Heterogeneity of Mismatch Repair Status and Microsatellite Instability between Primary Tumour and Metastasis and Its Implications for Immunotherapy in Colorectal Cancers

Abstract

Deficient mismatch repair system (dMMR)/microsatellite instability (MSI) is found in about 5% of metastatic colorectal cancers (mCRCs) with a major therapeutic impact for immune checkpoint inhibitor (ICI) use. We conducted a multicentre study including all consecutive patients with a dMMR/MSI mCRC. MSI status was determined using the Pentaplex panel and expression of the four MMR proteins was evaluated by immunohistochemistry (IHC). The primary endpoint was the rate of discordance of dMMR/MSI status between primary tumours and paired metastases. We included 99 patients with a dMMR/MSI primary CRC and 117 paired metastases. Only four discrepancies (3.4%) with a dMMR/MSI primary CRC and a pMMR/MSS metastasis were initially identified and reviewed by expert pathologists and molecular biologists. Two cases were false discrepancies due to human or technical errors. One discordant case could not be confirmed due to the low level of tumour cells. The last case had a confirmed discrepancy with a dMMR/MSI primary CRC and a pMMR/MSS peritoneal metastasis. Our study demonstrated a high concordance rate of dMMR/MSI status between primary CRCs and their metastases. The analysis of one sample, either from the primary tumour or metastasis, with consistent dMMR and MSI status seems to be sufficient prior to treatment with ICI.

Keywords: colorectal cancer; deficient mismatch repair; immunohistochemistry; inter-tumoral heterogeneity; microsatellite instability.

Figure 1

Flow chart. CRC: colorectal cancer; IHC: immunohistochemistry; MMR: mismatch repair; dMMR: deficient mismatch repair; MSI: microsatellite instability; MSS: microsatellite stability.

Figure 2

Pathological examination and MMR immunohistochemistry of the case number 1. (A) Primary left-sided colon cancer in 2003 with pMMR status ((A1), hematoxylin and eosin stain, 100× magnification) with an expression of the four MMR proteins (400× magnification; (A2) upper left: MLH1, upper right: MSH2, bottom left: MSH6 and bottom right: PMS2). (B). Primary right-sided colon cancer in 2005 with dMMR status ((B1), hematoxylin and eosin stain, 100× magnification) with a loss of MLH1 and PMS2 expression ((B2), upper left: MLH1 and upper right: PMS2) and MSH2 and MSH6 expression (bottom left: MSH2 and bottom right: MSH6). (C) Lung metastasis with pMMR status ((C1), hematoxylin and eosin stain, 100× magnification) with an expression of the four MMR proteins (400× magnification; (C2) upper left: MLH1, upper right: MSH2, bottom left: MSH6 and bottom right: PMS2). IHC: immunohistochemistry; MMR: mismatch repair.

Figure 2

Pathological examination and MMR immunohistochemistry of the case number 1. (A) Primary left-sided colon cancer in 2003 with pMMR status ((A1), hematoxylin and eosin stain, 100× magnification) with an expression of the four MMR proteins (400× magnification; (A2) upper left: MLH1, upper right: MSH2, bottom left: MSH6 and bottom right: PMS2). (B). Primary right-sided colon cancer in 2005 with dMMR status ((B1), hematoxylin and eosin stain, 100× magnification) with a loss of MLH1 and PMS2 expression ((B2), upper left: MLH1 and upper right: PMS2) and MSH2 and MSH6 expression (bottom left: MSH2 and bottom right: MSH6). (C) Lung metastasis with pMMR status ((C1), hematoxylin and eosin stain, 100× magnification) with an expression of the four MMR proteins (400× magnification; (C2) upper left: MLH1, upper right: MSH2, bottom left: MSH6 and bottom right: PMS2). IHC: immunohistochemistry; MMR: mismatch repair.

Figure 2

Pathological examination and MMR immunohistochemistry of the case number 1. (A) Primary left-sided colon cancer in 2003 with pMMR status ((A1), hematoxylin and eosin stain, 100× magnification) with an expression of the four MMR proteins (400× magnification; (A2) upper left: MLH1, upper right: MSH2, bottom left: MSH6 and bottom right: PMS2). (B). Primary right-sided colon cancer in 2005 with dMMR status ((B1), hematoxylin and eosin stain, 100× magnification) with a loss of MLH1 and PMS2 expression ((B2), upper left: MLH1 and upper right: PMS2) and MSH2 and MSH6 expression (bottom left: MSH2 and bottom right: MSH6). (C) Lung metastasis with pMMR status ((C1), hematoxylin and eosin stain, 100× magnification) with an expression of the four MMR proteins (400× magnification; (C2) upper left: MLH1, upper right: MSH2, bottom left: MSH6 and bottom right: PMS2). IHC: immunohistochemistry; MMR: mismatch repair.

Figure 3

Pathological examination and MMR immunohistochemistry of the discordant case number 4. (A) Primary tumor with dMMR status ((A1), hematoxylin and eosin stain, 200× magnification) with an expression of MSH2 (A2, up left) and MSH6 (A2, up right) protein expression and a loss of MLH1 (A2, down left) and PMS2 (A2, down right). (B) Metastatic synchronous peritoneal carcinomatosis with a pMMR status ((B1), hematoxylin and eosin stain, 200× magnification) with an expression of MSH6 ((B2, up left), 200× magnification), MSH2 ((B2, up right), 200× magnification), MLH1 ((B2, down left), 200× magnification), and PMS2( (B2, down right), 300× magnification). IHC: immunohistochemistry; MMR: mismatch repair.

Figure 3

Pathological examination and MMR immunohistochemistry of the discordant case number 4. (A) Primary tumor with dMMR status ((A1), hematoxylin and eosin stain, 200× magnification) with an expression of MSH2 (A2, up left) and MSH6 (A2, up right) protein expression and a loss of MLH1 (A2, down left) and PMS2 (A2, down right). (B) Metastatic synchronous peritoneal carcinomatosis with a pMMR status ((B1), hematoxylin and eosin stain, 200× magnification) with an expression of MSH6 ((B2, up left), 200× magnification), MSH2 ((B2, up right), 200× magnification), MLH1 ((B2, down left), 200× magnification), and PMS2( (B2, down right), 300× magnification). IHC: immunohistochemistry; MMR: mismatch repair.