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Zhang et al. J Cancer Metastasis Treat 2020;6:21 I http://dx.doi.org/10.20517/2394-4722.2020.40 Page 7 of 11

20)] [44,45] . In contrast, about 80% of mouse PCTs carry translocations of the IgH locus on mouse Chr 12
[13]
juxtaposed to the MYC locus on Chr 15 . Many myeloma mouse genetically engineered models have
focused on the dysregulation (overexpression or knock-out) of a particular gene or pathway, most notably,
the dysregulation of MYC or BCL2 [46,47] , as well as the earlier spontaneous 5T models that have M spikes
[48]
and develop bone lesions . Adoptive B cell transfer mouse models also provide a novel approach to study
[50]
[49]
MM pathogenesis . Vlummens et al. comprehensively reviewed numerous murine models, ranging
from xenografts to immunocompetent spontaneous and transgenic models, for studying both the etiology
and pathogenesis of MM. More recently, Rajagopalan et al. generated a Nras LSL Q61R/+ mouse which takes
[51]
advantage of crossing Vk*MYC mice to mice harboring a Q61R NRAS mutation (as found in WGS studies
of myeloma) [35,42,46] . This rapid model also develops both bone lesions and M spikes.

In contrast, the focus of our studies has been on genetically inherited alleles of genes in immunocompetent
strains of mice that predispose the mice to peritoneal plasmacytoma development. In the past several
years, more than 17 risk loci for multiple myeloma susceptibility have now been mapped to unique
regions of the human genome [52-57] . The one gene in common with our studies is the Cdkn2a locus; it
is a tumor susceptibility gene in both mouse plasma cell tumors by genetic linkage studies [4,6,15] and in
[52]
genome-wide association studies (GWAS) in human multiple myeloma . Much progress has been made
[58]
in understanding the omics of myeloma through GWAS [52-57] , eQTL , and WGS [35,42] studies of myeloma
patient samples. These studies have helped to identify new targets for intervention of myeloma disease
progression and form the basis for developing companion diagnostics for drug treatments.

In our studies, we have viewed cancer treatment through the lens of cancer as a complex genetic trait
[4]
by using pristane-induced mouse PCT as the model . A goal in the molecular identification of these
susceptibility/resistance genes has been to uncover the signaling pathways that are involved in promoting
or controlling B cell neoplasia and to understand how these pathways may act in concert to contribute
to or limit tumor progression. Tumor incidence data in congenic strains of mice, constructed to harbor
[5,9]
different combinations of resistance alleles , led to the hypothesis that drug combinations affecting these
pathways are likely to have at least an additive, if not synergistic effect in inhibiting tumor cell growth.
We investigated experimental therapeutic approaches to target myeloma; that led to the twin goals of
upregulating tumor suppressor activities and downregulating oncogenic processes simultaneously.

Our initial preclinical studies focused on Mtor inhibition, through targeting Mtor kinase activity, coupled
with HDAC inhibition, which inhibits histone deacetylation. HDAC inhibitors can target a broad spectrum
of genes involved in chromatin modification, including those that regulate the Rb1 and p16 pathways. Our
mechanistic analysis of the successful targeting of these two pathways, which induced synergistic anti-
tumor activity in susceptible tumors, identified MYC as an important upstream driver regulated by the
combined pathways through their cooperative effects on MYC protein degradation. Drug combinations
targeting the two signaling pathways (Cyclin D/CDK/Cdkn2a/Rb and PI3K/AKT/Mtor) identified by our
genetic analysis of PCT susceptibility, were indeed synergistic in their activity, not only for myeloma, but
also a variety of tumor types as shown in their broad synergistic activity in the panel of NCI-60 cell lines.
In fact, the only NCI-60 cell line for which this combination was antagonistic had a mutation in FBXW7
[19]
which is involved in MYC protein degradation .
MYC is often overexpressed and/or dysregulated in cancer, including mouse PCT, as well as human
myeloma and Burkitt’s lymphoma [30,59,60] . MYC has often been considered an undruggable target, yet many
researchers are pursuing a number of avenues to downregulate MYC, including drug combinations such
as the one described above to target post-translational steps, such as protein stability. In addition, MYC’s
transcription factor activity requires dimerization with its binding partner MAX (MYC-associated factor
x), and many efforts have focused on interrupting this complex to downregulate its transcription factor