Craniofacial shape variation in Twist1+/- mutant mice

In utero nicotine exposure affects murine palate development

OBJECTIVES

Despite data linking smoking to increased risk of fetal morbidity and mortality, 11% of pregnant women continue to smoke or use alternative nicotine products. Studies confirm that nicotine exposure during pregnancy increases the incidence of birth defects; however, little research has focused on specific anatomic areas based on timing of exposure. We aim to determine critical in utero and postnatal periods of nicotine exposure that affect craniofacial development, specifically palate growth. Malformation of the palatal structures can result in numerous complications including facial growth disturbance, or impeding airway function. We hypothesized that both in utero and postnatal nicotine exposure will alter palate development.

MATERIALS AND METHODS

We administered pregnant C57BL6 mice water supplemented with 100 μg/mL nicotine during early pregnancy, throughout pregnancy, during pregnancy and lactation, or lactation only. Postnatal day 15 pups underwent micro-computed tomography (μCT) analyses specific to the palate.

RESULTS

Resultant pups revealed significant differences in body weight from lactation-only nicotine exposure, and μCT investigation revealed several dimensions affected by lactation-only nicotine exposure, including palate width, palate and cranial base lengths, and mid-palatal suture width.

CONCLUSIONS

These results demonstrate the direct effects of nicotine on the developing palate beyond simple tobacco use. Nicotine exposure through tobacco alternatives, cessation methods, and electronic nicotine delivery systems (ENDS) may disrupt normal growth and development of the palate during development and the postnatal periods of breastfeeding. Due to the recent dramatic increase in the use of ENDS, future research will focus specifically on this nicotine delivery method.

In Utero Exposure to Maternal Electronic Nicotine Delivery System use Demonstrate Alterations to Craniofacial Development

OBJECTIVE

Develop a model for the study of Electronic Nicotine Device (ENDS) exposure on craniofacial development.

DESIGN

Experimental preclinical design followed as pregnant murine dams were randomized and exposed to filtered air exposure, carrier exposure consisting of 50% volume of propylene glycol and vegetable glycine (ENDS Carrier) respectively, or carrier exposure with 20 mg/ml of nicotine added to the liquid vaporizer (ENDS carrier with nicotine).

SETTING

Preclinical murine model exposure using the SciReq exposure system.

PARTICIPANTS

C57BL6 adult 8 week old female pregnant mice and exposed in utero litters.

INTERVENTIONS

Exposure to control filtered air, ENDS carrier or ENDS carrier with nicotine added throughout gestation at 1 puff/minute, 4 h/day, five days a week.

MAIN OUTCOME MEASURES

Cephalometric measures of post-natal day 15 pups born as exposed litters.

RESULTS

Data suggests alterations to several facial morphology parameters in the developing offspring, suggesting electronic nicotine device systems may alter facial growth if used during pregnancy.

CONCLUSIONS

Future research should concentrate on varied formulations and exposure regimens of ENDS to determine timing windows of exposures and ENDS formulations that may be harmful to craniofacial development.

Selective serotonin re-uptake inhibitors affect craniofacial structures in a mouse model

Selective serotonin re-uptake inhibitors (SSRI) widely used in the treatment of depression, anxiety, obsessive compulsive disorder, fibromyalgia, and migraine are among the most heavily prescribed drug class in the United States (US). Along with an overall rise in SSRI use, these medications are increasingly used by pregnant individuals and recent preclinical and clinical studies have indicated that SSRIs may increase the prevalence of congenital abnormalities and birth defects of the craniofacial region. Our group has developed pre-clinical models of study, including those that mimic the clinical use of SSRI in mice. Here we designed a study to interrogate a commonly prescribed SSRI drug, Citalopram, for its effects on craniofacial and dental development when introduced in utero. Pre-natal exposure to a clinically relevant dose of citalopram resulted in changes in craniofacial form identified by an increase in endocast volume in SSRI exposed postnatal day 15 mouse pups. More specifically, cranial length and synchondrosis length increased in SSRI exposed pups as compared to control pups of the same age. Additionally, growth center (synchondrosis) height and width and palate length and width decreased in SSRI exposed pups as compared to control un-exposed pups. Effects of SSRI on the molars was minimal. Craniofacial growth and development continue to be an area of interest in the investigation of in utero pharmaceutical drug exposure. Altogether these data indicate that prenatal SSRI exposure affects craniofacial form in multiple tissues and specifically at growth sites and centers of the skull.

Parallel evolution of integrated craniofacial traits in trophic specialist pupfishes

Populations may adapt to similar environments via parallel or non-parallel genetic changes, but the frequency of these alternative mechanisms and underlying contributing factors are still poorly understood outside model systems. We used QTL mapping to investigate the genetic basis of highly divergent craniofacial traits between the scale-eater (Cyprinodon desquamator) and molluscivore (C. brontotheroides) pupfish adapting to two different hypersaline lake environments on San Salvador Island, Bahamas. We lab-reared F2 scale-eater x molluscivore intercrosses from two different lake populations, estimated linkage maps, scanned for significant QTL for 29 skeletal and craniofacial traits, female mate preference, and sex. We compared the location of QTL between lakes to quantify parallel and non-parallel genetic changes. We detected significant QTL for six craniofacial traits in at least one lake. However, nearly all shared QTL loci were associated with a different craniofacial trait within each lake. Therefore, our estimate of parallel evolution of craniofacial genetic architecture could range from one out of six identical trait QTL (low parallelism) to five out of six integrated trait QTL (high parallelism). We suggest that pleiotropy and trait integration can affect estimates of parallel evolution, particularly within rapid radiations. We also observed increased adaptive introgression in shared QTL regions, suggesting that gene flow contributed to parallel evolution. Overall, our results suggest that the same genomic regions may contribute to parallel adaptation across integrated suites of craniofacial traits, rather than specific traits, and highlight the need for a more expansive definition of parallel evolution.

Synchondrosis fusion contributes to the progression of postnatal craniofacial dysmorphology in syndromic craniosynostosis

Syndromic craniosynostosis (CS) patients exhibit early, bony fusion of calvarial sutures and cranial synchondroses, resulting in craniofacial dysmorphology. In this study, we chronologically evaluated skull morphology change after abnormal fusion of the sutures and synchondroses in mouse models of syndromic CS for further understanding of the disease. We found fusion of the inter-sphenoid synchondrosis (ISS) in Apert syndrome model mice (Fgfr2^S252W/+ ) around 3 weeks old as seen in Crouzon syndrome model mice (Fgfr2c^C342Y/+ ). We then examined ontogenic trajectories of CS mouse models after 3 weeks of age using geometric morphometrics analyses. Antero-ventral growth of the face was affected in Fgfr2^S252W/+ and Fgfr2c^C342Y/+ mice, while Saethre-Chotzen syndrome model mice (Twist1^+/- ) did not show the ISS fusion and exhibited a similar growth pattern to that of control littermates. Further analysis revealed that the coronal suture synostosis in the CS mouse models induces only the brachycephalic phenotype as a shared morphological feature. Although previous studies suggest that the fusion of the facial sutures during neonatal period is associated with midface hypoplasia, the present study suggests that the progressive postnatal fusion of the cranial synchondrosis also contributes to craniofacial dysmorphology in mouse models of syndromic CS. These morphological trajectories increase our understanding of the progression of syndromic CS skull growth.

HDAC9 structural variants disrupting TWIST1 transcriptional regulation lead to craniofacial and limb malformations

Structural variants (SVs) can affect protein-coding sequences as well as gene regulatory elements. However, SVs disrupting protein-coding sequences that also function as cis-regulatory elements remain largely uncharacterized. Here, we show that craniosynostosis patients with SVs containing the histone deacetylase 9 (HDAC9) protein-coding sequence are associated with disruption of TWIST1 regulatory elements that reside within the HDAC9 sequence. Based on SVs within the HDAC9-TWIST1 locus, we defined the 3'-HDAC9 sequence as a critical TWIST1 regulatory region, encompassing craniofacial TWIST1 enhancers and CTCF sites. Deletions of either Twist1 enhancers (eTw5-7Δ/Δ) or CTCF site (CTCF-5Δ/Δ) within the Hdac9 protein-coding sequence led to decreased Twist1 expression and altered anterior/posterior limb expression patterns of SHH pathway genes. This decreased Twist1 expression results in a smaller sized and asymmetric skull and polydactyly that resembles Twist1+/- mouse phenotype. Chromatin conformation analysis revealed that the Twist1 promoter interacts with Hdac9 sequences that encompass Twist1 enhancers and a CTCF site, and that interactions depended on the presence of both regulatory regions. Finally, a large inversion of the entire Hdac9 sequence (Hdac9 INV/+) in mice that does not disrupt Hdac9 expression but repositions Twist1 regulatory elements showed decreased Twist1 expression and led to a craniosynostosis-like phenotype and polydactyly. Thus, our study elucidates essential components of TWIST1 transcriptional machinery that reside within the HDAC9 sequence. It suggests that SVs encompassing protein-coding sequences could lead to a phenotype that is not attributed to its protein function but rather to a disruption of the transcriptional regulation of a nearby gene.

Evaluation of dental maturity in Muenke syndrome, Saethre-Chotzen syndrome, and TCF12-related craniosynostosis

OBJECTIVES

To determine whether dental maturity (dental development) was delayed in patients with Muenke syndrome, Saethre-Chotzen syndrome, and TCF12-related craniosynostosis, compared with a Dutch control group without syndromes.

MATERIALS AND METHODS

This study included 60 patients (38 patients with Muenke syndrome, 17 patients with Saethre-Chotzen syndrome, and 5 with TCF12-related craniosynostosis), aged 5.8-16.8 years that were treated at the Department of Oral Maxillofacial Surgery, Special Dental Care, and Orthodontics, in Sophia Children's Hospital, Erasmus University Medical Center, Rotterdam, the Netherlands. Dental age was calculated according to Demirjian's index of dental maturity. The control group included 451 children without a syndrome.

RESULTS

Compared with the control group, dental development was delayed by an average of one year in 5- to 8-year-old patients with Muenke syndrome (P = 0.007) and in 8- to 10-year-old patients with Saethre-Chotzen syndrome (P = 0.044), but not in patients with TCF12-related craniosynostosis.

CONCLUSIONS

Our results indicated that dental development was delayed by one year, on average, in patients with Muenke syndrome and Saethre-Chotzen syndrome, compared with a Dutch control group without syndromes.

IMPLICATIONS

Our findings have improved the understanding of dental development in patients with Muenke and Saethre-Chotzen syndrome. These results can provide guidance on whether the orthodontist needs to consider growth disturbances related to dental development.

Quantitative Morphologic Analysis of Cranial Vault in Twist1+/- Mice: Implications in Craniosynostosis

BACKGROUND

The haploinsufficiency in the TWIST1 gene encoding a basic helix-loop-helix transcription factor is a cause of one of the craniosynostosis syndromes, Saethre-Chotzen syndrome. Patients with craniosynostosis usually require operative release of affected sutures, which makes it difficult to observe the long-term consequence of suture fusion on craniofacial growth.

METHODS

In this study, we performed quantitative analysis of morphologic changes of the skull in Twist1 heterozygously-deleted mice (Twist1+/-) with micro-computed tomographic images.

RESULTS

In Twist1+/- mice, fusion of the coronal suture began before postnatal day 14 and progressed until postnatal day 56, during which morphologic changes occurred. The growth of the skull was not achieved by a constant increase in the measured distances in wild type mice; some distances in the top-basal axis were decreased during the observation period. In the Twist1+/- mouse, growth in the top-basal axis was accelerated and that of the frontal cranium was reduced. In the unicoronal suture fusion mouse, the length of the zygomatic arch of affected side was shorter in the Twist1+/- mouse. In one postnatal day 56 Twist1+/- mouse with bilateral coronal suture fusion, asymmetric zygomatic arch length was identified.

CONCLUSION

The authors'results suggest that measuring the length of the left and right zygomatic arches may be useful for early diagnosis of coronal suture fusion and for estimation of the timing of synostosis, and that more detailed study on the growth pattern of the normal and the synostosed skull could provide prediction of the risk of resynostosis.

CLINICAL RELEVANCE STATEMENT

The data from this study can be useful to better understand the cranial growth pattern in patients with craniosynostosis.

Facial Dysmorphology in Saethre-Chotzen Syndrome

PURPOSE

Classic features of Saethre-Chotzen syndrome (SCS) described in the literature include a prominent nasal bridge, eyelid ptosis, telorbitism, maxillary hypoplasia, and mandibular prognathism. The purpose of this study was to evaluate objectively the bony features of SCS.

METHODS

Preoperative computer tomography scans of 15 SCS patients, 23 normal controls, 13 bicoronal nonsyndromic, and 7 unicoronal nonsyndromic craniosynostosis patients were included for analysis. Unaffected controls and nonsyndromic patients were age- and sex-matched to SCS patients. Morphometric cephalometrics were analyzed using three-dimensional computer tomography reconstructions. Mann-Whitney U were used to compare facial measurements between SCS and normal and nonsyndromic craniosynostosis controls.

RESULTS

Telorbitism was present in bicoronal SCS patients only (P = 0.04) but absent in the unicoronal and bicoronal/metopic cohorts. The angle of the nasal bone relative to the sella was not different between SCS and controls (P = 0.536), although the angle of the nasal bone relative to the forehead was decreased in SCS by 15.5° (P < 0.001). Saethre-Chotzen syndrome had a 2.6° maxillary retrusion relative to controls (P = 0.03). In addition, SCS patients aged 4 to 7 months had a wider (39.34 versus 35.04, P = 0.017) and anteroposteriorly foreshortened (32.12 versus 35.06, P = 0.039) maxilla. There was no difference in mandibular prognathism among SCS patients as measured by the sella-nasion-B point angle compared to controls (P = 0.705).

CONCLUSIONS

Despite classic descriptions, on morphometric analysis SCS patients did not demonstrate consistency across all suture subtypes in terms of telorbitism, a broad nasal bridge, or mandibular prognathism. Rather, SCS subtypes of SCS based on suture pathology more closely resemble nonsyndromic patients.

Facial Suture Pathology in Syndromic Craniosynostosis: Human and Animal Studies

BACKGROUND

Facial deformities in syndromic craniosynostosis are not only functionally, psychosocially, and aesthetically impairing but also notoriously challenging to reconstruct. Whether facial suture synostosis plays a significant role in the pathogenesis of these deformities is inadequately studied in human patients.

METHODS

The MEDLINE database was queried using a methodologically generated search term inventory. Article inclusion was adjudicated by 2 authors after independent review. Articles provided insight into facial suture involvement in either syndromic craniosynostosis patients or animal models of disease.

RESULTS

Comprehensive review yielded 19 relevant articles meeting inclusion criteria. Mid-20th century craniofacial biologists characterized how patent facial sutures are essential for normal postnatal facial development. They also posited that premature ossification disrupts growth vectors, causing significant dysmorphologies. Recently, facial suture synostosis was found to cause midfacial deformities independent of cranial base pathology in mouse models of syndromic craniosynostosis. Few recent studies have begun exploring facial suture involvement in patients, and although they have paved the way for future research, they bear significant limitations.

CONCLUSIONS

The hypothesis that facial suture synostosis acts in conjunction with cranial base pathology to produce the prominent, multifocal facial deformities in syndromic craniosynostosis may fundamentally alter surgical management and warrants further investigation. Methodically evaluating the literature, this review synthesizes all basic science and human clinical research thus far on the role of facial sutures in syndromic craniosynostosis and elucidates important topics for future research. We ultimately identify the need for rigorous imaging studies that longitudinally evaluate facial osteology across patients with various craniosynostosis syndromes.

Size changes in miR‑21 knockout mice: Geometric morphometrics on teeth, alveolar bone and mandible

MicroRNA‑21 (miR‑21) is a small non‑coding RNA that is differentially expressed during tooth development, particularly during amelogenesis. Although orthodontic tooth movement and the innate immune response are impaired, miR‑21 knockout mice demonstrate no obvious skeletal phenotype. However, the consequence of miR‑21 knockout on tooth phenotype and corresponding alveolar bone is unknown. The current study utilized landmark‑based geometric morphometrics to identify anatomical dissimilarities of the three lower and upper molars, and the corresponding alveolar bone, in miR‑21 knockout and wild‑type control mice. The anatomical structures were visualized by microcomputer tomography. A total of 36 and 38 landmarks were placed on mandibular and maxillary molars, respectively. For the alveolar bone, 16 landmarks were selected on both anatomical sites. General Procrustes analysis revealed significantly smaller molars and dimensions of the alveolar bone in the mandible of the miR‑21 knockout mice when compared with wild‑type controls (P=0.03 and P=0.04, respectively). The overall dimension of the mandible was reduced by the lack of miR‑21 (P=0.02). In the maxilla, the dimension of the alveolar bone was significant (P=0.02); however, this was not observed in the molars (P=0.36). Based on principal component analysis, no changes in shape for any of the anatomical sites were observed. Dental and skeletal jaw length were calculated and no prognathism was identified. However, the fluctuating asymmetry of the molars in the mandible and the maxilla was reduced in the miR‑21 knockout mice by 38 and 27%, respectively. Taken together, the results of the present study revealed that the molars in the mandible and the dimension of the respective alveolar bone were smaller in miR‑21 mice compared with wild‑type littermates, suggesting that miR‑21 influences tooth development.

Cranial Suture Regeneration Mitigates Skull and Neurocognitive Defects in Craniosynostosis

Craniosynostosis results from premature fusion of the cranial suture(s), which contain mesenchymal stem cells (MSCs) that are crucial for calvarial expansion in coordination with brain growth. Infants with craniosynostosis have skull dysmorphology, increased intracranial pressure, and complications such as neurocognitive impairment that compromise quality of life. Animal models recapitulating these phenotypes are lacking, hampering development of urgently needed innovative therapies. Here, we show that Twist1+/- mice with craniosynostosis have increased intracranial pressure and neurocognitive behavioral abnormalities, recapitulating features of human Saethre-Chotzen syndrome. Using a biodegradable material combined with MSCs, we successfully regenerated a functional cranial suture that corrects skull deformity, normalizes intracranial pressure, and rescues neurocognitive behavior deficits. The regenerated suture creates a niche into which endogenous MSCs migrated, sustaining calvarial bone homeostasis and repair. MSC-based cranial suture regeneration offers a paradigm shift in treatment to reverse skull and neurocognitive abnormalities in this devastating disease.

Loss of Extreme Long-Range Enhancers in Human Neural Crest Drives a Craniofacial Disorder

Non-coding mutations at the far end of a large gene desert surrounding the SOX9 gene result in a human craniofacial disorder called Pierre Robin sequence (PRS). Leveraging a human stem cell differentiation model, we identify two clusters of enhancers within the PRS-associated region that regulate SOX9 expression during a restricted window of facial progenitor development at distances up to 1.45 Mb. Enhancers within the 1.45 Mb cluster exhibit highly synergistic activity that is dependent on the Coordinator motif. Using mouse models, we demonstrate that PRS phenotypic specificity arises from the convergence of two mechanisms: confinement of Sox9 dosage perturbation to developing facial structures through context-specific enhancer activity and heightened sensitivity of the lower jaw to Sox9 expression reduction. Overall, we characterize the longest-range human enhancers involved in congenital malformations, directly demonstrate that PRS is an enhanceropathy, and illustrate how small changes in gene expression can lead to morphological variation.

Pharmacological exposures may precipitate craniosynostosis through targeted stem cell depletion

The Centers for Disease Control and Prevention, National Birth Defects Study suggests that environmental exposures including maternal thyroid diseases, maternal nicotine use, and use of selective serotonin reuptake inhibitors (SSRIs) may exacerbate incidence and or severity of craniofacial abnormalities including craniosynostosis. Premature fusion of a suture(s) of the skull defines the birth defect craniosynostosis which occurs in 1:1800–2500 births. A proposed mechanism of craniosynostosis is the disruption of proliferation and differentiation of cells in the perisutural area. Here, we hypothesize that pharmacological exposures including excess thyroid hormone, nicotine, and SSRIs lead to an alteration of stem cells within the sutures resulting in premature fusion. In utero exposure to nicotine and citalopram (SSRI) increased the risk of premature suture fusion in a wild-type murine model. Gli1⁺ stem cells were reduced, stem cell populations were depleted, and homeostasis of the suture mesenchyme was altered with exposure. Thus, although these pharmacological exposures can deplete calvarial stem cell populations leading to craniosynostosis, depletion of stem cells is not a unifying mechanism for pharmacological exposure associated craniosynostosis.

Creating diversity in mammalian facial morphology: a review of potential developmental mechanisms

Mammals (class Mammalia) have evolved diverse craniofacial morphology to adapt to a wide range of ecological niches. However, the genetic and developmental mechanisms underlying the diversification of mammalian craniofacial morphology remain largely unknown. In this paper, we focus on the facial length and orofacial clefts of mammals and deduce potential mechanisms that produced diversity in mammalian facial morphology. Small-scale changes in facial morphology from the common ancestor, such as slight changes in facial length and the evolution of the midline cleft in some lineages of bats, could be attributed to heterochrony in facial bone ossification. In contrast, large-scale changes of facial morphology from the common ancestor, such as a truncated, widened face as well as the evolution of the bilateral cleft possessed by some bat species, could be brought about by changes in growth and patterning of the facial primordium (the facial processes) at the early stages of embryogenesis.

Three-dimensional study of 31 cases of synostotic anterior plagiocephaly before and after surgical management the Lille protocol

Synostotic anterior plagiocephaly is a rare pathological cranial malformation. Therapeutic options are rarely studied due to the rarity of the malformation and difficulties in diagnosis and care management. The objective of this study was to analyze the results obtained with the Lille protocol based on 62 CT-scans done before and after surgery in 31 patients. A specific analysis was designed for this work. Nine cephalometric measures enabled to evidence on each CT-Scan the corrections made on the fronto-orbital bandeau and the potential impact of surgery on the craniofacial structures. Results show that surgical symmetry of the fronto-orbital bandeau in the transversal plane, according to the symmetrical axis of the semicircular canals, allows a normalization of the skull's growth and morphogenesis for the surgically affected structures but also adjacent ones.

Recombinant mouse periostin ameliorates coronal sutures fusion in Twist1+/- mice

BACKGROUND

Saethre-Chotzen syndrome is an autosomal dominantly inherited disorder caused by mutations in the twist family basic helix-loop-helix transcription factor 1 (TWIST1) gene. Surgical procedures are frequently required to reduce morphological and functional defects in patients with Saethre-Chotzen syndrome. Therefore, the development of noninvasive procedures to treat Saethre-Chotzen syndrome is critical. We identified that periostin, which is an extracellular matrix protein that plays an important role in both bone and connective tissues, is downregulated in craniosynostosis patients.

METHODS

We aimed to verify the effects of different concentrations (0, 50, 100, and 200 μg/l) of recombinant mouse periostin in Twist1^+/- mice (a mouse model of Saethre-Chotzen syndrome) coronal suture cells in vitro and in vivo. Cell proliferation, migration, and osteogenic differentiation were observed and detected. Twist1^+/- mice were also injected with recombinant mouse periostin to verify the treatment effects.

RESULTS

Cell Counting Kit-8 results showed that recombinant mouse periostin inhibited the proliferation of suture-derived cells in a time- and concentration-dependent manner. Cell migration was also suppressed when treated with recombinant mouse periostin. Real-time quantitative PCR and Western blotting results suggested that messenger ribonucleic acid and protein expression of alkaline phosphatase, bone sialoprotein, collagen type I, and osteocalcin were all downregulated after treatment with recombinant mouse periostin. However, the expression of Wnt-3a, Wnt-1, and β-catenin were upregulated. The in vivo results demonstrated that periostin-treated Twist1^+/- mice showed patent coronal sutures in comparison with non-treated Twist1^+/- mice which have coronal craniosynostosis.

CONCLUSION

Our results suggest that recombinant mouse periostin can inhibit coronal suture cell proliferation and migration and suppress osteogenic differentiation of suture-derived cells via Wnt canonical signaling, as well as ameliorate coronal suture fusion in Twist1^+/- mice.

Maternal environment and craniofacial growth: geometric morphometric analysis of mandibular shape changes with in utero thyroxine overexposure in mice

An estimated 3% of US pregnancies are affected by maternal thyroid dysfunction, with between one and three of every 1000 pregnancies being complicated by overactive maternal thyroid levels. Excess thyroid hormones are linked to neurological impairment and excessive craniofacial variation, affecting both endochondral and intramembranous bone. Using a geometric morphometric approach, this study evaluates the role of in utero thyroxine overexposure on the growth of offspring mandibles in a sample of 241 mice. Canonical variate analysis utilized 16 unilateral mandibular landmarks obtained from 3D micro-computed tomography to assess shape changes between unexposed controls (n = 63) and exposed mice (n = 178). By evaluating shape changes in the mandible among three age groups (15, 20 and 25 days postnatal) and different dosage levels (low, medium and high), this study found that excess maternal thyroxine alters offspring mandibular shape in both age- and dosage-dependent manners. Group differences in overall shape were significant (P < 0.001), and showed major changes in regions of the mandible associated with muscle attachment (coronoid process, gonial angle) and regions of growth largely governed by articulation with the cranial base (condyle) and occlusion (alveolus). These results compliment recent studies demonstrating that maternal thyroxine levels can alter the cranial base and cranial vault of offspring, contributing to a better understanding of both normal and abnormal mandibular development, as well as the medical implications of craniofacial growth and development.

Thyroxine Exposure Effects on the Cranial Base

Thyroid hormone is important for skull bone growth, which primarily occurs at the cranial sutures and synchondroses. Thyroid hormones regulate metabolism and act in all stages of cartilage and bone development and maintenance by interacting with growth hormone and regulating insulin-like growth factor. Aberrant thyroid hormone levels and exposure during development are exogenous factors that may exacerbate susceptibility to craniofacial abnormalities potentially through changes in growth at the synchondroses of the cranial base. To elucidate the direct effect of in utero therapeutic thyroxine exposure on the synchondroses in developing mice, we provided scaled doses of the thyroid replacement drug, levothyroxine, in drinking water to pregnant C57BL6 wild-type dams. The skulls of resulting pups were subjected to micro-computed tomography analysis revealing less bone volume relative to tissue volume in the synchondroses of mouse pups exposed in utero to levothyroxine. Histological assessment of the cranial base area indicated more active synchondroses as measured by metabolic factors including Igf1. The cranial base of the pups exposed to high levels of levothyroxine also contained more collagen fiber matrix and an increase in markers of bone formation. Such changes due to exposure to exogenous thyroid hormone may drive overall morphological changes. Thus, excess thyroid hormone exposure to the fetus during pregnancy may lead to altered craniofacial growth and increased risk of anomalies in offspring.

Genetic advances in craniosynostosis

Craniosynostosis, the premature ossification of one or more skull sutures, is a clinically and genetically heterogeneous congenital anomaly affecting approximately one in 2,500 live births. In most cases, it occurs as an isolated congenital anomaly, that is, nonsyndromic craniosynostosis (NCS), the genetic, and environmental causes of which remain largely unknown. Recent data suggest that, at least some of the midline NCS cases may be explained by two loci inheritance. In approximately 25-30% of patients, craniosynostosis presents as a feature of a genetic syndrome due to chromosomal defects or mutations in genes within interconnected signaling pathways. The aim of this review is to provide a detailed and comprehensive update on the genetic and environmental factors associated with NCS, integrating the scientific findings achieved during the last decade. Focus on the neurodevelopmental, imaging, and treatment aspects of NCS is also provided.

Effects of In Utero Thyroxine Exposure on Murine Cranial Suture Growth

Large scale surveillance studies, case studies, as well as cohort studies have identified the influence of thyroid hormones on calvarial growth and development. Surveillance data suggests maternal thyroid disorders (hyperthyroidism, hypothyroidism with pharmacological replacement, and Maternal Graves Disease) are linked to as much as a 2.5 fold increased risk for craniosynostosis. Craniosynostosis is the premature fusion of one or more calvarial growth sites (sutures) prior to the completion of brain expansion. Thyroid hormones maintain proper bone mineral densities by interacting with growth hormone and aiding in the regulation of insulin like growth factors (IGFs). Disruption of this hormonal control of bone physiology may lead to altered bone dynamics thereby increasing the risk for craniosynostosis. In order to elucidate the effect of exogenous thyroxine exposure on cranial suture growth and morphology, wild type C57BL6 mouse litters were exposed to thyroxine in utero (control = no treatment; low ~167 ng per day; high ~667 ng per day). Thyroxine exposed mice demonstrated craniofacial dysmorphology (brachycranic). High dose exposed mice showed diminished area of the coronal and widening of the sagittal sutures indicative of premature fusion and compensatory growth. Presence of thyroid receptors was confirmed for the murine cranial suture and markers of proliferation and osteogenesis were increased in sutures from exposed mice. Increased Htra1 and Igf1 gene expression were found in sutures from high dose exposed individuals. Pathways related to the HTRA1/IGF axis, specifically Akt and Wnt, demonstrated evidence of increased activity. Overall our data suggest that maternal exogenous thyroxine exposure can drive calvarial growth alterations and altered suture morphology.

Effects of thyroxine exposure on the Twist 1 +/- phenotype: A test of gene-environment interaction modeling for craniosynostosis

BACKGROUND

Craniosynostosis, the premature fusion of one or more of the cranial sutures, is estimated to occur in 1:1800 to 2500 births. Genetic murine models of craniosynostosis exist, but often imperfectly model human patients. Case, cohort, and surveillance studies have identified excess thyroid hormone as an agent that can either cause or exacerbate human cases of craniosynostosis.

METHODS

Here we investigate the influence of in utero and in vitro exogenous thyroid hormone exposure on a murine model of craniosynostosis, Twist 1 +/-.

RESULTS

By 15 days post-natal, there was evidence of coronal suture fusion in the Twist 1 +/- model, regardless of exposure. With the exception of craniofacial width, there were no significant effects of exposure; however, the Twist 1 +/- phenotype was significantly different from the wild-type control. Twist 1 +/- cranial suture cells did not respond to thyroxine treatment as measured by proliferation, osteogenic differentiation, and gene expression of osteogenic markers. However, treatment of these cells did result in modulation of thyroid associated gene expression.

CONCLUSION

Our findings suggest the phenotypic effects of the genetic mutation largely outweighed the effects of thyroxine exposure in the Twist 1 +/- model. These results highlight difficultly in experimentally modeling gene-environment interactions for craniosynostotic phenotypes. Birth Defects Research (Part A) 106:803-813, 2016. © 2016 Wiley Periodicals, Inc.

Understanding craniosynostosis as a growth disorder

Craniosynostosis is a condition of complex etiology that always involves the premature fusion of one or multiple cranial sutures and includes various anomalies of the soft and hard tissues of the head. Steady progress in the field has resulted in identifying gene mutations that recurrently cause craniosynostosis. There are now scores of mutations on many genes causally related to craniosynostosis syndromes, though the genetic basis for the majority of nonsyndromic cases is unknown. Identification of these genetic mutations has allowed significant progress in understanding the intrinsic properties of cranial sutures, including mechanisms responsible for normal suture patency and for pathogenesis of premature suture closure. An understanding of morphogenesis of cranial vault sutures is critical to understanding the pathophysiology of craniosynostosis conditions, but the field is now poised to recognize the repeated changes in additional skeletal and soft tissues of the head that typically accompany premature suture closure. We review the research that has brought an understanding of premature suture closure within our reach. We then enumerate the less well-studied, but equally challenging, nonsutural phenotypes of craniosynostosis conditions that are well characterized in available mouse models. We consider craniosynostosis as a complex growth disorder of multiple tissues of the developing head, whose growth is also targeted by identified mutations in ways that are poorly understood. Knowledge gained from studies of humans and mouse models for these conditions underscores the diverse, associated developmental anomalies of the head that contribute to the complex phenotypes of craniosynostosis conditions presenting novel challenges for future research. WIREs Dev Biol 2016, 5:429-459. doi: 10.1002/wdev.227 For further resources related to this article, please visit the WIREs website.