Any genotoxic agents can induce DNA damage and consequently result in oncogenic alterations. The importance of the DNA damage response (DDR) pathway in tumour suppression is well recognized. A defective DDR can result in genomic instability (GIN), which is generally considered to be central to any carcinogenic process.1,2
Alternatively, the presence of an activated DDR can be a hallmark of GIN, which may subsequently enhance the carcinogenic process.
The p53-binding protein 1 (53BP1) belongs to a family of evolutionarily conserved DDR proteins with C-terminal BRCA1 C-terminus domains.3,4
53BP1 is a nuclear protein that rapidly localizes at the sites of DNA double strand breaks (DSBs) and activates p53 along with other kinases.5–10
Activated p53 plays critical roles in the DDR, such as cell cycle arrest, DNA repair, and apoptosis.11,12
It has been well documented in vitro
with immunofluorescence that 53BP1 exhibits diffuse nuclear staining in untreated primary cells, whereas, after exposure to radiation, 53BP1 localizes at the sites of DSBs and forms discrete nuclear foci (NF).5,6,13,14
We have recently demonstrated that, with an immunofluorescence method, 53BP1 NF may serve as a valuable molecular marker of GIN during carcinogenesis.15,16
GIN seems to be induced at the precancerous stage during thyroid and skin carcinogenesis, as follicular adenoma and actinic keratosis show occasional 53BP1 NF.15,16
Given that one manifestation of GIN is the induction of endogenous DDR,17
we propose that immunofluorescence analysis of 53BP1 expression can be a useful tool with which to estimate the level of GIN as well as the malignant potential of human tumours.
Uterine cervical cancers are believed to develop through a multistep process. Furthermore, it is well established that persistent infections with high-risk human papillomavirus (HR-HPV) represent a necessary cause of high-grade premalignant lesions and subsequent invasive cancer of the uterine cervix. The HR-HPV viral oncogenes, E6
, have been shown to be the main contributors to the development of human papillomavirus (HPV)-induced cervical cancer, and increased expression resulting from integration of the viral DNA into the host genome has been detected in invasive cancers and a subset of high-grade lesions.18
It has been shown that E6
together cause polyploidy soon after they are introduced into cells, so GIN is thought to play an essential role in the cellular transformation of cervical epithelium during carcinogenesis. The most manifest function of the E6 protein is to promote the degradation of p53 through its interaction with a cellular protein, E6-associated protein, an E3 ubiquitin ligase.19
E7 is known to bind to the retinoblastoma tumour suppressor gene product, retinoblastoma protein (pRb). Phosphorylation of pRb by G1
cyclin-dependent kinases releases E2F, leading to cell cycle progression into the S phase. E7 is able to bind unphosphorylated pRb, and this may induce cells to prematurely enter the S phase by disrupting pRb–E2F complexes. One cyclin-dependent kinase inhibitor, p16INK4a
, which prevents the phosphorylation of pRb family members, is overexpressed when pRb is inactivated by E7.20
Normally, overexpression of p16INK4a
results in cell cycle arrest, but with E7 expression, this is overcome. Thus, overexpression of p16INK4a
has been suggested as a useful biomarker for evaluating HPV pathogenic activity in cervical lesions.
The present study analysed the presence of endogenous DSBs by immunofluorescence for 53BP1 expression in a series of cervical tissues from patients to evaluate the significance of GIN and its association with HPV infection and p16INK4a overexpression during cervical carcinogenesis. Like other tumours, GIN was shown to be induced in cervical epithelium at a precancerous stage, and increased significantly with progression to cancer.