Session # 37

Explaining an Atypical Immunohistochemical Pattern Using Paired Tumor/Germline Testing for Lynch syndrome

  • Paired tumor/germline testing fully explained atypical IHC results (abnormal staining of MLH1 and/or PMS2 in addition to abnormal staining of MSH6) in 10% of cases and partially explained the IHC pattern in an additional 33% of cases, while germline testing alone would not have fully explained any cases, showing that paired testing can improve clinical utility compared to germline testing alone in cases with atypical IHC results.
  • Our data supports previous research showing that somatic MSH6 coding microsatellite mutations can contribute to abnormal MSH6 IHC staining in cases that also demonstrate abnormal MLH1 and/or PMS2 staining, and paired testing results were concordant with the atypical IHC pattern in 37% of cases, but did not fully explain abnormal MSH6 staining due to the presence of only a single somatic CM mutation.
  • Our data confirms that this atypical IHC pattern can be fully explained by paired tumor/germline testing in 10% of cases, while other cases may be partially or completely unexplained due to possibly erroneous IHC results, potentially undetected germline or somatic alterations, or other causes.

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