The Classification of Leukaemia

 

Essentials

Leukaemia is a malignant neoplasm of haematopoietic cells originating in the marrow and spreading to the blood and other tissues, such as the lymph nodes, spleen, and liver. The characteristic feature of the neoplastic cells is that they retain the ability to proliferate but fail to differentiate normally into functional haematopoietic cells. This results in replacement of the normal bone marrow by the leukaemic cells.
 
General approach to classification of leukaemia

Leukaemias are subdivided into acute or chronic and lymphoid or myeloid on the basis of the cell lineage and stage of differentiation of the malignant cell. This is of fundamental importance because each subtype differs in clinical behaviour, prognosis, and response to treatment.

Clinical approach—morphological assessment of the blood film and (usually) bone marrow aspirate is essential. The cellular morphology may be diagnostic and is used to determine which ancillary tests are required for classification. Establishing the cell lineage and stage of differentiation is achieved by phenotyping the neoplastic cell using a panel of antibodies to cellular antigens associated with specific cell types (lymphoid or myeloid lineage; B-, T- or NK-cells) and degree of maturation. Determining the genotype of the malignant cell, e.g. by examination for chromosomal translocations and other genetic abnormalities, is the defining property for some acute leukaemias and can be prognostically significant.

WHO classification of acute and chronic leukaemias

The following classes of leukaemia are recognized: (1) precursor B- and T-cell neoplasms—lymphoblastic leukaemias/lymphomas; (2) acute myeloid leukaemias—including subtypes with recurrent genetic abnormalities, with myelodysplasia-related changes, therapy-related and ‘not otherwise specified’; (3) acute leukaemias of ambiguous lineage; (4) mature B-cell neoplasms—including chronic lymphocytic leukaemia; (5) mature T-cell and NK-cell neoplasms—including T-cell prolymphocytic leukaemia; (6) myeloproliferative neoplasms—including chronic myelogenous leukaemia; (7) myelodysplastic/myeloproliferative neoplasms.

This classification of leukaemia cannot be regarded as fixed and definitive—it will certainly need to be revised as new discoveries are made and new disease subtypes defined.

Introduction

Over the past 40 years there have been several leukaemia classifications. For a classification to be useful it must be reproducible, clinically relevant, and able to be used worldwide. The French–American–British (FAB) classification of acute leukaemias, introduced in the mid-1970s, was widely applied because of its ease of use and clinical applicability. FAB was based on morphology of the neoplastic cells in the blood and bone marrow together with cytochemical reactivity. The classification was revised to include cell phenotype for some entities, and a classification for chronic lymphoid leukaemias was introduced. In the 1990s it was realized that clearer definitions of leukaemic subtypes could be achieved by combining genetic data with morphology and cell phenotype. The World Health Organization (WHO) classification of haematopoietic neoplasms, which incorporates all this information, has now replaced the FAB classification.on

WHO classification

The World Health Organization Classification of Tumours of Haematopoietic and Lymphoid Tissues was published in 2001 and revised in 2008 (Bullet list 1). This is a comprehensive consensus classification of all haematological malignancies, including the leukaemias. It stratifies neoplasms by cell lineage and defines clinicopathological entities by integrating clinical features, morphology, cell phenotype, and genotype. For many entities, especially those of lymphoid origin, the cell of origin and stage of differentiation are postulated. The lymphoid classification identifies B-cell, T-cell, and natural killer (NK) cell types, and stratifies the neoplasms into precursor and mature neoplasms. For myeloid neoplasms, the cell of origin is most commonly the pluripotent stem cell. The myeloid disorders are divided into acute myeloid leukaemia, myeloproliferative neoplasms (MPN), myelodysplastic syndromes (MDS), and MDS/MPN overlap syndromes.

To diagnose and classify acute and chronic leukaemias, morphological assessment of the blood film and, usually, bone marrow aspirate are essential. The cellular morphology may be diagnostic and is used to determine which ancillary tests are required to classify the leukaemia. Establishing the cell lineage and stage of differentiation by phenotyping the neoplastic cell is implicit in the classification. Phenotyping is performed using a panel of antibodies to cellular antigens associated with specific cell types (lymphoid or myeloid lineage; B-, T-, or NK-cells) and stage of differentiation (Table 1 below). Chromosomal translocations and dysregulation of specific genes are important in the pathogenesis of leukaemia. Over 50% of leukaemias have genetic abnormalities, and many of these are associated with specific oncogenes in particular types of leukaemia. The genotype is the defining property for some acute leukaemias and can be prognostically significant. In this chapter the major acute and chronic leukaemia entities defined by the WHO will be described.

Bullet list .1 The classification of acute and chronic leukaemias (extract from WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues)

Precursor lymphoid neoplasms
  • B lymphoblastic leukaemia/lymphoma
  • T lymphoblastic leukaemia/lymphoma
Acute myeloid leukaemias

Acute myeloid leukaemia with recurrent genetic abnormalities

  • AML with t(8;21)(q22;q22); RUNX1-RUNX1T1
  • AML with inv(16)(p13.1q22) or t(16;16)(p13.1q22); CBFB-MYH11
  • Acute promyelocytic leukaemia with t(15;17)(q22;q12); PML-RARA
  • AML with t(9;11) (p22;q23); MLLT3-MLL
  • AML with t(6;9)(p23;q34); DEK-NUP214
  • AML with inv(3)(q21q26.2) or t(3;3)(q21;q26.2); RPN1-EVI1
  • AML (megakaryoblastic) with t(1;22)(p13;q13); RBM15-MKL1
  • AML with mutated NPM1
  • AML with mutated CEBPA

Acute myeloid leukaemia with myelodysplasia-related changes

Therapy-related myeloid neoplasms

Acute myeloid leukaemia, not otherwise specified

  • Acute myeloid leukaemia with minimal differentiation
  • Acute myeloid leukaemia without maturation
  • Acute myeloid leukaemia with maturation
  • Acute myelomonocytic leukaemia
  • Acute monoblastic and monocytic leukaemia
  • Acute erythroid leukemia
  • Acute megakaryoblastic leukaemia
  • Acute basophilic leukaemia
  • Acute panmyelosis with myelofibrosis

Myeloid sarcoma

Myeloid proliferations related to Down syndrome

Acute leukaemias of ambiguous lineageMature B-cell neoplasms
  • Chronic lymphocytic leukaemia/small lymphocytic lymphoma
  • B-cell prolymphocytic leukaemia
  • Hairy cell leukemia
  • Burkitt lymphoma
Mature T-cell and NK-cell neoplasms
  • T-cell prolymphocytic leukaemia
  • T-cell large granular lymphocytic leukaemia
  • Aggressive NK cell leukaemia
  • Adult T-cell leukaemia/lymphoma
Myeloproliferative neoplasms
  • Chronic myelogenous leukaemia, BCR-ABL1 positive
  • Chronic neutrophilic leukaemia
  • Chronic eosinophilic leukaemia, NOS
Myelodysplastic/myeloproliferative neoplasms
  • Chronic myelomonocytic leukaemia
  • Juvenile myelomonocytic leukaemia
  • Atypical chronic myeloid leukaemia, BCR-ABL1 negative
  • Mast cell leukaemia

Acute leukaemias

Acute leukaemias occur as a result of malignant transformation of a progenitor or stem cell, leading to the unregulated proliferation of immature (blast) cells, without differentiation, in the bone marrow. Blood involvement is common, and there may be organ infiltration, particularly of the spleen, liver, and lymph nodes. The diagnosis requires blast cells to constitute 20% or more of all of the nucleated cells in the blood or bone marrow. Generally the marrow is hypercellular and diffusely replaced by leukaemic cells. Acute leukaemia can be lymphoid or myeloid, based on blast cell lineage. 

Table 1 Monoclonal antibodies used for the classification of acute and chronic leukaemias

 

Disorder/cell type Monoclonal antibodies
B-cell malignancies Pan B-cell antigens CD19, CD20, CD22, CD79a, CD79b, IgM, κ, λ
B-lymphoblastic leukaemia CD10, CD34, CD45, TdT; myeloid antigens CD13, CD33
Chronic lymphocytic leukaemia CD5, CD23, CD38, ZAP70
B-cell prolymphocytic leukaemia CD5, CD23
Hairy cell leukaemia CD11c, CD25, CD103, CD123
Burkitt lymphoma CD10, BCL6
T-cell malignancies Pan T-cell antigens CD2, CD3, CD4, CD5, CD7, CD8
T-lymphoblastic leukaemia CD1a, TdT; myeloid antigens CD13, CD33
T-cell prolymphocytic leukaemia CD26
T-cell large granular lymphocytic leukaemia CD16, CD56, CD57, CD158a,b,e
Natural killer–cell leukaemia NK-cell antigens CD2, CD7, CD8, CD16, CD56, CD57, CD158a,b,e
Acute myeloid leukaemia Pan-myeloid antigens CD13, CD33, CD34, CD117, HLA-DR, MPO
AML with monocytoid differentiation CD11b, CD14, CD64, CD68, lysozyme
Acute promyelocytic leukaemia PML protein, CD2, CD9, CD68
Erythroleukaemia CD235, haemoglobin A
Acute megakaryoblastic leukaemia CD41, CD42, CD61

Precursor lymphoid neoplasms

Precursor lymphoid neoplasms are clonal proliferations of immature lymphoid precursors or lymphoblasts of B- or T-cell lineage. Lymphoblasts have a high nuclear to cytoplasmic ratio, fine chromatin, inconspicuous nucleoli, and usually lack cytoplasmic granules. They have an immature lymphoid cell phenotype including the presence of the intranuclear DNA polymerase terminal deoxynucleotidyl transferase (TdT).Precursor lymphoid neoplasms

B-lymphoblastic leukaemia

B-lymphoblastic leukaemia, previously known as B-cell acute lymphoblastic leukaemia (B-ALL), is a leukaemia of lymphoblasts committed to the B-cell lineage. This is most commonly a disease of children (75% of patients are under 6 years of age). The blood and bone marrow are involved in all cases, and there may be involvement of lymph nodes, liver, spleen, and extranodal sites. When patients present with primarily organ involvement, it is termed B-lymphoblastic lymphoma. The lymphoblasts are uniform in their appearance but can vary in size. They express B-cell–associated antigens such as CD19, cytoplasmic CD22 and CD79a, are TdT-positive, and commonly express the CD34 stem cell–associated antigen. There can be variation in the degree of differentiation of the blast cells, and this can be ascertained by the pattern of expression of the CD10 antigen. Early or ‘pro-B-ALL’, the least differentiated, is CD10-negative (10% of childhood and 30% of adult B-ALL cases); ‘common-ALL’ is CD10-positive (75% of childhood B-ALL); and ‘pre-B-ALL’, the most differentiated, is CD10-positive and expresses cytoplasmic μ immunoglobulin (Ig) heavy chains. The blast cells do not express surface heavy or light chains of Ig.

Cytogenetic abnormalities are important in defining prognosis. Hyperdiploidy (51–65 chromosomes) is common in children and has a good prognosis. Translocation t(12;21)(q21;q22), present in 15 to 30% of paediatric cases, but rarely in infants and adults, also has a good prognosis. This translocation generates a chimaeric TEL-AML1 fusion gene and is associated with aberrant myeloid antigen expression (e.g. CD13 or CD33). Other genetic abnormalities have a poorer prognosis. The t(1;19)(q23;p13) translocation, involving the PBX1 and E2A genes, accounts for 25% of B-ALL cases, the majority being of the pre-B-ALL type. These frequently have organomegaly and central nervous system involvement. The poorest prognosis B-ALL cases are associated with t(9;22) and MLL 11q23 gene rearrangements. The t(9;22)(q34;q11), resulting in the Philadelphia (Ph) chromosome and the BCR-ABL1 fusion gene, is more common in adult (20% of cases) than paediatric B-ALL where it is uncommon (2-5%) and the fusion gene generates a p190 chimaeric protein. In adults the fusion results in a p210 chimaeric protein, the same as seen in chronic myelogenous leukaemia (CML). Translocations involving the mixed lineage leukaemia (MLL) gene on chromosome band 11q23 have a particularly poor prognosis. These are commonly CD10 and TdT-negative (pro-B-ALL), and gene expression profiling has demonstrated that these are derived from an early haemopoietic progenitor cell. The most common translocation involving the MLL gene is t(4;11)(q21;q23) which generates a hybrid MLL-AF4 gene. This is common in infants (70% of cases) with over half of all cases under 4 years of age. The t(4;11) is associated with organomegaly and high blast cell counts at presentation. These patients go into complete remission but have an early and rapid relapse. The WHO classifies B lymphoblastic leukaemia based on these genetic features due to their distinct prognostic associations.

T-lymphoblastic leukaemia

T-lymphoblastic leukaemia or T-cell acute lymphoblastic leukaemia (T-ALL) is a leukaemia of lymphoblasts committed to the T-cell lineage. They comprise 15% of paediatric and 25% of adult ALL cases. T-ALL is more common in males than females, and is more common in adolescents than young children. It commonly presents with a high leucocyte count and organomegaly, particularly thymic enlargement, and can have less marrow involvement than B-ALL. When only nodal or extranodal sites are involved, it is termed T-lymphoblastic lymphoma. The blast cell morphology is similar to that of B-ALL. T-lymphoblasts have variable expression of T-cell–associated antigens, are TdT-positive, and rarely express CD34 antigen. T-ALL can be stratified into different stages of thymic differentiation by the pattern of T-cell antigens expressed. The most immature, or ‘pro-T-ALL’ (immature thymocyte), only express surface membrane CD7 and cytoplasmic CD3 antigens. Intermediate (common thymocyte), ‘pre-T-ALL’, express CD1a, CD2, CD5, CD7 and both CD4 and CD8 antigens. ‘Mature T-ALL’ (cortical thymocyte) express CD3 on the cell surface, are CD1a-negative and express either the CD4 or CD8 antigen. It remains controversial as to whether there are prognostic differences between these subtypes. Approximately one-third of T-ALL cases have chromosomal translocations involving the TCRα/δ loci at 14q11 and the TCRβ/γ loci at 7q34. Examples include t(1;14)(p32;q11) involving the TAL1 gene, t(10;14)(q24;q11) with the HOX11 gene, and t(11;14)(p13;q11) involving the RBTN2 gene. In 25% of T-ALL cases, the TAL1 locus (1p32) is dysregulated by submicroscopic deletion.

Acute myeloid leukaemias

Acute myeloid leukaemia (AML) is a clonal expansion of myeloid blast cells (myeloblasts) in the bone marrow. Myeloblasts vary in size from slightly larger than a lymphoblast to the size of a monocyte (12-15 μm). They have variable amounts of basophilic or light grey cytoplasm which may contain azurophilic granules or Auer rods, the latter being myeloid-specific. The nuclei vary from round to ovoid, indented or convoluted, and the nuclear chromatin is fine with one or more nucleoli. The blast cell morphology is typical in some AML subtypes, such as acute promyelocytic leukaemia. Myeloperoxidase (MPO), Sudan Black B, and esterase (nonspecific and α-naphthyl acetate) cytochemical stains are variably positive. The most useful phenotypic markers are antibodies to myeloid lineage–associated antigens such as CD13, CD33, CD68, CD117, and MPO. CD34 is expressed in over 50% of cases. The AML categories in the WHO classification incorporate genetic data that define biological entities and predict clinical behaviour and outcome.

Acute myeloid leukaemia with recurrent genetic abnormalities

This group includes AML with the recurrent genetic abnormalities t(8;21), t(15;17), inv(16) or t(16;16), t(9;11), t(6;9), inv(3) or t(3;3), t(1;22) and mutations of NPM1 and CEBPA. Cases with t(8;21), t(15;17) or inv(16) represent 20 to 25% of all AML, and have a significantly better prognosis than those with normal karyotypes or other genetic abnormalities.

AML with t(8;21)(q22;q22);AML1-ETO occurs in 5 to 10% of AML cases, most commonly younger patients, has a high complete remission rate and long-term disease-free survival. In some cases, predominantly those in children, it presents as a myeloid sarcoma and may have little or no bone marrow involvement. The blast cells show evidence of myeloid maturation, with significant azurophilic granulation and frequent Auer rods, and there are maturing myeloid cells present in the marrow. The blast cells express myeloid-associated antigens (CD13, CD33, MPO) and are usually CD34-positive. The B-lymphoid lineage antigen CD19 (70% of cases) and CD56 are commonly expressed. The translocation involves the AML1 (or RUNX1) and ETO genes.

AML with inv(16)(p13.1q22) or t(16;16)(p13;q22);CBFB-MYH11 represents approximately 10% of AML cases, particularly in young adults, and has a high complete remission rate and favourable prognosis. The blast cells have both granulocytic and monocytic (myelomonocytic) differentiation. Auer rods may be present. There is a variable eosinophilia, and the eosinophils have abnormal large granules. In addition to myeloid antigens, the blast cells frequently express monocyte-associated antigens (CD11b, CD11c, CD14, CD64, and lysozyme). Both inv(16)(p13.1q22) and t(16;16)(p13;q22) result in the fusion of the CBFβ gene (16q22) with the MYH11 gene (16p13).

Translocation t(15;17)(q22;q12), occurs in acute promyelocytic leukaemia (APML), and accounts for 5 to 8% of AML cases, predominantly in middle-aged adults. The leukaemic cells have characteristic morphology being large with reniform or bilobed nuclei, heavily granulated cytoplasm, and may contain bundles of Auer rods (‘faggot cells’). Less granulated cases occur and are called microgranular or hypogranular APML. The immunophenotype is characteristic with strong homogeneous CD33 expression, heterogeneous CD13, CD117, and MPO, and frequent expression of CD2 antigen. CD34 and HLA-DR are negative. The PML protein shows a nuclear microgranular pattern by immunofluorescent microscopy. The t(15;17) generates a PML-RARA fusion gene. APML is sensitive to all-trans retinoic acid, a differentiation-inducing agent, and has a favourable prognosis. Patients with less common variant translocations such as t(11;17)(q23;q12), which also involve RARA, usually lack the typical APML morphology and do not respond to all-trans retinoic acid.

AML with t(9;11); MLL T3-MLL have an intermediate prognosis. These occur in approximately 5% of AML cases and are most common in children, with a particularly high frequency in infants (aged under 1 year) and in therapy-related AML. They generally have monocytic and myelomonocytic morphology. The phenotype is nonspecific, expressing myeloid-associated antigens (CD13 and CD33) and monocyte-associated antigens (CD11b, CD14, CD64, and lysozyme); CD34 is negative. Variant MLL translocations also occur e.g. t(11;19); MLL-ELL.

In addition to chromosomal translocations, specific gene mutations also occur as recurrent genetic abnormalities in AML. Mutations in the NPM1 gene occur in approximately one-third of AML and are associated with monocytoid morphology, CD34-negative and better prognosis. They occur most commonly in middle-aged females. CEBPA mutations occur in 6-15% of AML and have no gender or morphologic association. Mutations in FLT3 are not associated with a specific AML type but have prognostic association.

Acute myeloid leukaemia with myelodysplasia-related changes

AML with myelodysplasia-related changes predominantly occur in elderly patients, presents with pancytopenia, and has a poor prognosis. It may evolve from a myelodysplastic syndrome or arise apparently de novo. In addition to blast cells, there is dysplasia in more than 50% of cells of two or more bone marrow lineages (i.e. erythroid, granulocyte, and megakaryocyte). The blast cells are generally CD34-positive myeloblasts (CD13, CD33 positive). The commonest chromosomal abnormalities are –5/del(5q–) and myeloid neoplasms –7/del(7q).

Therapy-related myeloid neoplasms

Therapy related AML occurs following exposure to cytotoxic chemotherapy and/or radiotherapy. These are thought to be the consequence of mutations secondary to cytotoxic therapy.

AML secondary to alkylating drugs or radiotherapy occur 5 to 6 years following exposure and may be preceded by myelodysplasia. Morphologically, these cases have over 20% myeloblasts and dysplasia of all myeloid cell lines. Abnormalities of chromosomes 5 and 7 are common.

AML following treatment with drugs targeting topoisomerase II (e.g. etoposide, teniposide, and anthracyclines) has a short latency (median 33 months) without a preceding myelodysplastic phase. Most cases have a monocytoid or myelomonocytic morphology and phenotype. They have balanced translocations and often involve the MLL gene.

Acute myeloid leukaemia not otherwise specified

This category in the WHO classification encompasses those cases that do not fulfil the criteria of the above-mentioned AML groups. They are largely classified according to FAB definitions, with some additions (Table 1 above ).

Acute myeloid leukaemia with minimal differentiation

These have no morphological or cytochemical evidence of myeloid differentiation. Their myeloid lineage can only be ascertained by the expression of one or more myeloid-associated antigens e.g. CD13, CD33. This entity, formerly known as FAB M0, comprises approximately 5% of AML cases, mostly adult.

Acute myeloid leukaemia without maturation

The blast cells are poorly differentiated (FAB M1) but do have some morphological (e.g. azurophilic granules and Auer rods) and cytochemical (MPO and Sudan Black B positivity) evidence of myeloid differentiation. They express myeloid-associated antigens but not monocyte antigens.

Acute myeloid leukaemia with maturation

The blast cells commonly have cytoplasmic granules and Auer rods, and there is morphological evidence of maturation to mature neutrophils (FAB M2). Many cases have t(8;21) and are therefore included in the group defined by the cytogenetic abnormality. Other cases may have a bone marrow basophilia together with abnormalities in chromosome 12p or t(6;9)(p23;q34) resulting in a chimaeric DEK-CAN fusion gene, one of the recurrent genetic abnormalities in AML.

Acute myelomonocytic leukaemia

This subtype (FAB M4) comprises 15 to 25% of AML cases and has blasts with both myeloblast and monoblast morphology. Neutrophils and their precursors, and monocytes and their precursors, each comprise at least 20% of marrow cells. Nonspecific genetic abnormalities are present in the majority of cases. Cases with inv(16) occur and are included in the group with recurrent genetic abnormalities.

Acute monoblastic and monocytic leukaemia

These are defined by over 80% of the blast cells having monocytoid features. In acute monoblastic leukaemia (FAB M5a) these are monoblasts, whereas in acute monocytic leukaemia (FAB M5b) they are more mature promonocytes. The blast cells may show haemophagocytosis. There is variable expression of monocyte-associated antigens; specifically CD64 is commonly expressed but CD14 only by more the mature monocytic cases. Chromosomal translocations involving chromosome band 11q23 are common.

Acute erythroid leukaemia

There are 2 rare AML subtypes with a predominant erythroid population (FAB M6). In erythroleukaemia (erythroid/myeloid) greater than 50% of cells in the marrow are erythroid progenitors, and, more than 20% of nonerythroid cells are myeloblasts. Pure erythroid leukaemia is a neoplastic proliferation of committed primitive erythroid cells (>80% of marrow cells) that express CD235 (glycophorin A) and haemoglobin A and in which there is no myeloblastic population.

Acute megakaryoblastic leukaemia

In this uncommon form of AML (FAB M7), the blast cell morphology is not diagnostic. The megakaryocyte lineage is identified by expression of one or more platelet glycoproteins (CD41, CD42, and CD61). In the marrow the blasts are commonly accompanied by dysplastic megakaryocytes. Other acute myeloid leukaemias

Other rare forms of AML in the WHO classification are acute basophilic leukaemia, acute panmyelosis with myelofibrosis, and myeloid sarcoma, a myeloid tumour developing in an extramedullary site. Acute leukaemias of ambiguous lineage are leukaemias where the morphology, cytochemistry, and phenotype either lack sufficient evidence to ascribe lineage (acute undifferentiated leukaemia), or the blast cells are of mixed lineage (biphenotypic or bilineal).

Chronic leukaemias

Chronic leukaemias are the proliferation of mature-appearing malignant cells with morphology resembling normal blood cells. As in the acute leukaemias, these can be lymphoid or myeloid (Box 1).

Mature B-cell neoplasms

Mature B-cell neoplasms include leukaemias (chronic B-cell leukaemias) and B-cell non-Hodgkin lymphomas. The chronic B-cell leukaemias are clonal proliferations of mature, surface immunoglobulin-positive B cells that involve the blood and bone marrow and are generally indolent. Four main types of chronic B-cell leukaemia, each with a relatively distinctive morphology and immunological profile will be described. Several types of B-cell non-Hodgkin lymphoma can present with, or evolve into, a leukaemic phase (i.e. circulating lymphoma cells >5 × 109/litre).

Chronic lymphocytic leukaemia

Chronic lymphocytic leukaemia (CLL) is the commonest leukaemia in the Western world and occurs in adults. It is characterized by monomorphic, small, round lymphocytes with clumped chromatin and minimal cytoplasm, together with a small number of prolymphocytes. The cells express B-cell antigens (CD19, weak CD20, and CD22), CD5, CD23, and weak IgM. Expression of ZAP70 and CD38, and genetic abnormalities (e.g. p53 and 11q deletions) assist in determining prognosis.

B-cell prolymphocytic leukaemia

B-cell prolymphocytic leukaemia (B-PLL) is rare in comparison with CLL. It presents in elderly patients, predominantly men, with marked splenomegaly and minimal lymphadenopathy, and has a poor prognosis. The neoplastic cell is larger than in CLL and is round with a centrally placed nucleus, single prominent nucleolus, and basophilic cytoplasm. The cells strongly express surface IgM and B-cell antigens; CD5 is present in one-third and CD23 is negative.

Hairy cell leukaemia

Hairy cell leukaemia (HCL) is a rare leukaemia that occurs primarily in middle-aged men, is indolent and potentially curable. It is typically associated with pancytopenia, splenomegaly, and fibrotic marrow. The leukaemic cells are of intermediate size due to abundant irregular grey cytoplasm with ‘hairy’ projections. The nuclei are eccentrically located and have a reniform shape. They have a characteristic phenotype with strong B-cell antigens, and express CD11c, CD103, CD123 and CD25 antigens. They have cytochemical tartrate-resistant acid phosphatase positivity.

Burkitt lymphoma

Burkitt lymphoma (BL), formerly classified as an acute leukaemia in the FAB classification, is a mature B-cell neoplasm derived from a germinal centre B-cell. It is highly aggressive, but potentially curable, and commonly has blood and bone marrow involvement. The cells are round, of intermediate size, and have intensely basophilic vacuolated cytoplasm and prominent nucleoli. They express B-cell associated antigens (CD19, CD20, strong Ig heavy and light chains), CD10, and BCL6. A characteristic feature of BL is that over 90% of cells are in cell cycle. They are TdT- and CD34-negative. BL is characterized by t(8;14)(q24;q32) resulting in a MYC-IgH fusion gene. Alternative translocations, t(2;8)(p12;q224) and t(8;22)(q24;q11), involve the MYC and κ and λ light chain genes, respectively. Gene expression profile analysis has shown BL to have a distinct molecular signature.

Mature T-cell and natural killer cell neoplasms

Mature T-cell neoplasms, derived from mature post-thymic T-cells, and NK-cell neoplasms are both rare and are grouped together in the WHO classification, as they are phenotypically similar. Those with a predominant leukaemic phase are termed chronic leukaemias. Sézary syndrome, a cutaneous T-cell lymphoma, and adult T-cell leukaemia/lymphoma are two T-cell non-Hodgkin’s lymphomas that commonly develop a leukaemic phase.

T-cell prolymphocytic leukaemia

T-cell prolymphocytic leukaemia (T-PLL) is an aggressive leukaemia of mature post-thymic T-cells characterized by hepatosplenomegaly and high leucocyte counts. The disease has a progressive course, but indolent forms are also reported. The neoplastic cells are small with irregular outline, convoluted or indented nuclei with a nucleolus, and basophilic cytoplasm with protrusions. T-PLL cells express T-cell antigens (CD2, CD3, CD5, strong CD7) and are generally CD4-positive. In 30% of cases both CD4 and CD8 antigens are expressed, and rarely, they express CD8 and lack CD4. Up to 90% of T-PLL have inversions of chromosome 14 with breakpoints at 14q11, the TCRα/β locus, and 14q32, the TCL1 and TCL1b loci.

Large granular lymphocytic leukaemias

Large granular lymphocytic leukaemias (LGL) of T- or NK-cell type are defined by a persistent (>6 months) unexplained lymphocytosis of large granular lymphocytes. T-LGL are associated with autoantibodies (25% of cases), have an indolent course, and morbidity is generally due to neutropenia. The cells resemble normal large granular lymphocytes, have a mature T-cell phenotype and have rearranged T-cell receptor genes. 80% of cases are CD8-positive and CD4-negative. They have variable expression of NK-cell–associated antigens, especially CD16 and CD57, and restricted expression of only one of the CD158 isoforms a, b, or e. Chronic NK-cell LGL has a similar morphology to T-cell LGL, expresses NK-associated antigens (CD16, CD56, CD57), but does not express T-cell–associated antigens or have rearranged T-cell receptor genes.

Aggressive NK-cell leukaemia

This is a rare, aggressive leukaemia, more common in Asia than the West, and the Epstein–Barr virus is thought to be pathogenic in many cases. The neoplastic cells are large and pleiomorphic with irregular hyperchromatic nuclei and basophilic granulated cytoplasm. They have variable expression of NK-cell antigens.

Chronic leukaemias of myeloid origin

Chronic leukaemias of myeloid origin fall into one of two WHO categories. The first, the myeloproliferative neoplasms (MPN), includes chronic myelogenous leukaemia (CML), chronic neutrophilic leukaemia (CNL), chronic eosinophilic leukaemia (CEL) and mast cell leukaemia. These are clonal haematopoietic stem cell disorders with essentially normal morphology of the endstage cell. The type of chronic leukaemia is defined by the lineage of the predominant proliferating cell. The second, the myelodysplastic/myeloproliferative neoplasms, consists of chronic myelomonocytic leukaemia (CMML), juvenile myelomonocytic leukaemia (JMML), and atypical chronic myeloid leukaemia (aCML). These all have dysplastic morphology as well as proliferative features.

Chronic myelogenous leukaemia, BCR-ABL1 positive

CML is a clonal myeloproliferative neoplasm defined by the presence of the hybrid BCR-ABL1 fusion gene which results from the reciprocal t(9;22)(q34;q11) translocation. Splenomegaly is characteristic. The blood has a leucocytosis with granulocytic cells at all stages of differentiation present. Myelocytes and mature neutrophils predominate, basophilia is characteristic, and eosinophilia common. Blast cells are present but make up fewer than 5% of cells in the chronic phase of the disease. The bone marrow is markedly hypercellular with granulocytic hyperplasia (myeloid to erythroid ratio >10:1) with normal differentiation. Megakaryocytes are increased and are characteristically small (‘dwarf’) with bilobed nuclei. The diagnosis is confirmed by the presence of the Ph chromosome and BCR-ABL1. Accelerated phase of CML is defined by one or more of the following: progressive leucocytosis, increasing spleen size, an increase in blast cells (10–19%), basophilia, persistent thrombocytopenia or unresponsive thrombocytosis, and cytogenetic evidence of clonal evolution. Blast phase resembles acute leukaemia with over 20% of blast cells in the marrow and can be of myeloblastic or lymphoblastic type.

Chronic eosinophilic leukaemia

CEL is a rare myeloproliferative neoplasm characterized by a blood eosinophilia with atypia. The eosinophils are hypogranular with abnormal cytoplasmic distribution of the granules. An internal deletion of chromosome band 4q12, which results in a FIP1L1-PDGFRA fusion gene, defines CEL, and these patients generally respond to tyrosine kinase inhibitors. CEL cases with the FIP1L1-PDGFRA mutation are included in the WHO entity Myeloid and Lymphoid Neoplasms with Eosinophilia and abnormalities of PDGFRA, PDGFRB or FGFR1 due to the clinical and therapeutic relevance of these recently described genetic mutations associated with eosinophilia.

Chronic neutrophilic leukaemia

CNL is a rare, slowly progressive myeloproliferative disorder with a marked neutrophil leucocytosis without immature forms. Some cases have the t(9;22), but, in contrast to CML, the BCR-ABL1 rearrangement results in a p230, rather than a p210, protein product.

Mast cell leukaemia

This is a rare disorder where neoplastic mast cells account for 10% or more of peripheral blood leucocytes and over 20% of nucleated cells in the marrow.

Chronic myelomonocytic leukaemia and juvenile myelomonocytic leukaemia

CMML is characterized by a blood monocytosis (>1 × 109/litre) with dysplastic features, granulocytic hyperplasia in the marrow, and less than 5% blast cells. It occurs in older adults, generally over 70 years. Although there are no characteristic chromosomal abnormalities, some patients have t(5;12) generating the PDGFRB-TEL fusion gene. Survival is variable and largely determined by the blast cell count. JMML occurs in children under 4 years of age, and the morphology is similar to CMML. Monosomy 7 occurs in 15% of cases, and 10% of affected children have neurofibromatosis. The prognosis of JMML is generally poor if untreated, and bone marrow transplantation is the only therapy that improves survival.

Atypical chronic myeloid leukaemia, BCR-ABL1 negative

Atypical CML defines those patients with high leucocyte counts who are Ph chromosome and BCR-ABL1 negative. The morphological features are more heterogeneous than CML, there is granulocytic dysplasia and a mild monocytosis, and, notably, there is no basophilia.

Conclusion

This article highlights that there are many subtypes of leukaemia which can be identified based on cell morphology and biological characteristics. It is important to distinguish between these entities because of therapeutic and prognostic differences. The WHO Classification has highlighted the significance of both cell phenotype and genetics in the definition of these diseases.

With the pace of progress of medical science, the WHO Classification will require periodical review and updating. New discoveries will continue to be made, and new disease subtypes will be defined, requiring inclusion in the classification. The WHO classification can, therefore, not be regarded as the definitive classification of leukaemia.