Diagnostic utility of p16(INK4a): a reappraisal of its use in cervical biopsies

p16(INK4a), an indirect marker of cell cycle dysregulation, is commonly expressed in cervical dysplasias and carcinomas associated with high risk human papillomavirus (HR-HPV) infections. Although p16(INK4a) immunohistology is routinely used as a cost effective surrogate marker, many of the published articles are confusing and contradictory. The discrepancies can be ascribed to a multitude of factors operating at the molecular, technical and interpretative levels. In the first place, our simplistic model of viral mediated oncogenesis is speculative and fails to account for all the known biomolecular changes. Unresolved technical issues include the variables of tissue fixation, antibody dilution, antibody isotype and clone, and the sensitivity of the particular detection method. Within any controlled staining method, strong diffuse or 'block' immunoreactivity in squamous cells may be found in moderate/severe dysplasia (CIN 2/3) and invasive squamous carcinoma. In contrast, focal or multifocal reactivity in squamous cells may be artefactual, related to low risk or HR-HPV. p16(INK4a) is less reliable when dealing with glandular lesions since considerable overlap exists between reactive and dysplastic lesions. In addition not all glandular dysplasias/carcinomas are HR-HPV related, nor are all p16(INK4a) immunoreactive lesions associated with HR-HPV. We conclude that p16(INK4a) immunoperoxidase shows greater specificity than sensitivity for squamous lesions; in comparison, glandular dysplasias/carcinomas show reduced specificity and sensitivity. Like all cell cycle regulatory proteins, the future diagnostic role of p16(INK4a) is limited. The ideal diagnostic molecular test for cervical dysplasias will detect a HR-HPV related product after, but not before, cell transformation and will reliably predict those cases yet to experience disease progression.