Background/Objectives: 8-Hydroxydeoxyguanosine (8-OHdG) is an indicator of oxidative stress and causes transversion mutations and carcinogenesis. 8-OHdG is excision repaired by 8-OHdG DNA glycosylase 1 (OGG1), which is classified as nuclear and mitochondrial subtypes. We aimed to clarify the role of OGG1 in pancreatic ductal adenocarcinoma (PDAC). Methods: Ninety-two patients with PDAC who had undergone surgical resection at multiple institutions were immunohistochemically analyzed. The OGG1 and 8-OHdG expression levels were scored using the Germann Immunoreactive Score. The cutoff values of OGG1, as well as that of 8-OHdG, were determined. Results: The low nuclear OGG1 expression group (n = 41) showed significantly higher carbohydrate antigen (CA)19–9 (p = 0.026), and higher s-pancreas antigen (SPAN)-1 (p = 0.017) than the high expression group (n = 51). Nuclear OGG1 expression has no effect on the prognosis. The low mitochondrial OGG1 expression group (n = 40) showed higher CA19-9 (p = 0.041), higher SPAN-1 (p = 0.032), and more histological perineural invasion (p = 0.037) than the high expression group (n = 52). The low mitochondrial OGG1 expression group had a significantly shorter recurrence-free survival (p = 0.0080) and overall survival (p = 0.0073) rates. The Cox proportional hazards model revealed that low mitochondrial OGG1 expression is an independent risk factor of the PDAC prognosis. OGG1 expression was negatively correlated with 8-OHdG expression (p = 0.0004), and high 8-OHdG expression shortened the recurrence-free survival of patients with PDAC. Conclusions: Low mitochondrial OGG1 expression might aggravate the PDAC prognosis.
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