Reply to Ultrastructural Study of the Clinically Uninvolved Skin in Familial Tumoral Calcinosis Caused by Mutations in GALNT3 (2006)

To the Editor: Familial tumoral calcinosis (FTC) is a debilitating hereditary metabolic disorder characterized by the progressive deposition of basic calcium phosphate crystals in juxta-articular soft tissues.1 The majority of patients have hyperphosphatemia due to increased renal reabsorption of phosphate, whereas in the remainder, phosphate homeostasis is normal.2 Mutations underlying hyperphosphatemic FTC (HFTC) were first identified in GALNT3 (MIM601756)3,4 which encodes UDP-N-acetyl-α-D-galactosamine: polypeptide N-acetylgalactosaminyl-transferase 3 (ppGalNacT3), a Golgi-associated enzyme that mediates mucin type O-linked glycan biosynthesis by instating GalNac residues on the protein scaffold, targeting serine or threonine residues.5 The nature of GalNacT3 physiologic substrate(s) is unknown and therefore it is still unclear how ppGalNacT3 deficiency due to deleterious mutations in GALNT3 leads to the HFTC phenotype. More recently, homozygous mutations in the fibroblast growth factor 23 gene have been demonstrated in some additional families with HFTC (MIM 307800).6,7 Fibroblast growth factor 23 is part of a group of modulators of phosphate circulating levels known as phosphatonins.8 It has been shown to inhibit phosphate reabsorption in the renal proximal tubules.9 HFTC is characterized by the formation of basic calcium phosphate crystals (hydroxy-apatite) in periarticular soft tissues.10–12 Histopathologic studies of biopsies obtained from calcifying lesions of FTC patients revealed erythrocytes extravasation and aggregation of crystal-containing foamy histiocytes and macrophages in the early stages of the disease, followed by the formation of cystic cavities lined by osteoclastlike giant cells and mononuclear cells. In late stages of the disease, massive calcifications were found within the cysts, in the cells lining the cysts, and in the extracellular spaces.13 The mineralized deposits closely resemble bone, although they are more heavily mineralized and contain larger hydroxyapatite crystals.11 Ultrastructurally, the crystals were found both extracellularly and intracellularly within mononuclear cells in close proximity to dilated rough endoplasmic reticulum11; in large vesicles in the cytoplasms of multinucleated osteoclastlike giant cells; within cystic cavities, and occasionally in the lumina of small blood vessels.13 Occasional hydroxyapatite crystals were observed within the mitochondrial matrix in cytoplasms of cells adjacent to areas of disintegration.14 Several ultrastructural studies have implicated intramitochondrial calcification as the principal calcifying process in tumoral calcinosis,14–16 but others have found no evidence of mitochondrial calcification.11 In a previous study published in this journal,17 we have studied the clinically uninvolved skin of a patient with HFTC caused by mutation in GALNT3. The biopsies were taken from the arm at a distance of at least 5 cm from the closest subcutaneous calcium deposit. The histologic and ultrastructural analysis failed to detect minimal and subclinical calcium deposits both intracellularly and extracellularly.17 This is despite the fact that immunostaining has shown complete absence of intraepidermal glycosyltransferase ppGalNacT3 expression in the epidermis of the uninvolved skin.17 We have recently obtained an additional punch biopsy from the clinically uninvolved skin overlying a nodule of tumoral calcinosis in the finger of another patient with HFTC caused by GALNT3 mutation. It failed too, to show any signs of subclinical calcium deposits within the cellular components of the epidermis and dermis, and in the dermal matrix. The initial phase of tumor formation in tumoral calcinosis is characterized by edema, collagen degeneration, proliferation of capillaries, hemorrhages, mucin deposits, hyalinization, and necrosis leading to the formation of cysts, irregular cavities, and calcification.12,13,18–20 The lack of subclinical calcification in the overlying and distant uninvolved skin as shown in our studies support the notion that calcification rather than being a primary event in the pathogenesis of HFTC may occur as a result of a delayed response to tissue injury. Reuven Bergman, MD* Orit Topaz, MSc* Theodore C. Iancu, MD† Eli Sprecher, MD, PhD* *Department of Dermatology, Rambam Medical Centre †The pediatric Research and Electron Microscopy Unit, Ruth and Bruce Rappaport Faculty of Medicine Technion-Israel Institute of Technology Haifa, Israel