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.

Raine syndrome, a lethal osteosclerotic bone dysplasia in humans, is caused by loss-of-function mutations in FAM20C; however, Fam20c deficiency in mice does not recapitulate the human disorder, so the underlying pathoetiological mechanisms remain poorly understood. Here we show that FAM20C, in addition to the reported casein kinase activity, also fine-tunes the biosynthesis of chondroitin sulfate (CS) chains to impact bone homeostasis. Specifically, FAM20C with Raine-originated mutations loses the ability to interact with chondroitin 4-O-sulfotransferase-1, and is associated with reduced 4-sulfation/6-sulfation (4S/6S) ratio of CS chains and upregulated biomineralization in human osteosarcoma cells. By contrast, overexpressing chondroitin 6-O-sulfotransferase-1 reduces CS 4S/6S ratio, and induces osteoblast differentiation in vitro and higher bone mineral density in transgenic mice. Meanwhile, a potential xylose kinase activity of FAM20C does not impact CS 4S/6S ratio, and is not associated with Raine syndrome mutations. Our results thus implicate CS 4S/6S ratio imbalances caused by FAM20C mutations as a contributor of Raine syndrome etiology.

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.

Raine Syndrome (RS) also referred to as lethal osteosclerotic bone dysplasia describes an exceptionally rare autosomal recessive disorder with an estimated prevalence of <1 in 1,000,000. Endocrinological manifestations such as hypophosphataemic rickets depict a recent finding within the phenotypic spectrum of nonlethal RS. The dental sequelae of hypophosphataemic rickets are significant. Spontaneous recurrent abscesses on noncarious teeth result in significant odontogenic pain and multiple dental interventions. The dental presentations of nonlethal RS are less widely described within the literature. Amelogenesis Imperfecta (AI), however, was recently postulated as a key characteristic. This article presents the dental manifestations and extensive restorative and oral surgical intervention of three siblings with hypophosphataemic rickets secondary to Raine Syndrome treated at Great Ormond Street Hospital for Children, a tertiary referral hospital.

Raine syndrome (RS) is a rare genetic disorder characterized by osteosclerotic bone dysplasia caused by a homozygous mutation, compound heterozygous mutation, or microdeletion in the FAM20C gene. In the present study, the MiSeq next-generation sequencing platform was used to perform the FAM20C gene sequence analysis. A novel homozygous variant c.1255T>C (p.W419R) in the FAM20C gene was diagnosed, and a nonlethal RS phenotype was confirmed, thus contributing to the expansion of the nonlethal RS phenotype. Since there is limited information about rare diseases, we believe that these studies will contribute to the literature and to the understanding of how these disorders develop and progress.