ABSTRACT Most cancer deaths result from progression of therapy resistant disease, yet our understanding of this phenotype is limited. Cancer therapies generate stress signals that act upon mitochondria to initiate apoptotic programs. We isolated mitochondria from neuroblastoma cell lines obtained from children at diagnosis and after relapse following failed therapy, and profiled responses to tBid and Bim, death effectors activated by therapeutic stress. Mitochondria from post-relapse models had markedly attenuated cytochrome c release (surrogate for apoptotic commitment) in comparison with patient-matched diagnostic models. Mitochondrial DNA content, size, and shape did not differ consistently. However, we used electron microscopy to identify reduced endoplasmic reticulum-mitochondria contacts (ERMCs) as correlated with therapy resistance. ERMCs form microdomains for the transfer of Ca 2+ to mitochondria. We confirmed reduced Ca 2+ transfer in resistant cells, with restoration by re-opposing ERMCs via genetically-encoded linkers. However, reduced Ca 2+ transfer was not present in all ERMC-reduced cancers with therapy resistance, supporting Ca 2+ -independent mechanisms. Genetically or biochemically reducing ERMCs in therapy sensitive tumors phenocopied resistance, validating these inter-organelle contacts as physiologic regulators of apoptosis. Our work confirms the importance of ERMCs in stress signaling and provides a previously unrecognized mechanism for cancer cell resistance that is not exclusive to other contributors.

Reduced ER-mitochondria connectivity promotes neuroblastoma multidrug resistance