8929

PHOX2B

NBLST2|NBPhox|PMX2B;paired-like homeobox 2b;PHOX2B;paired-like homeobox 2b

PHOX2B homeodomain protein|neuroblastoma Phox|neuroblastoma paired-type homeobox protein|paired mesoderm homeobox 2b|paired mesoderm homeobox protein 2B

4p12

protein-coding

Neuroblastoma (NB) is a frequent pediatric tumor for which recurrent somatic rearrangements are known. Germline mutations of predisposing gene(s) are suspected on the basis of rare familial cases and the association of NB with other genetically determined congenital malformations of neural crest-derived cells--namely, Hirschsprung disease (HSCR) and/or congenital central hypoventilation syndrome (CCHS). We recently identified the paired-like homeobox2B (PHOX2B) gene as the major disease-causing gene in isolated and syndromic CCHS, which prompted us to regard it as a candidate gene in NB. Here, we report on germline mutations of PHOX2B in both a familial case of NB and a patient withthe HSCR-NB association. PHOX2B, therefore, stands as the first gene for which germline mutations predispose to NB.

Neuroblastoma (NB), a childhood malignancy affecting neural crest deriving cell lineages, is characterized by great clinical variability and histological heterogeneity. As NB usually occurs as sporadic form, molecular studies were mainly carried out on tumor samples and derived cell lines, leading to the identification of several somatic alterations. Although familial NB is rare, linkage data obtained from different families have provided evidence of linkage to markers mapping to different chromosomal regions, indicating a remarkable genetic heterogeneity of NB. The first evidence of germline mutations in NB pedigrees has been recently reported in the paired-like homeobox 2B (PHOX2B) gene, involved in the development of neural crest deriving cells. Nevertheless, as only a few NB families but not others have been shown to carry PHOX2B mutations, the role of this gene in NB predisposition has still to be clarified.On the basis of the current data, familial NB cannot be modeled as a Mendelian monogenic trait. Instead, an oligogenic mode of inheritance might explain the existence of different NB loci genetically interacting to cause and/or modify the disease-phenotype.

Neuroblastomas (NBs) are frequent solid tumors in childhood for which no specific genetic marker linked to their development has been identified to date. PHOX2b, which regulates the autonomic neuron development, has been associated with the development of autonomic diseases, and has been considered a potential candidategene for neural crest-derived tumors such as NB. To ascertain the role of the PHOX2b gene in NB development, we have sequenced the complete PHOX2b coding region in tumors from 69 patients with sporadic NB, while 130 blood donors served as negative controls and 9 NB cell lines as positive controls. We found a missense deletion in exon 3 in a cell line. A further silent mutation in exon 3 (c.870C>A) was observed in 3 tumors but in none of the controls. A new polymorphism in intron 1 (IVS1-114 G>A) was observed in 31 tumor samples (44.9%)and in 68 controls (52.3%). We did not find any conclusive association of the polymorphisms or mutations in PHOX2b with the development of NB, although the large confidence intervals neither substantiate nor exclude a role for this genein the tumor etiology.

Wilms tumor and neuroblastoma are childhood tumors of the kidney and undifferentiated neural crest cells, respectively. Both disorders are primarily sporadic, but familial Wilms tumor pedigrees and familial neuroblastoma pedigrees are each well recognized and account for approximately 1-3% of each tumor type. Families with Wilms tumor and neuroblastoma in the same, or related individuals,have not been reported. Here, we present nine families with two or more individuals with Wilms tumor and/or neuroblastoma. The affected individuals wereotherwise well, without syndromic features. Although this co-occurrence might bedue to chance in some families, the coexistence of two rare embryonal tumors in related individuals of multiple families suggests an underlying genetic susceptibility to both tumors. We undertook mutational analysis of the genes known to predispose to non-syndromic familial Wilms tumor (WT1) or neuroblastoma(PHOX2B, ALK) which excluded these as the underlying predisposition genes in thenine families. We also excluded epigenetic and copy-number abnormalities at 11p15 which are known to predispose to embryonal tumors including Wilms tumor and neuroblastoma. Overall, these data suggest that families with both Wilms tumor and neuroblastoma represent a previously unrecognized familial cancer syndrome in which the underlying predisposition gene(s) remain to be determined.

Neuroblastoma is a pediatric tumor that is thought to arise from autonomic precursors in the neural crest. mutations in the PHOX2B gene have been observed in familial and sporadic forms of neuroblastoma and represent the first defined genetic predisposition for neuroblastoma. Here, we address the mechanisms that may underlie this predisposition, comparing the function of wild-type and mutantPhox2b proteins ectopically expressed in proliferating, embryonic sympathetic neurons. Phox2b displays a strong antiproliferative effect, which is lost in allPhox2b neuroblastoma variants analyzed. In contrast, an increase in sympathetic neuron proliferation is elicited by Phox2b variants with mutations in the homeodomain when endogenous Phox2b levels are lowered by siRNA-mediated knockdown to mimic the situation of heterozygous PHOX2B mutations in neuroblastoma. The increased proliferation is blocked by Hand2 knockdown and the antiproliferative Phox2b effects are rescued by Hand2 overexpression, implying Hand2 in Phox2b-mediated proliferation control. A Phox2b variant with a nonsense mutationin the homeodomain elicits, in addition, a decreased expression of characteristic marker genes. Together, these results suggest that PHOX2B mutations predispose to neuroblastoma by increasing proliferation and promoting dedifferentiation of cells in the sympathoadrenergic lineage.

BACKGROUND: Neuroblastoma (NB) is a severe pediatric tumor originating from neural crest derivatives and accounting for 15% of childhood cancer mortality. The heterogeneous and complex genetic etiology has been confirmed with the identification of mutations in two genes, encoding for the receptor tyrosine kinase Anaplastic Lymphoma Kinase (ALK) and the transcription factor Paired-likeHomeobox 2B (PHOX2B), in a limited proportion of NB patients. Interestingly, these two genes are overexpressed in the great majority of primary NB samples and cell lines. These observations led us to test the hypothesis of a regulatory or functional relationship between ALK and PHOX2B underlying NB pathogenesis. METHODOLOGY/PRINCIPAL FINDINGS: Following this possibility, we first confirmed astriking correlation between the transcription levels of ALK, PHOX2B and its direct target PHOX2A in a panel of NB cell lines. Then, we manipulated their expression in NB cell lines by siRNA-mediated knock-down and forced over-expression of each gene under analysis. Surprisingly, PHOX2B- and PHOX2A-directed siRNAs efficiently downregulated each other as well as ALK gene and, consistently, the enhanced expression of PHOX2B in NB cells yielded an increment of ALK protein. We finally demonstrated that PHOX2B drives ALK gene transcription by directly binding its promoter, which therefore represents a novel PHOX2B target. CONCLUSIONS/SIGNIFICANCE: These findings provide a compelling explanation of the concurrent involvement of these two genes in NB pathogenesis and are going to foster a better understanding of molecular interactions at the base of the disease. Moreover, this work opens new perspectives for NBs refractory to conventional therapies that may benefit from the design of novel therapeutic RNAi-based approaches for multiple gene targets.