Induction of medulloblastomas..

..in P53-null mutant mice by somatic inactivation of Rb in the external granular layer cells of the cerellum

 Silvia Marino et al. Published in GENES and DEVELOPMENT 14:994-1004, 2000.

This study sets out to analyse the role of loss of function of the tumour suppressor genes P53 and Rb in glial tumours of the CNS. This arises from the observation that humans with germ line mutations in the P53 gene have a high incidence of astrocytomas. However, the tumour prone P53 knock-out mice do not develop these glial tumours. Retinoblastoma (Rb) knock-out mice, on the other hand, die early in gestation and heterozygotes die young from pituitary tumours. In order to overcome these lethal phenotypes so that later effects on glial cells could be studied, the authors generated conditional knock-outs.

The approach described used the promoter of GFAP, a protein expressed primarily in astroglia, to drive the cre recombinase, and LoxP targeted P53 and Rb genes. Thus, either one or both genes could be selectively inactivated in glial cells during embryonic development. Surprisingly, GFAP driven cre expression was seen not only in astrocytes but also granule neuron precursors including those of the External Granule Layer (EGL) of the cerebellum, a germinal layer on the outer surface of the cerebellum which appears to lack endogenous GFAP protein. It is not clear whether this expression of cre in the EGL reflects a very low level of endogenous GFAP expression in these cells or this is ‘mis-expression’ due to the specific promoter fragment being used. Whatever the explanation, the cre activity resulted in loss of the Lox targeted genes in both glia and granule cell precursors.

Loss of function of either gene alone did not give rise to CNS tumours. The double mutants gave rise, not to tumours of astrocytes, but to tumours which originated in the EGL of the cerebellum. The cellular and molecular features of these tumours resemble human medulloblastomas, one of the most common and aggressive solid tumours of children. In particular tumour cells expressed markers of both neuronal and glial differentiation. The cellular origin of medulloblastomas has been a long standing question. Since EGL cells appear normally to be specified to produce only granule neurons, the observation of both neuronal and glial differentiation has led to the suggestion that they might be derived from multipotent cells of the subependymal layer. Alternatively, they might arise from EGL cells which are either not fully specified or are despecified through transformation. This study demonstrates that such tumours can clearly be generated from the EGL.

While this study does shed light upon the potential origin of medulloblastomas, the authors point out that neither P53 nor Rb genes are generally disrupted in sporadic medulloblastomas found in humans. They speculate that the loss of cell cycle control in sporadic cases may involve disruption of the pathways in which these proteins function at as yet unidentified points. As for a role in glial tumours, the authors suggest that there may have been insufficient time for these tumours to develop due to the premature sacrifice of the mice (due to medulloblastoma formation) or that mouse glial cells are remarkably resistant to neoplastic transformation.

Yi-Chuan Cheng and Paul J. Scotting