The PRAME immunostain is a specialized laboratory test used by pathologists to analyze tissue samples. PRAME stands for PReferentially expressed Antigen in MElanoma, a protein marker whose presence or absence offers information about cell nature. The immunostain employs antibodies to visually tag this specific protein, allowing it to be seen under a microscope. This technique is particularly valuable in skin pathology for distinguishing between harmless moles and serious skin cancers.
The Biology of the PRAME Marker
PRAME is classified as a cancer-testis antigen (CTA), a group of proteins whose expression is normally restricted to reproductive cells in adults. Under healthy conditions, PRAME is primarily found in the germ cells of the testes and ovaries, the placenta, and, to a lesser extent, the adrenal gland and endometrium. In most other normal tissues, PRAME expression is absent or very low, making the protein an attractive target for cancer detection.
The presence of PRAME outside of these normal tissues is considered abnormal and is a hallmark of various malignancies, including sarcomas, leukemias, and several epithelial tumors. In melanoma, where the protein was first identified, PRAME is frequently overexpressed. This overexpression contributes to cancer development by interfering with the retinoic acid signaling pathway, which normally regulates cell growth and differentiation.
By blocking this pathway, the PRAME protein helps cancer cells maintain an undifferentiated state, promoting uncontrolled proliferation and inhibiting the natural processes that would cause the cell to mature or die. This differential expression—present in cancer but absent in most normal cells—provides the scientific basis for using PRAME as a diagnostic marker. The switch from silent to highly active expression is an indicator of malignant change within a cell.
Understanding the Immunostaining Process
The PRAME immunostain is performed using immunohistochemistry (IHC), which translates the presence of the protein into a visible color change. The process begins with a tissue sample, typically a thin slice of a biopsy preserved in formalin and embedded in a paraffin wax block. This preparation maintains tissue structure while allowing for thin sections to be cut for staining.
The next step involves applying a specific antibody that has been engineered to recognize and bind only to the PRAME protein within the tissue. Once the PRAME-specific antibody has attached to the protein, a series of subsequent reactions are performed to make this binding visible. This usually involves adding a secondary antibody linked to an enzyme, which, when exposed to a specific chemical substrate (a chromogen), produces a distinct color.
For PRAME, the color reaction is expected to occur inside the cell nucleus, which is the protein’s usual location. A pathologist then examines the stained slide under a microscope to determine the pattern and extent of the nuclear staining. The result is typically reported based on both the intensity of the color and the percentage of cells that show staining.
A high percentage of cells showing strong, diffuse nuclear staining is generally interpreted as a positive result. Conversely, a negative result means the cells show no staining, or only very weak, isolated staining. Pathologists often use a threshold, such as nuclear staining in \(\geq 75\%\) of the melanocytic cells, to support a diagnosis of malignancy. This quantitative assessment helps standardize the interpretation of the immunostain.
PRAME’s Diagnostic Utility in Skin Pathology
The primary clinical role of the PRAME immunostain is to assist dermatopathologists in resolving diagnostically challenging skin lesions. Many moles (benign melanocytic nevi) can sometimes exhibit microscopic features that overlap with early-stage malignant melanoma, creating ambiguity for the pathologist. PRAME serves as an ancillary marker to help confidently steer the diagnosis toward either a benign or a malignant classification.
In practice, a positive PRAME result, defined by diffuse nuclear staining, strongly supports a diagnosis of melanoma because benign nevi are typically PRAME-negative or show only focal, non-diffuse staining. When a pathologist encounters a lesion with ambiguous features, a strong PRAME signal can provide objective evidence of malignancy, showing high specificity against benign lesions in some studies.
The test is particularly useful for evaluating melanocytic lesions that arise within a pre-existing mole, where distinguishing the evolving malignant component from the underlying benign component can be difficult. In these cases, PRAME can visually highlight the malignant cells that are overexpressing the protein while the surrounding benign nevus cells remain unstained. This clear delineation assists in accurate diagnosis and staging.
PRAME staining is used as an adjunctive test and is often interpreted alongside other standard melanocytic markers, such as SOX10 and Melan-A. These markers confirm the cell type but do not reliably differentiate between benign and malignant status. PRAME is also a tool for identifying metastatic melanoma, helping to distinguish cancer that has spread to lymph nodes from a benign accumulation of mole cells sometimes found in those nodes. Higher PRAME expression is associated with more aggressive tumor behavior, suggesting the marker may hold future potential as an indicator of prognosis.