by ecancer reporter Clare Sansom

Since the first human-sized genomes were sequenced, whole-genome and, more commonly, whole-exome sequencing of tumours has revealed an enormous amount about the mutations and processes involved in carcinogenesis.

In particular, it has become possible to associate mutational patterns in tumours with the environmental carcinogens to which patients were exposed.

However, there have been no studies as yet of the mutational landscape of carcinogen-induced mutations in mouse models of cancer.

A group of researchers led by Allan Balmain of the University of California San Francisco, California, USA has now investigated the mutational patterns arising in mouse models of non small cell lung cancer (NSCLC) arising from activation of the oncogene Kras or exposure to a carcinogen, urethrane or methyl-nitrosourea (MNU).

Both these carcinogens initiate cancer development through spontaneous mutation of Kras, which is also commonly mutated in human NSCLC.

Carcinogen-induced tumours in mice can be thought of as models for human tumours induced by smoking.

Balmain and his colleagues sequenced the complete exomes of 82 lung adenomas from the same mouse strain; 44 of these had been induced by urethrane, 26 by MNU, and 12 by Kras activation.

Mice with one functionally null Kras allele – Kras /- mice – were also included in the study; these develop more and larger tumours when treated with carcinogens than their wild-type littermates.

The carcinogen-induced tumours were found to contain more single nucleotide variations (SNVs) than those induced by Kras activation, which mimics the finding in humans that lung tumours from smokers carry more of these mutations than those from non-smokers.

The pattern of base substitutions differed significantly between the carcinogens, with urethrane inducing a range of A > T, G > A and A > G substitutions and MNU mainly G > A transitions.

The most frequent SNV in the Kras-activated tumours was CGN>A, which is a known signature of genomic instability.

In contrast, the most frequent SNV in the MNU-induced tumours was GGT>A, and 25 of the 26 tumours in this group were found to carry this substitution in codon 12 of Kras where it causes the amino acid substitution G12D.

All 44 Kras-activated tumours had mutations in codon 61 of Kras, which codes for glutamine, and this codon was also frequently mutated in the urethrane-induced tumours.

Interestingly, however, urethrane-induced tumours from wild-type mice almost exclusively carried the mutation Q61L, whereas those from Kras /- mice carried Q61R mutations.

This, with the finding that the single tumour from a wild-type mouse that carried the Q16R mutation had an additional loss-of-function Kras mutation, suggested that the selection of cells with one or other of the Q61 mutations is modulated by Kras status.

The researchers searched for additional driver mutations in all the mouse tumours, and validated SNVs in a total of 49 genes.

Most of these occurred in amino acids that are conserved between mouse and human.

Four different mutations were found in a candidate tumour suppressor gene, Mtus1; expression of the human gene MTUS1 is known to be positively associated with survival in lung adenocarcinoma.

Tumours initiated by Kras activation had higher rates of aneuploidy and copy number alteration (CNA) than those initiated by carcinogens.

Most (9-12) of these tumours showed amplification of Kras expression, mainly through a gain of one copy of chromosome 6; other copy number changes frequently observed were gains on chromosomes 2, 8, 12, 15 and 17, and deletions on chromosomes 4, 9, 11 and 17.

Interestingly, no Kras-induced tumour carried any SNVs in established driver genes, although some had copy number alterations involving these genes.

Taken together, these results indicate that there are significant differences in the pattern of genetic changes involved in mutation-driven and carcinogen-driven lung adenoma, and in particular that SNVs induced by carcinogens may reduce the requirement for copy number changes.

The researchers additionally sequenced the exomes of adenocarcinomas from the same mice, and found that these showed the same mutational patterns as the adenomas but with additional SNVs in driver genes in the carcinogen-induced tumours.

Many of these genes are the mouse equivalents of driver genes known to be frequently mutated in human lung adenocarcinoma.

Therefore, these results also suggest the importance of carcinogen-driven mouse models of cancer in understanding the complex patterns of human cancer development.

Reference

Westcott, P.M.K., Halliwill, K.D., To1, M.D. and 11 others (2014). The mutational landscapes of genetic and chemical models of Kras-driven lung cancer. Nature, published online ahead of print 6 November 2014.