Raine syndrome (RNS) is an autosomal recessive neonatal osteosclerotic bone dysplasia that usually results in death in the postnatal period. Patients present with abnormal craniofacial features and widespread periosteal osteosclerosis affecting the ribs, skull, and long bones. Nonlethal forms of RNS have recently been reported. Biallelic mutations in the FAM20C gene, which encodes a Golgi serine/threonine protein kinase that phosphorylates secretory pathway proteins, are responsible for RNS. In this study, we examined a Turkish family consisting of three patients by exome sequencing and found that the homozygous c.1071A > G transition at the second-to-last position of the 5' end of exon 5 gave rise to a synonymous variant (p.P357P) in FAM20C. Subsequent mRNA (cDNA) sequencing of FAM20C showed that the c.1071A > G substitution disrupted the splice junction and directed the splicing to a new location in intron 5, resulting in an in-frame 12 amino acid insertion into the protein. FAM20C is a serine/threonine protein kinase that localizes to the Golgi apparatus by forming a homo- or hetero-dimer and/or is secreted from the cell to phosphorylate secretory proteins. Functional analysis showed that the identified insertion did not disrupt either homo- or hetero-dimerization of the FAM20C protein. However, this variant protein failed to localize properly to the Golgi apparatus and exhibited poor secretion from the cell. All these findings suggest that the identified variant causing the 12 amino acid insertion is responsible for the nonlethal form of RNS in the family and provides mechanistic insight into the molecular pathogenesis of RNS.

Bi-allelic mutations in FAM20C gene are known to cause a rare genetic disorder- Raine syndrome (RS). The FAM20C protein binds calcium and phosphorylates proteins involved in biomineralization of bones and teeth. RS is recognized as an osteosclerotic bone dysplasia. It is characterized by distinctive facial features, generalized osteosclerosis and respiratory insufficiency along with periosteal bone formation. RS is typically described as being an aggressive skeletal dysplasia with death in the neonatal period or early infancy. However, in the recent past an increasing number of individuals having an extended life span along with a highly heterogeneous phenotype has led to classifying RS into short and extended lifespan categories. We report a case of RS with antenatal fractures, facial dysmorphism and osteosclerosis without significant respiratory manifestations. The child has a relatively extended lifespan, whereby she died at 17-months of age. Clinical exome sequencing revealed a previously known, homozygous, nonsense variant c.1680C > A (p.Cys560Ter) in exon 10 of FAM20C. Whilst the variant was initially classified as a variant of uncertain significance (VUS), through the latest release of gnomAD and GTEx data, this was subsequently re-classified as likely pathogenic. Furthermore, segregation analysis showed both parents to be carriers. In contrast, a previously reported case with the same variant had polyhydramnios, complex facial abnormalities and bright echogenic brain parenchyma with oval shaped skull and anterior flattening at 26 weeks of gestation. The variant identified has been previously reported as a VUS. The present case provides further evidence towards the pathogenicity of the variant. A plausible genotype-phenotype correlation based on the location of the variant has been verified, wherein the position of a nonsense variant in the terminal exon of FAM20C gene, could have had a partial effect on the protein function, thereby resulting in a relatively milder phenotype and extended lifespan. Furthermore, the vast phenotypic variation on clinical comparison current case and a previously reported case, despite having the same genotype, could suggest an oligogenic effect and/ or environmental influence.

The WNT/β-catenin pathway is central to the pathogenesis of various human diseases including those affecting bone development and tumor progression. To evaluate the role of a gain-of-function variant in CTNNB1 in a child with a sclerosing bone dysplasia and an adrenocortical adenoma. Whole exome sequencing with corroborative biochemical analyses. We recruited a child with a sclerosing bone dysplasia and an adrenocortical adenoma together with her unaffected parents. Whole exome sequencing and performance of immunoblotting and luciferase-based assays to assess the cellular consequences of a de novo variant in CTNNB1. A de novo variant in CTNNB1 (c.131C>T; p.[Pro44Leu]) was identified in a patient with a sclerosing bone dysplasia and an adrenocortical adenoma. A luciferase-based transcriptional assay of WNT signaling activity verified that the activity of β-catenin was increased in the cells transfected with a CTNNB1p.Pro44Leu construct (P = 4.00 × 10-5). The β-catenin p.Pro44Leu variant was also associated with a decrease in phosphorylation at Ser45 and Ser33/Ser37/Thr41 in comparison to a wild-type (WT) CTNNB1 construct (P = 2.16 × 10-3, P = 9.34 × 10-8 respectively). Increased β-catenin activity associated with a de novo gain-of-function CTNNB1 variant is associated with osteosclerotic phenotype and adrenocortical neoplasia.

Raine syndrome is a rare, autosomal recessive, osteosclerotic bone dysplasia due to pathogenic variants in FAM20C. The clinical phenotype is characterized by generalized osteosclerosis affecting all bones, cerebral calcifications, and craniofacial dysmorphism. Most cases present during the neonatal period with early lethality due to pulmonary hypoplasia and respiratory compromise while only few affected individuals have been reported to survive into adulthood. FAM20C is a ubiquitously expressed protein kinase that contains five functional domains including a catalytic domain, a binding pocket for FAM20A and three distinct N-glycosylation sites. We report a newborn infant with a history of prenatal onset fractures, generalized osteosclerosis, and craniofacial dysmorphism and early lethality. The clinical presentation was highly suggestive of Raine syndrome. A homozygous, novel missense variant in exon 5 of FAM20C (c.1007T>G; p.Met336Arg) was identified by targeted Sanger sequencing. Following in silico analysis and mapping of the variant on a three-dimensional (3D) model of FAM20C it is predicted to be deleterious and to affect N-glycosylation, protein folding, and subsequent secretion of FAM20C. In addition, we reviewed all published FAM20C mutations and observed that most pathogenic variants affect functional regions within the protein establishing evidence for an emerging genotype-phenotype correlation.

The group of sclerosing bone dysplasia's is a clinically and genetically heterogeneous group of rare bone disorders which, according to the latest Nosology and classification of genetic skeletal disorders (2015), can be subdivided in three subgroups; the neonatal osteosclerotic dysplasias, the osteopetroses and related disorders and the other sclerosing bone disorders. Here, we give an overview of the most important radiographic and clinical symptoms, the underlying genetic defect and potential treatment options of the different sclerosing dysplasias included in these subgroups.