Altered enhancer-promoter interaction leads to MNX1 expression in pediatric acute myeloid leukemia with t(7;12)(q36;p13) [ACT-Seq]
Acute myeloid leukemia (AML) with translocation t(7;12)(q36;p13) is a subgroup that occurs only in infants or very young children and is characterised by a poor outcome. Deep molecular characterization has been hampered by the rarity of this AML subtype and the lack of model systems. Approximately 50% of AML with t(7;12)(q36;p13) express an MNX1::ETV6 oncofusion transcript, but there is no evidence for the presence of an oncofusion protein. However, a universal feature is the strong RNA and protein expression of MNX1, a homeobox transcription factor that is normally not transcribed in haematopoietic cells. Here we use whole genome sequencing data to precisely map the translocation breakpoints on chromosomes 7 and 12 in these pediatric AML patients to a region downstream of MNX1 on chromosome 7 and either introns 1 or 2 of ETV6 on chromosome 12. We confirm the tight correlation of MNX1 overexpression in t(7;12)(q36;p13) AML in our own samples and three additional cohorts of pediatric AML. Using a CRISPR-engineered iPSC cell line, ChiPSC22t(7;12), harbouring the t(7;12)(q36;p13) translocation, we unravel an enhancer-hijacking event leading to MNX1 overexpression. Identification of hematopoietic enhancer regions in hematopoietic stem and progenitor cells derived from differentiated ChiPSC22t(7;12) cells allowed us to demonstrate their importance for MNX1 expression in knock-out experiments and by measuring the promoter-enhancer distance in confocal microscopy. In contrast to the prevailing dogma in AML, suggesting that translocations lead to the creation of oncofusion genes, t(7;12)(q36;p13) AML is characterized by an enhancer-hijacking event driving MNX1, a novel leukemia oncogene and the haploinsufficiency of ETV6. Overall design: We used the iPSC line CHIPSC22  (Cellartis/Takara Bio Europe AB, Gothenburg, Sweden), both WT and with a t(7;12)(q36;p13) translocation. We included both the iPSC CHIPSC22 and CHIPSC22 differentiated into hematopoietic stem and progenitor cells (HSPC). We used antibodies against H3K4me1, H3K27ac and IgG. For each condition, 3 replicates were used.