G.J.L. Kaspers

Impaired bortezomib binding to mutant beta 5 subunit of the proteasome is the underlying basis for bortezomib resistance in leukemia cells.

Proteasome inhibition is a novel treatment for several hematological malignancies. However, resistance to the proteasome inhibitor bortezomib (BTZ, Velcade) is an emerging clinical impediment. Mutations in the β5 subunit of the proteasome, the primary target of BTZ, have been associated with drug resistance. However, the exact mechanism by which these mutations contribute to BTZ resistance, is still largely unknown. Toward this end, we here developed BTZ-resistant multiple myeloma (8226) and acute lymphoblastic leukemia (CCRF-CEM) cell line models by exposure to stepwise increasing concentrations of BTZ. Characterization of the various BTZ-resistant cells revealed upregulation of mutant β5 subunit of the proteasome. These newly identified β5-subunit mutations, along with previously described mutations, formed a mutation cluster region in the BTZ-binding pocket of the β5 subunit, that of the S1 specificity pocket in particular. Moreover, we provide the first evidence that the mechanism underlying BTZ resistance in these tumor cells is impaired binding of BTZ to the mutant β5 subunit of the proteasome. We propose that proteasome subunit overexpression is an essential compensatory mechanism for the impaired catalytic activity of these mutant proteasomes. Our findings further suggest that second-generation proteasome inhibitors that target the α7 subunit of the proteasome can overcome this drug resistance modality.

Implementation of a multi-institutional diffuse intrinsic pontine glioma autopsy protocol and characterization of a primary cell culture

V. Caretti, M. H. A. Jansen, D. G. van Vuurden, T. Lagerweij, M. Bugiani, I. Horsman, H. Wessels, P. van der Valk, J. Cloos, D. P. Noske, W. P. Vandertop, P. Wesseling, T. Wurdinger, E. Hulleman and G. J. L. Kaspers (2013) Neuropathology and Applied Neurobiology39, 426–436

Multiregional Tumor Drug-Uptake Imaging by PET and Microvascular Morphology in End-Stage Diffuse Intrinsic Pontine Glioma

Inadequate tumor uptake of the vascular endothelial growth factor antibody bevacizumab could explain lack of effect in diffuse intrinsic pontine glioma. Methods: By combining data from a PET imaging study using 89Zr-labeled bevacizumab and an autopsy study, a 1-on-1 analysis of multiregional in vivo and ex vivo 89Zr-bevacizumab uptake, tumor histology, and vascular morphology in a diffuse intrinsic pontine glioma patient was performed. Results: In vivo 89Zr-bevacizumab measurements showed heterogeneity between lesions. Additional ex vivo measurements and immunohistochemistry of cervicomedullary metastasis samples showed uptake to be highest in the area with marked microvascular proliferation. In the primary pontine tumor, all samples showed similar vascular morphology. Other histologic features were similar between the samples studied. Conclusion: In vivo 89Zr-bevacizumab PET serves to identify heterogeneous uptake between tumor lesions, whereas subcentimeter intralesional heterogeneity could be identified only by ex vivo measurements. 89Zr-bevacizumab uptake is enhanced by vascular proliferation, although our results suggest it is not the only determinant of intralesional uptake heterogeneity.