Pax3 target gene recognition occurs through distinct modes that are differentially affected by disease-associated mutations

The PAX Genes: Roles in Development, Cancer, and Other Diseases

Since their 1986 discovery in Drosophila, Paired box (PAX) genes have been shown to play major roles in the early development of the eye, muscle, skeleton, kidney, and other organs. Consistent with their roles as master regulators of tissue formation, the PAX family members are evolutionarily conserved, regulate large transcriptional networks, and in turn can be regulated by a variety of mechanisms. Losses or mutations in these genes can result in developmental disorders or cancers. The precise mechanisms by which PAX genes control disease pathogenesis are well understood in some cases, but much remains to be explored. A deeper understanding of the biology of these genes, therefore, has the potential to aid in the improvement of disease diagnosis and the development of new treatments.

Pax3 deficiency diminishes melanocytes in the developing mouse cochlea

Cochlear melanocytes are intermediate cells in the stria vascularis that generate endocochlear potentials required for auditory function. Human PAX3 mutations cause Waardenburg syndrome and abnormalities of melanocytes, manifested as congenital hearing loss and hypopigmentation of skin, hair and eyes. However, the underlying mechanism of hearing loss remains unclear. During development, cochlear melanocytes in the stria vascularis are dually derived from Pax3-Cre+ melanoblasts migrating from neuroepithelial cells including neural crest cells and Plp1+ Schwann cell precursors originated from also neural crest cells, differentiating in a basal-apical manner. Here, using a Pax3-Cre mouse line, we found that Pax3 deficiency causes foreshortened cochlea, malformed vestibular apparatus, and neural tube defects. Lineage tracing and in situ hybridization show that Pax3-Cre derivatives contribute to S100+ , Kir4.1+ and Dct+ melanocytes (intermediate cells) in the developing stria vascularis, all significantly diminished in Pax3 mutant animals. Taken together, these results suggest that Pax3 is required for the development of neural crest cell-derived cochlear melanocytes, whose absence may contribute to congenital hearing loss of Waardenburg syndrome in human.

Human TYRP1: Two functions for a single gene?

In the animal kingdom, skin pigmentation is highly variable between species, and it contributes to phenotypes. In humans, skin pigmentation plays a part in sun protection. Skin pigmentation depends on the ratio of the two pigments pheomelanin and eumelanin, both synthesized by a specialized cell population, the melanocytes. In this review, we explore one important factor in pigmentation: the tyrosinase-related protein 1 (TYRP1) gene which is involved in eumelanin synthesis via the TYRP1 protein. Counterintuitively, high TYRP1 mRNA expression is associated with a poor clinical outcome for patients with metastatic melanomas. Recently, we were able to explain this unexpected TYRP1 function by demonstrating that TYRP1 mRNA sequesters microRNA-16, a tumor suppressor miRNA. Here, we focus on actors influencing TYRP1 mRNA abundance, particularly transcription factors, single nucleotide polymorphisms (SNPs), and miRNAs, as they all dictate the indirect oncogenic activity of TYRP1.

Identification and functional analysis of a novel missense mutation of PAX3 associated with Waardenburg syndrome type I

PURPOSE

Waardenburg syndrome type 1 (WS1) is a rare genetic disorder characterized by dystopia canthorum, abnormal iris pigmentation, and congenital hearing loss with variable penetrance.WS1 is caused by mutations in paired box gene 3 (PAX3). The current study aimed to investigate the genetic cause of hearing loss in a four-generation Chinese WS1 family.

METHODS

The phenotype of the study family was characterized using clinical evaluation and pedigree analysis. Target region high-throughput sequencing system was designed to screen the all coding exons and flanking intronic sequences of the six WS-associated genes. Sanger sequencing was used to identify the causative nucleotide changes and perform the co-segregating analysis. The expression, subcellular distribution, and transcriptional activity of the mutant PAX3 protein were analysis to reveal the functional consequences of the mutation.

RESULTS

Based on diagnostic criteria, the proband of this pedigree classified as WS1. We identified a novel missense mutation (c.117 C > A, p. Asn39Lys) in exon 2 of the PAX3 gene. In vitro, the Asn39Lys PAX3 retained nuclear distribution ability. However, it failed to activate the melanocyte inducing transcription factor (MITF) promoter and impaired the function of WT PAX3.

CONCLUSIONS

Our study reports a novel missense PAX3 mutation in a Chinese family and shows haploinsufficiency may be the underlying mechanism for the WS1 phenotype.

PAX3 is a biomarker and prognostic factor in melanoma: Database mining

Paired box 3 (PAX3) is a transcription factor and critical regulator of pigment cell development during embryonic development. However, while there have been several studies on PAX3, its expression patterns and precise role remain to be clarified. The present study is an in-depth computational study of tumor-associated gene information, with specific emphasis on the expression of PAX3 in melanoma, using Oncomine along with an investigation of corresponding expression profiles in an array of cancer cell lines through Cancer Cell Line Encyclopedia analysis. Based on Kaplan-Meier analysis, the prognostic value of high PAX3 expression in tissues from patients with melanoma compared with normal tissues was assessed. PAX3 was more highly expressed in male patients with melanoma compared with female patients with melanoma. Using Oncomine and Coexpedia analysis, it was demonstrated that PAX3 expression was clearly associated with SRY-box 10 expression. The survival analysis results revealed that high PAX3 mRNA expression was associated with worse survival rates in patients with melanoma. These results suggested that PAX3 may be a biomarker and essential prognostic factor for melanoma, and provided an important theoretical basis for the development of melanoma treatments.

Melanoma biomolecules: independently identified but functionally intertwined

The majority of patients diagnosed with melanoma present with thin lesions and generally these patients have a good prognosis. However, 5% of patients with early melanoma (<1 mm thick) will have recurrence and die within 10 years, despite no evidence of local or metastatic spread at the time of diagnosis. Thus, there is a need for additional prognostic markers to help identify those patients that may be at risk of recurrent disease. Many studies and several meta-analyses have compared gene and protein expression in melanocytes, naevi, primary, and metastatic melanoma in an attempt to find informative prognostic markers for these patients. However, although a large number of putative biomarkers have been described, few of these molecules are informative when used in isolation. The best approach is likely to involve a combination of molecules. We believe one approach could be to analyze the expression of a group of interacting proteins that regulate different aspects of the metastatic pathway. This is because a primary lesion expressing proteins involved in multiple stages of metastasis may be more likely to lead to secondary disease than one that does not. This review focuses on five putative biomarkers – melanoma cell adhesion molecule (MCAM), galectin-3 (gal-3), matrix metalloproteinase 2 (MMP-2), chondroitin sulfate proteoglycan 4 (CSPG4), and paired box 3 (PAX3). The goal is to provide context around what is known about the contribution of these biomarkers to melanoma biology and metastasis. Although each of these molecules have been independently identified as likely biomarkers, it is clear from our analyses that each are closely linked with each other, with intertwined roles in melanoma biology.

Functional dissection of Pax3 in paraxial mesoderm development and myogenesis

The paired box transcription factor Pax3 is well-known as a major regulator of embryonic myogenesis. Before Pax3 expression becomes restricted to the dermomyotome, this transcription factor is also expressed in the developing somites. The role of Pax3 at this early stage is unclear, in particular because of the scarce frequency of Pax3-positive cells in the early mouse embryo. Inducible gene expression in embryonic stem cells (ESCs) represents an excellent tool to overcome this limitation, since it can provide large quantities of otherwise rare embryonic populations expressing a factor of interest. Here we used engineered mouse ESCs to perform a functional analysis of Pax3 with the aim to identify the molecular determinants involved in the early functions of this transcription factor. We find that Pax3 induction during embryoid body differentiation results in the upregulation of genes expressed in the presomitic and somitic mesoderm. Moreover, we show that paraxial mesoderm induced by transient expression of Pax3 is not irreversibly committed to myogenesis rather requires sustained Pax3 expression. Using a series of deletion mutants of Pax3, which differentially affect its transcriptional activity, we map protein domains necessary for induction of paraxial mesoderm and induction of the myogenic program. The paired, homeo-, and transcriptional activation domains were each required for both processes, however, the paired-c-terminal RED domain showed a paraxial mesoderm-specific activity that was dispensable for myogenesis. These findings demonstrate and provide mechanistic insight into an early role for Pax3 in the generation of paraxial mesoderm.

Pax3 and Zic1 drive induction and differentiation of multipotent, migratory, and functional neural crest in Xenopus embryos

Defining which key factors control commitment of an embryonic lineage among a myriad of candidates is a longstanding challenge in developmental biology and an essential prerequisite for developing stem cell-based therapies. Commitment implies that the induced cells not only express early lineage markers but further undergo an autonomous differentiation into the lineage. The embryonic neural crest generates a highly diverse array of derivatives, including melanocytes, neurons, glia, cartilage, mesenchyme, and bone. A complex gene regulatory network has recently classified genes involved in the many steps of neural crest induction, specification, migration, and differentiation. However, which factor or combination of factors is sufficient to trigger full commitment of this multipotent lineage remains unknown. Here, we show that, in contrast to other potential combinations of candidate factors, coactivating transcription factors Pax3 and Zic1 not only initiate neural crest specification from various early embryonic lineages in Xenopus and chicken embryos but also trigger full neural crest determination. These two factors are sufficient to drive migration and differentiation of several neural crest derivatives in minimal culture conditions in vitro or ectopic locations in vivo. After transplantation, the induced cells migrate to and integrate into normal neural crest craniofacial target territories, indicating an efficient spatial recognition in vivo. Thus, Pax3 and Zic1 cooperate and execute a transcriptional switch sufficient to activate full multipotent neural crest development and differentiation.

A phosphatidylinositol 3-kinase-Pax3 axis regulates Brn-2 expression in melanoma

Deregulation of transcription arising from mutations in key signaling pathways is a hallmark of cancer. In melanoma, the most aggressive and lethal form of skin cancer, the Brn-2 transcription factor (POU3F2) regulates proliferation and invasiveness and lies downstream from mitogen-activated protein kinase (MAPK) and Wnt/β-catenin, two melanoma-associated signaling pathways. In vivo Brn-2 represses expression of the microphthalmia-associated transcription factor, MITF, to drive cells to a more stem cell-like and invasive phenotype. Given the key role of Brn-2 in regulating melanoma biology, understanding the signaling pathways that drive Brn-2 expression is an important issue. Here, we show that inhibition of phosphatidylinositol 3-kinase (PI3K) signaling reduces invasiveness of melanoma cells in culture and strongly inhibits Brn-2 expression. Pax3, a transcription factor regulating melanocyte lineage-specific genes, directly binds and regulates the Brn-2 promoter, and Pax3 expression is also decreased upon PI3K inhibition. Collectively, our results highlight a crucial role for PI3K in regulating Brn-2 and Pax3 expression, reveal a mechanism by which PI3K can regulate invasiveness, and imply that PI3K signaling is a key determinant of melanoma subpopulation diversity. Together with our previous work, the results presented here now place Brn-2 downstream of three melanoma-associated signaling pathways.

Functional analysis of Waardenburg syndrome-associated PAX3 and SOX10 mutations: report of a dominant-negative SOX10 mutation in Waardenburg syndrome type II

Waardenburg syndrome (WS) is an auditory-pigmentary disorder resulting from melanocyte defects, with varying combinations of sensorineural hearing loss and abnormal pigmentation of the hair, skin, and inner ear. WS is classified into four subtypes (WS1-WS4) based on additional symptoms. PAX3 and SOX10 are two transcription factors that can activate the expression of microphthalmia-associated transcription factor (MITF), a critical transcription factor for melanocyte development. Mutations of PAX3 are associated with WS1 and WS3, while mutations of SOX10 cause WS2 and WS4. Recently, we identified some novel WS-associated mutations in PAX3 and SOX10 in a cohort of Chinese WS patients. Here, we further identified an E248fsX30 SOX10 mutation in a family of WS2. We analyzed the subcellular distribution, expression and in vitro activity of two PAX3 mutations (p.H80D, p.H186fsX5) and four SOX10 mutations (p.E248fsX30, p.G37fsX58, p.G38fsX69 and p.R43X). Except H80D PAX3, which retained partial activity, the other mutants were unable to activate MITF promoter. The H80D PAX3 and E248fsX30 SOX10 were localized in the nucleus as wild type (WT) proteins, whereas the other mutant proteins were distributed in both cytoplasm and nucleus. Furthermore, E248fsX30 SOX10 protein retained the DNA-binding activity and showed dominant-negative effect on WT SOX10. However, E248fsX30 SOX10 protein seems to decay faster than the WT one, which may underlie the mild WS2 phenotype caused by this mutation.