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GALNT3
polypeptide N-acetylgalactosaminyltransferase 3
GALNT3 encodes a glycosyltransferase enzyme that regulates phosphate balance in the body by modifying a key signalling protein involved in kidney function. The GALNT3 gene provides instructions for producing ppGalNacT3, an enzyme responsible for attaching sugar molecules to fibroblast growth factor 23.
GALNT3 is located on the long (q) arm of chromosome 2, at band 2q24.3. Arm ratio per GRCh38 - banding schematic.
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Overview
GALNT3 (polypeptide N-acetylgalactosaminyltransferase 3) is located on chromosome 2q24.3 and encodes an enzyme that plays a central role in phosphate homeostasis. The ppGalNacT3 protein functions as a glycosyltransferase, meaning it attaches specific sugar groups to target proteins. Its primary substrate is fibroblast growth factor 23 (FGF23), a hormone produced in bone that signals the kidneys to reduce phosphate reabsorption when levels rise above normal.
Phosphate is critical for bone formation during childhood and preserves skeletal integrity in adulthood. The body tightly regulates phosphate concentrations through kidney function, and GALNT3 contributes to this regulatory system by ensuring FGF23 remains stable and functional. When GALNT3 function is compromised, phosphate control mechanisms can fail, leading to abnormal mineral accumulation in tissues.
What the gene does
The ppGalNacT3 enzyme catalyses the transfer of N-acetylgalactosamine sugar molecules from UDP-GalNAc donor substrates onto specific serine or threonine residues of target proteins. This glycosylation reaction occurs predominantly on fibroblast growth factor 23, a signalling molecule that regulates phosphate metabolism by acting on kidney cells. Sugar groups added by ppGalNacT3 facilitate the secretion of FGF23 from bone cells and shield it from enzymatic cleavage that would otherwise disable the hormone.
Rising phosphate concentrations trigger ppGalNacT3 to stabilise FGF23 through glycosylation, enabling the hormone to circulate and instruct kidney tubules to lower their phosphate retention. This regulatory loop helps normalise phosphate concentrations. The enzyme appears in multiple tissue types, underscoring its role in whole-body phosphate coordination. Loss of ppGalNacT3 activity results in rapid FGF23 breakdown, eliminating the body's ability to reduce phosphate reabsorption and causing hyperphosphataemia.
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Chromosome location
GALNT3 is positioned on the long arm of chromosome 2 at cytogenetic band 2q24.3. The gene encodes a protein of 633 amino acids. Detailed exon structure and transcript variant information have been catalogued in genomic databases, though the clinical significance of alternative splicing patterns remains an area of ongoing research.
Protein structure
The ppGalNacT3 protein contains distinct functional regions that enable its enzymatic activity. Catalytic subdomain A spans amino acids 184-293, whilst catalytic subdomain B occupies positions 356-418. These two subdomains together form the active site responsible for transferring sugar molecules onto target proteins. A ricin B-type lectin domain extends from amino acid 504 to 630 at the carboxy-terminal end. Ricin-type lectin domains typically mediate carbohydrate recognition and may contribute to substrate specificity or cellular localisation of the enzyme.
Key variants
Genetic changes in GALNT3 that reduce or eliminate enzyme function have been identified in clinical settings. At least 25 pathogenic variants have been reported, predominantly associated with loss of glycosyltransferase activity. These alterations include nonsense mutations that introduce premature stop codons, frameshift variants caused by small insertions or deletions, and missense changes affecting critical residues within the catalytic domains.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.1102dup | p.Ser368fs | Pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.1312C>T | p.Arg438Cys | Pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.149C>G | p.Ser50Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.1696C>T | p.Gln566Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.260_266del | p.Arg87fs | Pathogenic/Likely pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.505C>T | p.Arg169Ter | Pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.516-2A>G | - | Pathogenic/Likely pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.746_749del | p.Val249fs | Pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.892del | p.Tyr298fs | Pathogenic/Likely pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
c.985G>A | p.Gly329Arg | Pathogenic/Likely pathogenic | ★★☆☆ | Tumoral calcinosis, hyperphosphatemic, familial, 1 |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Pathogenic variants in GALNT3 cause hyperphosphataemic familial tumoural calcinosis, a rare autosomal recessive disorder characterised by raised phosphate concentrations in the bloodstream and abnormal deposits of calcium phosphate in soft tissues. Affected individuals typically develop calcified masses around large joints, which can cause pain and restrict movement. The condition results from failure to properly glycosylate fibroblast growth factor 23, leading to its rapid breakdown and loss of phosphate-lowering signals. Without effective FGF23 activity, the kidneys continue reabsorbing phosphate even when levels are already excessive, creating a cycle of progressive hyperphosphataemia and ectopic calcification.
No disease links recorded for this gene in our reference set.
UK clinical status
GALNT3 appears on several NHS Genomic Medicine Service gene panels with green (high evidence) ratings. It is included in the Familial tumoural calcinosis panel (R162), reflecting its established role in this specific phosphate disorder. The gene also features on broader panels including Congenital disorders of glycosylation, Likely inborn error of metabolism (R98), Skeletal dysplasia (R104), Pigmentary skin disorders (R236), and Undiagnosed metabolic disorders, acknowledging that GALNT3-related conditions may present with diverse clinical features affecting multiple organ systems.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What does the GALNT3 gene do?
GALNT3 encodes an enzyme that attaches sugar molecules to fibroblast growth factor 23, a hormone that controls phosphate levels by signalling the kidneys to excrete excess phosphate. This modification protects FGF23 from degradation and enables it to function properly.
How are GALNT3 variants inherited?
Pathogenic GALNT3 variants are inherited in an autosomal recessive pattern, meaning an individual must inherit altered copies from both parents to develop hyperphosphataemic familial tumoural calcinosis. Carriers with one altered copy typically do not show symptoms.
What is hyperphosphataemic familial tumoural calcinosis?
This rare condition results from GALNT3 variants that prevent proper regulation of phosphate levels, causing excessive phosphate in the blood and abnormal calcium-phosphate deposits in soft tissues around joints. The calcified masses can impair mobility and require clinical management.
References
- Yancovitch A, Hershkovitz D, Indelman M. Novel mutations in GALNT3 causing hyperphosphatemic familial tumoral calcinosis. Journal of bone and mineral metabolism. 2011. PMID: 21347749
- Farrow EG, Imel EA, White KE. Miscellaneous non-inflammatory musculoskeletal conditions. Hyperphosphatemic familial tumoral calcinosis (FGF23, GALNT3 and αKlotho). Best practice & research. Clinical rheumatology. 2011. PMID: 22142751
- Sprecher E. Familial tumoral calcinosis: from characterization of a rare phenotype to the pathogenesis of ectopic calcification. The Journal of investigative dermatology. 2010. PMID: 19865099
- Ichikawa S, Baujat G, Seyahi A. Clinical variability of familial tumoral calcinosis caused by novel GALNT3 mutations. American journal of medical genetics. Part A. 2010. PMID: 20358599
- Chefetz I, Kohno K, Izumi H. GALNT3, a gene associated with hyperphosphatemic familial tumoral calcinosis, is transcriptionally regulated by extracellular phosphate and modulates matrix metalloproteinase activity. Biochimica et biophysica acta. 2009. PMID: 18976705
- Chefetz I, Sprecher E. Familial tumoral calcinosis and the role of O-glycosylation in the maintenance of phosphate homeostasis. Biochimica et biophysica acta. 2009. PMID: 19013236