Acute lymphoblastic leukaemia: diagnosis and classification.

Acute lymphoblastic leukaemia (ALL) is a heterogeneous disease with distinct biological and prognostic groupings. Diagnosis relies on traditional cytomorphological and immunohistochemical evaluation of the leukaemic blasts. Subsequently, cytogenetic analysis identifies clonal numeric and/or structural chromosomal abnormalities that may be present, thus confirming the subtype classification and providing important prognostic information for treatment planning. The major chromosomal abnormalities in ALL are t(9;22)(q34;q11), t(12;21)(p13;q22), t(4;11)(q21;q23), t(1;19)(q23;p13), 8q24 translocations and hyperdiploidy. Generally, hyperdiploidy, occurring most frequently in paediatric cases, is associated with a good prognosis, while hypodiploidy confers a poor prognosis. Among structural chromosomal abnormalities, the t(9;22)(q34;q11) resulting in the BCR/ABL fusion protein, and rearrangements of the MLL gene, confer a poor prognosis in both children and adults, while t(12;21)(p13;q22), resulting in the TEL/AML1 fusion protein, and del (12p) confer a good prognosis. More recently, additional diagnostic and prognostic information has been gained from fluorescence in situ hybridization (FISH) and DNA microarray techniques.