Retained PAX2 expression associated with DNA mismatch repair deficiency in endometrial endometrioid adenocarcinoma

Aims Loss of expression of tumour suppressor PAX2 and MMR deficiency (dMMR) has been frequently seen in endometrial endometrioid adenocarcinoma (EEC). However, the relationship between PAX2 expression and MMR status is unknown. Methods and Results We studied the PAX2 expression and examined its association with MMR status at the protein and genetic levels in 180 cases of EEC. Overall, total loss of PAX2 expression was found in about 70%, while retained PAX2 expression was seen in 30% of EEC. Among 125 cases with loss of PAX2, 68.8% were found in EECs with pMMR, while 31.2% were seen in those with dMMR. Among 55 cases of EECs with retained PAX2 expression, 92.7% were EECs with dMMR and 7.3% were those with pMMR ( P < 0.001). While dMMR cases with MLH1 hypermethylation show almost equal retained or loss of PAX2 expression (52% versus 48%), dMMR with genetic alterations had significantly more retained PAX2 expression than loss of PAX2 (92.3% versus 7.7%), regardless of somatic or germline mutations. Loss of PAX2 was observed in 97.3% of dMMR with MLH1 hypermethylation compared to 2.7% of dMMR with genetic alterations ( P < 0.001). Aggressive features such as higher tumour grades (FIGO 2–3) and advanced clinical stage (T2–T4) were significantly more frequently seen in dMMR with retained PAX2 expression, compared those to pMMR with loss of PAX2 expression. Conclusion Our study demonstrates a close correlation between retained PAX2 expression and dMMR in EEC. The molecular mechanism and clinical significance linking these two pathways in EEC remains to be unravelled.