Redefining the biological basis of lineage-ambiguous leukemia through genomics: BCL11B deregulation in acute leukemias of ambiguous lineage

Acute leukemias of ambiguous lineage (ALAL), including mixed phenotype acute leukemia (MPAL) and related entities such as early T-cell precursor acute leukemia (ETP-ALL), remain diagnostic and clinical challenges due to limited understanding of pathogenesis, reliance of immunophenotyping to classify disease, and the lack of a rational approach to guide selection of appropriate therapy. Recent studies utilizing genomic sequencing and complementary approaches have provided key insights that are changing the way in which such leukemias are classified, and potentially, treated. Several recurrent genomic alterations define leukemias that straddle immunophenotypic entities, such as ZNF384-rearranged childhood B-ALL and B/myeloid MPAL, and BCL11B-rearranged T/myeloid MPAL, ETP-ALL and AML. In contrast, some cases of MPAL represent canonical ALL/AML entities exhibiting lineage aberrancy. For many cases of ALAL, experimental approaches indicate lineage aberrancy arises from acquisition of a founding genetic alteration into a hematopoietic stem or progenitor cell. Determination of optimal therapeutic approach requires genomic characterization of uniformly treated ALAL patients in prospective studies, but several approaches, including kinase inhibitors and BH3 mimetics may be efficacious in subsets of ALAL.     

Acute undifferentiated leukemia: implications for cellular origin and clonality suggested by analysis of surface markers and immunoglobulin gene rearrangement

Although B cell leukemias and, recently, T cell leukemias can be identified both by surface marker and molecular analysis, there remains a population of acute undifferentiated leukemias (AUL) that cannot be allocated definitively to a single cell lineage. AUL was diagnosed in nine patients according to stringent criteria. We combined both immunologic and molecular approaches to analyze further the ambiguous origin of AUL cells. Southern blot analysis revealed rearranged Ig heavy-chain genes in seven patients and indicated a biclonal or oligoclonal leukemic cell population in three of them, including one case of AUL with translocation (4;11). Analysis of cell surface markers showed expression of at least one early B cell-associated antigen (BA- 1, BA-2, B4, UL-38) in six of these seven patients, with coexpression of a myeloid antigen (VIM-2) in three patients. Leukemic cells of two other patients neither exhibited Ig chain gene rearrangements nor expressed B cell-associated antigens. T cell receptor beta-chain genes showed germline configuration in all nine cases. Our results demonstrate heterogeneity among AUL patients based on molecular and surface marker analyses and suggest that most AUL blast cells are derived from a precursor cell that shares phenotypic and genotypic characteristics of early B cells with certain surface antigens of myeloid cells, in some cases of AUL more than one abnormal cell clone or subclone may exist, and the cellular origin, at least of some AULs exhibiting t(4;11), may be truly B cell lineage committed.     

Cytogenetic abnormalities in acute leukaemia of ambiguous lineage: an overview

Acute leukaemia of ambiguous lineage (ALAL) is a rare complex entity with heterogeneous clinical, immunophenotypic, cytogenetic and molecular genetic features and adverse outcome. According to World Health Organization 2008 classification, ALAL encompasses those leukaemias that show no clear evidence of differentiation along a single lineage. The rarity of ALAL and the lack of uniform diagnostic criteria have made it difficult to establish its cytogenetic features, although cytogenetic analysis reveals clonal chromosomal abnormalities in 59–91% of patients. This article focuses on the significance of cytogenetic analysis in ALAL supporting the importance of cytogenetic analysis in the pathogenesis, diagnosis, prognosis, follow up and treatment selection of ALAL. It reviews in detail the types of chromosomal aberrations, their molecular background, their correlation with immunophenotype and age distribution and their prognostic relevance. It also summarizes some novel chromosome aberrations that have been observed only once. Furthermore, it highlights the ongoing and future research on ALAL in the field of cytogenetics.     

Analysis of the cellular DNA and RNA content in acute leukemias by flow cytometry

Summary In the present study bone marrow samples from 573 patients with newly diagnosed acute myeloid (AML) and lymphoblastic or undifferentiated leukemias (ALL/AUL), were analysed for their cellular DNA und DNA/RNA content, respectively, by means of flow cytometry. From 237 patients with AML 35.4% revealed aneuploid DNA stemlines. While no relation of DNA aneuploidy with other pretherapeutic parameters, including FAB subtype, white blood cell count, lactate dehydrogenase, S-phase index and percentage of blasts in the bone marrow, was observed, cases with aneuploid DNA stemlines revealed a tendency towards longer remission duration. In ALL/AUL 21.8% of 280 patients expressed DNA aneuploidies, which were less frequently found in T-cell ALL (11.1%) as compared to common(C)-ALL (21.4%) or null-cell(null)-ALL (23.5%). DNA aneuploidy was not related with other clinically defined risk factors such as age, white blood cell count, and rapid achievement of remission. Patients with DNA indices <1.0, however, tended to have shorter remissions. A significant difference in RNA indices was observed between AML and ALL/AUL with median values of 14.4 and 10.1, respectively (P<0.05). These data indicate the usefulness of flow cytometric analyses of cellular DNA and RNA content for the characterization and classification of acute leukemias, complementing the identification of clinical risk factors, immuno-phenotyping and cytogenetics.Abbreviations AML acute myeloid leukemia - ALL acute lymphoblastic leukemia - AUL acute undifferentiated leukemia - T-ALL, C-ALL and Null-ALL T-cell, common and Null-cell ALL     

Prognosis of children with mixed phenotype acute leukemia treated on the basis of consistent immunophenotypic criteria

Background

Mixed phenotype acute leukemia (MPAL) represents a diagnostic and therapeutic dilemma. The European Group for the Immunological Classification of Leukemias (EGIL) scoring system unambiguously defines MPAL expressing aberrant lineage markers. Discussions surrounding it have focused on scoring details, and information is limited regarding its biological, clinical and prognostic significance. The recent World Health Organization classification is simpler and could replace the EGIL scoring system after transformation into unambiguous guidelines.

Design and Methods

Simple immunophenotypic criteria were used to classify all cases of childhood acute leukemia in order to provide therapy directed against acute lymphoblastic leukemia or acute myeloid leukemia. Prognosis, genotype and immunoglobulin/T-cell receptor gene rearrangement status were analyzed.

Results

The incidences of MPAL were 28/582 and 4/107 for children treated with acute lymphoblastic leukemia and acute myeloid leukemia regimens, respectively. In immunophenotypic principal component analysis, MPAL treated as T-cell acute lymphoblastic leukemia clustered between cases of non-mixed T-cell acute lymphoblastic leukemia and acute myeloid leukemia, while other MPAL cases were included in the respective non-mixed B-cell progenitor acute lymphoblastic leukemia or acute myeloid leukemia clusters. Analogously, immunoglobulin/T-cell receptor gene rearrangements followed the expected pattern in patients treated as having acute myeloid leukemia (non-rearranged, 4/4) or as having B-cell progenitor acute lymphoblastic leukemia (rearranged, 20/20), but were missing in 3/5 analyzed cases of MPAL treated as having T-cell acute lymphobastic leukemia. In patients who received acute lymphoblastic leukemia treatment, the 5-year event-free survival of the MPAL cases was worse than that of the non-mixed cases (53±10% and 76±2% at 5 years, respectively, P=0.0075), with a more pronounced difference among B lineage cases. The small numbers of MPAL cases treated as T-cell acute lymphoblastic leukemia or as acute myeloid leukemia hampered separate statistics. We compared prognosis of all subsets with the prognosis of previously published cohorts.

Conclusions

Simple immunophenotypic criteria are useful for therapy decisions in MPAL. In B lineage leukemia, MPAL confers poorer prognosis. However, our data do not justify a preferential use of current acute myeloid leukemia-based therapy in MPAL.     

Some difficult problems in the classification of acute leukemia

The authors presented 5 cases of ANLL and 6 cases of ALL according to FAB classification and further classified them with leukemic cell morphology, cytochemistry and immunophenotyping. Five cases of them belonged to low grade peroxidase ANLL (LP-ANLL), 4 to hybrid acute leukemia (HAL) and 2 to acute undifferentiated leukemia (AUL). The importance of immunophenotyping of leukemic cells in classification of acute leukemia was emphasized and the criteria for diagnosis of LP-ANLL, HAL and AUL discussed. The authors are of the opinion that the diagnosis of these three kinds of acute leukemia is difficult and the prognosis is serious.     

Analysis of peroxidase-negative acute unclassifiable leukemias by monoclonal antibodies. 1. Acute myelogenous leukemia and acute myelomonocytic leukemia

In this study, pretreatment peripheral and/or bone marrow blasts from 12 patients with acute unclassifiable leukemia (AUL) expressing the myeloid-related cell-surface antigen (CD 11) were isolated for further analysis. Despite a lack of myeloperoxidase (MPO) activity, 1 patient's blasts contained cytoplasmic Auer rods. The circulating blasts from another patient expressed MPO while maintaining the same surface phenotype during 20 months of clinical follow-up. In addition, the blasts from 3 cases demonstrated both myelomonocytic and monocyte-specific surface antigens, whereas the remaining 9 cases completely lacked any monocyte-specific antigen detectable by monoclonal antibodies, Mo2, My4 and Leu M3 (CD 14). The first case eventually was diagnosed as acute myelomonocytic leukemia and the second as acute myelogenous leukemia by means of immunophenotypic analysis using flow cytometry (FACS IV). In addition, the presence of MPO protein was identified in the cytoplasm of blast cells from 5 patients with AUL by means of a cytoplasmic immunofluorescence test using a monoclonal antibody (MA1). Our study indicates that non-T, non-B AUL expressing OKM1 (CD 11) antigens include acute leukemias which are unequivocally identifiable as being of either myeloid or myelomonocytic origin. However, further investigations, including immunophenotypic and cytoplasmic analysis, ultrastructural cytochemistry and gene analysis with molecular probes (tests applicable to normal myeloid cells), are necessary in order to determine the actual origin of blasts and to recognize the differentiation stages of the various types of leukemic cells from patients with undifferentiated forms of leukemia.     

The diagnosis of acute leukemia with undifferentiated or minimally differentiated blasts

Summary A small group of acute leukemias can be classified by morphological, cytochemical, or immunological marker analysis neither as acute lymphatic leukemia nor as acute myeloid leukemia. These leukemias are referred to as acute undifferentiated leukemia (AUL) and make up 2%–7% of the acute nonmyeloid leukemias. These leukemias are poorly defined in the literature and are also sometimes referred to as AML-MO. Most of the definitions include in the morphological and cytochemical criteria the expression of myeloid antigens. Here the value of these markers and of other techniques used in the diagnosis of undifferentiated or minimally differentiated leukemia is discussed. More than half of the leukemias that are undifferentiated by morphology and cytochemistry on the light-microscopic level show a positive reaction for myeloperoxidase by electron microscopy, which points to an early myeloid differentiation of those leukemias. Immunological marker analysis in most cases is inconclusive. Most show a positive reaction for CD 13, CD 33 or other myeloid-associated markers. However, in about half of these leukemias co-expression of lymphatic markers is seen. In a small minority, only lymphatic markers are expressed. Cytogenetic abnormalities which are found in these leukemias vary in type, and antigen receptor rearrangements are not lineage specific. Receptor studies, gene expression, and in vitro culture studies may, in the near future, contribute substantially to our knowledge about the commitment of these undifferentiated or minimally differentiated blasts. Recent definitions for AUL and AML-MO based on these different techniques are discussed.     

Acute leukemias of ambiguous lineage in adults: molecular and clinical characterization

Acute leukemias of ambiguous lineage represent a heterogeneous group of rare, poorly characterized leukemias with adverse outcome. No larger studies have yet performed a combined approach of molecular and clinical characterization of acute undifferentiated leukemia (AUL) and biphenotypic acute leukemia (BAL) in adults. Here we describe 16 adults with AUL and 26 with BAL and performed mutational as well as expression studies of genes with prognostic impact in acute leukemia (BAALC, ERG, MN1, WT1, and IGFBP7). AUL showed overexpression of these genes compared to T-lymphoblastic leukemia (T-ALL), B-precursor ALL, and to acute myeloid leukemia (AML). Genotype alterations were not detectable in AUL. BAL samples were characterized by frequent WT1 mutations (18 %) and BCR-ABL translocations (30 %). ALL-based treatment protocols induced complete remissions in 40 % and AML-like therapies in 22 % of AUL/BAL patients. The outcome in both groups was very poor; a long-term survival was only observed in patients undergoing allogeneic stem cell transplantation (SCT). Our findings indicate that AUL and BAL share important molecular and high-risk features of both myeloid and lymphoid leukemias. BAL patients exhibited genetic alterations, which can be targeted therapeutically. Importantly, ALL therapy might be more effective than AML protocols and AUL/BAL patients should be considered for allogeneic SCT.     

Characterization of acute undifferentiated leukemia by combined analysis of plasma membrane-associated gamma-glutamyltranspeptidase and soluble terminal transferase

gamma-Glutamyltranspeptidase (gamma-GT) is a plasma membrane-associated enzyme present in blasts of certain acute leukemias. We analyzed 90 cases of undifferentiated and differentiated acute leukemias for gamma- GT, using a colorimetric assay. Blasts of all patients with common acute lymphoblastic leukemia (ALL) and T-ALL were negative for gamma-GT (less than 5 units). In contrast, gamma-GT was significantly elevated in acute myeloblastic or monoblastic leukemia blasts (P less than .001). In 16 cases of acute undifferentiated leukemia (AUL) studied, the levels of gamma-GT ranged from 0 to 93 units; in eight cases, gamma- GT was positive (greater than 5 units), and six of these had 2% to 5% Sudan black-positive leukemic cells in the blast-enriched suspension. Combined gamma-GT/TdT analysis revealed that both enzyme markers were mutually exclusive in 75% of AUL cases, suggesting that gamma-GT+/TdT- blasts are of nonlymphoid origin, and gamma-GT-/TdT+ blasts are of lymphoid origin. Two cases were devoid of both enzyme activities and could represent truly undifferentiated leukemia. Thus, combined gamma- GT/TdT analysis underlines the heterogeneity of AUL and appears to be useful in defining the lineage commitment of undifferentiated leukemic blasts.     

Acute unclassified leukemia. A clinicopathologic study with diagnostic implications of electron microscopy

By rigid cytological and cytochemical criteria, the diagnosis of acute and undifferentiated leukemia was established in 22 patients. According to defined criteria, the leukemic cells could not be classified by conventional light microscopic techniques employed in the study of hematopoietic tissue. Cytochemical studies including peroxidase, periodic acid schiff (PAS) and nonspecific esterase (alpha napthyl butyrate-reacting esterase) stains were done on fresh bone marrow samples, and the percentage of positive leukemia cells for each of these stains was determined on 200 cells. In this series of leukemias, cytochemistry at the light microscope level did not contribute to further classification. Subsequent electron microscopic examination of bone marrow samples from these patients confirmed the immaturity and nuclear/cytoplasmic asynchrony of the leukemic cells. Several in vivo neoplastic markers, such as nuclear blebs, increased nuclear bodies, and cytoplasmic fibrillar bundles could be demonstrated in these cells. Fourteen cases from this series exhibited peroxidase-positive developmental granule formation at the ultrastructural level and were reclassified as acute granulocytic leukemia (AGL). One case was reclassified as lymphoma (poor differentiated type), one case was diagnosed as acute monocytic leukemia (AMonoL), and six cases remained in the undifferentiated category (AUL). Clinical and laboratory features, response to treatment, and survival data were evaluated for these patients. This study demonstrated that electron microscopy is useful in the cytological diagnosis of human leukemia.     

Children Diagnosed as Mixed-Phenotype Acute Leukemia Didn’t Benefit from the CCLG-2008 Protocol,Retrospective Analysis from Single Center

Mixed phenotype acute leukemia (MPAL) is a rare type of acute leukemia with a poor clinical outcome which lacks specific therapy. To evaluate the therapeutic efficiency of CCLG-2008 protocol used for acute lymphoblastic leukemia (ALL) in China on MPAL children who were initially diagnosed as ALL by morphology, we reviewed patients’ database diagnosed as ALL and MPAL according to WHO classification and compared their outcomes from July 2008 to June 2012. Total newly enrolled ALL in this study were 309 cases by morphology, in which ten cases were identified as MPAL mainly by immunophenotyping: B+ myeloid (3/10), T+ myeloid (2/10), B + T (4/10), trilineage (1/10). Two cases were classified as intermediate risk (IR) and 8 cases were high risk (HR) according to the CCLG-2008 criteria. Only one case of IR survived and others died due to primary resistance of chemotherapy and relapse. Compared with MPAL, ALL children in IR and HR had a longer survival (28.1 vs 9.5 months, p < 0.0001) and lower relapse (16.3 vs 85.7 %, p = 0.0002). In a summary, our result indicated that MPAL in children is a poor-risk disease which needs personalized therapy to improve outcome.

Electronic supplementary material

The online version of this article (doi:10.1007/s12288-014-0372-6) contains supplementary material, which is available to authorized users.     

Surface markers in acute non-lymphoid leukemia: Analysis with a panel of 36 monoclonal antibodies

Summary The reactivity of a panel of monoclonal antibodies was studied in fifty-four cases of acute myeloid (AML) or undifferentiated (AUL) leukemias. Thirty-six antibodies from the Myeloid section of the Second Workshop on Human Leukocyte Differentiation Antigens were used in an indirect immunofluorescence assay. The antibodies could be classified into three groups recognizing respectively granulocytic, monocytic or granulomonocytic leukemias. Most antibodies stained erythroblastic and megakaryoblastic leukemias. In each group, it was possible to define antibodies staining either the less differentiated forms (FAB M 1 and M 5 a) or the more differentiated forms (M 2, M 3, M 4 and M 5 b). Six out of eight AUL were stained by some of the antibodies (mainly from the monocytic group). However, a heterogeneity of stainings in a same blast population was observed.     

An update on classification,genetics, and clinical approach to mixed phenotype acute leukemia (MPAL)

Mixed phenotype acute leukemia (MPAL) is an uncommon diagnosis, representing only about 2–5% of acute leukemia cases. The blast cells of MPAL express multilineage immunophenotypic markers and may have a shared B/T/myeloid phenotype. Due to historical ambiguity in the diagnosis of MPAL, the genetics and clinical features of this disease remain poorly characterized. Based on the 2008 and 2016 World Health Organization classifications, myeloid lineage is best determined by presence of myeloperoxidase, while B and T lymphoid lineages are demonstrated by CD19 and cytoplasmic CD3 expression. MPAL typically carries a worse prognosis than either acute myeloid leukemia (AML) or acute lymphoid leukemia (ALL). Given the rarity of MPAL, there is a lack of prospective trial data to guide therapy; treatment generally relies on ALL-like regimens followed by consolidation chemotherapy or hematopoietic stem cell transplant (HSCT). Here, we review the updated classification, biology, clinical features, and treatment approach to MPAL.     

Prognostic impact of Philadelphia chromosome in mixed phenotype acute leukemia (MPAL): A cancer registry analysis on real-world outcome

Mixed phenotype acute leukemia (MPAL) is thought to have poor outcome, and presence of the Philadelphia chromosome (Ph+) has been considered to be an adverse prognostic marker. However, most of these reports were in the pre-tyrosine kinase inhibitors (TKIs) era. Recent limited reports indicate improved outcomes for MPAL with the addition of TKIs. We examined the outcomes of 241 cases of MPAL according to the 2008 WHO classification from the Surveillance, Epidemiology, and End Results registry. The MLL+ patients had a median age of 6 years while other subtypes occurred mostly in adults and had comparable age. On multivariate analyses and after adjustment for age, year of diagnosis and chemotherapy status, Ph+ MPAL patients had reduced risk of death in comparison to Ph(–) MPAL patients (hazard ratio [HR] = 0.28, P = .002). So, MLL+ MPAL had the worst outcome with a 10-fold increased risk of death in comparison to Ph+ MPAL patients (HR = 10.2, P < .001). Importantly, the outcome of Ph+ MPAL was comparable to Ph+ acute lymphoblastic leukemia in a 1:1 matched case-control analysis. In conclusion, this is the largest registry study which examines the outcomes of MPAL subtypes. We confirm that MPAL is a heterogenous disease. Note, Ph+ MPAL nowadays has a better OS in comparison to other subtypes and is comparable to Ph+ ALL patients. This is most likely secondary to changes in practice and more utilization of TKIs. On the other hand, MLL rearrangement is associated with infantile MPAL and has a dismal prognosis.     

Acute undifferentiated leukemia (AUL): a case report and a proposed system of classification.

This report describes a case of HLADR+, CD34- acute undifferentiated leukemia (AUL) diagnosed in an 18-year-old male. A definition of AUL and a system for its classification are proposed on the basis of the current state of knowledge about phenotypic features of AUL cells and their clonal counterparts that exist during early stages of normal hematopoiesis.     

Ultrastructural demonstration of peroxidase expression in acute unclassified leukemias: correlation to immunophenotype and treatment outcome.

The lineage affinity of 57 cases of acute unclassified leukemias (AUL) was reevaluated by ultrastructural analysis of peroxidase expression (POEM) in combination with immunophenotyping and analysis of immunoglobulin gene configuration. Twenty-three cases of myeloid and three cases of megakaryocytic differentiation were identified by detection of ultrastructural myeloperoxidase (UMPO) and platelet peroxidase (UPPO). No significant correlation was noted between myeloid marker expression and POEM positivity, whereas presence of CD 19 or CD 24 antigen significantly correlated with POEM negativity (P = .001 and .023, respectively). Ig gene rearrangements including oligoclonal patterns were also recorded in 8 of 14 UMPO+ patients tested. Fourteen UMPO+ patients responded poorly to an ALL/AUL chemotherapy regimen with a low complete remission (CR) rate of 29% and a short median remission duration (MRD) of 5 months. The POEM- patients proved very heterogenous with respect to immunophenotype and Ig gene rearrangement. Seventeen of 21 patients tested had Ig gene rearrangements, including oligoclonal patterns. Combined data suggest that a proportion of these cases probably derive from a very immature lymphoid progenitor cell, particularly because 15 POEM- AUL patients showed a response to ALL/AUL chemotherapy comparable to that observed in patients with definitive acute lymphoblastic leukemia (ALL) (CR rate 80%, MRD 20 months). Thus, ultrastructural analysis of peroxidase expression can provide decisive prognostic information in AUL patients.     

MLL-rearranged leukemias: insights from gene expression profiling

Gene expression analysis of human leukemias has provided insight into disease classification and mechanisms of oncogenesis. Its success is particularly evident for acute leukemias with rearrangement of the mixed lineage leukemia (MLL) gene on chromosome 11q23. Unlike most other recurrent translocations, MLL rearrangements are found in leukemias classified as acute myelogenous leukemia (AML) or acute lymphoblastic leukemia (ALL). In addition, MLL-rearranged leukemias often express both myeloid- and lymphoid-associated genes. These unusual characteristics have generated much interest in the cell of origin and the mechanism of transformation by MLL rearrangements. Here we review insights gained from characterization of MLL-rearranged human leukemias by genome-wide expression profiling and compare these to data from model systems.     

Significance of lineage specific differentiation markers for complex classification of acute leukemias. II. Acute lymphoblastic leukemias

Acute lymphoblastic leukemias originate from cells committed either to the T-lineage (T-ALL) or B-lineage (nonT-ALL). Leukemic cells are allocated to these lineages according to the expression of lineage specific differentiation markers (LSDM), which are T-cell antigen receptors for the T-cell lineage and immunoglobulins for the B-cell lineage. T-ALL seem to be one type of disease, among nonT-ALL it is possible to distinguish several types of diseases according to the immunoglobulin expression and clinical findings. The specificity of monoclonal antibodies and other markers is discussed with regard to the classification of ALL and "hybrid acute leukemias" (hAL), the latter with cells differentiating into myeloid and lymphoid lineages. The existence of a single hAL has not yet been reliably proved with the use of LSDM. Short-term cultures of leukemic cells represent useful diagnostic tools particularly for acute unclassifiable leukemias. Present knowledge of karyotype findings in acute leukemias classified according to LSDM is reviewed and the necessity to introduce a complex classification on this basis stressed.     

Acute myeloid/T‐lymphoblastic leukaemia (AMTL): a distinct category of acute leukaemias with common pathogenesis in need of improved therapy

Advances in the classification of acute leukaemias have led to improved outcomes for a substantial fraction of patients. However, chemotherapy resistance remains a major problem for specific subsets of acute leukaemias. Here, we propose that a molecularly distinct subtype of acute leukaemia with shared myeloid and T cell lymphoblastic features, which we term acute myeloid/T‐lymphoblastic leukaemia (AMTL), is divided across 3 diagnostic categories owing to variable expression of markers deemed to be defining of myeloid and T‐lymphoid lineages, such as myeloperoxidase and CD3. This proposed diagnostic group is supported by (i) retained myeloid differentiation potential during early T cell lymphoid development, (ii) recognition that some cases of acute myeloid leukaemia (AML) harbour hallmarks of T cell development, such as T‐cell receptor gene rearrangements and (iii) common gene mutations in subsets of AML and T cell acute lymphoblastic leukaemia (T‐ALL), including WT1, PHF6, RUNX1 and BCL11B. This proposed diagnostic entity overlaps with early T cell precursor (ETP) T‐ALL and T cell/myeloid mixed phenotype acute leukaemias (MPALs), and also includes a subset of leukaemias currently classified as AML with features of T‐lymphoblastic development. The proposed classification of AMTL as a distinct entity would enable more precise prospective diagnosis and permit the development of improved therapies for patients whose treatment is inadequate with current approaches.