-Author name in bold denotes the presenting author
-Asterisk * with author name denotes a Non-ASH member
Clinically Relevant Abstract denotes an abstract that is clinically relevant.

PhD Trainee denotes that this is a recommended PHD Trainee Session.

Ticketed Session denotes that this is a ticketed session.

1871 The RNA Binding Protein IGF2BP3 Is Required for MLL-AF4 Mediated Leukemogenesis

Program: Oral and Poster Abstracts
Session: 603. Oncogenes and Tumor Suppressors: Poster II
Hematology Disease Topics & Pathways:
AML, Leukemia, Diseases, Biological Processes, Lymphoid Malignancies, Myeloid Malignancies, pathogenesis, pathways
Sunday, December 6, 2020, 7:00 AM-3:30 PM

Tiffany Tran, BS1*, Jaspal Bassi2*, Neha Nibber3*, Julia Philipp4*, Jolene Draper4*, Tasha Lin, MD5*, Jayanth Kumar Palanichamy, MD, MBBS6*, Amit Kumar2*, Oscar Silva, MD, PhD7*, May Paing2*, Jennifer King, MD8*, Sol Katzman4*, Jeremy Sanford4* and Dinesh S. Rao, MD, PhD1

1Pathology & Laboratory Medicine, UCLA, Los Angeles, CA
2Pathology & Laboratory Medicine, UCLA, Los Angeles
3UCLA, Los Angeles
4Molecular, Cellular, and Developmental Biology, UCSC, Santa Cruz
5Hematology/Oncology, UCLA, Los Angeles
6All India Institute of Medical Sciences, New Delhi, India
7Department of Pathology, Stanford University Medical Center, Stanford, CA
8Rheumatology, UCLA, Los Angeles, CA

Chromosomal rearrangements of the mixed-lineage leukemia (MLL) gene are observed in acute lymphoblastic leukemias (ALL), acute myeloid leukemias (AML), and in rare mixed-lineage leukemia. Despite recent progress in therapeutic approaches, patients with MLL-rearranged (MLLr) leukemias still have very poor outcomes and a high risk of relapse. Of more than 90 fusion partner genes, MLL-AF4 is the most common MLL fusion protein in patients. Previously, we found that the RNA binding protein IGF2BP3 was specifically overexpressed in MLL-rearranged B-ALL, and enforced expression in vivo led to a pathologic expansion of hematopoietic stem and progenitor cells resulting in B and myeloid cell leukocytosis in the periphery. However, the requirement of IGF2BP3 in MLL-AF4 mediated leukemogenesis remains to be determined.

Utilizing our previously generated list of differentially regulated targets with IGF2BP3 knockdown and a published dataset of MLL-Af4 targets, we determined that transcripts modulated by IGF2BP3 showed significant enrichment for MLL-Af4-bound genes. Furthermore, we observed that MLL-AF4 directly binds to and transcriptionally induces IGF2BP3. We performed ChIP-PCR assays on RS4;11 and SEM cell lines, human B-ALL cell lines that carry the MLL-AF4 translocation, and determined that the region in the first intron of IGF2BP3 is strongly bound by MLL-AF4. Furthermore, we observed a dose-dependent increase in luciferase reporter activity when we co-transfected a dual-luciferase reporter vector containing the promoter region of IGF2BP3 with increasing levels of MLL-AF4 expressing retroviral vector.

To determine the role of Igf2bp3 in MLL-Af4 driven leukemogenesis, we generated the first Igf2bp3 KO murine model. Surprisingly, Igf2bp3 KO mice maintain normal, steady-state hematopoiesis. However, in striking contrast, deletion of Igf2bp3 in the MLL-Af4 leukemia model, significantly increases the survival of MLL-Af4 transplanted mice and greatly attenuates the disease. Furthermore, Igf2bp3 deficiency significantly reduced the tumor burden and disease severity. We observed significant decreases in WBC counts, spleen weights, and infiltrating leukemic cells visualized in histopathological analysis of hematopoietic tissues and quantified by FACS analysis. Moreover, deletion of Igf2bp3 led to a leukemia-initiating cell (LIC) disadvantage in vivo, demonstrated by significantly reduced engraftment in primary transplanted mice and reconstitution of secondary serially transplanted mice.

To identify the transcripts directly regulated by Igf2bp3 in the context of MLL-Af4 driven leukemia, we carried out enhanced crosslinking and immunoprecipitation (eCLIP) transcriptome analysis of MLL-Af4 transformed early stem and progenitor cells and primary cells purified from splenic tumors of MLL-Af4 leukemic mice. We discovered an IGF2BP3-regulated post-transcriptional operon governing leukemic cell survival and proliferation, in which mRNA targets include the Hoxa locus and numerous genes within the Ras signaling pathway.

In our study, we provide evidence that Igf2bp3 is required for the initiation of MLL-Af4 driven leukemia. We determined that Igf2bp3 is necessary for the development of and function of MLL-Af4 LICs. Mechanistically, we show that Igf2bp3 binds to and modulates the expression of hundreds of critical target transcripts. In summary, we demonstrate that Igf2bp3 is a positive regulator of MLLr leukemogenesis by targeting Hoxa transcripts such as Hoxa9 and numerous Ras signaling pathway transcripts, thereby controlling multiple downstream effector pathways required for disease initiation and aggressiveness. Together, our findings identify IGF2BP3 as an important, potential therapeutic target in this disease.

Disclosures: No relevant conflicts of interest to declare.

Previous Abstract | Next Abstract >>
*signifies non-member of ASH