Ras mutations in human leukemia and related disorders

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Abstract
The clinical association of an increased incidence of acute myelogenous leukemia (AML) with previous chemoradiotherapy, the detection of specific karyotypic changes in these secondary (therapy‐induced) cases of AML and the discovery of increasing levels of oncogene‐specific RNA in leukemia cells suggest that one potential site of action of environmental agents might be the proto‐oncogenes in human hematopoietic stem cells. The location of human proto‐oncogenes at the sites of chromosome breaks and/or translocations in cells from some patients with leukemia or lymphoma is a striking observation. These data stimulated research into the mechanism of activation of specific oncogenes that change the biology of human hematopoietic cells. Recent investigations have focused upon several areas that might alter cell biology including: 1) translocation and/or inversion of chromosome fragments containing a proto‐oncogene to a location where other gene sequences can stimulate oncogene activation, 2) replication of copy number of proto‐oncogenes or increased transcriptional activity and 3) point mutation in proto‐oncogenes leading to a structurally altered protein. The third area of research has recently received significant attention with respect to the potential role of three ras genes (c‐Harvey‐rar, c‐Kirsten‐ras and N‐ras) in human leukemias and myelodysplastic syndromes. Recent studies have proposed a model for leukemogenic transformation of human hematopoietic cells by the product of a mutated ras oncogene. Mutations at codons 12, 13 or 61 of the first exon of its 4.7 Kb of DNA (for c‐Ha‐rar) have been described. Other data revealing an absence of such mutations in the ras genes of many human leukemias and the absence of detectable transcription of ras genes in many alkylating agent‐associated cases of AML, suggest that while ras mutations may be involved in some settings, there are probably multiple genetic pathways to leukemogenic transformation of human hematopoietic cells.