What is the role of Immunohistochemistry (IHC) in diagnosing Mismatch Repair Deficiency (MMRd) and p53, and Molecular studies in identifying POLE mutations?

From the Guidelines

Immunohistochemistry (IHC) is useful for diagnosing mismatch repair deficiency (MMRd) and p53 abnormalities, while molecular studies are needed for POLE mutations due to their different detection mechanisms. IHC works by using antibodies to detect protein expression patterns - loss of MMR proteins (MLH1, MSH2, MSH6, PMS2) indicates MMRd, while p53 shows either complete absence or overexpression when mutated. These protein-level changes are directly visible through IHC staining. In contrast, POLE mutations occur at the DNA level without consistent protein expression changes that can be detected by IHC. POLE mutations require direct DNA sequencing or next-generation sequencing to identify specific nucleotide changes in the polymerase epsilon gene. This distinction is clinically important because these molecular classifications (MMRd, p53 abnormal, POLE mutant) have prognostic and therapeutic implications, particularly in endometrial and colorectal cancers, where they guide treatment decisions including potential immunotherapy use for MMRd tumors or different surgical and adjuvant therapy approaches based on molecular subtype 1.

Some key points to consider:

• IHC has a sensitivity and specificity of 86-100% and 48-67%, respectively, for detecting MMRd 1.
• POLE mutations have prognostic implications and require direct DNA sequencing or next-generation sequencing for detection 1.
• The identification of MLH1 protein loss enables methylation analysis, which can exclude women with somatic MLH1 loss from unnecessary NGS 1.
• MMR IHC and/or MSI screening followed by analysis of MLH1 promoter hypermethylation is recommended for women with endometrial cancer, since 2-3% of endometrial cancers are associated with Lynch syndrome 1.

The most recent and highest quality study, published in 2019, recommends that tumor MMR or MSI status is used to identify women for germline MMR testing, and that IHC testing is tightly regulated and protocol-driven to ensure interpretable, quality-assured results 1. Therefore, IHC is a valuable tool for diagnosing MMRd and p53 abnormalities, while molecular studies are necessary for detecting POLE mutations.

From the Research

Immunohistochemistry (IHC) and Molecular Studies

• Immunohistochemistry (IHC) is useful for the diagnosis of MMRd (mismatch repair deficient) and p53, as it can detect the presence or absence of specific proteins in tissue samples 2, 3.
• IHC can help identify patients with Lynch syndrome, a hereditary colorectal cancer syndrome caused by germline mutations in DNA mismatch repair genes 2, 4.
• Molecular studies, such as those examining POLE mutations, can provide additional information on the genetic alterations present in a tumor 5.
• POLE mutations are associated with a distinct subtype of colorectal cancer, and their detection can have implications for treatment and prognosis.

Role of IHC in Diagnosing MMRd and p53

• IHC can detect the loss of mismatch repair protein expression, which is a hallmark of MMRd tumors 2, 3.
• The loss of p53 protein expression can also be detected by IHC, which can help diagnose tumors with TP53 mutations 5.
• IHC results can be used in conjunction with molecular studies to confirm the diagnosis of MMRd and p53 tumors.

Importance of Molecular Studies

• Molecular studies, such as next-generation sequencing, can detect genetic alterations, including POLE mutations, which can provide additional information on the tumor's biology and behavior 5.
• The detection of POLE mutations can help identify patients with a distinct subtype of colorectal cancer, which may have different treatment options and prognosis.
• Molecular studies can also help identify patients with Lynch syndrome, who may benefit from genetic counseling and germline testing 4.