Epitope-positive truncating MLH1 mutation and loss of PMS2: implications for IHC-directed genetic testing for lynch syndrome

Abstract

We assessed mismatch repair by immunohistochemistry (IHC) and microsatellite instability (MSI) analysis in an early onset endometrial cancer and a sister's colon cancer. We demonstrated high-level MSI and normal expression for MLH1, MSH2 and MSH6. PMS2 failed to stain in both tumors, strongly implicating a PMS2 defect. This family did not meet clinical criteria for Lynch syndrome. However, early onset endometrial cancers in the proband and her sister, a metachronous colorectal cancer in the sister as well as MSI in endometrial and colonic tumors suggested a heritable mismatch repair defect. PCR-based direct exonic sequencing and multiplex ligation-dependent probe amplification (MLPA) were undertaken to search for PMS2 mutations in the germline DNA from the proband and her sister. No mutation was identified in the PMS2 gene. However, PMS2 exons 3, 4, 13, 14, 15 were not evaluated by MLPA and as such, rearrangements involving those exons cannot be excluded. Clinical testing for MLH1 and MSH2 mutation revealed a germline deletion of MLH1 exons 14 and 15. This MLH1 germline deletion leads to an immunodetectable stable C-terminal truncated MLH1 protein which based on the IHC staining must abrogate PMS2 stabilization. To the best of our knowledge, loss of PMS2 in MLH1 truncating mutation carriers that express MLH1 in their tumors has not been previously reported. This family points to a potential limitation of IHC-directed gene testing for suspected Lynch syndrome and the need to consider comprehensive MLH1 testing for individuals whose tumors lack PMS2 but for whom PMS2 mutations are not identified.

Germline mutations inPMS2andMLH1in individuals with solitary loss of PMS2 expression in colorectal carcinomas from the Colon Cancer Family Registry Cohort

“…Both these examples suggest that it is the nature of the mutation rather than a technical anomaly associated with tissue fixation or staining quality that is the cause of this differential staining pattern. In support of this, Zighelboim et al 20 described two sisters who carried the same MLH1 mutation: one developed endometrial cancer at 48 years and the other CRC at 45 years and endometrial cancer at 53 years; all tumours showed solitary loss of PMS2 expression and the presence of MLH1 expression.…”

The Role of Immunohistochemistry Markers in Endometrial Cancer with Mismatch Repair Deficiency: A Systematic Review

“…We identified 57 articles analyzing MLH1 promoter methylation [ 19 , 26 , 32 , 33 , 34 , 36 , 40 , 42 , 46 , 47 , 52 , 55 , 57 , 58 , 59 , 69 , 71 , 75 , 79 , 81 , 82 , 83 , 84 , 85 , 86 , 90 , 91 , 94 , 95 , 96 , 99 , 100 , 112 , 113 , 116 , 122 , 124 , 125 , 127 , 128 , 129 , 136 , 137 , 139 , 140 , 141 , 142 , 143 , 144 , 145 , 146 , 147 , 148 , 149 ,…”

Identification of Individuals at Risk for Lynch Syndrome Using Targeted Evaluations and Genetic Testing: National Society of Genetic Counselors and the Collaborative Group of the Americas on Inherited Colorectal Cancer Joint Practice Guideline

“…Furthermore, there are multiple reports of MLH1 germline mutations when tumors show loss of staining for PMS2 only (De Jong et al 2004;Zighelboim et al 2009). The exact likelihood of finding an MLH1 mutation in patients with this IHC result is not known, but appears to be low.…”