A hit-and-run on sporadic cancer

Mouse models for tumorigenesis have traditionally mimicked hereditary rather than sporadic cancers, as it is relatively straightforward to breed mice with a germline mutation that predisposes them to cancer. However, the development of a modified 'hit-and-run' gene-targeting technique, described by Jacks and colleagues in Nature, is set to change this.

The technique targets the K-ras oncogene — which is mutated in 30% of human tumours — and the advance comes not in the 'hit' step, in which the mutated allele is inserted into the genome of embryonic stem cells, but in the 'run' step. The authors capitalized on the fact that mitotic recombination occurs 10⁻³–10⁻⁷ times per cell division. By building a construct that could recombine to produce an oncogenic ras gene, they can trigger tumorigenesis spontaneously in vivo — as would occur in sporadic cancer.

The authors generated two constructs, K-ras^LA1 and K-ras^LA2. Each carried two copies of exon 1, in which codon 12 of one or both of the copies, respectively, was mutated from glycine to aspartic acid, leading to a constitutively active K-ras protein. Recombination in cis resulted in oncogenic K-ras in 50% of recombination events in K-ras^LA1 mice, but in 100% of events in K-ras^LA2 mice. K-ras^LA mice had reduced lifespans — K-ras^LA2 mice the shortest — compared with wild-type mice, and developed many tumours. Both strains had a similar tumour spectrum, and all mutants developed tumours that histopathologically resembled human non-small-cell lung cancer. Approximately 30% of the mice also developed thymic lymphoma and skin papilloma. Biochemical analyses showed that tumour cells carried the recombined, oncogenic ras allele.

Surprisingly, the mice did not develop carcinomas of the colon or pancreas, cancers frequently associated with the K-ras oncogene in humans. As carcinomas with K-ras mutations also often have defects in the p53 pathway, the authors crossed K-ras^LA1 mice with Trp53 -deleted mice. Trp53^+/− and Trp53^−/− mice, when carrying the K-ras^LA1 allele, showed a further reduction in survival compared with parental strains. Although these mice had a broader range of tumours — 30% developed haemangiosarcomas and fibrosarcomas — colonic and pancreatic tumours still did not occur. Goals for the future will therefore be to understand why this oncogene so consistently results in lung cancer in mice and to refine the technique to create models of the tumour types that occur in humans with K-ras mutations.

With new initiatives to develop better mouse models of cancer, this is an exciting time for the cancer research community. This study by Johnson et al. is an early fruit of such an initiative, and no doubt more will follow in the future.