Leukemic transformation by the APL fusion protein PRKAR1A-RAR alpha critically depends on recruitment of RXR alpha

PRKAR1A (R1A)-retinoic acid receptor-alpha (R1A-RAR alpha) is the sixth RAR alpha-containing fusion protein in acute promyelocytic leukemia (APL). Using the murine bone-marrow retroviral transduction/transformation assay, we showed that R1A-RAR alpha fusion protein could transform bone-marrow progenitor/stem cells. In gel-shift assays, R1A-RAR alpha was able to bind to a panel of retinoic acid response elements both as a homodimer and as a heterodimer with RXR alpha, and demonstrated distinct DNA-binding characteristics compared with wild-type RAR alpha/RXR alpha or other X-RAR alpha chimeric proteins. The ratio of R1A-RAR alpha to RXR alpha proteins affected the retinoic acid response element interaction pattern of R1A-RAR alpha/RXR alpha complexes. Studies comparing R1A-RAR alpha with R1A-RAR alpha(Delta RIIa) demonstrated that the RIIa protein interaction domain located within R1A was responsible for R1A-RAR alpha homodimeric DNA binding and interaction with wild-type R1A protein. However, the RIIa domain was not required for R1A-RAR alpha-mediated transformation because its deletion in R1A-RAR alpha(Delta RIIa) did not compromise its transformation capability. In contrast, introduction of point mutations within the RAR alpha portion of either R1A-RAR alpha or R1A-RAR alpha(Delta RIIa), previously demonstrated to eliminate RXR alpha interaction or treatment of transduced cells with RXR alpha shRNA or a RXR alpha agonist, reduced transformation capability. Thus, leukemic transformation by APL fusion protein PRKAR1A-RAR alpha is critically dependent on RXR alpha, which suggests RXR alpha is a promising target for APL. (Blood. 2010; 115: 643-652)