Clinics in haematology最新文献

Secondary MDS, or AL induced by the treatment of another primary disease, occurs at an average of 48-71 months after that treatment. A high percentage of the 2 MDS convert to AL. Survival of either is less than 1 year. A constellation of morphological abnormalities from all three cell lines produces a unique appearance. The 2 AL are difficult to classify by the FAB system. With the exception of cytogenetic analysis, the biology of 2 MDS/AL remains largely unexplored. Alterations of chromosomes 5 and 7 predominate, but other associated cytogenetical abnormalities are increasingly being recognized. A review of the development of 2 MDS/AL in a variety of primary diseases generates the following tentative conclusions: many of the commonly used alkylating agents, and the non-classical alkylating agent procarbazine, are leukaemogens; procarbazine is probably the important leukaemogen in the MOPP programme; cyclophosphamide appears to be a less potent leukaemogen than other alkylating agents; the method in which a drug is administered probably influences its leukaemogenic potential; the duration of therapy with a drug, or the total amount of drug delivered, is probably an important factor in leukaemogenesis; irradiation alone appears to be a weak leukaemogen; irradiation has little or no synergism with chemotherapy in leukaemogenesis; the older patient treated with leukaemogenic drugs is at substantial risk to develop a 2 MDS/AL; most studies show no plateau in the actuarial incidence of developing a 2 MDS/AL, despite lengthy follow-up. Benzene is the only chemical agent for which strong evidence of leukaemogenesis exists. Nonetheless, the similarities in the karyotypic alterations of leukaemic cells between those whose occupations expose them to chemical hazard and those who are exposed to cytotoxic agents lend support to the idea that more environmental leukaemogens have yet to be discovered. Aggressive therapy should be considered for a patient of any age with an adequate performance status and a diagnosis of secondary AL, especially if the karyotype in the malignant cell is predictive of a high response rate. The therapy of 2 MDS remains investigational. To mitigate the development of a leukaemic complication, maintenance therapy should be restricted to diseases in which its efficacy is established or to an investigational setting, and consideration of the leukaemogenic potential of equally effective regimens should be part of the therapeutic planning in the older patient.

The assessment of clonogenic leukaemic precursors in cell culture has demonstrated considerable heterogeneity of patients with respect to their ability to give rise to leukaemic blast cell colonies. Specific growth patterns rather than the frequency of blast colonies, appear to be of prognostic value for the clinical outcome. In particular, blast cells with high self-renewal ability were associated with poor prognosis. A similar degree of heterogeneity among leukaemic blast cell populations can be identified by studies with monoclonal antibodies directed against various cell surface cytoplasmic determinants that are associated with haematopoietic precursors. Some of these reflect properties of pluripotent normal counterparts while others display patterns that are associated with precursors restricted to a single myeloid lineage. Blast cells examined after culture may differ in their marker expression from the original blast cell population. Usually, properties of more mature cells are acquired during the culture period. These changes reflect events of aberrant differentiation without leading to the full development of a morphologically normal phenotype and functional capability. The use of both technologies has provided considerable insight into the biology of leukaemic blast cell populations and it is anticipated that their future use will provide further information about the control mechanisms involved in their proliferation and their potential to differentiate normally.

Until recently, lineage fidelity was thought to be preserved in leukaemic cells, which by available tests showed surface markers and enzymatic patterns characteristic of an appropriate normal cell lineage and stage of differentiation. Our data indicate that this theory is too restrictive. If leukaemogenesis occurs in pluripotent progenitors in a relatively high percentage of cases, we would propose a model in which lymphoid and myeloid differentiation antigens are expressed simultaneously until the progenitor cell commits to a single lineage. Lineage commitment could involve external factors, e.g. growth factors (Sherr et al, 1985), that cause genes specific for the opposite lineage to be 'switched off'. The control of gene expression in mammalian cells and the specific chromosomal sites of genes coding for the various lineage-associated markers remain uncertain. However, recent studies indicate that most, if not all, leukaemic cells contain chromosomal abnormalities, many involving rearrangements of DNA (Williams et al, 1986). Since the control of eukaryotic gene expression is known to involve numerous sequence elements, some acting at a distance from the site of transcription (Dynan and Tjian, 1985), genetic perturbations within the cell (e.g. a reciprocal translocation) could be expected to deregulate certain genes, leading to their under- or overexpression analogous to activation of the c-myc oncogene by the 8;14 translocation in Burkitt's lymphoma. Thus, an almost infinite variety of cell lineage-related phenotypes could be expected from this mechanism alone, even if the transforming event did not involve a pluripotent stem cell. Also, we have hypothesized that enzymes such as TdT, a DNA polymerase that catalyses polymerization of deoxyribonucleotides without a DNA template, could serve as a modifier of DNA sequences, permitting otherwise inactive genes to be expressed (Stass and Mirro, 1985). It is interesting that most cases of childhood acute mixed-lineage leukaemia are TdT positive, even though this is not true for the chronic leukaemias of adults. It is now clear that unusual combinations of myeloid and lymphoid cell lineages are much more common in acute leukaemia than have been generally recognized or suspected. The traditional division of the acute leukaemias into ALL and AML may not be the most accurate way to represent this class of haematological malignancies. That mixed-lineage leukaemia may require alternative therapy is a clinically important observation and underscores the need for comprehensive testing of blast cells at diagnosis.(ABSTRACT TRUNCATED AT 400 WORDS)

Over the past ten years, there have been substantial advances in the treatment of AML. Intensive induction chemotherapy using seven-day courses of cytarabine and daunorubicin or amsacrine produce remission in 60-85% of patients. Median remission duration is 9-16 months. In some series, 20-40% of patients are in continuous remission for two years or more; many of these patients remain in remission for five years or longer and some may be cured. Bone marrow transplantation has evolved as a useful therapeutic modality capable of achieving long-term survival in some circumstances in which chemotherapy is relatively ineffective. Its precise role in the initial therapy of AML remains to be defined but it is likely to be beneficial in selected patients. These data indicate substantial recent progress in the treatment of this disease which was almost uniformly fatal 30 years ago. The fact that most patients relapse within 1-2 years reflects a lack of progress in developing effective postremission therapy. Maintenance chemotherapy, immunotherapy and CNS prophylaxis have little role in AML. It is unclear whether consolidation or intensification extend remissions or increase the proportion of long-term survivors; controlled randomized trials should answer this question within the next few years. Future progress in the treatment of AML awaits the development of more sensitive methods for detecting residual leukaemia, more effective use of current therapeutic modalities and the introduction of new effective drugs. Most data suggest that early intensive treatment is of key importance for achieving cures. We cannot presently distinguish, however, between patients cured by initial treatment and those who require further chemotherapy.