Toll-Like Receptor (TLR) Signaling Adaptor Protein MYD88 in Myelodysplastic Syndromes (MDS)

Abstract 556

We have documented that deregulation of the Toll-like Receptor 2 (TLR2) centered innate immunity signals contribute to the pathogenesis of myelodysplastic syndromes (MDS). Of relevance, oncogenically active mutations of MYD88, a signal adaptor protein for TLR signal, have recently been identified as recurrent genetic lesions in both B-cell lymphoma and in chronic lymphocytic leukemia (CLL) (Vu N et al. Nature 2010 and Puente et al. Nature 2011). This information further supports the importance of innate immunity deregulation in leukemogenesis. To further characterize this pathway in MDS, we analyzed potential genetic alteration and expression level of MYD88 in patients of MDS. In a cohort of 40 MDS whole bone marrow mononuclear cell DNA, we first performed pyrosequencing analysis focusing on a list of previously reported MYD88 mutations (V217, W218, S219, I220, S222, M232, S243, L265, and T294). We did not detect mutation of any these hotspots on MYD88 in MDS. We then expanded the sequencing efforts to the entire coding region of MYD88 using an approach that combines PCR amplification and massive parallel sequencing. Still, no mutation of MYD88 was detected using this technique. We then examined the expression of MYD88 in CD34+ hematopoietic stem cells from 65 patients with MDS. In comparison to healthy donors, 26% of MDS patients (N=17) presented a more than 2 fold increase of MYD88 RNA, and 15% (N=10) had a 30%–90% increase. In average, MYD88 RNA level was 1.7 fold increased compared to control. Of potential clinical relevance, patients with higher MYD88 RNA expression in bone marrow CD34+ cells (above median value) (N=33) had a propensity of shorter period (24.4 mo) of overall survival (OS) in comparison to patients with lower levels of MYD88 expression (N=32) (32 mo)(P=0.05). We also found that patients with higher levels of MYD88 expression (split at 0.8 fold to controls) tended to have higher WBC (P=0.02). We have previously shown that blockade of the TLR2 mediated innate immunity signaling in MDS CD34+ cells could positively regulate erythroid lineage differentiation. To evaluate the potential of MYD88 blockade, we applied a 26 AA MYD88 inhibitory peptide that blocks its homodimerization (Invivogen, San Diego, CA) on primary CD34+ cells isolated from patients with lower-risk MDS (N=5). Methylcellulose medium supported colony formation assays indicated that the presence of MYD88 inhibitor led to an average of 60% increase for the numbers of erythroid colonies and a 30% increase for the numbers of total colonies. At the same time, we did not observe these effects of MYD88 blockade on the CD34+ cells isolated from the patients of higher-risk MDS (N=3). IL-8 is one of the key downstream transcriptional targets of the TLR-MYD88-NFkB innate immunity signaling that was documented to be elevated in bone marrow plasma of MDS. ELISA assays also indicated that blockade MYD88 in cultured MDS CD34+ cells led to a decrease of IL-8 concentration in medium. Taken together, these results indicated that MYD88 is overexpressed in hematopoietic stem cells of MDS and that blockade of MYD88 mediated innate immunity signaling may have therapeutic potential in treating patients with MDS.