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Tn is a cryptantigen located on O-linked oligosaccharides of cell membrane glycoproteins and is composed of N-acetylgalactosamine a -linked to serine or threonine of the protein backbone. Tn is expressed on hemopoietic cells of individuals with the rare idiopathic Tn syndrome, characterized by a variable pattern of Tn expression suggestive of its somatic and clonal origin. Tn is also associated with overexpression in several autoimmune diseases and malignancies. Tn is a bioprecursor of the T cryptantigen and subsequently the disialotetrasaccharide units typical of O-linked oligosaccharides of RBC sialoglycoproteins. Tn results from defective oligosaccharide biosynthesis caused by the malfunction of T-synthase, a b1,3-galactosyltransferase. Recent evidence suggested that the activity of T-synthase is dependent on a molecular chaperone, Cosmc (Ju and Cummings, PNAS 2002; 99:16613–18). In view of this evidence, we investigated whether Cosmc is required for T-synthase activity and ultimately for Tn phenotype by obtaining material from 4 apparently healthy, unrelated Caucasian individuals with Tn phenotype. Case 1 was 93.9% Tn+ on an EBV-transformed lymphoblastoma cell line. Analysis of the Cosmc gene, C1GALT1C1, showed a homozygous 428C>T, Ala143Val mutation and Case 1 showed a complete lack of expression of C1GALT1C1cDNA. Case 2 revealed no C1GALT1C1mutations in DNA extracted from plasma, but in DNA of lymphocyte origin an apparent heterozygous 454G>A, Glu152Lys change was observed, in agreement with Ju and Cummings (Nature 2005, 437:1252). When Case 2 hemopoietic progenitor cells were expanded into the erythroid cell line, the mutation appeared homozygous. Tn expression varied from 76.1% in lymphocytes, 90.7% in RBCs to 96.9% in erythroblasts. Case 3 showed 19.4% Tn+ lymphocytes, compared to 97.0% Tn+ RBCs. In Case 3 we found a single point mutation 577T>C, Ser193Pro. Case 4 exhibited 46.5% Tn+ lymphocytes, 96.0% Tn+ RBCs and 90.2% Tn+ erythroblasts. DNA analysis revealed 3G>C, converting the translation-initiating methionine to isoleucine and predicting the loss of first 12 amino acids of the protein, potentially altering its morphology. The mutations found in Cases 2–4 reflected the clonal nature of Tn, appearing heterozygous in DNA of lymphocyte origin and homozygous in DNA of erythroid origin. To confirm that the observed mutations are indeed responsible for Tn phenotype, pBabe puro vector with Tn or wild type C1GALT1C1inserts was transfected into Jurkat cells. Untransfected cells and cells transfected with bare vector expressed Tn. Jurkat cells transfected with wild-type C1GALT1C1were Tn-negative while those transfected with C1GALT1C1from Cases 2–4 expressed Tn. From this evidence we postulate that Cosmc is directly involved in the expression of Tn phenotype. To investigate the involvement of other genes, we performed expression profiling of 3 Tn and 4 control samples hybridized to HG-U133A arrays. A list of 100 up-regulated and 173 down-regulated genes, with 1.5× fold difference in expression, was obtained. Some genes, relating to erythrocyte development/heme biosynthesis were upregulated, while down-regulated genes were related to cholesterol/lipid metabolism. Real-time Q-PCR on six differentially expressed genes of interest, down-regulated FABP5, CYP1B1and LRP8and up-regulated AQP1, AQP3and EPB42, confirmed the microarray results, and elevated expression of AQP3 on Tn + RBCs was detected serologically. The effects of C1GALT1C1mutations are wider than Tn expression on hemopoietic cells.