RNA-sequencing based linkage analysis identifies supt16h as a target gene from a forward genetic screen

Chromatin organization and accessibility are fundamental to how genes are transcriptionally controlled. We identify the first vertebrate mutant for supt16h, a component of the FACT (FAcilitates Chromatin Transcription) complex along with Ssrp1 known to reorganize nucleosomes and assist in transcriptional elongation. We demonstrate its importance in hematopoietic stem cell (HSC) specification by regulating the elongation of Notch genes. Unexpectedly, Assay for Transposase Accessible Chromatin (ATAC) sequencing revealed that loss of supt16h does not affect histone accessibility on a Notch-specific or global level. Although the majority of genes are unaffected, loss of supt16h alters chromatin accessibility significantly at the p53 locus, leading to its overexpression in mutants. Upon downregulation of p53, both loss of Notch and loss of HSC phenotypes are rescued. Notably, ssrp1 mutants possessed normal elongation of Notch genes, levels of P53, and specification of HSCs. Our results highlight the discrete effects of Supt16h and Ssrp1 during HSC specification. Additionally, we demonstrate the relationship between supt16h and p53 during transcriptional elongation to specify HSC fate via modulation of Notch signaling.