Abstract

Histone Deacetylase 9 (HDAC9) is a class IIa chromatin-modifying enzyme that, within the hematopoietic system, is preferentially expressed in the B-cell lineage. In our previous works, in order to identify HDAC9 function in the B cell lineage we developed mice that constitutively expressed human HDAC9 from early stages of B-cell development, under the control of the immunoglobulin heavy chain (IgH) enhancer. These mice developed lymphoproliferative disorders, including indolent Marginal Zone Lymphoma (MZL) and more aggressive post-Germinal Center (GC) lymphomas, demonstrating an oncogenic role for HDAC9 in B-cells.
In order to examine the relationship between diseases observed in the mouse model and human primary lymphoma, we have examined, using immunohistochemistry (IHC) the expression of full length HDAC9 isoform in a panel of various B-cell malignancies from human tumor samples. The study group included 59 non-Hodgkin lymphomas (NHL), and 3 classical HL. Non-HL consisted of 34 diffuse large B cell lymphoma (DLBCL), 9 follicular lymphoma (FL), 5 marginal zone lymphoma (MZL), 6 mantle cell lymphoma (MCL), and 2 small lymphocytic lymphomas (SLL). HDAC9 expression was assessed by IHC using tissue microarray and/or routine tissue sections. Protein expression was scored as negative (0), low (1), or high (2) depending on the staining signal intensity. Expression of HDAC9 in the nuclei of the tumor cells was compared with that seen in adenocarcinoma cells; if equal or higher, then expression of HDAC9 was considered high and if lower, then expression of HDAC9 was considered low. Five reactive lymph nodes were studied to assess the baseline expression of HDAC9. Rectal adenocarcinomas were used as positive controls. In reactive lymph nodes, HDAC9 was weakly expressed in a subset of germinal center cells, a subset of lymphoid cells in the paracortex as well as in endothelial cells.
HDAC9 expression was detected in all subsets of B-cell lymphomas analyzed and in most cases with a level of expression higher than those seen in reactive lymph nodes. DLBCL and MCL tumors had the highest frequency of high HDAC9 expression among the B-cell lymphomas analyzed, 77 and 83% (Fisher's exact test P = 1.0), respectively. No differences in HDAC9 expression were detected in DLBCL of GC and non-GC type. In contrast, most (69%) of the low-grade B cell lymphomas showed no or lower expression of HDAC9 (Fisher's exact test P = 0.004; as compared to DLBCL). Classical HL showed frequently low-expression of HDAC9 in the tumor cells.
In summary, HDAC9 is frequently expressed in B-cell lymphomas with the highest level of expression found in the most aggressive lymphomas such as DLBCL and MCL. These findings support the biological role of HDAC9 in the pathobiology of aggressive B cell neoplasms and highlight the need to further study HDAC9 function in these malignancies as well as its importance as a therapeutic target.