Special article
Relevance, Pathogenesis, and Testing Algorithm for Mismatch Repair–Defective Colorectal Carcinomas: A Report of the Association for Molecular Pathology

https://doi.org/10.1016/j.jmoldx.2011.11.001Get rights and content
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Loss-of-function defects in DNA mismatch repair (MMR), which manifest as high levels of microsatellite instability (MSI), occur in approximately 15% of all colorectal carcinomas (CRCs). This molecular subset of CRC characterizes patients with better stage-specific prognoses who experience no benefit from 5-fluorouracil chemotherapy. Most MMR-deficient (dMMR) CRCs are sporadic, but 15% to 20% are due to inherited predisposition (Lynch syndrome). High penetrance of CRCs in germline MMR gene mutation carriers emphasizes the importance of accurate diagnosis of Lynch syndrome carriers. Family-based (Amsterdam), patient/family-based (Bethesda), morphology-based, microsatellite-based, and IHC-based screening criteria do not individually detect all germline mutation carriers. These limitations support the use of multiple concurrent tests and the screening of all patients with newly diagnosed CRC. This approach is resource intensive but would increase detection of inherited and de novo germline mutations to guide family screening. Although CRC prognosis and prediction of 5-fluorouracil response are similar in both the Lynch and sporadic dMMR subgroups, these subgroups differ significantly with regard to the implications for family members. We recommend that new CRCs should be classified into sporadic MMR-proficient, sporadic dMMR, or Lynch dMMR subgroups. The concurrent use of MSI testing, MMR protein IHC, and BRAF c.1799T>A mutation analysis would detect almost all dMMR CRCs, would classify 94% of all new CRCs into these MMR subgroups, and would guide secondary molecular testing of the remainder.

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CME Disclosure: None of the authors disclosed any relevant financial relationships.

The Mismatch Repair-Defective CRC Working Group is a subcommittee of the Association for Molecular Pathology Clinical Practice Committee. The 2008–2011 Association for Molecular Pathology Clinical Practice Committee consisted of Aaron Bossler, M. Fernanda Sabato Charreun, Michelle Dolan, Christine A. Curtis, William K. Funkhouser, Julie M. Gastier-Foster, Jane S. Gibson, Cyrus V. Hedvat, Neal Lindeman, Janina Longtine (chair 2011), Ira Lubin, Kathleen T. Montone, Federico Monzon, Kasinathan Muralidharan, Narasimhan Nagan, Victoria Pratt (chair 2008), Joseph F. Pulliam, Daniel E. Sabath, Iris Schrijver (chair 2009–2010), Siby Sebastian, Patrik Vitazka, Jeffrey D. Wisotzkey, Donna M. Wolk, and Belinda Yen-Lieberman.

Standard of practice is not being defined by this article, and there may be alternatives.

The findings and conclusions in this study are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention or the Agency for Toxic Substances and Disease Registry.

Address reprint requests to the Association for Molecular Pathology, c/o Mary Williams, 9650 Rockville Pike, Bethesda, MD 20814-3993. Email: [email protected]