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Simple Summary: A mesiodens is an extra tooth located in the midline of the upper jaw. To investigate the genetic cause of mesiodens, clinical and radiographic examination were performed on 23 family members of a two-generation Hmong family. Whole exome or Sanger sequencing were performed in 22 family members. We found an extremely rare mutation (c.1807G>A;p.Glu603Lys) in the PTPN23 gene in seven family members who had mesiodens. However, six family members who did not have mesiodens also carried the mutation. The mode of inheritance appears to be autosomal dominance with incomplete penetrance (53.84%). The finding of a PTPN23 mutation as a cause of mesiodens phenotype is supported by the findings of two additional rare PTPN23 mutations in two unrelated Thai patients with mesiodens. PTPN23 is a regulator of endosomal trafficking functioning to move activated membrane receptors, such as EGFR, from the endosomal sorting complex for multivesicular body biogenesis, lysosomal degradation and subsequent downregulation of receptor signaling. Our immunohistochemical study and RNAscope on developing mouse embryos showed broad expression of PTPN23 in oral tissues, while immunofluorescence showed that EGFR was specifically concentrated in the midline epithelium. Our study showed for the first time that genetic variants in PTPN23 caused reduced phosphatase activity, disrupted midline signaling, and subsequent mesiodens formation. A mesiodens is a supernumerary tooth located in the midline of the premaxilla. To investigate the genetic cause of mesiodens, clinical and radiographic examination were performed on 23 family members of a two-generation Hmong family. Whole exome sequencing (WES) or Sanger sequencing were performed in 22 family members and two unrelated Thai patients with mesiodens. WES in the Hmong family revealed a missense mutation (c.1807G>A;p.Glu603Lys) in PTPN23 in seven affected members and six unaffected members. The mode of inheritance was autosomal dominance with incomplete penetrance (53.84%). Two additional mutations in PTPN23, c.2248C>G;p.Pro750Ala and c.3298C>T;p.Arg1100Cys were identified in two unrelated patients with mesiodens. PTPN23 is a regulator of endosomal trafficking functioning to move activated membrane receptors, such as EGFR, from the endosomal sorting complex towards the ESCRT-III complex for multivesicular body biogenesis, lysosomal degradation, and subsequent downregulation of receptor signaling. Immunohistochemical study and RNAscope on developing mouse embryos showed broad expression of PTPN23 in oral tissues, while immunofluorescence showed that EGFR was specifically concentrated in the midline epithelium. Importantly, PTPN23 mutant protein was shown to have reduced phosphatase activity. In conclusion, mesiodens were associated with genetic variants in PTPN23, suggesting that mesiodens may form due to defects in endosomal trafficking, leading to disrupted midline signaling. [ABSTRACT FROM AUTHOR]

High incidence of chronic otitis media is associated with human craniofacial syndromes, suggesting that defects in the formation of the middle ear and associated structures can have a knock-on effect on the susceptibility to middle ear inflammation. Patients with branchio-oto-renal (BOR) syndrome have several defects in the ear leading to both sensorineural and conductive hearing loss, including otitis media. 40% of BOR syndrome cases are due to Eya1 haploinsufficiency, with mouse models affecting Eya1, mimicking many of the defects found in patients. Here, we characterize the onset, consequences, and underlying causes of chronic otitis media in Eya1 heterozygous mice. Cavitation defects were evident in these mice from postnatal day (P) 11 onwards, with mesenchyme around the promontory and attic regions of the middle ear space. This mesenchyme was still prominent in adult Eya1 heterozygous mice, while the wild-type littermates had fully aerated ears from P14 onwards. MicroCT analysis highlighted a significantly smaller bulla, confirming the link between bulla size defects and the ability of the mesenchyme to retract successfully. Otitis media was observed from P14, often presenting unilaterally, resulting in hyperplasia of the middle ear mucosa, expansion of secretory cells, defects in the motile cilia, and changes in basal epithelial cell markers. A high incidence of otitis media was identified in older mice but only associated with ears with retained mesenchyme. To understand the impact of the environment, the mouse line was rederived onto a super-clean environment. Cavitation defects were still evident at early stages, but these generally resolved over time, and importantly, no signs of otitis media were observed at 6 weeks. In conclusion, we show that a small bulla size is closely linked to defects in cavitation and the presence of retained mesenchyme. A delay in retraction of the mesenchyme predates the onset of otitis media, making the ears susceptible to its development. Early exposure toOMappears to exacerbate the cavitation defect, with mesenchyme evident in the middle ear throughout the animal's life. This highlights that permanent damage to the middle ear can arise as a consequence of the early onset of OM. [ABSTRACT FROM AUTHOR]

Background: The egg tooth is a vital structure allowing hatchlings to escape from the egg. In squamates (snakes and lizards), the egg tooth is a real tooth that develops within the oral cavity at the top of the upper jaw. Most squamates have a single large midline egg tooth at hatching, but a few families, such as Gekkonidae, have two egg teeth. In snakes the egg tooth is significantly larger than the rest of the dentition and is one of the first teeth to develop. Results: We follow the development of the egg tooth in four snake species and show that the single egg tooth is formed by two tooth germs. These two tooth germs are united at the midline and grow together to produce a single tooth. In culture, this merging can be perturbed to give rise to separate smaller teeth, confirming the potential of the developing egg tooth to form two teeth. Conclusions: Our data agrees with previous hypotheses that during evolution one potential mechanism to generate a large tooth is through congrescence of multiple tooth germs and suggests that the ancestors of snakes could have had two egg teeth. Key Findings: Here we show that the midline egg tooth in a variety of snakes forms by the fusion of two closely placed tooth germs. Perturbation of the fusion process leads to the formation of two smaller teeth. We propose this is a conserved mechanism for the generation of a large tooth. [ABSTRACT FROM AUTHOR]