No abstract

Breathing is maintained and controlled by a network of neurons in the brainstem that generate respiratory rhythm and provide regulatory input. Central chemoreception, the mechanism for CO2 detection that provides an essential stimulatory input, is thought to involve neurons located near the medullary surface, whose nature is controversial. Good candidates are serotonergic medullary neurons and glutamatergic neurons in the parafacial region. Here, we show that mice bearing a mutation in Phox2b that causes congenital central hypoventilation syndrome in humans breathe irregularly, do not respond to an increase in CO2, and die soon after birth from central apnea. They specifically lack Phox2b-expressing glutamatergic neurons located in the parafacial region, whereas other sites known or supposed to be involved in the control of breathing are anatomically normal. These data provide genetic evidence for the essential role of a specific population of medullary interneurons in driving proper breathing at birth and will be instrumental in understanding the etiopathology of congenital central hypoventilation syndrome.brainstem ͉ congenital central hypoventilation syndrome ͉ neurodegenerative disease ͉ respiration

Neuroblastoma (NB) is a frequent pediatric tumor for which recurrent somatic rearrangements are known. Germline mutations of predisposing gene(s) are suspected on the basis of rare familial cases and the association of NB with other genetically determined congenital malformations of neural crest-derived cells--namely, Hirschsprung disease (HSCR) and/or congenital central hypoventilation syndrome (CCHS). We recently identified the paired-like homeobox 2B (PHOX2B) gene as the major disease-causing gene in isolated and syndromic CCHS, which prompted us to regard it as a candidate gene in NB. Here, we report on germline mutations of PHOX2B in both a familial case of NB and a patient with the HSCR-NB association. PHOX2B, therefore, stands as the first gene for which germline mutations predispose to NB.

The Phox2b gene is necessary for autonomic nervous-system development. Phox2b-/- mice die in utero with absent autonomic nervous system circuits, since autonomic nervous system neurons either fail to form or degenerate. We first identified the Phox2b human ortholog, PHOX2B, as the gene underlying congenital central hypoventilation syndrome (CCHS, or Ondine curse), with an autosomal dominant mode of inheritance and de novo mutation at the first generation. We have subsequently shown that heterozygous mutations of PHOX2B may account for several combined or isolated disorders of autonomic nervous-system development--namely, tumors of the sympathetic nervous system (TSNS), such as neuroblastoma and late-onset central hypoventilation syndrome. Here, we report the clinical and molecular assessments of a cohort of 188 probands with CCHS, either isolated or associated with Hirschsprung disease and/or TSNS. The mutation-detection rate was 92.6% (174/188) in our series, and the most prevalent mutation was an in-frame duplication leading to an expansion of +5 to +13 alanines in the 20-alanine stretch at the carboxy terminal of the protein. Such findings suggest PHOX2B mutation screening as a simple and reliable tool for the diagnosis of CCHS, independent of the clinically variable phenotype. In addition, somatic mosaicism was detected in 4.5% of parents. Most interestingly, analysis of genotype-phenotype interactions strongly supports the contention that patients with CCHS who develop malignant TSNS will harbor either a missense or a frameshift heterozygous mutation of the PHOX2B gene. These data further highlight the link between congenital malformations and tumor predisposition when a master gene in development is mutated.

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.

Beside the well-known polyglutamine expansions involved in several neurodegenerative disorders, convergent recent findings pointed to the expansion of polyalanine stretches as a disease mechanism in congenital malformations, skeletal dysplasia and nervous system anomalies. Polyalanine stretches have been predicted in roughly 500 human proteins among which nine have been ascribed to disease phenotype by expansion of polyalanines. The function of polyalanine stretches is largely unknown. This paper aims to review the rapidly growing evidences for a disease-causing mechanism common to expansion of homopolymeric tracts whatever the amino acid involved is.

The most common forms of neurocristopathy in the autonomic nervous system are Hirschsprung disease (HSCR), resulting in congenital loss of enteric ganglia, and neuroblastoma (NB), childhood tumors originating from the sympathetic ganglia and adrenal medulla. The risk for these diseases dramatically increases in patients with congenital central hypoventilation syndrome (CCHS) harboring a nonpolyalanine repeat expansion mutation of the Paired-like homeobox 2b (PHOX2B) gene, but the molecular mechanism of pathogenesis remains unknown. We found that introducing nonpolyalanine repeat expansion mutation of the PHOX2B into the mouse Phox2b locus recapitulates the clinical features of the CCHS associated with HSCR and NB. In mutant embryos, enteric and sympathetic ganglion progenitors showed sustained sex-determining region Y (SRY) box10 (Sox10) expression, with impaired proliferation and biased differentiation toward the glial lineage. Nonpolyalanine repeat expansion mutation of PHOX2B reduced transactivation of wild-type PHOX2B on its known target, dopamine β-hydroxylase (DBH), in a dominant-negative fashion. Moreover, the introduced mutation converted the transcriptional effect of PHOX2B on a Sox10 enhancer from repression to transactivation. Collectively, these data reveal that nonpolyalanine repeat expansion mutation of PHOX2B is both a dominant-negative and gain-of-function mutation. Our results also demonstrate that Sox10 regulation by PHOX2B is pivotal for the development and pathogenesis of the autonomic ganglia.

Late Onset Central Hypoventilation Syndrome associated with Hypothalamic Dysfunction (LO-CHS/HD) is a distinct entity among the clinical and genetic heterogeneous group of patients with late onset central hypoventilation. Here we report a series of 13 patients with LO-CHS/HD. Rapid onset obesity is the first symptom of HD followed by hypoventilation with a mean delay of 18 mos. The outcome remains poor for this group of patients and would benefit from early diagnosis to anticipate ventilation and possible metabolic disorders. Tumor predisposition is more frequent than initially suspected and as high as 40% in this series. These tumors of the sympathetic nervous system (TSNS) are usually differentiated and do not significantly worsen the prognosis. We report a familial case with recurrence in siblings. The cause underlying LO-CHS/HD remains poorly understood although recurrence in siblings argues for a monogenic disorder. We ruled out PHOX2B, ASCL1, and NECDIN as disease-causing genes by direct sequencing in our series of patients and discuss possible disease-causing mechanisms. A genetic basis for C-CHS was postulated because of, i) concordant monozygotic twins; ii) rare cases of recurrence in siblings and vertical transmission; and iii) the association with genetically determined neurocristopathies (i.e. Hirschsprung disease and tumors of the sympathetic nervous system). The screening of genes involved in the developmental cascade of the autonomic nervous system pointed to PHOX2B as the gene underlying C-CHS with an autosomal dominant mode of inheritance and de novo mutations in the first generation (4). We and others subsequently showed that PHOX2B mutations also account for a subset of CHS presenting later in life and named Late-Onset Central Hypoventilation Syndrome (LO-CHS) (5-7). LO-CHS is genetically and clinically heterogeneous and a subset of patients presenting hypothalamic dysfunction (HD) in association with LO-CHS is clearly recognized as a distinct entity (8).Single-case reports of LO-CHS/HD patients, one series and review of the literature have been reported and, thus, far (9 -22) for a total of 30 patients to date. Here we present a series of 13 additional patients with LO-CHS/HD, one being a familial case with recurrence in siblings. This observation strongly argues that LO-CHS/HD is a monogenic condition. In this series, PHOX2B, ASCL1, and NECDIN have been ruled out as disease-causing genes by direct sequencing. PATIENTS AND METHODSThirteen patients with LO-CHS/HD were referred to us between 2000 and 2007 and medical records were reviewed. Inclusion criteria were i) central hypoventilation confirmed by polysomnographic recordings with normal brain MRI and no lung or neuromuscular diseases and ii) at least one other sign of hypothamic dysfunction among the following: rapid-onset obesity due to hyperphagia, hyperprolactinemia, central hypothyroidism, water balance disorder, nonresponse to growth hormone stimulation test, corticotrophin deficiency, or abnormal puberty (precocious or dela...