The assessment of oncogene expression at cellular level is important in understanding the role of those genes in
carcinogenesis. Using in situ hybridization and immunohistochemistry, the expression of oncogenes can be visualized in topographic relation to tissue morphology. In the present study, c-myc overexpression was studied in ten
carcinomas of different origin (6 mammary
adenocarcinomas, 2 vulvar and 2 bronchial
squamous cell carcinomas) by in situ hybridization (ISH) with 35S-labeled
RNA probes and by immunohistochemistry (IHC).
DNA amplification and transcription of c-myc oncogene were also studied with polymerase chain reaction (PCR) using
beta-globin as an intrinsic standard for
DNA amplification. The effect of
formalin fixation of c-myc expression was simultaneously studied. Half of the tumours (5/10) demonstrated c-myc
mRNA overexpression by ISH performed on frozen sections and two of the samples were shown to over-express c-myc
protein by IHC. Only two samples fixed in
formalin showed positive signals for c-myc
mRNA. None of the biopsies showed
DNA amplifications either with ISH or PCR. The present results suggest that ISH with
RNA probes is a useful method for detecting the transcription of activated oncogenes in malignant tissues, especially when applied on frozen sections. The results also indicate that in some cases, c-myc gene may be adequately transcribed to
mRNA but the latter is not translated to the appropriate
oncoprotein.