Leukaemic B-cell precursors from ALL patients have been previously reported to constitutively express functional receptors for interleukin 1 (Uckun et al, 1989), interleukin 2 (Touw et al, 1985), and interleukin 7 (Uckun et al, 1991). Therefore, constitutive activation of the genes for one or more of the cytokines may lead to an apoptosis-resistant phenotype as well as an autocrine stimulation of the proliferative activity of leukaemic B-cell precursors. IL-7 activates the anti-apoptotic transcription factors STAT1, STAT3 and STAT5 in B-cell precursors and promotes their proliferation (Uckun et al, 1991; Van der Plas et al, 1996). Forced overexpression of IL-7 in transgenic mice or administration of exogenous IL-7 leads to an increase of the B-cell precursor pool (Morrissey et al, 1991). Nakayama et al (2009) recently reported that constitutive activation of JAK3/STAT5 signalling pathway in B-cell precursors by autocrine IL-7 production in the absence of BLNK protein, a direct inhibitor of JAK3 that functions as a tumour suppressor, causes B-precursor leukaemia in mice. Inhibition of IL-7 receptor signalling or JAK3/STAT5 activity resulted in apoptosis of B-precursor ALL cells (Nakayama et al, 2009). Our current study provided unprecedented evidence that primary leukaemia cells from infant ALL patients express significantly higher levels of multiple genes for anti-apoptotic and mitogenic cytokines that mediate their biological effects through stimulation of the JAK-STAT signal transduction pathway including interleukin 1a, interleukin 1b, interleukin 2, and interleukin 7. We further showed that the JAK/STAT signalling pathway is constitutively active in CD10− infant ALL cells and treatment with the JAK3 inhibitor WHI-P131 or the pan-JAK inhibitor AG-490 triggers their apoptosis. These findings demonstrate that JAK3 is an attractive molecular target for disrupting the constitutively degulated anti-apoptotic STAT3 and STAT5 signalling pathway in infant ALL cells.
Our comparative analysis of the gene expression profiles of primary leukaemic cells from infants with ALL versus those taken from children with ALL provided evidence that remarkably different pathognomonic transcriptomes dominate the biology of infant versus paediatric high risk B-precursor ALL. In particular, we identified eight signature genes that best discriminated infant ALL from childhood ALL cells: S100A8, S100A9, RFC3, RFC4, PDGFA, ERBB3, and IL2. The known biological functions of the protein products of these differentially expressed genes prompt the hypothesis that they are likely contributors to the apoptosis-resistance and high proliferative activity of infant leukaemia cells. S100A8 [also termed myeloid related protein (MRP)8] and S100A9 (MRP14) exist mainly as a S100A8/S100A9 heterodimer (termed calprotectin), which binds to and acts as an endogenous activator of toll-like receptor 4 (TLR4) (Ehrchen et al, 2009). S100A8/S100A9 tetramers have been shown to promote tubulin polymerization and bundle microtubules leading to stabilization of tubulin filaments (Leukert et al, 2006). Increased co-expression of S100A8 and S100A9 proteins synergistically promote cell survival by exhibiting marked anti-apoptotic activity (Nemeth et al, 2009). Increased expression of S100A8 and S100A9 have been associated with steroid resistance (Holleman et al, 2004) and relapse in childhood ALL (Bhojwani et al, 2006). The markedly enhanced expression of S100A8 and S100A9 in infant ALL suggests that S100 proteins may play an important role in leukemogenesis and/or adverse clinical course of ALL during infancy. Likewise, the SWI/SNF chromatin remodelling complexes, that play a critical role in the regulation of gene expression during lymphopoiesis (Wang et al, 1996), have been implicated in carcinogenesis as well as leukemogenesis (Klochendler-Yeivin et al, 2002; Park et al, 2009; Nie et al, 2003). MLL gene may cause altered chromatin remodelling and transcription by licencing histone acetylases and SWI/SNF-like helicases (Redner et al, 1999), and MLL-R may cause abnormal derepression of MLL-regulated genes by aberrant recruitment of SWI/SNF complexes (Rozenblatt-Rosen et al, 1998). In the present study, infant leukaemia cells showed 3·2-fold higher expression levels for the SNF-like SMARCA2 gene than leukaemic cells from children with high risk ALL. Notably, four studies (Yeoh et al, 2002; Ross et al, 2003; Fine et al, 2004; Tsutsumi et al, 2003) showed over expression of SMARCA2 in MLL-R+ ALL cells. Replication factor C (RFC) plays an important role in exonuclease 1-independent mismatch repair, an important DNA repair pathway responsible for correction of DNA replication errors (Kadyrov et al, 2009) as well as resistance to apoptosis (Arai et al, 2009). Increased expression of RFC3 and RFC4 has been associated with early relapse in childhood ALL (Bhojwani et al, 2006). The observed high level expression of RFC3 and RFC4 in infant leukaemia cells is a likely contributor of their resistance to apoptosis-promoting agents and high incidence of early relapse in infant ALL. Leukaemia cell lines derived from B-precursor ALL patients have been shown to express PDGFA gene and secrete PDGF A-chain (Tsai et al, 1994). The significance of the high level PDGF gene expression in infant ALL cells that was documented in the present study is not known, but it prompts the hypothesis that inhibitors of the PDGF-MAPK pathway may affect the proliferation and survival of infant ALL cells. Future studies will aim at deciphering the molecular mechanisms of cooperation between the protein products of the differentially expressed infant ALL signature genes and other putative leukemogenic proteins. Further evaluation of these proteins as potential molecular targets may provide the basis for therapeutic innovation against infant ALL.