THE MANAGEMENT OF PATIENTS WITH LEUKAEMIA: THE ROLE OF CYTOGENETICS IN THIS MOLECULAR ERA

In this era of advancing molecular technologies, cytogenetic investigation remains an essential requirement in the management of patients with leukaemia. Metaphase chromosome analysis of bone marrow cells enables the entire genome to be screened for evidence of chromosomal changes, which provide the chromosomal landmarks for the genes involved in leukaemogenesis. The majority of newly diagnosed leukaemias show chromosomal abnormalities. Up to 80% of patients with acute lymphoblastic leukaemia (ALL) and 70% of patients with acute myeloid leukaemia (AML) have abnormal karyotypes (Heim & Mitelman, 1995). A large number of these chromosomal changes are recurrent and have been associated with speci®c morphological types in both myeloid and lymphoid lineages. As a result, cytogenetics plays a major role in diagnosis and, notably, in the myeloproliferative and myelodysplastic disorders, the ®nding of an abnormal cytogenetic result may provide the de®nitive diagnosis. The greatest impact on patient management has been the demonstration that the cytogenetic result is an independent prognostic indicator. Certain karyotypes are associated with a good prognosis, whereas others indicate a poor outcome, leading to the administration of alternative therapies. The demand for chromosomal analysis of the majority of new cases of haematological malignancies from major referral centres and district hospitals re ̄ects the value placed on cytogenetics at diagnosis. The National External Quality Assessment Scheme (NEQAS) in Clinical Cytogenetics reported that, in 1997/98, more than 17 500 samples were referred for analysis to 36 cytogenetics laboratories in the UK, of which <2000 were new acute leukaemias (Waters & Rodgers, 1998). Evidence from NEQAS data over a number of years has demonstrated that participation in this scheme has made a major contribution to improvements in cytogenetic preparation quality and the standard of interpretation and reporting of results. Cytogenetic analysis is now a requirement for entry of patients to the current UK Medical Research Council treatment trials for childhood acute lymphoblastic leukaemia (ALL 97), adult ALL (UKALL XII), acute myeloid leukaemia in children and adults (AML 12) and patients >55 years (AML 14). Collection and co-ordination of cytogenetic results from patients entered into these trials is now well-established in the UK through karyotype databases for ALL and AML. These are funded by the Leukaemia Research Fund and the Kay Kendall Leukaemia Fund and are directed by experienced cytogeneticists. Their operation has led to a closer interaction between UK cytogenetics laboratories and the clinical trial co-ordinators, resulting in an improved understanding of the signi®cance of chromosomal changes in acute leukaemia. One advantage of cytogenetic analysis is that it is sensitive enough to detect abnormalities in small populations of cells. In fact, two metaphases with the same chromosomal gain or structural rearrangement, or three cells with the same chromosomal loss de®ne the presence of an abnormal clone (ISCN, 1995). The examination of individual cells enables secondary chromosomal changes and patterns of karyotypic evolution to be identi®ed. Cytogenetic analysis is highly informative in the majority of cases, although interpretation of the ®ndings may be dif®cult if the karyotype is complex or multiple clones are present. In spite of continual technical developments in methodology, some samples fail to produce a successful result because of a low mitotic index or poor chromosome morphology. The growth of techniques in molecular cytogenetics, or ̄uorescence in situ hybridization (FISH), has overcome some of these limitations. FISH has produced major improvements in the sensitivity, accuracy and reliability of cytogenetic analysis for both research and routine applications (Kearney, 1999). The introduction of interphase-FISH has made one of the most important contributions, allowing large numbers of non-dividing cells to be screened. Analysis is no longer restricted to cytogenetic cell suspensions (Bentz et al, 1993). The increasing availability of commercial probes means that FISH has become accessible to most routine cytogenetics laboratories. Molecular cytogenetics has become an integral part of the discipline of cytogenetics and is indispensable in accurate chromosomal analysis of leukaemias. The expansion of molecular technologies in the 1980s was seen as a serious potential rival to routine cytogenetic analysis, particularly when the speci®city and sensitivity of these techniques was realized. On the contrary, they were responsible for the progression of cytogenetics through developments in FISH. The introduction of Southern blotting allowed genetic rearrangements to be identi®ed in malignant tissue. However, it was the development of the polymerase chain reaction (PCR) (Saiki et al, 1988) and, subsequently, reverse transcriptase±PCR (RT±PCR) that transformed molecular technology. Novel fusion mRNAs transcribed by the fusion genes at the site of chromosomal rearrangements provide tumour-speci®c markers suitable for PCR ampli®cation. PCR is 400±4000 times more sensitive than Southern analysis, with the ability to detect one leukaemic British Journal of Haematology, 2000, 108, 19±30