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    Enamel defects in Acp4R110C/R110C mice and human ACP4 mutations
    (2022) Liang, Tian; Wang, Shih-Kai; Smith, Charles; Zhang, Hong; Hu, Yuanyuan; Seymen, Figen; Koruyucu, Mine; Kasımoğlu, Yelda; Kim, Jung-wook; Zhang, Chuhua; Saunders, Thomas L.; Simmer, James P.; Hu, Jan C-C.
    Human ACP4 (OMIM*606362) encodes a transmembrane protein that belongs to histidine acid phosphatase (ACP) family. Recessive mutations in ACP4 cause non-syndromic hypoplastic amelogenesis imperfecta (AI1J, OMIM#617297). While ACP activity has long been detected in developing teeth, its functions during tooth development and the pathogenesis of ACP4-associated AI remain largely unknown. Here, we characterized 2 AI1J families and identified a novel ACP4 disease-causing mutation: c.774_775del, p.Gly260Aspfs*29. To investigate the role of ACP4 during amelogenesis, we generated and characterized Acp4R110C mice that carry the p.(Arg110Cys) loss-of-function mutation. Mouse Acp4 expression was the strongest at secretory stage ameloblasts, and the protein localized primarily at Tomes' processes. While Acp4 heterozygous (Acp4+/R110C) mice showed no phenotypes, incisors and molars of homozygous (Acp4R110C/R110C) mice exhibited a thin layer of aplastic enamel with numerous ectopic mineralized nodules. Acp4R110C/R110C ameloblasts appeared normal initially but underwent pathology at mid-way of secretory stage. Ultrastructurally, sporadic enamel ribbons grew on mineralized dentin but failed to elongate, and aberrant needle-like crystals formed instead. Globs of organic matrix accumulated by the distal membranes of defective Tomes' processes. These results demonstrated a critical role for ACP4 in appositional growth of dental enamel probably by processing and regulating enamel matrix proteins around mineralization front apparatus.
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    FAM20A mutations and transcriptome analyses of dental pulp tissues of enamel renal syndrome
    (2023) Wang, Shih-Kai; Zhang, Hong; Wang, Yin-Lin; Lin, Hung-Ying; Seymen, Figen; Koruyucu, Mine; Wright, J. Timothy; Kim, Jung-Wook; Simmer, James P.; Hu, Jan C-C.
    Aim: Biallelic loss-of-function FAM20A mutations cause amelogenesis imperfecta (AI) type IG, better known as enamel renal syndrome (ERS), characterized by severe enamel hypoplasia, delayed/failed tooth eruption, intrapulpal calcifications, gingival hyperplasia, and nephrocalcinosis. FAM20A binds to FAM20C, the Golgi casein kinase (GCK), and potentiates its function to phosphorylate secreted proteins critical for biomineralization. While many FAM20A pathogenic mutations have been reported, the pathogeneses of orodental anomalies in ERS remain to be elucidated. This study aimed to identify disease-causing mutations for patients with ERS phenotypes and to discern the molecular mechanism underlying ERS intrapupal calcifications. Methodology: Phenotypic characterization and whole exome analyses were conducted for 8 families and 2 sporadic cases with hypoplastic AI. A minigene assay was performed to investigate the molecular consequences of a FAM20A splice-site variant. RNA sequencing followed by transcription profiling and gene ontology (GO) analyses were carried out for dental pulp tissues of ERS and the control. Results: Biallelic FAM20A mutations were demonstrated for each affected individual, including 7 novel pathogenic variants: c.590-5T>A, c.625T>A (p.Cys209Ser), c.771del (p.Gln258Argfs*28), c.832_835delinsTGTCCGACGGTGTCCGACGGTGTCCA (p.Val278Cysfs*29), c.1232G>A (p.Arg411Gln), c.1297A>G (p.Arg433Gly), and c.1351del (p.Gln451Serfs*4). The c.590-5T>A splice-site mutation caused Exon 3 skipping, which resulted in an in-frame deletion of a unique region of the FAM20A protein, p.(Asp197_Ile214delinsVal). Analyses of differentially expressed genes in ERS pulp tissues demonstrated that genes involved in biomineralization, particularly dentinogenesis, were significantly upregulated, such as DSPP, MMP9, MMP20, and WNT10A. Enrichment analyses indicated over-representation of gene sets associated with BMP and SMAD signaling pathways. In contrast, GO terms related to inflammation and axon development were under-represented. Among BMP signaling genes, BMP agonists GDF7, GDF15, BMP3, BMP8A, BMP8B, BMP4, and BMP6 were upregulated, while BMP antagonists GREM1, BMPER, and VWC2 showed decreased expression in ERS dental pulp tissues. Conclusions: Upregulation of BMP signaling underlies intrapulpal calcifications in ERS. FAM20A plays an essential role in pulp tissue homeostasis and prevention of ectopic mineralization in soft tissues. This critical function probably depends upon MGP (matrix Gla protein), a potent mineralization inhibitor that must be properly phosphorylated by FAM20A-FAM20C kinase complex.
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    Translated mutant DSPP mRNA expression level impacts the severity of dentin defects
    (2022) Kim, Youn Jung; Lee, Yejin; Zhang, Hong; Seymen, Figen; Koruyucu, Mine; Bayrak, Şule; Tüloğlu, Nuray; Simmer, James P.; Hu, Jan C-C.; Kim, Jung-Wook
    Hereditary dentin defects are conventionally classified into three types of dentinogenesis imperfecta (DGI) and two types of dentin dysplasia (DD). Mutations in the dentin sialophospho protein (DSPP) gene have been identified to cause DGI type II and III and DD type II; therefore, these are not three different conditions, but rather allelic disorders. In this study, we recruited three families with varying clinical phenotypes from DGI-III to DD-II and performed mutational analysis by candidate gene analysis or whole-exome sequencing. Three novel mutations including a silent mutation (NM_014208.3: c.52-2del, c.135+1G>C, and c.135G>A; p.(Gln45=)) were identified, all of which affected pre-mRNA splicing. Comparison of the splicing assay results revealed that the expression level of the DSPP exon 3 deletion transcript correlated with the severity of the dentin defects. This study did not only expand the mutational spectrum of DSPP gene, but also advanced our understanding of the molecular pathogenesis impacting the severity of hereditary dentin defects.