Serum Amyloid Protein A 1 (hSAA1) Is Overexpressed in Myelodysplastic Syndromes and Potentially Mediates Toll-Like Receptor 2 Innate Immunity Signaling in CD34+ Hematopoietic Stem Cells

Abstract 1703

Myelodysplastic syndromes (MDS) are a group of myeloid malignancies that originate from bone marrow hematopoietic stem/ progenitor cells (HSC). We have previously reported that deregulation of Toll-like Receptor (TLR) 2 mediated innate immunity signaling contributes to the pathogenesis of MDS through its impact on HSC. Several amyloid family peptides have been reported to be endogenous TLR2 agonists, including Ab-42 and the serum amyloid protein A (SAA1). To further dissect the role of this TLR2 centered molecular signaling in MDS, we characterized these amyloid peptides in MDS. Elisa assays indicated that the concentration of SAA1 protein in bone marrow plasma of MDS patients (N=17) was 3 fold higher (p-value 0.01) compared to normal controls, whereas in the same patient cohort the concentration of Ab-42 in MDS bone marrow plasma was elevated by ∼ 1.5 fold. We then examined expression of SAA1 in bone marrow CD34+ cells of MDS. RT-PCR results indicated that SAA1 RNA level was increased by 18 fold in MDS CD34+ cells compared to control (N=64, p-value 0.0001). While it is known that expression of SAA1 is highly sensitive to acute and chronic inflammation and can be regulated by multiple inflammatory cytokines such as IL-6 and IL-8, its regulation by TLR2 mediated signal in HSC is unknown. To examine the impact of TLR2 signal on SAA1 expression, we profiled RNA expression in an OCI-AML3 cell line that stably expresses the shRNA against TLR2. Microarray analysis indicated that in TLR2 shRNA expressing cells, RNA level of SAA1 decreased by ∼50%. Similar decrease of SAA1 RNA was also detected when primary CD34+ cells isolated from the bone marrow of MDS patients (N=4) were treated with retroviral mediated TLR2 shRNA. These results suggest that TLR2 deregulation may contribute to the overexpression SAA1 in MDS CD34+ cells. Further, we examined whether elevated bone marrow SAA1 level could affect HSC function. We treated in vitro cultured bone marrow CD34+ cells with a recombinant 105 amino acid SAA1 peptide and then analyzed its impact on hematopoietic colony formation in methylcellulose-medium. Results derived from four CD34+ HSC samples indicated that 1mg/ml SAA1 peptide led to a 16% decrease of hematopoietic colony numbers (p-value 0.02). This result suggests that SAA1 can negatively affect the hematopoietic capacity of HSC. Taken together, this study suggests that SAA1 may be part of the deregulated TLR2 mediated innate immunity signaling network that contributes to MDS pathogenesis. Previous studies have indicated that SAA level was elevated in HSC of the mice with telomere dysfunction (Sahin et al. Nature 2011). Therefore, we hypothesize that SAA is involved in the pathogenesis of MDS during ageing. Further molecular analysis of the role of SAA1, and its clinical impact (that is being analyzed), during ageing and MDS require further investigation.