Optimization of HSC transduction by lentiviral vectors. In Luigi Naldini’s lab in Torino, Italy, I contributed to the discovery that the addition of the HIV-1 polypurine tract to the lentiviral vector backbone greatly improved the efficiency of nuclear translocation of the pre-integration complex, and thus of lentiviral vector integration into non-dividing cells (published in Nature Genetics, 2000). This vector design is now used throughout the scientific community for a wide range of applications. In addition, I was the first to demonstrate by clonal analysis that the cells being transduced by lentiviral vectors were true self-renewing HSC, based on the ability of a single transduced clone to repopulate both primary and secondary NOD/SCID mice (published in Molecular Therapy, 2002).

Development of Wnt reporter lentiviral vectors. In Irving Weissman’s lab at Stanford University, I developed lentiviral vector reporters of activated Wnt signaling. These vectors contributed to two important publications from the Weissman lab; one showing that the Wnt pathway plays a role in mediating hematopoietic stem cell self-renewal (published in Nature, 2003), and one showing that the abnormal activation of the Wnt pathway in the myeloid progenitor compartment of chronic myeloid leukemia (CML) patients leads to their progession to acute phase of the disease (published in NEJM, 2004). The latter was the first demonstration (as well as a mechanism) of a normally non-self-renewing cell acquiring the properties of self-renewal, and leading to an aggressive malignant phenotype.

Identification of a subpopulation of cells with cancer stem cell properties in head and neck squamous cell carcinoma. I identified a subset of cells in human head and neck squamous cell carcinomas (HNSCC) that possess the unique properties of cancer stem cells (published in PNAS, 2007), and form unique histological microdomains that may provide a prognostic indicator for HNSCC patients. Identification of this subset of cells will allow further characterization at the molecular level, and should ultimately lead to cancer initiating cell-targeted therapies for HNSCC. More recently, we demonstrated that the frequency of cells with the phenotype of CICs correlates with disease-free survival (published in Head and Neck, 2012), demonstrating the clinical importance of CICs in this disease.

Identification of ovarian cancer initiating cells, and demonstration of their phenotypic heterogeneity and instability. In collaboration with the Neel Lab, we identified CD133 as a marker that allows isolation of cancer-initiating cells from several primary serous ovarian cancer (SOC) cases. However, in other cases, substantial TIC activity is found in both the CD133+ and CD133- fractions, whereas still other cases have exclusively CD133- TIC. Furthermore, the TIC phenotype can change in xenografts: primary tumors in which all TIC are CD133+ can give rise to xenografts that contain substantial numbers of CD133- TIC. Our results highlighted the need for quantitative rigor in the evaluation of TICs and for caution when using passaged xenografts for such studies. Furthermore, although our data suggest that while SOC conforms to the cancer stem cell hypothesis, the heterogeneity of the TIC phenotype may complicate its clinical application (Published in PNAS in 2011).

Development of a novel high-throughput flow cytometry platform for cell surface profiling. A platform for the systematic profiling of >360 cell surface proteins was developed and demonstrated to provide a sensitive, quantitative and highly reproducible resource for biomarker discovery, molecular classification of cancers, or identification of novel lineage-specific or stem cell markers (published in PLoS One in 2014).