Crispr Sgrnas Genome-Wide Screen Identifies the Proteasome Regulatory Subunit PSMC6 As a Bortezomib Resistance Gene in Human Multiple Myeloma Cells

Background: The proteasome inhibitor Bortezomib (BTZ) is an efficient treatment option for both, newly diagnosed and relapsed/refractory multiple myeloma (MM). Despite the effectiveness, most patients eventually acquire drug resistance for reasons not fully understood

Materials and methods: To better understand BTZ resistance mechanism we used a CRISPR library (GeCKO V2) targeting 19052 human genes, trying to identify genes responsible for BTZ resistance. CRISPR sgRNAs targeting the ERN1-XBP1 pathway were used as positive controls. We first infected RPMI 8226 myeloma cell line expressing Cas9 with the CRISPR library packaged into lenti-vectors and selected for resistance to BTZ. Surviving cells were subjected to next generation sequencing. Based on the initial screen results we constructed a second CRISPR sgRNA library including 31 genes, each gene targeted with four sgRNAs. After the second round screening and subsequent sequencing, we selected the top 20 genes for individual validation.

Results: Proteasome regulatory gene PSMC6 was identified as the only reproducible gene conferring BTZ resistance. Interestingly, the same gene was independently found by a second group using a CRISPR approach (Sheffer M et al. ASH Abstract 273, 2014). Resistance was reproducible using a PSMC6 knockout by three individual CRISPR sgRNAs targeting exonic regions and one pair of SgRNA targeting intron region flanking exon for the deletion of exon one. PSMC6 knockout was verified by PCR and Sanger sequencing. Sensitivity to BTZ was rescued by over expression of PSMC6 cDNA in RPMI 8226 cells harboring a deletion of PSMC6 exon 1. MM cells lacking PSMC6 also developed resistance against Carfilzomib. Resistance was reproduced on a second MM cell line, KMS11. We did not see any significant difference of toxicity in PSMC6 deleted cells for other chemicals tested (tunicamycin, staurosporine, dexamethasone and melphalan). We demonstrated that the sensitivity of chymotrypsin-like activity of proteasome against BTZ was significantly reduced in cells lacking PSMC6. Consequently, MM cells without the PSMC6 gene were relatively resistant to apoptosis induced by BTZ, which was verified by Western blot for caspase 8 degradation and luminescent assay for caspase 3/7 activities. Clinically we could not correlate the PSMC6 expression level with the outcome of BTZ treatment in BTZ naive patients using publically available gene expression data. We initially used CRISPR sgRNAs targeting ER stress pathway (ERN1 and XBP1) as a positive control. However we could not derive any resistant cells from the experiment. We also could not identify any sgRNAs targeting the ERN1-XBP1 pathway from our whole exome screen and next generation sequencing. Since this contradicts published data, we decided to knockout ERN1 and XBP1 genes individually. Clones of cells with successful knockout of ERN1 in three cell lines (RPMI 8226, KMS11 and JJN3) and XBP1 in two cell lines (RPMI 8226 and KMS11) were tested for response to BTZ and Carfilzomib , however we did not find any drug response difference between the knockout and parent cells. We also found that the ERN1-XBP1 knockout cells did not show difference in response to ERN1 specific inhibitors (4u8C and STF-038010) and ER stress inducers (tunicamycin and thapsigargin) compared to parental cells.

Conclusions: Human multiple myeloma cells lacking the PSMC6 gene develop significant resistance to apoptosis induced by BTZ. We have however not found a correlation of PSMC6 expression levels with outcome to BTZ treatment in BTZ naïve patients. We are therefore currently investigating the PSMC6 mutation rate in relapsed MM patients after proteasome treatment. In contrast to previous reports showing that progenitor MM cells lacing XBP-1 or ERN-1 invoked BTZ resistance, we were not able to demonstrate a change in sensitivity after full CRISPR knock out of either ERN1 or XBP1. It has long been believed that ERN1-XBP1 pathway plays an important role for MM treatment, leading to the development of ERN1 specific inhibitors. However, we demonstrated that the toxicity of two ERN1-specific inhibitors appears independent of the ERN1-XBP1 pathway. We also demonstrated that the toxicity of two important ER stress inducers, tunicamycin and thapsigargin, is independent of the ERN1-XBP1 pathway.