Identification of two novel mutations in the GALNT3 gene in a Chinese family with hyperphosphatemic familial tumoral calcinosis

Rare and Common Variants in GALNT3 May Affect Bone Mass Independently of Phosphate Metabolism

Anabolic treatment options for osteoporosis remain limited. One approach to discovering novel anabolic drug targets is to identify genetic causes of extreme high bone mass (HBM). We investigated a pedigree with unexplained HBM within the UK HBM study, a national cohort of probands with HBM and their relatives. Whole exome sequencing (WES) in a family with HBM identified a rare heterozygous missense variant (NM_004482.4:c.1657C > T, p.Arg553Trp) in GALNT3, segregating appropriately. Interrogation of data from the UK HBM study and the Anglo-Australasian Osteoporosis Genetics Consortium (AOGC) revealed an unrelated individual with HBM with another rare heterozygous variant (NM_004482.4:c.831 T > A, p.Asp277Glu) within the same gene. In silico protein modeling predicted that p.Arg553Trp would disrupt salt-bridge interactions, causing instability of GALNT3, and that p.Asp277Glu would disrupt manganese binding and consequently GALNT3 catalytic function. Bi-allelic loss-of-function GALNT3 mutations alter FGF23 metabolism, resulting in hyperphosphatemia and causing familial tumoral calcinosis (FTC). However, bone mineral density (BMD) in FTC cases, when reported, has been either normal or low. Common variants in the GALNT3 locus show genome-wide significant associations with lumbar, femoral neck, and total body BMD. However, no significant associations with BMD are observed at loci coding for FGF23, its receptor FGFR1, or coreceptor klotho. Mendelian randomization analysis, using expression quantitative trait loci (eQTL) data from primary human osteoblasts and genome-wide association studies data from UK Biobank, suggested increased expression of GALNT3 reduces total body, lumbar spine, and femoral neck BMD but has no effect on phosphate concentrations. In conclusion, rare heterozygous loss-of-function variants in GALNT3 may cause HBM without altering phosphate concentration. These findings suggest that GALNT3 may affect BMD through pathways other than FGF23 regulation, the identification of which may yield novel anabolic drug targets for osteoporosis. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Relationship between rs7586085, GALNT3 and CCDC170 gene polymorphisms and the risk of osteoporosis among the Chinese Han population

Osteoporosis (OP) has plagued many women for years, and bone density loss is an indicator of OP. The purpose of this study was to evaluate the relationship between the polymorphism of the rs7586085, CCDC170 and GALNT3 gene polymorphisms and the risk of OP in the Chinese Han population. Using the Agena MassArray method, we identified six candidate SNPs on chromosomes 2 and 6 in 515 patients with OP and 511 healthy controls. Genetic model analysis was performed to evaluate the significant association between variation and OP risk, and meanwhile, the multiple tests were corrected by false discovery rate (FDR). Haploview 4.2 was used for haplotype analysis. In stratified analysis of BMI ˃ 24, rs7586085, rs6726821, rs6710518, rs1346004, and rs1038304 were associated with the risk of OP based on the results of genetic models among females even after the correction of FDR (qd < 0.05). In people at age ≤ 60 years, rs1038304 was associated with an increased risk of OP under genetic models after the correction of FDR (qd < 0.05). Our study reported that GALNT3 and CCDC170 gene polymorphisms and rs7586085 are the effective risk factors for OP in the Chinese Han population.

A novel FGF23 mutation in hyperphosphatemic familial tumoral calcinosis and its deleterious effect on protein O-glycosylation

BACKGROUND

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare disease characterized by hyperphosphatemia and ectopic calcification, predominantly at periarticular locations. This study was performed to characterize the clinical profile of tumoral calcinosis and to identify gene mutations associated with HFTC and elucidated its pathogenic role.

METHODS

The three subjects (two male and one female) were aged 30, 25 and 15 years, respectively. The clinical features, histopathological findings, and outcomes of three subjects with HFTC were retrospectively reviewed. The three subjects were analyzed for FGF23, GALNT3 and KL mutations. Function of mutant gene was analyzed by western blotting and wheat germ agglutinin affinity chromatography.

RESULTS

All subjects had hyperphosphatemia and elevated calcium-phosphorus product. Calcinosis positions included the left shoulder, left index finger, and right hip. Bone and joint damage were present in two cases and multiple foci influenced body growth in one case. The histopathological features were firm, rubbery masses comprising multiple nodules of calcified material bordered by the proliferation of mononuclear or multinuclear macrophages, osteoclastic-like giant cells, fibroblasts, and chronic inflammatory cells. The novel mutation c.484A>G (p.N162D) in exon 3 of FGF23 was identified in one subject and his family members. Measurement of circulating FGF23 in the subject confirmed low intact FGF23 and increased C-terminal fragment. In vitro experiments showed that the mutant FGF23 proteins had defective O-glycosylation and impaired protein proteolysis protection.

CONCLUSION

We identified a novel FGF23 missense mutation, and confirmed its damaging role in FGF23 protein O-glycosylation. Our findings expand the current spectrum of FGF23 variations that influence phosphorus metabolism.

Polypeptide N-acetylgalactosaminyltransferase-Associated Phenotypes in Mammals

Mucin-type O-glycosylation involves the attachment of glycans to an initial O-linked N-acetylgalactosamine (GalNAc) on serine and threonine residues on proteins. This process in mammals is initiated and regulated by a large family of 20 UDP-GalNAc: polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts) (EC 2.4.1.41). The enzymes are encoded by a large gene family (GALNTs). Two of these genes, GALNT2 and GALNT3, are known as monogenic autosomal recessive inherited disease genes with well characterized phenotypes, whereas a broad spectrum of phenotypes is associated with the remaining 18 genes. Until recently, the overlapping functionality of the 20 members of the enzyme family has hindered characterizing the specific biological roles of individual enzymes. However, recent evidence suggests that these enzymes do not have full functional redundancy and may serve specific purposes that are found in the different phenotypes described. Here, we summarize the current knowledge of GALNT and associated phenotypes.

A novel homozygous variant in exon 10 of the GALNT3 gene causing hyperphosphatemic familial tumoral calcinosis in a family from North India

Hyperphosphatemic familial tumoral calcinosis (HFTC) is an extremely rare autosomal recessive disorder caused by variants in the GALNT3 (N-acetylgalactosaminyltransferase 3), FGF23 (Fibroblast Growth Factor-23) and αKL (α-Klotho) genes, which results in progressive calcification of soft tissues. We describe the case of a 9-year-old girl who presented with recurrent hard nodular swellings on her feet and knees which intermittently discharged chalky white material. Her younger brother also had a similar condition. Both siblings showed hyperphosphatemia, but the parentsbiochemical parameters were normal. The histological features of the material aspirated from a skin lesion were consistent with tumoral calcinosis. Sanger sequencing identified a novel homozygous non-synonymous sequence variant in exon 10 of the GALNT3 gene (NM_004482.3:c.[1681T>A];[1681T>A], NP_004473.2:p. [Cys561Ser];[Cys561Ser] in the proband and her affected brother. The parents were heterozygous carriers for the same sequence variant. In conclusion, we report a new variant in the GALNT3 gene that caused HFTC in a North Indian family.

Hyperphosphatemic familial tumoral calcinosis caused by a novel variant in the GALNT3 gene

AIM

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare endocrine disorder caused by autosomal recessive variants in GALNT3, FGF23, and KL leading to progressive calcification of soft tissues and subsequent clinical effects. The aim of this was to study the cause of HFTC in an Iranian family.

PATIENTS AND METHODS

Four generations of a family with HFTC were studied for understanding the genetic pattern of the disease. Whole exome sequencing was applied on genomic DNA of the proband. Based on its result, genetically altered sequences were checked in his family through sanger sequencing. Then bioinformatics approaches as well as co-segregation analysis were applied to validate the genetic alteration.

RESULTS

A novel homozygous variant in exon four of GALNT3, namely p.R261Q was found. The parents and sister were carriers.

CONCLUSION

To our knowledge, it is the first-reported Iranian family with GALNT3-CDG novel variant.

Clinical and genetic analysis of idiopathic normophosphatemic tumoral calcinosis in 19 patients

PURPOSE

Tumoral calcinosis is a rare clinicopathological entity characterized by ectopic soft-tissue calcification, typically periarticular. Normophosphatemic tumoral calcinosis is seldom reported in East Asian populations, and the preoperative diagnosis is often elusive. This study was performed to characterize the clinical profile of normophosphatemic tumoral calcinosis and investigate the presence of the SAMD9 gene mutation.

METHODS

The clinical features, pathological examination findings, and outcomes of 19 subjects were retrospectively reviewed. All patients were analyzed for SAMD9 gene mutation using paraffin-embedded tumoral calcinosis specimens.

RESULTS

Nineteen subjects were analyzed (7 males, 12 females). Their mean age at surgery, mean age at symptom onset, and median disease duration was 51.9 ± 17.3 (range 7-75) years, 49.1 ± 17.2 (range 7-74) years, and 1.3 (interquartile range 0.5-3.0) years, respectively. Lesions were located in the hand in 8 (42.1%) subjects; wrist in 5 (26.3%); shoulder in 2 (10.5%); and hip, knee, buttock, and scrotum in 1 (5.3%) subject each. The lesions in 17 (89.5%) subjects were located around the joints [small joints (hand and wrist) in 13 (68.4%) and large joints (shoulder, hip, and knee) in 4 (21.1%)]. Lesions occurred in the upper limbs in 15 (78.9%) subjects and in the lower limbs in 2 (10.5%). Multiple-lesion involvement (distal right index finger and middle finger) occurred in one (5.3%) subject. Symptoms included pain in 15 (78.9%) subjects, impaired mobility in 5 (26.3%), swelling in 5 (26.3%), numbness in 2 (10.5%), and an asymptomatic mass in 2 (10.5%). The serum inorganic phosphorus concentration was normal in all 19 subjects (mean 1.17 ± 0.15 mmol/L). The serum calcium concentration was normal in 18 subjects and low in 1. The serum alkaline phosphatase concentration was normal in all 19 subjects. Pathological examination indicated multiple nodules of calcified materials that manifested an amorphous or granular blue-purple crystal and were surrounded by proliferation of mononuclear or multinuclear macrophages, osteoclastic-like giant cells, fibroblasts, and chronic inflammatory cells. Notably, different phases of pathological manifestations were observed in the same microscopic field. During follow-up (0.5-65.0 months), no recurrence of tumoral calcinosis was observed in 18 (94.7%) subjects, but 1 subject developed in situ recurrence of an asymptomatic subcutaneous mass after 6 months postoperatively. Genetic analysis in all 19 subjects revealed no SAMD9 gene mutations.

CONCLUSIONS

Most subjects were females and developed calcinosis in adulthood. Small joints (hand and wrist) and the upper limbs were frequently involved. The presence of different phases of pathological features in the same subject suggests that about half of the study participants had been misdiagnosed with another condition (such as gout, osteoarthritis, etc.). Complete surgical excision led to cure without recurrence during follow-up in majority of the study participants.

Hyperphosphatemic familial tumoral calcinosis secondary to fibroblast growth factor 23 (FGF23) mutation: a report of two affected families and review of the literature

Hyperphosphatemic familial tumoral calcinosis (HFTC), secondary to fibroblast growth factor 23 (FGF23) gene mutation, is a rare genetic disorder characterized by recurrent calcified masses. We describe young Lebanese cousins presenting with HFTC, based on a retrospective chart review and a prospective case study. In addition, we present a comprehensive review on the topic, based on a literature search conducted in PubMed and Google Scholar, in 2014 and updated in December 2017. While the patients had the same previously reported FGF23 gene mutation (homozygous c.G367T variant in exon 3 leading to a missense mutation), they presented with variable severity and age of disease onset (at 4 years in patient 1 and at 23 years in patient 2). A review of the literature revealed several potential patho-physiologic pathways of HFTC clinical manifestations, some of which may be independent of hyperphosphatemia. Most available treatment options aim at reducing serum phosphate level, by stimulating renal excretion or by inhibiting intestinal absorption. HFTC is a challenging disease. While the available medical treatment has a limited and inconsistent effect on disease symptomatology, surgical resection of calcified masses remains the last resort. Research is needed to determine the safety and efficacy of FGF23 replacement or molecular therapy, targeting the specific genetic aberration. Hyperphosphatemic familial tumoral calcinosis is a rare genetic disorder characterized by recurrent calcified masses, in addition to other visceral, skeletal, and vascular manifestations. It remains a very challenging disease.

Roles of NHERF Family of PDZ-Binding Proteins in Regulating GPCR Functions

Multicellular organisms are equipped with an array of G-protein-coupled receptors (GPCRs) that mediate cell-cell signaling allowing them to adapt to environmental cues and ultimately survive. This is mechanistically possible through complex intracellular GPCR machinery that encompasses a vast network of proteins. Within this network, there is a group called scaffolding proteins that facilitate proper localization of signaling proteins for a quick and robust GPCR response. One protein family within this scaffolding group is the PSD-95/Dlg/ZO-1 (PDZ) family which is important for GPCR localization, internalization, recycling, and downstream signaling. Although the PDZ family of proteins regulate the functions of several receptors, this chapter focuses on a subfamily within the PDZ protein family called the Na⁺/H⁺ exchanger regulatory factors (NHERFs). Here we extensively review the predominantly characterized roles of NHERFs in renal phosphate absorption, intestinal ion regulation, cancer progression, and immune cell functions. Finally, we discuss the future perspectives and possible clinical application of targeting NHERFs in several disorders.