Oncogene (2015) 34, 2681–2689 © 2015 Macmillan Publishers Limited All rights reserved 0950-9232/15 www.nature.com/onc REVIEW PAX genes in childhood oncogenesis: developmental biology gone awry? P Mahajan1, PJ Leavey1 and RL Galindo1,2,3 Childhood solid tumors often arise from embryonal-like cells, which are distinct from the epithelial cancers observed in adults, and etiologically can be considered as ‘developmental patterning gone awry’. Paired-box (PAX) genes encode a family of evolutionarily conserved transcription factors that are important regulators of cell lineage specification, migration and tissue patterning. PAX loss-of-function mutations are well known to cause potent developmental phenotypes in animal models and underlie genetic disease in humans, whereas dysregulation and/or genetic modification of PAX genes have been shown to function as critical triggers for human tumorigenesis. Consequently, exploring PAX-related pathobiology generates insights into both normal developmental biology and key molecular mechanisms that underlie pediatric cancer, which are the topics of this review. Oncogene (2015) 34, 2681–2689; doi:10.1038/onc.2014.209; published online 21 July 2014 INTRODUCTION developmental mechanisms and PAX genes in medical (adult) The developmental mechanisms necessary to generate a fully oncology. patterned, complex organism from a nascent embryo are precise. Undifferentiated primordia undergo a vast array of cell lineage specification, migration and patterning, and differentiate into an STRUCTURAL MOTIFS DEFINE THE PAX FAMILY SUBGROUPS ensemble of interdependent connective, muscle, nervous and The mammalian PAX family of transcription factors is comprised of epithelial tissues. Dysregulation of these precise developmental nine members that function as ‘master regulators’ of organo- programs cause various diseases/disorders, including—and genesis4 (Figure 1). The structural motif that defines this unique relevant to this review—childhood cancer. family of molecules is the evolutionarily conserved paired domain Interestingly, the pathobiology of childhood cancer is different (PD). The PD, 128 amino acids in length, is a DNA-binding motif from adult neoplasia. Solid tumors in adults are most often that recognizes highly related DNA sequences (TCACGC/G; minor – epithelial in origin (for example, prostate and breast carcinoma), variability can be seen for each PAX ortholog).5 7 Seven of the whereas pediatric solid tumors are typically comprised nine PAX molecules, PAX2 through -8, additionally possess a of histologically primitive blastemal/embryonal-type cells (for second DNA-binding domain, a homeodomain (HD) motif 7,8 example, neuroblastoma, nephroblastoma, medulloblastoma (consensus binding sequence TAAT–ATTA), which adds func- and rhabdomyosarcoma (RMS)). Developmental dysregulation of tional complexity to these PAX orthologs, as the PD and HD can – 7,9 precursor cell maturation and terminal differentiation, or ‘devel- operate cooperatively (consensus sequence AATTA GTCACGC) opment gone awry,’ represents a seminal feature of childhood or independently of one another. Unlike the PD, the HD varies cancer. structurally among the HD-containing PAX proteins, and is used to In this review, we discuss in particular the misregulation of further organize the PAX orthologs into subfamilies (described in Paired-Box (PAX) gene-mediated developmental programs as more detail below). Finally, except for PAX4 and -6, PAX molecules important underpinnings of childhood cancer. The evolutionarily contain a conserved octapeptide motif (OP) positioned between the PD and HD. This eight-amino-acid element has been shown to conserved PAX transcription factors are master regulators of function as a transcriptional inhibitory motif evolutionarily related histogenesis and organogenesis. Spontaneous and targeted to the Drosophila-engrailed family eh1 repression domain and murine Pax gene mutations disrupt many aspects of tissue/organ Goosecoid protein, which behave as transcriptional repressors.10 patterning, as well as interfere with the maintenance of adult stem Thus, PAX molecules, depending on context and cofactors, can fi cells necessary for tissue-speci c repair/regeneration. Here we function either as transcriptional activators or repressors, under- profile how PAX genes normally direct precursor cell differentia- scoring even more so their experimentally problematic functional tion, whereas dysregulation and/or misexpression of PAX ortho- complexity. logs serve as critical participants in a broad spectrum of childhood Below, we discuss in more detail aspects of PAX-mediated malignancies (Table 1). development and pathobiology that pertain specifically to PAX- Of note, we refer interested readers to Li et al.,1 Lang et al.2 related pediatric malignancies, organized into sections corre- and Robson et al.,3 for more detailed reviews of PAX sponding to the four PAX family subgroups. 1Department of Pediatrics, University of Texas Southwestern Medical Center at Dallas, Dallas, TX, USA; 2Department of Pathology, University of Texas Southwestern Medical Center at Dallas, Dallas, TX, USA and 3Department of Molecular Biology, University of Texas Southwestern Medical Center at Dallas, Dallas, TX, USA. Correspondence: Dr Galindo, Department of Pathology, University of Texas Southwestern Medical Center at Dallas, 5323 Harry Hines Boulevard, Dallas, TX 75390-9072, USA. E-mail: [email protected] Received 3 April 2014; revised 10 June 2014; accepted 11 June 2014; published online 21 July 2014 PAX genes in childhood cancers P Mahajan et al 2682 Table 1. PAX genes in normal development and childhood cancers PAX Subgroup Normal Pediatric cancer genes development PAX3 III Skeletal muscle Rhabdomyosarcoma PAX7 development and regeneration PAX2 II Kidney Wilms tumor PAX5 Renal cell carcinoma PAX8 Thyroid Thyroid carcinoma B cell B-cell acute lymphoblastic leukemia Acute myeloblastic leukemia B-cell lymphomas T-lymphoblastic leukemia Medulloblastoma Neuroblastoma PAX4 IV Pancreas Retinoblastoma? PAX6 Eye PAX1 I Musculoskeletal Pediatric bone-related PAX9 malignancies? PAX-MEDIATED DEVELOPMENT, PATHOBIOLOGY AND ONCOGENICITY PAX3 and PAX7 are canonical proto-oncogenes PAX3 and PAX7, members of subgroup III, are structurally the most complex PAX orthologs, being the only two family members containing a PD, a three-helix-length HD and an OP motif. In addition to being master regulators of neural tube, neural crest and somatic muscle development, PAX3 and -7 are also the clearest examples of how PAX can function as proto-oncogenes and drive human tumorigenesis, as clearly illustrated in the context of the skeletal muscle-lineage soft tissue sarcoma, rhabdomyosarcoma (RMS). Figure 1. The mammalian family of PAX transcription factors. The PAX family of proteins is comprised of nine members, which are further classified into four distinct subgroups I–IV based on PAX3 and PAX7 in skeletal muscle development and regeneration. fi structural composition. All nine PAX proteins have a highly Mutation of the Pax3 locus was identi ed as the molecular basis for conserved PD. Group I includes PAX1 and 9; group II consists of the spontaneous-occurring semidominant lethal murine splotch PAX2, 5 and 8; group III includes PAX3 and 7; and PAX4 and 6 belong mutation.11 Splotch homozygous mice were initially characterized to group IV. PAX2, 3, 4, 5, 6, 7 and 8 contain a HD and all of the PAX for phenotypes related to neural tube- and neural crest-derived proteins, with the exception of PAX4 and 6, contain an OP. TAD, tissues. Further analysis showed that the splotch mouse also served transcription activation domain. as a model for skeletal muscle patterning (myogenesis). Develop- mental studies uncovered that Pax3 is expressed in the somites and monopotential stem cells, termed satellite cells, which are critically mesenchymal limb buds,12 and that splotch mice, although required for skeletal muscle physiologic growth, maintenance and 17,18 19 exhibiting a mild reduction in body wall muscle, showed a striking regeneration. Realix et al. further demonstrated that the loss failure to develop normal limb musculature.13 The observation that of satellite cells is progressive in Pax7 mutants, pointing toward an Pax3 mutation blocks the onset of myogenesis in a subset of antiapoptic role of Pax7 for which Pax3 is not compensatory. In somitic cells otherwise fated to become myoblasts (no expressions addition, Pax7 deletion in primary murine myoblasts and satellite 20 of the myogenic transcription factors Myogenin and MyoD,for cells resulted in cell cycle arrest and early differentiation, example, were detected in these precursors) clearly pointed to Pax3 highlighting that Pax7 is necessary for maintaining satellite cells as a critical upstream myogenesis determinant.14 The importance of in a stem cell-like state for ongoing muscle renewal. PAX3 in development, including myogenesis, is also seen in Of note, although numerous findings support the notion that humans: mutations in the PAX3 gene have been identified in Pax7 is indispensible for the proper establishment and function of patients with Waardenburg Syndrome, a rare autosomal dominant perinatal satellite cells, Cre/loxP lineage tracing and tamoxifen- disorder characterized by deafness, pigmentation anomalies and inducible conditional gene inactivation interestingly reveal that disruptions in myogenesis.15 loss of Pax7 in adult satellite cells does not compromise muscle 21 Similar to its ‘sibling’ Pax3, Pax7 is also