Frequent Detection of KRAS-G12C and PIK3CA-Q546K Mutations in MAP Tumors Highlights their Role in MUTYH Variants of Uncertain Significance Reclassification.

Biallelic pathogenic variants in MUTYH cause MUTYH-associated polyposis (MAP), a rare recessive colorectal cancer (CRC) predisposition syndrome characterized by somatic G:C > T:A transversions. The hotspot somatic mutations KRAS-G12C and PIK3CA-Q546K are highly enriched in MAP CRCs, and are rarely observed in sporadic cases, suggesting their potential utility in supporting the reclassification of variants of uncertain significance (VUS) in MUTYH. This study is aimed at evaluating the frequency of KRAS-G12C and PIK3CA-Q546K in adenomas and CRC from MAP patients and to demonstrate their relevance to reclassify MUTYH VUS. These hotspot mutations were evaluated using targeted NGS in adenomas and CRC tissues from 16 previously diagnosed MAP patients and three patients suspected of MAP, who harbored a VUS in either homozygosity or compound heterozygosity with a germline pathogenic MUTYH variant. KRAS-G12C and PIK3CA-Q546K were identified in 92.3% and 38.4% of 13 MAP adenocarcinomas, respectively. KRAS-G12C was also present in 47% of 17 MAP adenomas, whereas none of them harbored PIK3CA-Q546K. The detection of either mutation in CRC showed 100% sensitivity and 97% specificity for MAP (p = 0.00001). Among the three-suspected MAP cases with VUS, two harbored somatic KRAS-G12C and/or PIK3CA-Q546K, providing sufficient evidence to reclassify MUTYH VUS p.Pro301Arg and p.Trp113Arg as likely pathogenic based on ACMG/AMP criteria. These findings support the use of KRAS-G12C and PIK3CA-Q546K as cost-effective, accessible tumor biomarkers for aiding in MAP diagnosis and MUTYH VUS reclassification, particularly in settings with limited access to whole-exome/genome mutational signature analysis.