To explore the genetic basis for a Chinese pedigree affected with Brachydactyly type B1 (BDB1) through whole exome sequencing (WES). A BDB1 pedigree admitted to the Affiliated Women and Children's Hospital of Qingdao University on June 25, 2021 was selected as the study subject. Clinical data of the pedigree was collected with informed consent. WES was carried out for the proband, and candidate variant was verified by Sanger sequencing and bioinformatic analysis. WES and Sanger sequencing had identified a heterozygous c.2257delT variant in the ROR2 gene of the proband and his affected father, which has conformed to an autosomal dominant pattern of inheritance. Based on the guidelines from the American College of Medical Genetics and Genomics (ACMG), the variant was classified to be likely pathogenic (PVS1_Strong+PM2 Supporting+PP4). The c.2257delT variant of the ROR2 gene was unreported previously and is strongly correlated with the BDB1-like phenotype in this pedigree. Above finding has enriched the mutational spectrum of the ROR2 gene and facilitated the diagnosis and genetic counseling for this pedigree.

Non-canonical/β-catenin-independent Wnt signaling plays crucial roles in tissue/cell polarity in epithelia, but its functions have been less well studied in mesenchymal tissues, such as the skeleton. Mutations in non-canonical Wnt signaling pathway genes cause human skeletal diseases such as Robinow syndrome and Brachydactyly Type B1, which disrupt bone growth throughout the endochondral skeleton. Ror2 is one of several non-canonical Wnt receptor/co-receptors. Here, we show that ror2-/- mutant zebrafish have craniofacial skeletal defects, including disruptions of chondrocyte polarity. ror1-/- mutants appear to be phenotypically wild type, but loss of both ror1 and ror2 leads to more severe cartilage defects, indicating partial redundancy. Skeletal defects in ror1/2 double mutants resemble those of wnt5b-/- mutants, suggesting that Wnt5b is the primary Ror ligand in zebrafish. Surprisingly, the proline-rich domain of Ror2, but not its kinase domain, is required to rescue its function in mosaic transgenic experiments in ror2-/- mutants. These results suggest that endochondral bone defects in ROR-related human syndromes reflect defects in cartilage polarity and morphogenesis.

Brachydactyly type B is an autosomal dominant disorder that is characterized by hypoplasia of the distal phalanges and nails and can be divided into brachydactyly type B1 (BDB1) and brachydactyly type B2 (BDB2). BDB1 is the most severe form of brachydactyly and is caused by truncating variants in the receptor tyrosine kinase-like orphan receptor 2 (ROR2) gene. Here, we report a five-generation Chinese family with brachydactyly with or without syndactyly. The proband and her mother underwent digital separation in syndactyly, and the genetic analyses of the proband and her parents were provided. The novel heterozygous frameshift variant c.1320dupG, p.(Arg441Alafs*18) in the ROR2 gene was identified in the affected individuals by whole-exome sequencing and Sanger sequencing. The c.1320dupG variant in ROR2 is predicted to produce a truncated protein that lacks tyrosine kinase and serine/threonine- and proline-rich structures and remarkably alters the tertiary structures of the mutant ROR2 protein. The c.1320dupG, p.(Arg441Alafs*18) variant in the ROR2 gene has not been reported in any databases thus far and therefore is novel. Our study extends the gene variant spectrum of brachydactyly and may provide information for the genetic counselling of family members.

We present five members of a consanguineous Pakistani kinship with the most severe familial tetramelic transverse autopod deficiency reported to date and additionally having some of the common autosomal recessive Robinow syndrome-1 (RRS1) features including short stature, short neck, severe vertebral anomalies of kyphoscoliosis, hemivertebrae, fusion of thoracic vertebrae, broad forehead, and dental crowding. We mapped the locus of this atypical RRS and detected homozygous 8-nucleotide deletion c.1353_1360del (p.(Met452Alafs*4)) in ROR2, the gene responsible for RRS1. We did not find any other variant shared by all affected individuals that could possibly act as a modifier of limb defect. Autopods are affected in RRS1, but severe autopod deficiency is not a characteristic feature. Over 30 biallelic variants dispersed throughout the gene are known in ROR2-related RS, with no genotype-phenotype correlation for specific RRS1 features. Considering together with the sporadic case homozygous for variant p.(Arg442*) and the case homozygous for p.(Arg441Thrfs*16) in a family where heterozygous members have brachydactyly type B1, we propose that homozygous truncating variants that originate at residues 441-452 can cause severe autopod reduction anomalies, suggesting some genotype-phenotype correlation for this particular phenotype.

Mutations in the human receptor tyrosine kinase ROR2 are associated with Robinow syndrome (RRS) and brachydactyly type B1. Amongst others, the shortened limb phenotype associated with RRS is recapitulated in Ror2-/- mutant mice. In contrast, Ror1-/- mutant mice are viable and show no limb phenotype. Ror1-/- ;Ror2-/- double mutants are embryonic lethal, whereas double mutants containing a hypomorphic Ror1 allele (Ror1hyp ) survive up to birth and display a more severe shortened limb phenotype. Both orphan receptors have been shown to act as possible Wnt coreceptors and to mediate the Wnt5a signal. Here, we analyzed genetic interactions between the Wnt ligand, Wnt9a, and Ror2 or Ror1, as Wnt9a has also been implicated in skeletal development. Wnt9a-/- single mutants display a mild shortening of the long bones, whereas these are severely shortened in Ror2-/- mutants. Ror2-/- ;Wnt9a-/- double mutants displayed even more severely shortened long bones, and intermediate phenotypes were observed in compound Ror2;Wnt9a mutants. Long bones were also shorter in Ror1hyp/hyp ;Wnt9a-/- double mutants. In addition, Ror1hyp/hyp ;Wnt9a-/- double mutants displayed a secondary palate cleft phenotype, which was not present in the respective single mutants. Interestingly, 50% of compound mutant pups heterozygous for Ror2 and homozygous mutant for Ror1 also developed a secondary palate cleft phenotype.