Acute lymphoblastic leukaemia.

In acute lymphoblastic leukaemia (ALL) the karyotype provides important prognostic information which is beginning to have an impact on treatment. The most significant structural chromosomal changes include: the poor-risk abnormalities; t(9;22)(q34;q11), giving rise to the BCR/ABL fusion and rearrangements of the MLL gene; abnormalities previously designated as poor-risk; t(1;19)(q23;p13), producing the E2A/PBX1 and rearrangements of MYC with the immunoglobulin genes; and the probable good risk translocation t(12;21)(p13;q22), which results in the ETV6/AML1 fusion. These abnormalities occur most frequently in B-lineage leukaemias, while rearrangements of the T cell receptor genes are associated with T-lineage ALL. Abnormalities of the short arm of chromosome 9, in particular homozygous deletions involving the tumour suppressor gene (TSG) p16(INK4A), are associated with a poor outcome. Numerical chromosomal abnormalities are of particular importance in relation to prognosis. High hyperdiploidy (51-65 chromosomes) is associated with a good risk, whereas the outlook for patients with near haploidy (23-29 chromosomes) is extremely poor. In view of the introduction of risk-adjusted therapy into the UK childhood ALL treatment trials, an interphase FISH screening programme has been developed to reveal chromosomal abnormalities with prognostic significance in childhood ALL. Novel techniques in molecular cytogenetics are identifying new, cryptic abnormalities in small groups of patients which may lead to further improvements in future treatment protocols.