HSPA9/mortalin inhibition disrupts erythroid maturation through a TP53-dependent mechanism in human CD34+ hematopoietic progenitor cells.

Myelodysplastic syndromes (MDS) are a heterogeneous group of clonal hematopoietic stem cell malignancies characterized by abnormal hematopoietic cell maturation, increased apoptosis of bone marrow cells, and anemia. They are the most common myeloid blood cancers in American adults. The full complement of gene mutations that contribute to the phenotypes or clinical symptoms in MDS is not fully understood. Around 10%-25% of MDS patients harbor an interstitial heterozygous deletion on the long arm of chromosome 5 [del(5q)], creating haploinsufficiency for a large set of genes, including HSPA9. The HSPA9 gene encodes for the protein mortalin, a highly conserved heat shock protein predominantly localized in mitochondria. Our prior study showed that knockdown of HSPA9 induces TP53-dependent apoptosis in human CD34+ hematopoietic progenitor cells. In this study, we explored the role of HSPA9 in regulating erythroid maturation using human CD34+ cells. We inhibited the expression of HSPA9 using gene knockdown and pharmacological inhibition, and found that inhibition of HSPA9 disrupted erythroid maturation as well as increased expression of p53 in CD34+ cells. To test whether the molecular mechanism of HSPA9 regulating erythroid maturation is TP53-dependent, we knocked-down HSPA9 and TP53 individually or in combination in human CD34+ cells. We found that knock-down of TP53 partially rescued the erythroid maturation defect induced by HSPA9 knock-down, suggesting that the defect in cells with reduced HSPA9 expression is TP53-dependent. Collectively, these findings indicate that reduced levels of HSPA9 may contribute to the anemia observed in del(5q)-associated MDS patients due to the activation of TP53. SIGNIFICANCE STATEMENT: MDS is the most common adult myeloid blood cancer in the United States. A typical symptom of MDS patients includes fatigue that is associated with anemia. Some MDS patients harbor del(5q), with haploinsufficiency for a set of genes including HSPA9. We showed that inhibition of HSPA9 disrupts erythroid maturation in human CD34+ hematopoietic progenitor cells, through a TP53-dependent mechanism. Our findings not only indicate that reduced levels of HSPA9 may contribute to anemia observed in del(5q)-associated MDS patients, but also provide new insights into potential mechanisms of anemia.