MYC / BCL2 Protein Co-Expression (Dual-Expressor Phenotype) in Diffuse Large B-Cell Lymphoma

Introduction and a critical clarification

Among the more frequently misunderstood biomarkers in the diagnostic workup of diffuse large B-cell lymphoma (DLBCL) is the "dual-expressor" phenotype — the simultaneous overexpression of the MYC and BCL2 proteins by immunohistochemistry (IHC). Its name invites confusion with the genetically defined "double-hit" lymphoma, but the two are fundamentally different entities determined by different assays. This article aims to explain what the dual-expressor test measures, how it is performed and scored, what its results mean prognostically, and — importantly — where the boundaries of its interpretation lie.

The single most important point to establish up front: dual-expressor is a protein-expression phenotype detected by IHC, whereas double-hit is a genetic abnormality detected by cytogenetic or molecular methods. They are not synonyms, they are not interchangeable, and conflating them is a common source of diagnostic and reporting error.

What the test measures (brief biology)

MYC is a transcription factor that drives cellular proliferation, metabolism, and growth; its dysregulation is a recurrent theme across aggressive lymphomas. BCL2 is an anti-apoptotic protein that promotes cell survival by blocking programmed cell death. When a lymphoma expresses high levels of both proteins simultaneously, the tumor cells gain both a proliferative drive (MYC) and a survival advantage (BCL2). The evidence suggests this biological combination is associated with more aggressive clinical behavior [1].

Crucially, elevated protein levels can arise through multiple mechanisms — transcriptional upregulation, signaling activation, or, in some cases, underlying genetic rearrangement — but the IHC assay simply reports the amount of protein present, without specifying its cause. A tumor can be a dual-expressor by IHC while lacking any MYC or BCL2 gene rearrangement, and vice versa. The current WHO classification framework treats the genetically defined high-grade B-cell lymphomas with MYC and BCL2 rearrangements as a distinct category, separate from protein co-expression phenotypes [2].

How it is tested

Assay and specimen. Dual-expressor status is assessed by immunohistochemistry performed on formalin-fixed, paraffin-embedded (FFPE) tissue — the standard diagnostic block from a lymph node or extranodal biopsy. Two separate stains are run: one antibody directed against MYC and one against BCL2. Each is scored independently by the pathologist estimating the percentage of tumor cell nuclei (MYC) or tumor cells (BCL2) showing positive staining.

Scoring. In the widely applied scheme, a tumor is classified as a dual-expressor when MYC is expressed in ≥40% of tumor cells AND BCL2 is expressed in ≥50% of tumor cells [1]. Both thresholds must be met. If either falls below its cutoff, the case is designated not dual-expressor.

Preanalytic constraints. Because IHC quantification depends on preserved antigenicity, results are sensitive to preanalytic variables — fixation time and type, tissue handling, and antibody clone and staining protocol. MYC IHC in particular is known to be technically demanding and subject to interobserver variability in nuclear percentage estimation. Consistent laboratory validation and adherence to standardized protocols are therefore important for reproducible scoring.

What each result state means

Dual-expressor. The tumor overexpresses both MYC and BCL2 proteins above the defined thresholds. In DLBCL, this phenotype has been associated with worse outcomes compared with tumors lacking co-expression [1]. It is an adverse prognostic marker — a signal that the disease may behave more aggressively — but it is not, by itself, a distinct WHO disease entity, and it does not establish a genetic diagnosis.

Not dual-expressor. One or both proteins fall below the threshold. This does not confer any favorable "protective" status; it simply means the tumor lacks this particular adverse phenotype. Prognosis in these cases is determined by the full constellation of clinical, pathologic, and molecular features.

It bears repeating: a dual-expressor result does not mean the tumor is double-hit. Confirming or excluding MYC, BCL2, and BCL6 gene rearrangements requires cytogenetic testing (typically fluorescence in situ hybridization), which is a separate investigation from IHC protein staining.

What it determines for treatment eligibility

The dual-expressor phenotype is prognostic, not predictive. It stratifies risk by identifying a subgroup of DLBCL with statistically worse outcomes [1], and this information may contribute to overall risk assessment and clinical decision-making within the framework used by the treating team. It informs prognostic stratification rather than eligibility for a specific targeted drug class.

This article does not provide treatment recommendations. The presence or absence of the dual-expressor phenotype is one data point among many — including stage, performance status, molecular subtype, and genetic findings — that clinicians integrate. No patient should interpret a dual-expressor result as directing any particular therapy; those decisions belong to the treating oncology team.

Caveats and what is evolving

Several important limitations deserve emphasis:

• Cutoff variability. The ≥40% MYC / ≥50% BCL2 thresholds [1] are widely used but represent operationally defined boundaries; the literature contains variation, and cutoffs remain an area of ongoing methodological discussion. A case scoring just above or just below threshold should be interpreted with appropriate caution.
• Interobserver and technical reproducibility. Percentage-based scoring, especially for MYC, is subject to variability between observers and between laboratories.
• The dual-expressor / double-hit distinction. This remains the central conceptual pitfall. Dual-expressor is defined by protein IHC; double-hit (genetically defined high-grade B-cell lymphoma with MYC and BCL2 rearrangements) is defined by cytogenetics and is treated as a distinct entity in current classification [2]. A dual-expressor result cannot substitute for genetic testing, and genetic testing cannot be inferred from IHC.

Understanding the dual-expressor phenotype as a prognostic protein-expression signature — distinct from any genetic lesion — is essential to accurate reporting and to preventing downstream clinical misinterpretation.

References

1. Johnson NA, et al. Dual expression of MYC and BCL2 proteins predicts worse outcomes in diffuse large B-cell lymphoma. Journal of Clinical Oncology. 2015. PMID:26421520.
2. Alaggio R, Amador C, Anagnostopoulos I, et al. The 5th edition of the WHO Classification of Haematolymphoid Tumours: Lymphoid Neoplasms. Leukemia. 2022. doi:10.1038/s41375-022-01620-2.