Mutations of PHOX2B Gene in Patients of Obesity Hypoventilation Syndrome in Central India

Background Paired-like homeobox 2B (PHOX2B) gene on chromosome 4p12 codes for a transcription factor having a role in the formation of noradrenergic neuronal circuits. Its mutations have been linked to congenital central hypoventilation syndrome (CCHS). The clinical presentation of both, obesity hypoventilation syndrome (OHS) and CCHS in adults (named late-onset central hypoventilation syndrome), is quite similar. Because of this symptomatic similarity, multifactorial causation of OHS, the mutation of PHOX2B gene was studied in patients with OHS in this study. Methods A hospital-based cross-sectional study was performed on patients diagnosed with OHS. The deoxyribonucleic acid was extracted from 2 mL of venous blood and was further amplified, specific to exon 3. The amplified products were cast and run in 2% agarose gel and then subjected to Sanger sequencing. Results Thirty patients of OHS (21 male; 9 female) were enrolled in the present study, average age being 51.7 years. The Sanger sequencing of the samples revealed no apparent areas of deletions and no apparent mutations. Conclusion Primers for exon 3 were used for amplification in thermocycler, as exon 3 is the most frequently mutated exon for PHOX2B gene, as per existing literature. The entire gene needs to be studied for mutations and the sample size needs to be increased.

Congenital Central Hypoventilation Syndrome Caused by A de novo RET Pathogenic Variant

Introduction: Congenital central hypoventilation syndrome (CCHS) is a rare life-threatening disorder characterized by a failure in physiologic autonomic control of breathing resulting in apnoea, hypoxemia, and hypoventilation episodes, associated to a large spectrum of autonomic nervous system dysfunctions. Despite this condition is most likely caused by mutations in the PHOX2B gene, other genes have been found responsible for CCHS in rare cases. Case report: We report a 15-month-old toddler presenting episodes of central and obstructive apnoea with cyanosis, hypertonia, hypercapnia, and cyanotic breath-holding spells. The CCHS diagnosis was supported by central desaturating apnoea/hypopneas episodes during rapid eye movement sleep and obstructive apnoea in polysomnography, as well as by the presence of hypoxia and hypercapnia in arterial blood gas during critical episodes. Autonomic dysregulation with sporadic profuse sweating and gastrointestinal dysmotility resulting in gastro-oesophageal reflux and chronic constipation were also associated. Next Generation Sequencing revealed the missense variant p.Met918Thr in the RET gene.Conclusion: This case, identifying a de novo pathogenic variant in the RET gene, highlights the importance of using clinical exome sequencing or a panel of genes associated with the specific disease, rather than looking for mutations in the single most frequently correlated gene.

N-terminal deletion of specific phosphorylation sites on PHOX2B disrupts the formation of enteric neurons in vivo

Mutations in the paired-like homeobox 2b (PHOX2B) gene are associated with congenital central hypoventilation syndrome (CCHS), which is a rare condition in which both autonomic dysregulation with hypoventilation and an enteric neuropathy may occur. The majority of CCHS patients have a polyalanine repeat mutation (PARM) in PHOX2B, but a minority of patients have non-polyalanine repeat mutations (NPARM), some of which have been localized to exon 1. A PHOX2B-Y14X nonsense mutation previously generated in a human pluripotent stem cell (hPSC) line results in an N-terminus truncated product missing the first 17 or 20 amino acids, possibly due to translational reinitiation at an alternate ATG start site. This N-terminal truncation in the PHOX2B protein results in the loss of two key phosphorylation residues. Though the deletion does not affect the potential for PHOX2BY14X/Y14Xmutant hPSC to differentiate into enteric neural crest cells (ENCC) in culture, it impedes in vivo development of neurons in an in vivo model of human aganglionic small intestine.

STRUCTURE OF HEREDITARY AND CONGENITAL PATHOLOGY IN A MULTIDISCIPLINARY SPECIALIZED PEDIATRIC CLINIC: PERSONALIZED APPROACH TO DIAGNOSTICS

Hereditary and congenital pathologies make up a significant proportion in the structure of the general morbidity and mortality of the child population. In this regard, the problem of personalized medical care in multidisciplinary clinical centers seems very urgent. In the Scientific-Practical Center of specialized medical care for children the medical genetic service successfully functions since 1995. In 2017–2019, patients were consulted by geneticists, which accounted for 10,6% of all discharged patients; karyotype studies were performed in 463 (20,4%) consulted patients, chromosomal pathology was revealed in 28 (6,04%) of all karyotyped patients. DNA research was performed in 188 patients: exome sequencing – in 112; sequencing of exons of the SCN1A gene – in 39; search for mutations in the PHOX2B gene – in 17; search for frequent mutations in the FGFR1,2,3 genes – in 15 (the technique was introduced since September 2018); sequencing of SLC2A1 gene – in 5 patients. The high demand for medical genetic counseling and laboratory genetic diagnosis of hereditary and congenital diseases confirms the need for independent medical genetic units in multidisciplinary clinical hospitals, that should interact with medical genetic services at regional and federal levels.

A Common 3′UTR Variant of the PHOX2B Gene Is Associated With Infant Life-Threatening and Sudden Death Events in the Italian Population

Heterozygous mutations in the Paired like homeobox 2b (PHOX2B) gene are causative of congenital central hypoventilation syndrome (CCHS), a rare monogenic disorder belonging to the family of neurocristopathies and due to a defective development of the autonomic nervous system. Most patients manifest sudden symptoms within 1 year of birth, mainly represented by central apnea and cyanosis episodes. The sudden appearance of hypoxic manifestations in CCHS and their occurrence during sleep resemble two other unexplained perinatal disorders, apparent life-threatening event (ALTE) and sudden and unexpected infant death (SUID), among which the vast majority is represented by sudden infant death syndrome (SIDS). Differently from CCHS, characterized by Mendelian autosomal dominant inheritance, ALTE and SIDS are complex traits, where common genetic variants, together with external factors, may exert an additive effect with symptoms likely manifesting only over a “threshold.” Given the similarities observed among the three abovementioned perinatal disorders, in this work, we have analyzed the frequency of PHOX2B common variants in two groups of Italian idiopathic ALTE (IALTE) and SUIDs/SIDS patients. Here, we report that the c*161G>A (rs114290493) SNP of the 3′UTR PHOX2B (i) became overrepresented in the two sets of patients compared to population matched healthy controls, and (ii) associated with decreased PHOX2B gene expression, likely mediated by miR-204, a microRNA already known to bind the 3′UTR of the PHOX2B gene. Overall, these results suggest that, at least in the Italian population, the SNP c*161G>A (rs114290493) does contribute, presumably in association with others mutations or polymorphisms, to confer susceptibility to sudden unexplained perinatal life-threatening or fatal disorders by increasing the effect of miR-204 in inducing PHOX2B expression down-regulation. However, these are preliminary observations that need to be confirmed on larger cohorts to achieve a clinical relevance.

Recurrent apnoea and respiratory failure in an infant: congenital central hypoventilation syndrome with a novel PHOX2B gene variant

A 20-day-old term infant presented with recurrent apnoea, lethargy and respiratory failure. Examination revealed episodes of apnoea and desaturation to 85% without any signs of respiratory distress requiring initiation of non-invasive positive pressure ventilation (NPPV). Capillary blood gas was indicative of respiratory acidosis and serum bicarbonate was elevated at 35 mmol/L. Chest radiograph, echocardiogram and evaluations for infectious aetiologies resulted normal. Due to inability to wean off NPPV with ensuing apnoea and desaturation, polysomnogram was performed and showed central and obstructive sleep apnoea, hypoxaemia and hypoventilation. Central apnoeas and hypoventilation were worse in non-rapid eye movement sleep. Paired-like homeobox 2B genetic studies showed a novel non-polyalanine repeat mutation (c.429+1G>A) establishing the diagnosis of congenital central hypoventilation syndrome (CCHS). Our case highlights the utility of polysomnography in the evaluation of term infants with apnoea. Although rare, clinicians should consider a diagnosis of CCHS in the evaluation of infants with apnoea and hypoventilation.

Central Sleep Apnea

As part 3 of the three chapters on sleep-disordered breathing, this chapter reviews central sleep apnea (CSA). CSA is defined as recurrent apneic events during sleep in the absence of respiratory muscle effort and loss of neuronal output to respiratory muscles. The most common cause of CSA is compromised cardiac function. In patients with CSA whose ejection fraction is less than or equal to 45%, adaptive servo-ventilation (ASV) has been shown to increase mortality. With the abuse of opioids reaching epidemic proportions, it has been estimated that 30 to 75% of patients on chronic opioid therapy have a significant increased incidence of CSA. Opioid-induced sleep-disordered breathing can be treated with ASV. This review contains 1 figure, and 37 references. Key Words: adaptive servo-ventilation, central sleep apnea, Cheyne-Stokes respiration, congenital central hypoventilation syndrome, impaired central ventilatory drive, opioid abuse, PHOX2B gene, supplemental oxygen