DNA methylation-independent loss of RARA gene expression in acute myeloid leukemia

The retinoic acid receptor (RAR) alpha gene (RARA) encodes 2 major isoforms and mediates positive effects of all-trans retinoic acid (ATRA) on myelomonocytic differentiation. Expression of the ATRA-inducible (RAR alpha 2) isoform increases with myelomonocytic differentiation and appears to be down-regulated in many acute myeloid leukemia (AML) cell lines. Here, we demonstrate that relative to normal myeloid stem/progenitor cells, RAR alpha 2 expression is dramatically reduced in primary AML blasts. Expression of the RAR alpha 1 isoform is also significantly reduced in primary AML cells, but not in AML cell lines. Although the promoters directing expression of RAR alpha 1 and RAR alpha 2 are respectively unmethylated and methylated in AML cell lines, these regulatory regions are unmethylated in all the AML patient cell samples analyzed. Moreover, in primary AML cells, histones associated with the RAR alpha 2 promoter possessed diminished levels of H3 acetylation and lysine 4 methylation. These results underscore the complexities of the mechanisms responsible for deregulation of gene expression in AML and support the notion that diminished RARA expression contributes to leukemogenesis.