Abstract 4628: Alterations in FPGS splicing as a plausible underlying mechanism of MTX resistance in ALL.
Authors: Wojtuszkiewicz Anna , Assaraf Yehuda , Raz Shachar , Stark Michal , Sonneveld Edwin , Peters Godefridus , Jansen Gerrit , Kaspers Gertjan , Cloos Jacqueline 
Published: August 2014

Abstract
The antifolate methotrexate (MTX) is used in acute lymphoblastic leukemia (ALL) treatment. Cellular retention and consequent efficacy of MTX is enhanced by polyglutamylation, a reaction catalyzed by folylpolyglutamate synthetase (FPGS). It has been shown that loss of FPGS activity can confer antifolate resistance. However, the mechanism underlying this phenomenon remains elusive. We have recently shown that alterations in FPGS mRNA splicing may be a plausible determinant of MTX resistance but a comprehensive screen of the spectrum of splicing aberrations throughout the entire FPGS gene is lacking. The aim of the current study was to evaluate the pattern of FPGS mRNA splicing alterations as a predictor of MTX resistance and treatment failure in ALL. A PCR-based assay was devised that allowed for a screen of the entire FPGS mRNA sequence for splicing alterations. This method was subsequently used to screen antifolate-resistant human leukemia cell lines and ALL patient samples. Our results identified multiple FPGS mRNA splicing alterations (Table 1) in MTX resistant cell lines with marked loss of FPGS activity. Moreover, the level of partial retention of intron 8 was markedly increased upon a 24 hour exposure to MTX in the antifolate-resistant leukemia cell lines. Moreover, most of the detected FPGS splicing alterations either introduced a frame-shift or a premature stop codon, hence resulting in either transcripts targeted for the nonsense-mediated decay or truncated proteins devoid of FPGS activity. These in vitro FPGS splicing alterations where corroborated in vivo as aberrations detected in leukemic cell lines were also identified in a panel of 30 ALL patient specimens at diagnosis. In summary, impaired FPGS mRNA splicing could be responsible for loss of FPGS activity and might offer an opportunity for the design of reliable molecular tools allowing for identification of antifolate resistance in leukemia cells.
Table 1. FPGS mRNA splicing alterations in cell lines and ALL patient samples. Alteration Frame shift (yes/no) Early stop codon position (nucleotide) Frequency in primary ALL samples (%) Exon 2 skipping − − 71 Exon 5 skipping + 477 50 Exon 6 skipping − − 93 Exon 12 skipping + 1173 57 Exon 14 skipping + 1356 50 Intron 5 retention + 792 43 Intron 6 retention + 660 35 Intron 5 + Intron 6 retention − 879 50 Intron 8 partial retention − 870 93 Intron 6 retention + Intron 8 partial retention + 660 50
Citation Format: Anna Wojtuszkiewicz, Yehuda Assaraf, Shachar Raz, Michal Stark, Edwin Sonneveld, Godefridus Peters, Gerrit Jansen, Gertjan Kaspers, Jacqueline Cloos. Alterations in FPGS splicing as a plausible underlying mechanism of MTX resistance in ALL. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4628. doi:10.1158/1538-7445.AM2013-4628