The Murine Vk*MYC Myeloma Shares Defining Genetic Lesions with Human Multiple Myeloma.
Authors: Chesi Marta , Keats Jonathan J , Braggio Esteban , Bergsagel Peter Leif 
Published: October 2019

Abstract
Abstract 1808
Poster Board I-834
We have developed the first mouse model of multiple myeloma (MM), Vk*MYC, in an immune competent mouse, with indolent plasma cell (PC) growth confined to the bone marrow, and in which the major biological and clinical features of the human disease are recapitulated, including the post germinal center origin of the tumor cells, the presence of a M-spike in the serum, anemia, diffuse bone disease and drug sensitivity.
To further characterize karyotypically and phenotypically the Vk*MYC tumors, we performed gene expression profile (GEP) and aCGH analysis on CD138+ purified MM cells from Vk*MYC mice. For GEP, we analyzed 33 individual tumors on the Affymetrix 430_2 array for which publicly available datasets can be used to compare levels of gene expression. For aCGH, we used the Agilent 244K array to analyze 19 individual tumors.
By GEP we first confirmed the PC origin of the Vk*MYC tumors, that clearly express the typical PC markers, including CD138, IRF4, BLIMP, XBP1, IgG or IgA, but not B cell markers like IgM, IgD, PAX5, or BCL6. Next, we analyzed the expression of the three cyclin D, that are almost universally dysregulated in human myeloma patients and cell lines, but are not expressed in normal human or murine PCs. We found that the Vk*MYC tumors express high levels of cyclin D2, and occasionally cyclin D1, but never cyclin D3 that, instead, is commonly expressed in B cells. Interestingly, in one sample lacking any D type cyclin expression we detected loss of Rb expression (and homozygous deletion of the gene, see below), as in the “TC none” group of human MM patients, highlighting once again the importance of this pathway in both human and mouse myeloma.
aGCH analysis indicated that the genomic complexity of Vk*MYC tumors is low, and complex rearrangements were rarely observed. In contrast with the recurrent trisomy of odd number chromosomes present in approximately 50% of human MM patients, we found no evidences of hyperdiploidy in Vk*MYC mice, except perhaps in one tumor with trisomy 15 and 17. Instead, we detected monosomy 5 in 10/19, monosomy 14 in 4/19 and trisomy 16 in 4/19 tumors. Interestingly a large portion of the murine chr.14 is syntenic to the human chr.13, frequently lost in MM patients. After monosomy 13, the most frequent chromosomal change in human MM is loss of a sex chromosome. In the mouse, the most common numerical abnormality identified was loss of a sex chromosome: 10/11 male had loss of Y and 5/8 female had loss of X.
From the analysis of aCGH data publicly available on the MMRC portal, we have generated a list of the most common bi-allelic deletions present in human myeloma patients and cell lines. Interestingly, several of these lesions are also present in Vk*MYC tumors. The Rb1 gene, frequently affected by monosomy 13 in human MM, is bi-allelically lost in 3/19 Vk*MYC tumors. In one case, we also detected a focal monoallelic deletion encompassing Rb1, in addition to the four tumors with monosomy 14. Other focal bi-allelic deletion identified are: Cdkn2a (p16 and p19ARF), FHIT and PTEN. The latter was found in a Vk*MYC tumor selected in-vivo to be bortezomib resistant. On chr.X we identified recurrent (5/19 tumors) bi-allelic deletions of UTX, the histone demethylase gene recently reported to be deleted in various cancers, including MM tumors and human myeloma cell lines. While we are in the process of performing functional studies in human cell lines to identify a role for UTX in myeloma pathogenesis, we took advantage of the relative genetic uniformity of the Vk*MYC mice to identify by GEP a signature associated with UTX lost. We separated our cohort of Vk*MYC tumors for which GEP data are available in two groups based on expression of UTX and generated by volcano plot a list of genes that distinguished the two groups. Strikingly, loss of UTX was associated with loss of expression of a subset of lymphoid genes and acquisition of a “macrophage signature”, suggesting a role for UTX in lineage commitment. These data provide a genetic basis for tumor plasticity similar to epithelial mesenchymal transition observed in some solid tumors, in which the tumor gradually acquires supporting characteristics of its microenvironment.
We conclude that the Vk*MYC provide a remarkably faithful model of MM not only phenotypically but also genetically, that provides an excellent framework for functional characterization of the molecular pathogenesis of MM

Disclosures
Bergsagel: Genentech: Consultancy; Amgen: Consultancy; Celgene: Consultancy; Merck: Research Funding.