HMGA1 Drives Inflammatory Pathways, Cell Cycle Progression, and Embryonic Stem Cell Genes During Leukemic Transformation

Abstract 1371

Although the high mobility group AT-hook 1 (HMGA1) gene functions as a potent oncogene in experimental models and high expression of HMGA1 portends a poor prognosis in diverse tumors, its role in leukemogenesis has remained elusive. We showed previously that HMGA1 induces leukemic transformation in cultured cells and causes aggressive lymphoid leukemia in transgenic mice. Inhibiting HMGA1 expression blocks colony formation in human lymphoid leukemia cells in vitro. Moreover, high levels correlate with relapse in childhood acute lymphoblastic leukemia (ALL), suggesting that it plays an important role in ALL. Because HMGA1 functions as a chromatin remodeling protein that modulates gene expression, we hypothesized that it drives leukemogenesis by dysregulating specific genes and pathways. To identify genes and cellular pathways induced by HMGA1 that could be targeted in therapy, we performed global gene expression profile analysis from lymphoid cells from the HMGA1 transgenic mice at different stages in tumorigenesis. All HMGA1 transgenics succumb to lymphoid malignancy with complete penetrance by 8–12 months. Pooled RNA samples at 2 months (before tumors develop) and 12 months (after tumors are well-established) were analyzed for differential expression of >20,000 unique genes by microarray analysis (Affymetrix) using both a parametric and nonparametric approach. A subset of differentially expressed genes was confirmed using quantitative, RT-PCR. Differentially expressed genes were analyzed for cellular pathways and functions using Ingenuity Pathway Analysis (IPA; www.ingenuity.com) and Gene Set Enrichment Analysis. To determine if these genes and pathways were relevant in human ALL, we knocked down HMGA1 expression in human ALL cells and assessed expression of a subset of the differentially expressed genes.

Early in leukemogenesis (at 2 months), 113 genes were differentially expressed in the HMGA1 transgenics compared to controls. In established leukemia (12 months), 715 genes were differentially expressed. In established tumors, the dysregulated genes are involved in cancer, cell cycle regulation, and cell-mediated immune response by Ingenuity Pathway Analysis. Geneset enrichment showed that embryonic stem cell genes are enriched in the established leukemic cells. At both early and late stages in leukemogenesis, differentially regulated genes are involved in cellular development, hematopoiesis, and hematologic development. Early in leukemogenesis, most of the significantly dysregulated genes are involved in the inflammatory response and included NF-kappaB as a major node. In human ALL cells, knock-down of HMGA1 also resulted in knock-down of genes identified in our transgenic model, suggesting that these HMGA1 regulated genes are also relevant to human ALL.

In summary, we found that HMGA1 induces inflammatory pathways early in leukemogenesis and pathways involved in embryonic stem cells, cell cycle progression, and cancer in established tumors. HMGA1 also dysregulates genes involved in cellular development and hematopoiesis at both early and late stages of tumorigenesis. Some of these HMGA1 pathways were also present in human ALL cells. Moreover, these results provide mechanistic insight into HMGA1 function at different stages in leukemogenesis and point to cellular pathways that could serve as therapeutic targets in ALL.