Cooperative effects of INK4a and ras in melanoma susceptibility in vivo

Abstract

The familial melanoma gene (INK4a/MTS1/CDKN2) encodes potent tumor suppressor activity. Although mice null for the ink4a homolog develop a cancer-prone condition, a pathogenetic link to melanoma susceptibility has yet to be established. Here we report that mice with melanocyte-specific expression of activated H-ras G12V on an ink4a-deficient background develop spontaneous cutaneous melanomas after a short latency and with high penetrance. Consistent loss of the wild-type ink4a allele was observed in tumors arising in ink4a heterozygous transgenic mice. No homozygous deletion of the neighboring ink4b gene was detected. Moreover, as in human melanomas, the p53 gene remained in a wild-type configuration with no observed mutation or allelic loss. These results show that loss of ink4a and activation of Ras can cooperate to accelerate the development of melanoma and provide the first in vivo experimental evidence for a causal relationship between ink4a deficiency and the pathogenesis of melanoma. In addition, this mouse model affords a system in which to identify and analyze pathways involved in tumor progression against the backdrop of genetic alterations encountered in human melanomas.

Modeling malignant melanoma in mice: pathogenesis and maintenance

“…Signi®cantly, these rare melanomas showed spontaneous deletion of both INK4a alleles. Correspondingly, INK4a D2/3 mice of mixed genetic background developed spontaneous cutaneous melanomas with high penetrance after a short latency (Chin et al, 1997), providing unequivocal evidence that ink4a de®ciency can cooperate with oncogenic RAS to accelerate the genesis of melanoma in vivo. These tumors resembled human nodular melanomas in that they were amelanotic and highly vascular, and they exhibited strong immunoreactivity to tyrosinase related protein I (TRP1), thus con®rming their melanocytic origin.…”

“…In these cases, there was still an intact allele of ink4b that was in cis with the ink4a mutant allele. Furthermore, examination of melanomas arisen in INK4a 7/7 animal revealed no INK4b deletion, suggesting that once INK4a is eliminated, there was no longer a genetic pressure to delete sequences at the 9p21 locus (Chin et al, 1997). Together, these results would suggest, but could not rigorously prove, that Ink4b loss in melanomas simply re¯ects an innocent bystander e ect.…”

“…Against the wildtype background, spontaneous cutaneous melanomas emerged at a very low incidence and with a long latency (Chin et al, 1997). Signi®cantly, these rare melanomas showed spontaneous deletion of both INK4a alleles.…”

“…ARF functions as a potent growth suppressor (Quelle et al, 1995), *Correspondence: L Chin Oncogene (1999) 18, 5304 ± 5310 ã 1999 Stockton Press All rights reserved 0950 ± 9232/99 $15.00 http://www.stockton-press.co.uk/onc blocks oncogenic transformation and sustains p53-dependent apoptosis in RB null cells that have reentered the cell cycle in vivo (Pomerantz et al, 1998) or in the setting of hyperproliferative oncogenic signals (de Stanchina et al, 1998;Zindy et al, 1998;Radfar et al, 1998). That p19 ARF may reside in a p53 pathway was ®rst proposed by genetic studies in the INK4a D2/3 (p16 INK4a and p19 ARF ) null mouse showing an absence of p53 mutations in melanomas (Chin et al, 1997). Analogous genetic observations were demonstrated in p19 ARF -de®cient MEFs that retained wildtype p53 con®guration upon immortalization (Kamijo et al, 1997).…”

“…In addition, re-introduction of PTEN into PTEN-de®cient melanoma cells has been shown to suppress growth (Robertson and Jones, 1998). However, the existence of con¯icting data in the cases of RAS (references within Chin et al, 1997) and PTEN (Boni et al, 1998) has fueled debate surrounding the pathogenetic relevance of these genetic changes in melanoma development in vivo. Thus, there exists a strong need to provide genetic evidence that the mutations described above serve a critical role in transformation and malignant progression of melanocytes and are not simply a correlate of these processes.…”