We report 2 novel, cryptic chromosomal abnormalities in precursor B-cell acute lymphoblastic leukemia (BCP-ALL): a translocation, either t(X;14)(p22;q32) or t(Y;14)(p11; q32), in 33 patients and an interstitial deletion, either del(X)(p22.33p22.

Key Points• RAS pathway mutations are prevalent in relapsed childhood ALL, and KRAS mutations are associated with a poorer overall survival.• RAS pathway mutations confer sensitivity to mitogenactivated protein kinase kinase inhibitors.For most children who relapse with acute lymphoblastic leukemia (ALL), the prognosis is poor, and there is a need for novel therapies to improve outcome. We screened samples from children with B-lineage ALL entered into the ALL-REZ BFM 2002 clinical trial (www. clinicaltrials.gov, #NCT00114348) for somatic mutations activating the Ras pathway (KRAS, NRAS, FLT3, and PTPN11) and showed mutation to be highly prevalent (76 from 206). Clinically, they were associated with high-risk features including early relapse, central nervous system (CNS) involvement, and specifically for NRAS/KRAS mutations, chemoresistance. KRAS mutations were associated with a reduced overall survival. Mutation screening of the matched diagnostic samples found many to be wild type (WT); however, by using more sensitive allelic-specific assays, low-level mutated subpopulations were found in many cases, suggesting that they survived up-front therapy and subsequently emerged at relapse. Preclinical evaluation of the mitogen-activated protein kinase kinase 1/2 inhibitor selumetinib (AZD6244, ARRY-142886) showed significant differential sensitivity in Ras pathway-mutated ALL compared with WT cells both in vitro and in an orthotopic xenograft model engrafted with primary ALL; in the latter, reduced RAS-mutated CNS leukemia. Given these data, clinical evaluation of selumetinib may be warranted for Ras pathway-mutated relapsed ALL. (Blood. 2014;124(23):3420-3430)

Inactivation of the tumor suppressor gene, CDKN2A, can occur by deletion, methylation, or mutation. We assessed the principal mode of inactivation in childhood acute lymphoblastic leukemia (ALL) and frequency in biologically relevant subgroups. Mutation or methylation was rare, whereas genomic deletion occurred in 21% of B-cell precursor ALL and 50% of T-ALL patients. Single nucleotide polymorphism arrays revealed copy number neutral (CNN) loss of heterozygosity (LOH) in 8% of patients. Array-based comparative genomic hybridization demonstrated that the mean size of deletions was 14.8 Mb and biallelic deletions composed a large and small deletion (mean sizes, 23.3 Mb and 1.4 Mb). Among 86 patients, only 2 small deletions were below the resolution of detection by fluorescence in situ hybridization. Patients with high hyperdiploidy, ETV6-RUNX1, or 11q23/MLL rearrangements had low rates of deletion (11%, 15%, 13%), whereas patients with t(9;22), t(1;19), TLX3, or TLX1 rearrangements had higher frequencies (61%, 42%, 78%, and 89%). In conclusion, CDKN2A deletion is a significant secondary abnormality in childhood ALL strongly correlated with phenotype and genotype. The variation in the incidence of CDKN2A deletions by cytogenetic subgroup may explain its inconsistent association with outcome. CNN LOH without apparent CDKN2A inactivation suggests the presence of other relevant genes in this region. (Blood. 2009;113:100-107) IntroductionGenetic alterations including chromosomal translocation, promoter hypermethylation, somatic mutation, and gene deletion are thought to play a key role in oncogenesis. Alterations of the 9p21 locus have been implicated in many types of cancer, indicating a role for the tumor suppressor genes CDKN2A (MTS1) and CDKN2B (MTS2), which encode for p16 INK4a /p14 ARF and p15 INK4b , respectively. 1 Loss of cell proliferation control and regulation of the cell cycle are known to be critical to cancer development. 2 Both p16 INK4a and p15 INK4b specifically inhibit cyclin/CDK-4/6 complexes that block cell division during the G 1 /S phase of the cell cycle. 3 It has been reported that CDKN2A and CDKN2B are frequently inactivated in various hematologic malignancies. 1,4 Loss of heterozygosity (LOH) of chromosome arm 9p, including the CDKN2A locus, is one of the most frequent genetic events in childhood acute lymphoblastic leukemia (ALL), suggesting inactivation of the second allele or, possibly, haploinsufficiency. [5][6][7][8] Haploinsufficiency of a tumor suppressor gene, eg, CDKN2A, has been shown to be adequate to promote tumor progression. [9][10][11] Homozygous deletion of CDKN2A has been suggested as the dominant mechanism of its inactivation in leukemogenesis. 12 However, the reported frequencies of both heterozygous and homozygous deletions in childhood ALL vary, 9% to 27% and 6% to 33% in B-cell precursor (BCP) respectively. 13 Similarly, the frequency of hypermethylation of the CDKN2A promoter has been reported to vary from 0% to 40% in childhood ALL. [14][15][16][17][18][19] Althou...