GALNT3Udp-N-Acetyl-Alpha-D-Galactosamine:polypeptide N-Acetylgalactosaminyltransferase 3
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A novel homozygote P85Rfs*6 (c.254_255delCT) mutation in exon 1 of the GALNT3 gene was detected by next generation sequencing (NGS).
Hyperphosphatemia, elevated TmP/GFR ratio, family history, biopsy result and presence of the same homozygote P85Rfs*6 (c.254_255delCT) mutation in GALNT3 gene confirmed the diagnosis of HFTC.
Homozygote mutations in the GALNT3, FGF23 and KL genes were found in patients with the HHS phenotype.
In conclusion, we report two siblings with a novel homozygote GALNT3 mutation representing an HFTC phenotype.
In comparing the 2 cell sublines, we found that several GALNT genes were expressed in both, (PTX and BM neuroblasts expressed GALNT1, 2, 4, 6, 7,12, and 14), but the expression of 3 genes (GALNT3,5, and 10) were undetectable in these 2 cell lines.
[7] Human genes: MYCN, v-myc myelocytomatosis viral related oncogene, neuroblastoma derived (avian); GALNT3, UDP-N-acetyl-[alpha]-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3 (GALNAC-T3); B2M, [[beta].sub.2]-microglobulin.
The defective GALNT3 gene encodes UDP-Ga1NAc transferase 3 (GALNT3), responsible for the transfer of UDP-Ga1NAc to Thr/Ser to the protein backbone.
Mutations in GALNT3, encoding a protein involved in 0-linked glycosylation, cause familial tumoral calcinosis.
[6] Nonstandard abbreviafions: CDG, congenital disorders of glycosylation; TIEF, transferrin isoelectric focusing; ApoC-III, apolipoprotein C-III; IEF, isoelectric focusing; Ga1NAc, N-acetylgalactosamine; NeuAc, sialic acid; FTC, familial tumoral calcinosis (MIM 211900); HIBM, hereditary inclusion body myopathy (MIM 600737); MEB, muscle-eye-brain disease (MIM 253280); HUS, hemolyfic uremic syndrome; HGPS, Hutchinson Gilford progeria syndrome (MIM 176670), F5FSD, a combined deficiency of factor V and factor VIII (MIM 227300); ER, endoplasmic reficulum; COG7, conserved oligomeric Golgi complex; CMP-NeuAc: cytidine 5'monophospho-N-acetylneuraminic acid; GNE/MNK, uridine-5'-diphosphate-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase; GALNT3, UDP-Ga1NAc transferase 3.
Diagnosis: Recently immunostaining with a monoclonal antibody against GalNT3 revealed that the protein was absent in a frozen skin biopsy from a patient with FTC, whereas GalNT3 was strongly expressed in the epidermis of a healthy individual.
This becomes obvious in patients with FTC (GalNT3 deficiency), who present only with massive calcium deposits in the skin and subcutaneous tissues.
It is expected to give a normal result because GalNT3, the defective enzyme in FTC, is not expressed in liver tissue, where apoC-III is synthesized (94).