High Resolution Genomic Profiling Using Single Nucleotide Polymorphism Microarrays Identifies Multiple Novel Genomic Lesions in Pediatric Acute Lymphoblastic Leukemia.

To obtain a comprehensive registry of oncogenic lesions in pediatric acute lymphoblastic leukemia (ALL), we used Affymetrix single nucleotide polymorphism (SNP) arrays to examine changes in DNA copy number and loss-of heterozygosity (LOH) in leukemic blasts and matched remission samples from 250 ALLs. We studied B-progenitor ALLs with high hyperdiploidy, n=39; ETV6-RUNX1, n=47; MLL rearranged, n=11; TCF3-PBX1, n=17; BCR-ABL1, n=9; low hyperdiploidy, n=23; hypodiploidy, n=10; unclassified cases, n=42; and 50 T-lineage ALLs. Four arrays (50K Hind and Xba, 250K Sty and Nsp) were used to interrogate over 615,000 loci at a mean inter-marker distance of 4.8 kb. Data was analyzed using dChipSNP and a modified array normalization algorithm using only SNPs from regions known to be diploid by routine karyotyping. Copy number abnormalities were confirmed by FISH and genomic quantitative PCR. Complementary methylation analysis and sequencing of candidate genes was performed. 84% of B-ALLs and 96% of T-ALLs had at least one region of somatic deletion, and excluding cases with high hyperdiploidy, 68% of B-ALLs and 50% of T-ALLs had at least one region of somatic amplification. These included previously identified abnormalities including chromosomal duplications in hyperdiploid B-ALL; 1q duplication in TCF3-PBX1 ALL; and deletions of 9p21 (harboring CDKN2A/B, 70% of T-ALLs, 34.5% of B-ALL), 12p13 (ETV6; 25.5% of B-ALLs, 10% of T-ALLs), 6q16 (22 cases) and 11q (15 cases). The resolution of the arrays enabled precise mapping of the minimal regions of deletion at 9p21 to CDKN2A, and at 12p13 to ETV6. Combined LOH and copy number analysis identified several patterns of 9p21 abnormality: focal hemizygous deletion with corresponding LOH; focal homozygous and flanking hemizygous loss with corresponding LOH, indicating two focal deletional events; and focal homozygous loss with LOH of all of 9p or chromosome 9, indicating loss of the normal 9 or 9p and duplication of the chromosome or chromosomal arm containing the focal deletion. Copy-neutral LOH without any focal deletion in the affected region was uncommon. Deletions involving other genes with potential roles in leukemogenesis were identified including BTG1 (17 cases), ERG (10), FHIT (14), mir-16/-15a (19), MYB (5), NF1 (9), the glucocorticoid receptor NR3C1 (11), PTEN (4), and RB1 (20). Furthermore, deletions, translocations, amplifications, and point mutations of genes that regulate B-cell development and differentiation, including EBF, PAX5, Ikaros and Aiolos, were identified in 40% of B-ALL. For each of the listed genes, cases were identified that contained focal deletions limited to the specific gene. Overall, 73.6% B-ALL and 88% T-ALLs harbored deletions of one of the common lesions listed above, with 48% of B-ALLs and 48.5% of T-ALLs having multiple common lesions. The average number of deletions per case was 3.8 and 5.7 for B and T-lineage ALLs respectively. By contrast, when hyperdiploid cases were excluded, it was rare to find more than 2 regions of amplification in a single case, and the majority of cases contained no amplifications. These findings show the power of high-resolution copy number analysis for the identification of new genetic lesions in cancer, and demonstrate that multiple genetic abnormalities contribute to leukemogenesis in pediatric ALL.