Identification of New Genomic Lesions in Childhood ALL without Known Genetic Aberrations: A Microarray Study of Gene Expression and Genotype Data.

Abstract

Despite the risk stratification based on the prognostic relevant translocations, approximately 20–25% of childhood ALL patients cannot be classified according to genetic hallmarks. Recent studies based on single nucleotide polymorphism (SNP) analyses showed that Loss of Heterozygosity (LOH), with or without loss of genetic material, is frequently related to childhood ALL and AML. Moreover, there is an increasing interest in integrating the gene expression profiles with chromosomal localizations to identify new genetic subgroups. We applied an integrated approach composed of DNA index estimation, PCR (and/or RT-PCR) and cytogenetics, to exclude patients with known molecular and cytogenetics aberrations. Aim of the study was to identify criptic abnormalities in childhood ALL patients by performing an integrative analysis of gene expression and copy number changes (CNC) data. The 30 patients included in our study met the following inclusion criteria: a) B-cell precursor childhood ALL b) DNA index 1; c) negativity at t(4;11), t(12;21), t(9;22), t(1;19) RT-PCR screening; d) cytogenetics revealing normal karyotype or not technically feasible. Patients genomic DNA has been analyzed by the Affymetrix GeneChip Mapping 100K SNP arrays to identify genomic regions of LOH and CNC. In parallel, biotin-labeled cRNA has been synthesized and hybridized on HG-U133Plus2 Probe Arrays in accordance with the MILE (mircroarray innovation in leukemia) protocols; all samples were part of the B-ALL class without known genetic aberrations.The presence of del(9)(p21) was found in 9/30 (30%) patients, with an homozygous commonly deleted region involving CDKN2A and CDKN2B genes in 7/30. Hemizygous losses on 9p13 were found in two cases. Four patients (13,3%) showed the hemizygous deletion of chromosome 12p13.2, with a commonly deleted region involving the ETV6 gene. In one ETV6 deleted patient FISH analysis identified a translocation involving the first ETV6 exon and an unknown partner gene. Four patients, including three ETV6 deleted, showed the presence of the intrachromosomal amplification of chromosome 21 (iAMP21) with a common region of amplification between 31.3–43.5 Mb and a common region of deletion in 2/4 patients between 43.5–47 Mb. Four patients showed large regions of LOH not associated with CNC. Three patients did not show CNC and random microdeletions were found in single cases. Remission samples were analyzed for 5 patients carrying different aberrations, showing small regions of LOH in both the diagnosis and remission samples in 4 cases. No CNC was found in the remission samples. We found a statistically significant reduction for the expression of the CDKN2A transcript located within the (9)(p21) region. We are applying a locally adaptive statistical procedure to identify genes whose expression signals are specifically modulated by CNC. The integration of genomic variation and gene expression profiling might may be useful to identify new hidden genetic lesions, and to learn how critical regulators of tumor are linked to genomic alterations in cancer cells.