Identification of Chromatin Remodeling Genes Arid4a and Arid4b as Leukemia Suppressor Genes

Mei-Yi Wu; Karen W. Eldin; Arthur L. Beaudet

Disclosures

J Natl Cancer Inst. 2008;100(17):1247-1259. 

In This Article

Discussion

Arid4a and Arid4b knockout mice provide a suitable animal model for better understanding the progression of a premalignant hematologic disorder and the eventual transformation to AML. Using this model, we found disruption of myeloid homeostasis in the Arid4a -/- mice due to the increase of HSCs and downstream progenitors, which might be the principal components of the expansion of a transformed leukemic stem cell compartment with aggressive self-renewal properties. We have obtained some insight into the molecular mechanisms by defining the downstream regulatory impact on the Hox and Fox genes. Moreover, our results suggest that Arid4a controls chromatin modification necessary to support normal hematopoiesis, and the data are consistent with an important role of epigenetic regulation in cancer development.

H3K4me3 is a mark of transcriptionally active chromatin states, whereas the H3K9me3 and H4K20me3 modifications are usually found at repressive chromatin domains. However, we found that H3K4me3, H3K9me3, and H4K20me3 were all increased in bone marrow cells from mice deleted for Arid4a. We envision that these histone methylations will reach a combinatorial steady state that may mediate activation or repression of specific genomic loci or chromosome domains. We propose that ARID4A is a critical determinant for distinct histone methylation states and that it plays an important role in coordination between the modification marks at different chromatin regions. Further analysis of chromatin-specific components associated with ARID4A and/or ARID4B could more clearly address the contribution of these two proteins to the combinatorial pattern of histone modifications. A goal of future research should be to determine whether ARID4A and ARID4B contain any intrinsic enzyme activity and whether they mediate their effects either through direct interaction with chromatin complexes or through intermediate steps involving various pathways that are modulated through chromatin remodeling.

Our results suggest plausible molecular mechanisms for the hematologic disorder in Arid4a-deficient mice based on downregulation of a number of homeobox genes (Pitx2 and a cluster of Hoxb genes including Hoxb3, Hoxb5, Hoxb6, and Hoxb8) in Arid4a-/- bone marrow cells with or without Arid4b haploinsufficiency. A homeobox sequence encodes a protein domain that binds DNA. Homeobox genes encode transcription factors and are classified into two subgroups: Hox and non-Hox genes. Abnormal expression of both groups of homeobox genes plays a role in the pathogenesis of myeloid malignancies.[31,32] Little is known about the molecular mechanisms of homeobox gene regulation, but the MLL gene is reported to positively regulate multiple Hox genes.[28]MLL encodes a histone methyltransferase that methylates H3K4.[42] Leukemogenic MLL translocations define a specific group of leukemias (mixed lineage leukemia).[43] We suggest that Arid4a and Arid4b play a prominent role in leukemic transformation by controlling the trimethylation of H3K4 and H3K9. Although we did not find that mutations of Arid4a and Arid4b affected the expression of Mll, our results suggest plausible molecular mechanisms whereby Arid4a and Arid4b might regulate hematopoiesis by controlling the expression of specific homeodomain genes, such as Pitx2, required for normal erythropoiesis,[44] and a subset of Hoxb genes, including Hoxb6, whose disruption results in an increase of early erythrocyte progenitors;[45]Hoxb8, which affects lineage-specific development of hematopoietic progenitor cells;[46,47] and Hoxb3, whose deficiency impairs B lymphopoiesis[48] (Figure 7, B).

Our results also imply that Arid4a and Arid4b are involved in the regulation of a forkhead box gene Foxp3 (Figure 7, B), an X chromosome-encoded forkhead transcription factor family member that plays an important role in the development and function of natural regulatory T cells.[40] FOXP3 protein interacts with AML1 to control regulatory T-cell function.[39] Although Arid4a and Arid4b mutations did not affect the expression of AML1, we found decreased expression of Foxp3 in the Arid4a-/-Arid4b± mice. FOXP3 functions as a transcriptional regulator by assembling chromatin remodeling complexes involved in histone modification.[49] In humans, mutations of FOXP3 leads to an X-linked fatal autoimmune disease known as IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) syndrome and an analogous lymphoproliferative disease.[50] Deficiency of Foxp3 in mice leads to an early onset, highly aggressive and fatal autoimmune disease characterized by excessive proliferation of CD4+ T cells and extensive infiltration by leukocytes in multiple organs.[38,51,52] Recently, it has been reported that FOXP3 protein functions as a tumor suppressor involved in the development of breast cancer.[41] Decreased expression of Foxp3 may be involved in the disease mechanisms through epigenetic effects in the Arid4a -/- Arid4b ± mice, which develop an increased population of T cells in bone marrow and progress to AML and myeloid sarcoma with various tissues infiltrated by leukemic cells. It will be of interest to examine whether decreased expression of Foxp3 contributes to the high frequency of leukemia malignancies in the Arid4a -/- Arid4b ± mice.

We present evidence that ARID4A and ARID4B function as tumor suppressors and that a myelodysplastic/myeloproliferative disorder in mice carrying the Arid4a or/and Arid4b mutations progresses to hematologic malignancies, resembling human CMML and AML. However, we have not investigated whether mutations in ARID4A and ARID4B participate in genetic and/or epigenetic mechanisms of human CMML and AML, or other cancers. Although the ARID4A and ARID4B proteins have been identified as breast cancer-associated antigens[3,20,21] and Arid4a and Arid4b are involved in the regulation of Foxp3, which functions as a breast cancer suppressor gene,[41] we have not observed primary solid tumors in mice with ARID4 family deficiency, (although we have not performed a detailed search for breast cancer). Further study of the Arid4 gene family may advance our understanding of the connection between gene regulation, epigenetic control, disease development, and cancer formation. We also suggest that gene regulation by ARID4A and ARID4B should be examined for potential disease-related roles, not only in human malignancies, but also in other complex disease traits.

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