Bi-allelic deletions within 13q14 and transient trisomy 21 with absence of GATA1s in pediatric acute megakaryoblastic leukemia

Repression of TRIM13 by chromatin assembly factor CHAF1B is critical for AML development

Acute myeloid leukemia (AML) is an aggressive blood cancer that stems from the rapid expansion of immature leukemic blasts in the bone marrow. Mutations in epigenetic factors represent the largest category of genetic drivers of AML. The chromatin assembly factor CHAF1B is a master epigenetic regulator of transcription associated with self-renewal and the undifferentiated state of AML blasts. Upregulation of CHAF1B, as observed in almost all AML samples, promotes leukemic progression by repressing the transcription of differentiation factors and tumor suppressors. However, the specific factors regulated by CHAF1B and their contributions to leukemogenesis are unstudied. We analyzed RNA sequencing data from mouse MLL-AF9 leukemic cells and bone marrow aspirates, representing a diverse collection of pediatric AML samples and identified the E3 ubiquitin ligase TRIM13 as a target of CHAF1B-mediated transcriptional repression associated with leukemogenesis. We found that CHAF1B binds the promoter of TRIM13, resulting in its transcriptional repression. In turn, TRIM13 suppresses self-renewal of leukemic cells by promoting pernicious entry into the cell cycle through its nuclear localization and catalytic ubiquitination of cell cycle-promoting protein, CCNA1. Overexpression of TRIM13 initially prompted a proliferative burst in AML cells, which was followed by exhaustion, whereas loss of total TRIM13 or deletion of its catalytic domain enhanced leukemogenesis in AML cell lines and patient-derived xenografts. These data suggest that CHAF1B promotes leukemic development, in part, by repressing TRIM13 expression and that this relationship is necessary for leukemic progression.

Pediatric Acute Megakaryoblastic Leukemia: Multitasking Fusion Proteins and Oncogenic Cooperations

Pediatric leukemia presents specific clinical and genetic features from adult leukemia but the underpinning mechanisms of transformation are still unclear. Acute megakaryoblastic leukemia (AMKL) is the malignant accumulation of progenitors of the megakaryocyte lineage that normally produce blood platelets. AMKL is diagnosed de novo, in patients showing a poor prognosis, or in Down syndrome (DS) patients with a better prognosis. Recent data show that de novo AMKL is primarily associated with chromosomal alterations leading to the expression of fusions between transcriptional regulators. This review highlights the most recurrent genetic events found in de novo pediatric AMKL patients and, based on recent functional analyses, proposes a mechanism of leukemogenesis common to de novo and DS-AMKL.

Fluorescence In situ Hybridization: Cell-Based Genetic Diagnostic and Research Applications

Fluorescence in situ hybridization (FISH) is a macromolecule recognition technology based on the complementary nature of DNA or DNA/RNA double strands. Selected DNA strands incorporated with fluorophore-coupled nucleotides can be used as probes to hybridize onto the complementary sequences in tested cells and tissues and then visualized through a fluorescence microscope or an imaging system. This technology was initially developed as a physical mapping tool to delineate genes within chromosomes. Its high analytical resolution to a single gene level and high sensitivity and specificity enabled an immediate application for genetic diagnosis of constitutional common aneuploidies, microdeletion/microduplication syndromes, and subtelomeric rearrangements. FISH tests using panels of gene-specific probes for somatic recurrent losses, gains, and translocations have been routinely applied for hematologic and solid tumors and are one of the fastest-growing areas in cancer diagnosis. FISH has also been used to detect infectious microbias and parasites like malaria in human blood cells. Recent advances in FISH technology involve various methods for improving probe labeling efficiency and the use of super resolution imaging systems for direct visualization of intra-nuclear chromosomal organization and profiling of RNA transcription in single cells. Cas9-mediated FISH (CASFISH) allowed in situ labeling of repetitive sequences and single-copy sequences without the disruption of nuclear genomic organization in fixed or living cells. Using oligopaint-FISH and super-resolution imaging enabled in situ visualization of chromosome haplotypes from differentially specified single-nucleotide polymorphism loci. Single molecule RNA FISH (smRNA-FISH) using combinatorial labeling or sequential barcoding by multiple round of hybridization were applied to measure mRNA expression of multiple genes within single cells. Research applications of these single molecule single cells DNA and RNA FISH techniques have visualized intra-nuclear genomic structure and sub-cellular transcriptional dynamics of many genes and revealed their functions in various biological processes.