The Philadelphia chromosome (Ph) encodes the oncogenic BCR-ABL1 kinase and defines a subgroup of ALL with a particularly unfavorable prognosis. ALL cells derived from B cell precursors typically carry rearranged immunoglobulin heavy chain (IGH) variable (V) region genes devoid of somatic mutations. Somatic hypermutation is restricted to mature germinal center B cells and depends on expression of activationinduced cytidine deaminase (AID), which introduces single-strand breaks into target DNA. Since, at much lower frequency AID can also target non-Ig genes and may even act as a genome-wide mutator, we investigated whether AID was expressed in Ph+ ALL and blast crisis chronic myeloid leukemia (CML) patients. On 35 adult Ph+ ALL patients, we detected AID mRNA and protein in 16 (46%), moreover AID expression was also found in blast crisis B-lymphoid lineage CML patients (n=2) but not in myeloid lineage (n=5) or in chronic phase CML (n=30). AID expression correlated with the BCR-ABL1 transcript levels and with the frequency of mutations within the BCR-ABL1 kinase domain that confer resistance to the BCR-ABL1 kinase inhibitors and disappeared after treatment with imatinib at the time of remission. To investigate whether AID introduces DNA-SSB in Ph+ ALL, we performed a genome wide analysis by 250K NspI single nucleotide polymorphism (SNP) array (Affymetrix Inc., USA) which is able to detect at high resolution and throughout the genome copy number abnormalities. We identified region of high level amplification and homozygous deletion in all patients. Overall, deletions outnumbered amplifications 3:1 and on average, we found 10 lesions per case, with a median of 6 losses and 2 gains. Patients who expressed AID had a higher number of alterations respect to patients who were AID negative (median copy number alteration was 8.5, range 4-22, versus 4, range 1-10, respectively, p<0.004). Recurring copy number abnormalities were identified in genes with an established role in leukemogenesis such as CDKN2A, CDKN2B, GADD45, PAX5, ETV1, BTG1 and MDS1. In particular, we found that the pattern of AID expression correlated with a high frequency of DNA single-strand breaks within the tumor suppressor genes CDKN2A and CDKN2B, which were found in B cell lineage but not myeloid lineage subclones of blast crisis CML. These findings show that AID can act as a mutator outside the Ig gene loci in Ph+ ALL cells prommoting genetic instability in leukemia cells. Supported by European LeukemiaNet, AIL, AIRC, FIRB 2006, Fondazione del Monte di Bologna e Ravenna, Strategico di Ateneo.

ACTIVATION-INDUCED CYTIDINE DEAMINASE IS ABERRANTLY OVEREXPRESSED IN BCRABL1 POSITIVE ACUTE LYMPHOBLASTIC LEUKEMIA CELLS AND PROMOTES DNA-SINGLE STRAND BREAKS

IACOBUCCI, ILARIA;LONETTI, ANNALISA;SOVERINI, SIMONA;OTTAVIANI, EMANUELA;TESTONI, NICOLETTA;PAOLINI, STEFANIA;PAPAYANNIDIS, CRISTINA;MARTINELLI, GIOVANNI
2008

Abstract

The Philadelphia chromosome (Ph) encodes the oncogenic BCR-ABL1 kinase and defines a subgroup of ALL with a particularly unfavorable prognosis. ALL cells derived from B cell precursors typically carry rearranged immunoglobulin heavy chain (IGH) variable (V) region genes devoid of somatic mutations. Somatic hypermutation is restricted to mature germinal center B cells and depends on expression of activationinduced cytidine deaminase (AID), which introduces single-strand breaks into target DNA. Since, at much lower frequency AID can also target non-Ig genes and may even act as a genome-wide mutator, we investigated whether AID was expressed in Ph+ ALL and blast crisis chronic myeloid leukemia (CML) patients. On 35 adult Ph+ ALL patients, we detected AID mRNA and protein in 16 (46%), moreover AID expression was also found in blast crisis B-lymphoid lineage CML patients (n=2) but not in myeloid lineage (n=5) or in chronic phase CML (n=30). AID expression correlated with the BCR-ABL1 transcript levels and with the frequency of mutations within the BCR-ABL1 kinase domain that confer resistance to the BCR-ABL1 kinase inhibitors and disappeared after treatment with imatinib at the time of remission. To investigate whether AID introduces DNA-SSB in Ph+ ALL, we performed a genome wide analysis by 250K NspI single nucleotide polymorphism (SNP) array (Affymetrix Inc., USA) which is able to detect at high resolution and throughout the genome copy number abnormalities. We identified region of high level amplification and homozygous deletion in all patients. Overall, deletions outnumbered amplifications 3:1 and on average, we found 10 lesions per case, with a median of 6 losses and 2 gains. Patients who expressed AID had a higher number of alterations respect to patients who were AID negative (median copy number alteration was 8.5, range 4-22, versus 4, range 1-10, respectively, p<0.004). Recurring copy number abnormalities were identified in genes with an established role in leukemogenesis such as CDKN2A, CDKN2B, GADD45, PAX5, ETV1, BTG1 and MDS1. In particular, we found that the pattern of AID expression correlated with a high frequency of DNA single-strand breaks within the tumor suppressor genes CDKN2A and CDKN2B, which were found in B cell lineage but not myeloid lineage subclones of blast crisis CML. These findings show that AID can act as a mutator outside the Ig gene loci in Ph+ ALL cells prommoting genetic instability in leukemia cells. Supported by European LeukemiaNet, AIL, AIRC, FIRB 2006, Fondazione del Monte di Bologna e Ravenna, Strategico di Ateneo.
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Iacobucci I; Lonetti A;; Ferrari A; Soverini S; Ottaviani E; Testoni N; Colarossi S; Salmi F; Gnani A; Paolini S; Piccaluga PP; Papayannidis C; Panagiota G; Cilloni D; Messa F;; Pane F; Vitale A; Chiaretti S; Saglio G; Foà R; Baccarani M; Martinelli G
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/154725
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