Chromosome alterations and Cancer

Keywords: Chromosome stability, Radiation-induced tumours, Sarcoma, Glioma, Amplification, TP53, RB1, EGFR

Read the scientific activity report. (pdf 120Ko, last update 19th, january 2010)

Our team develops two main programs.

The first program looks for specific alterations that could discriminate between radiation-induced tumors (RIT) and sporadic tumors. To find such a molecular signature, we analyze the tumor cells of secondary cancer developed by some patients after radiotherapy. This occurs at a low rate - 0,03 to 0,1% of the treated cases. But radiotherapy is indicated for about 50 % of the cancer cases, and the rate of RITs is higher for genetically predisposed persons, notably children carrying a germ line mutation of the retinoblastoma gene (RB1).

The second program intends to elucidate the mechanism by which an oncogène is amplified in the tumoral cells of gliomas. Gliomas are the most frequent type of tumor of the central nervous system, and about half of the gliomas with amplification contains up to hundred copies of the epidermal growth factor receptor (EGFR) gene, nevertheless other genes are also recurrently amplified (MYC, MET...). These amplified DNA fragments (amplicons) can appear as intrachromosomic structures, named HSR for « homogeneously staining region » (Figure 1) or form a small, circular, autonomously replicating, extrachromosome called dmin, for « double minute » (Figure 2).

Fig. 1

Fig. 2

Looking for a molecular signature of the ionising radiation, in the radiation-induced tumors (RIS)

To be sure of the relationship of cause and effect, we analyzed radiation-induced sarcomas that develop in the field of irradiation in patients treated by radiotherapy for a primary cancer. Using molecular approachs, we observed in these RIS a high rate of small deletions in TP53 gene, as compared with mutations in sporadic tumors. In addition, two codons were found recurrently and specifically mutated.
Patients carrying a germ-line mutation in the retinoblastoma gene (RB1) are predisposed not only to retinoblastoma (a tumour of the retina) but also to radiation-induced sarcomas, suggesting that RB1 inactivation may be a pre-requisite for the development of RIS. RB1 is not a ‘target' of ionising radiation, since we observed no somatic mutation in these tumors. RB1 was, nevertheless, inactivated by a mechanism involving genetic instability. In retinoblastoma, TP53 is rarely inactivated by mutation ; the suppression of its function is due to activation of regulatory pathways after RB1 loss. We have shown that the same tumorigenesis pathways are used in retinal and mesenchymal cells, but in RIS, inactivation of TP53 is a consequence of irradiation, and not of the RB1 loss.
In the future, we intend to investigate the ‘mutation signature' of ionising radiation in various tumour types, and search for a ‘gene expression signature' of ionising radiation, by making use of global transcription and genome analyses.

Elucidating the extrachromosomal amplification mechanisms in gliomas

We have investigated the molecular structure of dmins containing only the EGFR gene in a series of gliomas using real-time quantitative PCR and fluorescence in situ hybridization. We conclude that the founding extrachromosomal DNA molecule was generated by a simple event that circularizes a chromosome fragment overlapping EGFR. The fusion of the two ends of this initial amplicon resulted from micro-homology-based non-homologous-end-joining. The corresponding chromosomal loci were not rearranged, which strongly suggests that a post-replicative event was responsible for the formation of each of these initial amplicons (Figure 3).
We now focus on the mechanisms of formation of extrachromosomal DNA molecules with complex amplicons formed by the fusion of genes coming from several loci in the genome. In addition, we study relationships between EGFR amplification and its overexpression, as well as the pathways affected by its amplification during the gliomas development.

Last update: July 2008

Key publications

2006

• Gonin-Laurent N, Gibaud A, Huygue M, Lefèvre SH, Le Bras N, Chauveinc L, Sastre-Garau X, Doz F, Lumbroso L, Chevillard S, Malfoy B.
  Specific TP53 mutation pattern in radiation-induced sarcomas
  Carcinogenesis, 27, 1266-1272

2005

• Lefèvre SH, Coquelle A, Gonin-Laurent N, Cör A,Vogt N, Chauveinc L, Anract P , Dutrillaux B, Chevillard S, Malfoy B.
  Non-homologous end-joining genes are not inactivated in human radiation-induced sarcomas with genomic instability
  J. Radiation Research, 46, 223-231