Homozygous mutation of the PHOX2B gene in congenital central hypoventilation syndrome (Ondine's Curse)

Knockdown of PHOX2B in the retrotrapezoid nucleus reduces the central CO2 chemoreflex in rats

PHOX2B is a transcription factor essential for the development of different classes of neurons in the central and peripheral nervous system. Heterozygous mutations in the PHOX2B coding region are responsible for the occurrence of Congenital Central Hypoventilation Syndrome (CCHS), a rare neurological disorder characterised by inadequate chemosensitivity and life-threatening sleep-related hypoventilation. Animal studies suggest that chemoreflex defects are caused in part by the improper development or function of PHOX2B expressing neurons in the retrotrapezoid nucleus (RTN), a central hub for CO2 chemosensitivity. Although the function of PHOX2B in rodents during development is well established, its role in the adult respiratory network remains unknown. In this study, we investigated whether reduction in PHOX2B expression in chemosensitive neuromedin-B (NMB) expressing neurons in the RTN altered respiratory function. Four weeks following local RTN injection of a lentiviral vector expressing the short hairpin RNA (shRNA) targeting Phox2b mRNA, a reduction of PHOX2B expression was observed in Nmb neurons compared to both naive rats and rats injected with the non-target shRNA. PHOX2B knockdown did not affect breathing in room air or under hypoxia, but ventilation was significantly impaired during hypercapnia. PHOX2B knockdown did not alter Nmb expression but it was associated with reduced expression of both Task2 and Gpr4, two CO2/pH sensors in the RTN. We conclude that PHOX2B in the adult brain has an important role in CO2 chemoreception and reduced PHOX2B expression in CCHS beyond the developmental period may contribute to the impaired central chemoreflex function.

Genetic study of a patient with congenital central hypoventilation syndrome in Iran: a case report

BACKGROUND

Congenital central hypoventilation syndrome (CCHS) is an extremely rare genetic disorder characterized by Autonomic nervous system dysregulation caused by mutations in the PHOX2B gene. Here we introduce the first genetic analysis of a one-month-old CCHS baby girl in Iran.

METHODS AND RESULTS

Genetic analysis of the PHOX2B gene was performed by Sanger sequencing and interpreted using the American College of Medical Genetics and Genomics (ACMG) guideline. The results showed a heterozygous duplication in exon 3, causing a polyalanine repeat expansion mutation to 27 repeats in thePHOX2B gene (20/27 genotype).The patient's parents did not demonstrate this mutation on genetic studies.

CONCLUSIONS

According to the ACMG guideline, the mutation is pathogenic, and it was a denovo mutation in the family. The genetic study can help the family for prenatal diagnosis or pre-implantation diagnosis if the parents have gonadal mosaicism.

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.

Guidelines for diagnosis and management of congenital central hypoventilation syndrome

BACKGROUND

Congenital Central Hypoventilation Syndrome (CCHS) is a rare condition characterized by an alveolar hypoventilation due to a deficient autonomic central control of ventilation and a global autonomic dysfunction. Paired-like homeobox 2B (PHOX2B) mutations are found in most of the patients with CCHS. In recent years, the condition has evolved from a life-threatening neonatal onset disorder to include broader and milder clinical presentations, affecting children, adults and families. Genes other than PHOX2B have been found responsible for CCHS in rare cases and there are as yet other unknown genes that may account for the disease. At present, management relies on lifelong ventilatory support and close follow up of dysautonomic progression. BODY: This paper provides a state-of-the-art comprehensive description of CCHS and of the components of diagnostic evaluation and multi-disciplinary management, as well as considerations for future research.

CONCLUSION

Awareness and knowledge of the diagnosis and management of this rare disease should be brought to a large health community including adult physicians and health carers.

Congenital central hypoventilation syndrome: Severe disease caused by co-occurrence of two PHOX2B variants inherited separately from asymptomatic family members

Congenital Central Hypoventilation Syndrome (CCHS) is a rare disease characterized by autonomic nervous system dysregulation. Central hypoventilation is the most prominent and clinically important presentation. CCHS is caused by mutations in paired-like homeobox 2b (PHOX2B) and is inherited in an autosomal dominant pattern. A co-occurrence of two asymptomatic PHOX2B variants with a classical CCHS presentation highlights the importance of clinical PHOX2B testing in parents and family members of all CCHS probands. Despite being an autosomal dominant disease, once a polyalanine repeat expansion mutation has been identified, sequencing of the other allele should also be considered.

Congenital central hypoventilation syndrome mimicking mitochondrial disease

Later-onset congenital central hypoventilation syndrome (LO-CCHS) does not present only breathing problems but can be present as episodic multiple organs involvement. Our unique case demonstrated LO-CCHS should be considered in the differential diagnosis of mitochondrial diseases and having nontypical polysomnography result.

Heterozygous 24-polyalanine repeats in the PHOX2B gene with different manifestations across three generations

Congenital central hypoventilation syndrome (CCHS) is an uncommon genetic disorder that is characterized by alveolar hypoventilation and autonomic dysregulation. More than 90% of the patients are heterozygous for polyalanine repeat expansion mutations in the paired-like homeobox 2b (PHOX2B) gene. The normal genotype has a 20-polyalanine sequence whereas expanded alleles are usually 25-33. Heterozygous 24-polyalanine repeat expansion mutation in the PHOX2B gene has rarely been reported. We report three consecutive generations harboring heterozygous 24-polyalanine repeats in the PHOX2B gene with manifestations ranging from apparently asymptomatic to alveolar hypoventilation and apnea requiring mechanical ventilation. The 3-year-old proband developed cor pulmonale and central hypoventilation following an upper respiratory tract infection. Our findings add to the accumulating evidence that the 24-polyalanine repeat in the PHOX2B is a disease-causing mutation. In addition, a high index of suspicion and careful monitoring after anesthesia, sedation, or respiratory illnesses should be exercised when evaluating asymptomatic family members with this genotype.

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.

Congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) is characterized by hypoventilation during sleep and impaired ventilatory responses to hypercapnia and hypoxemia. Most cases are sporadic and caused by de novo PHOX2B gene mutations, which are usually polyalanine repeat expansions. Physiological and neuroanatomical studies of genetically engineered mice and analyses of cellular responses to mutated Phox2b have shed light on the pathophysiological mechanisms of CCHS. Findings in Phox2b(27Ala/+) knock-in mice consisted of unstable breathing with apneas, absence of the ventilatory response to hypercapnia, death within a few hours after birth, and absence of the retrotrapezoid nucleus (RTN). Conditional mouse mutants in which Phox2b(27Ala) was targeted to the RTN also lacked the ventilatory response to hypercapnia at birth but survived to adulthood and developed a partial hypercapnia response. The therapeutic effects of desogestrel are being evaluated in clinical trials, and recent analyses of cellular responses to polyAla Phox2b aggregates have suggested new pharmacological approaches designed to counteract the toxic effects of mutated Phox2b.

Congenital central hypoventilation syndrome and sudden infant death syndrome: disorders of autonomic regulation

Long considered a rare and unique disorder of respiratory control, congenital central hypoventilation syndrome has recently been further distinguished as a disorder of autonomic regulation. Similarly, more recent evidence suggests that sudden infant death syndrome is also a disorder of autonomic regulation. Congenital central hypoventilation syndrome typically presents in the newborn period with alveolar hypoventilation, symptoms of autonomic dysregulation and, in a subset of cases, Hirschsprung disease or tumors of neural crest origin or both. Genetic investigation identified PHOX2B, a crucial gene during early autonomic development, as disease defining for congenital central hypoventilation syndrome. Although sudden infant death syndrome is most likely defined by complex multifactorial genetic and environmental interactions, it is also thought to result from central deficits in the control of breathing and autonomic regulation. The purpose of this article is to review the current understanding of these autonomic disorders and discuss the influence of this information on clinical practice and future research directions.

Anesthetic considerations for rapid-onset obesity, hypoventilation, hypothalamic dysfunction, and autonomic dysfunction (ROHHAD) syndrome in children

Rapid-onset obesity, hypoventilation, hypothalamic dysfunction, and autonomic dysfunction is an increasingly common diagnosis in patients who are being seen at tertiary care children's hospitals. We present two cases of anesthetics from the authors' own experience in addition to a comprehensive review of the disorder and anesthetic implications.

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.

[The congenital central hypoventilation syndrome (CCHS): a late presentation]

BACKGROUND

The congenital central hypoventilation syndrome (CCHS) or Ondine's curse is a rare autosomal dominant disease, characterized by disorders of the autonomic nervous system, with abnormal ventilatory responses to hypercapnia and hypoxia. PHOX2B has been identified as the major gene causing CCHS. It results from polyalanine repeat expansion mutations. It typically presents in the newborn period but some cases have been described in adults (late onset CCHS) reflecting the variable penetrance of PHOX2B mutations.

CASE REPORT

A 48 year-old woman presented, after ovarian cyst surgery, with severe hypoventilation requiring intubation. Arterial blood gases revealed a PaO2 of 6.6kPa (50mmHg), a PaCO2 of 10kPa (80mmHg) and a pH of 7.22. The past medical history revealed nocturnal symptoms for a few years. These included apnoeas, fitful sleep and awakening with headaches. Physical examination, pulmonary function tests, lung tomography and magnetic resonance imaging of the brainstem were all normal. Polysomnography revealed numerous central and obstructive apnoeas and hypopnoeas, with severe hypoxaemia and hypercapnia. Hypoxic and hypercapnic stimulation tests showed no adaptation of the ventilatory responses. Genetic analysis showed a heterozygous five alanine expansion mutation of the 20-residue polyalanine tract in exon 3 of the PHOX2B gene.

CONCLUSION

The diagnosis of late onset CCHS should be considered in patients with unexplained hypoventilation, and physiological evaluation should be undertaken to document the abnormal ventilatory responses. The presence of a PHOX2B mutation confirms the diagnosis.

PHOX2B mutations in patients with Ondine-Hirschsprung disease and a review of the literature

Congenital central hypoventilation syndrome (CCHS), also known as Ondine's curse, is characterized by idiopathic failure of autonomic breathing and is often associated with neurocristopathies such as Hirschsprung disease (HSCR). CCHS is caused by mutations in the paired-like homeobox 2B (PHOX2B) gene, often manifest as polyalanine repeat expansions. Herein, we report the cases of two unrelated Korean patients with Ondine-Hirschsprung disease. The patient's clinical manifestations were apnea and cyanosis requiring immediate endotracheal intubation, recurrent hypoventilation with hypercapnia, hypoxia after ventilator removal, and abdominal distension since birth. Intestinal biopsies were performed and the absence of ganglion cells in the colon was consistent with HSCR. We performed direct sequencing analysis in the PHOX2B and RET genes and fluorescence polymerase chain reaction in order to determine the polyalanine tract expansion in exon 3 of the PHOX2B gene. Expansion mutations were detected in both patients; one had 20/24 repeats and the other had 20/27 repeats. The 20/24 genotype has not been previously described in severe CCHS phenotypes and associated HSCR. We believe that the information in this report will improve our understanding of the phenotypic and genotypic heterogeneities of CCHS and HSCR.

Congenital central hypoventilation syndrome and the PHOX2B gene: a model of respiratory and autonomic dysregulation

The paired-like homeobox 2B gene (PHOX2B) is the disease-defining gene for congenital central hypoventilation syndrome (CCHS). Individuals with CCHS typically present in the newborn period with alveolar hypoventilation during sleep and often during wakefulness, altered respiratory control including reduced or absent ventilatory responses to hypercarbia and hypoxemia, and autonomic nervous system (ANS) dysregulation; however, a subset of individuals present well into adulthood. Thermoregulation is altered and perception of shortness of breath is absent, but voluntary breathing is retained. Structural and functional magnetic resonance imaging (MRI) and limited post-mortem studies in subjects with CCHS reveal abnormalities in both forebrain and brainstem. MRI changes appear in the hypothalamus (responsible for thermal drive to breathing), posterior thalamus and midbrain (mediating O(2) and oscillatory motor patterns), caudal raphé and locus coeruleus (regulating serotonergic and noradrenergic systems), the lateral medulla, parabrachial pons, and cerebellum (coordinating chemoreceptor and somatic afferent activity with breathing), and insular and cingulate cortices (mediating shortness of breath perception). Structural and functional alterations in these sites may result from PHOX2B mutations or be secondary to hypoxia/perfusion alterations from suboptimal management/compliance. The study of CCHS, with collaboration between physician-scientists and basic scientists, offers a rare opportunity to investigate control of breathing within the complex physiological network of the ANS.

An official ATS clinical policy statement: Congenital central hypoventilation syndrome: genetic basis, diagnosis, and management

BACKGROUND

Congenital central hypoventilation syndrome (CCHS) is characterized by alveolar hypoventilation and autonomic dysregulation.

PURPOSE

(1) To demonstrate the importance of PHOX2B testing in diagnosing and treating patients with CCHS, (2) to summarize recent advances in understanding how mutations in the PHOX2B gene lead to the CCHS phenotype, and (3) to provide an update on recommendations for diagnosis and treatment of patients with CCHS.

METHODS

Committee members were invited on the basis of their expertise in CCHS and asked to review the current state of the science by independently completing literature searches. Consensus on recommendations was reached by agreement among members of the Committee.

RESULTS

A review of pertinent literature allowed for the development of a document that summarizes recent advances in understanding CCHS and expert interpretation of the evidence for management of affected patients.

CONCLUSIONS

A PHOX2B mutation is required to confirm the diagnosis of CCHS. Knowledge of the specific PHOX2B mutation aids in anticipating the CCHS phenotype severity. Parents of patients with CCHS should be tested for PHOX2B mutations. Maintaining a high index of suspicion in cases of unexplained alveolar hypoventilation will likely identify a higher incidence of milder cases of CCHS. Recommended management options aimed toward maximizing safety and optimizing neurocognitive outcome include: (1) biannual then annual in-hospital comprehensive evaluation with (i) physiologic studies during awake and asleep states to assess ventilatory needs during varying levels of activity and concentration, in all stages of sleep, with spontaneous breathing, and with artificial ventilation, and to assess ventilatory responsiveness to physiologic challenges while awake and asleep, (ii) 72-hour Holter monitoring, (iii) echocardiogram, (iv) evaluation of ANS dysregulation across all organ systems affected by the ANS, and (v) formal neurocognitive assessment; (2) barium enema or manometry and/or full thickness rectal biopsy for patients with a history of constipation; and (3) imaging for neural crest tumors in individuals at greatest risk based on PHOX2B mutation.

Carbon dioxide chemoreception and hypoventilation syndromes with autonomic dysregulation

Respiratory and autonomic disorders of infancy, childhood, and adulthood are a group of disorders that have varying presentation, combined with a range of severity of respiratory control and autonomic nervous system dysfunction. Within this group, congenital central hypoventilation syndrome and rapid onset obesity with hypothalamic dysfunction, hypoventilation, and autonomic dysregulation, exhibit the greatest respiratory control deficits, requiring supported ventilation as a mainstay of care. The discovery of the key role of the paired-like homeobox 2B gene in autonomic nervous system development, along with the identification of paired-like homeobox 2B gene mutations causing congenital central hypoventilation syndrome, has led to a fruitful dialog between basic scientists and physician-scientists, producing an explosion of knowledge regarding genotype-phenotype correlations in this disorder, as well as important animal models of chemosensory regulation deficit. Though the etiology of rapid onset obesity with hypothalamic dysfunction, hypoventilation, and autonomic dysregulation is still to be determined, recent studies have begun to carefully delineate the phenotype, suggesting that it too may provide fertile ground for research that both advances our knowledge and improves patient care.

Association analysis of the PHOX2B gene with Hirschsprung disease in the Han Chinese population of Southeastern China

BACKGROUND

Hirschsprung disease (HSCR, OMIM 142623) is a complex congenital disorder characterized by intestinal obstructions caused by the absence of the intestinal ganglion cells of the nerve plexuses in variable lengths of the digestive tract. The PHOX2B gene is involved in neurogenesis and disruption of Phox2b in mice results in a HSCR-like phenotype. The first association study of the PHOX2B gene with HSCR derived from Chinese population in Hong Kong; here, we address the question of whether PHOX2B acts as a predisposing factor in HSCR pathogenesis in Chinese population in mainland.

METHODS

To investigate the contribution of PHOX2B to the HSCR phenotype, polymerase chain reaction amplification and direct sequencing were used to screen PHOX2B coding regions and intron/exon boundaries for mutations and polymorphisms in 102 patients with HSCR and 96 ethnically matched controls, in Han Chinese populations of Southeastern China.

RESULTS

In this study, we genotyped 4 single nucleotide polymorphisms (SNPs) (including 1 novel SNP) located within the PHOX2B gene. Statistically significant differences were found for c.701 A > G and IVS2 + 100 A > G, and the log-additive model was accepted as the best inheritance model (odds ratio [OR], 1.79; 95% confidence interval [CI], 1.11-2.87) for IVS2 + 100 A > G. We also showed that the haplotype-A G A N composed of 4 SNPs exhibited significant association with the disease (P = .03); this haplotype was more frequently observed in cases than in controls (OR, 2.31; 95% CI, 1.11-4.82).

CONCLUSIONS

Our study provided further evidence that the PHOX2B gene is involved in the susceptibility to HSCR in the Han Chinese population. Our findings are in accordance with the involvement of PHOX2B in the signaling pathways governing the development of enteric neurons.

Later-onset congenital central hypoventilation syndrome due to a heterozygous 24-polyalanine repeat expansion mutation in the PHOX2B gene

AIM

to describe a family with later onset congenital central hypoventilation syndrome (LO-CCHS) and heterozygosity for a 24-polyalanine repeat expansion mutation in the PHOX2B gene, rendered phenotypically apparent with exposure to anesthetics.

CASE SUMMARY

An otherwise healthy 2.75-year-old boy presented with alveolar hypoventilation after adenoidectomy and tonsillectomy for obstructive sleep apnea, requiring invasive ventilatory support during sleep. He had a heterozygous 24-polyalanine repeat expansion in the PHOX2B gene (20/24 genotype), a genotype that has not been previously described in association with CCHS or LO-CCHS symptoms. Clinical findings in members of the family with the same 20/24 genotype ranged from asymptomatic to prolonged sedation after benzodiazepines.

CONCLUSION

CCHS should be suspected in individuals presenting with unexplained hypoventilation and/or seizures after anesthetics or sedatives. This is the first report of LO-CCHS in a kindred with the PHOX2B 20/24 genotype. The incomplete penetrance observed in this family suggests a gene-environment interaction.