Myc Drives Chromosomal Gain in Acute Myeloid Leukemia

Acute Myeloid Leukemia (AML) is a disease characterized by diverse genetic pathogenesis, including both balanced and unbalanced chromosomal aberrations. Much is known regarding the pathogenic effects of balanced rearrangements in AML, whereas our understanding of how unbalanced aberrations contribute to leukemia is more limited. The balanced t(15;17) chromosomal rearrangement is a nearly constant feature of acute promyeloctyic leukemia (APL), a subtype AML. The translocation fuses the promyelocytic leukemia gene (PML) to the retinoic acid receptor α gene (RARA). Trisomy 8 is the most common secondary karyotypic lesion observed in APL, and it has been speculated but not proven that the MYC gene contributes to this chromosomal gain. We previously reported that mouse chromosome 15, which contains the mouse Myc gene in a region syntenic to human chromosome 8q24, is commonly gained in the MRP8 PML-RARA mouse model of APL. We now report our work to assess the hypothesis that increased MYC cooperates with PML-RARα to accelerate disease and that gain of MYC/Myc drives +8 in humans and +15 in mice. Expressing MYC with a retroviral vector in PML-RARA bone marrow led to the rapid development of APL-like leukemias (3 months vs. 8.5 months with PML-RARA alone). Chromosome 15 was not gained in any of the leukemias, although 70% had other clonal karyotypic abnormalities. This finding suggests that when MYC is overexpressed, there is no selective pressure to gain chromosome 15, supporting our hypothesis that Myc is driving this gain. We also generated PML-RARA mice haploinsufficient for Myc to examine the effect of decreasing MYC levels. The median latency among leukemic animals was 258 days for mice with PML-RARA and two wild-type Myc alleles, whereas the latency was increased to 339 days for PML-RARA Myc haploinsufficient mice. Hence, lower MYC expression served as a check on leukemic transformation. Furthermore, the majority of the leukemias that arose in Myc haploinsufficient mice had gained wild-type Myc. These data demonstrate a selective pressure for Myc gain. Additional experiments showed that

• as MYC expression increases there is a decrease in both latency and genetic complexity of leukemias that arise, that

• MYC and PML-RARα interact to disrupt myeloid differentiation in vivo and that

• although MYC cooperates with PML-RARα to cause leukemia, additional events are required for completing transformation even at high levels of MYC.

Altogether our studies of increased and decreased MYC expression in PML-RARA mice show a strong correlation between MYC dosage and leukemic transformation. Our results suggest that agents that target MYC might be useful for the treatment of AML.