Insights into the development of molecular therapies for craniosynostosis

Management of Lambdoid Craniosynostosis: A Comprehensive and Systematic Review

BACKGROUND

Craniosynostosis is a condition characterized by the premature fusion of 2 or more skull bones. Craniosynostosis of the lambdoid suture is one of the rarest forms, accounting for 1-4% of all craniosynostoses. Documented cases are separated into simple (single suture), complex (bilateral), and associated with adjacent synostoses ("Mercedes Benz" Pattern) or syndromes (i.e., Crouzon, Sathre-Chotzen, Antley-Bixler). This condition can manifest phenotypic deformities and neurological sequelae that can lead to impaired cognitive function if improperly treated or left undiagnosed. Preferred surgical techniques have varied over time but all maintain the common goals of establishing proper head shape and preventing of complications that could contribute to aforementioned sequelae.

SUMMARY

This comprehensive review highlights demographic distributions, embryological development, pathogenesis, clinical presentation, neurological sequelae, radiologic findings, surgical techniques, surgical outcomes, and postoperative considerations of patients with lambdoid craniosynostosis presentation. In addition, a systematic review was conducted to explore the operative management of lambdoid craniosynostosis using PubMed, Embase, and Scopus databases, with 38 articles included after screening. Key Messages: Due to a low volume of published cases, diagnosis and treatment can vary. Large overlap in presentation can occur in patients that display lambdoid craniosynostosis and posterior plagiocephaly, furthering the need for comprehensive analysis. Possessing the knowledge and tools to properly assess patients with lambdoid craniosynostosis will allow for more precise care and improved outcomes.

Spatial transcriptomics reveals a role for sensory nerves in preserving cranial suture patency through modulation of BMP/TGF-β signaling

The patterning and ossification of the mammalian skeleton requires the coordinated actions of both intrinsic bone morphogens and extrinsic neurovascular signals, which function in a temporal and spatial fashion to control mesenchymal progenitor cell (MPC) fate. Here, we show the genetic inhibition of tropomyosin receptor kinase A (TrkA) sensory nerve innervation of the developing cranium results in premature calvarial suture closure, associated with a decrease in suture MPC proliferation and increased mineralization. In vitro, axons from peripheral afferent neurons derived from dorsal root ganglions (DRGs) of wild-type mice induce MPC proliferation in a spatially restricted manner via a soluble factor when cocultured in microfluidic chambers. Comparative spatial transcriptomic analysis of the cranial sutures in vivo confirmed a positive association between sensory axons and proliferative MPCs. SpatialTime analysis across the developing suture revealed regional-specific alterations in bone morphogenetic protein (BMP) and TGF-β signaling pathway transcripts in response to TrkA inhibition. RNA sequencing of DRG cell bodies, following direct, axonal coculture with MPCs, confirmed the alterations in BMP/TGF-β signaling pathway transcripts. Among these, the BMP inhibitor follistatin-like 1 (FSTL1) replicated key features of the neural-to-bone influence, including mitogenic and anti-osteogenic effects via the inhibition of BMP/TGF-β signaling. Taken together, our results demonstrate that sensory nerve-derived signals, including FSTL1, function to coordinate cranial bone patterning by regulating MPC proliferation and differentiation in the suture mesenchyme.

Effect of Fibroblast Growth Factor 2 on Extraocular Muscle Structure and Function

Purpose

Mutations in the fibroblast growth factor (FGF) receptor can result in strabismus, but little is known about how FGFs affect extraocular muscle structure and function. These were assessed after short-term and long-term exposure to exogenously applied FGF2 to determine the effect of enhanced signaling.

Methods

One superior rectus muscle of adult rabbits received either a series of three injections of 500 ng, 1 µg, or 5 µg FGF2 and examined after 1 week, or received sustained treatment with FGF2 and examined after 1, 2, or 3 months. Muscles were assessed for alterations in force generation, myofiber size, and satellite cell number after each treatment.

Results

One week after the 5 µg FGF2 injections, treated muscles showed significantly increased force generation compared with naïve controls, which correlated with increased myofiber cross-sectional areas and Pax7-positive satellite cells. In contrast, 3 months of sustained FGF2 treatment resulted in decreased force generation, which correlated with decreased myofiber size and decreased satellite cells compared with naïve control and the untreated contralateral side.

Conclusions

FGF2 had distinctly different effects when short-term and long-term treatments were compared. The decreased size and ability to generate force correlated with decreased myofiber areas seen in individuals with Apert syndrome, where there is sustained activation of FGF signaling. Knowing more about signaling pathways critical for extraocular muscle function, development, and disease will pave the way for improved treatment options for strabismus patients with FGF abnormalities in craniofacial disease, which also may be applicable to other strabismus patients.

Neurodevelopmental risk evaluation of premature closure of the anterior fontanelle

PURPOSE

To evaluate neurological development of completely healthy children with anterior fontanelle premature closure via Denver Developmental Screening Test II and to compare the results with control group.

METHOD AND RESULTS

The records of 140 patients applied to Mersin University Pediatric Neurology Outpatient Clinic between 2011 and 2019 with the complaint of premature closure of the anterior fontanelle were retrospectively reviewed. Patients with microcephaly, craniosynostosis, infection, sequelae of hypoxia-ischemia, metabolic disorders, intracranial hemorrhage, epilepsy, endocrine problems, and dysmorphic features were excluded from the study. Sixty-six completely healthy children with anterior fontanelle premature closure were included in the study. Denver Developmental Screening Test II was performed by the same developmental specialist to the children with premature closure of the anterior fontanelle as well as to the healthy control group. For each child included in the case and the control group, 90% of the values for each development area were calculated and recorded. Then, the results were compared. Denver II Developmental Screening Test (p < 0.001) and gross motor subtest (p < 0.001) results showed statistically significant retardation in the case group compared with the control group.

CONCLUSIONS

The study was the first study in the literature on the gross motor development of children with premature closure of anterior fontanelle, and it has been found significantly undeveloped compared with the control group, and it has been concluded that similar patients should be evaluated from this view point in pediatric neurology department.

Physiologic closure time of the metopic suture in South Australian infants from 3D CT scans

Metopic synostosis is a craniofacial condition characterised by the premature fusion of the metopic suture. This early fusion restricts frontal bone growth [17] and has significant impacts on the developing infant during a critical phase of rapid growth and development [4]. Diagnosis of the condition is usually achieved by clinical assessment, followed by a three-dimensional computed tomography (3D CT) scan, verifying premature metopic suture fusion.

PURPOSE

This retrospective study aims to investigate the timing of metopic suture fusion in the developing infant in an Australian subpopulation.

METHODS

The study evaluates metopic suture fusion in 258 cranial 3D CT scans of children aged 0-24 months over a 5-year period (2011-2016), scanned at Women's and Children's Hospital.

RESULTS

The findings suggest that the age range over which physiologic metopic suture fusion occurs is larger than previously reported.

CONCLUSIONS

The approximate range for physiologic fusion was found to be 3-19 months and patients with fusion within this range can be considered normal. Complete suture fusion is expected by 19 months. Additionally, results indicate suture fusion prior to 3 months is abnormal and diagnostically indicative of metopic synostosis.

Adeno-Associated Virus-Mediated RNAi against Mutant Alleles Attenuates Abnormal Calvarial Phenotypes in an Apert Syndrome Mouse Model

Apert syndrome (AS), the most severe form of craniosynostosis, is caused by missense mutations including Pro253Arg(P253R) of fibroblast growth factor receptor 2 (FGFR2), which leads to enhanced FGF/FGFR2-signaling activity. Surgical correction of the deformed skull is the typical treatment for AS. Because of constant maldevelopment of sutures, the corrective surgery is often executed several times, resulting in increased patient challenge and complications. Biological therapies targeting the signaling of mutant FGFR2 allele, in combination with surgery, may bring better outcome. Here we screened and found a small interfering RNA (siRNA) specifically targeting the Fgfr2-P253R allele, and we revealed that it inhibited osteoblastic differentiation and matrix mineralization by reducing the signaling of ERK1/2 and P38 in cultured primary calvarial cells and calvarial explants from Apert mice (Fgfr2^+/P253R). Furthermore, AAV9 carrying short hairpin RNA (shRNA) (AAV9-Fgfr2-shRNA) against mutant Fgfr2 was delivered to the skulls of AS mice. Results demonstrate that AAV9-Fgfr2-shRNA attenuated the premature closure of coronal suture and the decreased calvarial bone volume of AS mice. Our study provides a novel practical biological approach, which will, in combination with other therapies, including surgeries, help treat patients with AS while providing experimental clues for the biological therapies of other genetic skeletal diseases.

Craniosynostosis - Recognition, clinical characteristics, and treatment

Craniosynostosis is a developmental craniofacial anomaly, resulting in impairment of brain development and abnormally shaped skull. The main cause of craniosynostosis is premature closure of one or more cranial sutures. It usually occurs as an isolated condition, but may also be associated with other malformations as part of complex syndromes. When left untreated, craniosynostosis can cause serious complications, such as developmental delay, facial abnormality, sensory, respiratory and neurological dysfunction, anomalies affecting the eye, and psychological disturbances. Thus, early diagnosis, expert surgical techniques, postoperative care, and adequate follow-up are of vital importance in treating craniosynostosis.

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.

Clinical genetics of craniosynostosis

PURPOSE OF REVIEW

When providing accurate clinical diagnosis and genetic counseling in craniosynostosis, the challenge is heightened by knowledge that etiology in any individual case may be entirely genetic, entirely environmental, or anything in between. This review will scope out how recent genetic discoveries from next-generation sequencing have impacted on the clinical genetic evaluation of craniosynostosis.

RECENT FINDINGS

Survey of a 13-year birth cohort of patients treated at a single craniofacial unit demonstrates that a genetic cause of craniosynostosis can be identified in one quarter of cases. The substantial contributions of mutations in two genes, TCF12 and ERF, is confirmed. Important recent discoveries are mutations of CDC45 and SMO in specific craniosynostosis syndromes, and of SMAD6 in nonsyndromic midline synostosis. The added value of exome or whole genome sequencing in the diagnosis of difficult cases is highlighted.

SUMMARY

Strategies to optimize clinical genetic diagnostic pathways by combining both targeted and next-generation sequencing are discussed. In addition to improved genetic counseling, recent discoveries spotlight the important roles of signaling through the bone morphogenetic protein and hedgehog pathways in cranial suture biogenesis, as well as a key requirement for adequate cell division in suture maintenance.

Deciphering the molecular mechanisms underlying the binding of the TWIST1/E12 complex to regulatory E-box sequences

The TWIST1 bHLH transcription factor controls embryonic development and cancer processes. Although molecular and genetic analyses have provided a wealth of data on the role of bHLH transcription factors, very little is known on the molecular mechanisms underlying their binding affinity to the E-box sequence of the promoter. Here, we used an in silico model of the TWIST1/E12 (TE) heterocomplex and performed molecular dynamics (MD) simulations of its binding to specific (TE-box) and modified E-box sequences. We focused on (i) active E-box and inactive E-box sequences, on (ii) modified active E-box sequences, as well as on (iii) two box sequences with modified adjacent bases the AT- and TA-boxes. Our in silico models were supported by functional in vitro binding assays. This exploration highlighted the predominant role of protein side-chain residues, close to the heart of the complex, at anchoring the dimer to DNA sequences, and unveiled a shift towards adjacent ((-1) and (-1*)) bases and conserved bases of modified E-box sequences. In conclusion, our study provides proof of the predictive value of these MD simulations, which may contribute to the characterization of specific inhibitors by docking approaches, and their use in pharmacological therapies by blocking the tumoral TWIST1/E12 function in cancers.