The proteasome is a validated target of cancer therapeutics. Inhibition of proteasome activity results in the activation of the unfolded protein response (UPR) characterized by phosphorylation of eukaryotic initiation factor 2α (eIF2α), global translational arrest, and increased expression of the proapoptotic CHOP (C/EBP homologous protein) protein. Defects in the UPR response has been reported to result in altered sensitivity of tumor cells to proteasome inhibitors. Here, we characterized the effects of the deubiquitinase (DUB) inhibitor VLX1570 on protein homeostasis, both at the level of the UPR and on protein translation, in acute lymphoblastic leukemia (ALL). Similar to the 20S inhibitor bortezomib, VLX1570 induced accumulation of polyubiquitinated proteins and increased expression of the chaperone Grp78/Bip in ALL cells. Both compounds induced cleavage of PARP (Poly (ADP-ribose) polymerase) in ALL cells, consistent with induction of apoptosis. However, and in contrast to bortezomib, VLX1570 treatment resulted in limited induction of the proapoptotic CHOP protein. Translational inhibition was observed by both bortezomib and VLX1570. We report that in distinction to bortezomib, suppression of translation by VXL1570 occurred at the level of elongation. Increased levels of Hsc70/Hsp70 proteins were observed on polysomes following exposure to VLX1570, possibly suggesting defects in nascent protein folding. Our findings demonstrate apoptosis induction in ALL cells that appears to be uncoupled from CHOP induction, and show that VLX1570 suppresses protein translation by a mechanism distinct from that of bortezomib.

Telomere biology disorders (TBDs) are characterized by short telomeres, premature aging, bone marrow failure and cancer predisposition. Germline mutations in NHP2, encoding for one component of the telomerase cofactor H/ACA RNA binding complex together with Dyskerin, NOP10 and GAR1, have been previously reported in rare cases of TBDs. Here, we report two novel NHP2 variants (NHP2-A39T and NHP2-T44M) identified in a compound heterozygous patient affected by premature aging, bone marrow failure/myelodysplastic syndrome and gastric cancer. Although still able to support cell viability, both variants reduce the levels of hTR, the telomerase RNA component, and telomerase activity, expanding the panel of NHP2 pathological variants. Furthermore, both variants fail to be incorporated in the H/ACA RNA binding complex when in competition with wild-type endogenous NHP2, and the lack of incorporation causes their drastic proteasomal degradation. By RoseTTAFold prediction followed by molecular dynamics simulations, we reveal a dramatic distortion of residues 33–41, which normally position on top of the NHP2 core, as the main defect of NHP2-A39T, and high flexibility and the misplacement of the N-terminal region (residues 1–24) in NHP2-T44M and, to a lower degree, in NHP2-A39T. Because deletion of amino acids 2–24 causes a reduction in NHP2 levels only in the presence of wild-type NHP2, while deletion of amino acids 2–38 completely disrupts NHP2 stability, we propose that the two variants are mis-incorporated into the H/ACA binding complex due to the altered dynamics of the first 23 amino acids and/or the distortion of the residues 25–41 loop.