Progressive Activation of YAP1 in Human Papillomavirus–Associated Squamous Intraepithelial Lesions and Squamous Cell Carcinoma

PURPOSE: The Hippo signaling pathway critically regulates cell proliferation and differentiation via the transcriptional coactivator Yes-associated protein 1 (YAP1), which is typically inactivated through phosphorylation and cytoplasmic sequestration. Disruption of Hippo signaling leads to YAP1 dephosphorylation, nuclear translocation, and enhanced cell proliferation. Experimental models suggest that human papillomavirus (HPV) E6 and E7 oncoproteins activate YAP1, contributing to cervical carcinogenesis, but systematic evaluation of YAP1 activation in human cervical squamous cell carcinoma (SCC) and its precursor squamous intraepithelial lesions (SIL) is lacking. MATERIALS AND METHODS: We assessed immunohistochemical (IHC) expression of total YAP1, which detects both inactive and active forms, and activated YAP1 (act-YAP1), which specifically recognizes the active form, in 118 cervical and vulvar cases. These included benign squamous epithelium (n = 15), low-grade SIL or cervical squamous neoplasia 1 (LSIL/CIN1; n = 30), high-grade SIL (HSIL/CIN2-3; n = 63), and invasive SCC (n = 10). We further correlated act-YAP1 expression with HPV genotype and IHC profiles of p16, p53, and Rb1. RESULTS: Act-YAP1 IHC showed greater specificity, reduced background, and improved interpretability compared with total YAP1. Act-YAP1 IHC demonstrated absence nuclear expression in most of benign squamous epithelium and most LSIL/CIN1, whereas focal and diffuse nuclear act-YAP1 staining was common and frequent in HSIL and SCC. Act-YAP1 expression increased progressively in both extent and intensity from CIN2 (76%) to CIN3 (84%) and to SCC (100%). No significant associations were found between YAP1 activation and HPV genotype or immunoprofiles of p16, p53, or Rb1. CONCLUSIONS: Our study demonstrates that YAP1 activation is a progressive and sustained event during HPV-associated squamous neoplastic progression, supporting Hippo pathway disruption as an important mechanism in cervical carcinogenesis.