The genetic basis and cell of origin of mixed phenotype acute leukaemia

Mixed phenotype acute leukaemia (MPAL) is a high-risk subtype of leukaemia with myeloid and lymphoid features, limited genetic characterization, and a lack of consensus regarding appropriate therapy. Here we show that the two principal subtypes of MPAL, T/myeloid (T/M) and B/myeloid (B/M), are genetically distinct. Rearrangement of ZNF384 is common in B/M MPAL, and biallelic WT1 alterations are common in T/M MPAL, which shares genomic features with early T-cell precursor acute lymphoblastic leukaemia. We show that the intratumoral immunophenotypic heterogeneity characteristic of MPAL is independent of somatic genetic variation, that founding lesions arise in primitive haematopoietic progenitors, and that individual phenotypic subpopulations can reconstitute the immunophenotypic diversity in vivo. These findings indicate that the cell of origin and founding lesions, rather than an accumulation of distinct genomic alterations, prime tumour cells for lineage promiscuity. Moreover, these findings position MPAL in the spectrum of immature leukaemias and provide a genetically informed framework for future clinical trials of potential treatments for MPAL.

[Study on the clinical characteristics of 32 patients with mixed phenotype acute leukemia]

OBJECTIVE

To investigate the clinical and biological characteristics and prognosis of mixed phenotype acute leukemia (MPAL).

METHODS

Thirty two patients were diagnosed as MPAL by bone marrow examination, immunophenotyping, cytogenetic and molecular assay and were treated with combined chemotherapy regimens for both acute lymphoblastic and acute myeloid leukemia. Two cases were received allogeneic hematopoietic stem cell transplantation (allo-HSCT).

RESULTS

(1) The incidence of MPAL in acute leukemias was 2.6%. There were 16 cases (50.0%) of mixed myeloid and B-lymphoid (M/B), 14(43.8%) myeloid and T-lymphoid (M/T), one each (3.1%) of trilineage (M/B/T) and B- and T-lymphoid (B/T) phenotype. (2) The positive rates of CD34 and HLA-DR were 87.5% and 62.5%, respectively. (3) Abnormal karyotypes were detected in 70.0% of 30 MPAL patients, which were structural and numerical abnormalities including t(9;22), 11q23 and complex karyotypes. (4) The total complete remission (CR) rate was 75.0% and the overall survival (OS) and disease-free survival (DFS) at 2 years were 14.8% and 14.2% respectively. The CR rates for M/B and M/T cases were 75.0% and 71.4% respectively. No statistical difference was observed in OS and DFS between M/B and M/T cases.

CONCLUSIONS

MPAL is a rare type of acute leukemia with a high heterogeneity. The unfavorable indicators of MPAL may be factors such as abnormal karyotypes, high expression of CD34 and extramedullary infiltration. Combined regimens and more intensive therapy including allo-HSCT might contribute to improving survival.

Diagnosis of biphenotypic acute leukemia: a paradigmatic approach

Biphenotypic acute leukemia (BAL), or acute leukemia with a single population of blasts coexpressing markers of two different lineages, is a rare clinical entity. To define BAL, a scoring system was proposed by the European Group of Immunological Markers for Leukemias (EGIL) in 1995. However, increasing evidence suggests that this system has limitations, as acknowledged by the 2008 World Health Organization (WHO) Classification of Tumors of Hematopoietic and Lymphoid Tissues. Although substantially improved in relation to the EGIL, the new WHO Classification is still not optimal for guiding the clinical management of patients with BAL. We propose a new paradigmatic approach to defining BAL based on recent clinical studies of BAL and advances in immunologic marker definition and cytogenetics, and applied our new approach to 8 cases of "BAL" among a cohort of 742 new acute leukemias in our Cancer Center. By our new criteria, 6 cases were reclassified as acute lymphoblastic leukemia (ALL), while only 2 were still classified as BAL. Our approach is also supported by analyses of the BAL cases previously reported by other institutions.

[Biphenotypic and bilineal acute leukemias]

Human acute leukemias (AL) are classified as myeloid or lymphoid according to cytomorphology and the expression of leukocyte differentiation antigens/CD-markers. However, in the minority of cases leukemic cells express markers of more than one lineage, which has led to the introduction of a new subgroup of acute leukemias termed mixed or biphenotypic acute leukemias (BAL). In an effort to distinguish between BAL and those AL with aberrant expression of markers of other lineage, the European Group for the Immunological Characterization of Acute Leukemias (EGIL) has proposed a scoring system in which CD-markers are assigned a score of 0.5, 1.0 or 2.0, depending on the specificity of a particular antigen for myeloid, B- and/or T-lymphoid lineage, respectively. The new WHO classification of hematologic tumors has adopted the EGIL criteria for BAL and introduced a new group of AL termed 'AL of ambiguous lineage'. In addition to BAL in which a single cell population expresses both myeloid and lymphoid differentiation markers, this new group of leukemias also comprises cases that present with two separate blast populations (acute bilineal leukemia, aBLL). In general, BAL accounts for less than 5% of all AL cases, whereas aBLL is a rare disease constituting 1%-2% of AL cases that contains B- or T-lymphoid along with myeloid blasts. Chromosome abnormalities are frequent in both entities with a relatively high incidence of Philadelphia chromosome and rearrangements involving 11q23, especially in cases with B- and myeloid involvement. Other biological features include CD34 expression and multi-drug resistance P-glycoprotein overexpression. The prognosis of BAL and aBLL is unfavorable, with poor prognostic factors being age, high WBC and the presence of Philadelphia chromosome. Unfortunately, optimal therapy is not known, although regimens designed for acute lymphoblastic leukemia may result in a better response rate. Collaborative studies are needed for better understanding of the biology of these entities and establishment of standard therapeutic protocols.

[Cytomorphology of acute mixed leukemia]

Biphenotypic acute leukemias (AL) with blasts expressing both myeloid and lymphoid antigens are grouped with undifferentiated AL and bilineal AL in the group of AL of ambiguous lineage. Not all AL with myeloid and lymphoid antigens (ALMy+Ly) are true biphenotypic AL. According to EGIL scoring system, true biphenotypic ALMy+Ly are those with a sum of antigens 2 or more points for both myeloid and lymphoid lineage or for B and T lineage. The aim of this study was to compare cytomorphology and immunophenotype of AL to better understand the relation of certain AL morphology, immunophenotype, cytogenetics and molecular biology of biphenotypic AL.

PATIENTS AND METHODS

The study included a group of 169 AL patients treated from 1985 till 1991, and a group of 102 AL patients treated from 1993 till 1996 at Zagreb University Hospital Center. Bone marrow and peripheral blood of the two groups of AL patients were analyzed according to Pappenheim (May-Grunwald-Giemsa), cytochemical and alkaline phosphatase-anti-alkaline phosphatase (APAAP) immunocytochemical staining. Flow cytometry immunophenotyping of bone marrow was also done in both patient groups.

RESULTS AND DISCUSSION

In the group of 169 adult AL patients, 116 were cytomorphologically classified as acute myeloblastic leukemias (AML), 35 as acute lymphoblastic leukemias (ALL) and 18 as acute undifferentiated leukemias (ANLM). In 6 (3.4%) of 169 AL patients, blasts expressed both myeloid and lymphoid antigens. In the group of 102 AL patients there were 19 (18.6%) ALMy+Ly. In 64 patients cytomorphologically classified into AML subgroup out of 102 AL patients, there were 15 (14.7%/102; 23.4%/64) AML with lymphoid antigens (AMLLy+). In 35 patients cytomorphologically diagnosed as ALL and 3 as ANLM out of 102 AL, there were 4 (3.9%/102; 10.5%/38) ALL with myeloid antigens (ALLMy+). The incidence of mixed AL in 102 AL was more consistent with other studies, pointing to the necessity of myeloperoxidase (MPO), CD7 and TdT determination as part of standard immunophenotyping for better recognition of mixed AL.

CONCLUSION

In both groups of 169 and 102 AL patients, the majority of AL cases were cytomorphologically classified as AML. In the group of 169 patients there were 5 AMLLy+ and in the group of 102 patients there were 15 AMLLy+. In one ANLM,My+ out of 169 AL and also one ANLM,My+ out of 102 AL, blasts were cytomorphologically undifferentiated; in 3 ALLMy+ of 102 AL blasts expressed lymphoid morphology. According to EGIL scoring system, among 15 AMLLy+ of 102 AL there were 4 true biphenotypic ALMy+Ly (1 M1, 2 M3, 1 M4), and in 4 ALMy+Ly with undifferentiated and lymphoid morphology there were 2 true biphenotypic AL (1 L2; 1 ANLM). In 3 ALLB+T out of 35 ALL, one was interlineal biphenotypic AL. These observations are consistent with other studies and WHO determinations indicating that the majority of true biphenotypic leukemias are associated with immature monoblastic or myeloid cytomorphology or with lymphoid or undifferentiated characteristics, but may also express any AML cytomorphology type. Thus, there is no direct correlation of leukemic cell cytomorphology and biphenotypic AL immunophenotype.

Definition of acute biphenotypic leukemia

BACKGROUND AND OBJECTIVE

A minority of acute leukemias have features characteristic of both the myeloid and lymphoid lineages and for this reason are designated mixed-lineage, hybrid or biphenotypic acute leukemias (BAL). There have been difficulties in establishing whether BAL represents a distinct clinico-biological entity due to a lack of objective criteria for distinguishing BAL from acute myeloid leukemias (AML) or acute lymphoblastic leukemias (ALL) with aberrant expression of a marker from another lineage. In this work we analyze diagnostic criteria for BAL.

METHODS

We describe the features of 26 patients (19 adults and 7 children) with BAL diagnosed at the Royal Marsden Hospital. BAL was defined according to a scoring system devised by our group and the European Group for the Immunological Classification of Leukemia (EGIL). This system is based on the number and degree of specificity of the markers (lymphoid and myeloid) expressed by the blasts.

RESULTS

According to the FAB criteria, BAL may present as "ALL" or as one of the "AML" subtypes, often M1. It is not infrequent to identify two distinct blast populations: one of small size resembling lymphoblasts and the other larger. The most common immunophenotype is coexpression of B-lymphoid and myeloid markers and less frequently, T-lymphoid and myeloid markers. Cases with a B and T lymphoid phenotype or with trilineage differentiation are rare. BAL has a high incidence of clonal chromosomal abnormalities, the most common being the t(9;22) (q34;q11) (Ph chromosome) and structural abnormalities involving 11q23. Data are emerging that BAL has a negative prognosis in both children and adults and this may be related to the underlying chromosome abnormalities.

INTERPRETATION AND CONCLUSIONS

In summary, BAL is an uncommon type of leukemia which probably arises from a multipotent progenitor cell and carries a poor prognosis. Although there are no uniform criteria about whether to treat these patients as ALL or AML, it is likely that an intensive approach with high-dose therapy followed by bone marrow transplantation will be required to eradicate the disease permanently.

Possibilities and limitations of cytochemical methods in diagnosis of acute leukemia

Accurate identification and classification of leukemic blast cells is a very important prerequisite of the precise diagnosis of acute leukemia and has a great impact on therapy and prognosis. The purpose of this review is to consider, in the broad sense of the word, the present possibilities and limitations of enzyme cytochemistry and to emphasize how cytochemistry may contribute, on integration with the other methods of study, to the final classification and differential diagnosis of acute leukemia, a highly variable hematological disorder. In this review, the role of conventional enzyme cytochemistry, either dominant or subsidiary, in the discrimination of acute leukemia subtypes is discussed. The survey confirms the absolute necessity of immunologic marker analysis in the accurate diagnosis of acute lymphoblastic leukemia, undifferentiated or minimally differentiated leukemia and mixed-lineage leukemia because in these cases, the cytochemical evaluation provides insufficiently relevant information regarding blast cell origin, specificity of leukemia subtypes and the discrete stages of leukemic cell maturation. On the other hand, cytochemical investigation is appreciated to be dominant over immunophenotyping in characterizing acute myeloid leukemia, because of the lack of specificity of the majority of immunological markers against myeloid antigens and, because of the availability of standardized and sufficiently specific cytochemical reactions. The cytogenetic, molecular biological and electron microscopic studies mentioned in this review supplement the important information for correct differential diagnosis of acute leukemia. The prognostic impact of enzyme cytochemistry in correlation to other methods is evaluated.

Correlation of morphological FAB classification and immunophenotyping: value in recognition of morphological, cytochemical and immunological characteristics of mixed leukaemias

Correlation between the FAB classification and immunophenotype was studied in 169 consecutive adult patients with acute leukaemia (AL). The lineage of leukaemic cells could be determined in the majority of cases, whereas 3 patients (1.8%) remained unclassified. In 22 out of 71 patients (31%) with acute myeloid leukaemia (AML) FAB M1 and M2 types, and in 5 out of 16 patients (31%) with chronic myeloid leukaemia (CML) in myeloid blast crisis, leukaemic cells did not express myeloid lineage-related markers, indicating asynchronous expression of cell markers in a substantial proportion of patients. Flow cytometric two-colour immunofluorescence revealed mixed AL immunophenotype in 6 out of 169 patients (3.4%). This group included five CD2+AML (5% of AML tested) and one undifferentiated AL expressing CD10(CALLA), CDw65(VIM-2). The former group included FAB M1, M2, M3 and M4 forms of AML with a single cell population, and an AML M2 patient with both cytochemically and immunologically two separate populations of leukaemic cells. This further illustrates the heterogeneity of the target cell(s) for leukaemogenesis and the level of differentiation of AML cells. However, there was no difference in the treatment response and the remission duration between AML patients and patients with mixed phenotype AML.

Hand mirror variant of adult acute lymphoblastic leukemia. Evidence for a mixed leukemia

Blast cells from a female patient with acute lymphoblastic leukemia-hand mirror variant were examined using various techniques, including light and ultrastructural morphologic examination, cytochemical analysis, surface antigen characterization, cytogenetic analysis, and gene rearrangement studies. The blast cells were found to be pre-B cells (CD19+ and Tdt+) that also expressed the myeloid antigens CD13 and CD33 and demonstrated a heavy chain immunoglobulin gene rearrangement. Cytogenetic studies revealed a t(11;19) translocation previously described in biphenotypic leukemias. A subset of acute lymphoblastic leukemia-hand mirror cells has been previously defined and includes predominately female patients with an indolent course. The authors' findings place this case, a mixed leukemia, within that subgroup. The possibility of mixed lineage should be considered in future cases of hand mirror variants of adult acute lymphoblastic leukemia. Furthermore, hand mirror morphologic features in any case of acute leukemia should alert the hematopathologist/hematologist to the possibility of mixed lineage.

[Acute mixed-cell human leukemia]

The incidence rate of acute mixed-lineal leukemias, distinguished in a group of patients with acute non-lymphoblastic leukemia, in children and adults was about the same: CALLA 10-20%, T-cell antigens 15-22%, Thy-1 15-17%. The incidence rate of acute lymphoblastic leukemia with myeloid (17-21%) and erythroid (12-13%) antigens did not significantly differ in children and adults. Distinguishing features have been proposed for actual acute mixed-lineal leukemias having markers of mature stages of varying cell line differentiation from cryptic lineal-different-marker leukemias of bi- or polypotent precursor-cell origin.

Undifferentiated acute leukemia and lineage infidelity (difficulties in classification and management)

The acute leukemias have been considered to represent a clonal expansion of a malignant transformed hematopoietic progenitor cell with adherence to either the myeloid or lymphoid lineage--"lineage fidelity." Lineage fidelity has been challenged by the demonstration of lineage switching or mixed-lineage leukemias. We describe a 7 year old male who presented with undifferentiated acute leukemia and nasopharyngeal and cervical masses. His blasts had the morphologic appearance of myeloblasts (FAB M1) and were positive solely for the myeloid antigen CD15. He entered a complete remission (CR) with acute nonlymphocytic leukemia therapy. At first relapse he had evidence of mixed-lineage leukemia with B-cell lymphoid and myeloid phenotypes. He again relapsed from a second CR with Burkitt-cell leukemia. Cytogenetic findings showed a consistent 14q+, 17p+ abnormality in the blasts and nasopharyngeal mass. The t(8;14) associated with Burkitt's lymphoma was found in the mass tissue only following passage in the nude mouse. Our patient demonstrates that limitations still exist in our ability to classify acute leukemia. That leukemic transformation occurred in a multipotential progenitor cell leading to undifferentiated leukemia at diagnosis and/or that chemotherapy can influence the genetic programs of leukemic cells leading to the evidence of mixed-lineage leukemia and lineage switching is supported.

Rearrangement of T-cell receptor and immunoglobulin heavy chain genes in childhood acute mixed lineage leukaemia

Rearrangement of the beta and gamma chain genes of the TCR gene complex and of the Ig heavy chain genes were examined in three cases of childhood acute mixed lineage leukaemia. Blast cells, classified morphologically as acute lymphoblastic leukaemia (ALL) in one child and acute non-lymphocytic leukaemia (ANLL) in the other two, all co-expressed markers associated with both T (CD7, TdT) and myeloid (CD33) cells. Cytogenetic analysis detected abnormalities associated with myeloid leukaemia. Immunoglobulin heavy chain genes were not rearranged in two patients but a novel rearrangement was seen in the third. No rearrangement of the beta or gamma chains of the T-cell receptor complex were seen. Acute mixed lineage leukaemia may thus arise from a pluripotent precursor cell capable of both lymphoid and myeloid differentiation.