MYC Activation Is a Common Transformation Event in Myeloma and Associated with Poor Prognosis.

Abstract 834

Multiple myeloma (MM) is an incurable bone marrow cancer. Events mediating transformation from the pre-malignant monoclonal gammopathy of undetermined significance (MGUS) to MM is unknown. We analyzed 2 gene expression datasets generated on the Affymetrix U133 platform. The test set consisted of 22 MGUS and 101 MM (GEO Accession GSE6477) and the validation set 50 MGUS and 351 MM (GEO Accession GSE2658 and GSE5900). The gene expression profiles of MM were compared to MGUS using gene-set enrichment analysis. Genes over-expressed in MM were enriched for cell cycle, proliferation and MYC activation gene-sets. We dissected the relationship between MYC and cell cycle, and identified a MYC activation signature dissociated from proliferation. We validated our MYC signature in publicly available mouse and human cell line gene expression dataset (GEO Accession GSE3151 and GSE3158 respectively), showing specific expression of our MYC signature in cell lines forced to expressed MYC but not when over-expressing other oncogene such as E2F, HER2. In these analyses, we noted that tumors with RAS mutation also consistently express this signature. Applying this signature to the test dataset, we showed that MYC is activated in 60% of myeloma but none of MGUS. This pattern is reproduced in an independent validation dataset. In order to validate the hypothesis that RAS mutation may also lead to the MYC activation signature, we correlated RAS mutation with MYC activation as measured by a MYC index calculated from the median expression of genes that constitute the MYC activation signature, and showed that almost all cases with RAS mutation have a high MYC index. Together with samples with very high expression of MYC mRNA corresponding to those with MYC translocations, these 2 mechanisms account for 67% of cases with MYC activation. To further confirm MYC activation, we performed immunohistochemistry using a validated MYC antibody together with CD138 double-staining. We can clearly identify CD138 positive plasma cells with and without nuclear MYC expression and found that nuclear expression of MYC, a marker of MYC activation, correlated strongly with the MYC signature, therefore providing clear evidence that MYC activation is present in majority of newly diagnosed MM but not in MGUS. MYC activation is not very well correlated with proliferation as assessed by the plasma cell labeling index. Among newly diagnosed myeloma patients with plasma cell labeling index less than 1, those with nuclear MYC expression have significantly shorter survival than those without (Median survival 77.7 months versus 37.9 months, log-rank p-value 0.04). Multiple pathways converge on MYC activation, which is a common transforming event in MM associated with poor prognosis. MYC nuclear staining by IHC can be a useful clinical surrogate. Targeting MYC can be a chemoprevention strategy.