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 reux 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 specic disease, rather than looking for mutations in the single most frequently correlated gene.


Introduction
Congenital central hypoventilation syndrome (CCHS) is a genetic disorder characterised by an idiopathic failure of the autonomic control of breathing resulting in hypoventilation, impaired ventilatory response to hypercapnia and hypoxemia episodes, more severe during sleep (1)(2). CCHS has been associated to a large spectrum of autonomic nervous system dysfunctions (2). It is considered a rare syndrome affecting approximately 1:200,000 live births, and it is usually diagnosed in the newborn period or in infancy, more rarely in childhood or adulthood (3). Many studies have reported that mutations in the PHOX2B gene represent the genetic cause in 90% of CCHS cases (2). Mutations of other genes involved in the development of neural crest cells, such as RET, GDNF, ASCL1 and EDN3 were identi ed in rare cases of CCHS (2)(3)(4).
Here, we report on the case of a 15-month-old toddler presenting with CCHS features, caused by a de novo missense variant in the RET gene.

Patient Information
A 15-month-old boy was admitted to the Pediatric Oncology Unit in January 2020 for several episodes of brief respiratory arrest characterized by cyanosis, limb hypertonia, xed gaze, altered level of responsiveness and resolution after strong stimulation manoeuvres. The symptoms were noticed by parents one month before the admission, more frequently during sleep. Parents referred also sporadic profuse sweating, and chronic constipation. When the patient was 3-month-old, he was diagnosed with cyanotic breath-holding spells. The mother told us that, after an uncomplicated pregnancy, a Caesarean section was performed due to obstetric problems. Birth weight was 2,750 g. The newborn physical examination revealed monolateral clubfoot, surgically treated in the early months of life.

Clinical Findings
Physical examination upon admission to our unit revealed mild hypotonia. Vital parameters monitoring (heart rate, respiratory rate, blood pressure and oxyhaemoglobin saturation [SO 2 ]) showed apnoeas and desaturation episodes (from 1 to 10 episodes per day) more frequently during sleep (SO 2 of 88-89% in ambient air), but also during wakefulness.

Therapeutic Intervention
The patient started proton pump inhibitor therapy (Lansoprazole 1 mg/kg/die) for one month without signi cant clinical improvement.

Follow-up and Outcomes
The child was moved to a specialised Bronchopneumology Unit in order to receive the best respiratory care.

Discussion
CCHS represents a rare disorder characterized by abnormal control of respiration in the absence of neuromuscular, lung or cardiac disease, or an identi able brainstem lesion. Although CCHS patients usually present apnoeas or breath holding spells since neonatal period (5), milder respiratory disorders might be detected in childhood or adulthood as late-onset CCHS, during sedation, anaesthesia, respiratory infection or sleep apnoea (2). The typical alveolar hypoventilation leads to a period of cyanosis during sleep induction, a decrease in SO2 and increase in the partial pressure of CO2 in arterial blood, with no physiological respiratory response or arousal re ex (6). Hypoventilation usually occurs during non-rapid eye movement (NREM) sleep, but it can also be present during REM sleep and wakefulness (1). Affected children may present neurocognitive decline related to the recurrent hypoxemia/hypercapnia episodes (7), tumours derived from neural crest cells (neuroblastoma, ganglioneuroma, and ganglioneuroblastoma), Hirschsprung's disease (HSCR), autonomic nervous system dysregulation with oesophageal dysmotility, constipation, cardiovascular and temperature regulation abnormalities. CCHS requires an early diagnosis in order to establish adequate ventilator management and to minimise neurocognitive defects related to the repeated hypoxia and hypercapnia episodes. CCHS represents a complex diagnosis due to the rarity and the phenotypic variability of this entity. CCHS diagnosis has to be suspected when children present with hypoventilation episodes, worsening during sleep, without primary pulmonary, metabolic, neurologic, cardiac disease, or brainstem lesions (8). In the present case, episodes of central and obstructive apnoea with cyanosis, hypertonia, hypercapnia, and cyanotic breath-holding spells were considered as features of CCHS (9). The diagnosis of CCHS was supported by central desaturating apnoea/hypopneas episodes during REM sleep and obstructive apnoea in PSG, as well as hypoxia and hypercapnia in ABG during critical episodes. Sporadic profuse sweating and gastrointestinal dysmotility resulting in gastrooesophageal re ux and chronic constipation were considered as further features of CCHS. Despite pathogenic variants in PHOX2B usually represent the cause of CCHS, some papers highlighted that RET-GDNF signalling and/or EDN3-endothelin receptor-B signalling pathways play a signi cant role in the development of the respiratory centre and could be associated with the pathophysiology of CCHS (3). CCHS patients present Hirschsprung's disease (HSCR) in 16-20% of cases. HSCR is a congenital absence of ganglion cells in the myenteric and submucosal plexuses of bowel. Pathogenic variants in the RET gene have been associated with up to 50% of familial and 35% of sporadic cases of HSCR (10)(11).
The reported association of CCHS with HSCR in humans, and a similar phenotype observed in RET knockout mice, supports that RET is a candidate gene for CCHS (6). NGS performed in this patient revealed the de novo p.Met918Thr heterozygous missense variant in the RET gene, whose allelic frequency accounts 0.00000398. As RET mutations correlate with HSCR, multiple endocrine neoplasia IIA and IIB, pheochromocytoma, and medullary thyroid carcinoma, we performed chest, neck and abdominal radiological examinations, serum tumour markers, and we recommended a close follow-up for the lateonset occurrence of tumours. 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 speci c disease rather than looking for mutations in the single most frequently correlated gene.

Patient Perspective
Parents were informed about clinical features of disease and they agreed with management.

Informed Consent
Parents gave informed consent for the composition of case report.

Declarations
Ethics approval and consent to participate achieved Consent for publication achieved Authors' contributions: Drs Silvestri, Dr Rinaldi, Drs Giansanti and Drs Russo conceptualized and designed the case report, drafted the initial manuscript and reviewed and revised the manuscript.
Dr Spalice and Dr Pizzuti contributed to conception and design of the manuscript, carried out the initial analyses, analysed and interpreted datas , supervised and coordinated the manuscript and reviewed and revised it.
Drs Schiavetti designed the data collection instruments, coordinated and supervised data collection, and critically reviewed the manuscript for important intellectual content.