Actionable loss of SLF2 drives B-cell lymphomagenesis and impairs the DNA damage response.

The DNA damage response (DDR) acts as a barrier to malignant transformation and is often impaired during tumorigenesis. Exploiting the impaired DDR can be a promising therapeutic strategy; however, the mechanisms of inactivation and corresponding biomarkers are incompletely understood. Starting from an unbiased screening approach, we identified the SMC5-SMC6 Complex Localization Factor 2 (SLF2) as a regulator of the DDR and biomarker for a B-cell lymphoma (BCL) patient subgroup with an adverse prognosis. SLF2-deficiency leads to loss of DDR factors including Claspin (CLSPN) and consequently impairs CHK1 activation. In line with this mechanism, genetic deletion of Slf2 drives lymphomagenesis in vivo. Tumor cells lacking SLF2 are characterized by a high level of DNA damage, which leads to alterations of the post-translational SUMOylation pathway as a safeguard. The resulting co-dependency confers synthetic lethality to a clinically applicable SUMOylation inhibitor (SUMOi), and inhibitors of the DDR pathway act highly synergistic with SUMOi. Together, our results identify SLF2 as a DDR regulator and reveal co-targeting of the DDR and SUMOylation as a promising strategy for treating aggressive lymphoma.

Initial evaluation of [18F]-FACBC for PET imaging of multiple myeloma.

RATIONALE: Multiple myeloma (MM) cells synthesize large amounts of paraproteins, making radiolabeled amino acids promising candidates for PET imaging of MM patients. METHODS: We compare tumor uptake of the two amino acid analogs [18F]-fluoroethyltyrosine and [18F]-FACBC in a MM xenograft model and show the feasibility of PET imaging with [18F]-FACBC in a MM patient. RESULTS: Preclinically [18F]-FACBC showed superior performance, mainly due to the uptake via the ASC-system. In a subsequent proof-of-concept investigation [18F]-FACBC PET was performed in a MM patient. It allowed identification of both lesions with and without CT correlate (SUVmean 8.0 or 7.9) based on higher uptake compared to normal bone marrow (SUVmean 5.7). Bone signal was elevated compared to non-MM patients, and, thus [18F]-FACBC potentially allows the assessment of bone marrow infiltration. CONCLUSION: The FDA/EMA approved PET agent [18F]-FACBC is promising for imaging MM and should be further evaluated in prospective clinical studies.

MCT1 is a predictive marker for lenalidomide maintenance therapy in multiple myeloma.

Biomarkers that predict response to lenalidomide maintenance therapy in patients with multiple myeloma (MM) have remained elusive. We have shown that immunomodulatory drugs (IMiDs) exert anti-MM activity via destabilization of MCT1 and CD147. In this study, cell samples of 654 patients with MM who received lenalidomide (n = 455), thalidomide (n = 98), or bortezomib (n = 101) maintenance were assessed by gene expression profiling and RNA sequencing, followed by correlation of MCT1 and CD147 expression with data for progression-free survival (PFS) and overall survival (OS). Patients with high expression levels of MCT1 showed significantly reduced PFS (31.9 months vs 48.2 months in MCT1high vs MCT1low; P = .03) and OS (75.9 months vs not reached [NR] in MCT1high vs MCT1low; P = .001) in cases with lenalidomide maintenance, whereas MCT1 expression had no significant impact on PFS or OS in cases with bortezomib maintenance. We validated the predictive role of MCT1 for IMiD-based maintenance in an independent cohort of patients who received thalidomide (OS, 83.6 months vs NR in MCT1high vs MCT1low; P = .03). Functional validation showed that MCT1 overexpression in human MM cell lines significantly reduced the efficacy of lenalidomide, whereas no change was observed with bortezomib treatment, either in vitro or in a MM xenograft model. Our findings have established MCT1 expression as a predictive marker for response to lenalidomide-based maintenance in patients with MM.

The IMiD target CRBN determines HSP90 activity toward transmembrane proteins essential in multiple myeloma.

The complex architecture of transmembrane proteins requires quality control (QC) of folding, membrane positioning, and trafficking as prerequisites for cellular homeostasis and intercellular communication. However, it has remained unclear whether transmembrane protein-specific QC hubs exist. Here we identify cereblon (CRBN), the target of immunomodulatory drugs (IMiDs), as a co-chaperone that specifically determines chaperone activity of HSP90 toward transmembrane proteins by means of counteracting AHA1. This function is abrogated by IMiDs, which disrupt the interaction of CRBN with HSP90. Among the multiple transmembrane protein clients of CRBN-AHA1-HSP90 revealed by cell surface proteomics, we identify the amino acid transporter LAT1/CD98hc as a determinant of IMiD activity in multiple myeloma (MM) and present an Anticalin-based CD98hc radiopharmaceutical for MM radio-theranostics. These data establish the CRBN-AHA1-HSP90 axis in the biogenesis of transmembrane proteins, link IMiD activity to tumor metabolism, and nominate CD98hc and LAT1 as attractive diagnostic and therapeutic targets in MM.