Diffuse large B-cell lymphoma (DLBCL)

The cytomorphology and especially the histology are indicative for guiding the downstream diagnostics. In addition, blood and bone marrow smears can be used to assess whether there is lymphoma washout (rare with this lymphoma entity).

The neoplastic cells express CD 45 and pan-B cell markers (CD19+, CD20+, CD22+, CD79a+, PAX5+), but in some circumstances one or more of these markers may be absent. The majority of DLBCL, NOS cases show expression of membrane-bound and cytosolic immunoglobulin (mostly IgM, less commonly IgG and IgA). The antigen CD10 is expressed in 30-50% of cases. The marker CD138 is typically negative, and CD30 is positive in only 10-20%. In EBV+ DLBCL, this marker is usually positive. Only 5-10% of DLBCL, NOS are positive for CD5, there is a strong association with de novo DLBCL (WHO 2022). Since lymphoma washout is rare in DLBCL, immunohistochemistry on lymph node biopsy is particularly useful to determine the immunophenotype.

Chromosomal analysis can detect rearrangements, gains/amplifications and losses. Chromosomal alterations are observed in the vast majority (87% and 89.1%, respectively) (Cigudosa et al. 1999, Zhao et al. 2013). In 92.8% of cases, the chromosomal aberrations are present as part of a complex karyotype (Zhao et al. 2013). While there is generally high genetic heterogeneity in the disease and the genetic landscape also differs, particularly between COO subtypes, there are genetic mechanisms that contribute significantly to pathogenesis. These include, in particular, deregulation of the BCL6 (3q27), BCL2 (18q21), and MYC (8q24) genes. MYC rearrangements are found in both the GCB and ABC subtypes, while BCL6 rearrangements occur more frequently in the ABC subtype (30%) than in GCB-DLBCL (10%). BCL2 rearrangements, on the other hand, are more common in ABC subtype (30%) than in GCB-DLBCL (10%) (WHO 2022).

In DLBCL, FISH is suitable to detect rearrangements, especially at interphase nuclei. Of particular diagnostic and prognostic relevance is the exclusion of DLBCL/HGBL with MYC and BCL2 rearrangements (WHO 2022). For this purpose, the probe against IGH::BCL2 as well as the partner gene independent probe against MYC are used. In addition, the FISH panel includes a probe against BCL6 as well as probes for del(13q14) (DLEU) and del(17p13)(TP53). Additional probes may also be used depending on the results of chromosome analysis to confirm other typical aberrations (e.g. del(6q21~23)(SEC63/MYB), del(11q22)(ATM), +12). Supporting FISH can be used to clarify or validate complicated chromosomal aberrations by chromosome painting or 24-color FISH in addition to classical metaphase analysis.

Prognostic panel: BCL2, CD79B, FOXO1, KLHL6, MYD88, NOTCH1, SGK1, SOCS1, STAT3, STAT6, TP53

The analysis of the prognostic effect of individual mutations in DLBCL is complex, not least because mutation frequencies are highly variable depending on the cohort. Accordingly, the prognostic relevance has not been fully elucidated for any of the mutations listed here and is subject to further debate (Lopez-Santillan et al. 2021).

MYD88 and CD79B mutations are strongly associated with the ABC subtype (MYD88: 35%, CD79B: 20-25%), but can also occur in rare cases in GCB-DLBCL (Swerdlow et al. 2017). In this context, the MYD88 L265P mutation is almost exclusive to the ABC subtype (Karube et al. 2018, Pasqualucci & Dalla-Favera 2018). There is evidence that, among others, MYD88 and TP53 mutations are prognostically unfavorable (Ren et al. 2024). For CD79B mutations, one study also observed an association with worsened prognosis (Reddy et al. 2017). MYD88 and CD79B mutations are also characteristic of a genetic signature that was found across studies and was associated with an unfavorable prognosis in the majority of studies (see Figure 1).

BCL2 translocations, as well as mutations, show a strong association with the GCB subtype. An association between BCL2 mutations and reduced progression-free survival was observed in a phase III study (Bolen et al. 2020). In contrast, the cluster associated with BCL2 alterations and EZH2-mutations-associated GCB is considered to be basically prognostically favorable (see Figure 1).

TP53 mutations are detectable in 20-25% of DLBCL patients (Swerdlow et al. 2017) and represent a prognostic risk factor (Young et al. 2008, Xu-Monette et al. 2012, Cao et al. 2016, Lacy et al. 2020, Landsburg et al. 2023). NOTCH1 mutations occur in circa 3% of cases, and this alteration also appears to be prognostically unfavorable (Karube et al. 2018, Schmitz et al. 2018, Lacy et al. 2020). There is also evidence of unfavorable prognostic significance for mutations of the KLHL6 gene (Karube et al. 2018) and the transcription factor FOXO1 (Trinh et al. 2013).

In contrast, the data situation is contradictory for SGK1. While SGK1 mutations had a prognostic negative impact in one study (Karube et al. 2018), two other studies found prognosis improved with SGK1 mutation (Reddy et al. 2017, Chapuy et al. 2018). SGK1 mutations could possibly be assigned to different signatures depending on the genetic context. Characteristic and cluster-defining for this would be SOCS1 or TET2 co-mutations, according to Lacy et al. In each case, there is an association with the GCB subtype and a favorable prognosis (see Figure 1). There is also evidence that SOCS1 mutations may also represent an independent prognostic favorable factor for progression-free survival (Juskevicius et al. 2017). SOCS1, like STAT3 and STAT6, are factors of the JAK-STAT pathway. Alterations in this pathway are associated with favorable prognosis (Karube et al. 2018).