Inhibition of Canonical WNT Signaling in the Bone Marrow Niche Prevents the Development of Disease in a Mouse MDS Model with Apc Haploinsufficiency

The myelodysplastic syndromes (MDS) include a spectrum of clonal hematopoietic stem cell (HSC) disorders that are characterized by ineffective hematopoiesis, peripheral cytopenia(s), morphologic dysplasia, and a variable propensity to transform to acute myeloid leukemia (AML). A del(5q) is the most common recurring cytogenetic abnormality in MDS, and is observed in 10-20% of primary MDS, and 40% of therapy-related MDS and AML. Our lab has had a long-standing interest in understanding how loss of function of tumor suppressor genes on 5q, such as APC, contributes to the pathogenesis of MDS and AML.

Apc is a key negative regulator of the canonical β-catenin (Ctnnb1)/WNT signaling pathway.WNT signaling has been implicated in self-renewal of HSCs and the regulation of hematopoiesis, and the deregulation of WNT signaling is associated with the development of hematological malignancies. Recent data suggests that WNT activation in the bone marrow (BM) microenvironment or niche may also contribute to the development of AML. Kode et al. (Nature 506:240, 2014) showed that expression of activated β-catenin in osteoblast cells in the BM microenvironment leads to AML in mice. Thus, aberrant WNT signaling in HSCs and/or the microenvironment that supports them may contribute to malignant transformation. We previously used a conditional mouse model (Mx1-Cre⁺, Apc ^fl/+, referred to as Apc^del/+) and showed that haploinsufficiency of Apc leads to ineffective hematopoiesis (Blood 115:3481, 2010). Interestingly, haploinsufficient loss of Apc in the niche stromal cells, but not the hematopoietic progenitors, led to the development of MDS, characterized by a severe macrocytic anemia, dyserythropoiesis, megakaryocytic proliferation, and mild granulocytic dysplasia (Blood 123:228, 2014). In this study, we sought to determine whether haploinsufficient loss of Apc, mediates this disease through modulating WNT signaling, versus other Apc functions, and whether inhibition of WNT signaling could prevent the development of MDS in Apc^del/+mice.

Here, we demonstrate thatsignaling through the canonical WNT pathway mediates the development of MDS in Apc haploinsufficient mice. Specifically, loss of one copy of Ctnnb1 is sufficient to prevent the development of MDS in Apc^del/+ mice. Compared to the median survival of Apc ^del/+ mice, which was 255 days, all Apc ^del/+, Ctnnb1 ^del/+ mice survived until the end of the study (~400 days) (P<0.001). In addition, we showed that altered canonical WNT signaling in the BM microenvironment is responsible for the disease. Using bone marrow transplantation of wild type BM cells, we showed that Apc ^del/+ recipients develop MDS and die of a fatal anemia by ~4 months of age. In contrast, Apc ^del/+, Ctnnb1 ^del/+ recipients survived on average 8-10 months longer (median survival 115 vs 413 days, P <0.0001). To determine if we could prevent the development of MDS, we treated mice with the FDA-approved antihelminth drug, Pyrvinium, an inhibitor of the WNT pathway via activation of casein kinase 1 alpha and β-catenin degradation. Apc ^del/+mice in the control DMSO-treated group developed disease with a median survival of 227 days; however, during this time period, none of the Pyrvinium-treated mice became moribund. The mice succumbed to MDS about 100 days after Pyrvinium treatment was suspended, indicating that the drug prevented the development of disease, and must be continuously administered. We also showed that Pyrvinium treatment does not have to be administered before anemia develops; however, the drug is more effective when it is started in animals that display mild-to-moderate anemia rather than severe anemia.

These studies have important implications for the treatment of certain forms of MDS. Kode et al. (Nature 506:240, 2014) showed nuclear accumulation of β-catenin in osteoblasts from ~40% of MDS/AML patients, supporting the idea that activated WNT signaling in the BM microenvironment may contribute to the development of disease. Targeting the tumor microenvironment is now recognized as a promising approach to complement current therapies, and our data highlight a potential new strategy of targeting the WNT signaling pathway for treating some forms of MDS.