PHOX2B mutations and ventilatory control
Introduction
Respiratory control impairments occurring early during development stages may compromise brain oxygenation, thereby leading to irreversible motor and cognitive disorders. The etiology and incidence of these impairments vary considerably, from apnea of prematurity, seen in 85% of infants born before 34 weeks of gestation (Schmidt et al., 2006), to Congenital Central Hypoventilation Syndrome (CCHS or Ondine's syndrome), a rare disorder typically presenting in the newborn period (Weese-Mayer et al., 1999). The incidence was estimated at 1 per 200,000 live births based on the French cohort of patients (Trang et al., 2005) but a more accurate estimate will be obtained in the near future from on-going international epidemiological studies of CCHS populations.
Mouse studies of respiratory phenotypes in mutant newborn mice have helped to identify candidate genes for developmental respiratory control disorders (Gaultier and Gallego, 2005). Regarding CCHS, several groups (Matera et al., 2002, Amiel et al., 2003b, Weese-Mayer et al., 2003) examined the RNX gene, as RNX knock-out mice exhibited respiratory control disorders, but found no mutations. Weese-Mayer et al. (2002) analyzed brain-derived neurotrophic factor (BDNF) as a potential candidate gene in CCHS after respiratory control disorders were observed in mice with Bdnf mutations (Erickson et al., 1996). Based on mouse studies showing cross-regulation of the Phox2b and Mash1 genes and impaired ventilatory responses to hypercapnia, the human ortholog of Mash1 (HASH-1) was considered an additional candidate gene for CCHS (de Pontual et al., 2003, Weese-Mayer et al., 2003).
Although rare, CCHS provides a unique opportunity to identify genetic factors involved in respiratory control development. This syndrome is characterized by sleep-related hypoventilation and apneas (especially in severe cases) with severe abnormalities in chemosensitivity, as well as by various autonomic disorders of widely variable penetrance (Weese-Mayer et al., 1999, Marazita et al., 2001, Weese-Mayer et al., 2001, Vanderlaan et al., 2004). However, a general limitation of most clinical descriptions of CCHS is the lack of genetic testing. The considerable inter-individual variability in the number and severity of associated symptoms probably reflects differences in the nature of the genetic defect (polyalanine expansion, missense or frameshift mutations, see below). Affected patients have absent or markedly reduced ventilatory responses to sustained hypercapnia (Paton et al., 1989) and, to a lesser extent, to sustained hypoxia (Paton et al., 1989). These abnormalities are generally ascribed to impaired central integration of chemosensory inputs to the brainstem, rather than to failure of the chemoreceptors, which are at least partially active (Marcus et al., 1991, Gozal et al., 1993, Spengler et al., 2001). Respiratory control disorders are difficult to analyze in the most severe forms of CCHS (e.g., patients who cannot breathe spontaneously even when awake). For this reason, previous studies may not reflect the entire spectrum of respiratory disorders in CCHS.
The identification of PHOX2B as the disease-causing gene in CCHS (Amiel et al., 2003a, Sasaki et al., 2003, Weese-Mayer et al., 2003, Matera et al., 2004) has stimulated considerable interest in the role for this gene not only in CCHS, but also in prevalent conditions such as sleep apnea. Recently, there have been several reports of adults exhibiting sleep-related hypoventilation and, in some instances, central apneas and severe hypoxemia, with PHOX2B mutations generally characterized by five additional alanine residues in PHOX2B (Matera et al., 2004, Trang et al., 2004, Trochet et al., 2005a, Weese-Mayer et al., 2005b, Antic et al., 2006, Barratt et al., 2007, Diedrich et al., 2007, Doherty et al., 2007, Trochet et al., 2008). These observations suggest that respiratory control disorders associated with PHOX2B mutations may be more prevalent than previously inferred from the very low incidence of CCHS in newborns. More generally, PHOX2B appears pivotal to the development of respiratory networks. To analyze the functional impact of PHOX2B mutations on breathing at the organism level, mouse models must be studied. The present review focuses on the respiratory phenotypes of mice lacking one Phox2b allele (Phox2b+/− mutants). The fact that Phox2b+/− pups develop normally is in striking contrast to the severe abnormalities seen in humans with CCHS. However, the purpose of this mouse model is to determine which components of respiratory control, if any, depend on Phox2b for normal development. This approach is critical to unraveling the pathogenesis of CCHS. The present review also describes the respiratory phenotype of mice carrying polyalanine expansions. We studied mice carrying a +7 alanine expansion (Phox2b27Ala/+ mice). This genotype was the most frequent in a cohort of 188 patients with CCHS occurring either as an isolated disorder or in combination with Hirschsprung disease (HSCR) and/or tumors of the sympathetic nervous system (Trochet et al., 2005a).
Section snippets
PHOX2B, the main disease-causing gene for CCHS
About 20% of patients with CCHS also have Hirschsprung disease, a developmental disorder of the enteric nervous system characterized by absence of ganglion cells in the distal colon (Weese-Mayer et al., 2001, Amiel et al., 2008). This association suggested shared pathophysiological mechanisms in CCHS and HSCR. Furthermore, most patients with CCHS exhibit a variety of autonomic disorders, and CCHS is associated with tumors developed from neural crest cells (neuroblastoma), in addition to
Early assessment of respiratory control in mice
To investigate the role for Phox2b in the development of respiratory control, the respiratory phenotype of neonatal mice with one invalidated Phox2b allele (Phox2b+/−) was determined. Homozygous Phox2b knock-out mice (Phox2b−/−) die in utero around embryonic day 14 (Pattyn et al., 2000b), whereas Phox2b+/− pups survive and are fertile. Body weight and mouth temperature were normal in 2-day-old Phox2b+/− mice (Dauger et al., 2003). At 5 days of age, however, the mutant pups had slightly lower
The ventilatory phenotype of Phox2b27Ala/+ mice
In contrast to Phox2b+/− mutant mice (carrying one invalidated Phox2b allele), Phox2b27Ala/+ mice carry a widespread CCHS-causing mutation, namely, a +7 alanine expansion of the 20-residue poly-Ala tract (the Phox2b27Ala allele) (Dubreuil et al., 2008). Heterozygous Phox2b27Ala/+ offspring of the chimeric founders (obtained using a knock-in approach) died within a few hours after birth. At birth, the weights and body temperatures (before and after plethysmographic recordings) of mutant and
Conclusion
Over the last few years, important advances have shed light on the function of PHOX2B in respiratory control and on its role in CCHS. These achievements are the result of multidisciplinary efforts combining genetic studies in humans with neurobiological and physiological studies in mouse models. Phox2b is required for the development of all major homeostatic functions (Brunet and Goridis, 2008). In particular, studies in adult rats indicated that Phox2b was pivotal to the provision of central
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2021, Sleep MedicineCitation Excerpt :To date, a limited number of investigations have been published on the issue of ventilatory control in OSA and those that do exist have reported contradictory results. However, it seems that the mutations reported in the Phox2b transcription factor gene are the most important cause of ventilatory disorders in OSA [52,53]. Obesity is intermediately linked to an increase in the risk of developing OSA, and population-based studies have found a close association between BMI and OSA.
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2012, Autonomic Neuroscience: Basic and ClinicalCitation Excerpt :The second objective of this study was to determine whether abnormalities in SIUD and SIDS cases are evident in the hRTN given the essential and critical role with respect to respiratory control of Phox2b RTN neurons in animal models, and the demonstrable role of PHOX2B mutations in disrupting breathing in humans (Weese-Mayer et al., 2003, 2009) and in mice (Abbott et al., 2009; Dubreuil et al., 2009). In rodents, the development of RTN neurons is uniquely sensitive to a PHOX2B mutation that causes a large decrease in the central chemoreflex and in breathing automaticity (Gallego and Dauger, 2008; Takakura et al., 2008). In the human, the paired like homeobox gene, PHOX2B, is the disease-defining gene for CCHS — an autosomal dominant genetic disorder characterized by alveolar hypoventilation in sleep and (in a subset of cases) wakefulness, altered response to hypoxemia and hypercarbia asleep and awake, and evidence of ANS dysregulation (Amiel et al., 2003; Weese-Mayer et al., 2008, 2009, 2010).
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2010, Paediatric Respiratory ReviewsCitation Excerpt :Expression of the most common human CCHS mutation in a mouse model tremendously increased understanding of the role of the RTN in ventilatory control. Several detailed reviews reveal the enormous potential of this approach for understanding ventilatory control and its development.37,38 Another clinical disorder with insights from genetically altered mouse models is Prader-Willi Syndrome (PWS).
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