Myeloid Sarcoma of the Testis in Children: Clinicopathologic and Immunohistochemical Characteristics With KMT2A (MLL) Gene Rearrangement Correlation.

Myeloid sarcoma (MS) is defined as an extramedullary mass-forming lesion composed of immature myeloid cells. It is a rare but well-known manifestation of acute myeloid leukemia. Pediatrics testicular MS may pose a possible diagnostic challenge, an issue that is underscored in the few testicular pediatric MS cases reported in the literature. Herein, we report a series of 5 cases of pediatric testicular MS that are evaluated at the morphologic and immunohistochemical levels with correlation with the KMT2A (MLL) rearrangement status. Three patients presented with no prior history of acute myeloid leukemia. All 5 cases showed monoblastic morphology; positive for CD33, CD43, CD68, CD163, CD4 (dim), and lysozyme; and negative for CD10, CD34, CD117, and myeloperoxidase. KMT2A (MLL) rearrangement was detected in 4 of the 5 cases. In the literature, 8 more cases of pediatric testicular lymphoma were reported. Most of them showed monocytic differentiation and KMT2A (MLL) rearrangement was reported in 3 of the cases. In conclusions, testicular MS in pediatric patients shows monoblastic differentiation which may be attributed to the KMT2A (MLL) rearrangement. We also highlight the importance of using an extended immunohistochemistry panel in the diagnosis of MS.

MEF2B is a member of the BCL6 gene transcriptional complex and induces its expression in diffuse large B-cell lymphoma of the germinal center B-cell-like type.

Myocyte enhancer-binding factor 2B (MEF2B) has been implicated as a transcriptional regulator for BCL6. However, details about the interaction between MEF2B and BCL6 during expression, as well as the relationship of MEF2B to the expression of other germinal center (GC) markers, have not yet been fully explained. Using germinal center B-cell-like diffuse large B-cell lymphoma (GC-DLBCL) and activated B-cell diffuse large B-cell lymphoma (ABC-DLBCL) cell lines, we analyzed the expression of MEF2B and its associations with BCL6, CD10, and ERK. Furthermore, small interfering RNA (siRNA) was used to study the possible effects of MEF2B knockdown on these proteins and cell growth. Analysis of the BCL6 transcriptional complex was performed using electrophoretic mobility shift assay. The correlation between MEF2B expression and the genetic type of DLBCL was assessed using immunohistochemistry on 111 patient samples, and via in silico analysis of publicly available microarray (Gene Expression Omnibus (GEO)) datasets. Our results indicate that the expression of MEF2B protein is important for the growth of GC-DLBCL cells, as evidenced by MEF2B knockdown inhibition of cell growth and the subsequent suppression of BCL6, CD10, and ERK phosphorylation. Analysis of BCL6 transcription factors in nuclear extracts of MEF2-expressing DLBCL cells showed involvement of MEF2B with AP-2α and BCL6 proteins in the formation of the BCL6 gene transcriptional complex. Indeed, differential expression of MEF2B in the GC-DLBCL is statistically significant compared to the ABC-DLBCL in the GEO datasets, as well as in tissue microarray, as indicated via immunohistochemistry (Visco-Young algorithm). Our findings indicate that MEF2B is an essential component of the BCL6 gene transcriptional complex for the regulation of DLBCL growth via the promotion of BCL6 expression. Beyond its regulatory role in DLBCL growth, MEF2B expression correlated positively with BCL6 and CD10 expression, and was preferentially expressed in the GBC-DLBCL group.