Supplementary Material for: Loss of RUNX3 Immunoreactivity in Non-Neoplastic Rectal Mucosa May Predict the Occurrence of Ulcerative Colitis-Associated Colorectal Cancer

<b><i>Background/Aims:</i></b> Runt-related transcription factor (RUNX) 3 is a tumor suppressor whose expression is reduced in non-neoplastic rectal mucosa of patients with ulcerative colitis (UC) with coexisting colitis-associated cancer (CAC). We aimed to evaluate RUNX3 utility as a predictive marker for CAC using immunohistochemistry (IHC) for non-neoplastic UC mucosa. <b><i>Methods:</i></b> We retrospectively compared the RUNX3 expression detected by IHC between non-neoplastic rectal biopsy specimens from 20 cases with invasive cancer (CAC group) and 20 cases selected from 138 patients without CAC (non-CAC group) that were treated during the same period (2006–2017) and were matched for sex, duration, extension, and age. We validated the results using tissue microarrays (TMA) of 44 operated cases with CAC. The RUNX3 expression level was determined by calculating the percentage of RUNX3-positive-cells. <b><i>Results:</i></b> The RUNX3 expression was lower in the CAC than that in the non-CAC group (35.6 vs. 70.7%, <i>p</i> = 0.03). For a cutoff value of 58%, the sensitivity and specificity for predicting CAC were 75.0 and 70.0% respectively. The immunostaining results for the TMA showed the same trend; 74% of cases with CAC were negative for the RUNX3 expression. <b><i>Conclusion:</i></b> RUNX3 immunostaining of non-neoplastic mucosa is useful for identifying UC patients at a high risk of developing CAC.