Modeling craniofacial and skeletal congenital birth defects to advance therapies

p75 neurotrophin receptor regulates craniofacial growth and morphology in postnatal development

Craniofacial abnormalities are among the most prevalent congenital defects, significantly affecting appearance, function, and quality of life. While the role of genetic mutations in craniofacial malformations is recognized, the underlying molecular mechanisms remain poorly understood. In this study, we investigate the role of p75 neurotrophin receptor (p75NTR) in craniofacial development by comparing wild-type (p75NTR+/+) mice against p75NTR-deficient (p75NTR-/-) knockout mice. We employed histology, micro-CT surface distance, volumetric analysis, and geometric morphometric analysis to assess craniofacial development and growth. On postnatal day 7 (P7), p75NTR-/- mice exhibited reduced skull length compared to wild-type controls. By P28, micro-CT analysis revealed significant reductions in calvarial bone volume and trabecular bone thickness in p75NTR-/- mice. Geometric morphometric analysis identified significant shape alterations in the nasal, parietal, and occipital regions, with p75NTR-/- mice showing a shortened cranium and tapered nasal bone morphology. These findings highlight the critical role of p75NTR in regulating postnatal craniofacial development. Disruption of p75NTR signaling impairs both the growth and morphological integrity of craniofacial structures, which may contribute to the pathogenesis of congenital craniofacial abnormalities. In the future, a better understanding of the molecular mechanisms through which p75NTR mediates craniofacial development may offer valuable insights for future targeted therapeutic strategies for craniofacial defects.

Resolving complex cartilage structures in developmental biology via deep learning-based automatic segmentation of X-ray computed microtomography images

The complex shape of embryonic cartilage represents a true challenge for phenotyping and basic understanding of skeletal development. X-ray computed microtomography (μCT) enables inspecting relevant tissues in all three dimensions; however, most 3D models are still created by manual segmentation, which is a time-consuming and tedious task. In this work, we utilised a convolutional neural network (CNN) to automatically segment the most complex cartilaginous system represented by the developing nasal capsule. The main challenges of this task stem from the large size of the image data (over a thousand pixels in each dimension) and a relatively small training database, including genetically modified mouse embryos, where the phenotype of the analysed structures differs from the norm. We propose a CNN-based segmentation model optimised for the large image size that we trained using a unique manually annotated database. The segmentation model was able to segment the cartilaginous nasal capsule with a median accuracy of 84.44% (Dice coefficient). The time necessary for segmentation of new samples shortened from approximately 8 h needed for manual segmentation to mere 130 s per sample. This will greatly accelerate the throughput of μCT analysis of cartilaginous skeletal elements in animal models of developmental diseases.

DNA glycosylase NEIL2 functions in multiple cellular processes

Cell survival largely depends on the faithful maintenance of genetic material since genomic DNA is constantly exposed to genotoxicants from both endogenous and exogenous sources. The evolutionarily conserved base excision repair (BER) pathway is critical for maintaining genome integrity by eliminating highly abundant and potentially mutagenic oxidized DNA base lesions. BER is a multistep process, which is initiated with recognition and excision of the DNA base lesion by a DNA glycosylase, followed by DNA end processing, gap filling and finally sealing of the nick. Besides genome maintenance by global BER, DNA glycosylases have been found to play additional roles, including preferential repair of oxidized lesions from transcribed genes, modulation of the immune response, participation in active DNA demethylation and maintenance of the mitochondrial genome. Central to these functions is the DNA glycosylase NEIL2. Its loss results in increased accumulation of oxidized base lesions in the transcribed genome, triggers an immune response and causes early neurodevelopmental defects, thus emphasizing the multitasking capabilities of this repair protein. Here we review the specialized functions of NEIL2 and discuss the consequences of its absence both in vitro and in vivo.

Craniofacial Development: Neural Crest in Molecular Embryology

Craniofacial development, one of the most complex sequences of developmental events in embryology, features a uniquely transient, pluripotent stem cell-like population known as the neural crest (NC). Neural crest cells (NCCs) originate from the dorsal aspect of the neural tube and migrate along pre-determined routes into the developing branchial arches and frontonasal plate. The exceptional rates of proliferation and migration of NCCs enable their diverse contribution to a wide variety of craniofacial structures. Subsequent differentiation of these cells gives rise to cartilage, bones, and a number of mesenchymally-derived tissues. Deficiencies in any stage of differentiation can result in facial clefts and abnormalities associated with craniofacial syndromes. A small number of conserved signaling pathways are involved in controlling NC differentiation and craniofacial development. They are used in a reiterated fashion to help define precise temporospatial cell and tissue formation. Although many aspects of their cellular and molecular control have yet to be described, it is clear that together they form intricately integrated signaling networks required for spatial orientation and developmental stability and plasticity, which are hallmarks of craniofacial development. Mutations that affect the functions of these signaling pathways are often directly or indirectly identified in congenital syndromes. Clinical applications of NC-derived mesenchymal stem/progenitor cells, persistent into adulthood, hold great promise for tissue repair and regeneration. Realization of NCC potential for regenerative therapies motivates understanding of the intricacies of cell communication and differentiation that underlie the complexities of NC-derived tissues.

Molecular Diagnostics and In Utero Therapeutics for Orofacial Clefts

Orofacial clefts and their management impose a substantial burden on patients, on their families, and on the health system. Under the current standard of care, affected patients are subjected to a lifelong journey of corrective surgeries and multidisciplinary management to replace bone and soft tissues, as well as restore esthetics and physiologic functions while restoring self-esteem and psychological health. Hence, a better understanding of the dynamic interplay of molecular signaling pathways at critical phases of palate development is necessary to pioneer novel prenatal interventions. Such pathways include transforming growth factor-β (Tgfβ), sonic hedgehog (Shh), wingless-integrated site (Wnt)/β-catenin, bone morphogenetic protein (Bmp), and fibroblast growth factor (Fgf) and its associated receptors, among others. Here, we summarize commonly used surgical methods used to correct cleft defects postnatally. We also review the advances made in prenatal diagnostics of clefts through imaging and genomics and the various in utero surgical corrections that have been attempted thus far. An overview of how key mediators of signaling that drive palatogenesis are emphasized in the context of the framework and rationale for the development and testing of therapeutics in animal model systems and in humans is provided. The pros and cons of in utero therapies that can potentially restore molecular homeostasis needed for the proper growth and fusion of palatal shelves are presented. The theme advanced throughout this review is the need to develop preclinical molecular therapies that could ultimately be translated into human trials that can correct orofacial clefts at earlier stages of development.

Testing the Cre-mediated genetic switch for the generation of conditional knock-in mice

The Cre-mediated genetic switch combines the ability of Cre recombinase to stably invert or excise a DNA fragment depending upon the orientation of flanking mutant loxP sites. In this work, we have tested this strategy in vivo with the aim to generate two conditional knock-in mice for missense mutations in the Impad1 and Clcn7 genes causing two different skeletal dysplasias. Targeting constructs were generated in which the Impad1 exon 2 and an inverted exon 2* and the Clcn7 exon 7 and an inverted exon 7* containing the point mutations were flanked by mutant loxP sites in a head-to-head orientation. When the Cre recombinase is present, the DNA flanked by the mutant loxP sites is expected to be stably inverted leading to the activation of the mutated exon.

The targeting vectors were used to generate heterozygous floxed mice in which inversion of the wild-type with the mutant exon has not occurred yet. To generate knock-in mice, floxed animals were mated to a global Cre-deleter mouse strain for stable inversion and activation of the mutation. Unexpectedly the phenotype of homozygous Impad1 knock-in animals overlaps with the lethal phenotype described previously in Impad1 knock-out mice. Similarly, the phenotype of homozygous Clcn7 floxed mice overlaps with Clcn7 knock-out mice. Expression studies by qPCR and RT-PCR demonstrated that mutant mRNA underwent abnormal splicing leading to the synthesis of non-functional proteins. Thus, the skeletal phenotypes in both murine strains were not caused by the missense mutations, but by aberrant splicing. Our data demonstrate that the Cre mediated genetic switch strategy should be considered cautiously for the generation of conditional knock-in mice.

Characterization of congenital craniofacial anomalies in a specialized hospital of Risaralda, Colombia. 2010-2014

Introducción. Los defectos craneofaciales congénitos pueden causar un impacto en la vida de los niños y de sus familias cuando comprometen el rostro. Además, pueden estar acompañados de alteración de las funciones cerebrales o de la apariencia facial. No se tienen datos concluyentes sobre la presencia de estos defectos en el Eje Cafetero.Objetivo. Identificar la frecuencia de las malformaciones craneofaciales congénitas en un periodo de cuatro años en una institución privada de la ciudad de Pereira, en Risaralda, Colombia.Materiales y métodos. Estudio trasversal retrospectivo. La información fue recolectada a través del sistema de información de historias clínicas de pacientes que consultaron por primera vez en una institución privada de salud. El análisis estadístico fue realizado mediante el software R y Microsoft Excel versión 2007.Resultados. Entre enero del 2010 y diciembre del 2014 se atendieron 1 807 pacientes con malformaciones craneofaciales congénitas, lo que corresponde al 19.5% del total de las anomalías congénitas. La hendidura labio-palatina fue la más frecuente.Conclusiones. Aunque las malformaciones craneofaciales congénitas se presentan con frecuencia, se sabe muy poco de su etiología. El diagnóstico temprano puede prevenir futuras complicaciones que deterioren la salud o que generen un sobrecosto para el sistema de salud.

EIF4A3 deficient human iPSCs and mouse models demonstrate neural crest defects that underlie Richieri-Costa-Pereira syndrome

Biallelic loss-of-function mutations in the RNA-binding protein EIF4A3 cause Richieri-Costa-Pereira syndrome (RCPS), an autosomal recessive condition mainly characterized by craniofacial and limb malformations. However, the pathogenic cellular mechanisms responsible for this syndrome are entirely unknown. Here, we used two complementary approaches, patient-derived induced pluripotent stem cells (iPSCs) and conditional Eif4a3 mouse models, to demonstrate that defective neural crest cell (NCC) development explains RCPS craniofacial abnormalities. RCPS iNCCs have decreased migratory capacity, a distinct phenotype relative to other craniofacial disorders. Eif4a3 haploinsufficient embryos presented altered mandibular process fusion and micrognathia, thus recapitulating the most penetrant phenotypes of the syndrome. These defects were evident in either ubiquitous or NCC-specific Eif4a3 haploinsufficient animals, demonstrating an autonomous requirement of Eif4a3 in NCCs. Notably, RCPS NCC-derived mesenchymal stem-like cells (nMSCs) showed premature bone differentiation, a phenotype paralleled by premature clavicle ossification in Eif4a3 haploinsufficient embryos. Likewise, nMSCs presented compromised in vitro chondrogenesis, and Meckel's cartilage was underdeveloped in vivo. These findings indicate novel and essential requirements of EIF4A3 for NCC migration and osteochondrogenic differentiation during craniofacial development. Altogether, complementary use of iPSCs and mouse models pinpoint unique cellular mechanisms by which EIF4A3 mutation causes RCPS, and provide a paradigm to study craniofacial disorders.