Abrupt Loss of MLH1 and PMS2 Expression in Endometrial Carcinoma

Abstract

Given that endometrial cancer (EC) is often the sentinel cancer for female Lynch syndrome patients, we have successfully implemented universal screening of ECs and have previously shown that this is the preferred method to identify these patients. However, during the course of universal screening of EC, we encountered 6 cases with an unusual pattern of mismatch-repair protein immunohistochemistry that has not been previously described in this setting. In these 6 cases, there was an abrupt loss of MLH1 and PMS2 expression in a portion of the tumor. In 3 cases, marked histologic differences were identified between the areas of the tumor with retained expression and areas with loss of expression. In 2 cases, the areas with loss of expression were of higher grade (1 demonstrated solid growth and the other demonstrated increased nuclear atypia with diffuse p53 expression). In 4 tumors, histologic features associated with microsatellite instability (MSI) were present, including increased intraepithelial lymphocytes. The areas with loss of and retained MLH1/PMS2 expression were separately microdissected and assessed for MSI and MLH1 promoter methylation. The areas with loss of MLH1 and PMS2 more commonly demonstrated MSI compared with the areas with intact expression (83% vs. 33%). MLH1 promoter methylation analysis demonstrated heterogenous hypermethylation, as all areas with loss of MLH1/PMS2 expression had more extensive methylation of MLH1 compared with those areas with retained expression. In summary, we describe the histologic and molecular features of 6 cases of EC with abrupt loss of MLH1 and PMS2 expression and demonstrate that heterogenous methylation of the MLH1 promoter results in this distinct and unusual pattern of immunohistochemical expression.

Unusual Mismatch Repair Immunohistochemical Patterns in Endometrial Carcinoma

“…[14][15][16][17][18][19][20] Furthermore, LS screening studies in gastrointestinal pathology suggest that colonic adenomas (precursor lesions for colonic adenocarcinoma) should not be screened routinely for LS, largely because the exact timing of somatic MMR mutations, and hence the acquisition of an MMR phenotype, in colorectal tumorigenesis is still controversial. [36][37][38][39][40][41][42][43] Unlike Pai et al, 22 we failed to find convincing evidence of histomorphologic differences across MMR-intact and MMR-deficient subclones, with 1 exception. A single case of mixed endometrioid grade 3 and serous carcinoma demonstrated subclonal loss of MLH1/PMS2 in the endometrioid component only.…”

“…Presumably, gains in methylation occur over time, ultimately resulting in complete silencing of MLH1. 22,31 Alternatively, subclonal loss of MLH1/PMS2 in the setting of MLH1 promoter methylation may result from a "second hit" (ie, somatic mutation) leading to a resultant loss of expression by IHC. 32 From a clinical perspective, identification of heterogenous MLH1/PMS2 loss is supportive of a sporadic deficiency rather than a germline mutation.…”

“…Joost et al 21 characterized 3 distinct "heterogenous" or "subclonal" patterns of staining in colorectal carcinoma: "intraglandular" loss (retained/lost staining within glands), "clonal" loss (distinct groups of glands/whole glands without staining), and "compartmental" loss (retained/lost staining between different blocks). Pai et al 22 recently reported "abrupt" loss of MLH1 and PMS2 in EC, akin to Joost's "clonal" loss, with variable MLH1 promoter methylation across the staining patterns.…”

MMR Deficiency is Homogeneous in Pancreatic Carcinoma and Associated with High Density of Cd8-Positive Lymphocytes

“…Intratumoral heterogeneity of MMR status has been described in some cases of colorectal and endometrial cancer but has so far not been analyzed in pancreatic adenocarcinoma. [26][27][28][29][30] To learn more on MSI heterogeneity in pancreatic cancer, a cohort of 597 operated pancreatic cancers was screened by IHC for loss of the MMR proteins MLH1, PMS2, MSH2, and/or MSH6 on a tissue microarray (TMA). Cases with suspected MSI were further analyzed by PCR and repeated IHC on large sections followed by a thorough analysis of all available cancer-containing tissue blocks for possible intratumoral heterogeneity.…”

Screening for Lynch Syndrome by Immunohistochemistry of Mismatch Repair Proteins: Significance of Indeterminate Result and Correlation With Mutational Studies

“…27 Although the majority of the indeterminate staining has been described in CRC, it has also been well documented in EC with more heterogeneity in the staining pattern, and subclonal loss as being the most frequent finding. [28][29][30][31] Indeterminate MMR staining in not unexpected because some gene mutations can be associated with intact protein expression, albeit a dysfunctional one, that can be detected by routine IHC. 8,32 Nontruncating point mutations or mutations in the noncoding region can also result in loss of enzymatic function with preservation of the defective protein that can cause weak staining.…”