Zinc-finger nucleases facilitate in vivo integration of transgenes into the albumin locus

A group from the Children's Hospital of Philadelphia published a unique way to ameliorate hereditary bleeding disorders and enzyme deficiencies. Katherine A. High and her group utilized adeno-associated virus 8 (AAV8) to deliver a pair of zinc-finger nucleases (ZFNs) along with an interchangeable transgene to the liver. The transgene cassette is promoterless but is flanked by terminal regions homologous to the albumin gene. Upon ZFN cleavage, the transgene integrates with low efficiency, but under a very strong promoter, which leads to phenotype correction in a model of hemophilia A and hemophilia B. The correction of hemophilia A is particularly significant, since the gene encoding for factor VIII is larger than the AAV capacity. Thus, the most important advantage of this current method is that it increases AAV coding capacity by obviating the need of the promoter. In contrast to Mark Kay's recent study in Nature, this study did not observe any gene expression in the sole presence of an AAV encoding for the promoterless transgene. The authors noted some off-target effects, which might add up over time, given the long-term expression of ZFNs from an AAV episome.

Read the paper on the Blood journal website: http://www.bloodjournal.org/content/early/2015/08/20/blood-2014-12-615492.long?sso-checked=true

2015 American Society of Gene and Cell Therapy Annual meeting in New Orleans, LA - Highlights - Long-term engraftment of zinc finger modified T-cells in HIV infection

Sangamo Biosciences (Richmond, CA, USA) presented novel clinical data regarding the SB-728-T-cell program for viral load control in chronic HIV infection. In this open-label phase 1 clinical trial, nine patients with low CD4+ T cell counts (200-500 cells/mm3) received 10-30 billion autologous, zinc finger nuclease modified CCR5 knockout T-cells.
Interestingly, modified T-cells were still detectable in the circulation after 3 years and generated long-lived T-memory stem cells. The authors demonstrated a sustained increase of CD4+ T cell counts in all treated subjects. T-memory stem cell count correlated with the decay of viral load.

Taken together, the SB-728-T-cell infusion unexpectedly resulted in very long term T-cell reconstitution, owing to the transformation of the zinc-finger edited T-cells to long lived memory stem cells. The program is now in Phase II, while a similar hemopoietic stem cell approach is already in phase I.