Mutations in BHD and TP53 genes, but not in HNF1beta gene, in a large series of sporadic chromophobe renal cell carcinoma

BHD, TP53, and HNF1β on chromosome 17 were studied in 92 cases of renal cell carcinoma (46 chromophobe, 19 clear cell, 18 oncocytoma, and nine papillary). Six, thirteen, and zero cases had, respectively BHD, TP53, and HNF1β mutations, (84% mutations involved chromophobe), suggesting a role for BHD and TP53 in chromophobe subtype.

Specific TP53 mutation pattern in radiation-induced sarcomas

The mutagenic properties of ionizing radiation are well known, but the presence of specific mutations in human radiation-induced tumours is not established. We have studied a series of 36 secondary sarcomas arising in the irradiation field of a primary tumour following radiotherapy. The allelic status and the presence of mutations of the TP53 gene were investigated. The mutation pattern was compared with data from sporadic sarcomas recorded in the IARC TP53 somatic mutations database. A high proportion (58%) of the radiation-induced sarcomas exhibited a somatic inactivating mutation for one allele of TP53, systematically associated with a loss of the other allele. The high frequency (52%) of short deletions observed in the mutation pattern of radiation-induced sarcomas may be related to the induction of DNA breaks by ionizing radiation. The lack of hyper-reactivity of CpG dinucleotides and the presence of recurrent sites of mutation at codons 135 and 237 seem also to be specific for radiation tumorigenesis.

Non-homologous end-joining genes are not inactivated in human radiation-induced sarcomas with genomic instability

DNA double-strand break (DSB) repair pathways are implicated in the maintenance of genomic stability. However the alterations of these pathways, as may occur in human tumor cells with strong genomic instability, remain poorly characterized. We analyzed the loss of heterozygosity (LOH) and the presence of mutations for a series of genes implicated in DSB repair by non-homologous end-joining in five radiation-induced sarcomas devoid of both active Tp53 and Rb1. LOH was recurrently observed for 8 of the 9 studied genes (KU70, KU80, XRCC4, LIG4, Artemis, MRE11, RAD50, NBS1) but not for DNA-PKcs. No mutation was found in the remaining allele of the genes with LOH and the mRNA expression did not correlate with the allelic status. Our findings suggest that non-homologous end-joining repair pathway alteration is unlikely to be involved in the high genomic instability observed in these tumors.

Genome instability in secondary solid tumors developing after radiotherapy of bilateral retinoblastoma

Genome alterations of seven secondary tumors (five osteosarcomas, one malignant peripheral sheath nerve tumor, one leiomyosarcoma) occurring in the field of irradiation of patients treated for bilateral retinoblastoma have been studied. These patients were predisposed to develop radiation-induced tumors because of the presence of a germ line mutation in the retinoblastoma gene (RB1). Tumor cells were characterized by a high chromosome instability whereas microsatellites and minisatellites were found to be stable. In all tumors, the normal RB1 allele was lost with the corresponding chromosome 13, whereas the germ line mutated allele was retained. The two alleles of TP53 were inactivated, one by deletion of the short arm of chromosome 17, the other by mutation. As compared with non-radiation-induced tumors, the observed panel of TP53 mutations was uncommon with sites not recurrently found otherwise and a high rate of deletions (3/7). In these predisposed patients, the loss of the single normal allele of RB1 is rather due to the radiation-induced chromosome instability than a direct effect of ionizing radiation.