Neuroblastoma is an aggressive pediatric
tumor that accounts for 15% of
cancer-related deaths in children. More than half of high-risk
neuroblastoma patients develop
tumor relapse that is lethal in most cases. A small population of tumor-initiating cells (
TICs), recently identified from high-risk
neuroblastoma patients as spheres, is believed to be responsible for
tumor relapse. Rab family
small G proteins are essential in controlling membrane traffic and their misregulation results in several
cancers. Rab15 was originally isolated as a brain-specific Rab
protein regulating the endocytic recycling pathway and was recently identified as a downstream target of the neural
transcription factor Atoh1. Previously, we identified two alternatively spliced Rab15
isoforms in
neuroblastoma cells and showed a significant correlation between Rab15 expression and neuronal differentiation. As aberrant alternative splicing is intimately associated with an increasing number of
cancers, its use as a new diagnostic and/or prognostic
biomarker has attracted considerable attention. In the present study, we explored
cancer-associated changes of Rab15 alternative splicing in
neuroblastoma TICs. We found that Rab15 alternative splicing generated two novel
isoforms designated as Rab15(AN2) and Rab15(AN3) in addition to two known
isoforms designated as Rab15(CN) and Rab15(AN1). Although both Rab15(AN2) and Rab15(AN3) contained
premature termination codons, they were detected in not only
neuroblastoma cells but also in normal human tissues. One
isoform was predominantly expressed in the brain and testis, while the other
isoform was more specifically expressed in the brain. In
neuroblastoma, Rab15
isoform balance measured by the Rab15(CN)/Rab15(AN1+AN2+AN3) ratio was significantly decreased in spheres compared to parental cells. These results suggest that Rab15 alternative splicing may serve as a
biomarker to discriminate
TICs from non-
TICs in
neuroblastoma.