The frequency and clinical significance of DNA polymerase beta (POLβ) expression in breast ductal carcinoma in situ (DCIS)

Background The prediction of clinical behaviour of breast ductal carcinoma in situ (DCIS) and its progression to invasive disease remains a challenge. Alterations of DNA damage repair mechanisms are associated with invasive breast cancer (BC). This study aims to assess the role of base excision repair (BER) DNA Polymerase Beta (POL beta) in DCIS. Methods A cohort of DCIS comprising pure DCIS (n = 776) and DCIS coexisting with invasive BC (n = 239) were prepared as tissue microarrays. POL beta protein expression was assessed using immunohistochemistry and correlated with clinicopathological parameters and patient outcome. Preclinically, we investigated the impact of POL beta depletion on stem cell markers in representative DCIS cell line models. Results Reduced POL beta expression was associated with aggressive DCIS features including high nuclear grade, comedo necrosis, larger tumour size, hormonal receptor negativity, HER2 overexpression and high Ki67 index. Combined low nuclear/low cytoplasmic POL beta expression showed the strongest association with the features' characteristics of aggressive behaviour. There was a gradual reduction in the POL beta expression from normal breast tissue, to DCIS, with the lowest expression observed in the invasive BC. Low POL beta expression was an independent predictor of recurrence in DCIS patients treated with breast conserving surgery (BCS). POL beta knockdown was associated with a significant increase in cell stemness markers including SOX2, NANOG and OCT4 levels in MCF10-DCIS cell lines. Conclusion Loss of POL beta in DCIS is associated with aggressive behaviour and it can predict recurrence. POL beta expression in DCIS provides an additional feature for patients' risk stratification for personalised therapy.