Cooperating leukemogenic events in MLLrearranged (MLL-r) infant acute lymphoblastic leukemia (ALL) are largely unknown. We explored the role of promoter CpG island hypermethylation in the biology and therapeutic targeting of MLL-r infant ALL. The HELP (HpaII tiny fragment enrichment by ligation-mediated polymerase chain reaction [PCR]) assay was used to examine genome-wide methylation of a cohort of MLL-r infant leukemia samples (n ؍ 5), other common childhood ALLs (n ؍ 5), and normals (n ؍ 5). Unsupervised analysis showed tight clustering of samples into their known biologic groups, indicating large differences in methylation patterns. Global hypermethylation was seen in the MLL-r cohort compared with both the normals and the others, with ratios of significantly (P < .001) hypermethylated to hypomethylated CpGs of 1.7 and 2.9, respectively. A subset of 7 differentially hypermethylated genes was assayed by quantitative reverse-transcription (
IntroductionThe 5-year event-free survival of childhood acute lymphocytic leukemia (ALL) now exceeds 80%. 1 However, the outcome for infants with ALL is substantially worse. Despite intensified treatment, the event-free survival for children younger than 12 months with ALL is only 30% to 40%. 2 Because of this, distinct mechanisms of leukemogenesis need to be explored to develop a rational basis for the design of novel therapies in this disease.Infant ALL has long been known to be an outlier from other childhood lymphocytic leukemias. It is clinically distinct in presentation and is less sensitive to effective ALL chemotherapy. [3][4][5] In addition, although morphologically and histochemically lymphoblastic, it is immunophenotypically unique, as it usually lacks the early lymphocyte antigen CD10 and has a propensity to coexpress myeloid antigens including CD15 and CD65. 6 Finally, it is characterized by an extremely high incidence of reciprocal translocations involving the mixed-lineage leukemia (MLL, ALL1, HRX) gene. 7 In comparison with ALL in older children, where MLL rearrangements (MLL-r's) are demonstrated in only 8% of cases, 8 most studies demonstrate the rate in infants to be 70% to 80%, particularly for younger infants. 8,9 Considered together, although there are now more than 60 characterized fusion partners of MLL, 10 these findings have established MLL-r ALL to be a unique leukemia. This has been corroborated by gene expression profiling, which has shown MLL-r ALL to be easily differentiated from MLL wild-type (MLL-wt) ALL and acute myeloid leukemia (AML). [11][12][13][14] The origin of MLL-r leukemia is an area of active investigation but several well-established mechanisms of leukemogenesis in other leukemias have been ruled out as contributing to the disease in infants. For example, a recent large genome-wide, high-density, single nucleotide polymorphism study of childhood ALL demonstrated a striking paucity of copy number alterations in MLL-r cases in comparison with other leukemias. 15 In addition, although activating JAK mutations have been demo...