Abstract 4798: The niche specific role of CD44 splice isoform expression in blast crisis leukemia stem cell generation

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Introduction Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm initiated in hematopoietic stem cells by expression of the BCR-ABL1 fusion oncogene and its protein product, which enhances ABL1 kinase activity. Targeted BCR-ABL tyrosine kinase inhibitors (TKIs), such as Imatinib, and the second generation TKIs like Nilotinib as well as the dual specific SCR and ABL inhibitor Dasatinib have significantly slowed disease progression by eradicating the bulk of BCR-ABL1 expressing cells in the circulation. However, progression to a therapeutically resistant blast crisis (BC) phase is driven by CD34+CD38+Lin- progenitors that co-opt stem cell properties, such as enhanced self-renewal and survival, albeit in a deregulated manner. These BC leukemia stem cells (BC LSC) harbor enhanced BCL2 and beta-catenin expression. Seminal research shows that a key Wnt/beta-catenin target gene, CD44, plays a vital role in cancer stem cell survival and retention in the malignant microenvironment. Because alternative mRNA splicing in humans generates many CD44 isoforms, we investigated CD44 variant expression and retention of human BC LSC in specific hematopoietic niches. Methods and Results We performed whole transcriptome RNA sequencing (RNA Seq) of FACS sorted progenitors (Lin-CD34+CD38+) from chronic phase (CP)(n=8) and blast crisis (BC)(n=8) CML patients as well as from normal cord blood (CB) (n=7) and adult peripheral blood (NPB)(n=4). A number of CD44 transcript variants were detected: 3, 4 (CD44s), 5, 6, 7, 8 plus additional variants. RNA seq analysis uncovered a higher overall CD44 expression in BC compared to CP progenitors. Notably, one isoform of CD44, variant 3, was highly upregulated in BC compared to CP progenitors and minimally expressed by their normal counterparts. Moreover, BC progenitors that engrafted in RAG2-/-γc-/- harbored CD44v3 expression in the bone marrow and the splenic niche and this differential BC LSC isoform expression was reduced after dasatinib treatment. In addition, BC LSC in the spleen showed a reduction of migration specific markers, such as RHAMM, ICAM-1 and Osteopontin, compared with the bone marrow resident BC LSC. Furthermore, a comparative splice isoform specific qRT-PCR analysis of CD44 variants expression in young versus old bone marrow showed no correlation with aging. In fact, human embryonic stem cells harbored variant 3 expression. Conclusions These data suggest that CD44 transcript variant 3 upregulation occurs following malignant reprogramming of hematopoietic progenitors that enables them to adopt features of an embryonic transcriptional program in select microenvironments. Detection of CD44 variant 3 has the potential to more precisely identify patients at risk for relapse and progression, suggest that combined therapeutic strategies involving a BCR-ABL specific TKI and a CD44 monoclonal antibody may decrease relapse and BC transformation rates. Citation Format: Frida L. Holm, Eva Hellqvist, Cayla Mason, Christian Barrett, Kelly A. Frazer, Anil Sadarangani, Catriona HM Jamieson. The niche specific role of CD44 splice isoform expression in blast crisis leukemia stem cell generation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4798. doi:10.1158/1538-7445.AM2014-4798