Little is known about the biology of MEPN. Clinical trial design requires a better understanding of tumor biology. This is particularly relevant to pediatric MEPN, which have a higher recurrence rate than adult MEPN. The present study uses microarray technology to identify aberrantly expressed genes in pediatric MEPN compared to pediatric intracranial EPN. The overexpression of three genes of interest, HOXB13, NEFL and PDGFRα, was recapitulated by IHC.
Numerous HOX family genes were found to be overexpressed in MEPN compared to intracranial EPN. This finding was verified in part by immunohistochemical staining of HOXB13. As predicted by the microarray data, HOXB13 protein was more highly expressed in MEPN than intracranial EPN. Overexpression of homeobox genes by microarray analysis of mRNA has been previously reported in spinal EPN but not MEPN. A study by Taylor et al
reported HOX family genes to be overexpressed in spinal EPN from three adults, compared to intracranial pediatric and adult EPN (39)
. Palm et al
also reported significant overexpression of HOX family genes in 14 spinal EPN and two EPN from the filum terminale (27)
. The authors did not stipulate whether their filum terminale examples had MEPN morphology. Although the present study did not include spinal EPN in the microarray analysis, HOXB13 protein expression was not observed in 7 of the 8 spinal EPN. This discrepancy may be due to differences in experimental approach, i.e. protein expression by IHC versus gene expression by mRNA microarray analysis.
The HOX family of genes specifies the patterning of body segments along the anterior–posterior axis by encoding homeodomain transcription factors essential for embryonic development (4
). HOX groups 10–13 are associated with the lumbar/sacral region where MEPN arise (43)
. Accordingly, the four most upregulated HOX genes in MEPN were HOXB13, HOXC10, HOXA13 and HOXD10. Thus, overexpression of HOX groups 10–13 in MEPN may potentially be a function of location. However, normal adult and fetal spinal ependyma and filum terminale were negative for HOXB13 by IHC. Relatively late gestational time periods were available for study (i.e. three fetuses of 18, 23 and 35 weeks' gestation) and HOXB13 might well have been present at earlier gestational time periods. It is probable that HOX groups 10–13 are expressed in lumbar ependyma early in fetal development and then switched off following segmentation. That HOXB13 was not found in normal fetal and adult ependyma suggests that HOX genes are aberrantly expressed in MEPN.
Growing evidence indicates that abnormal HOX gene expression may be involved in oncogenesis. Aberrant HOX gene expression has been noted in acute myeloid and mixed lineage leukemia (9
), breast (29)
, cervical (34)
, non-small cell lung (28)
, ovarian (6)
, prostate (42)
, skin (24)
and thyroid cancers (38)
. The overexpression of HOX genes in MEPN combined with their potential oncogenic function suggests that the HOX family should be evaluated as a potential therapeutic target. The design of HOX family inhibitors is currently being studied, but is hindered by the functional redundancy of HOX transcription factors and the common co-expression of multiple HOX genes (28)
NEFL was highly overexpressed in MEPN. Neurofilament light polypeptide (68 kDa) is a Class IV intermediate filament expressed in neurons (44)
. Two homeobox genes that are significantly overexpressed in MEPN compared to intracranial EPN, HOXA3 and HOXA9, have transcription-factor binding sites near the NEFL gene (40)
. Further research is needed to determine if the concurrent expression of these genes is related. Although the high expression of NEFL is significant to the molecular biology of MEPN, it is a poor therapeutic target due to its ubiquitous expression in axonal processes.
Upregulation of PDGFRα in MEPN suggests that therapeutic targeting of this receptor tyrosine kinase may be an appropriate candidate for future clinical trials. Several PDGFRα inhibitors are FDA-approved, including imatinib mesylate, sorafenib and sunitinib (1
). Although MEPN response to PDGFRα inhibitors is currently unknown, Fakhrai et al
reported a case study in which a recurrent spinal ependymoma with positive PDGFRα IHC demonstrated partial remission during treatment with imatinib mesylate (15)
An acknowledged limitation of this study is the relatively small number of pediatric MEPN available for gene expression profiling (n = 5). It should be noted, however, that rapidly frozen surgical specimens are required for this gene expression microarray technique and MEPN are not common in the pediatric age group. Indeed, at the busy tertiary care pediatric hospital where the specimens were obtained, the five MEPN represent virtually every MEPN seen at the institution and accrued over the past 10 years. Thus, it is not surprising that the current study presents the largest cohort to date in the literature that examined pediatric MEPN. The number and diversity of EPN types was increased by including adult patients for the IHC portion of this study, as compared to the gene expression microarray portion of the study which was confined to pediatric MEPN and pediatric EPN. This allowed us to include two adult EPN types that are seldom found in pediatric patients, i.e. SEPN and spinal EPN.
Literature on the molecular biology of MEPN is scarce and impedes progress in the treatment of recurrent tumors. The present study sought to address a specific clinical problem in a grade of tumor that has received relatively little attention despite its ability to recur and disseminate. The study reports the top over- and underexpressed genes in pediatric MEPN that distinguish them from intracranial EPN and identifies at least one potential therapeutic target for recurrent MEPN. Future studies with larger cohorts are needed to confirm our microarray and protein expression findings.