A male infant born in a tertiary maternity facility was noted to have microretrognathia, a small mouth and macroglossia at delivery. He was born limp and apnoeic and required multiple attempts at intubation before a definitive airway was eventually sited. Chest X-rays, while in the paediatric intensive care unit, demonstrated dysplastic ribs with associated 'high-riding' clavicles. A later X-ray was reported as showing interrupted posterior ribs. A tracheostomy was formed on day of life 9 given the immediate risk to the baby's airway. Further imaging of the facial bones, skull and brain showed generous CSF spaces over the cerebral convexities and also marked hypoplasia of the mandible and mid-face. The baby's middle ear cavities were shown to be completely opacified. Genetic testing eventually went on to confirm a diagnosis of cerebrocostomandibular syndrome, with the detection of a pathogenic variant of the small nuclear ribonucleoprotein polypeptide B gene.

Although gene splicing occurs throughout the body, the phenotype of spliceosomal defects is largely limited to specific tissues. Cerebro-costo-mandibular syndrome (CCMS) is one such spliceosomal disease, which presents as congenital skeletal dysmorphism and is caused by mutations of SNRPB gene encoding Small Nuclear Ribonucleoprotein Polypeptides B/B' (SmB/B'). This study employed in vitro cell cultures to monitor osteo- and chondro-differentiation and examined the role of SmB/B' in the differentiation process. We found that low levels of SmB/B' by knockdown or mutations of SNRPB led to suppressed osteodifferentiation in Saos-2 osteoprogenitor-like cells, which was accompanied by affected splicing of Dlx5. On the other hand, low SmB/B' led to promoted chondrogenesis in HEPM mesenchymal stem cells. Consistent with other reports, osteogenesis was promoted by the Wnt/β-catenin pathway activator and suppressed by Wnt and BMP blockers, whereas chondrogenesis was promoted by Wnt inhibitors. Suppressed osteogenic markers by SNRPB knockdown were partly rescued by Wnt/β-catenin pathway activation. Reporter analysis revealed that suppression of SNRPB results in attenuated Wnt pathway and/or enhanced BMP pathway activities. SNRPB knockdown altered splicing of TCF7L2 which impacts Wnt/β-catenin pathway activities. This work helps unravel the mechanism underlying CCMS whereby reduced expression of spliceosomal proteins causes skeletal phenotypes.

Cerebro-costo-mandibular syndrome (CCMS) is a congenital condition with skeletal and orofacial abnormalities that often results in respiratory distress in neonates. The three main phenotypes in the thorax are posterior rib gaps, abnormal costovertebral articulation and absent ribs. Although the condition can be lethal, accurate diagnosis, and subsequent management help improve the survival rate. Mutations in the causative gene SNRPB have been identified, however, the mechanism whereby the skeletal phenotypes affect respiratory function is not well-studied due to the multiple skeletal phenotypes, lack of anatomy-based studies into the condition and rarity of CCMS cases. This review aims to clarify the extent to which the three main skeletal phenotypes in the thorax contribute to respiratory distress in neonates with CCMS. Despite the posterior rib gaps being unique to this condition and visually striking on radiographic images, anatomical consideration, and meta-analyses suggested that they might not be the significant factor in causing respiratory distress in neonates. Rather, the increase in chest wall compliance due to the rib gaps and the decrease in compliance at the costovertebral complex was considered to result in an equilibrium, minimizing the impact of these abnormalities. The absence of floating ribs is likely insignificant as seen in the general population; however, a further absence of ribs or vestigial rib formation is associated with respiratory distress and increased lethality. Based on these, we propose to evaluate the number of absent or vestigial ribs as a priority indicator to develop a personalized treatment plan based on the phenotypes exhibited.

Cerebro-costo-mandibular syndrome (CCMS) is a rare congenital syndrome consisting of the main features of micrognathia and posterior rib gaps. Due to multiple abnormalities, patients almost have difficulty breathing with upper airway obstruction, decreased thoracic capacity, spina bifida, and scoliosis. We describe a case of a late preterm neonate boy presenting with low Apgar, respiratory distress, and complicated orofacial anomalies that had a poor outcome. His radiographic findings showed mandibular hypoplasia (micrognathia), chest deformity, multiple posterior rib gap defects, and abnormal costotransverse articulation. Based on physical examination and radiologic findings, the diagnosis of CCMS confirmed for the patient. Physicians should always consider the diagnosis of CCMS in all infants with micrognathia and rib-gap defects. These infants need careful respiratory function monitoring. Early airway management improves growth and development. In addition, their physical and psychological development should be assessed regularly.

Neurodevelopmental disorders (NDDs) often result from rare genetic variation, but genomic testing yield for NDDs remains below 50%, suggesting that clinically relevant variants may be missed by standard analyses. Here, we analyze "poison exons" (PEs), which are evolutionarily conserved alternative exons often absent from standard gene annotations. Variants that alter PE inclusion can lead to loss of function and may be highly penetrant contributors to disease. We curated published RNA sequencing data from developing mouse cortex to define 1937 conserved PE regions potentially relevant to NDDs, and we analyzed variants found by genome sequencing in multiple NDD cohorts. Across 2999 probands, we found 6 novel clinically relevant variants in PE regions. Five of these variants are in genes that are part of the sodium voltage-gated channel alpha subunit family (SCN1A, SCN2A, and SCN8A), which is associated with epilepsies. One variant is in SNRPB, associated with cerebrocostomandibular syndrome. These variants have moderate to high computational impact assessments, are absent from population variant databases, and in genes with gene-phenotype associations consistent with each probands reported features. With a very minimal increase in variant analysis burden (average of 0.77 variants per proband), annotation of PEs can improve diagnostic yield for NDDs and likely other congenital conditions.