Paired-like homeodomain proteins Phox2a/Arix and Phox2b/NBPhox have similar genetic organization and independently regulate dopamine beta-hydroxylase gene transcription

Developmental disorders affecting the respiratory system: CCHS and ROHHAD

Rapid-onset Obesity with Hypothalamic dysfunction, Hypoventilation, and Autonomic Dysregulation (ROHHAD) and Congenital Central Hypoventilation Syndrome (CCHS) are ultra-rare distinct clinical disorders with overlapping symptoms including altered respiratory control and autonomic regulation. Although both disorders have been considered for decades to be on the same spectrum with necessity of artificial ventilation as life-support, recent acquisition of specific knowledge concerning the genetic basis of CCHS coupled with an elusive etiology for ROHHAD have definitely established that the two disorders are different. CCHS is an autosomal dominant neurocristopathy characterized by alveolar hypoventilation resulting in hypoxemia/hypercarbia and features of autonomic nervous system dysregulation (ANSD), with presentation typically in the newborn period. It is caused by paired-like homeobox 2B (PHOX2B) variants, with known genotype-phenotype correlation but pathogenic mechanism(s) are yet unknown. ROHHAD is characterized by rapid weight gain, followed by hypothalamic dysfunction, then hypoventilation followed by ANSD, in seemingly normal children ages 1.5-7 years. Postmortem neuroanatomical studies, thorough clinical characterization, pathophysiological assessment, and extensive genetic inquiry have failed to identify a cause attributable to a traditional genetic basis, somatic mosaicism, epigenetic mechanism, environmental trigger, or other. To find the key to the ROHHAD pathogenesis and to improve its clinical management, in the present chapter, we have carefully compared CCHS and ROHHAD.

Transcriptional Profiling of Early and Late Phases of Bovine Tuberculosis

Bovine tuberculosis, caused by Mycobacterium tuberculosis var. bovis (M. bovis), is an important enzootic disease affecting mainly cattle, worldwide. Despite the implementation of national campaigns to eliminate the disease, bovine tuberculosis remains recalcitrant to eradication in several countries. Characterizing the host response to M. bovis infection is crucial for understanding the immunopathogenesis of the disease and for developing better control strategies. To profile the host responses to M. bovis infection, we analyzed the transcriptome of whole blood cells collected from experimentally infected calves with a virulent strain of M. bovis using RNA transcriptome sequencing (RNAseq). Comparative analysis of calf transcriptomes at early (8 weeks) vs. late (20 weeks) aerosol infection with M. bovis revealed divergent and unique profile for each stage of infection. Notably, at the early time point, transcriptional upregulation was observed among several of the top-ranking canonical pathways involved in T-cell chemotaxis. At the late time point, enrichment in the cell mediated cytotoxicity (e.g. Granzyme B) was the predominant host response. These results showed significant change in bovine transcriptional profiles and identified networks of chemokine receptors and monocyte chemoattractant protein (CCL) co-regulated genes that underline the host-mycobacterial interactions during progression of bovine tuberculosis in cattle. Further analysis of the transcriptomic profiles identified potential biomarker targets for early and late phases of tuberculosis in cattle. Overall, the identified profiles better characterized identified novel immunomodulatory mechanisms and provided a list of targets for further development of potential diagnostics for tuberculosis in cattle.

Research Advances on Therapeutic Approaches to Congenital Central Hypoventilation Syndrome (CCHS)

Congenital central hypoventilation syndrome (CCHS) is a genetic disorder of neurodevelopment, with an autosomal dominant transmission, caused by heterozygous mutations in the PHOX2B gene. CCHS is a rare disorder characterized by hypoventilation due to the failure of autonomic control of breathing. Until now no curative treatment has been found. PHOX2B is a transcription factor that plays a crucial role in the development (and maintenance) of the autonomic nervous system, and in particular the neuronal structures involved in respiratory reflexes. The underlying pathogenetic mechanism is still unclear, although studies in vivo and in CCHS patients indicate that some neuronal structures may be damaged. Moreover, in vitro experimental data suggest that transcriptional dysregulation and protein misfolding may be key pathogenic mechanisms. This review summarizes latest researches that improved the comprehension of the molecular pathogenetic mechanisms responsible for CCHS and discusses the search for therapeutic intervention in light of the current knowledge about PHOX2B function.

A Novel c.676_677insG PHOX2B Mutation in Congenital Central Hypoventilation Syndrome

ABSTRACT

Paired-like homeobox (PHOX)2B is considered to be the causative gene of congenital central hypoventilation syndrome (CCHS), a dominant genetic disorder that results in abnormal central respiratory control with resulting hypoventilation during sleep. In this study, we report a novel c.676_677insG (p.Ala226fs) mutation in a patient with severe CCHS, and we evaluated the function of this mutation. The mutation reduced the translation of the mutant PHOX2B protein and impaired its ability to activate the PHOX2A promoter, due to a haploinsufficiency effect. The mutant PHOX2B was able to interact with wildtype PHOX2B, resulting in retention of PHOX2B on the nuclear membrane, which may impair the normal function of the nuclear membrane, and leading to cellular morbidity. Our study provides useful information for the functional studies of PHOX2B and understanding the pathogenesis of CCHS, and thus is beneficial for the prognosis of, genetic counseling for, and development of pharmaceuticals for PHOX2B-associated diseases.

Structural and functional differences in PHOX2B frameshift mutations underlie isolated or syndromic congenital central hypoventilation syndrome

Heterozygous mutations in the PHOX2B gene are causative of congenital central hypoventilation syndrome (CCHS), a neurocristopathy characterized by defective autonomic control of breathing due to the impaired differentiation of neural crest cells. Among PHOX2B mutations, polyalanine (polyAla) expansions are almost exclusively associated with isolated CCHS, whereas frameshift variants, although less frequent, are often more severe than polyAla expansions and identified in syndromic CCHS. This article provides a complete review of all the frameshift mutations identified in cases of isolated and syndromic CCHS reported in the literature as well as those identified by us and not yet published. These were considered in terms of both their structure, whether the underlying indels induced frameshifts of either 1 or 2 steps ("frame 2" and "frame 3" mutations respectively), and clinical associations. Furthermore, we evaluated the structural and functional effects of one "frame 3" mutation identified in a patient with isolated CCHS, and one "frame 2" mutation identified in a patient with syndromic CCHS, also affected with Hirschsprung's disease and neuroblastoma. The data thus obtained confirm that the type of translational frame affects the severity of the transcriptional dysfunction and the predisposition to isolated or syndromic CCHS.

Nonsense pathogenic variants in exon 1 of PHOX2B lead to translational reinitiation in congenital central hypoventilation syndrome

Pathogenic variants in PHOX2B lead to congenital central hypoventilation syndrome (CCHS), a rare disorder of the nervous system characterized by autonomic dysregulation and hypoventilation typically presenting in the neonatal period, although a milder late-onset (LO) presentation has been reported. More than 90% of cases are caused by polyalanine repeat mutations (PARMs) in the C-terminus of the protein; however non-polyalanine repeat mutations (NPARMs) have been reported. Most NPARMs are located in exon 3 of PHOX2B and result in a more severe clinical presentation including Hirschsprung disease (HSCR) and/or peripheral neuroblastic tumors (PNTs). A previously reported nonsense pathogenic variant in exon 1 of a patient with LO-CCHS and no HSCR or PNTs leads to translational reinitiation at a downstream AUG codon producing an N-terminally truncated protein. Here we report additional individuals with nonsense pathogenic variants in exon 1 of PHOX2B. In vitro analyses were used to determine if these and other reported nonsense variants in PHOX2B exon 1 produced N-terminally truncated proteins. We found that all tested nonsense variants in PHOX2B exon 1 produced a truncated protein of the same size. This truncated protein localized to the nucleus and transactivated a target promoter. These data suggest that nonsense pathogenic variants in the first exon of PHOX2B likely escape nonsense mediated decay (NMD) and produce N-terminally truncated proteins functionally distinct from those produced by the more common PARMs.

Alanine Expansions Associated with Congenital Central Hypoventilation Syndrome Impair PHOX2B Homeodomain-mediated Dimerization and Nuclear Import

Heterozygous mutations of the human PHOX2B gene, a key regulator of autonomic nervous system development, lead to congenital central hypoventilation syndrome (CCHS), a neurodevelopmental disorder characterized by a failure in the autonomic control of breathing. Polyalanine expansions in the 20-residues region of the C terminus of PHOX2B are the major mutations responsible for CCHS. Elongation of the alanine stretch in PHOX2B leads to a protein with altered DNA binding, transcriptional activity, and nuclear localization and the possible formation of cytoplasmic aggregates; furthermore, the findings of various studies support the idea that CCHS is not due to a pure loss of function mechanism but also involves a dominant negative effect and/or toxic gain of function for PHOX2B mutations. Because PHOX2B forms homodimers and heterodimers with its paralogue PHOX2A in vitro, we tested the hypothesis that the dominant negative effects of the mutated proteins are due to non-functional interactions with the wild-type protein or PHOX2A using a co-immunoprecipitation assay and the mammalian two-hybrid system. Our findings show that PHOX2B forms homodimers and heterodimerizes weakly with mutated proteins, exclude the direct involvement of the polyalanine tract in dimer formation, and indicate that mutated proteins retain partial ability to form heterodimers with PHOX2A. Moreover, in this study, we investigated the effects of the longest polyalanine expansions on the homeodomain-mediated nuclear import, and our data clearly show that the expanded C terminus interferes with this process. These results provide novel insights into the effects of the alanine tract expansion on PHOX2B folding and activity.

Key Role of Amino Acid Repeat Expansions in the Functional Diversification of Duplicated Transcription Factors

The high regulatory complexity of vertebrates has been related to two rounds of whole genome duplication (2R-WGD) that occurred before the divergence of the major vertebrate groups. Following these events, many developmental transcription factors (TFs) were retained in multiple copies and subsequently specialized in diverse functions, whereas others reverted to their singleton state. TFs are known to be generally rich in amino acid repeats or low-complexity regions (LCRs), such as polyalanine or polyglutamine runs, which can evolve rapidly and potentially influence the transcriptional activity of the protein. Here we test the hypothesis that LCRs have played a major role in the diversification of TF gene duplicates. We find that nearly half of the TF gene families originated during the 2R-WGD contains LCRs. The number of gene duplicates with LCRs is 155 out of 550 analyzed (28%), about twice as many as the number of single copy genes with LCRs (15 out of 115, 13%). In addition, duplicated TFs preferentially accumulate certain LCR types, the most prominent of which are alanine repeats. We experimentally test the role of alanine-rich LCRs in two different TF gene families, PHOX2A/PHOX2B and LHX2/LHX9. In both cases, the presence of the alanine-rich LCR in one of the copies (PHOX2B and LHX2) significantly increases the capacity of the TF to activate transcription. Taken together, the results provide strong evidence that LCRs are important driving forces of evolutionary change in duplicated genes.

Mutations that disrupt PHOXB interaction with the neuronal calcium sensor HPCAL1 impede cellular differentiation in neuroblastoma

Heterozygous germline mutations in PHOX2B, a transcriptional regulator of sympathetic neuronal differentiation, predispose to diseases of the sympathetic nervous system, including neuroblastoma and congenital central hypoventilation syndrome (CCHS). Although the PHOX2B variants in CCHS largely involve expansions of the second polyalanine repeat within the C-terminus of the protein, those associated with neuroblastic tumors are nearly always frameshift and truncation mutations. To test the hypothesis that the neuroblastoma-associated variants exert their effects through loss or gain of protein-protein interactions, we performed a large-scale yeast two-hybrid screen using both wild-type (WT) and six different mutant PHOX2B proteins against over 10 000 human genes. The neuronal calcium sensor protein HPCAL1 (VILIP-3) exhibited strong binding to WT PHOX2B and a CCHS-associated polyalanine expansion mutant but only weakly or not at all to neuroblastoma-associated frameshift and truncation variants. We demonstrate that both WT PHOX2B and the neuroblastoma-associated R100L missense and the CCHS-associated alanine expansion variants induce nuclear translocation of HPCAL1 in a Ca(2+)-independent manner, while the neuroblastoma-associated 676delG frameshift and K155X truncation mutants impair subcellular localization of HPCAL1, causing it to remain in the cytoplasm. HPCAL1 did not appreciably influence the ability of WT PHOX2B to transactivate the DBH promoter, nor did it alter the decreased transactivation potential of PHOX2B variants in 293T cells. Abrogation of the PHOX2B-HPCAL1 interaction by shRNA knockdown of HPCAL1 in neuroblastoma cells expressing PHOX2B led to impaired neurite outgrowth with transcriptional profiles indicative of inhibited sympathetic neuronal differentiation. Our results suggest that certain PHOX2B variants associated with neuroblastoma pathogenesis, because of their inability to bind to key interacting proteins such as HPCAL1, may predispose to this malignancy by impeding the differentiation of immature sympathetic neurons.

Transcriptional dysregulation and impairment of PHOX2B auto-regulatory mechanism induced by polyalanine expansion mutations associated with congenital central hypoventilation syndrome

The PHOX2B transcription factor plays a crucial role in autonomic nervous system development. In humans, heterozygous mutations of the PHOX2B gene lead to congenital central hypoventilation syndrome (CCHS), a rare disorder characterized by a broad variety of symptoms of autonomic nervous system dysfunction including inadequate control of breathing. The vast majority of patients with CCHS are heterozygous for a polyalanine repeat expansion mutation involving a polyalanine tract of twenty residues in the C-terminus of PHOX2B. Although several lines of evidence support a dominant-negative mechanism for PHOX2B mutations in CCHS, the molecular effects of PHOX2B mutant proteins on the transcriptional activity of the wild-type protein have not yet been elucidated. As one of the targets of PHOX2B is the PHOX2B gene itself, we tested the transcriptional activity of wild-type and mutant proteins on the PHOX2B gene promoter, and found that the transactivation ability of proteins with polyalanine expansions decreased as a function of the length of the expansion, whereas DNA binding was severely affected only in the case of the mutant with the longest polyalanine tract (+13 alanine). Co-transfection experiments using equimolar amounts of PHOX2B wild-type and mutant proteins in order to simulate a heterozygous state in vitro and four different PHOX2B target gene regulatory regions (PHOX2B, PHOX2A, DBH, TLX2) clearly showed that the polyalanine expanded proteins alter the transcriptional activity of wild-type protein in a promoter-specific manner, without any clear correlation with the length of the expansion. Moreover, although reduced transactivation may be caused by retention of the wild-type protein in the cytoplasm or in nuclear aggregates, this mechanism can only be partially responsible for the pathogenesis of CCHS because of the reduction in cytoplasmic and nuclear accumulation when the +13 alanine mutant is co-expressed with wild-type protein, and the fact that the shortest polyalanine expansions do not form visible cytoplasmic aggregates. Deletion of the C-terminal of PHOX2B leads to a protein that correctly localizes in the nucleus but impairs PHOX2B wild-type transcriptional activity, thus suggesting that protein mislocalization is not the only mechanism leading to CCHS. The results of this study provide novel in vitro experimental evidence of a transcriptional dominant-negative effect of PHOX2B polyalanine mutant proteins on wild-type protein on two different PHOX2B target genes.

Transcription factor Phox2 upregulates expression of norepinephrine transporter and dopamine β-hydroxylase in adult rat brains

Degeneration of the noradrenergic locus coeruleus (LC) in aging and neurodegenerative diseases is well documented. Slowing or reversing this effect may have therapeutic implications. Phox2a and Phox2b are homeodomain transcriptional factors that function as determinants of the noradrenergic phenotype during embryogenesis. In the present study, recombinant lentiviral eGFP-Phox2a and -Phox2b (vPhox2a and vPhox2b) were constructed to study the effects of Phox2a/2b over-expression on dopamine β-hydroxylase (DBH) and norepinephrine transporter (NET) levels in central noradrenergic neurons. Microinjection of vPhox2 into the LC of adult rats significantly increased Phox2 mRNA levels in the LC region. Over-expression of either Phox2a or Phox2b in the LC was paralleled by significant increases in mRNA and protein levels of DBH and NET in the LC. Similar increases in DBH and NET protein levels were observed in the hippocampus following vPhox2 microinjection. In the frontal cortex, only NET protein levels were significantly increased by vPhox2 microinjection. Over-expression of Phox2 genes resulted in a significant increase in BrdU-positive cells in the hippocampal dentate gyrus. The present study demonstrates an upregulatory effect of Phox2a and Phox2b on the expression of DBH and NET in noradrenergic neurons of rat brains, an effect not previously shown in adult animals. Phox2 genes may play an important role in maintaining the function of the noradrenergic neurons after birth, and regulation of Phox2 gene expression may have therapeutic utility in aging or disorders involving degeneration of noradrenergic neurons.

In vitro drug treatments reduce the deleterious effects of aggregates containing polyAla expanded PHOX2B proteins

Heterozygous in frame duplications of the PHOX2B gene, leading to polyalanine (polyAla) expansions ranging from +5 to +13 residues of a 20-alanine stretch, have been identified in the vast majority of patients affected with Congenital Central Hypoventilation Syndrome (CCHS), a rare neurocristopathy characterized by absence of adequate autonomic control of respiration with decreased sensitivity to hypoxia and hypercapnia. Ventilatory supports such as tracheostomy, nasal mask or diaphragm pacing represent the only options available for affected. We have already shown that the severity of the CCHS phenotype correlates with the length of polyAla expansions, ultimately leading to formation of toxic intracytoplasmic aggregates and impaired PHOX2B mediated transactivation of target gene promoters, such as DBH. At present, there is no specific treatment to reduce cell aggregates and to ameliorate patients' respiration. In this work, we have undertaken in vitro analyses aimed at assessing the effects of molecules on the cellular response to polyAla PHOX2B aggregates. In particular, we tested 17-AAG, ibuprofen, 4-PBA, curcumin, trehalose, congo red and chrysamine G for their ability to i) recover the nuclear localisation of polyAla expanded PHOX2B, ii) rescue of PHOX2B mediated transactivation of the DBH promoter, and iii) clearance of PHOX2B (+13 Ala) aggregates. Our data have suggested that 17-AAG and curcumin are effective in vitro in both rescuing the nuclear localization and transactivation activity of PHOX2B carrying the largest expansion of polyAla and promoting the clearance of aggregates of these mutant proteins inducing molecular mechanisms such as ubiquitin-proteasome (UPS), autophagy and heat shock protein (HSP) systems.

Cytokines inhibit norepinephrine transporter expression by decreasing Hand2

Functional noradrenergic transmission requires the coordinate expression of enzymes involved in norepinephrine (NE) synthesis, as well as the norepinephrine transporter (NET) which removes NE from the synapse. Inflammatory cytokines acting through gp130 can suppress the noradrenergic phenotype in sympathetic neurons. This occurs in a subset of sympathetic neurons during development and also occurs in adult neurons after injury. For example, cytokines suppress noradrenergic function in sympathetic neurons after axotomy and during heart failure. The molecular basis for suppression of noradrenergic genes is not well understood, but previous studies implicated a reduction of Phox2a in cytokine suppression of dopamine beta hydroxylase. We used sympathetic neurons and neuroblastoma cells to investigate the role of Phox2a in cytokine suppression of NET transcription. Chromatin immunoprecipitation experiments revealed that Phox2a did not bind the NET promoter, and overexpression of Phox2a did not prevent cytokine suppression of NET transcription. Hand2 and Gata3 are transcription factors that induce noradrenergic genes during development and are present in mature sympathetic neurons. Both Hand2 and Gata3 were decreased by cytokines in sympathetic neurons and neuroblastoma cells. Overexpression of either Hand2 or Gata3 was sufficient to rescue NET transcription following suppression by cytokines. We examined expression of these genes following axotomy to determine if their expression was altered following nerve injury. NET and Hand2 mRNAs decreased significantly in sympathetic neurons 48 h after axotomy, but Gata3 mRNA was unchanged. These data suggest that cytokines can inhibit NET expression through downregulation of Hand2 or Gata3 in cultured sympathetic neurons, but axotomy in adult animals selectively suppresses Hand2 expression.

PHOX2B-mediated regulation of ALK expression: in vitro identification of a functional relationship between two genes involved in neuroblastoma

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-like Homeobox 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 a striking 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.

Neuroblastoma phox2b variants stimulate proliferation and dedifferentiation of immature sympathetic neurons

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 mutant Phox2b proteins ectopically expressed in proliferating, embryonic sympathetic neurons. Phox2b displays a strong antiproliferative effect, which is lost in all Phox2b 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 mutation in 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.

Effects of transcription factors Phox2 on expression of norepinephrine transporter and dopamine beta-hydroxylase in SK-N-BE(2)C cells

Phox2a and Phox2b are two homeodomain proteins that control the differentiation of noradrenergic neurons during embryogenesis. In the present study, we examined the possible effect of Phox2a/2b on the in vitro expression of the norepinephrine transporter (NET) and dopamine beta-hydroxylase (DBH), two important markers of the noradrenergic system. SK-N-BE(2)C cells were transfected with cDNAs or short hairpin RNAs specific to the human Phox2a and Phox2b genes. Transfection of 0.1 to 5 mug of cDNAs of Phox2a or Phox2b significantly increased mRNA and protein levels of NET and DBH in a concentration-dependent manner. As a consequence of the enhanced expression of NET after transfection, there was a parallel increase in the uptake of [(3)H]norepinephrine. Co-transfection of Phox2a and Phox2b did not further increase the expression of noradrenergic markers when compared with transfection of either Phox2a or Phox2b alone. Transfection of shRNAs specific to Phox2a or Phox2b genes significantly reduced mRNA and protein levels of NET and DBH after shutdown of endogenous Phox2, which was accompanied by a decreased [(3)H]norepinephrine uptake. Furthermore, there was an additive effect after cotransfection with both shRNAs specific to Phox2a or Phox2b genes on NET mRNA levels. Finally, the reduced DBH expression caused by the shRNA specific to Phox2a could be reversed by transfection with Phox2b cDNA and vice versa. The present findings verify the determinant role of Phox2a and Phox2b on the expression and function of NET and DBH in vitro. Further clarifying the regulatory role of these two transcription factors on key proteins of the noradrenergic system may open a new avenue for therapeutics of aging-caused dysfunction of the noradrenergic system.

Congenital central hypoventilation syndrome from past to future: model for translational and transitional autonomic medicine

The modern story of CCHS began in 1970 with the first description by Mellins et al., came most visibly to the public eye with the ATS Statement in 1999, and continues with increasingly fast paced advances in genetics. Affected individuals have diffuse autonomic nervous system dysregulation (ANSD). The paired-like homeobox gene PHOX2B is the disease-defining gene for CCHS; a mutation in the PHOX2B gene is requisite to the diagnosis of CCHS. Approximately 90% of individuals with the CCHS phenotype will be heterozygous for a polyalanine repeat expansion mutation (PARM); the normal allele will have 20 alanines and the affected allele will have 24-33 alanines (genotypes 20/24-20/33). The remaining approximately 10% of individuals with CCHS will have a non-PARM (NPARM), in the PHOX2B gene; these will be missense, nonsense, or frameshift. CCHS and PHOX2B are inherited in an autosomal dominant manner with a stable mutation. Approximately 8% of parents of a CCHS proband will be mosaic for the PHOX2B mutation. A growing number of cases of CCHS are identified after the newborn period, with presentation from infancy into adulthood. An improved understanding of the molecular basis of the PHOX2B mutations and of the PHOX2B genotype/CCHS phenotype relationship will allow physicians to anticipate the clinical phenotype for each affected individual. To best convey the remarkable history of CCHS, and to describe the value of recognizing CCHS as a model for translational and transitional autonomic medicine, we present this review article in the format of a chronological story, from 1970 to the present day.

A 2-year old with no ventilator requirement but who cannot be extubated

A 2-year-old boy was intubated during treatment for pneumonia. After resolution of the infection, he had no pulmonary requirement for ventilation and could function without it while awake. When he slept, however, he would have decreasing respiratory effort, increasing hypercapnia, and episodic apnea. This report provides an example of late-onset congenital central hypoventilation syndrome.

Trim11 increases expression of dopamine beta-hydroxylase gene by interacting with Phox2b

The homeodomain transcription factor Phox2b is one of the key determinants involved in the development of noradrenergic (NA) neurons in both the central nervous system (CNS) and the peripheral nervous system (PNS). Using yeast two-hybrid screening, we isolated a Phox2b interacting protein, Trim11, which belongs to TRIM (Tripartite motif) or RBCC proteins family, and contains a RING domain, B-boxes, a coiled-coil domain, and the B30.2/SPRY domain. Protein-protein interaction assays showed that Phox2b was able to physically interact with Trim11. The B30.2/SPRY domain of Trim11 was required for the interaction with Phox2b. Expression of Phox2b and Trim11 was detected in the sympathetic ganglia (SG) of mouse embryos. Forced expression of Trim11 with Phox2b further increased mRNA levels of dopamine beta-hydroxylase (DBH) gene in primary avian neural crest stem cell (NCSC) culture. This study suggests a potential role for Trim11 in the specification of NA phenotype by interaction with Phox2b.

Transcriptional regulation of TLX2 and impaired intestinal innervation: possible role of the PHOX2A and PHOX2B genes

TLX2 (also known as HOX11L1, Ncx and Enx) is a transcription factor playing a crucial role in the development of the enteric nervous system, as confirmed by mice models exhibiting intestinal hyperganglionosis and pseudo-obstruction. However, congenital defects of TLX2 have been excluded as a major cause of intestinal motility disorders in patients affected with intestinal neuronal dysplasia (IND) or pseudo-obstruction. After demonstrating the direct regulation of TLX2 expression by the homeoprotein PHOX2B, in the present work, we have focused on its paralogue PHOX2A. By co-transfections, electrophoretic mobility shift assays and chromatin immunoprecipitation, we have demonstrated that PHOX2A, like PHOX2B, is involved in the cascade leading to TLX2 transactivation and presumably in the intestinal neuronal differentiation. Based on the hypothesis that missed activation of the TLX2 gene induces the development of enteric nervous system defects, PHOX2A and PHOX2B have been regarded as novel candidate genes involved in IND and pseudo-obstruction and consequently analyzed for mutations in a specific set of 26 patients. We have identified one still unreported PHOX2A variant; however, absence of any functional effect on TLX2 transactivation suggests that regulators or effectors other than the PHOX2 genes must act in the same pathway, likely playing a non redundant and direct role in the pathogenesis of such enteric disorders.

Geldanamycin promotes nuclear localisation and clearance of PHOX2B misfolded proteins containing polyalanine expansions

Polyalanine expansions in the PHOX2B gene have been detected in the vast majority of patients affected with congenital central hypoventilation syndrome, a neurocristopathy characterized by absence of adequate control of breathing, especially during sleep, with decreased sensitivity to hypoxia and hypercapnia. The correlation between length of the alanine expanded tracts and severity of congenital central hypoventilation syndrome respiratory phenotype has been confirmed by length-dependent cytoplasmic PHOX2B retention with formation of aggregates. To deepen into the molecular mechanisms mediating the effects of PHOX2B polyalanine expansions, we have set up experiments aimed at assessing the fate of cells characterized by PHOX2B polyalanine aggregates. In particular, we have observed that activation of the heat shock response by the drug geldanamycin is efficient both in preventing formation and in inducing clearance of PHOX2B pre-formed polyalanine aggregates in COS-7 cells expressing PHOX2B-GFP fused proteins, and ultimately also in rescuing the PHOX2B ability to transactivate the Dopamine-beta-Hydroxilase promoter. In addition, we have demonstrated elimination of PHOX2B mutant proteins by the proteasome and autophagy, two cellular mechanisms already been involved in the clearance of proteins containing expanded polyglutamine and polyalanine tracts. Moreover, our data suggest that geldanamycin effects on PHOX2B aggregates may be also mediated by the proteasome pathway. Finally, analysis of cellular toxicity due to polyalanine aggregates has confirmed the occurrence of cell apoptosis consequent to expression of PHOX2B carrying the longest expanded alanine tract and shown that geldanamycin can delay cell progression toward the most advanced apoptotic stages.

Forced expression of Phox2 homeodomain transcription factors induces a branchio-visceromotor axonal phenotype

What causes motor neurons to project into the periphery is not well understood. We here show that forced expression of the homeodomain protein Phox2b, shown previously to be necessary and sufficient for branchio-visceromotor neuron development, and of its paralogue Phox2a imposes a branchiomotor-like axonal phenotype in the spinal cord. Many Phox2-transfected neurons, whose axons would normally stay within the confines of the neural tube, now project into the periphery. Once outside the neural tube, a fraction of the ectopic axons join the spinal accessory nerve, a branchiomotor nerve which, as shown here, does not develop in the absence of Phox2b. Explant studies show that the axons of Phox2-transfected neurons need attractive cues to leave the neural tube and that their outgrowth is promoted by tissues, to which branchio-visceromotor fibers normally grow. Hence, Phox2 expression is a key step in determining the peripheral axonal phenotype and thus the decision to stay within the neural tube or to project out of it.

Phox2 and dHAND transcription factors select shared and unique target genes in the noradrenergic cell type

The noradrenergic cell type is characterized by the expression of proteins involved in the biosynthesis, transport, and secretion of noradrenaline and is dependent on the sequential and combinatorial expression of numerous transcription factors, including Phox2a, Phox2b, dHAND, GATA2, GATA3, and MASH1. Phox2a and Phox2b transactivate the promoter of the gene encoding the noradrenergic biosynthetic enzyme, dopamine beta-hydroxylase (DBH), and dHAND potentiates the activity of Phox2a. In this study, we use chromatin immunoprecipitation assays to identify target genes of the Phox2 proteins and dHAND. All three proteins are bound to the DBH and PHOX2B promoter regions in SH-SY5Y neuroblastoma cells. The interaction between Phox2a and dHAND is analyzed by fluorescent anisotropy, which demonstrates that dHAND causes an eightfold increase in the affinity of Phox2a for its recognition sites on the DBH promoter region. The Phox2 proteins are not found on the genes encoding other noradrenergic enzymatic or transport proteins but are reciprocally bound to each other's promoters in SH-SY5Y cells. Together with Phox2a and Phox2b, dHAND is bound to the PHOX2B promoter and is also associated with the GATA2 and eHAND genes in the absence of the Phox2 proteins. These results demonstrate the direct interactions of the Phox2 and dHAND transcription factors within a noradrenergic cell type. The Phox2 proteins were found to share all target genes, whereas dHAND binds to genes independently of Phox2a.

Reciprocal gene replacements reveal unique functions for Phox2 genes during neural differentiation

The paralogous paired-like homeobox genes Phox2a and Phox2b are involved in the development of specific neural subtypes in the central and peripheral nervous systems. The different phenotypes of Phox2 knockout mutants, together with their asynchronous onset of expression, prompted us to generate two knock-in mutant mice, in which Phox2a is replaced by the Phox2b coding sequence, and vice versa. Our results indicate that Phox2a and Phox2b are not functionally equivalent, as only Phox2b can fulfill the role of Phox2a in the structures that depend on both genes. Furthermore, we demonstrate unique roles of Phox2 genes in the differentiation of specific motor neurons. Whereas the oculomotor and the trochlear neurons require Phox2a for their proper development, the migration of the facial branchiomotor neurons depends on Phox2b. Therefore, our analysis strongly indicates that biochemical differences between the proteins rather than temporal regulation of their expression account for the specific function of each paralogue.

Germline mutations of the paired-like homeobox 2B (PHOX2B) gene in neuroblastoma

Hereditary predisposition to neuroblastoma accounts for less than 5% of neuroblastomas and is probably heterogeneous. Recently, a predisposition gene has been mapped to 16p12-p13, but has not yet been identified. Occurrence of neuroblastoma in association with congenital central hypoventilation and Hirschsprung's disease suggests that genes, involved in the development of neural-crest-derived cells, may be altered in these conditions. The recent identification of PHOX2B as the major disease-causing gene in congenital central hypoventilation prompted us to test it as a candidate gene in familial neuroblastoma. We report a family with three first-degree relatives with neuroblastic tumours (namely two ganglioneuromas and one neuroblastoma) in one branch and two siblings with Hirschsprung's disease in another branch. A constitutional R100L PHOX2B mutation was identified in all three patients affected with tumours. We also report a germline PHOX2B mutation in one patient treated for Hirschsprung's disease who subsequently developed a multifocal neuroblastoma in infancy. Both mutations disrupt the homeodomain of the PHOX2B protein. No loss of heterozygosity at the PHOX2B locus was observed in the tumour, suggesting that haplo-insufficiency, gain of function or dominant negative effects may account for the oncogenic effects of these mutations. These observations identify PHOX2B as the first predisposing gene to hereditary neuroblastic tumours.

ERK1/2 is a negative regulator of homeodomain protein Arix/Phox2a

The homeodomain protein Arix/Phox2a plays a role in the development and maintenance of the noradrenergic cell type by regulating the transcription of genes involved in the biosynthesis and metabolism of noradrenaline. Previous work has shown that Arix/Phox2a is a phosphoprotein, and the phosphorylated form of Arix/Phox2a exhibits poorer DNA-binding activity than does the dephosphorylated form. Here, we demonstrate that Arix/Phox2a is phosphorylated by extracellular signal-related kinase (ERK)1/2 at two sites within the N-terminal transactivation domain. The phosphorylation level of Arix in cultured SH-SY5Y neuroblastoma cells is reduced when cells are treated with the mitogen activated protein kinase kinase 1 (MEK1) inhibitor UO126. Treatment of sympathetic neurons with the MEK1 inhibitor, PD98059, results in an elevation of mRNAs encoding noradrenergic proteins, dopamine beta-hydroxylase (DBH) and norepinephrine transporter (NET), but not tyrosine hydroyxlase (TH). Treatment of neuroblastoma cultures with PD98059 increases the interaction of Arix with DBH and NET genes, but not the TH gene. Together, these results suggest that phosphorylation of Arix by ERK1/2 inhibits its ability to interact with target genes, and that both specificity of expression and modulation by external stimuli are monitored through the same transcription factor.

A segment of the Mecp2 promoter is sufficient to drive expression in neurons

Rett syndrome (RTT) is caused by mutations in the gene encoding methyl CpG-binding protein 2 (MeCP2). Although MeCP2 shows widespread expression in both neuronal and non-neuronal tissues, the symptoms of RTT are largely neurological. Herein, we have identified the regulatory region of the mouse Mecp2 gene that is sufficient for its restricted expression in neurons. A segment of the Mecp2 gene (-677/+56) exhibited strong promoter activity in neuronal cell lines and cortical neurons, but was inactive in non-neuronal cells and glia. The region necessary for neuronal-specific promoter activity was located within a 19 bp region (-63/-45). Several nuclear factors were found to bind to this region and some of these factors were enriched in nuclear extracts prepared from the brain. To examine the activity of the Mecp2 promoter in vivo, we generated transgenic mice expressing the LacZ reporter driven by the -677/+56 region of the Mecp2 gene. The transgene was expressed in the mesencephalon as early as embryonic day 10 and in the hindbrain and spinal cord by E12. Interestingly, a marked induction of transgene expression was observed postnatally throughout the brain, similar to that of endogenous MeCP2. However, expression of the transgene was absent in non-neuronal tissues that are known to express Mecp2. Taken together, these data indicate that the -677/+56 region of the Mecp2 promoter partially recapitulates the native expression pattern of the Mecp2 gene, which possesses restricted expression in neurons of the central nervous system.

Molecular consequences of PHOX2B missense, frameshift and alanine expansion mutations leading to autonomic dysfunction

Heterozygous mutations of the PHOX2B gene account for a broad variety of disorders of the autonomic nervous system, either isolated or combined, including congenital central hypoventilation syndrome (CCHS), tumours of the sympathetic nervous system and Hirschsprung disease. In CCHS, the prevalent mutation is an expansion of a 20-alanine stretch ranging from +5 to +13 alanines, whereas frameshift and missense mutations are found occasionally. To determine the molecular basis of impaired PHOX2B function, we assayed the transactivation and DNA binding properties of wild-type and mutant PHOX2B proteins. Furthermore, we investigated aggregate formation by proteins with polyalanine tract expansions ranging from +5 to +13 alanines using immunofluorescence of transfected cells and gel filtration of in vitro translated proteins. We found that transactivation of the dopamine beta-hydroxylase promoter by PHOX2B proteins with frameshift and missense mutations was abolished or severely curtailed, as was in vitro DNA binding although the proteins localized to the nucleus. The transactivation potential of proteins with polyalanine tract expansions declined with increasing length of the polyalanine stretch, and DNA binding was affected for an expansion of +9 alanines and above. Cytoplasmic aggregation in transfected cells was only observed for the longest expansions, whereas even the short expansion mutants were prone to form multimers in vitro. Such a tendency to protein misfolding could explain loss of transactivation for alanine expansion mutations. However, additional mechanisms such as toxic gain-of-function may play a role in the pathogenic process.

Distinct pathogenetic mechanisms for PHOX2B associated polyalanine expansions and frameshift mutations in congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) is a rare neurocristopathy characterized by absence of adequate autonomic control of respiration with decreased sensitivity to hypoxia and hypercapnia. Frameshift mutations and polyalanine triplet expansions in the coding region of PHOX2B have been identified in the vast majority of CCHS patients and a correlation between length of the expanded region and severity of CCHS has been reported. In this work, we have undertaken in vitro analyses aimed at identifying the pathogenetic mechanisms which underlie the effects of PHOX2B mutations in CCHS. According to the known role of this gene, a transcription factor expressed during autonomic nervous system development, we have tested the transcriptional activity of WT and mutant PHOX2B expression constructs on the regulatory regions of two target genes, DbetaH and PHOX2A. We observed that the two sets of mutations play different roles in the transcriptional regulation of these genes, showing a correlation between the length of polyalanine expansions and the severity of reduced transcriptional activity. In particular, although reduced transactivation due to polyalanine expansions may be caused by retention of the mutated protein in the cytoplasm or in the nuclear aggregates, frameshift mutations did not impair the PHOX2B nuclear income, suggesting a different mechanism through which they would exert the observed effects on target promoters. Moreover, the frameshift due to deletion of a cytosine residue seems to cause sequestration of the corresponding mutant PHOX2B in the nucleolar compartment.

Molecular cloning and characterization of the promoter region of the human Phox2b gene

The closely related homeodomain transcription factors, Phox2a and Phox2b, are restrictively expressed in central and peripheral noradrenergic (NA) neurons in an overlapping but distinct manner, and critically regulate the differentiation and neurotransmitter identity of NA neurons. The structure and function of the human Phox2a (hPhox2a) promoter has recently been reported. Towards the long-term goal of delineating the regulatory cascade of NA neuron differentiation, we isolated a human Phox2b (hPhox2b) genomic clone encompassing approximately 7.8 kb of the 5' upstream promoter region, the entire exon-intron structure and 4.5 kb of the 3' flanking region. Two transcription start sites are identified to reside 115 and 110 nucleotides upstream of the start codon, based on both primer extension and 5'-rapid amplification of the cDNA ends analyses. In addition, transient transfection assays indicate that 1.1 kb or longer upstream sequences of the hPhox2b gene may confer cell type-specific gene expression in certain, but not all cell lines. The promoter activity of the hPhox2b gene is modestly transactivated by forced co-expression of Phox2b and the hPhox2b gene promoter contains a high-affinity binding site at -320 to -295 bp. This study provides a frame to further elucidate the molecular mechanisms underlying the regulation of Phox2a and Phox2b gene expression and its relation to NA differentiation.

ARIX gene polymorphisms in patients with congenital superior oblique muscle palsy

AIM

To identify ARIX gene polymorphisms in patients with congenital superior oblique muscle palsy and to find the relation between the ARIX gene and congenital superior oblique muscle palsy.

METHODS

The three exons of the ARIX gene were sequenced by genomic DNA amplification with polymerase chain reaction (PCR) and direct sequencing in 15 patients with superior oblique muscle palsy (13 with congenital and two with acquired palsy) and 54 normal individuals. PCR products cloned into plasmids were also sequenced. A family with father and a daughter each having congenital superior oblique muscle palsy was also involved in this study.

RESULTS

Four patients with congenital superior oblique muscle palsy carried heterozygous nucleotide changes in the ARIX gene. One patient with the absence of the superior oblique muscle had T7C in the 5'-UTR of the exon 1 and C-44A in the promoter region, both of which were located on the same strand. Another unrelated patient with congenital superior oblique muscle palsy had C76G in the 5'-UTR of the exon 1 and C-9A in the promoter region on the same strand. G153A in the 5'-UTR of exon 1 was found in common in two affected members of a family with congenital superior oblique muscle palsy. This G153A in the 5'-UTR of exon 1 was also present in four unrelated normal individuals. No other heterozygous nucleotide changes were found in normal individuals.

CONCLUSIONS

The nucleotide change (G153A) in the 5'-UTR of exon 1 co-segregated with congenital superior oblique muscle palsy in one family. Four other nucleotide changes in the exon 1 or the promoter region were found only in patients with congenital superior oblique muscle palsy. These nucleotide polymorphisms may be one of the risk factors for the development of congenital superior oblique muscle palsy.

The interaction between dHAND and Arix at the dopamine beta-hydroxylase promoter region is independent of direct dHAND binding to DNA

Dopamine beta-hydroxylase (DBH) catalyzes the production of norepinephrine, and its expression defines the noradrenergic phenotype. Transcription factors dHAND, a basic helix-loop-helix protein, and Arix/Phox2a, a homeoprotein, have been demonstrated to play a role in the differentiation and maintenance of catecholaminergic neurons. Three Arix regulatory sites have been identified in the DBH promoter proximal region, but there is no such evidence for dHAND. Cotransfection with a DBH promoter-luciferase reporter construct plus dHAND or dHAND-E12 expression plasmids did not alter luciferase activity, whereas transfection with Arix resulted in a 2.5-fold stimulation of luciferase activity. However, a 5.5-fold increase was observed when Arix and dHAND were combined, and an 8-fold level of expression was observed when Arix was transfected with a dHAND mutant lacking the basic DNA-binding domain. When the homeodomain sites in the DBH promoter proximal region were mutated, all activity was lost, demonstrating dependence upon Arix-DNA interaction for transcriptional activation. In electrophoretic mobility shift assays, the addition of dHAND decreased the amount of Arix needed to elicit a mobility shift with the DBH homeodomain sites, and the dHAND basic mutant potentiated Arix binding in a manner similar to wild-type dHAND. The dHAND-Arix complex was dissociated upon the addition of an unlabeled competitor containing a homeodomain, but not upon the addition of a competitor containing E-boxes. Arix coprecipitated with antisera directed against recombinant dHAND, demonstrating direct protein-protein interactions. These results indicate that the activation of the DBH promoter by Arix is potentiated by dHAND via a mechanism independent of a direct interaction of dHAND with DNA.

11q13 alterations in two cases of hibernoma: large heterozygous deletions and rearrangement breakpoints near GARP in 11q13.5

Hibernomas are rare, benign tumors with a histological appearance resembling that of brown adipose tissue. The diagnosis of hibernomas may be difficult because some of them contain only a small number of the characteristic multivacuolated fat cells and can be mistakenly classified as well-differentiated liposarcomas. Cytogenetic information has been reported for 10 cases, showing that these tumors are characterized by structural rearrangements involving 11q13. Previous fluorescence in situ hybridization (FISH) studies revealed consistent and sometimes cryptic losses of the MEN1 region in 11q13.1. Here, we describe the molecular cytogenetic analysis of two new hibernoma cases. Both tumors showed complex rearrangements, simultaneously including translocations, inversions, and deletions affecting the pair of chromosomes 11. The translocation partners were chromosome 5 in one case and chromosomes 16 and 22 in the other case. The 11q13 region was concomitantly rearranged on both chromosomes 11. FISH studies revealed large heterozygous deletions within the 11q13 band, from 11q13.1 to 11q13.5. Genes such as PYGM, MEN1, CCND1, FGF3, ARIX, and GARP were deleted, showing that the size of the 11q13 altered region was larger than previously reported. Furthermore, both tumors had breakpoints in 11q13.5, one of them in the immediate proximity of the GARP gene. Our results suggest that rearrangements of GARP or a neighboring gene may be important for the pathogenesis of hibernomas.

The role of Phox2b in synchronizing pan-neuronal and type-specific aspects of neurogenesis

Within the developing vertebrate nervous system, specific subclasses of neurons are produced in vastly different numbers at defined times and locations. This implies the concomitant activation of a program that controls pan-neuronal differentiation and of a program that specifies neuronal subtype identity, but how these programs are coordinated in time and space is not well understood. Our previous loss- and gain-of-function studies have defined Phox2b as a homeodomain transcription factor that coordinately regulates generic and type-specific neuronal properties. It is necessary and sufficient to impose differentiation towards a branchio- and viscero-motoneuronal phenotype and at the same time promotes generic neuronal differentiation. We have examined the underlying genetic interactions. We show thatPhox2b has a dual action on pan-neuronal differentiation. It upregulates the expression of proneural genes (Ngn2) when expressed alone and upregulates the expression of Mash1 when expressed in combination with Nkx2.2. By a separate pathway, Phox2brepresses expression of the inhibitors of neurogenesis Hes5 andId2. The role of Phox2b in the specification of neuronal subtype identity appears to depend in part on its capacity to act as a patterning gene in the progenitor domain. Phox2b misexpression represses the Pax6 and Olig2 genes, which should inhibit a branchiomotor fate, and induces Nkx6.1 and Nkx6.2, which are expressed in branchiomotor progenitors. We further show that Phox2b behaves like a transcriptional activator in the promotion of both, generic neuronal differentiation and expression of the motoneuronal marker Islet1. These results provide insights into the mechanisms by which a homeodomain transcription factor through interaction with other factors controls both generic and type-specific features of neuronal differentiation.

Phox2 genes - from patterning to connectivity

In the developing brain, many transcription factors are expressed in complex patterns and dynamics, and drive the differentiation of many classes of neurons. How does the spatio-temporal landscape of transcription factor expression map onto the bewildering variety of neuronal types, and, for each of them, the variety of developmental stages they go through? In other words, what is the logic in the transcriptional control of neuronal differentiation? Here, we review what recent work on the two neuronal-type-specific transcription factors Phox2a and Phox2b has contributed to our understanding of this broad question.

Specification of catecholaminergic and serotonergic neurons

The specification of neurotransmitter phenotype is an important aspect of neuronal fate determination. Substantial progress has been made in uncovering key extracellular signals and transcriptional regulators that control the mode of neurotransmission in several model systems, among which catecholaminergic and serotonergic neurons feature prominently. Here, we review our current knowledge of the regulatory circuits that direct neurotransmitter choice, and discuss the development of well-studied types of catecholaminergic and serotonergic neurons. One emerging concept is that different types of neuron use a similar core programme to control shared modes of neurotransmission, but recruit different factors that are specific for each neuronal type. Another is that most factors that specify neurotransmitter identity also control other features of the neuronal phenotype.

The paired-like homeodomain protein, Arix, mediates protein kinase A-stimulated dopamine beta-hydroxylase gene transcription through its phosphorylation status

The homeodomain transcription factor Arix/Phox2a plays a critical role in the specification of noradrenergic neurons by inducing the expression of dopamine beta-hydroxylase (DBH), the terminal enzyme for noradrenaline biosynthesis. In reporter assays, Arix together with activation of cAMP-dependent protein kinase (PKA) potentiates DBH gene transcription. We have evaluated whether post-translational modification of Arix regulates PKA-mediated DBH gene transcription. We found that Arix is constitutively phosphorylated in vivo at the basal level and that the phosphorylation level is substantially decreased upon stimulation of the PKA pathway. The change in the Arix phosphorylation state coincides with DNA binding activity of Arix. Treatment of cells with forskolin results in a robust enhancement of the DNA binding of Arix, which is reversed by treatment with serine/threonine and tyrosine phosphatase inhibitors. Consistent with the DNA binding activity of Arix, treatment of cultured cells with phosphatase inhibitors diminishes transcriptional activation with Arix plus forskolin. Amino acid analysis demonstrates the presence of phosphoserine within Arix. The results collectively suggest that dephosphorylation of Arix is a necessary event to fully activate PKA-mediated DBH transcription. Thus, the present study demonstrates that Arix can integrate extrinsic signals through post-translational modification, regulating DBH gene transcription in response to activation of the PKA pathway.

Enteric nervous system: development and developmental disturbances--part 1

This review, which is presented in two parts, summarizes and synthesizes current views on the genetic, molecular, and cell biological underpinnings of the early embryonic phases of enteric nervous system (ENS) formation and its defects. In the first part, we describe the critical features of two principal abnormalities of ENS development: Hirschsprung's disease (HSCR) and intestinal neuronal dysplasia type B (INDB) in humans, and the similar abnormalities in animals. These represent the extremes of the diagnostic spectrum: HSCR has agreed and unequivocal diagnostic criteria, whereas the diagnosis and even existence of INDB as a clinical entity is highly controversial. The difficulties in diagnosis and treatment of both these conditions are discussed. We then review the genes now known which, when mutated or deleted, may cause defects of ENS development. Many of these genetic abnormalities in animal models give a phenotype similar or identical to HSCR, and were discovered by studies of humans and of mouse mutants with similar defects. The most important of these genes are those coding for molecules in the GDNF intercellular signaling system, and those coding for molecules in the ET-3 signaling system. However, a range of other genes for different signaling systems and for transcription factors also disturb ENS formation when they are deleted or mutated. In addition, a large proportion of HSCR cases have not been ascribed to the currently known genes, suggesting that additional genes for ENS development await discovery.

Applications of molecular genetics to the understanding of congenital ocular motility disorders

The congenital fibrosis syndromes (CFS), including congenital fibrosis of the extraocular muscles (CFEOM) and Duane syndrome (DS), are rare congenital strabismus syndromes that present with nonprogressive restrictive ophthalmoplegia with or without ptosis. Although historically believed to result from primary extraocular muscle (EOM) fibrosis, our laboratory's work is based on the hypothesis that these disorders result from distinct, but analogous, developmental defects of the oculomotor (nIII), trochlear (nIV), and abducens (nVI) nuclei. We have defined three inherited CFEOM phenotypes (CFEOM1-3) and have mapped each phenotype to a distinct genetic locus (FEOM1-3). Individuals with CFEOM1 are born with bilateral ptosis and both eyes fixed in a downward position with absent upgaze and aberrant horizontal gaze. This disorder maps to the FEOM1 locus on chromosome 12cen.(1,2) Neuropathology studies of CFEOM1 reveal the absence of the superior division of oculomotor nerve and its corresponding alpha motor neurons in the midbrain, with abnormalities of target EOMs.(3) These neuropathology findings parallel those previously identified in Duane syndrome, in which there is an absence of nVI and the abducens nerve.(4,5) Individuals with CFEOM2 are born with bilateral ptosis and exotropia. This atypical form of CFEOM maps to the FEOM2 locus on chromosome 11q13 and results from mutations in ARIX (PHOX2A).(6,7) ARIX encodes a homeodomain transcription factor protein previously shown to be required for nIII/nIV development in mouse and zebrafish.(8,9) Together, these findings support the hypothesis that the congenital fibrosis syndromes result from parallel defects in nIII, nIV, and nVI nuclear development. Functional studies of the CFEOM genes should provide additional insight into the unique features of the extraocular lower motor neuron axis in health and disease. (For full (refs. 1-9), see reference list of the main paper.)

Structural and functional characterization of the 5' upstream promoter of the human Phox2a gene: possible direct transactivation by transcription factor Phox2b

The specification of neurotransmitter identity is a critical step in neural development. Recent progresses have indicated that the closely related homeodomain factors Phox2a and 2b are essential for development of noradrenergic (NA) neuron differentiation, and may directly determine the neurotransmitter identity. With a long-term goal of understanding the regulatory cascade of NA phenotype determination, we isolated and characterized a hPhox2a genomic clone encompassing approximately 7.5 kb of the 5' upstream promoter region, the entire exon-intron structure, and approximately 4 kb of the 3' flanking region. Using mRNAs isolated from the Phox2a-expressing human cell line, both primer extension and 5'-rapid amplification of cDNA ends analyses identified a single transcription start site that resides 172 nucleotides upstream of the start codon. The transcription start site was preceded by a TATA-like sequence motif and transcripts from this site contained an additional G residue at the 5' position, supporting the authenticity of this site as the transcriptional start site of hPhox2a. We assembled hPhox2a-luciferase reporter constructs containing different lengths of the 5' upstream sequences. Transient transfection assays of these reporter constructs in both hPhox2a-positive and -negative cell lines show that 1.3-kb or longer upstream sequences of the hPhox2a gene may confer NA cell-specific reporter gene expression. Furthermore, cotransfection assays in the Phox2a-negative HeLa cell line show that forced expression of Phox2b, but not that of Phox2a or MASH1, significantly transactivates the transcriptional activity of hPhox2a. This study will provide a frame to further delineate the regulatory cascade of NA neuron differentiation.

Homozygous mutations in ARIX(PHOX2A) result in congenital fibrosis of the extraocular muscles type 2

Isolated strabismus affects 1-5% of the general population. Most forms of strabismus are multifactorial in origin; although there is probably an inherited component, the genetics of these disorders remain unclear. The congenital fibrosis syndromes (CFS) represent a subset of monogenic isolated strabismic disorders that are characterized by restrictive ophthalmoplegia, and include congenital fibrosis of the extraocular muscles (CFEOM) and Duane syndrome (DURS). Neuropathologic studies indicate that these disorders may result from the maldevelopment of the oculomotor (nIII), trochlear (nIV) and abducens (nVI) cranial nerve nuclei. To date, five CFS loci have been mapped (FEOM1, FEOM2, FEOM3, DURS1 and DURS2), but no genes have been identified. Here, we report three mutations in ARIX (also known as PHOX2A) in four CFEOM2 pedigrees. ARIX encodes a homeodomain transcription factor protein previously shown to be required for nIII/nIV development in mouse and zebrafish. Two of the mutations are predicted to disrupt splicing, whereas the third alters an amino acid within the conserved brachyury-like domain. These findings confirm the hypothesis that CFEOM2 results from the abnormal development of nIII/nIV (ref. 7) and emphasize a critical role for ARIX in the development of these midbrain motor nuclei.