Differential Contributions of c-Kit Activating Mutations to Promotion of AML1-ETO Associated Neoplasia

Abstract 4195

The t(8;21) AML1-ETO translocation is one of the most frequent cytogenetic abnormalities observed in acute myeloid leukemia (AML), occurring in 10–12% of cases. The t(8;21) fuses the DNA binding domain of AML1 with the transcriptional repressor gene ETO. AML1-ETO dominantly blocks wild-type AML1 function, preventing activation of AML1 target genes and impairing myeloid differentiation. However, expression of AML1-ETO in mice does not promote AML in the absence of additional mutations. Another commonly mutated gene detected in AML1-ETO⁺ patient samples is the receptor tyrosine kinase c-KIT. Two classes of activating c-KIT mutations have been reported; one where mutations like D814V occur in the tyrosine kinase domain and the other involving a deletion within Exon 8 (E8D419) in a region that mediates receptor dimerization. These mutations are not randomly distributed with t(8;21) or with other chromosomal translocations found in AML. For instance, c-KIT^D814V is observed in 11–44% of t(8;21) patients, whereas c-KIT^E8Æ419 with t(8;21) is reported in 2–13% of cases. These observations suggest a degree of specificity with which activating c-KIT mutations can cooperate with AML1-ETO to promote AML. To test this hypothesis, we used a retroviral transduction/transplantation model to co-express AML1-ETO with c-Kit^D814V or c-Kit^E8Æ419 in murine bone marrow (BM) progenitor cells used to reconstitute lethally irradiated mice. Analysis of reconstituted mice has shown that simultaneous expression of AML1-ETO with c-Kit^D814V results in three non-overlapping phenotypes. The most common phenotype, occurring in 50% of animals, is a transplantable AML characterized by a relatively short disease latency that may or may not present with an associated granulocytic sarcoma. Second, a rapidly fatal non-transplantable myeloproliferative phenotype was observed in 25% of animals. Lastly, several mice (25%) developed a lethal, short latency B-cell leukemia that was transplantable to secondary recipient animals. The heterogeneity of disease phenotypes that develop in AML1-ETO;c-KIT^D814V mice may reflect the specific hematopoietic progenitor cell type that was initially transduced and capable of reconstituting the recipient mouse or differences in AML1-ETO and/or c-Kit^D814V expression levels. In contrast, co-expression of AML1-ETO with c-Kit^E8Æ419 promoted a transplantable AML phenotype characterized by a relatively long disease latency in a fraction (43.7%) of reconstituted mice. AML1-ETO; c-Kit^E8Æ419–expressing cells could persist for months without showing signs of selective expansion, which suggests that additional genetic changes are necessary for overt transformation to AML in this context. These observations indicate that when overlaid on the AML1-ETO genetic lesion, c-Kit^D814V promotes a more aggressive disease phenotype than c-Kit^E8Æ419, which displays a longer latency and reduced penetrance. These differences may account for the observed disparity in the frequencies of each c-KIT mutation concurrent with AML1-ETO in human AML cases.