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Haematologica 2018 Volume 103(5):830-839
Ferrata Storti Foundation
Acute Lymphoblastic Leukemia
MSH6 haploinsufficiency at relapse contributes to the development of thiopurine resistance in pediatric B-lymphoblastic leukemia
Nikki A. Evensen,1 P. Pallavi Madhusoodhan,1 Julia Meyer,2 Jason Saliba,1 Ashfiyah Chowdhury,1 David J. Araten,3 Jacob Nersting,4 Teena Bhatla,1 Tiffaney L. Vincent,5 David Teachey,5 Stephen P. Hunger,5 Jun Yang,6 Kjeld Schmiegelow4 and William L. Carroll1
1Departments of Pediatrics and Pathology, Perlmutter Cancer Center, NYU-Langone Medical Center, New York, NY, USA; 2Huntsman Cancer Institute, University of Utah Medical Center, Salt Lake City, USA; 3Department of Medicine, Perlmutter Cancer Center, NYU-Langone Medical Center, New York NY, USA; 4Department of Pediatrics and Adolescent Medicine, The University Hospital Rigshospitalet, Copenhagen, Denmark; 5Department of Pediatrics and the Center for Childhood Cancer Research, Children’s Hospital of Philadelphia and The Perelman School of Medicine at The University of Pennsylvania, Philadelphia, PA, USA and 6St. Jude Children’s Research Hospital, Memphis, TN, USA
ABSTRACT
Survival of children with relapsed acute lymphoblastic leukemia is poor, and understanding mechanisms underlying resistance is essential to developing new therapy. Relapse-specific heterozygous deletions in MSH6, a crucial part of DNA mismatch repair, are frequent- ly detected. Our aim was to determine whether MSH6 deletion results in a hypermutator phenotype associated with generation of secondary mutations involved in drug resistance, or if it leads to a failure to initiate apoptosis directly in response to chemotherapeutic agents. We knocked down MSH6 in mismatch repair proficient cell lines (697 and UOCB1) and showed significant increases in IC50s to 6-thioguanine and 6-mer- captopurine (697: 26- and 9-fold; UOCB1: 5- and 8-fold) in vitro, as well as increased resistance to 6-mercaptopurine treatment in vivo. No shift in IC50 was observed in deficient cells (Reh and RS4;11). 697 MSH6 knock- down resulted in increased DNA thioguanine nucleotide levels com- pared to non-targeted cells (3070 vs. 1722 fmol/μg DNA) with no differ- ence observed in mismatch repair deficient cells. Loss of MSH6 did not give rise to microsatellite instability in cell lines or clinical samples, nor did it significantly increase mutation rate, but rather resulted in a defect in cell cycle arrest upon thiopurine exposure. MSH6 knockdown cells showed minimal activation of checkpoint regulator CHK1, γH2AX (DNA damage marker) and p53 levels upon treatment with thiopurines, consistent with intrinsic chemoresistance due to failure to recognize thioguanine nucleotide mismatching and initiate mismatch repair. Aberrant MSH6 adds to the list of alterations/mutations associated with acquired resistance to purine analogs emphasizing the importance of thiopurine therapy.
Introduction
Relapsed B-precursor acute lymphoblastic leukemia (B-ALL) is a leading cause of cancer mortality amongst children. Development of chemoresistance is a crucial factor contributing to relapse, therefore understanding the biological mechanisms underlying this resistance is imperative for discovering innovative treatment strate- gies.1 Recent work has begun to highlight the direct role of relapse specific/enriched genetic alterations in the emergence of clones that have gained a selective advantage under the pressure of specific chemotherapeutics, such as NT5C2, TBL1XR1, PRPS1, and CREBBP.1-5 Many of these mutations cause resist- ance specifically to thiopurines, which are the backbone of maintenance therapy
Correspondence:
william.carroll@nyumc.org
Received: July 13, 2017. Accepted: February 7, 2018. Pre-published: February 15, 2018.
doi:10.3324/haematol.2017.176362
Check the online version for the most updated information on this article, online supplements, and information on authorship & disclosures: www.haematologica.org/content/103/5/830
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