Myeloid malignancies and chromosome 5 deletions.

Deletions of chromosome 5 were initially reported as a consistently occurring chromosomal abnormality in 5q- syndrome. They have since been recognized to occur in other myeloid malignancies such as therapy-related leukaemia and de novo AML as well. The variability of the deletions, and the heterogeneity of the clinical syndromes, have made it difficult to describe a single clinical-molecular entity such as we see with chromosomal translocations described elsewhere in this volume. Translocations in leukaemogenesis often have a dominant effect leading to activation of oncogenes or the production of a modified protein. Consistently occurring chromosomal deletions in human tumours, however, have been regarded as evidence that the affected regions contain tumour suppressor genes. Loss of function of these tumour suppressor genes or 'recessive oncogenes' leads to cancer. Deletions in the long arm of chromosome 5 in myeloid malignancies are thought to signal the existence of a recessive oncogene on 5q, which is homozygously inactivated in these malignancies. Here we describe the clinical and molecular features of the diseases associated with deletions of chromosome 5 in an attempt to propose a unified approach to identifying the genes on 5q which are involved in leukaemogenesis. It is likely that the clinical heterogeneity of these disorders will not be understood until the relevant genes are cloned and their role in the initiation or progression of leukaemia is known.