CCLG prizewinner: RNA helicase A is essential for 1p36 gene KIF1Bb tumour suppression in neuroblastomas


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Zhi Xiong Chen1
1Ludwig Institute for Cancer Research, Solna, Sweden and National University of Singapore, Singapore, Singapore

Abstract

Background

Developmental apoptosis of neuronal precursors is crucial in determining the final number of terminally differentiated cells. During neural development, cells undergo apoptosis as growth factors such as NGF becomes limiting. Abnormal NGF signaling or aberrant developmental apoptosis is implicated in pediatric nervous system tumors. Several genes act upon a developmental apoptotic pathway that is activated when NGF becomes limiting for neuronal progenitors and requires KIF1Bb. KIF1Bb is necessary and sufficient for neuronal apoptosis during NGF withdrawal. KIF1Bb maps to 1p36.2, a region that is frequently deleted in neural crest-derived tumors including neuroblastomas.

Method

Large-scale immunoprecipitation followed by mass spectrometry, cloning and mutagenesis studies, apoptosis assays, immunofluorescence, lentiviral expression or shRNA-based studies, siRNA silencing, RNA-SEQ, RT-PCR, immunohistochemistry, NGF withdrawal experiments, patient studies and mouse developmental models are the key methods used.

Results

We identified a transcriptional basis for KIF1Bb-induced apoptosis, which requires a RNA/DNA helicase known as RNA helicase A (DHX9). KIF1Bb interacts with DHX9 to promote translocation of cytoplasmic DHX9 into the nucleus, resulting in transcription of apoptotic XIAP-associated factor 1 (XAF1). Transcription-impaired or nuclear localization-impaired DHX9 is unable to potentiate KIF1Bb-induced cell death. Knockdown of DHX9 also protects from KIF1Bb-induced cell death whereas KIF1Bb negative mutants are unable to translocate cytoplasmic DHX9 into the nucleus. Furthermore, XAF1 silencing protects from KIF1Bb-induced apoptosis. In addition, a genome-wide shRNA library loss-of-function screen revealed a DHX9-interacting transcription factor ZIC2 that is deemed crucial for KIF1Bb-induced apoptosis. This further suggests that a DHX9-dependent transcriptional program initiated by KIF1Bb is required to induce apoptosis in neuroblastomas.

Conclusion

Recent literature strongly pointed to KIF1Bb as a bonafide tumor suppressor. Our findings provide a mechanistic understanding of this role, whereby KIF1Bb interacts with cytoplasmic DHX9 leading to its accumulation in the nucleus to initiate a unique transcriptional signature that includes apoptotic effectors such as XAF1.