Developmental apoptosis of neural crest precursors is crucial in determining the final number of terminally differentiated sympathetic neurons. An aberrant developmental apoptotic pathway is implicated in neuroblastoma. During development, when NGF becomes limiting, neuroblasts undergo apoptosis via KIF1Bb, a gene that resides on chr1p36.2 that is frequently deleted in neuroblastoma.
In order to understand the mechanism behind KIF1Bb-induced apoptosis in neuroblastoma, we employed various cell and molecular techniques, unbiased screens, next-generation sequencing, animal models and patient studies.
We identified that KIF1Bb-induced apoptosis requires RNA/DNA helicase DHX9. KIF1Bb interacts with DHX9 to enhance translocation and accumulation of cytoplasmic DHX9 in the nucleus, resulting in expression of apoptotic XAF1. Transcription-incompetent DHX9 is unable to potentiate KIF1Bb-induced cell death. Knockdown of DHX9 also protects from KIF1Bb-induced cell death whereas KIF1Bb loss-of-function domains or patient-associated point mutants are unable to translocate and accumulate cytoplasmic DHX9 in the nucleus, impairing XAF1 expression. Furthermore, XAF1 silencing protects from KIF1Bb-induced and NGF withdrawal-dependent apoptosis in vitro and ex vivo as well as promoting tumor growth in vivo whereas XAF1 overexpression is necessary and sufficient to induce apoptosis in vitro and delays tumor growth in vivo. Conditional knockout of KIF1Bb in the superior cervical ganglia neuroblasts of mouse pups specifically ablates XAF1 expression in vivo and ex vivo, suggesting that KIF1Bb and XAF1 act along the same pathway. Importantly, analysis of tissue microarray data of neuroblastoma patients in conjunction with clinical correlates revealed that XAF1 expression might have prognostic value in determining survival outcomes for post-treatment and overall neuroblastoma cases.
Our findings provide a mechanistic understanding on neuroblastoma development that opens a window of opportunity for personalized therapeutic intervention and prognostication of 1p36-deleted neuroblastoma patients based on KIF1Bb/DHX9/XAF1 biology.