Monoclonal antibodies to transthyretin

Transthyretin expression in medulloblastomas and medulloblastoma cell lines

Transthyretin is a protein crucial to the transport of lipophilic molecules such as thyroid hormones and retinoids. In the central nervous system, large amounts of transthyretin are synthesized by the choroid plexus and are secreted into the cerebrospinal fluid. The choroid plexus is the only site of transthyretin synthesis in the brain. Transthyretin is expressed by most benign and malignant choroid plexus tumours while gliomas and meningiomas do not express transthyretin. Other major sites of transthyretin synthesis are the retinal pigment epithelium and hepatocytes. Medulloblastoma is the prototypical primitive neuroectodermal tumour of the cerebellum and can show multiple lines of differentiation, including the expression of retinal markers. In this study, we examined transthyretin expression both at the RNA and protein level in four medulloblastomas and six medulloblastoma cell lines using Northern and Western blot analysis, reverse transcription polymerase chain reaction (PCR), RNA in situ hybridization, and immunohistochemistry. All four medulloblastomas and five of the six medulloblastoma cell lines expressed transthyretin-mRNA as demonstrated by reverse PCR and in situ hybridization while three medulloblastomas and one cell line were positive on Northern blot. The medulloblastoma with the most abundant RNA expression was transthyretin-immunoreactive on cryosections and the medulloblastoma cell line that was positive on Northern blot also expressed transthyretin at levels detectable by Western blot. No transthyretin-immunoreactivity was seen in 16 additional medulloblastomas studied on paraffin sections. These findings indicate that low-level expression of transthyretin-mRNA is common in medulloblastomas and medulloblastoma cell lines. Expression of transthyretin protein occurs rarely but can reach significant levels. Transthyretin expression in medulloblastoma is consistent with retinal pigment epithelium differentiation in medulloblastomas and reflects their pluripotential nature. Furthermore, the potential for transthyretin-immunoreactivity in medulloblastoma should be kept in mind when performing immunohistochemical studies on poorly differentiated cerebellar tumours.

Transthyretin and cystatin C are catabolized in proximal tubular epithelial cells and the proteins are not useful as markers for renal cell carcinomas

Ten human kidney specimens and thirty-two renal cell carcinomas were investigated for the presence of transthyretin mRNA and cystatin C mRNA using Northern blot analysis. Five of ten kidney specimens and 15 of 32 renal carcinomas were also immunohistochemically investigated for the presence of the corresponding proteins. Transthyretin mRNA could not be detected in any of the normal or neoplastic tissue preparations, whereas low amounts of cystatin C mRNA were found in nine of ten normal kidneys and in 24 of 32 renal cell carcinomas. Immunoreactive transthyretin and cystatin C were present in proximal tubular epithelial cells of all kidney specimens, whereas neither of the proteins was detected the tumour cells of the renal carcinomas. Immunoreactive cystatin C was, however, demonstrated in scattered monocyte/macrophage-like cells. We conclude that the presence of immunoreactive transthyretin and cystatin C in proximal tubular cells of the kidney is most likely due to reabsorption of the proteins from the primary urine. The small amounts of cystatin C mRNA in some of the normal and neoplastic renal preparations are probably due to cystatin C synthesis in macrophages. Transthyretin has been recommended as an immunohistochemical marker for renal cell carcinomas. Our results, however, clearly indicate that neither transthyretin nor cystatin C constitutes a useful marker for such neoplasms.

Cystatin C in the human pancreas and gut: an immunohistochemical study of normal and neoplastic tissues

The occurrence of immunoreactive cystatin C (CC) in normal and neoplastic cells of the human pancreas and gut was investigated using an indirect streptavidin-biotin method on formaldehyde-fixed and paraffin-embedded tissues. Virtually all pancreatic islet cells and many neuroendocrine cells throughout the gastrointestinal tract showed strong CC immunoreactivity and a granular cytoplasmic staining pattern. All 14 endocrine pancreatic tumours (insulinomas, glucagonomas, gastrinomas and non-producing tumours), as well as 16 of 17 gut carcinoid tumours, were also strongly CC immunoreactive. In addition, non-endocrine epithelial cells of pancreatic ducts and the gastrointestinal mucosa and 20 of the 24 adenocarcinomas from these sites showed weak CC immunoreactivity. Thus, CC cannot be used as a reliable immunohistochemical marker for endocrine gastro-entero-pancreatic tumours despite the fact that the protein is strongly expressed in a majority of such tumours.

Monoclonal antibody studies of the antigenic determinants of human plasma retinol-binding protein

A battery of monoclonal antibodies (MoAbs) against human retinol-binding protein (RBP) was produced to obtain useful probes for the study of the antigenic determinants of RBP. The 12 antibodies all reacted with human RBP by immunoblotting. Based on antibody cross-competition radioimmunoassays, four distinct and different groups of antibodies were identified: group I, 1A4 and 2F4; group II, 1G10, 5C5, 6F4, and 7G3; group III, 5H6, 6C7, 10G5, and 14E3; and group IV, 5H9 and 13A1. Information about the epitopes of RBP recognized by these MoAbs was obtained by testing the reactivity of each antibody with human, rabbit, and rat RBPs by immunoblotting. Group I and group IV antibodies reacted to a similar extent with human, rabbit, and rat RBPs. Group II antibodies reacted strongly with human and rabbit RBPs, but reacted very weakly with rat RBP. Group III antibodies reacted strongly with human RBP, but did not react with rabbit or rat RBP. Thus, the epitopes for group I and group IV antibodies appear to be regions of the RBP molecule that are conserved across the three species, whereas group III antibodies recognized only human RBP. In a preliminary study, the reactivity of each antibody with purified cyanogen bromide fragments of RBP was tested by slot immunoblotting. None of the MoAbs reacted with any of the cyanogen bromide fragments. This study shows that MoAbs specific for at least four different regions of the RBP molecule can be produced; hence, RBP contains at least four major antigenic domains.

Localization of transthyretin-mRNA and of immunoreactive transthyretin in the human fetus

Five human fetuses at mid-term (16-20 weeks) and one with a gestational age of 8 weeks were investigated. The cellular localization of transthyretin (TTR)-mRNA in different organs was demonstrated by in situ hybridization with a 35S-labelled, single-stranded RNA probe. Immunoreactive TTR (TTR-IR) was localized with a monoclonal antibody to TTR. In all fetuses, choroid plexus epithelial cells demonstrated intense labelling for TTR-mRNA as well as strong TTR-IR. Hepatocytes, on the other hand, showed weak in situ labelling and weak or, in some cases, non-demonstrable TTR-IR. In all mid-term fetuses, but not in the 8 week fetus, TTR-mRNA and TTR were also expressed in pancreatic endocrine A-cells. The degree of in situ labelling in these A-cells was moderate, whereas that of TTR-IR was strong. Despite negative findings for TTR-mRNA in the gut and the kidney, endocrine cells of the gut and epithelial cells of the renal proximal convoluted tubules showed TTR-IR in some of the fetuses. The investigation provides evidence for TTR synthesis in the human fetal choroid plexus, liver and endocrine pancreas. However, further studies are required to demonstrate TTR synthesis in the gut and the kidney.

In situ localization of transthyretin-mRNA in the adult human liver, choroid plexus and pancreatic islets and in endocrine tumours of the pancreas and gut

In situ hybridization with 35S-labeled single stranded RNA probes was used on sections from formaldehyde-fixed and paraffin-embedded tissue specimens to provide semiquantitative data on the occurrence of transthyretin(TTR)-mRNA in human liver, choroid plexus and pancreatic islets as well as in 15 endocrine tumours of the pancreas and gut. A monoclonal antibody to TTR was used for immunocytochemical identification of the protein in consecutive sections. The amount of TTR-mRNA in hepatocytes was found to be much less than that in epithelial cells of the choroid plexus. Glucagon cells of the pancreatic islets were also specifically labeled and the level of TTR-mRNA in these cells was intermediate between that of hepatocytes and choroid plexus epithelial cells. Four glucagonomas, one malignant insulinoma and two midgut carcinoids were shown to contain TTR-mRNA. The 'in situ' labeled cells were also found to be TTR immunoreactive. These findings present the first conclusive evidence for TTR synthesis in pancreatic islets and in endocrine tumours. They also establish that the high serum concentration of TTR found in some patients with endocrine tumours (notably glucagonomas) is most likely due to tumour production of TTR.