Polydactyly

Enhancers control the location and timing of gene expression and contain the majority of variants associated with disease1-3. The ZRS is arguably the most well-studied vertebrate enhancer and mediates the expression of Shh in the developing limb4. Thirty-one human single-nucleotide variants (SNVs) within the ZRS are associated with polydactyly4-6. However, how this enhancer encodes tissue-specific activity, and the mechanisms by which SNVs alter the number of digits, are poorly understood. Here we show that the ETS sites within the ZRS are low affinity, and identify a functional ETS site, ETS-A, with extremely low affinity. Two human SNVs and a synthetic variant optimize the binding affinity of ETS-A subtly from 15% to around 25% relative to the strongest ETS binding sequence, and cause polydactyly with the same penetrance and severity. A greater increase in affinity results in phenotypes that are more penetrant and more severe. Affinity-optimizing SNVs in other ETS sites in the ZRS, as well as in ETS, interferon regulatory factor (IRF), HOX and activator protein 1 (AP-1) sites within a wide variety of enhancers, cause gain-of-function gene expression. The prevalence of binding sites with suboptimal affinity in enhancers creates a vulnerability in genomes whereby SNVs that optimize affinity, even slightly, can be pathogenic. Searching for affinity-optimizing SNVs in genomes could provide a mechanistic approach to identify causal variants that underlie enhanceropathies.

In the limb bud, patterning along the anterior-posterior (A-P) axis is controlled by Sonic Hedgehog (Shh), a signaling molecule secreted by the "Zone of Polarizing Activity", an organizer tissue located in the posterior margin of the limb bud. We have found that the transcription factors GATA4 and GATA6, which are key regulators of cell identity, are expressed in an anterior to posterior gradient in the early limb bud, raising the possibility that GATA transcription factors may play an additional role in patterning this tissue. While both GATA4 and GATA6 are expressed in an A-P gradient in the forelimb buds, the hindlimb buds principally express GATA6 in an A-P gradient. Thus, to specifically examine the role of GATA6 in limb patterning we generated Prx1-Cre; GATA6(fl/fl) mice, which conditionally delete GATA6 from their developing limb buds. We found that these animals display ectopic expression of both Shh and its transcriptional targets specifically in the anterior mesenchyme of the hindlimb buds. Loss of GATA6 in the developing limbs results in the formation of preaxial polydactyly in the hindlimbs. Conversely, forced expression of GATA6 throughout the limb bud represses expression of Shh and results in hypomorphic limbs. We have found that GATA6 can bind to chromatin (isolated from limb buds) encoding either Shh or Gli1 regulatory elements that drive expression of these genes in this tissue, and demonstrated that GATA6 works synergistically with FOG co-factors to repress expression of luciferase reporters driven by these sequences. Most significantly, we have found that conditional loss of Shh in limb buds lacking GATA6 prevents development of hindlimb polydactyly in these compound mutant embryos, indicating that GATA6 expression in the anterior region of the limb bud blocks hindlimb polydactyly by repressing ectopic expression of Shh.

Desbuquois dysplasia type 1 (DBQD1) is a chondrodysplasia caused by mutations in CANT1 gene encoding an ER/Golgi calcium activated nucleotidase 1 that hydrolyses UDP. Here, using Cant1 knock-in and knock-out mice recapitulating DBQD1 phenotype, we report that CANT1 plays a crucial role in cartilage proteoglycan synthesis and in endochondral ossification. Specifically, the glycosaminoglycan synthesis was decreased in chondrocytes from Cant1 knock-out mice and their hydrodynamic size was reduced, whilst the sulfation was increased and the overall proteoglycan secretion was delayed. Interestingly, knock-out chondrocytes had dilated ER cisternae suggesting delayed protein secretion and cellular stress; however, no canonical ER stress response was detected using microarray analysis, Xbp1 splicing and protein levels of BiP and ATF4. The observed proteoglycan defects caused deregulated chondrocyte proliferation and maturation in the growth plate resulting in the reduced skeletal growth. In conclusion, the pathogenic mechanism of DBQD1 comprises deregulated chondrocyte performance due to defective intracellular proteoglycan synthesis and altered proteoglycan properties in the extracellular matrix.

BackgroundCuller-Jones syndrome is a rare autosomal dominant disease caused by nucleotide sequence changes in the GLI2 gene. The prevalence of this pathology is unknown, as the number of observations is small, some carriers of variants in the GLI2 gene have no manifestations of the disease. The clinical phenotype of the disease is heterogeneous and includes hypopituitarism, malformations of internal organs, facial dysmorphisms, and polydactyly. Since the discovery of the GLI2 gene by Roessler E. et al. in 2003, the spectrum of clinical manifestations, as well as the understanding of the pathogenesis of the disease components, has expanded considerably. Incomplete penetrance has been described for the GLI2 gene, and the clinical phenotype of the disease differs even among members of the same family with the same nucleotide variant.AimStudy clinical and molecular genetic polymorphism in patients with Culler-Jones syndrome.Materials and methodsA single-center, non-interventional, single-stage, non-comparative study was conducted. Children with Culler-Jones syndrome with a confirmed genetic cause of the disease were examined. All patients underwent a comprehensive examination, including laboratory and instrumental diagnostic methods and sequencing (by NGS (next-generation sequencing).Results18 children (7 girls; 11 boys) with variants in the GLI2 gene were included in the study. The age at the time of examination was 8.95 years [4,6; 12.4]. Growth hormone deficiency was noted in all children at age of 2 years [1; 6,5]. Central hypothyroidism was diagnosed in 13 children at the age of 1.5 years [1; 3.5]. Free thyroxine level at the time of diagnosis was 8.9 pmol/L [7.5; 11.3]. Secondary hypoadrenocorticism was diagnosed in 10 children at the age of 2 years [1.5; 2.8], with a cortisol level of 84 nmol/L at the time of diagnosis [47; 152]. Extrahypophyseal manifestations characteristic of the syndrome were detected in half of the patients and included maxillofacial anomalies, malformations of the cardiovascular and urinary systems and eye malformations. Polydactyly was detected in two children.ConclusionThe present study demonstrates the clinical polymorphism of Culler-Jones syndrome and the lack of genotype-phenotype correlation for this disease.