Haploinsufficiency for Phox2b in mice causes dilated pupils and atrophy of the ciliary ganglion: mechanistic insights into human congenital central hypoventilation syndrome

A Newborn Infant with Congenital Central Hypoventilation Syndrome and Pupillary Abnormalities: A Literature Review

We present a neonate with early onset apnea and bradycardia in the absence of primary cardiorespiratory and central nervous system disorders that eventually required chronic ventilator support starting at 6 hours of life. Molecular testing of paired-like homeobox 2b (PHOX2B) gene mutation confirmed the diagnosis of congenital central hypoventilation syndrome (CCHS). CCHS is a rare genetic disorder characterized by impaired central respiratory control with or without broad spectrum of autonomic nervous system (ANS) dysregulations. Ocular ANS dysregulation is a rare finding in CCHS individuals, and it is usually discovered later in life. However, the ophthalmic evaluation of this neonate on first day of life revealed persistent mild dilated oval pupils with limited light reactivity.

Area Postrema: Fetal Maturation, Tumors, Vomiting Center, Growth, Role in Neuromyelitis Optica

INTRODUCTION

The area postrema in the caudal fourth ventricular floor is highly vascular without blood-brain or blood-cerebrospinal fluid barrier. In addition to its function as vomiting center, several others are part of the circumventricular organs for vasomotor/angiotensin II regulation, role in neuromyelitis optica related to aquaporin-4, and somatic growth and appetite regulation. Functions are immature at birth. The purpose was to demonstrate neuronal, synaptic, glial, or ependymal maturation in the area postrema of normal fetuses. We describe three area postrema tumors.

METHODS

Sections of caudal fourth ventricle of 12 normal human fetal brains at autopsy aged six to 40 weeks and three infants aged three to 18 months were examined. Immunocytochemical neuronal and glial markers were applied to paraffin sections. Two infants with area postrema tumors and another with neurocutaneous melanocytosis and pernicious vomiting also studied.

RESULTS

Area postrema neurons exhibited cytologic maturity and synaptic circuitry by 14 weeks'. Astrocytes coexpressed vimentin, glial fibrillary acidic protein, and S-100β protein. The ependyma is thin over area postrema, with fetal ependymocytic basal processes. A glial layer separates area postrema from medullary tegmentum. Melanocytes infiltrated area postrema in the toddler with pernicious vomiting; two children had primary area postrema pilocytic astrocytomas.

CONCLUSIONS

Although area postrema is cytologically mature by 14 weeks, growth increases and functions mature during postnatal months. We recommend neuroimaging for patients with unexplained vomiting and that area postrema neuropathology includes synaptophysin and microtubule-associated protein-2 in patients with suspected dysfunction.

Expanding the phenotype of congenital central hypoventilation syndrome impacts management decisions

Congenital central hypoventilation syndrome (CCHS) is a neurocristopathy caused by pathogenic heterozygous variants in the gene paired-like homeobox 2b (PHOX2B). It is characterized by severe infantile alveolar hypoventilation. Individuals may also have diffuse autonomic nervous system dysfunction, Hirschsprung disease and neural crest tumors. We report three individuals with CCHS due to an 8-base pair duplication in PHOX2B; c.691_698dupGGCCCGGG (p.Gly234Alafs*78) with a predominant enteral and neural crest phenotype and a relatively mild respiratory phenotype. The attenuated respiratory phenotype reported here and elsewhere suggests an emergent genotype:phenotype correlation which challenges the current paradigm of invoking mechanical ventilation for all infants diagnosed with CCHS. Best treatment requires careful clinical judgment and ideally the assistance of a care team with expertise in CCHS.

Congenital Tonic Pupils Associated With Congenital Central Hypoventilation Syndrome and Hirschsprung Disease

Autonomic dysfunction can be associated with pupillary abnormalities. We describe a rare association of tonic pupils, congenital central hypoventilation syndrome, and Hirschsprung disease in a newborn with a mutation in the PHOX2B gene, a key regulator of neural crest cells. Hirschsprung disease is characterized by the congenital absence of neural crest-derived intrinsic ganglion cells. Tonic pupils may result from an abnormality of the ciliary ganglion, another structure of neural crest origin. The close association of these conditions in this child suggests a common abnormality in neural crest migration and differentiation.

Area postrema undergoes dynamic postnatal changes in mice and humans

The postnatal period in mammals represents a developmental epoch of significant change in the autonomic nervous system (ANS). This study focuses on postnatal development of the area postrema, a crucial ANS structure that regulates temperature, breathing, and satiety, among other activities. We find that the human area postrema undergoes significant developmental changes during postnatal development. To characterize these changes further, we used transgenic mouse reagents to delineate neuronal circuitry. We discovered that, although a well-formed ANS scaffold exists early in embryonic development, the area postrema shows a delayed maturation. Specifically, postnatal days 0-7 in mice show no significant change in area postrema volume or synaptic input from PHOX2B-derived neurons. In contrast, postnatal days 7-20 show a significant increase in volume and synaptic input from PHOX2B-derived neurons. We conclude that key ANS structures show unexpected dynamic developmental changes during postnatal development. These data provide a basis for understanding ANS dysfunction and disease predisposition in premature and postnatal humans.

Emotional disorders in adult mice heterozygous for the transcription factor Phox2b

Phox2b is an essential transcription factor for the development of the autonomic nervous system. Mice carrying one invalidated Phox2b allele (Phox2b(+/-)) show mild autonomic disorders including sleep apneas, and impairments in chemosensitivity and thermoregulation that recover within 10days of postnatal age. Because Phox2b is not expressed above the pons nor in the cerebellum, this mutation is not expected to affect brain development and cognitive functioning directly. However, the transient physiological disorders in Phox2b(+/-) mice might impair neurodevelopment. To examine this possibility, we conducted a behavioral test battery of emotional, motor, and cognitive functioning in adult Phox2b(+/-) mice and their wildtype littermates (Phox2b(+/+)). Adult Phox2b(+/-) mice showed altered exploratory behavior in the open field and in the elevated plus maze, both indicative of anxiety. Phox2b(+/-) mice did not show cognitive or motor impairments. These results suggest that also mild autonomic control deficits may disturb long-term emotional development.

Genetic diseases: congenital central hypoventilation, Rett, and Prader-Willi syndromes

The present review summarizes current knowledge on three rare genetic disorders of respiratory control, congenital central hypoventilation syndrome (CCHS), Rett syndrome (RTT), and Prader-Willi syndrome (PWS). CCHS is characterized by lack of ventilatory chemosensitivity caused by PHOX2B gene abnormalities consisting mainly of alanine expansions. RTT is associated with episodes of tachypneic and irregular breathing intermixed with breathholds and apneas and is caused by mutations in the X-linked MECP2 gene encoding methyl-CpG-binding protein. PWS manifests as sleep-disordered breathing with apneas and episodes of hypoventilation and is caused by the loss of a group of paternally inherited genes on chromosome 15. CCHS is the most specific disorder of respiratory control, whereas the breathing disorders in RTT and PWS are components of a more general developmental disorder. The main clinical features of these three disorders are reviewed with special emphasis on the associated brain abnormalities. In all three syndromes, disease-causing genetic defects have been identified, allowing the development of genetically engineered mouse models. New directions for future therapies based on these models or, in some cases, on clinical experience are delineated. Studies of CCHS, RTT, and PWS extend our knowledge of the molecular and cellular aspects of respiratory rhythm generation and suggest possible pharmacological approaches to respiratory control disorders. This knowledge is relevant for the clinical management of many respiratory disorders that are far more prevalent than the rare diseases discussed here.

Distinct neuroblastoma-associated alterations of PHOX2B impair sympathetic neuronal differentiation in zebrafish models

Heterozygous germline mutations and deletions in PHOX2B, a key regulator of autonomic neuron development, predispose to neuroblastoma, a tumor of the peripheral sympathetic nervous system. To gain insight into the oncogenic mechanisms engaged by these changes, we used zebrafish models to study the functional consequences of aberrant PHOX2B expression in the cells of the developing sympathetic nervous system. Allelic deficiency, modeled by phox2b morpholino knockdown, led to a decrease in the terminal differentiation markers th and dbh in sympathetic ganglion cells. The same effect was seen on overexpression of two distinct neuroblastoma-associated frameshift mutations, 676delG and K155X - but not the R100L missense mutation - in the presence of endogenous Phox2b, pointing to their dominant-negative effects. We demonstrate that Phox2b is capable of regulating itself as well as ascl1, and that phox2b deficiency uncouples this autoregulatory mechanism, leading to inhibition of sympathetic neuron differentiation. This effect on terminal differentiation is associated with an increased number of phox2b⁺, ascl1⁺, elavl3⁻ cells that respond poorly to retinoic acid. These findings suggest that a reduced dosage of PHOX2B during development, through either a heterozygous deletion or dominant-negative mutation, imposes a block in the differentiation of sympathetic neuronal precursors, resulting in a cell population that is likely to be susceptible to secondary transforming events.

Neuroblastoma, a tumor of the peripheral sympathetic nervous system, is the most common cancer diagnosed in infancy. Although most cases arise sporadically, familial predisposition also occurs in association with mutations in a single copy of the PHOX2B gene, a “master regulator” of sympathetic neuronal development. The exact mechanisms by which these mutations increase susceptibility to neuroblastoma are unclear, primarily because of the paucity of optimal models in which to study very early development of the sympathetic nervous system. We took advantage of the ex vivo development and transparent nature of zebrafish embryos to study the roles of both normal and mutated PHOX2B in development of the sympathetic nervous system. We present data indicating that aberrant PHOX2B expression causes an arrest in the normal maturation of sympathetic neurons, leading to immature cells that are resistant to drug-induced differentiation. Indeed, we demonstrate that phox2b gene “dosage” is important for normal differentiation of sympathetic neurons in the zebrafish and suggest that the population of immature cells resulting from a decreased dosage of this pivotal factor may be susceptible to secondary mutations that could ultimately lead to neuroblastoma.

[Malformation syndromes associated with childhood cancer: an update]

Biology, genetics and environment of childhood solid tumours set them apart from adult solid tumours. The nature of the progenitor cells from which these tumours arise, and their immature tissue environment, allows childhood solid tumours to develop with fewer defects in cell regulatory processes. Constitutional molecular defects are known to play a role in childhood solid tumours, as shown by the increased incidence of embryonic cancers in children carrying malformations associated with childhood cancer. These rare diagnoses are commonly missed. In this article, we reviewed the spectrum of these tumour predisposition syndromes.

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.

A developmental and genetic classification for midbrain-hindbrain malformations

Advances in neuroimaging, developmental biology and molecular genetics have increased the understanding of developmental disorders affecting the midbrain and hindbrain, both as isolated anomalies and as part of larger malformation syndromes. However, the understanding of these malformations and their relationships with other malformations, within the central nervous system and in the rest of the body, remains limited. A new classification system is proposed, based wherever possible, upon embryology and genetics. Proposed categories include: (i) malformations secondary to early anteroposterior and dorsoventral patterning defects, or to misspecification of mid-hindbrain germinal zones; (ii) malformations associated with later generalized developmental disorders that significantly affect the brainstem and cerebellum (and have a pathogenesis that is at least partly understood); (iii) localized brain malformations that significantly affect the brain stem and cerebellum (pathogenesis partly or largely understood, includes local proliferation, cell specification, migration and axonal guidance); and (iv) combined hypoplasia and atrophy of putative prenatal onset degenerative disorders. Pertinent embryology is discussed and the classification is justified. This classification will prove useful for both physicians who diagnose and treat patients with these disorders and for clinical scientists who wish to understand better the perturbations of developmental processes that produce them. Importantly, both the classification and its framework remain flexible enough to be easily modified when new embryologic processes are described or new malformations discovered.

Breathing with phox2b

In the last few years, elucidation of the architecture of breathing control centres has reached the cellular level. This has been facilitated by increasing knowledge of the molecular signatures of various classes of hindbrain neurons. Here, we review the advances achieved by studying the homeodomain factor Phox2b, a transcriptional determinant of neuronal identity in the central and peripheral nervous systems. Evidence from human genetics, neurophysiology and mouse reverse genetics converges to implicate a small population of Phox2b-dependent neurons, located in the retrotrapezoid nucleus, in the detection of CO(2), which is a paramount source of the 'drive to breathe'. Moreover, the same and other studies suggest that an overlapping or identical neuronal population, the parafacial respiratory group, might contribute to the respiratory rhythm at least in some circumstances, such as for the initiation of breathing following birth. Together with the previously established Phox2b dependency of other respiratory neurons (which we review briefly here), our new data highlight a key role of this transcription factor in setting up the circuits for breathing automaticity.

PHOX2B mutations and ventilatory control

The transcription factor PHOX2B is essential for the development of the autonomic nervous system. In humans, polyalanine expansion mutations in PHOX2B cause Congenital Central Hypoventilation Syndrome (CCHS), a rare life-threatening disorder characterized by hypoventilation during sleep and impaired chemosensitivity. CCHS is combined with comparatively less severe impairments of autonomic functions including thermoregulation, cardiac rhythm, and digestive motility. Respiratory phenotype analyses of mice carrying an invalidated Phox2b allele (Phox2b+/- mutant mice) or the Phox2b mutation (+7 alanine expansion) found in patients with CCHS (Phox2b(27Ala/+) mice) have shed light on the role for PHOX2B in breathing control and on the pathophysiological mechanisms underlying CCHS. Newborn mice that lacked one Phox2b allele (Phox2b+/-) had sleep apneas and depressed sensitivity to hypercapnia. However, these impairments resolved rapidly, whereas the CCHS phenotype is irreversible. Heterozygous Phox2b(27Ala/+) pups exhibited a lack of responsiveness to hypercapnia and unstable breathing; they died within the first few postnatal hours. The generation of mouse models of CCHS provides tools for evaluating treatments aimed at alleviating both the respiratory symptoms and all other autonomic symptoms of CCHS.

A Histone2BCerulean BAC transgene identifies differential expression of Phox2b in migrating enteric neural crest derivatives and enteric glia

The mammalian enteric nervous system (ENS) derives from migratory enteric neural crest-derived cells (ENCC) that express the transcription factor Phox2b. Studies of these enteric progenitors have typically relied on immunohistochemical (IHC) detection. To circumvent complicating factors of IHC, we have generated a mouse BAC transgenic line that drives a Histone2BCerulean (H2BCFP) reporter from Phox2b regulatory regions. This construct does not alter the endogenous Phox2b locus and enables studies of normal neural crest (NC) derivatives. The Phox2b-H2BCFP transgene expresses the H2BCFP reporter in patterns that recapitulate expression of endogenous Phox2b. Our studies reveal Phox2b expression in mature enteric glia at levels below that of enteric neurons. Moreover, we also observe differential expression of the transgene reporter within the leading ENCC that traverse the gut. Our findings indicate that the wavefront of migrating enteric progenitors is not homogeneous, and suggest these cells may be fate-specified before expression of mature lineage markers appears.

Central nervous system distribution of the transcription factor Phox2b in the adult rat

Phox2b is required for development of the peripheral autonomic nervous system and a subset of cranial nerves and lower brainstem nuclei. Phox2b mutations in man cause diffuse autonomic dysfunction and deficits in the automatic control of breathing. Here we study the distribution of Phox2b in the adult rat hindbrain to determine whether this protein is selectively expressed by neurons involved in respiratory and autonomic control. In the medulla oblongata, Phox2b-immunoreactive nuclei were present in the dorsal vagal complex, intermediate reticular nucleus, dorsomedial spinal trigeminal nucleus, nucleus ambiguus, catecholaminergic neurons, and retrotrapezoid nucleus (RTN). Phox2b was expressed by both central excitatory relays of the sympathetic baroreflex (nucleus of the solitary tract and C1 neurons) but not by the inhibitory relay of this reflex. Phox2b was absent from the ventral respiratory column (VRC) caudal to RTN and rare within the parabrachial nuclei. In the pons, Phox2b was confined to cholinergic efferent neurons (salivary, vestibulocochlear) and noncholinergic peritrigeminal neurons. Rostral to the pons, Phox2b was detected only in the oculomotor complex. In adult rats, Phox2b is neither a comprehensive nor a selective marker of hindbrain autonomic pathways. This marker identifies a subset of hindbrain neurons that control orofacial movements (dorsomedial spinal trigeminal nucleus, pontine peritrigeminal neurons), balance and auditory function (vestibulocochlear efferents), the eyes, and both divisions of the autonomic efferent system. Phox2b is virtually absent from the respiratory rhythm and pattern generator (VRC and dorsolateral pons) but is highly expressed by neurons involved in the chemical drive and reflex regulation of this oscillator.

[Ondine's Curse and rare oculomotor abnormalities: a case report]

Ondine's Curse or congenital central hypoventilation syndrome (CCHS) is a neurocristopathy (failure of migration or differentiation of neural crest-derived precursor cells) and is characterized by hypoventilation or apnea, which is most pronounced during sleep, with no other abnormalities of the neuro-respiratory system. Because of respiratory distress soon after birth, patients must be intubated and ventilated for a long time. This disorder may be associated with other symptoms of neurocristopathy (Hirschsprung disease, neuroblastoma, neuroganglioma) and other abnormalities of the autonomic nervous system (vasomotor dysfunctions or ophthalmic abnormalities: abnormal pupils, insufficient convergence, strabismus, or ptosis). We report the original case of a CCHS patient who presented with alternative ptosis of both the right and left eyes and esotropia. The ocular findings should lead to earlier diagnosis and speedier adequate treatment.

Cardiac malformations and midline skeletal defects in mice lacking filamin A

The X-linked gene filamin A (Flna) encodes a widely expressed actin-binding protein that crosslinks actin into orthogonal networks and interacts with a variety of other proteins including membrane proteins, integrins, transmembrane receptor complexes and second messengers, thus forming an important intracellular signalling scaffold. Heterozygous loss of function of human FLNA causes periventricular nodular heterotopia in females and is generally lethal (cause unknown) in hemizygous males. Missense FLNA mutations underlie a spectrum of disorders affecting both sexes that feature skeletal dysplasia accompanied by a variety of other abnormalities. Dilp2 is an X-linked male-lethal mouse mutation that was induced by N-ethyl-N-nitrosourea. We report here that Dilp2 is caused by a T-to-A transversion that converts a tyrosine codon to a stop codon in the Flna gene (Y2388X), leading to absence of the Flna protein and male lethality because of incomplete septation of the outflow tract of the heart, which produces common arterial trunk. A proportion of both male and female mutant mice have other cardiac defects including ventricular septal defect. In addition, mutant males have midline fusion defects manifesting as sternum and palate abnormalities. Carrier females exhibit milder sternum and palate defects and misshapen pupils. These results define crucial roles for Flna in development, demonstrate that X-linked male lethal mutations can be recovered from ENU mutagenesis screens and suggest possible explanations for lethality of human males hemizygous for null alleles of FLNA.

Mutations of the RET gene in isolated and syndromic Hirschsprung's disease in human disclose major and modifier alleles at a single locus

BACKGROUND

In Hirschsprung's disease (HSCR), a hypomorphic allele of a major gene, RET, accounts for most isolated (non-syndromic) cases, along with other autosomal susceptibility loci under a multiplicative model. However, some syndromic forms of HSCR are monogenic entities, for which the disease causing gene is known.

OBJECTIVE

To determine whether RET could be considered a modifier gene for the enteric phenotype on the background of a monogenic trait.

METHODS

The syndromic HSCR entities studied were congenital central hypoventilation (CCHS) and Mowat-Wilson syndrome (MWS), caused by PHOX2B and ZFHX1B gene mutations, respectively. The RET locus was genotyped in 143 CCHS patients, among whom 44 had HSCR, and in 30 MWS patients, among whom 20 had HSCR. The distribution of alleles, genotypes, and haplotypes was compared within the different groups. To test the interaction in vivo, heterozygous mice were bred for a null allele of Phox2b and Ret genes.

RESULTS

RET was shown to act as a modifier gene for the HSCR phenotype in patients with CCHS but not with MWS. The intestine of double heterozygote mice was indistinguishable from their littermates. A loss of over 50% of each gene function seemed necessary in the mouse model for an enteric phenotype to occur.

CONCLUSIONS

In CCHS patients, the weak predisposing haplotype of the RET gene can be regarded as a quantitative trait, being a risk factor for the HSCR phenotype, while in MWS, for which the HSCR penetrance is high, the role of the RET predisposing haplotype is not significant. It seems likely that there are both RET dependent and RET independent HSCR cases.

Sensitive detection of polyalanine expansions in PHOX2B by polymerase chain reaction using bisulfite-converted DNA

Congenital central hypoventilation syndrome, also known as Ondine's curse, is characterized by idiopathic abnormal control of respiration during sleep. Recent studies indicate that a polyalanine expansion of PHOX2B is relevant to the pathogenesis of this disorder. However, it is difficult to detect the repeated tract because its high GC content inhibits conventional polymerase chain reaction (PCR) amplification. Here, we describe a bisulfite treatment for DNA in which uracil is obtained by deamination of unmethylated cytosine residues. Deamination of DNA permitted direct PCR amplification that yielded a product of 123 bp for the common 20-residue repetitive tract with replacement of C with T by sequencing. It settled allele dropouts accompanied by insufficient amplification of expanded alleles. The defined procedure dramatically improved detection of expansions to 9 of 10 congenital central hypoventilation syndrome patients examined in a previous study. The chemical conversion of DNA before PCR amplification facilitates effective detection of GC-rich polyalanine tracts.

Ventilatory response to hyperoxia in newborn mice heterozygous for the transcription factor Phox2b

Heterozygous mutations of the transcription factor PHOX2B have been found in most patients with central congenital hypoventilation syndrome, a rare disease characterized by sleep-related hypoventilation and impaired chemosensitivity to sustained hypercapnia and sustained hypoxia. PHOX2B is a master regulator of autonomic reflex pathways, including peripheral chemosensitive pathways. In the present study, we used hyperoxic tests to assess the strength of the peripheral chemoreceptor tonic drive in Phox2b+/-newborn mice. We exposed 69 wild-type and 67 mutant mice to two hyperoxic tests (12-min air followed by 3-min 100% O2) 2 days after birth. Breathing variables were measured noninvasively using whole body flow plethysmography. The initial minute ventilation decrease was larger in mutant pups than in wild-type pups: -37% (SD 13) and -25% (SD 18), respectively, P<0.0001. Furthermore, minute ventilation remained depressed throughout O2 exposure in mutants, possibly because of their previously reported impaired CO2 chemosensitivity, whereas it returned rapidly to the normoxic level in wild-type pups. Hyperoxia considerably increased total apnea duration in mutant compared with wild-type pups (P=0.0001). A complementary experiment established that body temperature was not influenced by hyperoxia in either genotype group and, therefore, did not account for genotype-related differences in the hyperoxic ventilatory response. Thus partial loss of Phox2b function by heterozygosity did not diminish the tonic drive from peripheral chemoreceptors.

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.

In pursuit (and discovery) of a genetic basis for congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) typically presents in the newborn period with a phenotype including alveolar hypoventilation, symptoms of autonomic nervous system dysregulation, and in a subset of cases Hirschsprung disease and later tumors of neural crest origin. Study of genes related to the autonomic dysregulation and the embryologic origin of the neural crest has led to identification of the genetic basis for CCHS, the mode of inheritance, and the presence of mosaicism in a subset of parents. Polyalanine expansion mutations in PHOX2B have been identified to be the disease-defining mutation in CCHS, with a small subset of patients having other mutations in PHOX2B. Further, the size of the polyalanine repeat mutation in PHOX2B is correlated with the severity of the phenotype in CCHS, and non-polyalanine repeat mutations appear to, in general, result in CCHS phenotypes at the severe end of the spectrum. These studies highlight the utility of PHOX2B genetic testing for confirmation of the CCHS diagnosis, for prenatal diagnosis, and for identification of previously undiagnosed adults with unexplained hypercarbia or control of breathing deficits. This diagnostic approach may be a consideration for other complex, seemingly undecipherable diseases that affect infants and children. The purpose of this article is to provide a comprehensive review of current research into the genetic basis for CCHS, an explanation for how these studies evolved, recent studies that begin to explain the mechanisms through which mutations in PHOX2B exert their effects, and clinical application of the genetic testing.

Sleep-disordered Breathing in Newborn Mice Heterozygous for the Transcription Factor Phox2b

RATIONALE

Central congenital hypoventilation syndrome (CCHS) is a rare autosomal dominant syndrome present from birth, and characterized by depressed ventilation during sleep. Heterozygous mutations of the homeobox gene Phox2b were recently found in a very high proportion of patients.

OBJECTIVES

To determine whether newborn mice with heterozygous targeted deletion of the transcription factor Phox2b would display sleep-disordered breathing.

METHODS

We measured breathing pattern using whole-body plethysmography in wild-type and mutant 5-day-old mice, and we classified sleep-wake states using nuchal EMG and behavioral scores.

RESULTS

We found that sleep apnea total time was approximately six times longer (8.9 +/- 12 vs. 1.5 +/- 2.2 seconds, p < 0.0015), and ventilation during active sleep was 21% lower (18.4 +/- 5.1 vs. 23.3 +/- 5.5 ml/g/second, p < 0.006) in mutant than in wild-type pups. During wakefulness, apnea time and ventilation were not significantly different between mutant and wild-type pups. Mutant and wild-type pups showed highly similar sleep-wake states.

CONCLUSION

Although their respiratory phenotype was much less severe than CCHS, the Phox2b(+/-) mutant mice showed sleep-disordered breathing, which partially modeled the key feature of CCHS.