EXPRESSION OF TRANSCOBALAMIN-II MESSENGER-RNA IN HUMAN TISSUES AND CULTURED FIBROBLASTS FROM NORMAL AND TRANSCOBALAMIN-II-DEFICIENT PATIENTS

Transcobalamin II (TCII) is an important plasma transporter of cobalam in (Cbl; vitamin B-12). In the present study, TCII gene expression in human and rat tissues and in the fibroblasts of patients with TCII def iciency was investigated. Northern-blot analyses revealed expression o f TCII mRNA in many human and rat tissues. In humans, this was 14-fold higher in the kidney than in liver, whereas in the rat the levels of expression were similar in the kidney and liver. Southern-blot analysi s of genomic DNA from several species revealed sequence similarity in TCII across species. Metabolic labelling and ribonuclease protection a ssay revealed a 43 kDa TCII protein and a fully protected TCII mRNA ba nd in normal fibroblasts but not in fibroblasts from three TCII-defici ent patients. Southern-blot analysis of genomic DNA from all these fib roblasts revealed identical restriction patterns on BamHI, HindIII, Kp nI, MspI and EcoRI digestion. On the basis of these results, we sugges t that TCII is expressed in multiple tissues, and its level of express ion in tissues varies within the same and across species. Furthermore, the TCII deficiency characterized in this study is due to the absence of TCII protein which in turn is due to the absence or extremely low levels of its mRNA and not to detectable gross alterations in the gene structure.