In humans, PTEN mutations are typically found in high-grade primary glioblastoma multiforme (GBM), whereas TP53 mutations are more common in secondary GBM; however, tumors in mice lacking both Pten and Trp53 in the CNS bear a striking resemblance to primary GBM in humans.
The p53 and PTEN pathways have established roles in gliomagenesis, which prompted Ron DePinho and colleagues to create mice with central nervous system (CNS)-specific deletion of the two genes. In humans, PTEN mutations are typically found in high-grade primary glioblastoma multiforme (GBM), whereas TP53 mutations are more common in secondary GBM, which develops following progression from low-grade disease. So, the authors were somewhat surprised to find that tumours in mice lacking both Pten and Trp53 in the CNS bore a striking resemblance to primary GBM in humans.
Because CNS deletion of Pten is lethal, the authors examined Pten-/+;Trp53-/- mice. Of 57 mice, 42 (73%) presented with high-grade malignant gliomas with classical clinical, pathological and molecular features of primary GBM disease; 66% of these were classified as high-grade anaplastic astrocytomas and 34% as GBM. Although loss of p53 has been thought to be infrequent in primary GBM, re-sequencing of 35 human primary GBM samples showed TP53 mutations in 10, 6 of which also had concomitant deletion or mutation of PTEN. This analysis agrees with recent large-scale data from The Cancer Genome Atlas.
What features of the GBM tumours in Pten-/+;Trp53-/- mice resemble those of human GBM? PTEN expression was lost in tumour cells but not in the surrounding normal cells, and Pten loss of heterozygosity was confirmed in 6 of 7 tumours, findings that correspond with previously observed PTEN loss of heterozygosity in human primary GBM. Human GBM also displays morphological and lineage heterogeneity within tumours, possibly reflecting the acquisition of an immature developmental state. Supporting this, all of the tumours from Pten-/+;Trp53-/- mice expressed neural stem cell (NSC) markers and generated tumour neurospheres. Cultures of primary NSCs from Pten-/-;Trp53-/- mice showed higher proliferation and self-renewal and significantly lower differentiation potential than did NSCs lacking only Pten or Trp53.
Why are Pten-/-;Trp53-/- NSCs unable to differentiate? Promoter analyses using transcriptomic profiling and The Cancer Genome Atlas database revealed enrichment for MYC binding elements in Pten-/-;Trp53-/- NSCs and human GBM tumours. Levels of MYC were indeed higher in the double-null NSCs, and short hairpin RNA-mediated knockdown of MYC to physiological levels in these cells and in cells derived from Pten-/+;Trp53-/- tumour neurospheres restored their ability to differentiate and inhibited self-renewal and proliferation. Of 10 immunocompromised mice injected with tumour neurospheres carrying Myc short hairpin RNA, 9 survived for more than 3 months, whereas 10 of 10 control mice died from gliomas within 1 month.
So, restoration of the differentiation capacity of Pten-/+;Trp53-/- tumour cells by reduction of MYC expression reduces their tumorigenic potential. Importantly, these cells differ from normal NSCs, which readily respond to differentiation cues, suggesting a therapeutic strategy of restoring differentiation, possibly through inhibition of MYC, in human GBM.
Sarah Seton-Rogers
References
Zheng, H. et al. p53 and PTEN control neural and glioma stem/progenitor cell renewal and differentiation. Nature455, 1129–1133 (2008) | Article | PubMed |
The Cancer Genome Atlas Research Network. Comprehensive genomic characterization defines human glioblastoma genes and core pathways. Nature455, 1061–1068 (2008). | Article | PubMed |
