Analysis of Risk and Mechanism of Insertional Oncogenesis After Gene Transfer Into Hematopoietic Progenitors with Integrating Viral Vectors

Abstract 2049

Gene transfer into hematopoietic stem cells has been used successfully to treat a variety of human genetic diseases. Although protocols have shown positive clinical outcomes, the successes of clinical trials have been tempered by adverse events in which the integration of the viral vectors increased transcription of cancer-related genes and thereby contributed to development of leukemias. The use of gamma-retroviral vectors containing full-length, long terminal repeats (LTRs) with strong promoter and enhancer activity has been well documented to have the potential of resulting in activation of expression of genes neighboring the vector insertion site. Assessing safety of integrating viral vectors for future clinical use is therefore of paramount importance.

In preparation for gene therapy approaches for the Wiskott-Aldrich syndrome (WAS), we used an in vitro assay of murine bone marrow (BM) cell immortalization to compare the consequences of hematopoietic stem cell transduction by three different kinds of viral vectors, including Moloney murine leukemia virus (MMLV), lentivirus (LV), and foamy virus (FV) constructs. To evaluate critical elements for cell immortalization by MMLV vectors, we also tested five different MMLV LTR forms: unmodified (full-MMLV), deleted of most of the two 75-bp repeats associated with the viral enhancer (delE1), deleted of all the two 75-bp repeats and negative control region (NCR) (delE2), deleted of the viral promoter sequences (delP), and with full deletion of enhancer and promoter sequences (delEP). All vectors carried an internal expression cassette including the eGFP gene under the control of a UCOE (ubiquitously acting chromatin opening element) or the WAS endogenous promoter (WASp). In this assay, BM cells are harvested from C57BL6 mice, exposed to retroviral supernatants and cultured long-term. Derived lines are considered immortalized based on their ability to continue to grow in vitro for more than six weeks in the presence of interleukin-3 and stem cell factor. Real-time PCR was performed to verify comparable transduction efficiency of bone marrow cells by different vectors.

To date, full-MMLV and delE1 transduction of 123 and 132 cultures, respectively, has given rise to 48 and 43 immortalized lines (39.0% and 32.5% immortalization rate, respectively). The difference in immortalization rate between full-MMLV and delE1 was not statistically significant. In contrast, transduction of 114 and 62 cultures with LV and FV vectors, respectively, resulted in no immortalized lines.

In our analysis of MMLV LTR mutants, full-MMLV and delE1 transduction of 56 and 72 cultures, respectively, has given rise to 24 and 26 immortalized lines (43% and 36% immortalization rate). Again, the difference in immortalization rate between full-MMLV and delE1 was not statistically significant. In contrast, delE2, delP and delEP transduction of 24 cultures each has given rise to 2, 5 and 3 immortalized lines (8.3%, 21% and 13% immortalized ratio, respectively). The difference between the immortalization caused by delE1 and delE2 vectors was statistically significant (p<0.01), while there was no significant difference between the full-MMLV and delP vectors.

These preliminary results confirm that gamma-retroviral vectors are prone to causing immortalization of hematopoietic cells and indicate that deletion of viral enhancer and/or promoter sequences may not be adequate to eliminate the insertional oncogenesis risk. Importantly, our data point to the NCR as a crucial element for immortalization and justify additional studies to evaluate its specific role in MMLV-mediated insertional oncogenesis. Finally, our results suggest that vectors based on LV and FV backbones are safer alternatives for clinical gene transfer into hematopoietic stem cells.