinical diagnosis, RT-PCR analysis
was performed using PCR primers which amplify the t(11;22) hybrid
transcript believed to be specific for Ewing's family of
tumors.3
Following amplification, a positive PCR product
was obtained from this tumor sample (Figure 3
, lane 6), consistent with the positive
result obtained from the other Ewing's sarcoma samples included in
this study. The differing sizes of the products reflect differences in
the length of the hybrid transcripts, which can vary depending on the
location of the translocation breakpoints.3,5
Sequencing
of the PCR product from tumor no. AG confirmed a Ewing's-specific
transcript (data not shown).

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Figure 3. RT-PCR analysis of the Ewing's
t(11;22) transcript.
Lanes 15: Five cytogenetically confirmed Ewing's sarcoma
specimens. Lane 6: Tumor no. AG. W, water control.
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Discussion
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The increased survival rates observed over recent decades in
pediatric malignancies have resulted largely from the development and
streamlining of specific therapies for specific tumor
types.2
Accordingly, accurate diagnosis has become
increasingly important in order to achieve optimal treatment. Despite
improvements in molecular genetic techniques, differential diagnosis
can still present a difficulty, particularly among the small round-cell
tumors of childhood. For neuroblastoma, there have been reports of this
disease being misdiagnosed as acute leukemia; often the correct
diagnosis is made only postmortem.12,13
Similarly, Kessler
et al14
reported 5 cases of intrarenal neuroblastoma that
were originally misdiagnosed as Wilms' tumor, despite the fact that
Wilms' tumor is not included in the small round-cell tumors of
childhood. Thus, there is a continuing need for the development of
methods to improve the differential diagnosis of these malignancies.
The present findings suggest that among the small round-cell tumors of
childhood, coexpression of genes involved in the catecholamine
biosynthetic pathway is a molecular characteristic specific to
neuroblastoma.
Tyrosine hydroxylase is the first and rate-limiting enzyme in the
catecholamine biosynthetic pathway, thus suggesting a requirement for
tight regulatory control of the gene encoding this enzyme. This indeed
appears to be the case, since the use of tyrosine hydroxylase
expression as a specific marker of residual neuroblastoma cells has
been demonstrated in numerous studies involving the detection of
circulating tumor cells in patient's peripheral blood and bone marrow
samples.15-19
In the present study, the expression of
tyrosine hydroxylase in the non-neuroblastoma tumor specimens was at
very low levels, suggesting that it is unlikely to play any significant
biological role. Such low-level expression might be due either to
contamination of the tumor sample by surrounding tissue or to the
phenomenon known as illegitimate transcription.20
Illegitimate transcription is the expression of otherwise
tissue-specific genes in any cell type, with the detection of such
expression resulting from the increased sensitivity afforded by
PCR.20,21
Interestingly, of the two genes investigated in
the present study, expression of dopa decarboxylase demonstrated
greater specificity for neuroblastoma than did expression of tyrosine
hydroxylase. Using coexpression of two target genes as a specific
marker of neuroblastoma affords increased confidence in the accuracy of
the differential diagnosis. In this regard, Hoon et al22
have recently reported that RT-PCR analysis performed with more than
one target gene provides a more sensitive and reliable means of
detecting residual melanoma than assays using only a single target
gene.
A number of chromosomal abnormalities have proven useful in the
characterization of individual pediatric disease types, as has been
demonstrated in the present context by the use of the t(11;22)
translocation for the diagnosis of Ewing's sarcoma. Similarly, for
rhabdomyosarcoma, recent studies have shown that the t(2;13) and
t(1;13) translocations are valuable in the diagnosis of alveolar
rhabdomyosarcoma,23,24
whereas a number of genetic
rearrangements occurring in leukemia have also proven useful as markers
for the detection of that disease.25,26
For neuroblastoma,
although aggressive tumors are often characterized by deletions of
chromosome 1p and amplification of the N-myc oncogene, these
abnormalities occur, on average, in 30% or less of
tumors.27,28
In contrast, the present results suggest that
coexpression of tyrosine hydroxylase and dopa decarboxylase is
ubiquitous in neuroblastoma and hence a useful molecular diagnostic
marker for this malignancy.
As molecular techniques become increasingly valuable in the diagnosis
and detection of childhood malignancies, their limitations are also
becoming evident. For example, recent reports have identified the
t(11;22) translocation in confirmed cases of
rhabdomyosarcoma29
as well as
neuroblastoma,10
indicating that it is possible for tumor
markers to be present in more than one disease. Such findings highlight
the need for a panel of molecular markers to maximize the likelihood of
making the correct differential diagnosis. The results presented here
suggest that expression of the tyrosine hydroxylase and dopa
decarboxylase genes may prove a valuable aid in the differential
diagnosis of neuroblastoma from other small round-cell tumors of
childhood.
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Acknowledgements
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We thank the Neuroblastoma Biology Subcommittee of the Pediatric
Oncology Group for the review and approval of this research project and
for the samples of neuroblastoma tumors provided for analysis.
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Footnotes
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Address reprint requests to A/Prof. M. D. Norris, Children's Cancer Institute Australia for Medical Research, High Street (P.O. Box 81), Randwick, New South Wales, Australia, 2031.
Supported by grants to M. D. N., M. H., and G. M. M. from the National Health and Medical Research Council of Australia and the New South Wales Cancer Council. J. G. is the recipient of an Australian Postgraduate Award.
Children's Cancer Institute Australia for Medical Research is affiliated with the University of New South Wales and Sydney Children's Hospital.
Accepted for publication April 9, 1999.
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