Humanized mouse model for the study of MM

A new humanized mouse model has been developed by a team of researchers at Yale University, in order to study Multiple Myeloma (MM) in-vivo. Rituparna Das, along with Professors Richard A. Flavell and Madhav V. Dhodapkar, published their findings in Nature Medicine in November 2016. Notably, this model enables growth of pre-neoplastic plasma cells as well as malignant cells, and therefore also allows study of the precursor disease state of monoclonal gammopathy of undetermined significance (MGUS) and asymptomatic MM (AMM). The authors found that both xenografts from patients with MM or cells from the precursor state were supported in this mouse model, signifying a strong role for extrinsic cues from the microenvironment.

Key Highlights

• Mice (Rag2-deficient and IL-2Rϒ-deficient) were genetically engineered to carry human versions of six genes important for innate immune cell and myeloma cell development: IL-6; M-CSF; IL-3/GM-CSF; hSIRPα; TPO (thrombopoeitin); such mice were termed MIS^(KI)TRG6

Growth of Human IL6-dependent cells:

• INA-6 cells (human IL-6 dependent MM cell line) injected into the bones of mice expressing human IL-6 led to IL-6 dependent tumor growth, increased soluble IL-6 and bone destruction
• Human IL-6 (hIL-6), but not mouse IL-6, supported growth of INA-6 and resultant bone destruction in vivo
• INA-6 cells also grew in mice expressing hIL-6 and hSIRPα 

Growth of primary MM cells:

• Primary MM cells isolated from bone marrow of patients and injected into the bones of MIS^(KI)TRG6 mice grew within the bone, but not in the spleen; non-malignant cells were detected in the spleen as well
• Injection of both CD138⁺ cells or a CD138-depleted cell population of bone marrow mononuclear cells (BMMNCs) into MIS^(KI)TRG6 mice, led to the growth of CD138+ tumor cells in-vivo, indicating that both compartments can re-populate tumors
• Residual non-tumor cells (T-cells, myeloid cells, NK cells and B cells) also underwent expansion in these mice

Microenvironment-dependent growth of MM and precursor states:

• Injection of CD3-depleted BMMNCs from patients with either MGUS or AMM into MIS^(KI)TRG6 mice led to the growth of tumor cells in-vivo, primarily within the bone
• Samples from relapsed and refractory patients showed enhanced ability to grow in the contralateral bone; extramedullary growth was observed with more aggressive plasma cell leukemia (PCL)
• Percentage of clonal plasma cells in xenograft from pts with pre-neoplastic gammopathy were higher than in the primary samples: 4.9% vs 27.4% (p=0.06)
• The reliable growth of pre-neoplastic stages is a strong advantage of this model over the SCID-hu mice and other existing MM models

Genomic Diversity of Xenografts:

• To test whether MM cells grown in the MIS^(KI)TRG6 mouse retain the clonal diversity of the parental tumors, DNA from sort-purified tumor cells were analyzed by whole exome sequencing and compared to the parental cells
• Loss of heterozygosity (LOH) patterns were retained from the parental tumors to the xenograft, although additional LOH changes were observed
• Somatic copy number alterations (CNA) revealed a similar pattern, and both LOH and CNA changes were identical in individual mice transplanted with the same parental tumor 

In conclusion, the advantage of MIS^(KI)TRG6 mice over other MM models is the ability to support growth of primary tumor cells, in both pre-neoplastic and more advanced malignant states, in-vivo. In addition, growth of tumor cells is largely restricted to the bone marrow, which is consistent with human MM. Ultimately, this mouse model will enable detailed studies of MM biology and development from early precursor states, as well as pre-clinical testing and the development of personalized therapies.  

Abstract

Most human cancers, including myeloma, are preceded by a precursor state. There is an unmet need for in vivo models to study the interaction of human preneoplastic cells in the bone marrow microenvironment with non-malignant cells. Here, we genetically humanized mice to permit the growth of primary human preneoplastic and malignant plasma cells together with non-malignant cells in vivo. Growth was largely restricted to the bone marrow, mirroring the pattern in patients with myeloma. Xenografts captured the genomic complexity of parental tumors and revealed additional somatic changes. Moreover, xenografts from patients with preneoplastic gammopathy showed progressive growth, suggesting that the clinical stability of these lesions may in part be due to growth controls extrinsic to tumor cells. These data demonstrate a new approach to investigate the entire spectrum of human plasma cell neoplasia and illustrate the utility of humanized models for understanding the functional diversity of human tumors.