Do Chromosome 17 Centromere Copy Numbers Predict Polysomy? A Fluorescence In Situ Hybridization and Microarray-Based CGH Analysis
C Marchio, MBK Lambros, P Gugliotta, L Verdun Di Cartogno, C Botta, B Pasini, KK Shiu, A Mackay, K Fenwick, N Tamber, A Ashworth, A Sapino, JS Reis-Filho. ICR, London, United Kingdom; University of Turin, Turin, Italy
Background: Approximately 8% of breast cancers show increased numbers of chromosome 17 centromere(CEP17) by fluorescence in situ hybridisation(FISH; i.e. average CEP17>3.0 per nucleus). Currently, this pattern is considered to represent polysomy of chromosome 17. Chromosome 17 displays complex patterns of genetic aberrations, in particular involving its long arm, in HER2 amplified cancers. We hypothesised that increases in copy number of CEP17 may not necessarily represent chromosome 17 polysomy and that these could result from gains of centromeric/ pericentromeric regions of chromosome 17 or aneusomy of 17q with involvement of CEP17.
Design: Eighteen CEP17 polysomic cases, of which 7 displayed HER2:CEP17 ratios>2.2, and a control group of 10 CEP17 disomic cases. HER2 gene status was determined using the LSI HER2/CEP17 dual-colour probe(Vysis) and analysed with the motorised Metafer Scanning System. Analysis of HER2/CEP17 probes was performed automatically by Metafer through the PathVysion V2 software. Microarray-based comparative genomic hybridisation (aCGH) was performed in all cases after microdissection to ensure >90% of purity of cancer cells. A 32K tiling path bacterial artificial chromosome microarray platform(resolution 50kb) was employed. Data were normalised and smoothed using adaptive weight smoothing(aws). Smoothed data were converted into categorical values: losses - <-0.12; gains - >0.12 and <0.45; amplification 0.45; no changes - -0.12 and 0.12. Gains of the long or the short arm of chromosome 17 were defined as >50% of clones with aws ratios >0.12.
Results: A perfect concordance between FISH assay and aCGH analysis was found in assessing HER2 gene amplification. Out of the 18 polysomic cases, only two were polysomic (ie displayed gains of both short and long arm of chromosome 17). 11 polysomic cases displayed gains of 17q with involvement of part of the centromere. The remaining five cases that did not harbour gains of 17q displayed amplification of the centromeric region.
Conclusions: Our results demonstrate that CEP17 copy numbers>3.0 do not reliably identify chromosome 17 polysomic cases and that gains of CEP17 copies are more often caused by aneusomy of 17q and/or amplification of CEP17.
Tuesday, March 10, 2009 11:30 AM
Platform Session: Section B, Tuesday Morning