Thyroid hormone action on rat calvarial sutures

Epidemiology, Genetics, and Pathophysiology of Craniosynostosis

Craniosynostosis, the premature fusion of the infant cranial skulls, can be recognized by characteristic head shape differences that worsen with head growth. Craniosynostosis can be syndromic or nonsyndromic and can involve one suture or multiple sutures. Timely cranial vault surgery is recommended to expand and reshape the skull, with a goal of preventing increased intracranial pressure and providing sufficient space for brain growth. Several gene variants and environmental exposures are known to increase the risk of single suture craniosynostosis (SSC), including in utero constraint, exposure to specific toxins and medications, and medical conditions such as thyroid dysregulation and metabolic bone disorders.

Role of thyroid hormones in craniofacial development

The development of the craniofacial skeleton relies on complex temporospatial organization of diverse cell types by key signalling molecules. Even minor disruptions to these processes can result in deleterious consequences for the structure and function of the skull. Thyroid hormone deficiency causes delayed craniofacial and tooth development, dysplastic facial features and delayed development of the ossicles in the middle ear. Thyroid hormone excess, by contrast, accelerates development of the skull and, in severe cases, might lead to craniosynostosis with neurological sequelae and facial hypoplasia. The pathogenesis of these important abnormalities remains poorly understood and underinvestigated. The orchestration of craniofacial development and regulation of suture and synchondrosis growth is dependent on several critical signalling pathways. The underlying mechanisms by which these key pathways regulate craniofacial growth and maturation are largely unclear, but studies of single-gene disorders resulting in craniofacial malformations have identified a number of critical signalling molecules and receptors. The craniofacial consequences resulting from gain-of-function and loss-of-function mutations affecting insulin-like growth factor 1, fibroblast growth factor receptor and WNT signalling are similar to the effects of altered thyroid status and mutations affecting thyroid hormone action, suggesting that these critical pathways interact in the regulation of craniofacial development.

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.

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.

Squamosal Suture Craniosynostosis Due to Hyperthyroidism Caused by an Activating Thyrotropin Receptor Mutation (T632I)

BACKGROUND

Congenital hyperthyroidism can be a cause of failure to thrive, hyperactivity, developmental delay, and craniosynostosis during infancy. Most commonly, the condition occurs in the setting of maternal autoimmune thyroid disease. Rarely, congenital hyperthyroidism can also occur secondary to activating mutations within the thyrotropin (TSH) receptor.

PATIENT FINDINGS

A Hispanic male infant presented at age 6 months with severe thyrotoxicosis. At the time of presentation he was being evaluated for squamosal suture synostosis and he was noted to have significant developmental delays.

SUMMARY

The patient's thyrotoxicosis was initially treated with antithyroid medication, and he subsequently underwent craniosynostosis repair leading to neurodevelopmental improvement. DNA from the patient and his parents was submitted for mutational analysis of exons 9 and 10 of the TSH receptor. He was found to carry a monoallelic transition 1895C>T in exon 10 that resulted in the substitution of threonine at position 632 by isoleucine (T32I). This mutation resulted in constitutive activation of the TSH receptor. Neither parent carried this mutation indicating that the child has acquired a de novo germline mutation.

CONCLUSIONS

We report the first case of squamosal suture craniosynostosis in a patient with non-autoimmune hyperthyroidism. Squamosal suture craniosynotosis is rare, often has a subtle presentation, and should be considered in all patients with this condition because prompt treatment of hyperthyroidism and craniosynotosis repair can lead to normalization of neurodevelopment.

Craniosynostosis and risk factors related to thyroid dysfunction

Thyroid disease is a common problem among women of reproductive age but often goes undiagnosed. Maternal thyroid disease has been associated with increased risk of craniosynostosis. We hypothesized that known risk factors for thyroid disease would be associated with risk of craniosynostosis among women not diagnosed with thyroid disease. Analyses included mothers of 1,067 cases and 8,494 population-based controls who were interviewed for the National Birth Defects Prevention Study. We used multivariable logistic regression to estimate adjusted odds ratios (AOR) and 95% confidence intervals (CI). After excluding women with diagnosed thyroid disease, younger maternal age (AOR 0.7, 95% CI 0.6-0.9, for <25 years versus 25-29), black or other race-ethnicity (AOR 0.3, 95% CI 0.2-0.4 and AOR 0.6, 95% CI 0.4-0.8, respectively, relative to non-Hispanic whites), fertility medications or procedures (AOR 1.5, 95% CI 1.2-2.0), and alcohol consumption (AOR 0.8, 95% CI 0.7-0.9) were associated with risk of craniosynostosis, based on confidence intervals that excluded 1.0. These associations with craniosynostosis are consistent with the direction of their association with thyroid dysfunction (i.e., younger age, black race-ethnicity and alcohol consumption are associated with reduced risk and fertility problems are associated with increased risk of thyroid disease). This study thus provides support for the hypothesis that risk factors associated with thyroid dysfunction are also associated with risk of craniosynostosis. Improved understanding of the potential association between maternal thyroid function and craniosynostosis among offspring is important given that craniosynostosis carries significant morbidity and that thyroid disease is under-diagnosed and potentially modifiable.

Maternal thyroid disease, thyroid medication use, and selected birth defects in the National Birth Defects Prevention Study

BACKGROUND

Although thyroid disorders are present in approximately 3% of pregnant women, little is known about the association between maternal thyroid disease and birth defects.

METHODS

We assessed the association between maternal thyroid disease, thyroid medication use, and 38 types of birth defects among 14,067 cases and 5875 controls in the National Birth Defects Prevention Study, a multisite, population-based, case-control study. Infants in this study were born between October 1997 and December 2004. Information on exposures including maternal diseases and use of medications was collected by telephone interview.

RESULTS

We found statistically significant associations between maternal thyroid disease and left ventricular outflow tract obstruction heart defects (1.5; 95% CI, 1.0-2.3), hydrocephaly (2.9; 95% CI, 1.6-5.2), hypospadias (1.6; 95% CI, 1.0-2.5), and isolated anorectal atresia (2.4; 95% CI, 1.2-4.6). Estimates for the association between periconceptional use of thyroxine and specific types of birth defects were similar to estimates for any thyroid disease. Given that antithyroid medication use was rare, we could not adequately assess risks for their use for most case groups.

CONCLUSIONS

Our results are consistent with the positive associations between maternal thyroid disease or thyroid medication use and both hydrocephaly and hypospadias observed in some previous studies. New associations with left ventricular outflow tract obstruction heart defects and anorectal atresia may be chance findings.

Maternal thyroid disease as a risk factor for craniosynostosis

OBJECTIVE

To study the relationship between maternal thyroid disease and craniosynostosis using data from the National Birth Defects Prevention Study, a multisite, case-control study.

METHODS

Case infants (n=431) were identified through population-based birth defects surveillance systems at eight sites and had craniosynostosis verified by radiographic imaging. Control infants (n=4,094) consisted of a random sample of live births with no major birth defects from the same population as the case infants. Information on thyroid disease was based on self-report: mothers who reported either a thyroid disorder or use of a medication to treat a thyroid disorder during pregnancy were considered to have thyroid disease. Using an unconditional logistic regression model, we considered potential confounding factors (maternal age, race or ethnicity, smoking, body mass index, preexisting diabetes, plurality, gravidity, family history, infant sex).

RESULTS

Among case mothers, 19 (4.4%) were classified as having thyroid disease, compared with 65 (1.6%) of control mothers. Maternal thyroid disease was associated with craniosynostosis after controlling for maternal age (adjusted odds ratio 2.47, 95% confidence interval 1.46-4.18), the only factor that remained significant in the final model.

CONCLUSION

These data provide additional evidence that maternal thyroid disease (most likely Graves' disease) or its treatment is associated with craniosynostosis. Given the frequency of maternal thyroid disease, this association warrants further investigation.

LEVEL OF EVIDENCE

II.

Novel two-dimensional morphometric maps and quantitative analysis reveal marked growth and structural recovery of the rat hippocampal regions from early hypothyroid retardation

Effects of postnatal hypothyroidism and recovery from this condition on regional growth of the rat hippocampus (HC) were studied using two-dimensional (2D) foldout, morphometric maps of HC and its constituent CA1-CA4 regions. The maps were derived from unfolding serial coronal sections of the rat forebrain, consisting of the entire rostrocaudal extent of HC pyramidal cell layer in the normal control and hypothyroid weanling (P25, postnatal day 25) and young adult (P90) male rats, as well as animals allowed to recover from hypothyroid-induced growth retardation at weaning. The maps revealed novel views of HC regions for assessment of topological relationships and measurement of surface areas of the HC cortical sheet (pyramidal cell layer). In normal control P90 rats, the unfolded HC on each side extended 4 times more laterally than rostrocaudally; total HC surface area was about 40 mm(2), compared to 30 mm(2) in the weanling, indicating 35% growth from P25 to P90; CA1 took up 52% of the total HC surface area, followed by CA3 (31%) and CA2 and CA4, 8% each. Hypothyroidism resulted in significant (p<0.01) 11% and 20% reductions in the HC surface area in P25 and P90 rats, respectively; CA1 and CA4 regions suffered the most reductions while CA3 and CA2 regions the least. Recovering rats examined at P90 exhibited remarkable growth plasticity and recovery in HC regions, as evident by their near normal HC cortical surface area values, compared to age-matched controls. The 2D maps also revealed growth deficits in all HC regions of the hypothyroid rats; recovery in these parameters occurred across all dimensions, although the anterior-posterior growth was more severely affected than the mediolateral one. These results are confirmed and extended by volumetric analysis of laminar volumes of HC regions presented in a companion paper [Farahvar, A., Darwish, N., Sladek, S., Meisami, E., in press. Marked recovery of functional metabolic activity and laminar volumes in the rat hippocampus and dentate gyrus following postnatal hypothyroid growth retardation: a quantitative cytochrome oxidase study. Exp. Neurol.]. These results imply that HC regions, in contrast to whole brain, possess exceptional growth plasticity, as shown by ability to dramatically recover from early hypothyroid retardation; also 2D morphometric maps are useful tools to visualize complex and convoluted regional sheet of HC cortex and depict quantitative aspects of growth in normal and experimental conditions.

A thyrotoxic skeletal phenotype of advanced bone formation in mice with resistance to thyroid hormone

Thyroid hormone (T3) regulates bone turnover and mineralization in adults and is essential for skeletal development during childhood. Hyperthyroidism is an established risk factor for osteoporosis. Nevertheless, T3 actions in bone remain poorly understood. Patients with resistance to thyroid hormone, due to mutations of the T3-receptor beta (TRbeta) gene, display variable phenotypic abnormalities, particularly in the skeleton. To investigate the actions of T3 during bone development, we characterized the skeleton in TRbetaPV mutant mice. TRbetaPV mice harbor a targeted resistance to thyroid hormone mutation in TRbeta and recapitulate the human condition. A severe phenotype, which includes shortened body length, was evident in homozygous TRbetaPV/PV animals. Accelerated growth in utero was associated with advanced endochondral and intramembranous ossification. Advanced bone formation resulted in postnatal growth retardation, premature quiescence of the growth plates, and shortened bone length, together with increased bone mineralization and craniosynostosis. In situ hybridization demonstrated increased expression of fibroblast growth factor receptor-1, a T3-regulated gene in bone, in TRbetaPV/PV perichondrium, growth plate chondrocytes, and osteoblasts. Thus, the skeleton in TRbetaPV/PV mice is thyrotoxic and displays phenotypic features typical of juvenile hyperthyroidism.

The effect of unilateral partial facial paralysis on nasofrontal sutural growth: an experimental study in the rabbit

In a previous study in the rabbit, it was demonstrated that paralysis of the midfacial musculature results in decreased anteroposterior growth of the snout. At the end of growth, these animals showed macroscopically striking similarities to animals with unilateral fusion of the nasofrontal suture. In this study, whether nasofrontal sutural growth is unilaterally restricted in animals with unilateral partial facial paralysis was investigated. A left-sided partial facial paralysis was induced in sixteen 12-day-old New Zealand White rabbits. At the ages of 5, 9, 12, and 17 weeks, four animals were randomly assigned to be killed for analysis of nasofrontal sutural growth. In each animal, the left experimental side was compared with the right control side. By means of histomorphometric measurements, it was shown that diminished sutural growth activity was present on the left paralyzed side in periods of rapid growth. On the other hand, no significant alterations in sutural width were found. These findings seem to explain some of the macroscopic growth alterations (i.e., diminished anterior maxillary length) observed in rabbits with unilateral partial facial paralysis.

Congenital hyperthyroidism: autopsy report

We report the autopsy of a stillborn fetus with congenital hyperthyroidism born to a mother with untreated Graves' disease, whose cause of death was congestive heart failure. The major findings concerned the skull, thyroid, heart, and placenta. The cranial sutures were closed, with overlapping skull bones. The thyroid was increased in volume and had intense blood congestion. Histological examination showed hyperactive follicles. The heart was enlarged and softened, with dilated cavities and hemorrhagic suffusions in the epicardium. The placenta had infarctions that involved at least 20% of its surface, and the vessels of the umbilical cord were fully exposed due to a decrease in Wharton's jelly. Hyperthyroidism was confirmed by the maternal clinical data, the fetal findings of exophthalmia, craniosynostosis, and goiter with signs of follicular hyperactivity. Craniosynostosis is caused by the anabolic action of thyroid hormones in bone formation during the initial stages of development. The delayed initiation of treatment in the present case contributed to the severity of fetal hyperthyroidism and consequent fetal death.

Identification of IGF-I in the calvarial suture of young rats: histochemical analysis of the cranial sagittal sutures in a hyperthyroid rat model

Premature closure of cranial sutures has been known as one of the complications of juvenile or congenital hyperthyroidism. Thyroid hormone is an anabolic agent for bone formation in the early stages of childhood development. In children, excess thyroid hormone acts as an acceleration factor for the skeletal bone, whereas in adult hyperthyroidism, it causes bone mineral loss due to the high turnover rate of bone formation and consequently bone resorption. In addition, there are numerous literature descriptions concerning the interactions among bone metabolism, hormones, and growth factors, among which insulin-like growth factor I (IGF-I) is the most abundantly found growth factor in osteoblasts and in bone models in vivo. We therefore investigated whether or not the cranial sutures show accelerated closure and how the local growth factors or cytokines participate and function in local bone metabolism after administration of exogenous excess thyroid hormone in a rat model. A total of 60 female Wistar rats, aged 10 days, were divided into two groups, the triiodothyronine (T3)-treated group (n = 30, T3 0.1 microgram/gm of body weight per day) and the control group (n = 30, saline vehicle only), and were maintained and subsequently sacrificed at 15, 30, and 60 days. The parameters of cranial width derived from the morphologic measurements of the skull indicated that the lambda-asterion distance at 30 days and the pterion-bregma distance at 60 days in the T3-treated group were significantly decreased compared with those of the control group. Furthermore, the fluorescent histologic findings showed fluorescent labeling with no interruption along the suture edges, suggesting continuous bone formation, and displayed narrowing of the suture gap of the sagittal suture in the T3-treated group. Tartrate resistant acid phosphatase staining showed very little osteoclastic activity in the sagittal suture, especially in the T3-treated group. The intensity of immunohistochemical staining of IGF-I was markedly increased in the suture margins of the T3-treated group. There were no significant differences observed either in the skull base measurements or in the histologic and histochemical findings of the skull base or the coronal suture between the groups. More significantly, excess administration of thyroid hormone enhanced the cranial sagittal suture closure; therefore, it was proposed that local IGF-I plays an important role in sagittal sutural bone formation.

Craniofacial deformities associated with juvenile hyperthyroidism

Three juvenile cases of thyrotoxic craniosynostosis are reported. Two patients had small heads with posterior slanted foreheads. Skull expansion was performed in one patient. Various degrees of ocular proptosis and shallow orbits without enlargement of the extraocular muscles were seen in all cases. Zygomatic repositioning for zygomatic hypoplasia and exophthalmos was performed in two patients. The facial skeleton was involved in thyrotoxic craniosynostosis in these children, and their clinical features seemed to resemble craniofacial dysostosis.