Genetic barcoding uncovers the clonal makeup of solid and liquid biopsies

Intra-tumoral heterogeneity (ITH) is fueling tumor progression in breast cancer, as specific clones present within a tumor may have a selective advantage to colonize distant organs and escape therapy. Accurate sampling of ITH is therefore a pressing challenge in clinical oncology, to adequately predict recurrence and inform rational and personalized therapies. Here we used cellular barcoding to track the spatio-temporal composition of human breast cancer clones in six preclinical models - across two cell lines and four patient derived xenografts (PDXs). This allowed direct side-by-side quantitative comparison not only of intra-tumor clonal composition, but also of how that composition was reflected in needle biopsies and cell-free DNA (cfDNA). These analyses highlighted several clinically relevant findings. First, the use of orthogonal genetic and optical barcoding revealed that clonal diversity in the center of non-necrotic primary tumors was significantly higher compared to their periphery. Second, cfDNA barcode analysis suggested that DNA shedding in the vasculature varied largely, not only depending on necrosis and tumor burden, but also between models. Third, combining information captured in both solid and liquid biopsies can provide a more robust assessment of tumor clonal composition. Taken together, these results showcases the utility of these barcoded models to optimize the use of solid and liquid biopsies as surrogate markers of ITH.