FOXP3+ regulatory T cells, mismatch repair proteins and BRAF V600E status in young-onset colorectal cancer.

Introduction: Young onset colorectal cancer (YOCRC) is a heterogenous CRC phenotype with an increasing trend globally. This study aims to determine FOXP3+ Treg cells, Mismatch Repair (MMR) proteins, and proto-oncogene B-Raf (BRAF) V600E status among YOCRC patients at Hospital Universiti Sains Malaysia.

Materials and methods: This was a retrospective study of YOCRC (<50 years) over 8 years (January 2013 to December 2021). Immunohistochemistry staining of FOXP3, BRAFV600E, and MMR protein expression was performed using monoclonal antibodies. The staining intensity and percentage of positive cells were used to evaluate the staining using immunoreactive scoring. All data were analysed using descriptive and correlation statistics. A p-value of ≤ 0.05 was taken as statistically significant.

Results: A total of 65 YOCRC patients were diagnosed, out of which 53.8% had proficient MMR (pMMR) with a mean age of 41, while 46.2% had deficient MMR (dMMR) with a mean age of 35.5. The pMMR with the BRAFV600E+ group expressed higher FOXP3+Tregs (54.2%) than the dMMR with the BRAFV600E+ group (22.9%). Patients with lower FOXP3+Tregs were observed more in dMMR with BRAFV600E- (47%) than in pMMR with BRAFV600E- (5.9%). There was a statistically significant association between the density of expressed FOXP3+Tregs with MMR and BRAFV600E status (p=0.002).

Conclusion: While most of the YOCRC had pMMR, others exhibited dMMR with loss of one or more MMR proteins. The presence of BRAFV600E demonstrated the YOCRC's sporadic nature. A high FOXP3+Treg expression was significantly associated with MMR and BRAFV600E status. Future research must be expanded to cover other hospitals to increase the sample size and include MLH1 hypermethylation testing.