PROGRESSION OF PREGNANCY-DEPENDENT MMTV-INDUCED MAMMARY TUMORS OCCURS BY SELECTION OF PRE-EXISTENT INSERTIONAL EVENTS

GATTELLI ALBANA; ZIMBERLIN MARÍA; MEISS ROBERTO; CASTILLA LUCIO; KORDON EDITH

Anaheim, Orange County, California, USA

Congreso; 2005 AACR Annual Meeting; 2005

AACR

We have previously demonstrated that MMTV-induced poly-clonal pregnancy-dependent (PD) RE+RP+ mammary tumors progress to mono-clonal pregnancy-independent (PI) RE-RP- neoplasias. This progression is associated with the appearance of viral insertions detected by Southern blot analysis in the PI tumors, which are absent in the PD tumors from which they evolved. The goal of the present work was to determine whether the occurrence of each of these insertions results in a rapid progression to hormone-autonomy or, on the contrary, if these mutations occur early in the PD tumor and then, eventually, the small subpopulation containing such insertions are selected and become predominant in later PI passages. For this purpose, in five independent in vivo tumor lines, six different insertion sites associated to PI phenotype were cloned-out by inverse PCR (IPCR). Once we have determined the host DNA sequences adjacent to the MMTV-LTR, primers were designed in order to detect these viral insertions in early PD tumor passages. Our results show that in all cases, MMTV insertion sites found in PI tumors were also detected in the original PD transplants. By Real-Time PCR it was calculated that during PD to PI transition, the relative increase of subpopulations containing the isolated insertion was higher than 70%. In addition, we have found that, after three pregnancies, DNA from MMTV(LA) infected BALB/c hyperplastic-dysplastic (but not neoplastic) mouse mammary glands showed a high number of MMTV insertional events, similar to the quantity found in PD and PI tumors. Therefore, our results indicate that in MMTV-induced mammary neoplasias, the occurrence of new insertional events is not required, and does not seem to be of common occurrence, during progression towards autonomy. Alternatively, this process would mainly happen by selection of a small population already present in the early stages of tumor evolution.