Abstract B76: Manipulating tumor suppressors to improve thymus function following hematopoietic stem cell transplantation
Authors: Garfin Phillip M. , Min Dullei , Weinber Kenneth I. , Sage Julien 
Published: May 2015

Hematopoietic stem cell transplantation (HSCT) is an important therapy for many pediatric malignancies and pre-malignant conditions. HSCT is used regularly to treat relapsed leukemia and lymphoma, myelodysplastic syndrome, high-risk neuroblastoma, some brain tumors, and is now incorporated into trials for metastatic Ewing's Sarcoma. While HSCT can successfully treat cancer, the post-transplant course is often complicated. Many of these complications, including increased risk of infection, increased rate of relapse, and potentially also graft versus host disease can be attributed to damage to the thymus. The thymus is the site of T cell maturation and selection and is essential for T cell mediated immune function. Unfortunately, the thymus is damaged by the high-dose chemotherapy and radiation used in most pre-HSCT conditioning regimens. Preventing this damage or promoting rapid thymic regeneration could reduce the incidence and severity of many complications that follow HSCT.

Previously, we described a novel RB-E2F-FOXN1 module that regulates thymic size and function in mice. This regulatory module plays an important role regulating the proliferation and differentiation of thymic epithelial cells (TECs). Deletion of the RB family of proteins (RB, P130, and P107) in very young mice prevents thymic involution, supports continued thymic growth, and increases of T cell production throughout the life of the mice. Importantly, we demonstrated that this effect persists following HSCT. We also have evidence that this regulatory module exists in human epithelial cells.

Here we identify the RB-E2F module in TECs as a target of Keratinocyte Growth Factor (KGF, trade name: Palifermin). Thus, the thymic-protective effects of Palifermin may be due to its ability to regulate the RB-E2F-FOXN1 module. We also investigate the role of the RB-E2F-FOXN1 module in thymic regeneration. Using timed deletion of the RB family we have found that deletion of the RB family in mice older than three months does not support significant thymic regeneration. We are in the process of deleting the RB family at progressively earlier ages in order to determine the potential of RB inactivation to regenerate involuted thymus. In addition, we have taken advantage of the different growth characteristics of TECs in RB and FONX1 mutant mice to begin to identify additional modules important for the regulation of thymic size and function. We have preliminary evidence that the HIPPO/YAP pathway is present in TECs and that its activity can be regulated. We do not yet know the role of this pathway in vivo. Because this pathway is a more traditional kinase-cascade, it may provide some druggable targets to improve thymic function following HSCT.

In summary, we have found that we regulate thymic size and improve T cell output, even following HSCT. We have identified a pathway that may play a role in thymic recovery post-transplantation and we are in the process of describing new pathways that potentially could be targeted in the development of therapies aimed at improving recovery following HSCT and improving outcomes for children with high-risk cancers.

Citation Format: Phillip M. Garfin, Dullei Min, Kenneth I. Weinber, Julien Sage. Manipulating tumor suppressors to improve thymus function following hematopoietic stem cell transplantation. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr B76.