Pax-5 expression in nonhematopoietic tissues

The transcription factor PAX5 activates human LINE1 retrotransposons to induce cellular senescence

As a hallmark of senescent cells, the derepression of Long Interspersed Elements 1 (LINE1) transcription results in accumulated LINE1 cDNA, which triggers the secretion of the senescence-associated secretory phenotype (SASP) and paracrine senescence in a cGAS-STING pathway-dependent manner. However, transcription factors that govern senescence-associated LINE1 reactivation remain ill-defined. Here, we predict several transcription factors that bind to human LINE1 elements to regulate their transcription by analyzing the conserved binding motifs in the 5'-untranslated regions (UTR) of the commonly upregulated LINE1 elements in different types of senescent cells. Further analysis reveals that PAX5 directly binds to LINE1 5'-UTR and the binding is enhanced in senescent cells. The enrichment of PAX5 at the 5'-UTR promotes cellular senescence and SASP by activating LINE1. We also demonstrate that the longevity gene SIRT6 suppresses PAX5 transcription by directly binding to the PAX5 promoter, and overexpressing PAX5 abrogates the suppressive effect of SIRT6 on stress-dependent cellular senescence. Our work suggests that PAX5 could serve as a potential target for drug development aiming to suppress LINE1 activation and treat senescence-associated diseases.

The Pleiotropy of PAX5 Gene Products and Function

PAX5, a member of the Paired Box (PAX) transcription factor family, is an essential factor for B-lineage identity during lymphoid differentiation. Mechanistically, PAX5 controls gene expression profiles, which are pivotal to cellular processes such as viability, proliferation, and differentiation. Given its crucial function in B-cell development, PAX5 aberrant expression also correlates with hallmark cancer processes leading to hematological and other types of cancer lesions. Despite the well-established association of PAX5 in the development, maintenance, and progression of cancer disease, the use of PAX5 as a cancer biomarker or therapeutic target has yet to be implemented. This may be partly due to the assortment of PAX5 expressed products, which layers the complexity of their function and role in various regulatory networks and biological processes. In this review, we provide an overview of the reported data describing PAX5 products, their regulation, and function in cellular processes, cellular biology, and neoplasm.

Pax-5 Protein Expression Is Regulated by Transcriptional 3'UTR Editing

The Pax-5 gene encodes a transcription factor that is essential for B-cell commitment and maturation. However, Pax-5 deregulation is associated with various cancer lesions, notably hematopoietic cancers. Mechanistically, studies have characterized genetic alterations within the Pax-5 locus that result in either dominant oncogenic function or haploinsufficiency-inducing mutations leading to oncogenesis. Apart from these mutations, some examples of aberrant Pax-5 expression cannot be associated with genetic alterations. In the present study, we set out to elucidate potential alterations in post-transcriptional regulation of Pax-5 expression and establish that Pax-5 transcript editing represents an important means to aberrant expression. Upon the profiling of Pax-5 mRNA in leukemic cells, we found that the 3′end of the Pax-5 transcript is submitted to alternative polyadenylation (APA) and alternative splicing events. Using rapid amplification of cDNA ends (3′RACE) from polysomal fractions, we found that Pax-5 3′ untranslated region (UTR) shortening correlates with increased ribosomal occupancy for translation. These observations were also validated using reporter gene assays with truncated 3′UTR regions cloned downstream of a luciferase gene. We also showed that Pax-5 3′UTR editing has direct repercussions on regulatory elements such as miRNAs, which in turn impact Pax-5 protein expression. More importantly, we found that advanced staging of various hematopoietic cancer lesions relates to shorter Pax-5 3′UTRs. Altogether, our findings identify novel molecular mechanisms that account for aberrant expression and function of the Pax-5 oncogene in cancer cells. These findings also present new avenues for strategic intervention in Pax-5-mediated cancers.

Overview of PAX gene family: analysis of human tissue-specific variant expression and involvement in human disease

Paired-box (PAX) genes encode a family of highly conserved transcription factors found in vertebrates and invertebrates. PAX proteins are defined by the presence of a paired domain that is evolutionarily conserved across phylogenies. Inclusion of a homeodomain and/or an octapeptide linker subdivides PAX proteins into four groups. Often termed "master regulators", PAX proteins orchestrate tissue and organ development throughout cell differentiation and lineage determination, and are essential for tissue structure and function through maintenance of cell identity. Mutations in PAX genes are associated with myriad human diseases (e.g., microphthalmia, anophthalmia, coloboma, hypothyroidism, acute lymphoblastic leukemia). Transcriptional regulation by PAX proteins is, in part, modulated by expression of alternatively spliced transcripts. Herein, we provide a genomics update on the nine human PAX family members and PAX homologs in 16 additional species. We also present a comprehensive summary of human tissue-specific PAX transcript variant expression and describe potential functional significance of PAX isoforms. While the functional roles of PAX proteins in developmental diseases and cancer are well characterized, much remains to be understood regarding the functional roles of PAX isoforms in human health. We anticipate the analysis of tissue-specific PAX transcript variant expression presented herein can serve as a starting point for such research endeavors.

Immunohistochemical identification of T and B lymphocytes in formalin-fixed paraffin-embedded chicken tissues using commercial antibodies

Immunohistochemical method to detect avian lymphocytes is an efficient and reliable tool for accurate diagnosis, and immunological analysis of avian diseases. However, there are scarce studies reporting immunohistochemistry (IHC) using commercially available antibodies in formalin-fixed paraffin-embedded (FFPE) chicken tissues. In the present study, we established an immunohistochemical method to identify chicken T and B lymphocytes in FFPE chicken tissues using commercial antibodies against chicken or human antigens. For this IHC method, the five tested anti-T lymphocyte antibodies reacted with chicken T lymphocytes on the FFPE sections. Further, 10 commercial anti-B lymphocyte antibodies were tested; of these, three successfully detected chicken B lymphocytes for IHC. In particular, anti-human CD3 (clone F7.2.38) antibody was most suitable for the detection of chicken T lymphocytes, whereas anti-chicken B cell activating factor receptor (BAFF-R) antibody (clone 2C4) was most suitable for the detection of chicken B lymphocytes under our IHC staining conditions. These two antibodies reacted with numerous lymphocytes of all representative lymphoid tissues without problematic background staining and nonspecific reactions. Our results indicate that T and B lymphocytes in FFPE chicken tissues can be immunohistochemically detected using commercial antibodies.

Novel Finding of Paired Box 5 (PAX5) Cytoplasmic Staining in Well-differentiated Rectal Neuroendocrine Tumors (Carcinoids) and Its Diagnostic and Potentially Prognostic Utility

Although nuclear immunostaining for paired box protein (PAX5) is widely used in practice, its cytoplasmic localization has not been evaluated. Recently we encountered cytoplasmic granular PAX5 staining in rectal well-differentiated neuroendocrine tumor (WD-NET) in the absence of nuclear staining. We investigated the specificity of this staining pattern for rectal NET (n=21) in comparison with 108 NETs, 1 WD rectal NET with elevated proliferation (WD-NET G3), and 40 poorly differentiated neuroendocrine carcinomas from the gastrointestinal and pancreatobiliary tract and liver. Representative tumor sections were subject to immunohistochemical stain for PAX5 antibody. Immunohistochemistry for 3 L-cell markers, glucagon-like peptide 1 and 2, and peptide YY, was performed on all rectal and appendiceal NETs and all other NETs with cytoplasmic PAX5 staining. Cytoplasmic PAX5 staining was observed in 90% (19/21) of rectal NET, 27% (3/11) of appendiceal, 14% (2/14) of pancreatic, 7% (2/29) of lung, 25% (3/12) metastatic NET in the liver, and 100% (1/1) of renal NET. No PAX5 cytoplasmic staining was seen in all grades of NET in other organs, rectal WD-NET G3, and all neuroendocrine carcinoma. L-cell marker staining was observed in all 21 (100%) rectal, in 3 of 3 (100%) PAX5-positive, and 1 of 7 (14%) PAX5-negative appendiceal NET. Cytoplasmic PAX5 staining is specific for rectal carcinoids. The sensitivity and specificity of PAX5 to detect L-cell type rectal carcinoids is 90% (19/21) and 100% (21/21), respectively. Cytoplasmic localization of the PAX5 protein may be utilized as a surrogate marker to detect L-cell type rectal carcinoids.

Paired Box 5 (PAX5) Expression in Poorly Differentiated Neuroendocrine Carcinoma of the Gastrointestinal and Pancreatobiliary Tract: Diagnostic and Potentially Therapeutic Implications

Paired Box 5 (PAX5), a well-established B-cell marker, is preferentially expressed in small cell lung carcinoma and regulates the transcription of c-Met, offering a potential for therapeutic target. Its expression in poorly differentiated neuroendocrine carcinoma (PDNEC) of the digestive system has not been systemically evaluated. Archived pathology materials from 38 PDNEC in the gastrointestinal (GI) and pancreatobiliary (PB) tract were reviewed. Representative tumor sections were subject to immunohistochemical stain for PAX5, c-Met, and CD20. The extent of the staining [focal (<10%), patchy (10% to 50%), and diffuse (>50%)] and intensity (1+ to 3+) was evaluated. In total, 38 cases of well-differentiated neuroendocrine tumors from GI/PB tract served as controls. Nuclear PAX5 staining was observed in 16 (42%) cases in total, in 46% (11/24) of large cell neuroendocrine carcinoma, 67% (4/6) of small cell neuroendocrine carcinoma, and 13% (1/8) of mixed adenoneuroendocrine carcinoma, with diffuse (8), patchy (4), or focal (4) staining. The intensity was 3+ (2), 2+ (6), and 1+ (8). PAX5 expression was common in ampullary (4/5) and gastroesophageal junctional/esophageal (5/9) PDNEC. Two (5%) of 38 well-differentiated neuroendocrine tumors were positive for PAX5. Three PAX5 positive PDNEC showed weak cytoplasmic c-Met immunolabeling. CD20 was negative in all tumors. Our data show that PAX5 is commonly expressed in PDNEC of the GI/PB tract including small cell neuroendocrine carcinoma. This observation warrants a cautious approach when interpreting small biopsy of poorly differentiated neoplasms, especially when lymphoma is considered in the differentials. Further study of PAX5/c-Met signaling pathway and its potential therapeutic value in GI/PB PDNEC is warranted.

PAX-5 Positivity in Anaplastic Lymphoma Kinase-Negative Anaplastic Large Cell Lymphoma: A Case Report and Review of Literature

Anaplastic lymphoma kinase (ALK)-negative anaplastic large cell lymphoma (ALCL) is a subtype of T-cell lymphomas that may mimic several other malignancies morphologically. With the help of immunohistochemistry, most cases of ALCL can be diagnosed on the basis of expression of T-cell lineage associated antigens. However, aberrations in the expression of immunohistochemical markers pose diagnostic challenges. The morphological and immunophenotypic features of ALCL show considerable overlap with classical Hodgkin lymphoma (CHL), which is a B-cell lymphoma. The 2008 World Health Organization Classification of Tumours of Haematopoietic and Lymphoid Tissues suggests that staining for the B-cell transcription factor, paired box 5 (PAX-5), is helpful in differentiating between them, as it is weakly positive in most CHL and should be negative in ALCL. We report a rare case of ALK-negative ALCL, which was positive for PAX-5 and CD15, mimicking CHL by immunohistochemistry, resulting in a diagnostic dilemma.

A case of basal cell carcinoma of the nictitating membrane in a dog

A case of a basal cell carcinoma (BCC) of the nictitating membrane (NM) in a 9-year-old female spayed dachshund is reported. Computed tomography and resection of the NM followed by cryosurgery was performed. Although uncommon, BCC should be considered as a differential diagnosis for tumors of the NM.

Expression of SOX11, PAX5, TTF-1 and ISL-1 in medulloblastoma

The aim of our study was to evaluate the immunohistochemical expression of SOX11, PAX5, TTF-1 and ISL-1 in medulloblastoma (MB) to investigate their diagnostic usefulness.

METHODS

Immunohistochemical expression of PAX5 (two antibodies: Dako, DAK-Pax5; and BD, clone 24), TTF-1 (Dako, 8G7G3/1), SOX11 (CL0142; Abcam) and ISL-1 (1 H9, Abcam) was analyzed using the h-score and Remmele score in 25 cases of MB.

RESULTS

There were 18 MBs of classic and 7 of desmoplastic type. SOX11 was strongly expressed in all tumors. The expression of PAX5 was higher and more frequent in a case of DAK-Pax5 clone (25/25) than clone 24 (6/25). ISL-1 was positive in 11 (44%) and TTF-1 in 3 (12%) cases. ISL-1 expression correlated positively (p<0.001), while TTF-1 correlated negatively with the age of patients (p=0.039). PAX5 expression correlated with ISL-1 (p=0.039) and showed a trend toward higher expression in the desmoplastic subtype (p=0.069).

CONCLUSIONS

SOX11 is strongly and robustly expressed in MBs. PAX5 expression pattern differs substantially among two antibody clones. TTF-1 and ISL-1 is associated with the age of patients.

PAX5 interacts with RIP2 to promote NF-κB activation and drug-resistance of B-lymphoproliferative disorders

Paired box protein 5 (PAX5) plays a lineage determination role in B-cell development. However, high expression of PAX5 has been also found in various malignant diseases including B-lymphoproliferative disorders (B-LPDs), but its functions and mechanisms in these diseases are still unclear. Here, we show that PAX5 induces drug-resistance through association and activation of receptor-interacting serine/threonine-protein kinase2 (RIP2) and subsequent activation of NF-κB signaling and anti-apoptosis genes expression in B-lymphoproliferative cells. Furthermore, PAX5 is able to interact with RIP1-3, modulating both RIP1- mediated TNFR and RIP2-mediated NOD1 and NOD2 pathways. Our findings describe a novel function of PAX5 in regulating RIP1 and RIP2 activation, which is at least involved in chemo drug-resistance in B-LPDs.

Applicability of the methylated CpG sites of paired box 5 (PAX5) promoter for prediction the prognosis of gastric cancer

Paired box gene 5 (PAX5), a member of the paired box gene family, is involved in control of organ development and tissue differentiation. In previous study, PAX5 promoter methylation was found in gastric cancer (GC) cells and tissues. At present study, we found that the inconsistently methylated levels of PAX5 promoter were identified in the different GC tissues. The methylated CpG site count and the methylated statuses of four CpG sites (-236, -183, -162, and -152) were significantly associated with the survival of 460 GC patients, respectively. Ultimately, the methylated CpG -236 was the optimal prognostic predictor of patients identified by using the Cox regression with AIC value calculation. These findings indicated that the methylated CpG -236 of PAX5 promoter has the potential applicability for clinical evaluation the prognosis of GC.

The role of Pax5 in leukemia: diagnosis and prognosis significance

Pax5 transcription factor, also known as B-cell specific activator protein (BSAP), plays a dual role in the hematopoietic system. Pax5 expression is essential in B-cell precursors for normal differentiation and maturation of B-cells. On the other hand, it inhibits the differentiation and progress toward other lineages. The expression of this factor is involved in several aspects of B-cell differentiation, including commitment, immunoglobulin gene rearrangement, BCR signal transduction and B-cell survival, so that the deletion or inactivating mutations of Pax5 cause cell arrest in Pro-B-cell stage. In recent years, point mutations, deletions and various rearrangements in Pax5 gene have been reported in several types of human cancers. However, no clear relationship has been found between these aberrations and disease prognosis. Specific expression of Pax5 in B-cells can raise it as a marker for the diagnosis and differentiation of B-cell leukemias and lymphomas as well as account for remission or relapse. Extensive studies on Pax5 along with other genes and immunomarkers are necessary for decisive results in this regard.

Merkel cell carcinoma with partial B-cell blastic immunophenotype: a potential mimic of cutaneous richter transformation in a patient with chronic lymphocytic lymphoma

Merkel cell polyomavirus (MCPyV) is a DNA virus whose pathogenic mechanisms in Merkel cell carcinoma (MCC) are still being unraveled. Emerging reports of an association between MCPyV and chronic lymphocytic lymphoma (CLL) have begun to broaden our understanding of the oncogenic mechanisms of this virus and the known association between these 2 malignancies. Herein, we report a case of MCC demonstrating a B-cell immunophenotype arising in a patient with CLL being treated with rituximab. In this context, we discuss the differential diagnostic considerations, especially with cutaneous Richter transformation (diffuse large B-cell lymphoma). We also assessed for the presence of MCPyV in both the patient's MCC and the CLL. Finally, we provide a large meta-analysis of patients with CLL and MCC. Patients with both MCC and CLL have a dismal prognosis, with greater than 50% overall mortality within the first year and a half after MCC diagnosis.

PAX immunoreactivity in poorly differentiated small round cell tumors of childhood

Paired box (PAX) gene antibodies have made it into the mainstream of tumor diagnosis in the recent years. We report the immunoreactivity expression patterns of three PAX genes (PAX2, PAX5 and PAX8) in poorly differentiated small round cell tumors of childhood for possible useful diagnostic applications. We collected and analyzed 123 cases of poorly differentiated small round cell tumors of childhood for their PAX immunoexpression patterns. The results indicated that PAX2 was strongly positive in all alveolar rhabdomyosarcomas and in two-thirds of the kidney clear cell sarcomas, and displayed variable expression in one-half of the embryonal rhabdomyosarcomas. PAX8 immunoexpression was noticed in five and three cases of alveolar rhabdomyosarcomas and embryonal rhabdomyosarcomas, respectively. About one-third of malignant rhabdoid tumors were PAX2-positive and PAX8-positive. All of the Ewing sarcoma and neuroblastoma cases stained negative with all three PAX stains.

Key tumor suppressor genes inactivated by "greater promoter" methylation and somatic mutations in head and neck cancer

Tumor suppressor genes (TSGs) are commonly inactivated by somatic mutation and/or promoter methylation; yet, recent high-throughput genomic studies have not identified key TSGs inactivated by both mechanisms. We pursued an integrated molecular analysis based on methylation binding domain sequencing (MBD-seq), 450K Methylation arrays, whole exome sequencing, and whole genome gene expression arrays in primary head and neck squamous cell carcinoma (HNSCC) tumors and matched uvulopalatopharyngoplasty tissue samples (UPPPs). We uncovered 186 downregulated genes harboring cancer specific promoter methylation including PAX1 and PAX5 and we identified 10 key tumor suppressor genes (GABRB3, HOXC12, PARP15, SLCO4C1, CDKN2A, PAX1, PIK3AP1, HOXC6, PLCB1, and ZIC4) inactivated by both promoter methylation and/or somatic mutation. Among the novel tumor suppressor genes discovered with dual mechanisms of inactivation, we found a high frequency of genomic and epigenomic alterations in the PAX gene family of transcription factors, which selectively impact canonical NOTCH and TP53 pathways to determine cell fate, cell survival, and genome maintenance. Our results highlight the importance of assessing TSGs at the genomic and epigenomic level to identify key pathways in HNSCC, deregulated by simultaneous promoter methylation and somatic mutations.

PAX immunoreactivity patterns in anaplastic wilms tumors

This immunohistochemistry study explores particular differential immuno expression patterns of PAX genes in unfavorable histology Wilms tumor (UH) and favorable histology Wilms tumor (FH). Some unique features in unfavorable histology Wilms tumor were discovered and they may provide insightful clues of this unique and both clinically and biologically important phenomenon.

Biphenotypic B-lymphoid/myeloid cells expressing low levels of Pax5: potential targets of BAL development

The expression of Pax5 commits common lymphoid progenitor cells to B-lymphoid lineage differentiation. Little is known of possible variations in the levels of Pax5 expression and their influences on hematopoietic development. We have developed a retroviral transduction system that allows for the study of possible intermediate stages of this commitment by controlling the levels of Pax5 expressed in Pax5-deficient progenitors in vitro and in vivo. Retroviral transduction of Pax5-deficient pro-/pre-B cell lines with a doxycycline-inducible (TetON) form of the human Pax5 (huPax5) gene yielded cell clones that could be induced to different levels of huPax5 expression. Clones inducible to high levels developed B220+/CD19+/IgM+ B cells, while clones with low levels differentiated to B220+/CD19−/CD11b+/Gr-1− B-lymphoid/myeloid biphenotypic cells in vitro and in vivo. Microarray analyses of genes expressed at these lower levels of huPax5 identified C/ebpα, C/ebpδ, Pu.1, Csf1r, Csf2r, and Gata-3 as myeloid-related genes selectively expressed in the pro-/pre-B cells that can develop under myeloid/lymphoid conditions to biphenotypic cells. Therefore, reduced expression of huPax5 during the induction of early lymphoid progenitors to B-lineage–committed cells can fix this cellular development at a stage that has previously been seen during embryonic development and in acute lymphoblastic lymphoma–like biphenotypic acute leukemias.

The utility of immunohistochemistry for the identification of hematopoietic and lymphoid cells in normal tissues and interpretation of proliferative and inflammatory lesions of mice and rats

Expression of antigens in cells and tissues can be readily studied immunohistochemically with the use of antibodies. A panel of antibodies to cell-specific markers can be used to diagnose lesions, including tumors, in the hematopoietic and lymphoid systems. This review discusses the use of readily available antibodies and procedures to identify antigens expressed in normal tissues and in proliferative and inflammatory lesions in formalin-fixed, paraffin-embedded (FFPE) murine specimens.

Characterization of a New Monoclonal Antibody Against PAX5/BASP in 1525 Paraffin-embedded Human and Animal Tissue Samples

INTRODUCTION

We describe the newly generated DAK-PAX5 monoclonal antibody raised against a fixation-resistant epitope of the human PAX5/BSAP molecule.

MATERIALS AND METHODS

Following Western-blot, absorption, and chess-board titration tests, and optimization of antigen-retrieval and detection methods, DAK-Pax5 was used in parallel with a reference antibody (clone 24) on tissue micro-arrays (TMAs) constructed from normal human and animal tissues and from hematologic and nonhematologic human malignancies. Such TMAs were also tested with an anti-PAX2 antibody.

RESULTS

DAK-Pax5 reacted with normal human and animal B-cells and with 460/473 B-cell non-Hodgkin lymphomas (B-NHLs). All plasmacytomas/plasmablastic tumors (n=13) and T/NK-cell neoplasms (n=264) turned out consistently negative as did acute myelogenous leukaemias (n=19) except 2 carrying t(8;21). Positivity was found in 6/6 and 155/169 lymphocyte predominant and classical HLs, respectively, although the staining intensity varied through cases. Among 521 nonhematologic malignancies, DAK-Pax5 reacted with 22/399 carcinomas (4/11 neuroendocrine, 2/4 Merkel-cell, 4/21 prostatic, 1/11 urothelial, 1/26 renal, 2/12 cervical squamous-cell, 3/13 ovarian, and 5/75 colonic). When compared with clone 24, DAK-Pax5 produced a stronger positivity in most if not all B-NHLs and HLs. No cross-reactivity with the anti-PAX2 antibody was recorded.

DISCUSSION

DAK-Pax5 represents a new reliable tool for diagnostics and research.

PAX-5: a valuable immunohistochemical marker in the differential diagnosis of lymphoid neoplasms

OBJECTIVE

Undifferentiated tumors and hematolymphoid neoplasms can be diagnostically challenging due to potential overlap of morphologic features and variant antigen expression. PAX-5, a transcription factor expressed throughout B-cell maturation, is detected in most B-cell neoplasms including those that lack expression of mature B-cell markers, such as classical Hodgkin lymphoma (cHL), B-lymphoblastic leukemia and B-cell lymphomas following rituximab therapy. The lack of PAX-5 expression in most CD30-positive non-hematopoietic malignancies (embryonal carcinoma and seminoma) and T-cell lymphomas, such as anaplastic large cell lymphoma (ALCL), suggests that the absence of PAX-5 may be used to confirm non-B-cell lineage. The goal of this study was to retrospectively assess PAX-5 immunoreactivity in diagnostic samples of hematolymphoid and other non-hematopoietic malignancies.

DESIGN

Diagnostic lymph node, decalcified core bone marrow biopsies and tissue sections from 111 archived paraffin-embedded tissue blocks and a tissue lymphoma microarray were immunostained using a monoclonal antibody to PAX-5. The corresponding hematoxylin and eosin stained tissue sections and additional immunostains were simultaneously evaluated. PAX-5 immunoreactivity in neoplastic cells was scored as positive or negative. This study was exempted by the Institutional Review Board for Human Research.

RESULTS

Nuclear PAX-5 immunoreactivity was detected in 88% (36/41) of Hodgkin lymphoma, all cases of diffuse large B-cell lymphoma (n=72), small B-cell lymphomas (n=5), B-lymphoblastic leukemia/lymphoma and mixed phenotype acute leukemia with B-cell lineage (n=5). PAX-5 was not detected in ALCL (n=22), T-cell lymphoblastic leukemia/lymphoma, mixed phenotype acute leukemia with T-cell lineage (n=5), acute myeloid leukemia (n=4), carcinoid tumors with typical morphology (n=5), melanoma (n=3), and undifferentiated/metastatic tumors (n=8). Non-neoplastic bone marrow sections showed scattered nuclear staining in small B-cell lymphocytes/hematogones. The detection of PAX-5 immunoreactivity resulted in the reclassification of two cases of ALCL to cHL.

CONCLUSION

Overall, our results demonstrate that including PAX-5 in a panel with other immunomarkers helps establish B-cell lineage and increases diagnostic yield.

PAX immunoreactivity identifies alveolar rhabdomyosarcoma

PAX5 is a member of the paired box transcription factors involved in development and its expression has been well characterized among hematopoietic malignancies of B-cell lineage. Its expression has also been reported in a subset of neuroendocrine carcinomas, urothelial tumors, Merkel cell carcinoma, glioblastoma, and neuroblastoma cell lines. As such, we sought to assess it as a diagnostic marker in the evaluation of pediatric small round blue cell tumors. Tumors selected for evaluation included embryonal rhabdomyosarcoma (55 cases), alveolar rhabdomyosarcoma (ARMS) (51 cases), neuroblastoma (22 cases), Wilms tumor (18 cases), Ewing Family of Tumors (11 cases), lymphoblastic lymphoma (8 cases), hepatoblastoma (6 cases), and granulocytic sarcoma (3 cases) as either cores in a tissue microarray or whole mount sections. All cases were immunostained using an antibody directed toward PAX5 and immunoreactivity was scored semiquantitatively according to percentage of nuclear staining. As expected, all B-cell lymphoblastic lymphomas were strongly immunoreactive against PAX5. Additionally, all Wilms tumors showed staining of variable intensity, most intensely in the epithelial component. Of the rhabdomyosarcoma cases, 34 of 51 (67%) ARMS were immunoreactive whereas none of the 55 embryonal rhabdomyosarcoma cases stained. No other tumor type on the array was immunoreactive toward PAX5. Genetic information was available on 7 ARMS, 5 of which had characteristic translocations involving PAX genes, either t(2:13) or t(1;13). Of the translocation-positive cases, all showed nuclear reactivity toward PAX5, and both the translocation-negative cases did not. Possible explanations of PAX5 staining include aberrant expression of the PAX5 transcription factor, PAX5 expression in normal tissue at the time the tumors most closely recapitulates in development or crossreactivity with another member of the PAX family. PAX3 and PAX7 fusion genes characterize the majority of ARMS making crossreactivity with these proteins an attractive theory, and suggest that PAX5 immunoreactivity may be specific for translocation-positive ARMS. Further study in a larger series of rhabdomyosarcomas is warranted to assess the sensitivity and specificity of PAX5 immunoreactivity for the ARMS variant.

Pax genes in embryogenesis and oncogenesis

The paired box genes are a family of nine developmental control genes, which in human beings (PAX) and mice (Pax) encode nuclear transcription factors. The temporal and spatial expressions of these highly conserved genes are tightly regulated during foetal development including organogenesis. PAY/Paxgenes are switched off during the terminal differentiation of most structures. Specific mutations within a number of PAX/Pax genes lead to developmental abnormalities in both human beings and mice. Mutation in PAX3 causes Waardenburg syndrome, and craniofacial-deafness-hand syndrome. The Splotch phenotype in mouse exhibits defects in neural crest derivatives such as, pigment cells, sympathetic ganglia and cardiac neural crest-derived structures. The PAX family also plays key roles in several human malignancies. In particular, PAX3 is involved in rhabdomyosarcoma and tumours of neural crest origin, including melanoma and neuroblastoma. This review critically evaluates the roles of PAX/Pax in oncogenesis. It especially highlights recent advances in knowledge of how their genetic alterations directly interfere in the transcriptional networks that regulate cell differentiation, proliferation, migration and survival and may contribute to oncogenesis.

Diagnostic uses of Pax5 immunohistochemistry

Pax5, or B-cell-specific activator protein, is a nuclear protein in the paired-box containing (PAX) family of transcription factors involved in control of organ development and tissue differentiation. Pax5 is mostly expressed in B lymphocytes and B-cell lymphomas, although recent data have shown expression in the developing central nervous system, some neuroendocrine tumors, and occasional myeloid leukemias. Pax5 immunohistochemistry shows robust nuclear staining, and has become a valuable tool in the diagnosis and subclassification of lymphomas. Pax5 staining is positive in most Hodgkin and B-cell non-Hodgkin lymphomas, and also precursor B-cell lymphoblastic neoplasms. Plasma cell neoplasms, multiple myeloma, and plasmablastic lymphomas typically are negative. T-cell lymphomas are, to date, consistently negative. Recently, Pax5 expression has been described in the majority of small cell carcinomas and Merkel cell carcinomas. Rare cases of Pax5 expression in other carcinomas have been reported. With these exceptions, Pax5 immunohistochemistry is fairly specific for B-cell lineage and is a valuable addition to the armamentarium of markers available for lymphoma subtyping.

The utility of PAX5 immunohistochemistry in the diagnosis of undifferentiated malignant neoplasms

PAX5 is a B-cell transcription factor whose expression at the protein level is reliably detected by immunohistochemistry in routine biopsies. The purpose of this study was to investigate whether PAX5 immunohistochemistry has diagnostic benefit as a B-cell marker in the work-up of undifferentiated malignant neoplasms. Twenty-five cases previously diagnosed as undifferentiated malignant neoplasms were selected. In addition, 59 hematolymphoid and 884 non-hematolymphoid malignancies were studied such that the specificity of PAX5 immunohistochemistry could be addressed. Two of the 25 (8%) undifferentiated neoplasms showed diffuse staining for PAX5, which indicated a B-cell derivation for these neoplasms that was not appreciated at the time of initial diagnosis. PAX5 staining was detected in the vast majority of hematolymphoid tumors of B-cell derivation but only in 5 of 884 (less than 1%) non-hematolymphoid tumors. Our results further show that PAX5 may be the only detectable marker of B lineage in lymphomas that lack or show equivocal CD45RB and CD20 expression. We conclude that the addition of PAX5 to a panel of immunohistologic markers used in the interrogation of undifferentiated neoplasms is of diagnostic benefit. Its expression can also facilitate the diagnosis of classical and nodular lymphocyte-predominant Hodgkin lymphoma with atypical morphologic and immunohistologic features. Lastly, we have shown that the lack of its expression at the protein level in many epithelial and mesenchymal neoplasms renders PAX5 expression an extremely specific marker of the B lineage.

Rare expression of BSAP (PAX-5) in mature T-cell lymphomas

Lineage determination in lymphomas is based on the assessment of lineage-specific markers, such as the B-cell-specific activator protein of the paired box family (BSAP, PAX-5) for the B-cell lineage. BSAP is thought to be expressed exclusively in B cells from the pro-B- to the mature B-cell stage and then silenced in plasma cells. BSAP has oncogenic potential and experimental evidence shows that the T-cell lineage is prone to this effect. Herein, we report on a BSAP-positive peripheral T-cell lymphoma with monoclonal T-cell receptor gamma-gene rearrangement. To assess the relative frequency of BSAP expression in mature T-cell lymphomas, we constructed and examined a tissue microarray consisting of 43 angioimmunoblastic T-cell lymphomas and peripheral T-cell lymphomas and detected no additional BSAP-positive cases. To conclude, BSAP can probably contribute to T-cell lymphomagenesis not only in vitro, but also in vivo. It is rarely expressed in peripheral T-cell lymphoma, thus its detection on lymphoid malignancies cannot be considered definitively lineage specific.