| Result | Possible interpretation | Explanation/comments | Next steps to consider |
| Loss of MLH1 only (rare) | - MLH1 germline mutation.
- MLH1 promoter hypermethylation.
- Bi-allelic (double) somatic MLH1 or PMS2 inactivation.
| - Epigenetic silencing of the MLH1 gene can occur through MLH1 promoter hypermethylation.
- Bi-allelic (double) inactivation of MLH1 can rarely cause isolated MLH1 loss.
| - BRAF V600E mutation testing (if negative, proceed to #2).
- Tumor testing for MLH1 promoter hypermethylation testing.¶
- Germline panel testing for at least the MMR genes if family history is suggestive of Lynch syndrome.*
- Tumor sequencing to evaluate for bi-allelic (double) somatic MLH1 inactivation due to mutation and/or LOH.
|
| Loss of MLH1 and PMS2 (common) | - MLH1 germline mutation.
- PMS2 germline mutation.
- MLH1 promoter hypermethylation.
- Bi-allelic (double) somatic MLH1 inactivation through mutation and/or LOH.
- Bi-allelic (double) somatic PMS2 inactivation through mutation and/or LOH.
| - Because the MLH1 and PMS2 proteins form a heterodimer, altered expression of either the MLH1 or PMS2 protein due to germline mutations frequently leads to loss of both MLH1 and PMS2 expression.
- Altered expression of either MLH1 or PMS2 due to bi-allelic (double) somatic mutation or LOH of the MLH1 or PMS2 gene can also lead to this pattern.
| - BRAF V600E mutation testing (if negative, proceed to #2).
- Tumor testing for MLH1 promoter hypermethylation.¶
- Tumor testing for bi-allelic (double) somatic MLH1 or PMS2 inactivation due to mutation and/or LOH.
- Germline panel testing for at least the MMR genes if family history is suggestive of Lynch syndrome.*
|
| Loss of PMS2 only (less common) | - PMS2 germline mutation.
- Bi-allelic (double) somatic PMS2 inactivation through mutation and/or LOH.
- MLH1 germline mutation.
- Bi-allelic (double) somatic MLH1 inactivation through mutation and/or LOH.
| - Because the MLH1 protein has heterodimer partner proteins other than the PMS2 protein, germline PMS2 mutations may not cause loss of MLH1 expression by IHC.
- PMS2 protein expression by IHC can be interpreted as equivocal—not clearly positive or negative.
- MLH1 germline mutations have rarely been identified when tumors show loss of staining of PMS2 only; bi-allelic double somatic MLH1 mutation or LOH could also mimic this.
| - Germline MMR panel testing.*
- If panel testing is negative consider tumor testing for the MMR genes to identify bi-allelic (double) somatic PMS2 or MLH1 inactivation due to mutation and/or LOH.
- If PMS2 expression by IHC is equivocal consider secondary MSI testing to confirm or rule out presence of dMMR.
|
| Loss of MSH2 only (rare) | - MSH2 germline mutation.
- EPCAM germline mutation.
- Bi-allelic (double) somatic inactivation of MSH2 through mutation and/or LOH.
| - Strong likelihood of germline MSH2 or EPCAM.
- Bi-allelic (double) somatic inactivation of MSH2 is rare but has been reported.
| - Germline MMR panel testing.*
- If panel testing is negative consider tumor testing for the MMR genes to identify bi-allelic (double) somatic MSH2 inactivation due to mutation and/or LOH.
|
| Loss of MSH2 and MSH6 (common) | - MSH6 germline mutation.
- MSH2 germline mutation.
- EPCAM germline mutation.
- Bi-allelic (double) somatic inactivation of MSH2 through mutation and/or LOH.
- Bi-allelic (double) somatic inactivation of MSH6 through mutation and/or LOH.
| - Because the MSH2 and MSH6 proteins form a heterodimer, alterations of expression of either the MSH2 or MSH6 protein due to germline or bi-allelic (double) somatic mutations in either the MSH2 or MSH6 genes could cause this pattern on IHC.
| - Germline MMR panel testing.*
- If panel testing is negative consider tumor testing for the MMR genes to identify bi-allelic (double) somatic MSH2 or MSH6 inactivation due to mutation and/or LOH.
|
| Loss of MSH6 (common) | - MSH6 germline mutation.
- Bi-allelic (double) somatic inactivation of MSH6 through mutation and/or LOH.
- MSH2 germline mutation.
- Bi-allelic (double) somatic inactivation of MSH2 through mutation and/or LOH.
| - Because MSH2 has heterodimer partners other than MSH6, germline MSH6 mutations may not cause loss of MSH2 expression by IHC and frequently cause loss of staining of MSH6 only; similar reasoning holds for bi-allelic (double) somatic MSH6 inactivation through mutation and/or LOH.
| - Germline MMR panel testing.*
- If panel testing is negative consider tumor testing for the MMR genes to identify bi-allelic (double) somatic MSH6 inactivation due to mutation and/or LOH.
|
| Loss of all 4 MMR proteins | - Likely an artifact of tissue fixation and IHC staining procedures.
- Germline mutation in any one of the Lynch syndrome genes
| | - Consider MSI testing to confirm or rule out dMMR.
- Germline MMR panel testing if family history warrants testing.*
|
| All proteins expressed | - Likely not Lynch syndrome if MSS.
- If MSI-H, rare missense germline mutations in MLH1, MSH2, MSH6, and PMS2 can demonstrate normal IHC.
- If MSI-H, rare patients with bi-allelic (double) somatic alterations due to mutation and/or LOH.
| Normal IHC is only approximately 85% sensitive for Lynch syndrome. | - Consider MSI testing to confirm or rule out dMMR.
- If MSI-H, germline MMR panel testing if family history warrants testing.*
- If MSI-H and germline panel testing are negative consider tumor testing for the MMR genes to identify bi-allelic (double) somatic inactivation due to mutation and/or LOH.
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