Ectodermal dysplasias: Classification and organization by phenotype, genotype and molecular pathway

An international advisory group met at the National Institutes of Health in Bethesda, Maryland in 2017, to discuss a new classification system for the ectodermal dysplasias (EDs) that would integrate both clinical and molecular information. We propose the following, a working definition of the EDs building on previous classification systems and incorporating current approaches to diagnosis: EDs are genetic conditions affecting the development and/or homeostasis of two or more ectodermal derivatives, including hair, teeth, nails, and certain glands. Genetic variations in genes known to be associated with EDs that affect only one derivative of the ectoderm (attenuated phenotype) will be grouped as non‐syndromic traits of the causative gene (e.g., non‐syndromic hypodontia or missing teeth associated with pathogenic variants of EDA “ectodysplasin”). Information for categorization and cataloging includes the phenotypic features, Online Mendelian Inheritance in Man number, mode of inheritance, genetic alteration, major developmental pathways involved (e.g., EDA, WNT “wingless‐type,” TP63 “tumor protein p63”) or the components of complex molecular structures (e.g., connexins, keratins, cadherins)

Functional analysis of Ectodysplasin-A mutations causing selective tooth agenesis.

Mutations of the Ectodysplasin-A (EDA) gene are generally associated with the syndrome hypohidrotic ectodermal dysplasia (MIM 305100), but they can also manifest as selective, non-syndromic tooth agenesis (MIM300606). We have performed an in vitro functional analysis of six selective tooth agenesis-causing EDA mutations (one novel and five known) that are located in the C-terminal tumor necrosis factor homology domain of the protein. Our study reveals that expression, receptor binding or signaling capability of the mutant EDA1 proteins is only impaired in contrast to syndrome-causing mutations, which we have previously shown to abolish EDA1 expression, receptor binding or signaling. Our results support a model in which the development of the human dentition, especially of anterior teeth, requires the highest level of EDA-receptor signaling, whereas other ectodermal appendages, including posterior teeth, have less stringent requirements and form normally in response to EDA mutations with reduced activity

Scaffolds to Control Inflammation and Facilitate Dental Pulp Regeneration

In dentistry, the maintenance of a vital dental pulp is of paramount importance because teeth devitalized by root canal treatment may become more brittle and prone to structural failure over time. Advanced carious lesions can irreversibly damage the dental pulp by propagating a sustained inflammatory response throughout the tissue. Although the inflammatory response initially drives tissue repair, sustained inflammation has an enormously destructive effect on the vital pulp, eventually leading to total necrosis of the tissue and necessitating its removal. The implications of tooth devitalization have driven significant interest in the development of bioactive materials that facilitate the regeneration of damaged pulp tissues by harnessing the capacity of the dental pulp for self-repair. In considering the process by which pulpitis drives tissue destruction, it is clear that an important step in supporting the regeneration of pulpal tissues is the attenuation of inflammation. Macrophages, key mediators of the immune response, may play a critical role in the resolution of pulpitis because of their ability to switch to a proresolution phenotype. This process can be driven by the resolvins, a family of molecules derived from fatty acids that show great promise as therapeutic agents. In this review, we outline the importance of preserving the capacity of the dental pulp to self-repair through the rapid attenuation of inflammation. Potential treatment modalities, such as shifting macrophages to a proresolving phenotype with resolvins are described, and a range of materials known to support the regeneration of dental pulp are presented

The WNT10A Gene in Ectodermal Dysplasias and Selective Tooth Agenesis

Mutations in the WNT10A gene were first detected in the rare syndrome odonto-onycho-dermal dysplasia (OODD, OMIM257980) but have now also been found to cause about 35-50% of selective tooth agenesis (STHAG4, OMIM150400), a common disorder that mostly affects the permanent dentition. In our random sample of tooth agenesis patients, 40% had at least one mutation in the WNT10A gene. The WNT10A Phe228Ile variant alone reached an allele frequency of 0.21 in the tooth agenesis cohort, about 10 times higher than the allele frequency reported in large SNP databases for Caucasian populations. Patients with bi-allelic WNT10A mutations have severe tooth agenesis while heterozygous individuals are either unaffected or have a mild phenotype. Mutations in the coding areas of the WNT10B gene, which is co-expressed with WNT10A during odontogenesis, and the WNT6 gene which is located at the same chromosomal locus as WNT10A in humans, do not contribute to the tooth agenesis phenotype. (C) 2014 Wiley Periodicals, Inc

Twist1- and Twist2-Haploinsufficiency Results in Reduced Bone Formation

<div><p>Background</p><p>Twist1 and Twist2 are highly homologous bHLH transcription factors that exhibit extensive highly overlapping expression profiles during development. While both proteins have been shown to inhibit osteogenesis, only Twist1 haploinsufficiency is associated with the premature synostosis of cranial sutures in mice and humans. On the other hand, biallelic Twist2 deficiency causes only a focal facial dermal dysplasia syndrome or additional cachexia and perinatal lethality in certain mouse strains. It is unclear how these proteins cooperate to synergistically regulate bone formation.</p><p>Methods</p><p>Twist1 floxed mice (<i>Twist1</i>^f/f) were bred with Twist2-Cre knock-in mice (<i>Twist2</i>^Cre/+) to generate Twist1 and Twist2 haploinsufficient mice (<i>Twist1</i>^f/+; <i>Twist2</i>^Cre/+). X-radiography, micro-CT scans, alcian blue/alizarin red staining, trap staining, BrdU labeling, immunohistochemistry, <i>in situ</i> hybridizations, real-time PCR and dual luciferase assay were employed to investigate the overall skeletal defects and the bone-associated molecular and cellular changes of <i>Twist1</i>^f/+;<i>Twist2</i>^Cre/+ mice.</p><p>Results</p><p>Twist1 and Twist2 haploinsufficient mice did not present with premature ossification and craniosynostosis; instead they displayed reduced bone formation, impaired proliferation and differentiation of osteoprogenitors. These mice exhibited decreased expressions of <i>Fgf2</i> and <i>Fgfr1–4</i> in bone, resulting in a down-regulation of FGF signaling. Furthermore, <i>in vitro</i> studies indicated that both Twist1 and Twist2 stimulated 4.9 kb <i>Fgfr2</i> promoter activity in the presence of E12, a Twist binding partner.</p><p>Conclusion</p><p>These data demonstrated that <i>Twist1</i>- and <i>Twist2</i>-haploinsufficiency caused reduced bone formation due to compromised FGF signaling.</p></div

Reduced bone formation in <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice.

<p>(A) Skeletons of 6-day-old control (left) and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> (right) mice stained with alcian blue (cartilage) and alizarin red (bone). The skeleton of the <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mouse is remarkably smaller. (B) Alcian blue- and alizarin red-stained skull from 6-day-old <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice (right) showed delayed fusion of interfrontal suture and open posterior fontanel (arrows), compared with the control mice (left). (C) Alcian blue- and alizarin red-stained hind foot of 6-day-old control (left) and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> (right) mice. Note the delayed ossification in metatarsals (mt) and phalanges (pl), and an additional toe (arrow) originating from the same (or duplicated) metatarsal as the hallux in <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice. (D) Plain X-radiography of the tibiae from 6-day-old control (left) and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice (right). The <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice had shorter tibiae and reduced radiopacity, compared to the control mice. (E) Representative three-dimensional μ-CT images of tibiae from 6-day-old control (left) and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> (right) mice. The <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice showed reduced trabecular (arrowheads) and cortical bones (arrows). (F–H) Quantitative μ-CT data showing that the 6-day-old <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice had a significant decrease in the ratio of bone volume (BV)/total volume (TV) (F) and in apparent bone density (G), compared to the control mice (n = 6, P<0.001). The <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice also presented reduced material density although no statistically significant difference was observed (H).</p

Histological examination of <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice.

<p>(A–B) Femur sections of 6-day-old control and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice were stained with H&E. The <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice displayed reduced metaphyseal trabecular bone (A, red arrows) and a decreased thickness of the periosteum (B, blue arrows) and cortical bone (B, red arrows). (C) TRAP staining of femur sections of 6-day-old control and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice. Note that the osteoclasts (red arrows) appeared to be similar in size and distribution in the control and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice. The osteoclast densities were 0.55±0.06/0.01 mm² in the controls (<i>n</i> = 5) and 0.60±0.02/0.01 mm² in the <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice (<i>n</i> = 5, <i>P</i>>0.05). (D–F) <i>In situ</i> hybridization analyses (signal in blue) of the transcripts of <i>Alp</i> (D), <i>Ocn</i> (E) and <i>Dmp1</i> (F) in the femurs of one-week-old control and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice. (G, H) Immunohistochemical analyses (signal in brown) of the osterix (G) and biglycan (H) protein levels in the femurs of the 6-day-old control and <i>Twist1^flox/+</i>; <i>Twist2^Cre/+</i> mice. Scale bar = 100 µm.</p