ABSTRACT© F e r r a t a S t o r t i F o u n d a t i o n . only part of this activity is transferred to the p53 pathway, leading to pro-apoptotic signaling. Monitoring of ATM function may be a feasible tool for disclosing ATM mutations. Several slightly modified tests have been suggested, based on monitoring p53 and p21 accumulation after cell exposure to ionizing radiation (IR). 17,[24][25][26] An alternative approach utilizing etoposide and nutlin-3a was also used which enabled efficient differentiation of TP53 and ATM defects. 27Despite undeniable progress, ATM mutation identification in CLL remains challenging due to: i) an extreme gene size (62 coding exons) with lack of well-characterized (hot-spot) mutations; ii) the difficult interpretation of polymorphisms and pathogenic mutations resulting from only vague information about their functional consequences. Therefore, relevant information is lacking about the clinical impact of ATM mutations, including the response of affected patients to chemoimmunotherapy.In our CLL patient cohort, we found that all ATM defects involving mutation(s) resulted in disruption of ATM activity towards p53 pathway activation. In addition, we present a novel functional test based on monitoring p21 induction after parallel treatment of CLL cells with doxorubicin and fludarabine that have different DNA damage mechanisms. This test proved to be an effective means to search for ATM mutations, which had been selected in a dominant proportion of leukemic cells. Table 1. The cohort consisted of predominantly high-risk CLL patients (harboring TP53 defect and/or 11q-and/or unmutated IGHV), and 45% of patients were treated with various regimens before ATM mutation analysis (Online Supplementary Table S1). Design and Methods Patients' samples ATM mutation analysisCustom resequencing microarray (Affymetrix, CA, USA) was used to detect 1-nt substitutions in all 62 coding exons and splicing sites of the ATM gene. The resequencing procedure was carried out according to the manufacturer's protocol (Affymetrix GeneChip Custom Resequencing Array Protocol). The resequencing principle is based on allele-specific hybridization. The hybridization of fluorescently labeled DNA fragments to particular positions determined the nucleotides in sequence with the ability to distinguish between homozygous and heterozygous state. Final sequence data was acquired using The GeneChip Sequence Analysis Software (GSEQ) processing fluorescence intensity files.Direct Sanger sequencing (3130xl Genetic Analyzer, Applied Biosystems) was used to: i) confirm ATM alterations detected by microarray analysis; ii) identify other mutations indicated by functional test and/or Western blot (WB) through screening of all 62 coding exons and splicing sites. A search was made for all identified sequence variations in appropriate databases of single nucleotide polymorphisms (SNPs) and mutations.Variations detected by the resequencing array but not confirmed by Sanger sequencing were evaluated by next-generation deep sequencing techno...

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Treatment options for TP53-mutated lymphoid tumors are very limited. In experimental models, TP53-mutated lymphomas were sensitive to direct inhibition of checkpoint kinase 1 (Chk1), a pivotal regulator of replication. We initially tested the potential of the highly specific Chk1 inhibitor SCH900776 to synergize with nucleoside analogs (NAs) fludarabine, cytarabine and gemcitabine in cell lines derived from B-cell malignancies. In p53-proficient NALM-6 cells, SCH900776 added to NAs enhanced signaling towards Chk1 (pSer317/pSer345), effectively blocked Chk1 activation (Ser296 autophosphorylation), increased replication stress (p53 and γ-H2AX accumulation) and temporarily potentiated apoptosis. In p53-defective MEC-1 cell line representing adverse chronic lymphocytic leukemia (CLL), Chk1 inhibition together with NAs led to enhanced and sustained replication stress and significantly potentiated apoptosis. Altogether, among 17 tested cell lines SCH900776 sensitized four of them to all three NAs. Focusing further on MEC-1 and co-treatment of SCH900776 with fludarabine, we disclosed chromosome pulverization in cells undergoing aberrant mitoses. SCH900776 also increased the effect of fludarabine in a proportion of primary CLL samples treated with pro-proliferative stimuli, including those with TP53 disruption. Finally, we observed a fludarabine potentiation by SCH900776 in a T-cell leukemia 1 (TCL1)-driven mouse model of CLL. Collectively, we have substantiated the significant potential of Chk1 inhibition in B-lymphoid cells.