Ectopic Expression of HDAC9 in Murine Lymphoid System Leads to Altered Lymphocyte Numbers and Proliferation as Well as Predisposition to Tumorigenesis.

Reversible acetylation of lysine residues on histone tails is associated with changes to chromatin structure and plays a key role in regulation of gene expression. In this process, histone hypoacetylation is generally associated with gene silencing and pharmacological inhibition of histone deacetylases (HDACs) leads usually to activation of gene expression. Decreased histone acetylation is a hallmark of cancer cells and increased HDAC expression or their mistargetting to specific gene promoters has been associated with a variety of tumors. In the past we have identified and cloned class IIa HDAC9. The HDAC9 gene is located in chromosome 7p21, which is frequently amplified in B-cell tumours such as mantle cell lymphoma (MCL) and in B-cell non-Hodgkin’s lymphoma cell lines. Consistently, initial analysis of patient samples and/or publicly available microarray data highlighted high levels of HDAC9 expression in chronic lymphocytic leukemia, folicullar lymphoma and MCL. Within the normal lymphoid system, HDAC9 is co-expressed with BCL-6 in germinal center B-cells (∼60% of cells). HDAC9 is also expressed in marginal zone B cells and a fraction of CD38 or CD27 positive subepithelial tonsilar cells. In order to examine the role of HDAC9 in the lymphoid development and pathogenesis of lymphoid malignancies we used Ig heavy chain enhancer (Eμ), which drives gene expression from early stages of B-cell development, to ectopically express HDAC9 in transgenic mice. Hemizygous and homozygous mice expressing Flag epitope tagged human HDAC9 (fHDAC9) transgene display throughout their lifespan altered B-cell development. Immunophenotypic analysis of B-cells isolated from bone marrow (BM) revealed an absence of cells expressing the pre-B/immature-B cell markers normally associated with C-E Hardy’s fractions. In vitro functional clonogenic assays for IL-7 responsive BM-derived B-cell progenitors demonstrated an increase (∼50%) in colony numbers in the transgenic BM. Moreover, morphologic and flow cytometric analyses of the transgenic colonies, but not those derived from normal BM, revealed the presence of granulocyte/macrophage colony forming units expressing the HDAC9 transgene, suggesting a lympho-myeloid lineage switch. This correlates with the finding that extramedullary myelopoiesis occurs in a fraction of mice presenting splenomegaly (44%). Furthermore, a subgroup of homozygous Eμ-fHDAC9 mice (n=16) developed tumours (81%) at middle age, and present with enlarged lymph nodes (6%) and abnormal hematopoietic elements in peripheral blood and BM. Taken together these data suggest that HDAC9 plays a role in B-cell maturation and its ectopic expression in early B-cells leads to perturbation of normal B-cell development, possibly predisposing transgenic mice to tumorigenesis.