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.