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September 2011 VOL 2, NO 5

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Original Research

Incidence and Characteristics of BRAF V600E Mutation in Colorectal Cancer with Mismatch Repair Protein Defect Due to Loss of MLH1: A Prospective Evaluation of 104 Consecutive Patients

Sameer Mahesh, MD 

Background: Mismatch repair (MMR) protein –deficient colorectal cancer (CRC), which accounts for 15% to 20% of all CRC, can be sporadic or genetic, due to Lynch syndrome (LS). Loss of MMR proteins can be detected by their absence of staining on immunohistochemistry (IHC). Previously, all MMR-deficient CRC were tested for LS by genetic testing. With the advent of BRAF testing, genetic testing for LS can be avoided. In addition, detection of BRAF mutations may have therapeutic implications. We sought to investigate the contribution of BRAF V600E mutations due to MLH1-deficient CRC and incidence of LS among all newly diagnosed CRC.

Methods: All CRC between March 2010 and Dec ember 2010 were tested for MMR protein (MLH1, MSH2, MSH6, and PMS2) deficiency by IHC. Absence of MLH1 prompted BRAF V600E mutation testing. If BRAF was mutant, further testing was stopped. Absence of MSH2, MSH6, and MLH1 with normal BRAF prompted complete mutational analysis of the missing gene. If patients had MMR-proficient tumors but met modified Bethesda criteria, LS genetic testing was pursued.

Results: Results are in the Table.

RESULTS OF MMR PROTEIN–DEFICIENCY TESTING IN CRC

Conclusion: BRAF was mutated in 77% of patients with absence of MLH1. These patients were elderly and predominantly women with rightsided CRC

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