TP53 Alterations Including Missense Mutations, Aberrant Exon-Junctions and Internal Intron Retentions Are Frequent and May Contribute to MDM2 Antagonist-Resistance in B-Acute Lymphoblastic leukemia

Abstract 1484

MDM2, a p53-inducible phosphoprotein, binds to the N-terminus of the p53 and negatively regulates its transcriptional activity. New MDM2 antagonists, such as RO5045337 (Roche) and NSC-66811 (Merck), are now available for Phase I/II clinical development, but their activity is dependent on TP53 mutation status. Therefore, in order to efficiently treat B-progenitor acute lymphoblastic leukemia (ALL) patients with an MDM2 antagonist, we set up a sensitive assay to identify TP53 lesions.

Deletions and uniparental disomy (UPD) involving TP53 were assessed on 146 DNA samples from Philadelphia-positive (Ph+)(n = 126) and Ph-negative (n = 20) ALL patients by Genome-Wide Human SNP 6.0 array (Affymetrix). No 17p UPD events were detected whereas losses were identified in 2% of cases.

Mutations of TP53 were thereafter investigated in 67 samples including 60 Ph+ and 7 Ph-negative cases. Since the majority of the studies in leukemia were focused on genomic alterations and resulted in low rate of TP53 mutations, we aimed to identify RNA mutations and aberrant isoforms due to other mechanisms, such as RNA editing. To this purpose three overlapping shorter amplicons covering the entire coding cDNA sequence (GenBank accession number NM_000546.4) and the untranslated exon 1 [amplicon 1 (491 bp): exons 1–5; amplicon 2 (482 bp): exons 5–8; amplicon 3 (498 bp): exons 8–11)] and a longer amplicon (1,317 bp) starting from exon 1 and ending to exon 11 were sequenced by Sanger method. TP53 mutations were detected in only 6 cases (8.9%), suggesting that these alterations are apparently rare events in B-ALL. They included 4 missense point mutations in the DNA binding domain and in the carboxyl-terminal tetramerization and regulatory domain: C135Y (ex 5), A234T (ex 7), R290C (ex 8) and A347T (ex 10). Interestingly, in two cases we identified aberrant transcripts: 1) a TP53 isoform characterized by retention of introns 5–6–7 and predicted to encode for a truncated protein due a premature stop codon; 2) a TP53 isoform in which the DNA binding domain is lost due to an exon conjunction between the exon 4 and the 3' untraslated region (UTR)(ex4-3'UTR: 7579533–7572842, according to GRCh37/hg19).

Next, in order to investigate if low rate of mutations were detectable, we also analyzed our whole transcriptome data obtained using next generation sequencing technology (Illumina/Solexa Genome Analyzer) on 3 Ph+ ALL patients. Curiously, all patients harbored clones ranging from 45% to 94% with TP53 mutations in the DNA binding and tetramerization domains: C182W (ex 5), T231A (ex 7), L330R (ex 9) in the first patient and Stop394S, D393V/H and G389Y/V (ex 11) in the second one. Moreover, in the first and third patient we detected 10 and 13 base exchanges, respectively, located in intron 6 within 7578166–7578142 region, suggesting a retention of this intron in the primary transcript and the dysfunction of the DNA-binding domain. The mechanism of intron retention (with or without mutations) was particularly intrigued since it could be a new mechanism of functional inactivation of TP53. To address this hypothesis we performed amplification of TP53 cDNA followed by single cell cloning and subsequent direct sequencing in 4 patients previously resulted wild-type by Sanger sequencing for TP53. By this approach, all patients showed cDNA alterations. In one case we identified the missense mutation S90P (ex 4) and an aberrant isoform lacking the DNA binding domain and caused by an exon-junction between exons 2 and 7 (ex2-7: 7579866–7577510). In a second patient the P190S (ex 6) and N235S (ex 7) missense mutations were detected. Moreover, an aberrant isoform lacking the DNA binding domain and characterized by an exon-junction between the first part of exon 4 and the last part of exon 7 (ex4-7: 7579581–7577532) was also identified. In the third patient the E285G (ex 8) was found associated with a 3'-UTR base exchange, which was also detected in the remaining fourth patient. In conclusion, we demonstrate for the first time that TP53 alterations at the RNA level, including missense mutations, aberrant exon junctions and internal intron retentions are highly frequent in B-ALL patients and that testing for TP53 mutations with sensitive assay based on RNA analysis is absolutely required. Supported by European LeukemiaNet, AIL, AIRC, Fondazione Del Monte di Bologna e Ravenna, FIRB 2006, PRIN 2009, Ateneo RFO grants, PIO program, Programma di Ricerca Regione – Università 2007 – 2009.