Correlation between [3H]thymidine and proliferating cell nuclear antigen (PCNA)/cyclin indices in archival, formaldehyde-fixed human colorectal tissues.

Sections were obtained from archival colorectal tissue samples preserved in paraffin since 1974, after an in vitro incubation with [3H]thymidine and fixation in formaldehyde. These sections were submitted to immunohistochemical staining with the 19A2 monoclonal antibody against proliferating cell nuclear antigen (PCNA) (i.e. the PCNA identified as the auxiliary protein to DNA polymerase delta), followed by autoradiography. Analysis of this double-labelled material revealed an excess of PCNA-labelled over 3H-labelled nuclei, as expected from our previous studies with this fixative. On the other hand, PCNA positive nuclei showed the same overall topographical distribution as the [3H]thymidine-labelled ones, eventually revealing the same heterogeneity or abnormality in the spatial distribution of proliferative cells. Finally, there was a highly significant correlation (r = 0.898; P < 0.0001) between the [3H]thymidine labelling index (TLI) and the proportion of PCNA-positive nuclei (PCNAF-LI). PCNA immunostaining after formaldehyde fixation thus appears as a valid approach for mapping the proliferative compartment and demonstrating tumour heterogeneity or abnormalities in the distribution of proliferative cells. The excellent correlation between the PCNAF-LI and the TLI also makes PCNA immunostaining a simple tool for retrospective or prospective studies on pathological material aimed at evaluating the potential relevance of proliferative indices to clinical prognosis or prediction of cancer risk.