Diagnostic Approach to Deficient Mismatch Repair with MLH1/PMS2 Loss

Immediate Diagnostic Algorithm

For a patient under 50 with tumor showing loss of MLH1 and PMS2 expression, the American College of Gastroenterology recommends immediate pursuit of MLH1 promoter hypermethylation testing and BRAF V600E mutation analysis to distinguish Lynch syndrome from sporadic cancer, as this distinction fundamentally changes management 1, 2
The loss of MLH1 and PMS2 together is the expected pattern, accounting for approximately 93% of dMMR tumors, according to the National Comprehensive Cancer Network 1

BRAF V600E mutation testing is necessary, as the BRAF V600E point mutation is found in most tumors with acquired MLH1 promoter methylation and is extremely rare in Lynch syndrome colorectal cancers, with a strength of evidence level of high, according to the American Society of Clinical Oncology 1, 2
MLH1 promoter hypermethylation testing is also necessary, as demonstrating promoter hypermethylation supports a somatic (non-inherited) event, with a strength of evidence level of moderate, according to the American Gastroenterological Association 1

The patient's age under 50 is a major red flag for Lynch syndrome, meeting the revised Bethesda guidelines and significantly increasing the probability of hereditary disease, with a strength of evidence level of high, according to the American College of Medical Genetics and Genomics 1, 3

The patient is eligible for immune checkpoint inhibitor therapy with pembrolizumab, which received FDA approval for unresectable or metastatic MSI-H or dMMR solid tumors, with an objective response rate of 39.6% (95% CI 31.7% to 47.9%), according to the National Cancer Institute 3
The VENTANA MMR IHC assay is the FDA-approved companion diagnostic for determining MMR status for treatment with pembrolizumab, with a strength of evidence level of high, according to the FDA 3

If germline MLH1 mutation is identified, the patient requires intensive lifelong surveillance, including colonoscopy every 1-2 years starting at age 20-25 years, with a strength of evidence level of high, according to the American College of Gastroenterology 4
Colonoscopic surveillance has demonstrated a 72% decrease in mortality from colorectal cancer in Lynch syndrome patients, with a strength of evidence level of high, according to the American Gastroenterological Association 4
Endometrial cancer surveillance is necessary, as MLH1 carriers have a 30-51% lifetime risk of endometrial cancer, with a strength of evidence level of moderate, according to the American College of Obstetricians and Gynecologists 2

If Lynch syndrome is confirmed, first-degree relatives have a 50% chance of carrying the same mutation and require genetic counseling and testing, with a strength of evidence level of high, according to the American Society of Clinical Oncology 2, 3

The MLH1 gene encodes a protein essential for DNA mismatch repair (MMR), playing a critical role in maintaining genomic integrity by correcting errors that occur during DNA replication 5

MLH1 is one of four primary genes that are critical components of the DNA mismatch repair system, which restores DNA integrity after mismatching errors occur 5, 6
The MLH1 protein functions as part of heterodimeric complexes, primarily partnering with PMS2 to form the MLH1-PMS2 complex that is essential for correcting single base mismatches or short insertions and deletions in DNA 5, 7

Germline mutations in MLH1 are responsible for approximately 50% of hereditary non-polyposis colorectal cancer (HNPCC) cases, also known as Lynch syndrome 8, 9
Inactivation of MLH1 can occur due to germline mutations, somatic mutations, or epigenetic silencing, resulting in defective MMR (dMMR) mechanism 5, 10
Defects in MLH1 lead to microsatellite instability (MSI), a condition of genetic hypermutability characterized by clustering of mutations in microsatellites 5, 6

MLH1 mutations are associated with various cancer types, particularly colorectal and endometrial cancers 11, 12
Tumors with MLH1 deficiency typically show loss of both MLH1 and PMS2 proteins when assessed by immunohistochemistry 10
MLH1-deficient tumors often have a better prognosis and may respond differently to certain chemotherapies compared to microsatellite-stable tumors 8, 10

Testing for MLH1 deficiency is commonly performed using immunohistochemistry to assess protein expression or through molecular tests for microsatellite instability 8, 10
The National Comprehensive Cancer Network recommends testing for MLH1 deficiency in patients with colorectal cancer diagnosed before age 50, as they are more likely to have Lynch syndrome 8

Biallelic (homozygous) mutations in MLH1 cause constitutional mismatch repair deficiency syndrome, a rare and aggressive cancer predisposition syndrome 13
This condition is characterized by early-onset colorectal neoplasia, brain tumors, hematologic malignancies, and café au lait spots 13