Chromosome Instability Underlies Hematopoietic Stem Cell Dysfunction and Lymphoid Neoplasia Associated with Impaired Fbw7-mediated Cyclin E Regulation

The Fbw7 ubiquitin ligase critically regulates hematopoietic stem cell (HSC) function, though the precise contribution of individual substrate ubiquitination pathways to HSC homeostasis is unknown. In the work reported here, we used a mouse model in which we introduced two knock-in mutations (T74A and T393A [changes of threonine to alanine at positions 74 and 393]) to disrupt Fbw7-dependent regulation of cyclin E, its prototypic substrate, and to examine the consequences of cyclin E dysregulation for HSC function. Serial transplantation revealed that cyclin ET74A T393A HSCs self-renewed normally; however, we identified defects in their multi-lineage reconstituting capacity. By inducing hematologic stress, we exposed an impaired self-renewal phenotype in cyclin E knock-in HSCs that was associated with defective cell cycle exit and the emergence of chromosome instability (CIN). Importantly, deregulated cyclin E in hematopoietic cells induced p53. Thus, we studied the effect of p53-loss on cyclin ET74A T393A HSCs. Cyclin ET74A T393A; p53-null HSCs exhibited defects in both self-renewal and multi-lineage reconstitution. By enumerating chromosomes in metaphase spreads, we found cyclin ET74A T393A; p53-null hematopoietic stem and progenitor cells (HSPCs) demonstrated significant CIN. Moreover, CIN was a characteristic feature of fatal T-cell malignancies that ultimately developed in recipients of cyclin ET74A T393A; p53-null HSCs. In thymocytes isolated from recipients of cyclin ET74A T393A ; p53-null HSCs, CIN was associated with the marked potentiation of cyclin E kinase activity by p53- or p21-loss. Together, our findings demonstrate the importance of Fbw7-dependent cyclin E control to the hematopoietic system and highlight CIN as a characteristic feature of HSC dysfunction and malignancy induced by deregulated cyclin E in vivo.