BSAP, also designated Pax-5 (protein), 52 kDa, is a nuclear protein, member of the paired-box transcription factors, involved in control of organ development and tissue differentiation. The Pax-5 gene is located on chromosome 9p21.1-p13.3. The activation of the Pax-5 gene as well as other genes required for the development of B-cells is controlled by "Early B-cell Factor" (EBF). Together with E2A and EBF, BSAP has a key function in the control of B-lineage commitment. The protein is required for many aspects of B-lymphopoiesis including lineage commitment, immunoglobulin rearrangement, pre-BCR (B-cell receptor) signalling and survival of mature B cells. The Pax5 protein controls the commitment to B-cell lineage by binding to promoter regions of specific genes and activating their expression (e.g. genes coding for CD19, CD20 and regulatory regions of the immunoglobulin heavy-chain locus genes) as well as by suppressing the expression of other genes associated with non-B cell differentiation. During embryogenesis, BSAP is transiently expressed in developing CNS but later its expression correlates with the B-cell lymphopoiesis. Nuclear expression of the protein is found in early progenitor B-cells and continues through pre-B-cell and all mature B lymphocytes. Down-regulation of BSAP expression in addition to up regulation of B-lymphocyte-induced maturation protein 1 (PRDM1) is important for normal development of mature plasma cells. Expression of BSAP is also found in human epididymis and the testis of adult mouse. Thus, the transcription factor BSAP plays an important role not only in B-cell differentiation and function but also may be important for neural development and probably spermatogenesis. Abnormalities: In cultured B-cells the deficiency of BSAP leads to slower cell growth, decreased surface IgM expression and complete loss of B-cell receptor (BCR) signalling as well as promoting the plasma cell differentiation. No similar relationship in human primary immunodeficiency has been observed. The deletion of the Pax-5 gene in mice allows mature B-cells from peripheral lymphoid organs to dedifferentiate to uncommitted lymphoid cell progenitors in bone marrow. These mice develop also aggressive lymphomas with gene profiles of the progenitor cell. Mutations of Pax-5 gene mostly in non-coding regions are detected in almost 60% of human diffuse large B-cell lymphomas. Pax-5 gene is also found mutated in 40% of paediatric ALL often resulting in lower levels of Pax-5 and disruption of Pax-5 signalling which may contribute to pathogenesis of B-cell progenitor ALL. Translocations involving Pax-5 gene t(9;14) might also contribute to pathogenesis of lymphoplasmacytic lymphoma and probably marginal zone lymphoma.
BSAP is expressed in most of non-Hodgkin B-cell lymphomas including precursor B-cell neoplasms. Reed-Sternberg and Hodgkin cells in classic Hodgkin lymphoma are also positive showing usually weaker staining than surrounding B-cells while nuclear staining of L&H cells in nodular lymphocytic predominance Hodgkin lymphoma is as a rule strongly positive. Plasma cell neoplasms and plasmablastic lymphomas are typically negative. The only exception are plasma cell malignancies with translocation t(11;14), which show positive in more than 70% of cases positive Pax5 staining in addition to positive staining for CD20 and Cyclin D1. T cell lymphomas are consistently negative with only single cases reported with aberrant Pax5/BSAP positive staining in addition to the expression of typical T-cell antigens. 30-100% of AML cases with t(8;21) are reported positive for Pax5/BSAP while AML cases without this translocation are usually negative. Positive staining, however with more heterogeneous intensity than in B-cell lymphomas, has been reported in a majority (70-100%) of small cell neuroendocrine carcinomas of internal organs (mostly lung) and Merkel cell carcinoma of the skin. Positive staining is also reported in high percentage of mesonephric hyperplasia and mesonephric carcinomas. Only rare cases carcinomas of different organs are reported positive often with focal weak staining (including urothelial carcinoma of urinary bladder, some types of uterine cervix adenocarcinoma, endometrioid carcinoma and non small cell neuroendocrine). Contradictory results are reported in cases of medulloblastomas.
Pax5 is a sensitive but not 100% specific marker of B-cell differentiation used in panels for subtyping of lymphomas and lymphatic leukaemias or confirmation B-cell origin of lymphomas treated with anti-CD20 antibodies (equally to CD79a). Positive staining in cases non-lymphatic tumours may be a potential diagnostic pitfall.
Tonsil and colon/appendix are recommended as positive and negative tissue controls for BSAP. In the tonsil, protocols must be calibrated to provide a distinct and strong nuclear staining reaction in virtually all mantle zone B-cells, germinal centre B-cells and interfollicular B-cells. In colon/appendix dispersed B-cells in lamina propria must be strongly stained. A weak cytoplasmic staining reaction in B-cells must be accepted.
No staining reaction must be seen in other cells including T-cells, stromal cells, epithelial cells of the tonsil and colon/appendix. As a supplement to tonsil and colon/appendix, especially in the technical calibration phase, it is recommended to verify the protocol on Hodgkin lymphoma, classical subtype often displaying weak to moderate nuclear expression of the neoplastic cells.
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