Research Papers:

Expression of MLL-AF4 or AF4-MLL fusions does not impact the efficiency of DNA damage repair

Julio Castaño, Ana B. Herrero, Aldeheid Bursen, Federico González, Rolf Marschalek, Norma C. Gutiérrez and Pablo Menendez _

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Oncotarget. 2016; 7:30440-30452. https://doi.org/10.18632/oncotarget.8938

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Julio Castaño1, Ana B. Herrero2, Aldeheid Bursen3, Federico González4, Rolf Marschalek3, Norma C. Gutiérrez2 and Pablo Menendez1,5

1 Josep Carreras Leukemia Research Institute, Department of Biomedicine, School of Medicine, University of Barcelona, Barcelona, Spain

2 Hematology Department, University Hospital of Salamanca, IBSAL, IBMCC (USAL-CSIC), Salamanca, Spain

3 Institute Pharmaceutical Biology, Goethe-University, Frankfurt/Main, Germany

4 IBEC - Institute for Bioengineering of Catalonia, Barcelona, Spain

5 Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain

Correspondence to:

Pablo Menendez, email:

Keywords: MLL.AF4, AF4.MLL, t(4;11), DSB, infant leukemia

Received: April 11, 2016 Accepted: April 13, 2016 Published: April 22, 2016


The most frequent rearrangement of the human MLL gene fuses MLL to AF4 resulting in high-risk infant B-cell acute lymphoblastic leukemia (B-ALL). MLL fusions are also hallmark oncogenic events in secondary acute myeloid leukemia. They are a direct consequence of mis-repaired DNA double strand breaks (DNA-DSBs) due to defects in the DNA damage response associated with exposure to topoisomerase-II poisons such as etoposide. It has been suggested that MLL fusions render cells susceptible to additional chromosomal damage upon exposure to etoposide. Conversely, the genome-wide mutational landscape in MLL-rearranged infant B-ALL has been reported silent. Thus, whether MLL fusions compromise the recognition and/or repair of DNA damage remains unanswered. Here, the fusion proteins MLL-AF4 (MA4) and AF4-MLL (A4M) were CRISPR/Cas9-genome edited in the AAVS1 locus of HEK293 cells as a model to study MLL fusion-mediated DNA-DSB formation/repair. Repair kinetics of etoposide- and ionizing radiation-induced DSBs was identical in WT, MA4- and A4M-expressing cells, as revealed by flow cytometry, by immunoblot for γH2AX and by comet assay. Accordingly, no differences were observed between WT, MA4- and A4M-expressing cells in the presence of master proteins involved in non-homologous end-joining (NHEJ; i.e.KU86, KU70), alternative-NHEJ (Alt-NHEJ; i.e.LigIIIa, WRN and PARP1), and homologous recombination (HR, i.e.RAD51). Moreover, functional assays revealed identical NHEJ and HR efficiency irrespective of the genotype. Treatment with etoposide consistently induced cell cycle arrest in S/G2/M independent of MA4/A4M expression, revealing a proper activation of the DNA damage checkpoints. Collectively, expression of MA4 or A4M does neither influence DNA signaling nor DNA-DSB repair.

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