Immunohistochemistry for PMS2 and MSH6 alone can replace a four antibody panel for mismatch repair deficiency screening in colorectal adenocarcinoma.
OBJECTIVE
Currently, testing for mismatch repair deficiency in colorectal cancers is initiated by performing immunohistochemistry with four antibodies (MLH1, PMS2, MSH2 and MSH6). If any one of these stains is negative the tumour is considered microsatellite unstable and, if clinical circumstances warrant it, the patient is offered genetic testing for Lynch's syndrome. Due to the binding properties of the mismatch repair heterodimer complexes, gene mutation and loss of MLH1 and MSH2 invariably result in the degradation of PMS2 and MSH6, respectively, but the converse is not true. We propose that staining for PMS2 and MSH6 alone will be sufficient to detect all cases of mismatch repair deficiency and should replace routine screening with all four antibodies.


METHODS
The electronic database of the department of Anatomical Pathology, Royal North Shore Hospital, Sydney, Australia, was searched for all colorectal carcinomas on which a four panel immunohistochemical microsatellite instability screen was performed. An audit of the slides for concordant loss of MLH1-PMS2 and MSH2-MSH6 was then undertaken. Unusual or discordant cases were reviewed and, in some cases, re-stained to confirm the staining pattern.


RESULTS
Of 344 cases of colorectal cancer which underwent four antibody immunohistochemistry, 104 displayed loss of at least one mismatch repair protein. Of these, 100 showed concordant mismatch repair loss (i.e., loss of MLH1 and PMS2 or loss of MSH2 and MSH6). The four discordant cases comprised two single negative cases (1 MSH6 negative/MSH2 positive case, 1 PMS2 negative/MLH1 positive) and two triple negative (both MLH1/PMS2/MSH6 negative). The microsatellite instability (MSI) group showed a relatively high median age (69.3 years) due to the departmental policy of testing all cases with possible MSI morphology regardless of age.


CONCLUSIONS
The sensitivity and specificity of a two panel test comprised of PMS2 and MSH6, compared to a four panel test, is 100%. No false negatives or positives were identified. We conclude that the two panel test should replace a four panel protocol for immunohistochemical screening for mismatch repair deficiency.
Hall
Geoffrey
G
Institute of Clinical Pathology and Medical Research, Department of Anatomical Pathology, Westmead Public Hospital, Westmead, Australia.
Clarkson
Adele
A
Shi
Amanda
A
Langford
Eileen
E
Leung
Helen
H
Eckstein
Robert P
RP
Gill
Anthony J
AJ
D000230:Adenocarcinoma / Q000235:genetics / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000276:immunology / Q000378:metabolism*; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D014408:Biomarkers, Tumor / Q000378:metabolism; D015179:Colorectal Neoplasms / Q000235:genetics / Q000378:metabolism*; D053843:DNA Mismatch Repair / Q000235:genetics; D045643:DNA Repair Enzymes / Q000172:deficiency / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000276:immunology / Q000378:metabolism*; D005260:Female; D006801:Humans; D007124:Immunoenzyme Techniques; D008297:Male; D008875:Middle Aged; D000070976:Mismatch Repair Endonuclease PMS2; D051718:MutS Homolog 2 Protein / Q000276:immunology / Q000378:metabolism*; D011237:Predictive Value of Tests; D055815:Young Adult
D014408:Biomarkers, Tumor; D004268:DNA-Binding Proteins; D000251:Adenosine Triphosphatases; C094934:PMS2 protein, human; C497172:MSH2 protein, human; D000070976:Mismatch Repair Endonuclease PMS2; D051718:MutS Homolog 2 Protein; D045643:DNA Repair Enzymes