Germinal Center Specific Activation of K-Ras, Common In Multiple Myeloma, Is Selected Against and Is Not Sufficient to Initiate Plasma Cell Transformation In Mice

Abstract 137

Activating mutations in Ras (N- and K-) are the most common mutations known in multiple myeloma (MM), and are associated with advanced clinical stage and poor outcomes. We re-sequenced coding regions of highly expressed cytokine signaling candidate genes from MM patient samples and found rare novel mutations. Still, the most common recurrent somatic mutations we found affected N and K-Ras. Previous studies of MM patient samples suggest that Ras mutations may be late/progression events, but the role of Ras activation in MM pathogenesis has not been tested directly. Toward this end, we generated double-transgenic mice by crossing Lox-stop-lox(LSL)- K-ras G12D knock-in mice (K-ras^+/G12D) with Cγ1-Cre knock-in mice (Cγ1^+/Cre) expressing the Cre recominase specifically in germinal center (GC) B-cells. K-ras^+/+/Cγ1^+/Cre mice served as negative controls. We found 58% of unstimulated K-ras^+/G12D/Cγ1^+/Cre mice developed fatal double-positive CD4/8 T-cell lymphomas (n=12) and 42% developed lung adenocarcinomas (n=12). The median survival of these mice with lymphomas was 19 weeks, compared to the 300 day end point of unstimulated K-ras^+/+/Cγ1^+/Cre control mice. Tumor cells from these mice demonstrated successful recomination of the K-Ras locus. In contrast, no evidence of multiple myeloma or monoclonal gammopathy was found by serum ELISA, SPEP, flow cytometry or histologic examination. We isolated B-cell subsets from K-ras^+/G12D/Cγ1^+/Cre mice and documented successful recombination in splenic germinal center cells (B220+, GL-7+) but not in bone marrow plasma cells. Together, these data suggest that Ras activation by “leaky” off-target Cre expression was sufficient to initiate tumorigenesis in two non-B-cell compartments, but that activation of the K-rasG12D allele in germinal center B-cells was associated instead with clearance of recombined cells. We conclude that activating Ras mutations are disease progression events in MM and that earlier mutations are required before primary plasma cells are transformed. This novel mouse model will be useful for identifying and testing early genetic events that drive MM development in cooperation with Ras.