A Truncating Variant of CHRNG as a Cause of Escobar Syndrome: A Multiple Pterygium Syndrome Subtype

Abstract

Escobar syndrome is a milder variant of multiple pterygium syndrome characterized by pterygia, scoliosis, and multiple congenital contractures. It is most frequently due to a genetic variant in CHRNG , which encodes the γ-subunit of the nicotinic acetylcholine receptor. Though the subunit is considered a “fetal” form and transitions to the “adult” ε-subunit by 33 weeks' gestation, the pathogenic musculoskeletal effects during fetal development render children with this condition permanently affected. We report a neonate with homozygous CHRNG c.117dupC and discuss some of the downstream clinical effects we observed with this variant.

Background

Arthrogryposis multiplex congenita (AMC) describes a group of congenital conditions characterized by joint contractures in two or more body areas at birth. ^{1 2} Although AMC could be due to decreased fetal movement during early development, the underlying etiology can be quite varied. Amyoplasia, distal arthrogryposis, and syndromic AMC due to primary neurological or muscle disorders are the common types. ^{1 2} One form is the multiple pterygium syndrome (MPS), that is characterized by, but not limited to, fetal growth retardation, cleft palate, multiple pterygia involving the neck, axilla, fingers, antecubital and popliteal regions, club feet, “rocker-bottom” feet (congenital vertical talus), congenital diaphragmatic weakness, and multiple joint contractures at birth. ³ There are two forms of MPS, a lethal variant and a nonlethal variant. In 1978, Escobar et al described the minimal diagnostic criteria of the nonlethal variant, as earlier, later earning the name “Escobar syndrome.” ⁴

The pathogenesis of MPS has become more transparent over the years with molecular genetics, proteomics, and immunohistochemistry, which have demonstrated in utero neuromuscular junction (NMJ) dysfunction leading to akinesia and myasthenia. ⁵ Several genotypic abnormalities have been described in Escobar syndrome with CHRNG gene mutations being the most common. Herein, we describe a molecularly confirmed neonatal case of Escobar syndrome and discuss the genetic, molecular, and musculoskeletal consequences of this pathogenic variant.

Case Presentation

A term male infant was born to a 21-year-old primigravid African mother via forceps-assisted vaginal delivery with a birth weight of 2,740 g (<5th percentile), length of 40 cm (<10th percentile), and head circumference of 36 cm (25th percentile). Fetal ultrasound showed polyhydramnios and left upper and lower extremity contractures. There was no history of consanguinity and family had no history of congenital abnormalities. There was no history of exposure to teratogens in pregnancy. Physical examination at birth showed facial dysmorphism manifested with down slanting palpebral fissures and mild ptosis ( Fig. 1A ), high arched palate, retrognathia, and cleft of the soft palate. Musculoskeletal examination was remarkable for contractures of multiple joints including shoulders, elbows, and knees accompanied by multiple pterygia most evident in the axilla and neck ( Fig. 1B ). Additional findings included bilateral camptodactyly ( Fig. 1C ), left club foot, and bilaterally prominent calcanei. At birth, he immediately required mechanical ventilation for respiratory insufficiency. Chest radiograph demonstrated an elevated right diaphragm suggestive of diaphragmatic eventration ( Fig. 1D ). Due to arthrogryposis and complex anatomy, he had difficult airway access for endotracheal intubation, which was compounded by the fact that nondepolarizing neuromuscular blocker such as rocuronium and muscle relaxants did not cause the desired muscle relaxation or paralyze the infant. Computed tomography of the chest demonstrated small volume lungs suggestive of hypoplasia with relative superior positioning of the anterior intra-abdominal structures compatible with right diaphragmatic eventration. Further evaluation including magnetic resonance imaging of the brain, formal ophthalmologic examination, and renal ultrasound did not reveal any abnormalities, though echocardiogram showed a small ventricular septal defect. Chromosomal evaluation showed 46,XY karyotype and normal microarray. Whole exome sequencing revealed a homozygous c.117dupC in exon 2 of CHRNG , causing p.Asn40Glnfs ^* 96 of CHRNG , suggestive of the diagnosis of Escobar syndrome and confirmed by Sanger ( Supplementary Figs. S1 , S2 , S3 , available in the online version). Due to social constraints, we were unable to obtain any parental exome.

Fig. 1.

Examination findings in Escobar syndrome. (A) Facial dysmorphism, (B) contracture of elbow and shoulder, (C) camptodactyly, (D) left club foot, (E) anteroposterior chest radiograph revealing elevated right hemidiaphragm, (F) lateral chest radiograph demonstrating an elevated right diaphragm suggestive of diaphragmatic eventration.

The hospital stay was marked by persistent respiratory distress with the inability to wean the patient from continuous positive airway pressure. Because of his respiratory insufficiency, he was incapable of oral feeding and at 3 months of age, the patient underwent surgery for right diaphragmatic plication and gastrostomy. Tracheostomy, mandibular distraction, and cleft palate repair were performed at 5 months of age and the patient was transferred to a long-term dependent care facility and no further follow-up could be obtained.

Discussion

Neuromuscular signaling is a complex electrophysiologic process involving a motor nerve action potential reaching and activating the NMJ. ⁶ The muscle-specific nicotinic acetylcholine receptor (AChR) is a transmembrane pentameric protein expressed on postsynaptic skeletal muscle fibers of the NMJ. This is critical for signaling between neurons and muscle cells whose five subunits are coded by CHRNA1, CHRNB, CHRND, and CHRNG/CHRNE. The γ-subunit of AChR encoded by CHRNG is expressed during fetal development. ^{1 7} Expression of the γ-subunit gene ( CHRNG ) is downregulated and subsequently lost around 33 weeks' gestation, replaced with the epsilon (ε) subunit to form the “adult” subtype nAChR, which is produced by the CHRNE gene. ^{8 9 10 11} Proper functioning of these integral membrane proteins is essential in utero for normal musculoskeletal development and maturation. CHRNG gene mutations which result in a lack of a functional γ-subunit prevents the fetal AChR protein from being assembled in the muscle cell membrane leading impaired neuromuscular communication in the developing fetus—fetal akinesia and results in MPS. There are more than 14 different mutations and 2 of which lead to the lethal form and nonlethal Escobar variant. Though Escobar syndrome does not directly lead to myasthenic symptoms or abnormal muscle histopathology after the fetal period (as the gamma subunit is only expressed during fetal life), congenital diaphragmatic muscle weakness, diffuse myopathy, and contractures reported in these patients are indirectly attributed to the variant γ-subunit in muscle organogenesis and maturation.

There are many potential causes of AMC, which arises from the lack of appropriate spontaneous prenatal muscle contractions. ^{12 13} The long-term sequelae of the various causes tend to be similar, regardless of the underlying pathology: intrauterine growth restriction, craniofacial anomalies, and limb contractures due to lack of somatic skeletal muscle movement, pulmonary hypoplasia due to diaphragmatic paralysis, short gut and major feeding difficulties due to lack of fetal swallowing, and ocular hypertelorism, low set ears, short webbed neck, and high arched palate, but there is wide variability in the clinical phenotype. ^{14 15} FADS is the most severe form of AMC and not all cases of MPS fall into this.

Escobar syndrome is due to a mutation in CHRNG which prevents normal fetal movement and subsequently leads to MPS. MPS is clinically and genetically heterogeneous disorder. ¹⁴ In addition to arthrogryposis of the joints and pterygia of the neck and joints, other phenotypic features such as short stature, genital abnormalities, craniofacial abnormalities, clubfoot, kyphoscoliosis, and cardiac abnormalities can be seen. ¹⁴ MPS can occur in autosomal-recessive, autosomal-dominant, or X-linked-dominant transmission. ¹⁴ Critical to the pathophysiology of Escobar syndrome, based on animal data, the dysfunctional γ-subunit also prevents or limits the expression of the nAChR, neuromuscular synaptogenesis, and NMJ organization. ⁵ Our patient demonstrated homozygous single nucleotide duplication at c.117 (exon 2) causing a frameshift that led to an early termination codon at codon 96; this rendered the γ-subunit either nonfunctional or unable to undergo translocation into the postsynaptic membrane of the NMJ during fetal development. Despite “receptor rescue” after the γ–ε switch presumed by 33 weeks' gestation, our patient might have also had disorganized NMJs with abnormal distribution of AChRs and acetylcholinesterase. ⁵ As mechanism of action for nondepolarizing paralytics involves AChRs, neuromuscular blockade failure in our patient during attempts at securing airway was likely due to his underlying condition.

There is one prior study on genetic causes of arthrogryposis that identified a single patient with CHRNG c.117dupC. ¹⁶ Therefore, we believe this is the first dedicated report on this specific variant of CHRNG , and thus provides a complete clinical examination and the consequences of this variant for future clinicians to reference when they confirm this variant in a patient. We acknowledge that parental exomes would have provided imperative information regarding the inheritance pattern of CHRNG , but we are left to speculate that like most cases of Escobar syndrome, this was likely inherited in an autosomal recessive fashion.

Conclusion

Airway management has been known to be difficult in the MPS Escobar variant patients and a cautionary approach with adequate precautions should be taken prior to securing the airway with the choice of medications and having emergent tracheostomy available.

Funding Statement

Funding None.