Diamond Blackfan Anemia Defects In Rps19-Mutant Embryonic Stem Cells: Rescue by Gene Replacement but Not Glucocorticoid Treatment
Authors: Singh Sharon , Melgar Katelyn , Goldberg Tracie A. , Nihrane Abdallah , Lipton Jeffrey Michael , Ellis Steven , Liu Johnson M. 
Published: October 2019

Abstract
Abstract 2237

Background:
Diamond Blackfan anemia (DBA) is one of the rare inherited bone marrow failure syndromes (IBMFS), characterized by erythroid hypoplasia, congenital anomalies and a cancer predisposition. DBA is caused by ribosomal protein haploinsufficiency, which somehow triggers apoptosis of erythroid precursors, possibly through activation of p53. Some DBA patients show a response to steroid treatment, while others remain transfusion-dependent. While the mechanism of action of steroids in DBA is unclear, recent work has suggested that p53 may antagonize glucocorticoid-induced proliferation of normal erythroid progenitors1.

Objective:
Our goal was to create a murine embryonic stem (ES) cell model of DBA with a mutation in Rps19 to study the pathophysiology of DBA and to test glucocorticoid responsiveness.

Methods:
The Rps19-mutated murine embryonic stem cell line, S17-10H1 was created using a gene trap strategy. The ES cells were induced to undergo primary differentiation into embryoid bodies (EBs). Day 9–11 EBs, representing definitive hematopoiesis, were re-plated with hematopoietic cytokines (stem cell factor, interleukin (IL)-3, IL-6 and erythropoietin) in methylcellulose, and secondary differentiated colonies were scored on day 10. S17-10H1 cells were transfected by electroporation with a plasmid vector expressing either wild-type Rps19 cDNA or an empty vector control. Protein and mRNA levels of the tumor suppressor p53 were measured at the ES cell stage. Secondary differentiated hematopoietic colonies were grown with and without glucocorticoids (either dexamethasone at 1nM, 10nM, 100nM, 1μM or hydrocortisone at 50nM, 2.5μM concentration).

Results:
Western blot analyses confirmed S17-10H1 Rps19-haploinsufficiency. The mutant cell line had reduced EB formation (hematopoietic and non-hematopoietic) following primary differentiation. Significant defects in both erythroid (BFU-E) and myeloid (CFU-GM) formation were found following secondary hematopoietic differentiation of day 9–11 EBs. These defects were specific to Rps19 haploinsufficiency since all defects were rescued by stable transfection of the mutant cell lines with an Rps19-expressing correction vector. However, glucocorticoid treatment was unable to rescue defects in secondary differentiation (hematopoietic colony formation). Finally, we also examined baseline levels of the tumor suppressor p53, which may be induced following abortive ribosome assembly resulting from ribosomal protein haploinsufficiency. Although there was no difference in transcription levels of p53 mRNA between mutant and control cells as determined by quantitative PCR, baseline levels of p53 protein were significantly increased in the mutant ES cells as compared to controls.

Conclusions:
(1) In our model system, Rps19 mutation was associated with a relatively early defect in hematopoietic progenitors, since both erythroid and myeloid (or mixed) colony formation was diminished.
(2) Hematopoietic defects in our murine ES cell model of DBA were rescued by Rps19 gene transfer but not by glucocorticoid therapy.
(3) We found significant accumulation of baseline p53 protein (but not mRNA) levels in mutant ES cells, suggesting that Rps19 haploinsufficiency leads to inhibition of the p53-degradation pathway.
(4) Taken together, these results suggest that p53 may antagonize glucocorticoid-induced proliferation in our Rps19-mutant ES cell model. Experiments to test this hypothesis are in progress.

Reference:
Ganguli G, Back J, Sengupta S, Wasylyk B. The p53 tumour suppressor inhibits glucocorticoid-induced proliferation of erythroid progenitors. EMBO Rep. 2002;3:569-574.

Disclosures:
No relevant conflicts of interest to declare.