FAM20C Overview: Classic and Novel Targets, Pathogenic Variants and Raine Syndrome Phenotypes

Knocking out FAM20C in pre-osteoblasts leads to up-regulation of osteoclast differentiation to affect long bone development

Family with sequence similarity 20 member C (FAM20C) is a Golgi casein kinase that phosphorylates extracellularly-secreted regulatory proteins involved in bone development and mineralization, but its specific role in bone development is still largely unknown. In this study, to examine the specific mechanisms that FAM20C influences bone development, we cross-bred Osx-Cre with FAM20Cflox/flox mice to establish a Osx-Cre; FAM20Cflox/flox knockout (oKO) mouse model; FAM20C was KO in pre-osteoblasts. oKO development was examined at 1-10 weeks, in which compared to control FAM20Cflox/flox, they had lower body weights and bone tissue mineralization. Furthermore, oKO had lower bone volume fractions, thickness, and trabecular numbers, along with higher degrees of trabecular separation. These mice also had decreased femoral metaphyseal cartilage proliferation layer, along with thickened hypertrophic layer and increased apoptotic cell counts. Transcriptomic analysis found that differentially-expressed genes in oKO were concentrated in the osteoclast differentiation pathway, in line with increased osteoclast presence. Additionally, up-regulation of osteoclast-related, and down-regulation of osteogenesis-related genes, were identified, in which the most up-regulated genes were signal regulatory protein β-1 family (Sirpb1a-c) and mitogen-activated protein kinase 13. Overall, FAM20C KO in pre-osteoblasts leads to abnormal long bone development, likely due to subsequent up-regulation of osteoclast differentiation-associated genes.

FAM20A: a potential diagnostic biomarker for lung squamous cell carcinoma

Background

Lung squamous cell carcinoma (LUSC) ranks among the carcinomas with the highest incidence and dismal survival rates, suffering from a lack of effective therapeutic strategies. Consequently, biomarkers facilitating early diagnosis of LUSC could significantly enhance patient survival. This study aims to identify novel biomarkers for LUSC.

Methods

Utilizing the TCGA, GTEx, and CGGA databases, we focused on the gene encoding Family with Sequence Similarity 20, Member A (FAM20A) across various cancers. We then corroborated these bioinformatic predictions with clinical samples. A range of analytical tools, including Kaplan-Meier, MethSurv database, Wilcoxon rank-sum, Kruskal-Wallis tests, Gene Set Enrichment Analysis, and TIMER database, were employed to assess the diagnostic and prognostic value of FAM20A in LUSC. These tools also helped evaluate immune cell infiltration, immune checkpoint genes, DNA repair-related genes, DNA methylation, and tumor-related pathways.

Results

FAM20A expression was found to be significantly reduced in LUSC, correlating with lower survival rates. It exhibited a negative correlation with key proteins in DNA repair signaling pathways, potentially contributing to LUSC's radiotherapy resistance. Additionally, FAM20A showed a positive correlation with immune checkpoints like CTLA-4, indicating potential heightened sensitivity to immunotherapies targeting these checkpoints.

Conclusion

FAM20A emerges as a promising diagnostic and prognostic biomarker for LUSC, offering potential clinical applications.

Genetic causes of hypophosphatemia

Phosphate is a key component of mineralized tissues and is also part of many organic compounds. Phosphorus homeostasis depends especially upon intestinal absorption, and renal excretion, which are regulated by various hormones, such as PTH, 1,25-dihydroxyvitamin D, and fibroblast growth factor 23. In this review we provide an update of several genetic disorders that affect phosphate transporters through cell membranes or the phosphate-regulating hormones, and, consequently, result in hypophosphatemia.

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

Gingival proteomics reveals the role of TGF beta and YAP/TAZ signaling in Raine syndrome fibrosis

Raine syndrome (RNS) is a rare autosomal recessive osteosclerotic dysplasia. RNS is caused by loss-of-function disease-causative variants of the FAM20C gene that encodes a kinase that phosphorylates most of the secreted proteins found in the body fluids and extracellular matrix. The most common RNS clinical features are generalized osteosclerosis, facial dysmorphism, intracerebral calcifications and respiratory defects. In non-lethal RNS forms, oral traits include a well-studied hypoplastic amelogenesis imperfecta (AI) and a much less characterized gingival phenotype. We used immunomorphological, biochemical, and siRNA approaches to analyze gingival tissues and primary cultures of gingival fibroblasts of two unrelated, previously reported RNS patients. We showed that fibrosis, pathological gingival calcifications and increased expression of various profibrotic and pro-osteogenic proteins such as POSTN, SPARC and VIM were common findings. Proteomic analysis of differentially expressed proteins demonstrated that proteins involved in extracellular matrix (ECM) regulation and related to the TGFβ/SMAD signaling pathway were increased. Functional analyses confirmed the upregulation of TGFβ/SMAD signaling and subsequently uncovered the involvement of two closely related transcription cofactors important in fibrogenesis, Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). Knocking down of FAM20C confirmed the TGFβ-YAP/TAZ interplay indicating that a profibrotic loop enabled gingival fibrosis in RNS patients. In summary, our in vivo and in vitro data provide a detailed description of the RNS gingival phenotype. They show that gingival fibrosis and calcifications are associated with, and most likely caused by excessed ECM production and disorganization. They furthermore uncover the contribution of increased TGFβ-YAP/TAZ signaling in the pathogenesis of the gingival fibrosis.

Mutations of family with sequence similarity 20-member C gene causing lethal and nonlethal Raine syndrome causes hypophosphatemia rickets

Family with sequence similarity 20-member C (FAM20C) is a kinase specific to most of the secreted phosphoproteome. FAM20C has been identified as the causative gene of Raine syndrome, initially characterized by lethal osteosclerosis bone dysplasia. However, since the identification of the cases of nonlethal Raine syndrome characterized by hypophosphatemia rickets, the previous definition of Raine syndrome has become debatable and raised a question about the role of mutations of FAM20C in controversial skeletal manifestation in the two forms of the disease. In this study, we aimed to investigate the influence of FAM20C mutations on skeletogenesis. We developed transgenic mice expressing Fam20c mutations mimicking those associated with human lethal and nonlethal Raine syndrome. The results revealed that transgenic mice expressing the mutant Fam20c found in the lethal (KO;G374R) and nonlethal (KO;D446N) Raine syndrome exhibited osteomalacia without osteosclerotic features. Additionally, both mutants significantly increased the expression of the Fgf23, indicating that Fam20c deficiency in skeletal compartments causes hypophosphatemia rickets. Furthermore, as FAM20C kinase activity catalyzes the phosphorylation of secreted proteomes other than those in the skeletal system, global FAM20C deficiency may trigger alterations in other systems resulting in osteosclerosis secondary to hypophosphatemia rickets. Together, the findings of this study suggest that FAM20C deficiency primarily causes hypophosphatemia rickets or osteomalacia; however, the heterogeneous skeletal manifestation in Raine syndrome was not determined solely by specific mutations of FAM20C. The findings also implicated that rickets or osteomalacia caused by FAM20C deficiency would deteriorate into osteosclerosis by the defects from other systems or environmental impacts.

The odontoblastic differentiation of dental mesenchymal stem cells: molecular regulation mechanism and related genetic syndromes

Dental mesenchymal stem cells (DMSCs) are multipotent progenitor cells that can differentiate into multiple lineages including odontoblasts, osteoblasts, chondrocytes, neural cells, myocytes, cardiomyocytes, adipocytes, endothelial cells, melanocytes, and hepatocytes. Odontoblastic differentiation of DMSCs is pivotal in dentinogenesis, a delicate and dynamic process regulated at the molecular level by signaling pathways, transcription factors, and posttranscriptional and epigenetic regulation. Mutations or dysregulation of related genes may contribute to genetic diseases with dentin defects caused by impaired odontoblastic differentiation, including tricho-dento-osseous (TDO) syndrome, X-linked hypophosphatemic rickets (XLH), Raine syndrome (RS), hypophosphatasia (HPP), Schimke immuno-osseous dysplasia (SIOD), and Elsahy-Waters syndrome (EWS). Herein, recent progress in the molecular regulation of the odontoblastic differentiation of DMSCs is summarized. In addition, genetic syndromes associated with disorders of odontoblastic differentiation of DMSCs are discussed. An improved understanding of the molecular regulation and related genetic syndromes may help clinicians better understand the etiology and pathogenesis of dentin lesions in systematic diseases and identify novel treatment targets.

Comparative N-Glycoproteomic Investigation of Eggshell Cuticle and Mineralized Layer Proteins

The eggshell cuticle layer (ECL) and eggshell mineralized layer (EML) contain amounts of glycoproteins and proteoglycans. However, there were few comprehensive reports about the effect of post-translational modifications on protein structure and function which requires investigation. Therefore, we used comparative N-glycoproteomics to study glycoproteins in the ECL and EML. We identified a total of 272 glycoproteins in this experiment and found that glycoproteins located in EML were more than that in ECL. Moreover, they showed distinct functional difference between both layers. As N-glycosylation of ovocleidin-17 and ovocleidin-116 in the EML affected eggshell mineralization, some glycoproteins located in ECL, like ovotransferrin and ovostatin-like, possessed antibacterial activity. The several regulated glycoproteins in the EML may pertain to the regulation of mineralization, while glycosylated proteins in the ECL may contribute to molecular adhesion and defense against microbial invasion. This study provides new insights into the eggshell matrix protein contents of the ECL and EML.

Fam20c regulates the calpain proteolysis system through phosphorylating Calpasatatin to maintain cell homeostasis

BACKGROUND

The family with sequence similarity 20-member C (FAM20C) kinase, a Golgi casein kinase, which is responsible for phosphorylating the majority of the extracellular phosphoproteins within S-x-E/pS motifs, and is fundamentally associated with multiple biological processes to maintain cell proliferation, biomineralization, migration, adhesion, and phosphate homeostasis. In dissecting how FAM20C regulates downstream molecules and potential mechanisms, however, there are multiple target molecules of FAM20C, particularly many phenomena remain elusive, such as changes in cell-autonomous behaviors, incompatibility in genotypes and phenotypes, and others.

METHODS

Here, assay for transposase-accessible chromatin using sequencing (ATAC-seq), RNA sequencing (RNA-seq), proteomics, and phosphoproteomics were performed in Fam20c-dificient osteoblasts and to facilitate an integrated analysis and determine the impact of chromatin accessibility, genomic expression, protein alterations, signaling pathway, and post translational modifcations.

RESULTS

By combining ATAC-seq and RNA-seq, we identified TCF4 and Wnt signaling pathway as the key regulators in Fam20c-dificient cells. Further, we showed Calpastatin/Calpain proteolysis system as a novel target axis for FAM20C to regulate cell migration and F-actin cytoskeleton by integrated analysis of proteomics and phosphoproteomics. Furthermore, Calpastatin/Calpain proteolysis system could negatively regulate the Wnt signaling pathway.

CONCLUSION

These observations implied that Fam20c knockout osteoblasts would cause cell homeostatic imbalance, involving changes in multiple signaling pathways in the conduction system.

Potential Role of Protein Kinase FAM20C on the Brain in Raine Syndrome, an In Silico Analysis

FAM20C (family with sequence similarity 20, member C) is a serine/threonine-specific protein kinase that is ubiquitously expressed and mainly associated with biomineralization and phosphatemia regulation. It is mostly known due to pathogenic variants causing its deficiency, which results in Raine syndrome (RNS), a sclerosing bone dysplasia with hypophosphatemia. The phenotype is recognized by the skeletal features, which are related to hypophosphorylation of different FAM20C bone-target proteins. However, FAM20C has many targets, including brain proteins and the cerebrospinal fluid phosphoproteome. Individuals with RNS can have developmental delay, intellectual disability, seizures, and structural brain defects, but little is known about FAM20C brain-target-protein dysregulation or about a potential pathogenesis associated with neurologic features. In order to identify the potential FAM20C actions on the brain, an in silico analysis was conducted. Structural and functional defects reported in RNS were described; FAM20C targets and interactors were identified, including their brain expression. Gene ontology of molecular processes, function, and components was completed for these targets, as well as for potential involved signaling pathways and diseases. The BioGRID and Human Protein Atlas databases, the Gorilla tool, and the PANTHER and DisGeNET databases were used. Results show that genes with high expression in the brain are involved in cholesterol and lipoprotein processes, plus axo-dendritic transport and the neuron part. These results could highlight some proteins involved in the neurologic pathogenesis of RNS.

Intracranial calcification in Fam20c-deficient mice recapitulates human Raine syndrome

FAM20C (family with sequence similarity 20-member C) is a protein kinase that phosphorylates secretory proteins, including the proteins that are essential to the formation and mineralization of calcified tissues. FAM20C loss-of-function mutations cause Raine syndrome in humans, characterized by generalized osteosclerosis, distinctive craniofacial dysmorphism, along with extensive intracranial calcification. Our previous studies revealed that inactivation of Fam20c in mice led to hypophosphatemic rickets. In this study, we examined the expression of Fam20c in the mouse brain and investigated brain calcification in Fam20c-deficient mice. Reverse transcription polymerase chain reaction (RT-PCR), Western-blotting and in situ hybridization analyses demonstrated the broad expression of Fam20c in the mouse brain tissue. X-ray and histological analyses showed that the global deletion of Fam20c (mediated by Sox2-cre) resulted in brain calcification in mice after postnatal 3 months and that the calcifications were bilaterally distributed within the brain. There was mild perifocal microgliosis as well as astrogliosis around calcospherites. The calcifications were first observed in the thalamus, and later in the forebrain and hindbrain. Furthermore, brain-specific deletion (mediated by Nestin-cre) of Fam20c in mice also led to cerebral calcification at an older age (postnatal 6 months), but no obvious skeletal or dental defects. Our results suggest that the local loss of FAM20C function in the brain may directly account for intracranial calcification. We propose that FAM20C plays an essential role in maintaining normal brain homeostasis and preventing ectopic brain calcification.

Epigenetic and transcriptional activation of the secretory kinase FAM20C as an oncogene in glioma

Gliomas are the most prevalent and aggressive malignancies of the nervous system. Previous bioinformatic studies have revealed the crucial role of the secretory pathway kinase FAM20C in the prediction of glioma invasion and malignancy. However, little is known about the pathogenesis of FAM20C in the regulation of glioma. Here, we construct the full-length transcriptome atlas in paired gliomas and observe that 22 genes are upregulated by full-length transcriptome and differential APA analysis. Analysis of ATAC-seq data reveals that both FAM20C and NPTN are the hub genes with chromatin openness and differential expression. Further, in vitro and in vivo studies suggest that FAM20C stimulates the proliferation and metastasis of glioma cells. Meanwhile, NPTN, a novel cancer suppressor gene, counteracts the function of FAM20C by inhibiting both the proliferation and migration of glioma. The blockade of FAM20C by neutralizing antibodies results in the regression of xenograft tumors. Moreover, MAX, BRD4, MYC, and REST are found to be the potential trans-active factors for the regulation of FAM20C. Taken together, our results uncover the oncogenic role of FAM20C in glioma and shed new light on the treatment of glioma by abolishing FAM20C.

Compound heterozygous FAM20C gene variants in a patient with severe Raine syndrome: a case report

Raine syndrome is a congenital disorder caused by biallelic mutations in the FAM20C gene. While most diagnosed cases of the syndrome are lethal in the first few months of life, there are also reports of non-lethal cases with Raine syndrome. The characteristic of this syndrome is typical facial dysmorphism and generalized osteosclerosis, as well as possible intracranial calcification, hearing loss, and seizures. We report a case of a 4-day-old patient at the time of examination, born with a distinct facial dysmorphism, short neck, narrow chest, and curved tibia. The parents, affirmative gypsy and non-consanguineous, had a previous male child born with the same phenotype who died at 4 months old. The computed tomography scan revealed choanal atresia, while transfontanelar ultrasound showed hypoplasia of the frontal and temporal lobes, corpus callosum dysgenesis, and multiple areas of intracranial hyperechogenicity. The chest X-Ray revealed generalized increased bone density. A skeletal disorders gene panel was performed which identified two variants in the FAM20C gene: a pathogenic variant c.1291C>T (p.Gln431*) and a likely pathogenic variant (c.1135G>A) (p.Gly379Arg), confirming the clinical diagnosis. The parents were also tested, and each was found to carry one of the variants. The particularity of this case is the severe phenotype in a compound heterozygous case that consists of FAM20C c.1291C>T (p.Gln431*) variant that has recently been reported in the literature. Also, our case is one of the few compound-heterozygous mutations in the FAM20C gene that has been described in a non-consanguineous marriage.

Fam20C in Human Diseases: Emerging Biological Functions and Therapeutic Implications

Fam20C, a typical member of Fam20 family, has been well-known as a Golgi casein kinase, which is closely associated with Raine Syndrome (RS). It can phosphorylate many secreted proteins and multiple substrates, and thereby plays a crucial role in biological functions. More importantly, Fam20C has also been found to enhance the metastasis of several types of human cancers, such as breast cancer, indicating that Fam20C may be a promising therapeutic target. Accordingly, some small-molecule inhibitors of Fam20C have been reported in cancer. Taken together, these inspiring findings would shed new light on exploiting Fam20C as a potential therapeutic target and inhibiting Fam20C with small-molecule compounds would provide a clue on discovery of more candidate small-molecule drugs for fighting with human diseases.

Osteocytes and the pathogenesis of hypophosphatemic rickets

Since phosphorus is a component of hydroxyapatite, its prolonged deprivation affects bone mineralization. Fibroblast growth factor 23 (FGF23) is essential for maintaining phosphate homeostasis and is mainly produced by osteocytes. FGF23 increases the excretion of inorganic phosphate (Pi) and decreases the production of 1,25-dihydroxyvitamin D in the kidneys. Osteocytes are cells of osteoblastic lineage that have undergone terminal differentiation and become embedded in mineralized bone matrix. Osteocytes express FGF23 and other multiple genes responsible for hereditary hypophosphatemic rickets, which include phosphate-regulating gene homologous to endopeptidase on X chromosome (PHEX), dentin matrix protein 1 (DMP1), and family with sequence similarity 20, member C (FAM20C). Since inactivating mutations in PHEX, DMP1, and FAM20C boost the production of FGF23, these molecules might be considered as local negative regulators of FGF23. Mouse studies have suggested that enhanced FGF receptor (FGFR) signaling is involved in the overproduction of FGF23 in PHEX-deficient X-linked hypophosphatemic rickets (XLH) and DMP1-deficient autosomal recessive hypophosphatemic rickets type 1. Since FGFR is involved in the transduction of signals evoked by extracellular Pi, Pi sensing in osteocytes may be abnormal in these diseases. Serum levels of sclerostin, an inhibitor Wnt/β-catenin signaling secreted by osteocytes, are increased in XLH patients, and mouse studies have suggested the potential of inhibiting sclerostin as a new therapeutic option for the disease. The elucidation of complex abnormalities in the osteocytes of FGF23-related hypophosphatemic diseases will provide a more detailed understanding of their pathogenesis and more effective treatments.