PAX5 expression in acute leukemias: higher B-lineage specificity than CD79a and selective association with t(8;21)-acute myelogenous leukemia

Single-cell transcriptomic profiling reveals immune cell heterogeneity in acute myeloid leukaemia peripheral blood mononuclear cells after chemotherapy

PURPOSE

Acute myeloid leukaemia (AML) is a heterogeneous disease characterised by the rapid clonal expansion of abnormally differentiated myeloid progenitor cells residing in a complex microenvironment. However, the immune cell types, status, and genome profile of the peripheral blood mononuclear cell (PBMC) microenvironment in AML patients after chemotherapy are poorly understood. In order to explore the immune microenvironment of AML patients after chemotherapy, we conducted this study for providing insights into precision medicine and immunotherapy of AML.

METHODS

In this study, we used single-cell RNA sequencing (scRNA-seq) to analyse the PBMC microenvironment from five AML patients treated with different chemotherapy regimens and six healthy donors. We compared the cell compositions in AML patients and healthy donors, and performed gene set enrichment analysis (GSEA), CellPhoneDB, and copy number variation (CNV) analysis.

RESULTS

Using scRNA-seq technology, 91,772 high quality cells of 44,950 PBMCs from AML patients and 46,822 PBMCs from healthy donors were classified as 14 major cell clusters. Our study revealed the sub-cluster diversity of T cells, natural killer (NK) cells, monocytes, dendritic cells (DCs), and haematopoietic stem cell progenitors (HSC-Prog) in AML patients under chemotherapy. NK cells and monocyte-DCs showed significant changes in transcription factor expression and chromosome copy number variation (CNV). We also observed significant heterogeneity in CNV and intercellular interaction networks in HSC-Prog cells.

CONCLUSION

Our results elucidated the PBMC single-cell landscape and provided insights into precision medicine and immunotherapy for treating AML.

Progression in Myeloid Neoplasms: Beyond the Myeloblast

Disease progression in myelodysplastic syndromes (MDS), myelodysplastic-myeloproliferative neoplasms (MDS/MPN), and myeloproliferative neoplasms (MPN), altogether referred to as myeloid neoplasms (MN), is a major source of mortality. Apart from transformation to acute myeloid leukemia, the clinical progression of MN is mostly due to the overgrowth of pre-existing hematopoiesis by the MN without an additional transforming event. Still, MN may evolve along other recurrent yet less well-known scenarios: (1) acquisition of MPN features in MDS or (2) MDS features in MPN, (3) progressive myelofibrosis (MF), (4) acquisition of chronic myelomonocytic leukemia (CMML)-like characteristics in MPN or MDS, (5) development of myeloid sarcoma (MS), (6) lymphoblastic (LB) transformation, (7) histiocytic/dendritic outgrowths. These MN-transformation types exhibit a propensity for extramedullary sites (e.g., skin, lymph nodes, liver), highlighting the importance of lesional biopsies in diagnosis. Gain of distinct mutations/mutational patterns seems to be causative or at least accompanying several of the above-mentioned scenarios. MDS developing MPN features often acquire MPN driver mutations (usually JAK2), and MF. Conversely, MPN gaining MDS features develop, e.g., ASXL1, IDH1/2, SF3B1, and/or SRSF2 mutations. Mutations of RAS-genes are often detected in CMML-like MPN progression. MS ex MN is characterized by complex karyotypes, FLT3 and/or NPM1 mutations, and often monoblastic phenotype. MN with LB transformation is associated with secondary genetic events linked to lineage reprogramming leading to the deregulation of ETV6, IKZF1, PAX5, PU.1, and RUNX1. Finally, the acquisition of MAPK-pathway gene mutations may shape MN toward histiocytic differentiation. Awareness of all these less well-known MN-progression types is important to guide optimal individual patient management.

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.

Immunohematological features of free-living Alouatta belzebul (Linnaeus, 1766) red-handed howler monkeys in the Eastern Amazon

The red-handed howler monkey (Alouatta belzebul) is one of the 35 threatened Brazilian primate species found in two highly endangered Brazilian biomes. Their Amazonian native populations have been declining due to exponential deforestation associated with human activities, especially the construction of dams. The studied population (n = 27) was located in the Belo Monte dam Area of Influence. For the first time, we presented hematological parameters and the basic profile of T (CD3) and B (BSAP PAX5) cells by immunocytochemistry. The results supported the hypothesis that the immuno-hematological profile is influenced by sex, age, and season. Eosinophils were significantly higher in females (p = 0.03), monocytes statistically greater in juveniles (p = 0.04), and total plasma protein increased significantly (p > 0.001) during the dry season. Furthermore, adults showed a statistically higher average absolute number of B lymphocytes than young individuals (p = 0.03), in contrast to T lymphocytes. Even without knowing the full history of antigenic exposure, these results not only contribute to elucidating the boundaries between health and disease but may help lay the groundwork for future research into the effects of anthropogenic stress on immune activation.

Mixed-Phenotype Acute Leukemia: Clinical Diagnosis and Therapeutic Strategies

Mixed-phenotype acute leukemia (MPAL) comprises a heterogenous group of leukemias that are genetically, immunophenotypically, and clinically, diverse. Given the rarity of the disease, the diagnosis and treatment of MPAL is extremely challenging. Recent collaborative efforts have made significant progress in understanding the complex genomic landscape of MPAL. Some retrospective studies support starting ALL-type induction followed by an allogeneic stem cell transplant(allo-sct) in the first complete remission; however, due to the inherent bias of retrospective data and small case series, a prospective validation of AML- and ALL-based regimen, and the incorporation of targeted therapies based on genetics and immunophenotype are warranted. The prognosis of adults and children with MPAL varies; this justifies modulating the intensity of therapy, including the use of allo-sct as a consolidation strategy.

AML1/ETO and its function as a regulator of gene transcription via epigenetic mechanisms

The chromosomal translocation t(8;21) and the resulting oncofusion gene AML1/ETO have long served as a prototypical genetic lesion to model and understand leukemogenesis. In this review, we describe the wide-ranging role of AML1/ETO in AML leukemogenesis, with a particular focus on the aberrant epigenetic regulation of gene transcription driven by this AML-defining mutation. We begin by analyzing how structural changes secondary to distinct genomic breakpoints and splice changes, as well as posttranscriptional modifications, influence AML1/ETO protein function. Next, we characterize how AML1/ETO recruits chromatin-modifying enzymes to target genes and how the oncofusion protein alters chromatin marks, transcription factor binding, and gene expression. We explore the specific impact of these global changes in the epigenetic network facilitated by the AML1/ETO oncofusion on cellular processes and leukemic growth. Furthermore, we define the genetic landscape of AML1/ETO-positive AML, presenting the current literature concerning the incidence of cooperating mutations in genes such as KIT, FLT3, and NRAS. Finally, we outline how alterations in transcriptional regulation patterns create potential vulnerabilities that may be exploited by epigenetically active agents and other therapeutics.

Survival prognostic factors in patients with acute myeloid leukemia using machine learning techniques

This paper identifies prognosis factors for survival in patients with acute myeloid leukemia (AML) using machine learning techniques. We have integrated machine learning with feature selection methods and have compared their performances to identify the most suitable factors in assessing the survival of AML patients. Here, six data mining algorithms including Decision Tree, Random Forrest, Logistic Regression, Naive Bayes, W-Bayes Net, and Gradient Boosted Tree (GBT) are employed for the detection model and implemented using the common data mining tool RapidMiner and open-source R package. To improve the predictive ability of our model, a set of features were selected by employing multiple feature selection methods. The accuracy of classification was obtained using 10-fold cross-validation for the various combinations of the feature selection methods and machine learning algorithms. The performance of the models was assessed by various measurement indexes including accuracy, kappa, sensitivity, specificity, positive predictive value, negative predictive value, and area under the ROC curve (AUC). Our results showed that GBT with an accuracy of 85.17%, AUC of 0.930, and the feature selection via the Relief algorithm has the best performance in predicting the survival rate of AML patients.

Update on Acute Leukemias of Ambiguous Lineage

Acute leukemias of ambiguous lineage are a heterogenous group of diseases that include acute undifferentiated leukemias and mixed-phenotype acute leukemias (MPALs). These leukemias pose a challenge for pathologists and clinicians alike in diagnosis, treatment, and further management. Recent genetic characterization has provided insights into their underlying biology and classification, and has offered potential for targeted therapies. This article addresses diagnosis of MPALs with examples of the most common pitfalls, recent comprehensive molecular studies, and advancement in treatment and follow-up modalities.

Progression, transformation, and unusual manifestations of myelodysplastic syndromes and myelodysplastic-myeloproliferative neoplasms: lessons learned from the XIV European Bone Marrow Working Group Course 2019

Disease progression in myelodysplastic syndromes (MDS) and myelodysplastic-myeloproliferative neoplasms (MDS/MPN) is a major source of mortality. The European Bone Marrow Working Group organized a dedicated workshop to address MDS and MDS/MPN progression, and myeloid neoplasms with histiocytic and lymphoblastic outgrowths in 2019 in Frankfurt, Germany. In this report, we summarize clinical, histopathological, and molecular features of 28 cases. Most cases illustrate that prognostic mutational profiles change during follow-up due to accumulation of high-risk mutations in the trunk clone, and that results from repeated molecular testing can often explain the clinical progression, suggesting that regular genetic testing may predict transformation by early detection of aggressive clones. Importantly, identical mutations can be linked to different clinical behaviors or risks of fibrotic progression and/or transformation in a context-dependent manner, i.e., MDS or MDS/MPN. Moreover, the order of mutational acquisition and the involved cell lineages matter. Several cases exemplify that histiocytic outgrowths in myeloid neoplasms are usually accompanied by a more aggressive clinical course and may be considered harbinger of disease progression. Exceptionally, lymphoblastic transformations can be seen. As best estimable, the histiocytic and lymphoblastic compounds in all occasions were clonally related to the myeloid compound and-where studied-displayed genomic alterations of, e.g., transcription factor genes or genes involved in MAPK signaling that might be mechanistically linked to the respective type of non-myeloid outgrowth.

Pathogenesis of pediatric B-cell acute lymphoblastic leukemia: Molecular pathways and disease treatments

B-cell acute lymphoblastic lymphoma (B-ALL) is a disease found mainly in children and in young adults. B-ALL is characterized by the rapid proliferation of poorly differentiated lymphoid progenitor cells inside the bone marrow. In the United States, ~4,000 of these patients are diagnosed each year, accounting for ~30% of childhood cancer types. The tumorigenesis of the disease involves a number of abnormal gene expressions (including TEL-AML1, BCR-ABL-1, RAS and PI3K) leading to dysregulated cell cycle. Risk factors of B-ALL are the history of parvovirus B 19 infection, high birth weight and exposure to environmental toxins. These risk factors can induce abnormal DNA methylation and DNA damages. Treatment procedures are divided into three phases: Induction, consolidation and maintenance. The goal of treatment is complete remission without relapses. Apart from traditional treatments, newly developed approaches include gene targeting therapy, with the aim of wiping out leukemic cells through the inhibition of mitogen-activated protein kinases and via c-Myb inhibition enhancing sensitivity to chemotherapy. To evaluate the efficacy of ongoing treatments, several indicators are currently used. The indicators include the expression levels of microRNAs (miRs) miR-146a, miR-155, miR-181a and miR-195, and soluble interleukin 2 receptor. Multiple drug resistance and levels of glutathione reductase can affect treatment efficacy through the increased efflux of anti-cancer drugs and weakening the effect of chemotherapy through the reduction of intracellular reactive oxygen species. The present review appraised recent studies on B-ALL regarding its pathogenesis, risk factors, treatments, treatment evaluation and causes of disease relapse. Understanding the mechanisms of B-ALL initiation and causes of treatment failure can help physicians improve disease management and reduce relapses.

Practical Application of Lineage-Specific Immunohistochemistry Markers: Transcription Factors (Sometimes) Behaving Badly

CONTEXT.—

Transcription factors (TFs) are proteins that regulate gene expression and control RNA transcription from DNA. Lineage-specific TFs have increasingly been used by pathologists to determine tumor lineage, especially in the setting of metastatic tumors of unknown primary, among other uses. With experience gathered from its daily application and increasing pitfalls reported from immunohistochemical studies, these often-touted highly specific TFs are not as reliable as once thought.

OBJECTIVES.—

To summarize the established roles of many of the commonly used TFs in clinical practice and to discuss known and potential sources for error (eg, false-positivity from cross-reactivity, aberrant, and overlap "lineage-specific" expression) in their application and interpretation.

DATA SOURCES.—

Literature review and the authors' personal practice experience were used. Several examples selected from the University Health Network (Toronto, Ontario, Canada) are illustrated.

CONCLUSIONS.—

The application of TF diagnostic immunohistochemistry has enabled pathologists to better assess the lineage/origin of primary and metastatic tumors. However, the awareness of potential pitfalls is essential to avoid misdiagnosis.

Negative CD19 expression is associated with inferior relapse-free survival in children with RUNX1-RUNX1T1-positive acute myeloid leukaemia: results from the Japanese Paediatric Leukaemia/Lymphoma Study Group AML-05 study

We performed a retrospective analysis of leukaemic surface antigen expression and genomic data from a total of 100 RUNX1-RUNX1T1-positive paediatric acute myeloid leukaemia (AML) patients enrolled in the Japanese Paediatric Leukaemia/Lymphoma Study Group (JPLSG) AML-05 protocol to determine risk factors for relapse. In univariate analysis, the KIT exon 17 mutation (n = 21) and CD19 negativity (n = 59) were significant risk factors for relapse (P = 0·01). In multivariate analysis, CD19 negativity was the sole significant risk factor for relapse (hazard ratio, 3·09; 95% confidence interval, 1·26-7·59; P < 0·01), suggesting that biological differences between CD19-positive and CD19-negative RUNX1-RUNX1T1 AML patients should be investigated.

RUNX1 Mutations Can Lead to Aberrant Expression of CD79a and PAX5 in Acute Myelogenous Leukemias: A Potential Diagnostic Pitfall

BACKGROUND

RUNX1 is a crucial transcription factor for hematological stem cells and well-known for its association with acute lymphoblastic leukemia (ALL) and acute myelogenous leukemia (AML). Besides the translocation t(8; 21) that leads to the RUNX1-RUNX1T1 fusion, somatic mutations of RUNX1 have been discovered.

METHODS

Four bone marrow trephine biopsies of patients with CD79a-positive and/or PAX5-positive acute leukemias were investigated by immunohistochemistry (IHC), karyotyping, and next-generation sequencing-based genetic analysis. Data were then compared to a historical collective of AML (n = 42) and 42 cases of AML newly diagnosed at our institution between June 2017 and May 2018.

RESULTS

We report on 4 cases of acute leukemia with an equivocal immunophenotype showing expression of CD79a and/or PAX5, which led to a preliminary histopathologic classification as probable ALL/unclassifiable acute leukemia. All cases were positive for CD34 and TdT but negative for several myeloid markers on IHC. Mutational analysis revealed point mutations and indels of RUNX1 and further mutations typical for AML such as TET2, DNMT3A, and SRSF2, and 2 cases had tetrasomy 13 characteristic of RUNX1 mutant AML.

CONCLUSION

Aberrant CD79a and/or PAX5 expression can be found in AML cases with RUNX1 mutations even without the translocation t(8; 21). Our series shows the expression of CD79a and PAX5 to be a potential pitfall in the classification of RUNX1 mutant acute leukemia.

Deacetylase activity-independent transcriptional activation by HDAC2 during TPA-induced HL-60 cell differentiation

The human myeloid leukemia cell line HL-60 differentiate into monocytes following treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). However, the mechanism underlying the differentiation of these cells in response to TPA has not been fully elucidated. In this study, we performed ChIP-seq profiling of RNA Pol II, HDAC2, Acetyl H3 (AcH3), and H3K27me3 and analyzed differential chromatin state changes during TPA-induced differentiation of HL-60 cells. We focused on atypically active genes, which showed enhanced H3 acetylation despite increased HDAC2 recruitment. We found that HDAC2 positively regulates the expression of these genes in a histone deacetylase activity-independent manner. HDAC2 interacted with and recruited paired box 5 (PAX5) to the promoters of the target genes and regulated HL-60 cell differentiation by PAX5-mediated gene activation. Taken together, these data elucidated the specific-chromatin status during HL-60 cell differentiation following TPA exposure and suggested that HDAC2 can activate transcription of certain genes through interactions with PAX5 in a deacetylase activity-independent pathway.

Mixed-Phenotype Acute Leukemia: Diagnostic Criteria and Pitfalls

Mixed-phenotype acute leukemia (MPAL) is a heterogeneous category in the World Health Organization classification that comprises acute leukemias with discrete admixed populations of myeloid and lymphoid blasts (“bilineal”) or with extensive coexpression of lymphoid and myeloid markers in a single blast population (“biphenotypic”). Flow cytometric findings suggestive of MPAL are often met with consternation by pathologists and oncologists alike, owing to unfamiliarity with the disease and uncertainty about how MPAL fits into established paradigms for treatment of acute leukemia. The purpose of this review is to explain the diagnostic criteria for MPAL, summarize its biological and clinical features, and address common diagnostic pitfalls of these unusual leukemias.

Lineage-specific and single-cell chromatin accessibility charts human hematopoiesis and leukemia evolution

We define the chromatin accessibility and transcriptional landscapes in 13 human primary blood cell types that span the hematopoietic hierarchy. Exploiting the finding that the enhancer landscape better reflects cell identity than mRNA levels, we enable 'enhancer cytometry' for enumeration of pure cell types from complex populations. We identify regulators governing hematopoietic differentiation and further show the lineage ontogeny of genetic elements linked to diverse human diseases. In acute myeloid leukemia (AML), chromatin accessibility uncovers unique regulatory evolution in cancer cells with a progressively increasing mutation burden. Single AML cells exhibit distinctive mixed regulome profiles corresponding to disparate developmental stages. A method to account for this regulatory heterogeneity identified cancer-specific deviations and implicated HOX factors as key regulators of preleukemic hematopoietic stem cell characteristics. Thus, regulome dynamics can provide diverse insights into hematopoietic development and disease.

Oct2 and Bob1 are sensitive and specific markers in lineage determination of B cell lymphomas with no expression of conventional B cell markers

AIMS

Rare cases of B cell lymphomas do not express conventional B cell markers (CD20, CD79a and PAX5), and these types of lymphomas include anaplastic lymphoma kinase (ALK)-positive large B cell lymphoma, plasmablastic lymphoma, primary effusion lymphoma and the solid variant of primary effusion lymphoma, extracavitary human herpesvirus 8 (HHV8)-positive large B cell lymphoma. Establishing accurate diagnoses of these B cell lymphomas can be challenging, and often requires a large panel of immunohistochemical stains, molecular assays and cytogenetic studies. B cell-specific transcription factors, Oct2 and Bob1, have been shown to be expressed consistently in most, if not all, B cell lymphomas, and therefore we investigated the utility of Oct2 and Bob1 immunohistochemistry in lineage determination of the aforementioned B cell lymphomas.

METHODS AND RESULTS

We selected 34 cases of previously diagnosed B cell lymphomas with no or weak expression of CD20, CD79a and PAX5. Oct2 and Bob1 were positive in 74% (25 of 34) and 85% (29 of 34) of the cases, respectively. When we combined the results of these two immunostains, 94% (32 of 34) cases expressed at least one of these two markers. We also included 51 control cases of non-B cell neoplasms, and none of them expressed either Oct2 or Bob1.

CONCLUSIONS

Oct2 and Bob1 are very reliable in determining B cell lineage in the absence of expression of other pan-B cell markers, and it should provide great diagnostic benefit to include them both in a panel of immunohistochemistry to assess undifferentiated malignant neoplasms.

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.

B-cell transcription factor expression and immunoglobulin gene rearrangement frequency in acute myeloid leukemia with t(8;21)(q22;q22)

OBJECTIVES

To assess a large series of patients with acute myeloid leukemia (AML) with t(8;21) for both IGH@ and IGK@ B-cell gene rearrangements and for expression of PAX5, OCT2, and Bob.1 by immunohistochemistry and expression of CD19, CD79a, CD20, and CD22 by flow cytometry immunophenotyping.

METHODS

A total of 48 cases of AML with t(8;21)(q22;q22) were evaluated by immunohistochemistry and/or heavy chain and light chain immunoglobulin rearrangement studies where paraffin-embedded and/or fresh frozen material was available for study; previously performed flow cytometry studies were also reviewed in available cases.

RESULTS

Our study yielded 1 of 19 cases of AML with t(8;21) with an IGH@ gene rearrangement; blasts were associated with weak PAX5 expression. In addition, expression of antigens CD79a by flow cytometry and OCT2 by immunohistochemistry were highly associated with PAX5 expression, and CD19 was expressed in most cases assessed.

CONCLUSIONS

Although B-cell antigen and B-cell transcription factor expression is seen in the majority of AMLs with t(8;21)(q22;q22) and correlates with PAX5 expression, immunoglobulin gene rearrangements are an uncommon event in this group of leukemias.

Lineage-inappropriate PAX5 expression in t(8;21) acute myeloid leukemia requires signaling-mediated abrogation of polycomb repression

The activation of B-cell-specific genes, such as CD19 and PAX5, is a hallmark of t(8;21) acute myeloid leukemia (AML) which expresses the translocation product RUNX1/ETO. PAX5 is an important regulator of B-lymphoid development and blocks myeloid differentiation when ectopically expressed. To understand the molecular mechanism of PAX5 deregulation, we examined its chromatin structure and regulation in t(8;21) AML cells, non-t(8;21) myeloid precursor control cells, and pre-B cells. In non-t(8;21) myeloid precursors, PAX5 is poised for transcription, but is repressed by polycomb complexes. In t(8;21) AML, PAX5 is not directly activated by RUNX1/ETO, but expression requires constitutive mitogen-activated protein (MAP) kinase signaling. Using a model of t(8;21) carrying an activating KIT mutation, we demonstrate that deregulated MAP kinase signaling in t(8;21) AML abrogates the association of polycomb complexes to PAX5 and leads to aberrant gene activation. Our findings therefore suggest a novel role of activating tyrosine kinase mutations in lineage-inappropriate gene expression in AML.

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.

B-lineage transcription factors and cooperating gene lesions required for leukemia development

Differentiation of hematopoietic stem cells into B lymphocytes requires the concerted action of specific transcription factors, such as RUNX1, IKZF1, E2A, EBF1 and PAX5. As key determinants of normal B-cell development, B-lineage transcription factors are frequently deregulated in hematological malignancies, such as B-cell precursor acute lymphoblastic leukemia (BCP-ALL), and affected by either chromosomal translocations, gene deletions or point mutations. However, genetic aberrations in this developmental pathway are generally insufficient to induce BCP-ALL, and often complemented by genetic defects in cytokine receptors and tyrosine kinases (IL-7Rα, CRLF2, JAK2 and c-ABL1), transcriptional cofactors (TBL1XR1, CBP and BTG1), as well as the regulatory pathways that mediate cell-cycle control (pRB and INK4A/B). Here we provide a detailed overview of the genetic pathways that interact with these B-lineage specification factors, and describe how mutations affecting these master regulators together with cooperating lesions drive leukemia development.

EuroFlow antibody panels for standardized n-dimensional flow cytometric immunophenotyping of normal, reactive and malignant leukocytes

Most consensus leukemia & lymphoma antibody panels consist of lists of markers based on expert opinions, but they have not been validated. Here we present the validated EuroFlow 8-color antibody panels for immunophenotyping of hematological malignancies. The single-tube screening panels and multi-tube classification panels fit into the EuroFlow diagnostic algorithm with entries defined by clinical and laboratory parameters. The panels were constructed in 2-7 sequential design-evaluation-redesign rounds, using novel Infinicyt software tools for multivariate data analysis. Two groups of markers are combined in each 8-color tube: (i) backbone markers to identify distinct cell populations in a sample, and (ii) markers for characterization of specific cell populations. In multi-tube panels, the backbone markers were optimally placed at the same fluorochrome position in every tube, to provide identical multidimensional localization of the target cell population(s). The characterization markers were positioned according to the diagnostic utility of the combined markers. Each proposed antibody combination was tested against reference databases of normal and malignant cells from healthy subjects and WHO-based disease entities, respectively. The EuroFlow studies resulted in validated and flexible 8-color antibody panels for multidimensional identification and characterization of normal and aberrant cells, optimally suited for immunophenotypic screening and classification of hematological malignancies.

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.

Acute myeloid leukemia with the t(8;21) translocation: clinical consequences and biological implications

The t(8;21) abnormality occurs in a minority of acute myeloid leukemia (AML) patients. The translocation results in an in-frame fusion of two genes, resulting in a fusion protein of one N-terminal domain from the AML1 gene and four C-terminal domains from the ETO gene. This protein has multiple effects on the regulation of the proliferation, the differentiation, and the viability of leukemic cells. The translocation can be detected as the only genetic abnormality or as part of more complex abnormalities. If t(8;21) is detected in a patient with bone marrow pathology, the diagnosis AML can be made based on this abnormality alone. t(8;21) is usually associated with a good prognosis. Whether the detection of the fusion gene can be used for evaluation of minimal residual disease and risk of leukemia relapse remains to be clarified. To conclude, detection of t(8;21) is essential for optimal handling of these patients as it has both diagnostic, prognostic, and therapeutic implications.

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.

Surrogate marker profiles for genetic lesions in acute leukemias

The basic hypothesis of surrogate marker profiles is that individual genetic lesions result in characteristic distortions of the cellular phenotype with some predictable consistency that can be exploited by sophisticated immunophenotyping. While cytogenetic and molecular aberrancies currently are accepted prognostic predictors in acute leukemias, single antigen expression and even antigenic profiles rarely impact on prognosis. However, increasingly, phenotypes are delineated which can serve as surrogates for underlying genetic aberrations of clinical importance. This development is of particular significance as antileukemic therapy becomes available that targets any component of the disturbed molecular pathways associated with these genetic lesions. This chapter will focus on established surrogate marker profiles, such as those for PML/RARα, AML1/ETO, FLT3-gene mutated acute lymphocytic leukemia (ALL), and BCR/ABL(POS) ALL. As the list of therapeutic targets grows, the role of surrogate antigen profiles will grow, as they can predict for the efficacy of targeted approaches in lieu of expensive, time-consuming and not always accessible genetic analyses.

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.

Common clonal origin of an acute B-lymphoblastic leukemia and a Langerhans' cell sarcoma: evidence for hematopoietic plasticity

BACKGROUND

The hierarchical organization of hematopoiesis with unidirectional lineage determination has become a questionable tenet in view of the experimental evidence of reprogramming and transdifferentiation of lineage-determined cells. Clinical examples of hematopoietic lineage plasticity are rare. Here we report on a patient who presented with an acute B-lymphoblastic leukemia and developed a Langerhans' cell sarcoma 9 years later. We provide evidence that the second neoplasm is the result of transdifferentiation.

DESIGN AND METHODS

B-cell acute lymphoblastic leukemia was diagnosed in an 11-year old boy in 1996. Treatment according to the ALL-BFM-1995 protocol resulted in a complete remission. Nine years later, in 2005, Langerhans' cell sarcoma was diagnosed in a supraclavicular lymph node. Despite treatment with different chemotherapy protocols the patient had progressive disease. Finally, he received an allogeneic peripheral blood stem cell transplant and achieved a continuous remission. Molecular studies of IGH- and TCRG-gene rearrangements were performed with DNA from the Langerhans' cell sarcoma and the cryopreserved cells from the acute B-lymphoblastic leukemia. The expression of PAX5 and ID2 was analyzed with real-time reverse transcriptase polymerase chain reaction.

RESULTS

Identical IGH-rearrangements were demonstrated in the acute B-lymphoblastic leukemia and the Langerhans' cell sarcoma. The key factors required for B-cell and dendritic cell development, PAX5 and ID2, were differentially expressed, with a strong PAX5 signal in the acute B-lymphoblastic leukemia and only a weak expression in the Langerhans' cell sarcoma, whereas ID2 showed an opposite pattern.

CONCLUSIONS

The identical IGH-rearrangement in both neoplasms indicates transdifferentiation of the acute B-lymphoblastic leukemia into a Langerhans' cell sarcoma. Loss of PAX5 and the acquisition of ID2 suggest that these key factors are involved in the transdifferentiation from a B-cell phenotype into a Langerhans'/dendritic cell phenotype. (Clinical trial registration at: Deutsches KrebsStudienRegister, http://www.studien.de, study-ID:8).

PAX5-positive T-cell anaplastic large cell lymphomas associated with extra copies of the PAX5 gene locus

Cell lineage is the major criterion by which lymphomas are classified. Immunohistochemistry has greatly facilitated lymphoma diagnosis by detecting expression of lineage-associated antigens. However, loss or aberrant expression of these antigens may present diagnostic challenges. Anaplastic large cell lymphoma is a T-cell lymphoma that shows morphologic and phenotypic overlap with classical Hodgkin's lymphoma, which is a tumor of B-cell derivation. Staining for the B-cell transcription factor, paired box 5 (PAX5), has been suggested to be helpful in this differential, as it is positive in most classical Hodgkin's lymphomas, but absent in anaplastic large cell lymphomas. In this study we report four systemic T-cell anaplastic large cell lymphomas that were positive for PAX5 by immunohistochemistry, with weak staining intensity similar to that observed in classical Hodgkin's lymphoma. All diagnoses were confirmed by a combination of morphologic, phenotypic, and molecular criteria. Three cases were anaplastic lymphoma kinase (ALK) negative and one was ALK positive. PAX5 immunohistochemistry was negative in 198 additional peripheral T-cell lymphomas, including 66 anaplastic large cell lymphomas. Unexpectedly, although PAX5 translocations were absent, all evaluable PAX5-positive anaplastic large cell lymphomas showed extra copies of the PAX5 gene locus by fluorescence in situ hybridization (FISH). In contrast, only 4% of PAX5-negative peripheral T-cell lymphomas had extra copies of PAX5. We conclude that aberrant expression of PAX5 occurs rarely in T-cell anaplastic large cell lymphomas, and may be associated with extra copies of the PAX5 gene. PAX5-positive lymphomas with morphologic features overlapping different lymphoma types should be evaluated with an extensive immunohistochemical panel and/or molecular studies to avoid diagnostic errors that could lead to inappropriate treatment. As PAX5 overexpression causes T-cell neoplasms in experimental models, PAX5 may have contributed to lymphomagenesis in our cases.

Expression profiling of transcription factors in B- or T-acute lymphoblastic leukemia/lymphoma and burkitt lymphoma: usefulness of PAX5 immunostaining as pan-Pre-B-cell marker

The optimal use of transcription factors to determine B-lineage specificity in B-acute lymphoblastic leukemia/lymphoma (B-ALL) has not been fully investigated. We undertook an extensive immunohistochemical study of a panel of B-cell transcription factors in B- and T-ALL and Burkitt lymphoma to evaluate those with the best specificity and sensitivity. Tissue microarrays were constructed from 34 B-ALL, 19 T-ALL, and 30 Burkitt lymphoma samples. All 34 (100%) cases of B-ALL expressed PAX5; 32 (94%), BOB.1; 33 (97%), PU.1; 29 (85%), CD79a; 27 (79%), CD22; 2 (6%), CD20; 9 (26%), OCT-2; and 3 (9%), MUM1. Burkitt lymphoma cases were positive for PAX5 (30/30 [100%]), BOB.1 (27/30 [90%]), PU.1 (23/30 [77%]), CD79a (29/30 [97%]), CD22 (14/30 [47%]), CD20 (30/30 [100%]), OCT-2 (23/30 [77%]), and MUM1 (5/30 [17%]). T-ALLs were only positive for PU.1 (15/19 [79%]) and BOB.1 (12/19 [63%]). PAX5 demonstrated better specificity for B-lineage determination than BOB.1 and PU.1 and better sensitivity than CD79a, CD22, and CD20. These findings suggest that PAX5 has the greatest diagnostic usefulness and lineage determination in B-ALL, especially in cases with an inadequate specimen for flow cytometric analysis.

Myeloid sarcoma: extramedullary manifestation of myeloid disorders

Myeloid sarcoma (MS), also termed extramedullary acute myeloid leukemia, extramedullary myeloid tumor, and granulocytic sarcoma or chloroma, is a rare manifestation that is characterized by the occurrence of 1 or more tumor myeloid masses occurring at an extramedullary site. The wide spectrum of this disorder and the conditions that it overlaps diagnostically were well reflected in the 25 cases submitted to the Society for Hematopathology/European Association for Haematopathology Workshop held in Indianapolis, IN, in November 2007. This review, on the one hand, focuses on the definition and most recent achievements on the pathobiology of MS, and on the other, also in the light of the revised World Health Organization classification, summarizes the main features of a representative series of this condition aiming to provide readers a useful document for daily practice.

Diagnostic utility of the B-cell lineage markers CD20, CD79a, PAX5, and CD19 in paraffin-embedded tissues from lymphoid neoplasms

The specificity and sensitivity of CD19, CD20, CD79a, and PAX5 for detection of B-cell lineage lymphoma/leukemia derivation was determined on tissue microarrays containing 148 Hodgkin lymphomas, 358 B-cell and 16 T-cell lymphomas, 50 myelomas, and 69 acute leukemias. In mature lymphoid neoplasms, receiver-operating characteristic curve analysis showed CD20 to be the most sensitive, and CD20 and CD79a the most specific markers for B-lineage derivation. CD19 had the weakest specificity, because it was expressed in 3 T-cell lymphomas, but its sensitivity was better than CD79a. In Hodgkin lymphoma cases, the presence of B-cell markers in Hodgkin and Reed-Sternberg cells decreased in the following order: PAX5>CD20>CD79a>CD19. CD19 and PAX5 were not detectable in myelomas. In acute leukemia, CD20 turned to be the most specific, and PAX5 and CD19 the most sensitive markers for B-lineage derivation. In conclusion, an optimal B-cell lineage panel for daily routine on paraffin-embedded tissues should consist of CD20 and CD79a, and eventually, PAX5 for mature lymphoid neoplasms and PAX5 and CD19, and eventually, CD20 in (acute) precursor cell leukemias, because they cover most of the sensitivity and specificity needed.

MLL rearrangements in pediatric acute lymphoblastic and myeloblastic leukemias: MLL specific and lineage specific signatures

Background

The presence of MLL rearrangements in acute leukemia results in a complex number of biological modifications that still remain largely unexplained. Armstrong et al. proposed MLL rearrangement positive ALL as a distinct subgroup, separated from acute lymphoblastic (ALL) and myeloblastic leukemia (AML), with a specific gene expression profile. Here we show that MLL, from both ALL and AML origin, share a signature identified by a small set of genes suggesting a common genetic disregulation that could be at the basis of mixed lineage leukemia in both phenotypes.

Methods

Using Affymetrix^® HG-U133 Plus 2.0 platform, gene expression data from 140 (training set) + 78 (test set) ALL and AML patients with (24+13) and without (116+65) MLL rearrangements have been investigated performing class comparison (SAM) and class prediction (PAM) analyses.

Results

We identified a MLL translocation-specific (379 probes) signature and a phenotype-specific (622 probes) signature which have been tested using unsupervised methods. A final subset of 14 genes grants the characterization of acute leukemia patients with and without MLL rearrangements.

Conclusion

Our study demonstrated that a small subset of genes identifies MLL-specific rearrangements and clearly separates acute leukemia samples according to lineage origin. The subset included well-known genes and newly discovered markers that identified ALL and AML subgroups, with and without MLL rearrangements.

Efficient retroviral transduction of human B-lymphoid and myeloid progenitors: marked inhibition of their growth by the Pax5 transgene

We applied a coculture system for the genetic manipulation of human B-lymphoid and myeloid progenitor cells using murine bone marrow stromal cell support, and investigated the effects of forced Pax5 expression in both cell types. Cytokine-stimulated cord blood CD34⁺ cells could be transduced at 85% efficiency and 95% cell viability by a single 24-h infection with RD114-pseudotyped retroviral vectors, produced by the packaging cell line Plat-F and bicistronic vector plasmids pMXs-Ig, pMYs-Ig, or pMCs-Ig, encoding EGFP. Infected CD34⁺ cells were seeded onto HESS-5 cells in the presence of stem cell factor and granulocyte colony-stimulating factor, allowing the extensive production of B progenitors and granulocytic cells. We examined the cell number and CD34, CD33, CD19, and CD20 lambda and kappa expressions by flow cytometry. Ectopic expression of Pax5 in CD34⁺ cells resulted in small myeloid progenitors coexpressing CD33 and CD19 and inhibited myeloid differentiation. After 6 weeks, the number of Pax5-transduced CD19⁺ cells was 40-fold lower than that of control cells. However, the expression of CD20 and the κ/λ chain on Pax5-transduced CD19⁺ cells suggests that the Pax5 transgene may not interfere with their differentiation. This report is the first to describe the effects of forced Pax5 expression in human hematopoietic progenitors.