Drug resistance remains a major obstacle to successful cancer treatment. A database of drug-associated gene expression profiles was screened for molecules whose profile overlapped with a gene expression signature of glucocorticoid (GC) sensitivity/resistance in acute lymphoblastic leukemia (ALL) cells. The screen indicated that the mTOR inhibitor rapamycin profile matched the signature of GC sensitivity. We tested the hypothesis that rapamycin would induce GC sensitivity in lymphoid malignancy cells and found that it sensitized to GC-induced apoptosis via modulation of antiapoptotic MCL1. These data indicate that MCL1 is an important regulator of GC-induced apoptosis and that the combination of rapamycin and glucocorticoids has potential utility in lymphoid malignancies. Furthermore, this approach represents a strategy for identification of promising combination therapies for cancer.

Summary We examined the leukemic stem cell potential of blasts at different stages of maturation in childhood acute lymphoblastic leukemia. Human leukemic bone marrow was transplanted intrafemorally into NOD/scid mice. Cells sorted using the B precursor differentiation markers CD19, CD20 and CD34 were isolated from patient samples and engrafted mice before serial transplantation into primary or subsequent (up to quaternary) recipients. Surprisingly, blasts representative of all the different maturational stages were able to reconstitute and re-establish the complete leukemic phenotype in vivo. Sorted blast populations mirrored normal B precursor cells with transcription of a number of stage-appropriate genes. These observations have informed a model for leukemia-propagating stem cells in childhood ALL.

Acute lymphoblastic leukemia (ALL) in infants (< 1 year) is characterized by a poor prognosis and a high incidence of MLL translocations. Several studies demonstrated the unique gene expression profile associated with MLL-rearranged ALL, but generally small cohorts were analyzed as uniform patient groups regardless of the type of MLL translocation, whereas the analysis of translocationnegative infant ALL remained unacknowledged. Here we generated and analyzed primary infant ALL expression profiles (n ‫؍‬ 73) typified by translocations t(4;11), t(11;19), and t(9;11), or the absence of MLL translocations. Our data show that MLL germline infant ALL specifies a gene expression pattern that is different from both MLL-rearranged infant ALL and pediatric precursor B-ALL. Moreover, we demonstrate that, apart from a fundamental signature shared by all MLL-rearranged infant ALL samples, each type of MLL translocation is associated with a translocation-specific gene expression signature. Finally, we show the existence of

MLL-rearranged infant acute lymphoblastic leukemia (ALL) remains the most aggressive type of childhood leukemia, displaying a unique gene expression profile. Here we hypothesized that this characteristic gene expression signature may have been established by potentially reversible epigenetic modifications. To test this hypothesis, we used differential methylation hybridization to explore the DNA methylation patterns underlying MLLrearranged ALL in infants. The obtained results were correlated with gene expression data to confirm gene silencing as a result of promoter hypermethylation. Distinct promoter CpG island methylation patterns separated different genetic subtypes of MLL-rearranged ALL in infants. MLL translocations t(4;11) and t(11;19) characterized extensively hypermethylated leukemias, whereas t(9;11)-positive infant ALL and infant ALL carrying wildtype MLL genes epigenetically resembled normal bone marrow. Furthermore, the degree of promoter hypermethylation among infant ALL patients carrying t(4; 11) or t(11;19) appeared to influence relapse-free survival, with patients displaying accentuated methylation being at high relapse risk. Finally, we show that the demethylating agent zebularine reverses aberrant DNA methylation and effectively induces apoptosis in MLLrearranged ALL cells. Collectively these data suggest that aberrant DNA methylation occurs in the majority of MLLrearranged infant ALL cases and guides clinical outcome. Therefore, inhibition of aberrant DNA methylation may be an important novel therapeutic strategy for MLL-rearranged ALL in infants. (Blood. 2009;114:5490-5498) IntroductionAlthough long-term survival rates in childhood acute lymphoblastic leukemia (ALL) exceed 80%, 1 the survival chances of infants (Ͻ 1 year of age) still range between 20% and 50%. 2 Approximately 80% of infants with ALL carry chromosomal translocations involving the MLL (mixed lineage leukemia) gene, 3 fusing the N-terminal portion of the MLL gene to the C-terminal region of one of its translocation partner genes. The most frequent MLL translocations among infant ALL patients are t(4;11), t(11;19), and t(9;11), 2,4 giving rise to the fusion proteins MLL-AF4, MLL-ENL, and MLL-AF9. These chimeric MLL fusion proteins exhibit pronounced transforming capacities 5 and independently contribute to an unfavorable prognosis. 2,6 As a member of the trithorax gene family, MLL is involved in transcriptional regulation. 7 Therefore, structural alterations of this gene may be expected to affect its function, presumably leading to transcriptional deregulation. Not surprisingly, in recent gene expression profiling studies, 8,9 the authors characterized MLL-rearranged ALL as a unique type of leukemia that is genetically clearly separable from other ALL subtypes. Because epigenetic modifications affect gene expression patterns, 10 we hypothesized that the specific gene expression profiles associated with MLL-rearranged infant ALL may well be driven by epigenetic changes, which recently have been established to play important role...

Infant B-cell acute lymphoblastic leukemia (B-ALL) accounts for 10% of childhood ALL. The genetic hallmark of most infant B-ALL is chromosomal rearrangements of the mixed-lineage leukemia (MLL) gene. Despite improvement in the clinical management and survival (∼85-90%) of childhood B-ALL, the outcome of infants with MLL-rearranged (MLL-r) B-ALL remains dismal, with overall survival <35%. Among MLL-r infant B-ALL, t(4;11)+ patients harboring the fusion MLL-AF4 (MA4) display a particularly poor prognosis and a pro-B/mixed phenotype. Studies in monozygotic twins and archived blood spots have provided compelling evidence of a single cell of prenatal origin as the target for MA4 fusion, explaining the brief leukemia latency. Despite its aggressiveness and short latency, current progress on its etiology, pathogenesis, and cellular origin is limited as evidenced by the lack of mouse/human models recapitulating the disease phenotype/latency. We propose this is because infant cancer is from an etiologic and pathogenesis standpoint distinct from adult cancer and should be seen as a developmental disease. This is supported by whole-genome sequencing studies suggesting that opposite to the view of cancer as a “multiple-and-sequential-hit” model, t(4;11) alone might be sufficient to spawn leukemia. The stable genome of these patients suggests that, in infant developmental cancer, one “big-hit” might be sufficient for overt disease and supports a key contribution of epigenetics and a prenatal cell of origin during a critical developmental window of stem cell vulnerability in the leukemia pathogenesis. Here, we revisit the biology of t(4;11)+ infant B-ALL with an emphasis on its origin, genetics, and disease models.