Mismatch-repair deficiency testing plays a critical role in the identification of proband in
Lynch Syndrome families and triaging patients with high stage or recurrent solid
malignancies for check point inhibitor (
Pembrolizumab)
immunotherapy. We compared microsatellite shift patterns of
microsatellite instability PCR analysis at 5 NCI recommended loci between
microsatellite instability high
endometrial carcinoma (n = 50) and
microsatellite instability high
colorectal cancer (n = 19). The
endometrial cancer cohort included 45 endometrioid, 1 serous, and 4 clear cell
carcinomas. Overall, 52% (26/50) of
microsatellite instability high
endometrial cancers showed minimal microsatellite shift (defined as a one to three
nucleotide repeat shift at an involved locus) observed at least at one locus. Among
microsatellite instability high
endometrial cancers with minimal microsatellite shift, the frequencies at each involved locus were D2S123 (21/21, 100%), D17S250 (10/11, 89%), D5S346 (11/12, 92%), BAT25 (9/12, 80%), and BAT26 (8/21, 45%). Noticeably, 11 of the 26 cases (42%) showed only minimal shift. Among
microsatellite instability high
endometrial cancers with minimal microsatellite shift, 65% (17/26) had combined MLH1 and PMS2 loss, 8% (2/26) had combined MSH2 and MSH6 loss, 13% (3/26) had MSH6 loss and 15% (4/26) had loss of PMS2 by immunohistochemistry. In contrast, only 16% (3/19) had minimal microsatellite shift seen in
colorectal cancer cohort with corresponding loss of MLH1/PMS2, MSH2/MSH6, or MSH6. Overall, 15% (7/50) of
microsatellite instability high
endometrial carcinomas showed isolated loss of MSH6 in contrast to 7% (1/15) seen in
microsatellite instability high
colorectal carcinomas. In conclusion,
microsatellite instability high
endometrial carcinomas have a significantly higher frequency of minimal microsatellite shift that coincides with a high percentage of combined loss of MLH1/PMS2.
Microsatellite instability high
endometrial cancers also have more frequent loss of MSH-6. Diagnostically, recognition of minimal microsatellite shift is crucial for accurate interpretation of
microsatellite instability PCR data of
endometrial carcinoma.