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Pure Discrete Punctate Nuclear Staining Pattern for MLH1 Protein Does Not Represent Intact Nuclear Expression
Qingzhao Zhang1, Gloria Q Young2, Zhaohai Yang1
1Penn State Health Milton S. Hershey Medical Center, Hershey, PA, USA.
Abstract:
Immunohistochemical staining for DNA mismatch repair (MMR) proteins is commonly used to screen for Lynch syndrome. Several laboratories have noticed a discrete punctate nuclear staining pattern for MLH1 that caused confusion in interpretation. This study was designed to investigate whether this particular staining pattern represents intact nuclear expression of MLH1. MMR proteins immunostaining and follow-up testing in 161 consecutive colorectal adenocarcinoma cases (86 biopsies, 75 resections) were retrospectively reviewed. Both discrete punctate nuclear staining and diffuse nuclear staining patterns for MLH1 were observed in internal control cells in 76 biopsies and 27 resections. Only diffuse nuclear staining was seen in the remaining 10 biopsies and 48 resections (P < .0001). Pure discrete punctate nuclear staining pattern for MLH1 was observed in 11 tumors (9 biopsies, 2 resections), and completely negative staining was seen in 13 tumors (2 biopsies, 11 resections; P = .003). Those 24 tumors (21 patients) invariably showed loss of PMS2. Three patients whose biopsies showed pure punctate staining for MLH1 underwent repeat testing on resections: 1 retained the punctate staining and 2 showed complete loss of MLH1. Nine patients who showed loss of PMS2 and pure punctate MLH1 staining underwent molecular testing: 4 had BRAF V600E mutations and 1 had MLH1 gene mutation. Our data showed that discrete punctate nuclear staining for MLH1 is more commonly seen in biopsy specimens. Pure discrete punctate staining pattern is paired with loss of PMS2 expression and may be associated with BRAF or MLH1 gene mutation, thus it should not be interpreted as intact nuclear expression.
Insights
Discrete punctate nuclear staining for MLH1 in Lynch syndrome screening is not intact expression. This pattern, often seen in biopsies, indicates loss of PMS2 and potential BRAF or MLH1 mutations.
Area of Science:
- Oncology
- Molecular Pathology
- Gastroenterology
Background:
- Immunohistochemical staining for DNA mismatch repair (MMR) proteins is standard for Lynch syndrome screening.
- A confusing discrete punctate nuclear staining pattern for MLH1 has been observed in laboratories.
Purpose of the Study:
- To investigate if the discrete punctate nuclear staining pattern for MLH1 signifies intact nuclear expression.
- To clarify the diagnostic implications of this staining pattern in colorectal cancer.
Main Methods:
- Retrospective review of MMR protein immunostaining and follow-up testing in 161 colorectal adenocarcinoma cases (biopsies and resections).
- Analysis of MLH1 and PMS2 staining patterns, including discrete punctate, diffuse nuclear, and negative staining.
- Correlation with molecular testing (BRAF V600E, MLH1 gene mutations) in relevant cases.
Main Results:
- Discrete punctate MLH1 staining was more common in biopsy specimens compared to resections.
- Pure discrete punctate MLH1 staining was invariably associated with loss of PMS2 expression.
- Molecular testing revealed BRAF V600E mutations in 4 patients and MLH1 gene mutation in 1 patient with pure punctate MLH1 and loss of PMS2.
Conclusions:
- Discrete punctate nuclear staining for MLH1 should not be interpreted as intact nuclear expression.
- This pattern, particularly in biopsies, suggests potential Lynch syndrome and warrants further investigation.
- The association with PMS2 loss and genetic mutations highlights its significance in diagnostic interpretation.
