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Novel insights into the role of aberrantly expressed MNX1 (HLXB9) in infant acute myeloid leukemia
Juerg Schwaller
University Children’s Hospital beider Basel (UKBB), Department of Biomedicine, University of Basel Childhood Leukemia Group ZLF, Switzerland.
E-mail: J.Schwaller@unibas.ch doi:10.3324/haematol.2018.205971
Almost two decades ago, the molecular characteriza- tion of a t(7;12)(q36;p13) chromosomal translocation in very young children with acute myeloid leukemia (AML) and poor outcome identified a fusion mRNA poten- tially encoding for a chimeric protein that contains the point- ed (PNT) and ETS domains of the ETS variant 6 (ETV6) gene, also known as TEL1 (Translocating E26 transforming-specific leukemia 1) on 12p13, joined to the regulatory sequences and first exons of the HLXB9 homeobox gene.1 Previous work reported a series of infant AML patients with t(7;12)(q36;p13) with blasts carrying a potential ETV6 translocation (revealed by a split FISH signal).2,3 In fact, the entire HLBX9 gene seems to be transferred onto the der(12) without disruption of the gene itself. A fusion transcript may result from a yet to be identified long-range splicing mechanism and can be found in about 50% of cases. Expression of any reciprocal ETV6- HLXB9 fusion transcript has not been reported. These find- ings suggested that a position effect triggers stable overex- pression of HLXB9 in t(7;12)(q36;p13)+ AML.4,5 The Nordic Society for Pediatric Hematology and Oncology (NOPHO) recently reported additional 7 patients to the previously 35 published t(7;12)(q36;p13)+ AML patients. Overall leukemic blasts from 20-30% of infant AML patients (<2 years) carry this translocation associated with trisomy 19 in 86% of the cases.6 In addition to t(7;12)(q36;p13), another translocation, t(6;7)(q23;q35), was identified in an AML cell line (GDM-1) which also resulted in aberrantly high HLXB9 expression via juxtaposition with regions of the MYB gene.7
The homeobox HLXB9 (or HB9) protein is today refer- enced as MNX1 (motor neuron and pancreas homeobox 1, OMIM: 142994). Putative loss-of-function mutations of MNX1 were identified as the molecular correlates of heredi- tary sacral agenesis.8 Gene targeting experiments in mice revealed MNX1 to be a critical regulator for normal pan- creas9,10 as well as motor neuron development.11 Loss-of-func- tion MNX1 mutations in neonatal diabetes confirmed its role in pancreas development in humans.12
There is increasing evidence to suggest an important role of this critical developmental homeobox transcription factor not only in hematologic malignancies but also in solid cancers;13,14 however, the functional consequences and molecular mecha- nisms of aberrantly high MNX1 expression for malignant transformation remain poorly understood.
In a study published in this edition of Haematologica, Ingenhag et al.15 addressed the oncogenic potential of increased MNX1 expression in non-hematopoietic and hematopoietic cells. They found that lentiviral MNX1 over- expression in human HT1080 fibrosarcoma or mouse NIH- 3T3 fibroblasts resulted in growth arrest with stalling at the G1/G2 phase of cell cycle, morphological signs of senescence, and increased senescence-associated β-galactosidase activity (SA-β-gal) associated with activation of the tumor suppressor
TP53 and its target the cyclin-dependent kinase inhibitor 1A (CDKN1A, aka p21WAF1/CIP1). As oncogene-induced senescence is a hallmark of early malignant transformation of solid tumors, this finding suggests that MNX1 overexpression may result in a pre-cancerous state.16 However, one has to keep in mind that both of the models used are immortalized solid cancer cell lines that may carry potent oncogenes such as mutated NRASQ61K present in HT-1080 (https://portals.broadinsti- tute.org/ccle/page?cell_line= HT1080_SOFT_TISSUE). Nevertheless, previous work has shown that overexpression of well-characterized AML-associated fusion oncogenes (e.g. PML-RARA, RUNX1-ETO, CBFB-MYH11) induces DNA damage, and activates a CDKN1A-dependent cell cycle arrest and DNA repair in mouse hematopoietic stem and progenitor cells (HSPC).17 It will, therefore, be interesting to verify whether overexpression of MNX1 may also lead to DNA damage activating the TP53-CDKN1A pathway in HSPC.
To address the oncogenic potential in hematopoietic cells, Ingenhag et al.15 also lentivirally expressed MNX1 in lineage marker-depleted mouse bone marrow (BM)-derived HSPC. Reconstitution of lethally irradiated syngenic recipients resulted in a reduced overall peripheral blood cellularity with very low contribution to any mature (CD19+ B cells; CD3+ T cells; CD11b+ neutrophils) cell lineage. Although the total cell number was also reduced in the BM, MNX1 expressing cells mostly contributed to the megakaryocytic-erythrocyte pro- genitor (MEP) cell compartment, whereas no distinct MNX1 expressing population was detected within the granulocyte- monocyte progenitor (GMP) compartment. Although some MNX1 expressing immature B cells (B220+CD19+CD93+) were found, no signal for MNX1 expression was found in mature B cells and immature or mature T cells. None of the transplanted mice developed any signs of a hematologic dis- ease during a relatively short observation period of 6.5 months. These observations show for the first time that MNX1 overexpression impacts hematopoietic differentiation of HSPC in vivo but may not be sufficient to induce a hema- tologic disease.
To study the potential oncogenic activity in human hematopoietic cells, Ingenhag et al.15 were able to lentivirally express MNX1 in CD34+ HSPC at comparable levels to what was observed in primary t(7;12)(q36;p13)+ AML cells. Gene expression profiling revealed that, in addition to MNX1, 116 significantly aberrantly regulated genes including de novo expression of HBZ (Hemoglobin Subunit Zeta), HBE1 (Hemoglobin Subunit Epsilon 1) SLC4A1 (Solute Carrier Family 4 Member 1), all tightly associated with erythroid differentiation. Notably, gene set enrichment analysis revealed upregulation of genes related to cell cycle progression, and downregulation of cytoskeleton organization and various intracellular signaling pathways. Previous work by the same group has already sug- gested that elevated MNX1 levels affect expression of genes
haematologica | 2019; 104(1)
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