Comparative FISH analysis of numerical chromosome 7 abnormalities in 5-mu m and 15-mu m paraffin-embedded tissue sections from prostatic carcinoma

Interphase fluorescence in situ hybridization (FISH) was performed on 15-mu m-thick paraffin sections from prostatic carcinomas using a chromosome 7-specific alpha-satellite DNA probe. A confocal laser scanning microscope (CLSM) was used for optical sectioning of the thick sections and reconstruction of 3D images. The number of FISH signals was determined by a gallery of optical sections evaluating only complete nuclei. To investiate the influence of section thickness and truncation and nuclei on scoring results, we compared the FISH data from 15-mu m sections with signal counts obtained from 5-mu m sections. The latter were evaluated by conventional fluorescence microscopy in the same tumor regions previously defined and marked on the slides. After statistical analysis of spot frequencies in tumor and non-tumorous cells (chi(2) test), we transferred the signal frequencies into a cytogenetic classification (-7, +7, polysomy 7). Based an this classification, most cases showed more than one chromosome 7 aberration type. Trisomy 7 (+7) became apparent in 15-mu m-thick sections in all 19 tumors, polysomy 7 (>3 spots) in 18/19 cases, and monosomy 7 (-7) in 13/19 cases. In 5-mu m sections, however, trisomy 7 and polysomy 7 were found in only 7/19 and 13/19 cases, respectively, and monosomy 7 in 7/19 cases. When comparing the classification results of tumor cells of the same tumor regions originating either from 5-mu m or 15-mu m sections, the following discrepancies were noted: in 15-mu m sections exclusively, in 12/19 tumors, trisomy 7 was found; in 6/19 cases, polysomy 7; in 8/19 cases, monosomy 7. The high proportion of cases with tumor nuclei expressing only one hybridization signal of chromosome 7 in 15-mu m sections could be confirmed as monosomy 7 in five selected cases by double-hybridization using centromere-specific probes for chromosomes 7 and 12. These results demonstrate that numerical chromosome 7 aberrations are more frequently observed in thick (15-mu m) paraffin-embedded tissue sections by evaluating only complete nuclei. The use of routine sections (5-mu m) for interphase cytogenetic analyses is compromised by a remarkable underestimation of the real chromosome copy numbers.