Assessment of Potential Side Effects Related To RAB27A Gene Therapy in Stem Cells

RAB27A plays an essential role in the regulation of exocytosis and intracellular vesicle trafficking. Loss-of-function mutations in the RAB27A gene cause dysfunctional immune cells and Griscelli Syndrome Type 2 (GS-2), whereas upregulation of RAB27A in cancer cells is associated with a worse prognosis and increased metastasis. Here, we wanted to assess the potential side effects of overexpression of RAB27A in different types of healthy stem cells as preparation for the development of gene therapy for GS-2. Bone marrow mesenchymal stem cells (BM-MSCs) were obtained from GS-2 patients and healthy donors. Healthy murine bone marrow-derived and human cord blood-derived hematopoietic stem/progenitor cells (HSPCs) were transduced with different lentiviral vectors carrying a codon-optimized RAB27A (RAB27Aco) transgene. Cells were used for in vitro functional assays and assessed using flow cytometry, Western Blot and RT-PCR. In vivo transplantation assays in mice were used to assess the effect of RAB27A on stem cell function. Engraftment was assessed using flow cytometry, sections of BM-MSC injection sites were analyzed using histological staining. Overexpression of RAB27A resulted in phenotypic changes in BM-MSCs and decreased colony-forming capacity of HSPCs. Transplantation of RAB27A + stem cells was not associated with any tumorigenesis. Despite high expression of RAB27A in HSPCs before transplantation, RAB27A levels in peripheral blood, bone marrow, and spleen cells remained low, indicating overexpression of RAB27A may have affected the long-term reconstitution potential. Development of gene therapy for GS-2 may require fine-tuning of RAB27A expression but is not likely to be complicated by RAB27A-induced tumorigenesis.