Molecular Genetics in the Diagnosis and Biology of Lymphoid Neoplasms

2017 Society for Hematopathology/European Association for Haematopathology Workshop Report

Megan S. Lim, MD, PhD; Nathanael G. Bailey, MD; Rebecca L. King, MD; Miguel Piris, MD

Disclosures

Am J Clin Pathol. 2019;152(3):277-301. 

In This Article

Small B-cell Lymphomas

Small/low-grade B-cell lymphoma diagnosis takes benefit from the identification of multiple genetic events that help to define specific lymphoma types and to facilitate routine diagnosis. These molecular events need to be interpreted in the context of the clinical, morphological, and other molecular data, because otherwise they can lead to inappropriate diagnoses. Workshop discussion illustrated this concept and also helped to refine the diagnostic features of some newly defined entities.

Thirteen cases Table 1 raised interesting discussions about the role of specific molecular markers in the recognition of different lymphoma entities. In general, case discussions illustrated the increasing role of specific genetic changes (single-nucleotide variants, copy number variations, translocations) in both disease definition and routine clinical diagnosis.

MYD88 L265P in Small B-cell Lymphomas

Sequencing has identified a recurrent and potentially targetable somatic mutation in MYD88 consisting of a single nucleotide change (T>C) in chromosome 3p22.2 that results in change from leucine to proline (L265P). The mutation results in constitutive activation of MYD88-dependent signaling in the Toll-like receptor pathway, leading to nuclear factor (NF-κB) activation.[1,2] Initially described in activated B-cell subtype diffuse large B-cell lymphoma (ABC DLBCL; 29%) and mucosa-associated lymphoid tissue (MALT) lymphomas (9%), the presence of MYD88 L265P is most strongly associated with lymphoplasmacytic lymphoma (LPL; 90% of cases).[3–5] MYD88 L265P mutation may also play a role in the prediction of sensitivity to BTK inhibitors for DLBCL.[6]

Cases submitted to the workshop examined the role of MYD88 L265P mutation in the diagnosis of marginal zone lymphoma (MZL). Some reports have described isolated MZL cases carrying the MYD88 L265P mutation,[7] particularly in orbital and ocular adnexal MALT/MZL, where the frequency of mutated MYD88 may reach 25%.[8,9] Case 112 Image 1, a conjunctival lymphoma with plasmacytic differentiation and MYD88 L265P mutation, provides a nice example of the clinical presentation and histology consistent with MZL, with plasmacytic differentiation arising in a specific, localized organ site (ocular conjunctiva) and associated with the presence of MYD88 L265P mutation. Additionally, MYD88 L265P may be found in chronic lymphocytic leukemia (CLL), SMZL, and other B-cell lymphomas, where it may be associated with specific clinical and diagnostic features.[10,11] Thus, the MYD88 L265P mutation can be found in varied clinicopathologic contexts and does not necessarily mandate a diagnosis of LPL, which requires the integration of clinical presentation, monoclonal gammopathy (almost always IgM), biphasic lymphoplasmacytoid cytology, and MYD88 L265P mutation.

Image 1.

Case 112. Conjunctival marginal zone lymphoma, mucosa-associated lymphoid tissue-type, with plasmacytic differentiation and MYD88 L265P mutation. A, The neoplastic cells are composed of lymphocytes and lymphoplasmacytoid cells (×400). B, Higher magnification demonstrates focally very prominent Dutcher bodies and numerous atypical plasma cells (×600). C, In situ hybridization studies demonstrate κ light chain restriction (×100). D, In situ hybridization studies for λ light chain are negative (×100).

Follicular Lymphoma With 1p36 Deletion

A variant of t(14;18)-negative nodal diffuse follicular lymphoma (FL) with 1p36 deletion has been proposed in the 2017 World Health Organization (WHO) classification of lymphoid neoplasms.[12] Most of these cases occur in inguinal lymph nodes and show 1p36/TNFRSF14 abnormalities, STAT6 mutation, and CD23 expression, a finding consistent with the data showing that CD23 is a STAT6 target gene, whose expression increases in STAT6 mutated lymphomas.[13,14] However, in isolation none of these findings are specific for this condition and any of them can be seen at different frequencies in classical FL cases. Case 334 Image 2 fulfilled the criteria for this condition, also associated with in situ follicular neoplasia with t(14;18) translocation. Interestingly, the patient has been in complete remission after local radiation therapy for 4 years. Discussion of the case and other published series highlighted the interest in analyzing whether similar cases diagnosed at higher clinical stages also show a favorable outcome.

Image 2.

Case 334. Follicular lymphoma with a diffuse pattern involving inguinal lymph nodes with CD23 positivity. A, H&E section shows an area with follicular lymphoma (diffuse areas), in situ follicular neoplasia-like colonization of germinal centers, and reactive follicles. B, CD23 is expressed in a subset of the neoplastic cells. CD10 (C), and BCL2 (D) demonstrate focal in situ follicular neoplasia. E, Single nucleotide polymorphism array analysis demonstrated loss of heterozygosity in 1p35.1-p36.33 that harbors the TNFRSF14 gene. (A-D, ×40).

Splenic B-cell Lymphomas Distinct From Splenic Marginal Zone Lymphoma

Splenic involvement by lymphoma and diagnostic criteria for splenic B-cell lymphoma were also controversial issues. Splenic diffuse red pulp small B-cell lymphoma (SDRPL) is a relatively rare entity characterized by diffuse infiltration of the splenic red pulp by small B cells that frequently show increased cyclin D3 expression.[15] Case 22 Image 3 fulfills all the criteria for this diagnosis, providing data that illustrate how increased cyclin D3 expression may be the consequence of CCND3 PEST domain mutations.[15]

Image 3.

Case 022. Splenic red pulp small B-cell lymphoma. A, Peripheral blood smear demonstrates atypical lymphocyte with bland nuclear chromatin and irregular membranous projections (×1,200). B, Diffuse infiltration of the red pulp by mature small lymphocytes (×40). C, Higher power examination demonstrate a sinusoidal growth pattern (×400). D, Cyclin D3 highlights both the neoplastic lymphoid cells and the sinusoidal lining cells (×200).

One of the most difficult diagnostic distinctions that needs to be established is between SDRPL and hairy cell leukemia (HCL) variant (HCLv), a condition initially termed the prolymphocytoid variant of HCL. Morphological examination of peripheral blood smears plays a key role in establishing the diagnosis. The terminology of this disorder implies similarity to HCL; however, immunophenotypic and molecular studies show that HCLv is an entirely different disease, with absence of CD25, CD123, annexin A1, cyclin D1, and tartrate-resistant acid phosphatase expression, wild-type BRAF, and resistance to conventional HCL therapy. Case 162 Image 4 provided an excellent illustration of HCLv; a MAPK1 mutation was found in this case, which is characteristic for this disorder.[16] Other cases (cases 198 and 208) raised the issue of the differential diagnostic considerations of B-cell lymphoproliferative processes involving the spleen and the necessity for an integrated approach using clinical, morphological, molecular, and immunophenotypic data.

Image 4.

Case 162. Hairy cell leukemia variant. A, Peripheral blood smear demonstrates atypical intermediate-sized lymphocytes with abundant grey pale cytoplasm with blunt cytoplasmic blebs or projections (×1,000). B, Macroscopic appearance of the spleen which demonstrates numerous infarcts and diffuse infiltration. C, Diffuse involvement of the splenic red and white pulp by intermediate-sized slightly irregular lymphocytes (×400). D, Fluorescence in situ hybridization of the spleen shows TP53 deletion in 62% of cells. Immunohistochemistry on this case demonstrated expression of CD20 without expression of BRAF VE1, CD25, CD123, CD3, CD5, CD23, and BCL6. Proliferation index highlighted by MIB-1 was 5% to 10% (×1,000).

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