Identification and characterization of candidate genes for neuroblastoma development

Abstract

Aim: Neuroblastoma (NB) is the most common tumor during infancy. It arises from undifferentiated cells in the sympathetic nervous system and is characterized by both clinical and genetic heterogeneity. One of the features of NBs with unfavorable outcome is loss on distal chromosome 1p. The aim of this thesis was to identify genetic factors with relevance for NB tumorigenesis, with special emphasis on the tumor suppressor candidate region on chromosome 1p36.2.Results: All known genes (UBE4B, KIF1B, PGD, CORT, DFFA and PEX14) in the hotspot NB tumor suppressor gene region on 1p36.22 were screened for mutations by capillary-based sequencing of DNA from a large series of NB tumors. The potential tumor suppressor properties of the genes ENO1, ICAT and CASP9, located close to this region, were also analyzed. A few mutations with predicted deleterious effects on protein function were found in the apoptotic regulator DFFA and in the UBE4B gene, which is involved in selective protein degradation. We were also able to show that expression of ENO1, DFFA, UBE4B and CASP9, critical for induction of the apoptotic cascade, is down-regulated in tumors with unfavorable outcome. We identified a novel gene, denoted APITD1, in 1p36.22; two alternative transcripts of this gene were abundantly expressed in normal adult and fetal tissues. Primary NB tumors showed either very weak or no measurable APITD1 expression and the lowest levels were detected in tumors with unfavorable outcome. The coding sequence of APITD1 was well-conserved in different species and no coding variations were found in primary NB tumors. Anti-proliferative and apoptotic effects were observed after over-expression of APITD1 and ENO1 in NB cell lines and in embryonal kidney- derived cells. A mutated ENO1 construct was used to show that the glycolytic enzyme á-enolase had as strong an effect on cell proliferation as the other translational product, MBP1, which is a negative regulator of the c-Myc oncogene. Biallelic inactivation of the transcription factor PHOX2B on chromosome 4p was found in a patient with multifocal NB, emphasizing the importance of early embryologic differentiation pathways in tumors of neural crest origin. Conclusions: A few rare mutations were found in genes with possible tumor suppressor functions. Several genes on 1p36.2 were also shown to be down-regulated in aggressive tumors. We therefore propose that the poor prognosis for NB patients with 1p deletions is a syngenic effect of mutations or transcriptional silencing of several genes in this region.