Characterisation of genomic instability in Dukes B2 colorectal cancer. D. Marchetti¹, M.R. Iascone¹, G.D. Beretta², A.R. Lincesso¹, S. Mosconi², R. Labianca². 1) Laboratorio di Genetica Molecolare; 2) USC Oncologia Medica; Ospedali Riuniti, Bergamo, Italy.
   Colorectal cancer (CRC) is one of the most common malignancies in developed countries. The TNM system represents the main tool for identifying prognostic differences among patients. Although the Dukes B2 CRC is an homogeneous histopathological class, 30% of patients has recurrences during lifetime and the role of adjuvant therapy is still unclear in this class. Genomic instability is a driving force in the initiation of CRC development. The molecular genetics of human cancers can be used to categorise CRC in two major types: chromosomal instability and microsatellite instability (MSI). The aim of our study is the evaluation of genomic instability in Dukes B2 CRC patients. After histopathological classification, we studied specimens from 73 Dukes B2 CRC patients. Genomic DNA was extracted from normal and neoplastic microdissected paraffin-embedded tissues. Ten microsatellite markers were used to detect MSI and LOH at 18q and 17q, the chromosomes most frequently affected by LOH in CRC. The product of PCR was sized by WAVE System DNASep Cartridge (Transgenomics) eluted through a linear gradient of TEAA at 50 degree. Four samples were not analysed for scarcity of tumour tissue, 9 (13%) showed MSI, whereas LOH was observed in 25 (36%) at 18q, 5 (7%)at 17q and 15 (22%) at both loci. Fifteen of 69 Dukes B2 CRC (22%) don't show neither MSI neither LOH (noMSI-noLOH). This finding suggests that alternative pathways may be involved in CRC development. To evaluate this hypothesis, 2 noMSI-noLOH cases were tested by 32K array-CGH (TechnoGenetics). The analysis confirmed the absence of LOH at 18q and 17q and showed a common amplification at 13q12-q14 and several different chromosomal gains/losses. This preliminary finding could indicate that 13q12-q14 region may be critical for the progression of CRC but other rearrangements could characterise each tumour progression. Further studies are necessary to verify this hypothesis and to evaluate its clinical utility for the risk stratification of Dukes B2 patients.