Abstract A37: PDZD2 is essential for steady-state hematopoiesis and its 37-kDa secreted product, sPDZD2, functions as a soluble tumor suppressor in AML

Abstract PDZ domain-containing protein 2 (PDZD2) is almost universally silenced through hypermethylation in myeloid malignancies. PDZD2 encodes for a 300kDa protein that it is cleaved into a 37kDa protein that is secreted to the tissue microenvironment (sPDZD2). Given that sPDZD2 has been reported to function as a tumor suppressor in solid tumors and its almost universal loss in AML, we hypothesized that PDZD2 is required for normal hematopoiesis and that sPDZD2 functions as a soluble tumor suppressor in the hematopoietic system. To determine if sPDZD2 is secreted to the bone marrow (BM) microenvironment we analyzed sPDZD2 levels in BM plasma isolated from primary AMLs and healthy donors using ELISA. We confirmed sPDZD2 secretion by normal CD34+ HSPC and detected lower levels in AMLs. In addition, treatment of a panel of AML cell lines (n=9) and primary human AML cells (n=4) with recombinant sPDZD2 (r-sPDZD2; dose: 100nM-300nM/day) led to growth inhibition, upregulation of differentiation markers (CD15/CD11b) and cell cycle arrest in G0/G1. To identify signaling pathways downstream of sPDZD2, we treated MV4-11 cells with 100nM r-sPDZD2 and performed phospho-kinase protein array followed by immunoblot. In vitro treatment with r-sPDZD2 led to inhibition of CREB(S133) phosphorylation, as well as increased GSK3b(S9), RSK1/2(S221/S227) and p53(S46) phosphorylation. In order to determine whether PDZD2 plays a role in normal hematopoiesis we performed in vitro differentiation in CRISPR-edited human HSPCs. We used a Cy3-labeled recombinant Cas9 protein and specific single guide RNAs (gRNAs) targeted against PDZD2. Cy3-positive cells were then used for in vitro myeloid or erythroid differentiation, and cell surface markers were evaluated by flow-cytometry analysis over a period of 11 days. While PDZD2-edited cells (PDZD2 KO) displayed a minor defect on myeloid differentiation due to insufficient upregulation of CD15, erythropoiesis appears to be more compromised, with a significant failure to adequately upregulate both CD235a and CD7. To determine the role of Pdzd2 in vivo, we took advantage of a Pdzd2Gt/Gt gene-trap knock-out mouse model and performed peripheral blood (PB) and BM analysis. PB counts at 8-weeks of age showed anemia, with a significant decrease in both red blood cell counts and hemoglobin levels, an observation compatible with the impaired erythropoiesis seen in vitro after PDZD2 KO in human HSPCs. In addition, we observed a trend to decreased MEPs and short-term HSCs, in the BM of 12-week-old mice (n=3), though the small size of the cohort prevented robust statistical analysis. In summary, our findings are the first to shed light on a previously unrecognized role for PDZD2 in hematopoiesis. Loss of PDZD2 in HSPC resulted in impaired erythroid differentiation both in vitro and in vivo. Moreover, our findings validate a soluble tumor suppressor role for sPDZD2 in AML, similar to that seen in solid tumors. Importantly, we further demonstrate a potential role for recombinant sPDZD2 as a novel therapeutic approach for AML. Citation Format: Marlise L Guerrero Schimpf, Courtney C Sparger, Hsuan-Ting Huang, Dean Wade, Maria E. Figueroa. PDZD2 is essential for steady-state hematopoiesis and its 37-kDa secreted product, sPDZD2, functions as a soluble tumor suppressor in AML [abstract]. In: Proceedings of the AACR Special Conference: Acute Myeloid Leukemia and Myelodysplastic Syndrome; 2023 Jan 23-25; Austin, TX. Philadelphia (PA): AACR; Blood Cancer Discov 2023;4(3_Suppl):Abstract nr A37.