Mismatch repair endonuclease PMS2

Colorectal adenocarcinoma with MMR - PMS2

The mismatch repair endonuclease PMS2 is a crucial enzyme involved in the DNA mismatch repair (MMR) system, which is responsible for maintaining genomic stability by correcting errors that occur during DNA replication. PMS2 is part of the MutL homolog family and plays a significant role in the repair of base-base mismatches and insertion-deletion loops.

Structure and Function

PMS2 forms a heterodimer with MLH1, another key protein in the mismatch repair pathway. This complex is essential for the repair process, as it interacts with other proteins such as MSH2 and MSH6 to recognize and bind to mismatched DNA. Once bound, the PMS2-MLH1 complex recruits additional factors that excise the erroneous DNA segment, allowing for the correct sequence to be synthesized.

The endonuclease activity of PMS2 is critical for introducing nicks in the newly synthesized DNA strand, which serves as a signal for the excision of the mismatch. This activity is tightly regulated to ensure that only the incorrect strand is targeted, preserving the integrity of the genetic material.

Clinical Significance

Mutations in the PMS2 gene are associated with Lynch syndrome, also known as hereditary nonpolyposis colorectal cancer (HNPCC). This condition significantly increases the risk of developing colorectal cancer and other types of cancer, such as endometrial cancer.

Sequential sections of a colon crypt showing normal high expression of PMS2

Individuals with Lynch syndrome often have germline mutations in one of the mismatch repair genes, including PMS2. These mutations lead to microsatellite instability (MSI), a hallmark of MMR deficiency, which results in an increased mutation rate and cancer susceptibility.

Diagnostic and Therapeutic Implications

Testing for PMS2 expression is a critical component of the diagnostic workup for Lynch syndrome. Immunohistochemistry (IHC) is commonly used to assess the presence of PMS2 protein in tumor tissues. Loss of PMS2 expression, as shown in

Sequential sections of colon epithelium near a cancer showing reduced or absent expression of PMS2

IHC, can indicate a defect in the mismatch repair system and suggest the need for further genetic testing.

Therapeutically, understanding the status of PMS2 and other MMR proteins can guide treatment decisions. Tumors with MMR deficiency, including those with PMS2 loss, may respond differently to certain chemotherapeutic agents and are often more susceptible to immune checkpoint inhibitors.

Related Pages

• DNA mismatch repair
• Lynch syndrome
• Colorectal cancer
• MLH1
• MSH2