Background: Disseminated tumor cells (DTCs) found in the bone marrow (BM) of patients with breast cancer
portend a poor prognosis and are thought to be intermediaries in the metastatic process. To assess the clinical
relevance of a mouse model for identifying possible prognostic and predictive biomarkers of these cells, we have
employed patient-derived xenografts (PDX) for propagating and molecularly profiling human DTCs.
Methods: Previously developed mouse xenografts from five breast cancer patients were further passaged by
implantation into NOD/SCID mouse mammary fat pads. BM was collected from long bones at early, serial
passages and analyzed for human-specific gene expression by qRT-PCR as a surrogate biomarker for the
detection of DTCs. Microarray-based gene expression analyses were performed to compare expression profiles
between primary xenografts, solid metastasis, and populations of BM DTCs. Differential patterns of gene
expression were then compared to previously generated microarray data from primary human BM aspirates
from patients with breast cancer and healthy volunteers.
Results: Human-specific gene expression of SNAI1, GSC, FOXC2, KRT19, and STAM2, presumably originating from DTCs,
was detected in the BM of all xenograft mice that also developed metastatic tumors. Human-specific gene expression
was undetectable in the BM of those xenograft lines with no evidence of distant metastases and in non-transplanted
control mice. Comparative gene expression analysis of BM DTCs versus the primary tumor of one mouse line identified
multiple gene transcripts associated with epithelial-mesenchymal transition, aggressive clinical phenotype, and
metastatic disease development. Sixteen of the PDX BM associated genes also demonstrated a statistically
significant difference in expression in the BM of healthy volunteers versus the BM of breast cancer patients
with distant metastatic disease.
Conclusion: Unique and reproducible patterns of differential gene expression can be identified that presumably
originate from BM DTCs in mouse PDX lines. Several of these identified genes are also detected in the BM of patients
with breast cancer who develop early metastases, which suggests that they may be clinically relevant biomarkers. The
PDX model may also provide a clinically relevant system for analyzing and targeting these intermediaries of metastases

• Identifying biomarkers of breast cancer micrometastatic disease in bone marrow using a patient-derived xenograft mouse model