Histopathological study comparing upstream binding factor expression and AgNOR staining

Abstract

AgNOR staining has been in past years, the subject of numerous publications, which have failed to reach agreement regarding its usefulness as a proliferation marker. This silver staining method does not react with NORs (actual chromosome regions containing rRNA (ribosomal RNA) genes), but with proteins associated with them, whose quantity increases in parallel with ribosome biogenesis. The transcription factor UBF (upstream binding factor) is associated with NORs and has an important regulatory role in rRNA synthesis as cofactor of RNA polymerase I. Recent research has revealed an additional cytoarchitectural function of UBF in decondensing r-chromatin (ribosomal-chromatin). Immune detection of UBF expression and AgNOR counts are closely correlated as both techniques identify substrates in or closely adjacent to NORs. However, contrary to AgNOR dots, the UBF signal disappears in cells which undergo apoptosis or terminal differentiation. These features imply that UBF evaluation would reflect tumour cell proliferation (growth fraction) more accurately than AgNOR counts. Here we also show that immunohistochemical staining of UBF may reveal distinct active NORs with open, decondensed chromatin and we hypothesize that the large stretches of decondensed r-chromatin revealed by UBF staining may correspond to clusters seen after silver staining and, conversely, shorter areas of decondensed r-chromatin should match the small AgNOR grains typically found in some tumour types. The length of decondensed r-chromatin may be a reflection of the ratio of active to silent r-RNA genes.