Dissecting the Roles of PPM1D and TP53 in Hematopoietic Stem Cells and Clonal Hematopoiesis

About This Grant

The acquisition and selection of recurrent, somatic mutations in hematopoietic stem cells (HSCs), termed clonal hematopoiesis (CH), has emerged as a significant risk for the development of myeloid neoplasia and mortality. Mutant HSCs can undergo clonal expansion by outcompeting wild-type HSCs and clonal evolution upon the acquisition of additional genetic alterations. Exposure to cytotoxic therapy is a significant risk factor for developing CH, and CH that arises after such exposures is more likely to carry mutations in genes that regulate the DNA damage response (DDR), with TP53 and PPM1D being the most common. PPM1D encodes a serine/threonine phosphatase that suppresses the DDR and p53 activation. Accordingly, PPM1D is mutationally activated in CH whereas loss-of-function mutations occur in TP53. Despite seemingly convergent functions, there are important biological and phenotypic differences between PPM1D- and TP53-mutant CH which suggests that PPM1D and p53 may drive HSC clonal changes through distinct processes. In addition to directly inactivating p53 via dephosphorylation, PPM1D can regulate the DDR upstream of p53 and changes in cell cycle, senescence, and apoptosis downstream of p53. Though PPM1D-mutant CH is more common and expands more rapidly in both aging and cytotoxic therapy-exposed individuals, TP53-mutant CH confers a significantly higher risk of malignancy. Accordingly, PPM1D- and TP53-mutant clonal evolution are associated with different co-mutation and karyotypic features. Using our genetically modified mouse models (GEMMs), we discovered that mutations in PPM1D and TP53 drive HSC expansion in different ways and hypothesize that the distinct phenotypes of PPM1D- and TP53-mutant CH are due to differences in the activation of the DNA damage response and p53-dependent cellular programs in the setting of cytotoxic therapy and accumulation of genetic alterations. To test this hypothesis, advance mechanistic insights, and guide therapeutic efforts we propose two aims. In Aim 1 we will quantify differences in how Ppm1d activation and Trp53 inactivation alter the immunophenotypic composition of the hematopoietic stem and progenitor cell pools (HSPCs) of our GEMMs at baseline and after genotoxic stress. In Aim 2 we will determine the effects of Ppm1d activation and Trp53 inactivation on the HSPC cell states and transcriptional programs under homeostatic conditions and after a DNA damaging insult. Completing these aims will define the distinct ways PPM1D and p53 shape the immunophenotypic and transcriptional landscape of hematopoiesis and highlight the cellular states and transcriptional programs that may underlie clonal changes. In so doing, we seek to improve prognostication and treatment strategies for CH and clonal evolution. .