BM failure and leukemia at a young age

BM failure and leukemia at a young age are the hallmarks of human FA patients, which made the study of FA of high interest for understanding the biology of HSC defect and malignant transformation. Mouse models have been developed for several FA TAE226 (Parmar et al., 2009). Despite the mild phenotypes and absence of BM failure in a steady-state condition, FA mouse models present hypersensitivity to DNA-damaging agents, oxidative stress and a deficiency of HSC repopulation abilities (Parmar et al., 2009). Studies of these mouse models have allowed advances in understanding factors and pathways involved in HSC normal functions and BM failure. For example, the study of FANCD2 interaction partners has demonstrated the importance of FOXO3a/FANCD2 interaction in HSC maintenance and resistance against oxidative stress (Li et al., 2010, 2015). Recently, a study emphasizing the relationship between DNA damage and HSC impairment has shown that repeated stresses led to the accumulation of DNA damage and acceleration of functional impairment in FA HSCs (Walter et al., 2015). Despite this accumulation of knowledge about BM failure, potent treatment for FA patients is still needed. Unbiased genomic approaches by short hairpin RNA (shRNA) screens have been successfully used to discover new mechanisms in different pathologies (Mohr et al., 2014), and can be conducted at a stem cell level (Hope et al., 2010; Wang et al., 2012). Using such a strategy, several tumor suppressors and checkpoint factors have been highlighted, such as p53 and p21. Interestingly, several of these proteins have also been implicated in tissue aging and stem cell functions. In this study, we took advantage of the well-established defective repopulation of FA HSCs and carried out an in vivo shRNA screening in the Fancd2 mouse model to identify genes whose inhibition would improve HSC functions during replicative stress. We demonstrated that deregulated Pparγ activity is a limiting factor for HSC self-renewal and repopulating capacity. Pparγ could be a potent target for improving and delaying FA BM failure and HSC aging in general.
Results
Discussion The current study provides several pieces of evidence that abnormal Pparγ activation in HSCs leads to HSC function impairment under replicative stress. (1) shRNA against Pparγ and Pparγ-related genes were enriched in Fancd2-deficient HSPCs undergoing hematopoietic repopulation. (2) Specific knockdown of Pparγ improved the repopulation capacity of Fancd2-deficient HSCs. (3) Genetic or pharmacological inhibition of Pparγ decreased expression of Cdkna1a in mouse and human FA-deficient cells and improved the repopulation capacity of Fancd2-deficient HSCs. PPARγ is a central transcription factor regulating adipocyte differentiation and energy metabolism (Siersbaek et al., 2010). Interestingly, increased population of adipocytes in the BM has been observed in FA patients and mouse models (Pulliam-Leath et al., 2010), suggesting a possible dysregulation of PPARγ. Moreover, it has been shown that adipocytes are deleterious for HSC self-renewal (Naveiras et al., 2009). These observations are in accordance with our current study demonstrating that pharmacological or intrinsic activation of PPARγ impaired hematopoietic repopulation of Fancd2-deficient HSCs as well as WT HSCs. In this context, it is noteworthy that our recent study (Amarachintha et al., 2015) with Fanca- and Fancd2-deficient BM mesenchymal stromal cells revealed upregulation of PPARγ activity and Pparγc1a expression and adipocyte differentiation in FA, suggesting that hyperactivation of PPARγ could be a common feature of FA cells. In accordance with a deleterious effect of PPARγ on the hematopoietic system, development of anemia in humans has been shown to be a side effect after the introduction of a PPARγ agonist drug on the market (Werner and Travaglini, 2001). In addition, FA patients are frequently treated with androgens, which delay BM failure (Dokal, 2003). Interestingly, androgen and PPARγ signaling inhibit each other (Dokal, 2003; Du et al., 2009; Singh et al., 2006). Thus, the effectiveness of androgen treatment in FA patients may be in part related to PPARγ inhibition, although this hypothesis has to be investigated.