SEQUENTIAL MUTATIONS IN THE INTERLEUKIN-3 (IL3) GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR/IL5 RECEPTOR BETA-SUBUNIT GENES ARE NECESSARY FOR THE COMPLETE CONVERSION TO GROWTH AUTONOMY MEDIATED BY A TRUNCATED BETA(C) SUBUNIT/

An amino-terminally truncated beta(C) receptor (beta(C)-R) subunit of the interleukin-3 (IL3)/granulocyte-macrophage colony-stimulating fact or/IL5 receptor complex mediates factor-independent and tumorigenic gr owth in two spontaneous mutants of a promyelocytic cell line. The cons titutive activation of the JAK2 protein kinase in these mutants confir ms that signaling occurs through the truncated receptor protein. Notew orthily, in addition to a 10-kb deletion in the beta(C)-R subunit gene encoding the truncated receptor, several secondary and independent mu tations that result in the deletion or functional inactivation of the allelic beta(C)-R subunit and the closely related beta(IL3)-R subunit genes were observed in both mutants, suggesting that such mutations ar e necessary for the full oncogenic penetrance of the truncated beta(C) -R subunit. Reversion of these mutations by the expression of the wild -type beta(C)-R in the two mutants resulted in a fivefold decrease in cloning efficiency of the mutants in the absence of IL3, confirming a functional interaction between the wild-type and truncated proteins. F urthermore, expression of the truncated beta(C)-R subunit in factor-de pendent myeloid cells did not immediately render the cells autonomous but increased the spontaneous frequency to factor-independent growth b y 4 orders of magnitude. Implications for both leukemogenic progressio n and receptor-subunit interaction and signaling are discussed.