Photoreceptor differentiation in cerebellar medulloblastoma: evidence for a functional photopigment and authentic S-antigen (arrestin)

Integrated genomics identifies five medulloblastoma subtypes with distinct genetic profiles, pathway signatures and clinicopathological features

BACKGROUND

Medulloblastoma is the most common malignant brain tumor in children. Despite recent improvements in cure rates, prediction of disease outcome remains a major challenge and survivors suffer from serious therapy-related side-effects. Recent data showed that patients with WNT-activated tumors have a favorable prognosis, suggesting that these patients could be treated less intensively, thereby reducing the side-effects. This illustrates the potential benefits of a robust classification of medulloblastoma patients and a detailed knowledge of associated biological mechanisms.

METHODS AND FINDINGS

To get a better insight into the molecular biology of medulloblastoma we established mRNA expression profiles of 62 medulloblastomas and analyzed 52 of them also by comparative genomic hybridization (CGH) arrays. Five molecular subtypes were identified, characterized by WNT signaling (A; 9 cases), SHH signaling (B; 15 cases), expression of neuronal differentiation genes (C and D; 16 and 11 cases, respectively) or photoreceptor genes (D and E; both 11 cases). Mutations in beta-catenin were identified in all 9 type A tumors, but not in any other tumor. PTCH1 mutations were exclusively identified in type B tumors. CGH analysis identified several fully or partly subtype-specific chromosomal aberrations. Monosomy of chromosome 6 occurred only in type A tumors, loss of 9q mostly occurred in type B tumors, whereas chromosome 17 aberrations, most common in medulloblastoma, were strongly associated with type C or D tumors. Loss of the inactivated X-chromosome was highly specific for female cases of type C, D and E tumors. Gene expression levels faithfully reflected the chromosomal copy number changes. Clinicopathological features significantly different between the 5 subtypes included metastatic disease and age at diagnosis and histology. Metastatic disease at diagnosis was significantly associated with subtypes C and D and most strongly with subtype E. Patients below 3 yrs of age had type B, D, or E tumors. Type B included most desmoplastic cases. We validated and confirmed the molecular subtypes and their associated clinicopathological features with expression data from a second independent series of 46 medulloblastomas.

CONCLUSIONS

The new medulloblastoma classification presented in this study will greatly enhance the understanding of this heterogeneous disease. It will enable a better selection and evaluation of patients in clinical trials, and it will support the development of new molecular targeted therapies. Ultimately, our results may lead to more individualized therapies with improved cure rates and a better quality of life.

Epithelial differentiation in medulloblastoma: Comparison with other embryonal tumors of neuroectodermal origin

Three cases of medulloblastoma characterized by epithelial differentiation are described in patients 6-months-, 1-month- and 8-years-old. Histologically, tumors from the two infant patients showed a perivascular arrangement without apparent radiated cytoplasmic processes from the vessels. Tumor cells displayed round and/or pleomorphic vesicular nuclei and a more abundant eosinophilic cytoplasm than that found in classic medulloblastoma. Neither Homer-Wright rosettes nor ependymal or ependymoblastic rosettes were noted in these tumors. The tumor in the 8-year-old patient exhibited a classic medulloblastoma component intermingled with abundant eosinophilic cytoplasm forming a tubular structure. Immunohistochemically, tumor cells in all cases were positive for cytokeratin, synaptophysin, and vimentin. In the third case involving the 8-year-old patient, epithelial tumor cells were positive for cytokeratin, whereas classic medulloblastoma components were negative for cytokeratin. Positive staining for melanoma-specific antigen was seen only in the third case, where strong reactivity of tumor cells formed a tubulus. However, the classic medulloblastoma component was negative for melanoma-specific antigen. Ultrastructurally, basal laminae were observed around tumor cells in the 6-month-old patient. These morphological and immunohistochemical features suggest that medulloblastoma with epithelial differentiation is a rare but distinct variant of medulloblastoma, and that some of these tumors should show differentiation in ocular pigment epithelium.

Pineal rhythms are synchronized to light-dark cycles in congenitally anophthalmic mutant rats

Genetically mutant anophthalmic rats lacking a complete visual system due to the absence of eyeballs and optic nerves up to the optic chiasma were used as a model to study photo-regulated physiological activities. The photoreception in these mutant rats was determined by measuring the neuroendocrine response of the pineal gland-melatonin levels in the serum, and synaptic ribbon complexes (SRCs) in the pinealocytes. These parameters were studied in both normal and anophthalmic rats maintained under light-dark (LD 12:12), continuous dark (DD) and light (LL) conditions. Both normal and mutant anophthalmic animals showed nocturnal increases in serum melatonin levels and in the number and diameter of SRC and their vesicles in the pinealocytes in LD. The daily rhythms persisted even upon transfer to DD both in normal and mutant rats, whereas in LL, the nocturnal elevation of both the parameters disappeared. These observations suggested that congenitally blind rats can perceive light. The studies of these parameters in both normal and mutant rats in reversed-LD conditions confirmed that pineal rhythms can be entrained by light-dark cycles in congenitally anophthalmic mutant rats through a nonvisual system for light perception.

Spectroscopic characterization of arrestin interactions with competitive ligands: study of heparin and phytic acid binding

A combination of intrinsic fluorescence and circular dichroic (CD) spectroscopy has been used to characterize the complexes formed between bovine retinal arrestin and heparin or phytic acid, two ligands that are known to mimic the structural changes in arrestin attending receptor binding. No changes in the CD spectra were observed upon ligand binding, nor did the degree of tryptophan fluorescence quenching change significantly in the complexes. These data argue against any large-scale changes in protein secondary or tertiary structure accompanying ligand binding. The change in tyrosine fluorescence intensity was used to determine the dissociation constants for the heparin and phytic acid complexes of arrestin. The only change observed was a saturable diminution of tyrosine fluorescence signal from the protein. For both ligands, the data suggest two distinct binding interactions with the protein--a high--affinity interaction with Kd between 200 and 300 nM, and a lower affinity interaction with Kd between 2 and 8 microM. Study of collisional quenching of tyrosine fluorescence in free arrestin and the ligand-replete complexes indicates that 10 of the 14 tyrosine residues of the protein are solvent-exposed in the free protein; this value drops to between 5 and 6 solvent-exposed residues in the high-affinity complexes of the two ligands. These data suggest that ligand binding leads to direct occlusion of between 4 and 5 tyrosine residues on the solvent-exposed surface of the protein, but not to any large-scale changes in protein structure. The large activation energy previously reported to be associated with arrestin-receptor interactions may therefore reflect localized movements of the N- and C-termini of arrestin, which are proposed to interact in the free protein through electrostatic interactions. Binding of the anionic ligands heparin, phytic acid, or phosphorylated rhodopsin may compete with the C-terminus of arrestin for these electrostatic interactions, thus allowing the C-terminus to swing out of the binding region.

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.

Pineal parenchymal tumors: cell differentiation and prognosis

Eleven pineal parenchymal tumors were studied using various antibodies specific to the central nervous system and cell-proliferation-related antigen MIB-1 in order to examine the divergent types of cell differentiation and also evaluate prognosis. Electron microscopy was also performed. All tumors were immunohistochemically positive to chromogranin A and alpha B crystallin, and were also highly positive to retinal S protein. Pineocytoma cells contained microtubules, intermediate filaments, glial bundles, clear-centered vesicles and synaptic apparati. Pineoblastoma cells also had microtubules and neurofilaments, but glial filaments and definite synapses were not identifiable. Pineal parenchymal tumors were considered to be of pinealocyte origin, and there was a continuous spectrum of divergent cell differentiation between pineocytoma and pineoblastoma cells. The MIB-1 labeling index correlated well with histological malignancy, neuronal differentiation evaluated immunohistochemically by both neurofilament protein and synaptophysin, and cases with seeding potentials. Although histopathological features of neuronal development were, until recently, seen as the hallmark of benign prognosis in pineal parenchymal tumors, they are now thought to be only one of the pieces of evidence that may be used for purposes of prognosis.

Rod-opsin immunoreaction in the pineal organ of the pigmented mouse does not indicate the presence of a functional photopigment

The aim of the present study was to characterize the rod-opsin immunoreaction in the mammalian pineal organ. Pigmented mice (strain C57BL) were selected as the animal model. Immunocytochemical investigations involving the use of highly specific polyclonal and monoclonal antibodies against bovine rod-opsin (the apoprotein of the photopigment rhodopsin) showed that approximately 25% of all pinealocytes were rod-opsin immunoreactive. Immunoblotting techniques revealed three protein bands of approximately 40, 75, and 110 kDa; these were detected by the monoclonal antibody and the polyclonal antiserum in retinal and pineal extracts. These protein bands presumably represented the monomeric, dimeric and trimeric forms of rod-opsin. The amount of rod-opsin in retina and pineal organ was quantified by means of an enzyme-linked immunosorbent assay. This yielded 570 +/- 30 pmoles rod-opsin per eye and 0.3 +/- 0.05 pmoles rod-opsin per pineal organ. High pressure liquid chromatography analysis of whole eye extracts demonstrated the chromophoric group of the photopigment rhodopsin, 11-cis retinal, and its isomer, all-trans-retinal. A shift from 11-cis retinal to all-trans-retinal was found upon light adaptation. No retinals were detected in the pineal organ. Autoradiographic investigations showed that 3H-retinol, intraperitoneally injected into the animals, was incorporated into the outer and inner segments of retinal photoreceptors, but not into the pineal organ. It is concluded that the mouse pineal organ contains the authentic apoprotein of rhodopsin but that it lacks retinal derivatives as essential components of all known vertebrate photopigments.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical demonstration of interphotoreceptor retinoid-binding protein in cerebellar medulloblastoma

Previously, immunoreactive rod-opsin and S-antigen (arrestin), two highly characteristic markers of retinal photoreceptors and pinealocytes, were shown to be present in certain medulloblastoma cells. It, thus, has been suggested that such cells differentiate along the photoreceptor lineage. This is corroborated in the present immunocytochemical investigation using antibodies against another photoreceptor-cell marker, the interphotoreceptor retinoid-binding protein (IRBP). As shown in preparations of human retina and pineal organ, IRBP can be successfully demonstrated in formalin-fixed and paraffin-embedded tissue: the IRBP immunoreaction is located to the outer and inner segments of retinal photoreceptor cells and to perikarya of certain pinealocytes. Examination of formalin-fixed, paraffin-embedded biopsy specimens of 66 cerebellar medullo-blastomas revealed varying numbers of IRBP-immuno-reactive tumor cells in 19 cases that were formerly shown to contain rod-opsin and S-antigen immunoreaction. IRBP-immunoreactive tumor cells were also found in a retinoblastoma and a pineocytoma, but not in neuroblastoma, ganglioneuroblastoma, glioblastoma, oligodendroglioma and astrocytoma. The results indicate: (1) cerebellar medulloblastomas are heterogeneous in their differentiation potential; (2) one type of medulloblastoma displays photoreceptor characteristics; (3) this type appears to be closely related to retinoblastoma and pineal cell tumors; and (4) all three types of tumors may display additional common features to be explored in future studies.

A pineal tumour with features of "pineal anlage tumour"

A primitive tumour of the pineal region in a 9 month-old boy caused hydrocephalus, ataxic gait and paraplegia leading to death after 9 months of evolution. Histological and histochemical examination revealed a pinealoblastoma with melanotic cells as well as cartilaginous formations. The leptomeningeal seeding of the tumour showed neuroblastic and neuronal differentiation. A diagnosis of pineoblastoma with neuroepithelial and ectomesenchymal differentiation was made. This tumour belongs to a recently defined category of pineal tumours, called pineal anlage tumours.