Embryology of the upper limb

Recent advances in research technology have allowed a rapid progression in the understanding of upper limb embryogenesis. Limb development occurs between 4 and 8 weeks of gestation and progresses along three axes, as controlled by three centres: proximal–distal by the apical ectodermal ridge, anterior–posterior by the zone of polarizing activity, and dorsal–ventral by the dorsal ectoderm. In addition, research into molecular anatomy has revealed the central role of the sonic hedgehog protein and the complex interconnections linking all three axes of development. Despite these advances in understanding normal limb development, the field of pathembryogenesis remained limited. Recent discoveries about the roles of HOX and TBX genes, as well as bone morphogenetic proteins and GLI transcription factors, have allowed important insights into the causations of common clinical conditions. This expanding field of knowledge has facilitated the establishment of a new classification system based entirely on embryogenesis, which should catalyse closer surgeon/scientist collaborations.

Spiny and soft-rayed fin domains in acanthomorph fish are established through a BMP-gremlin-shh signaling network

With over 18,000 species, the Acanthomorpha, or spiny-rayed fishes, form the largest and arguably most diverse radiation of vertebrates. One of the key novelties that contributed to their evolutionary success are the spiny rays in their fins that serve as a defense mechanism. We investigated the patterning mechanisms underlying the differentiation of median fin Anlagen into discrete spiny and soft-rayed domains during the ontogeny of the direct-developing cichlid fish Astatotilapia burtoni. Distinct transcription factor signatures characterize these two fin domains, whereby mutually exclusive expression of hoxa13a/b with alx4a/b and tbx2b marks the spine to soft-ray boundary. The soft-ray domain is established by BMP inhibition via gremlin1b, which synergizes in the posterior fin with shh secreted from a zone of polarizing activity. Modulation of BMP signaling by chemical inhibition or gremlin1b CRISPR/Cas9 knockout induces homeotic transformations of spines into soft rays and vice versa. The expression of spine and soft-ray genes in nonacanthomorph fins indicates that a combination of exaptation and posterior expansion of an ancestral developmental program for the anterior fin margin allowed the evolution of robustly individuated spiny and soft-rayed domains. We propose that a repeated exaptation of such pattern might underly the convergent evolution of anterior spiny-fin elements across fishes.

Variable expression of subclinical phenotypes instead of reduced penetrance in families with mild triphalangeal thumb phenotypes

BackgroundThe of zone of polarizing activity regulatory sequence (ZRS) is a regulatory element residing in intron 5 of LMBR1 and regulates Sonic Hedgehog expression in the limb bud. Variants in the ZRS are generally fully penetrant and can cause triphalangeal thumb (TPT) and polydactyly in affected families.ObjectiveIn this report, we describe two families with mild phenotypical presentation.MethodsWe performed a field study for clinical evaluation and sequenced the ZRS for variantsusing Sanger sequencing.ResultsIn family I, a novel 165A>G variant in the ZRS (g.156584405A>G, GRCh37/Hg19) was found. In family II, we identified a 295T>C variant in the ZRS (g.156584535T>C, GRCh37/Hg19). Family members of both families who were presumed to be unaffected shared the variant in the ZRS with affected family members, suggesting reduced penetrance of the genotype. However, clinical examination of these unaffected family members revealed minor anomalies like broad thumbs and lack of thumb opposition. As the phenotype in affected patients is remarkably mild, we suggest that these ZRS variants are minimally disruptive for Sonic Hedgehog expression and therefore can result in subclinical phenotypes.ConclusionOur study underlines the importance of accurate clinical examination and appropriate genetic counselling in families with mild cases of TPT.

Hedgehog signaling dynamics in mouse embryos determined by a bioluminiscent reporter

Determination of cellular signaling in live embryos is key to understand the molecular processes that drive development. Here, we show that a transgenic mouse line carrying a luciferase-based gene reporter of Gli-mediated transcriptional activation (Gli-Luc) displays sonic hedgehog (Shh) signaling in discrete developmental processes during short-term cultures of whole embryos or embryo explants. The bioluminescence in E9.5 embryos was detected in regions in which Shh activity has been demonstrated. Later, in E10.5 embryos, bioluminescence intensity markedly increased, mostly corresponding to the high Shh activity of the developing midbrain and limb. Notably, the dynamic range of the Gli-Luc reporter in the developing limb revealed the progressive emergence of bioluminescence in the zone of polarizing activity, where reporter activity locally increased and spatially spread in agreement with the signaling gradient expected for Shh. In the midbrain of E9.5 mouse embryos, bioluminescence was not detected along the ventral region as expected but, instead, Shh-dependent anterior and posterior bioluminescence foci emerged by E10.5 indicating that the Gli-Luc reporter can only respond transcriptionally to relatively high levels of GliA and/or without the interaction with other transcription factors. The present work supports the use of bioluminescence to identify and study the dynamics of centers of morphogen signaling during mouse embryogenesis.

Zone of Polarizing Activity Regulatory Sequence Mutations/Duplications with Preaxial Polydactyly and Longitudinal Preaxial Ray Deficiency in the Phenotype: A Review of Human Cases, Animal Models, and Insights Regarding the Pathogenesis

Clinicians and scientists interested in developmental biology have viewed preaxial polydactyly (PPD) and longitudinal preaxial ray deficiency (LPAD) as two different entities. Point mutations and duplications in the zone of polarizing activity regulatory sequence (ZRS) are associated with anterior ectopic expression of Sonic Hedgehog (SHH) in the limb bud and usually result in a PPD phenotype. However, some of these mutations/duplications also have LPAD in the phenotype. This unusual PPD-LPAD association in ZRS mutations/duplications has not been specifically reviewed in the literature. The author reviews this unusual entity and gives insights regarding its pathogenesis.

Fibroblast growth factors (FGFs) prime the limb specific Shh enhancer for chromatin changes that balance histone acetylation mediated by E26 transformation-specific (ETS) factors

Sonic hedgehog (Shh) expression in the limb bud organizing centre called the zone of polarizing activity is regulated by the ZRS enhancer. Here, we examine in mouse and in a mouse limb-derived cell line the dynamic events that activate and restrict the spatial activity of the ZRS. Fibroblast growth factor (FGF) signalling in the distal limb primes the ZRS at early embryonic stages maintaining a poised, but inactive state broadly across the distal limb mesenchyme. The E26 transformation-specific transcription factor, ETV4, which is induced by FGF signalling and acts as a repressor of ZRS activity, interacts with the histone deacetylase HDAC2 and ensures that the poised ZRS remains transcriptionally inactive. Conversely, GABPα, an activator of the ZRS, recruits p300, which is associated with histone acetylation (H3K27ac) indicative of an active enhancer. Hence, the primed but inactive state of the ZRS is induced by FGF signalling and in combination with balanced histone modification events establishes the restricted, active enhancer responsible for patterning the limb bud during development.

Shh and ZRS enhancer co-localisation is specific to the zone of polarizing activity

Limb-specific Shh expression is regulated by the (~1 Mb distant) ZRS enhancer. In the mouse, limb bud restricted spatiotemporal expression of Shh occurs from ~E10-E11.5 at the distal posterior margin and is essential for correct autopod formation. Here, we have analysed the higher-order chromatin conformation of Shh in expressing and non-expressing tissues, both by fluorescence in situ hybridisation (FISH) and by chromosome conformation capture (5C). Conventional and super-resolution light microscopy identified significantly elevated frequencies of Shh/ZRS co-localisation only in the Shh expressing regions of the limb bud. However, Shh-ZRS spatial distances were consistently shorter than intervening distances to a neural enhancer in all tissues and developmental stages analysed. 5C identified a topologically associating domain (TAD) over the Shh/ZRS genomic region and enriched interactions between Shh and ZRS throughout E11.5 embryos. Shh/ZRS co-localisation, therefore, correlates with the spatiotemporal domain of limb bud-specific Shh expression, but close Shh/ZRS proximity in the nucleus occurs regardless of whether the gene or enhancer is active. We suggest that this constrained chromatin configuration optimises the opportunity for the active enhancer to locate and instigate Shh expression.

A Novel ZRS Mutation in a Chinese Patient with Preaxial Polydactyly and Triphalangeal Thumb

Preaxial polydactyly (PPD; OMIM 603596), which is characterized as having supernumerary fingers, is an unusual congenital hand abnormality. Triphalangeal thumb (TPT; OMIM 190600) is identified by an extra phalangeal bone and is often found in association with PPD. When in combination, the disease is referred to as PPD type II (PPD II; OMIM 174500). Previous studies have demonstrated that variations in the zone of polarizing activity regulatory sequence (ZRS; chr7:156,583,796-156,584,569; hg19) region are associated with PPD II. In this study, our patient was diagnosed with PPD II, having bilateral thumb duplication and unilateral TPT (on the right hand). Further investigation of possible causative genes identified a de novo heterozygous ZRS mutation (ZRS 428T>A). This novel mutation was neither found in 200 normal controls nor reported in online databases. Moreover, the bioinformatics program Genomic Evolutionary Rate Profiling (GERP) revealed this site (ZRS428) to be evolutionarily highly conserved, and the 428T>A point mutation was predicted to be deleterious by MutationTaster. In conclusion, the affected individual shows bilateral thumb duplication, but unilateral TPT making this case special. Thus, our findings not only further support the important role of ZRS in limb morphogenesis and expand the spectrum of ZRS mutations, but also emphasize the significance of genetic diagnosis and counseling of families with digit number and identity alterations as well.