Mitochondrial DNA D‑loop lesions with the enhancement of DNA repair contribute to gastrointestinal cancer progression.

Gastrointestinal cancer (GIC) is a worldwide public health problem with a high mortality rate. Mitochondrial DNA (mtDNA) mutations in the displacement loop (D-loop) region are quite common in various types of primary human cancers; however, their role in the pathogenesis of GIC is controversial. In the present study, tumor and para-tumor tissues were selected from 18 patients with gastric cancer (GC), 21 patients with colon cancer (CC) and 30 patients with rectal cancer (RC). The mtDNA D-loop was analyzed by sequencing and reverse transcription-quantitative polymerase chain reaction. Furthermore, DNA oxidative damage and DNA repair functioning were detected by immunohistochemistry. The results demonstrated that increased mtDNA deletion was not evident in GIC; however, significant DNA oxidative damage was significant in RC by detecting 8-hydroxyguanine expression. In addition, over-activated DNA repair was identified in CC and RC through the detection of 8-oxo-20-deoxyguanosine glycosylase 1 expression. The mtDNA D-loop had a specific mutation hotspot region, and the level of mtDNA D-loop mutations was correlated with the progression of the GIC. The mutations of the mtDNA D-loop were primarily homoplasmic in GIC and often transitioned at pyrimidine sites. Mitochondrial microsatellite instability, including the formation of poly-cytidine stretches, was common in GIC. These results demonstrate the occurrence of mtDNA D-loop mutations in GIC in Chinese patients and support the correlation of these mutations with carcinoma progression. Over-activated DNA repair function possibly repairs the GIC mtDNA lesions.