Copy number alterations of the polycomb gene BMI1 in gliomas

Gliomas are heterogeneous tumours that grow in an uninhibited fashion, and these brain tumour cells share numerous characteristics with neural stem cells. The BMI1 gene encodes a component of the polycomb protein complex regulating epigenetically gene activity via histone modification. It functions for instance during the development of the central nervous system and maturation of neural cells. BMI-1 protein expression is deregulated in several forms of cancer and gene amplification has been identified in mantle cell lymphomas. Since BMI1 is located at chromosome 10p, a region implicated frequently in brain tumourigenesis, we investigated the genetic status and the corresponding expression patterns of BMI1 in a series of 100 low- and high-grade primary and recurrent gliomas. Chromogenic in situ hybridisation (CISH) with probes directed against BMI1 at 10p13 and the centromere of chromosome 10 was used in the analyses. Of all gliomas, 59% demonstrated aberrant copy numbers of BMI1. Deletions of the BMI1 locus were found in most types of tumours, and in a univariate survival analysis these cases had poor prognosis. Increased copy numbers of the BMI1 locus (3–5 copies) were found in all histological types, especially in high-grade astrocytomas. No difference in prognosis between cases with normal copy numbers and cases with increased copy numbers could be observed. This data suggests that BMI1 gene is aberrant at the chromosomal level in a subset of gliomas, and possibly contributes to brain tumour pathogenesis.