Chromosomal instability in the progression from rectal adenoma to carcinoma

H. Morreau, T. van Wezel, P. Eilers, E de Graaf, T. Karsten, P Kuppen and R.Tollenaar

The recently introduced transanal endoscopic microsurgery (TEM) is a surgical approach to curatively treat patients with rectal adenomas. However, after TEM 10-30% of lesions appear to be at least a stage T1 carcinoma, with risk of lymph node metastasis. In such cases an additional total mesorectal excision procedure (TME) is needed, with increased morbidity and mortality of these often-elderly patients. Therefore, it would be desirable to have a preoperative molecular classifier for the aggressiveness of the rectal lesions. As these tumors often show chromosomal instability, we determined genome wide chromosomal aberrations and loss of heterozygosity (LOH), to identify genetic changes underlying the aggressive behavior of tumors (advanced stage, possible lymph node metastasis). We analyzed 46 TEM treated tumors and 35 TME treated tumors (obtained from a Dutch surgical trial for the treatment of rectal cancer) with Affymetrix 10K SNP arrays. This enabled us to simultaneously determine chromosomal loss and gains and LOH. The group of TEM samples consisted of pure adenomas (n=24), adenomas with a carcinoma focus (n=8), and carcinomas (n=14). From TME patients we had T1 and T2 tumors with (n=11) and without lymph node metastases (n=24). Overall, most cases (91%) had one or more chromosomal aberrations. Cluster analysis demonstrated four distinct groups: two groups mainly consisting of adenomas, and two groups exclusively consisting of carcinomas. In this way it was possible to discern some early and late events in rectal carcinogenesis. LOH and copy number changes occurred mostly in the carcinoma. Having both the LOH and copy number analysis data, it is possible to say something about the underlying mechanism of changes. In some regions LOH was associated with physical loss, while in other regions LOH was caused by a copy number neutral event, like homologous recombination. From these results we conclude that rectal tumors acquire different chromosomal aberrations during their malignant transformation. Early events, already present in adenoma, were identified and later events indicative for the transition from an adenoma to an invasive carcinoma. With those data in hand we want to identify the cases with a more aggressive behaviour than concluded on basis of the preoperative tumor biopsies analyzed by standard microscopy.

Funded by the Dutch Cancer Society Grant UL 2003-2807