Different strokes

Lung cancer can come in one of two forms — small cell and non small cell — that are initiated by different sets of genetic lesions and that have very different phenotypes. Mouse models have previously proven useful in identifying the changes that are required to induce non-small-cell lung cancer, and Anton Berns and colleagues have used the same approach to investigate the changes that cause small-cell lung cancer (SCLC). Interestingly, the changes are very different: instead of an activating mutation in Kras , both the Trp53 and Rb tumour-suppressor genes must be inactivated.

The authors investigated the role of Rb and p53 because they are frequently mutated in human SCLC. They generated mice with conditional alleles of the two genes, which could be inactivated in lung epithelial cells by administering Ad-Cre by either intratracheal injection or intubation. The lungs of these mice were examined after 2–5 months and were found to contain areas of neoplasia in which both the Rb and Trp53 alleles had been lost. These cells expressed two neuroendocrine (NE) cell markers — synaptophysin and the neural cell adhesion molecule 1 (Ncam1) — which indicates that they resemble SCLC, as SCLC is derived from cells that share the NE phenotype.

Mice that were not examined at this early stage were sacrificed when they became moribund. Three strains were investigated — Trp53^f/f;Rb^f/f (in which all alleles were conditional), Trp53^+/f;Rb^f/f and Trp53^f/f;Rb^+/f (in which one of the tumour suppressors retains a wild-type allele) — which differed in several respects. The Trp53^f/f;Rb^f/f mice had the shortest median tumour-free survival time of 210 days, with the Trp53^+/f;Rb^f/f and Trp53^f/f;Rb^+/f mice remaining tumour free until 364 and 575 days, respectively. Tumours from the Trp53^f/f;Rb^f/f mice frequently metastasized to bone, brain, adrenal glands, ovaries and liver, whereas tumours from Trp53^f/f;Rb^+/f mice rarely did and from Trp53^+/f;Rb^f/f mice never did.

The primary tumour types that developed were also somewhat different. The Trp53^f/f;Rb^f/f mice developed mostly SCLC — histological analysis confirmed their resemblance to human SCLC — but the other strains also developed some pulmonary adenocarcinomas. The longer survival time of the Trp53^+/f;Rb^f/f and Trp53^f/f;Rb^+/f mice is probably related to the delay before the remaining wild-type allele is lost, as Southern blotting of the loci confirmed that loss of heterozygosity had occurred in all SCLC cases. Interestingly, this was not the case for pulmonary adenocarcinomas; PCR analysis of microdissected tissue from Trp53^f/f;Rb^+/f mice showed that loss of Rb was not required for development of the adenocarcinomas.

As well as NE markers, the achaete scute complex homologue-like 1 (ASCL1) transcription factor is frequently expressed in human SCLC, and it is thought to determine the onset of NE differentiation in lung epithelia and lung cancers with NE features. Indeed, the mouse SCLCs and their liver metastases generally stained positive for Ascl1 and the NE markers synaptophysin and Cgrp.

So, this mouse model closely resembles human SCLC, and shows that inactivation of both p53 and Rb are required for the initiation of SCLC. The applications of the model are yet to be determined, but should include identification of contributing genetic lesions and the testing of treatment strategies.