Molecular Basis and Targeted Inhibition of CBFβ-SMMHC Acute Myeloid Leukemia

Proteogenomic analysis reveals cytoplasmic sequestration of RUNX1 by the acute myeloid leukemia-initiating CBFB::MYH11 oncofusion protein

Several canonical translocations produce oncofusion genes that can initiate acute myeloid leukemia (AML). Although each translocation is associated with unique features, the mechanisms responsible remain unclear. While proteins interacting with each oncofusion are known to be relevant for how they act, these interactions have not yet been systematically defined. To address this issue in an unbiased fashion, we fused a promiscuous biotin ligase (TurboID) in-frame with 3 favorable-risk AML oncofusion cDNAs (PML::RARA, RUNX1::RUNX1T1, and CBFB::MYH11) and identified their interacting proteins in primary murine hematopoietic cells. The PML::RARA- and RUNX1::RUNX1T1-TurboID fusion proteins labeled common and unique nuclear repressor complexes, implying their nuclear localization. However, CBFB::MYH11-TurboID-interacting proteins were largely cytoplasmic, probably because of an interaction of the MYH11 domain with several cytoplasmic myosin-related proteins. Using a variety of methods, we showed that the CBFB domain of CBFB::MYH11 sequesters RUNX1 in cytoplasmic aggregates; these findings were confirmed in primary human AML cells. Paradoxically, CBFB::MYH11 expression was associated with increased RUNX1/2 expression, suggesting the presence of a sensor for reduced functional RUNX1 protein, and a feedback loop that may attempt to compensate by increasing RUNX1/2 transcription. These findings may have broad implications for AML pathogenesis.

PPP1R7 Is a Novel Translocation Partner of CBFB via t(2;16)(q37;q22) in Acute Myeloid Leukemia

In a subset of acute myeloid leukemia (AML) cases, the core binding factor beta subunit gene (CBFB) was rearranged via inv(16)(p13.1q22) or t(16;16)(p13.1;q22), in which the smooth muscle myosin heavy chain 11 gene (MYH11) was the partner (CBFB::MYH11). Rare variants of CBFB rearrangement occurring via non-classic chromosomal aberrations have been reported, such as t(1;16), t(2;16), t(3;16), t(5;16), and t(16;19), but the partners of CBFB have not been characterized. We report a case of AML with a complex karyotype, including t(2;16)(q37;q22), in which the protein phosphatase 1 regulatory subunit 7 gene (PPP1R7) at chromosome 2q37 was rearranged with CBFB (CBFB::PPP1R7). This abnormality was inconspicuous by conventional karyotype and interphase fluorescence in situ hybridization (FISH), thus leading to an initial interpretation of inv(16)(p13.1q22); however, metaphase FISH showed that the CBFB rearrangement involved chromosome 2. Using whole genome and Sanger sequencing, the breakpoints were identified as being located in intron 5 of CBFB and intron 7 of PPP1R7. A microhomology of CAG was found in the break and reconnection sites of CBFB and PPP1R7, thus supporting the formation of CBFB::PPP1R7 by microhomology-mediated end joining.

Molecular Approaches to Treating Pediatric Leukemias

Over the past decades, striking progress has been made in the treatment of pediatric leukemia, approaching 90% overall survival in children with acute lymphoblastic leukemia (ALL) and 75% in children with acute myeloid leukemia (AML). This has mainly been achieved through multiagent chemotherapy including CNS prophylaxis and risk-adapted therapy within collaborative clinical trials. However, prognosis in children with refractory or relapsed leukemia remains poor and has not significantly improved despite great efforts. Hence, more effective and less toxic therapies are urgently needed. Our understanding of disease biology, molecular drivers, drug resistance and, thus, the possibility to identify children at high-risk for treatment failure has significantly improved in recent years. Moreover, several new drugs targeting key molecular pathways involved in leukemia development, cell growth, and proliferation have been developed and approved. These striking achievements are linked to the great hope to further improve survival in children with refractory and relapsed leukemia. This review gives an overview on current molecularly targeted therapies in children with leukemia, including kinase, and proteasome inhibitors, epigenetic and enzyme targeting, as well as apoptosis regulators among others.