CARD9 Forms an Alternative CBM Complex in Richter Syndrome.

Julia Maier,André Lechel,Thomas F E Barth,Kevin Mellert,Peter Möller,Ralf Marienfeld

doi:10.3390/cancers14030531

Abstract

Simple SummaryThe transformation process of chronic lymphocytic leukemia into an aggressive lymphoma, called Richter syndrome (RS), is incompletely understood, and therapeutic options are limited. Here, we report CARD9 to be expressed in a subset of RS tissue specimen and in the first and only available RS cell line, U-RT1. In U-RT1, CARD9 attaches to BCL10 and MALT1, and knockdown of CARD9 leads to a significant reduction in cell viability. We hypothesized that CARD9 plays an oncogenic role in RS through the activation of NF-κB signaling. Our findings may help to extend the current knowledge about the pathogenesis of RS and promote the development of targeted therapies for this aggressive disease.Richter syndrome (RS) is defined as the transformation of chronic lymphocytic leukemia (CLL) into an aggressive lymphoma, mostly diffuse large B-cell lymphoma (DLBCL). Despite intensive therapy, patients with RS have an unfavorable clinical outcome. The detailed pathobiology of Richter transformation still needs to be elucidated. Here, we report high mRNA and protein levels of CARD9 in the RS cell line U-RT1. Co-immunoprecipitation revealed the assembly of a CBM complex using CARD9 instead of CARD11. CARD9 is known to be an activator of NF-кB signaling in myeloid cells. U-RT1 Western blot analyses showed phosphorylation of IκB as well as IKK, indicating a constitutively active canonical NF-кB pathway. This was further supported by the significant reduction in cell viability and CYLD cleavage products after CARD9 siRNA knockdown. We also showed immunostaining for CARD9 in 53% of cases analyzed in a series of RS tissue specimens, whereas other lymphomas rarely show CARD9 expression. This is the first report on ectopic expression and function of CARD9 in an aggressive B-cell lymphoma. Our findings suggest that CARD9 may contribute to the pathogenesis of RS.

Highlights

Chronic lymphocytic leukemia is an indolent B-cell neoplasm that, in 5–10% of cases, will undergo malignant transformation into an aggressive B-cell lymphoma, most commonly a diffuse large B-cell lymphoma [1,2], termed Richter syndrome (RS) [3]
Knockdown of CARD9 significantly reduced U-RT1 cell viability and the quantity of CYLD cleavage products as an indicator of impaired MALT paracaspase activity, which is essential for NF-кB pathway activation. These results underlined the relevance of CARD9 in promoting the survival of U-RT1 cells, possibly via NF-кB activation
The size of our RS cohort was moderate, and further studies with more cases will be required, our results suggested that the expression of CARD9 is neither a feature of chronic lymphocytic leukemia (CLL), de novo diffuse large B-cell lymphoma (DLBCL) or classical Hodgkin lymphoma (cHL), nor does it represent a general event in the transformation of indolent lymphoma to aggressive lymphoma

Summary

Introduction

Chronic lymphocytic leukemia is an indolent B-cell neoplasm that, in 5–10% of cases, will undergo malignant transformation into an aggressive B-cell lymphoma, most commonly a diffuse large B-cell lymphoma [1,2], termed Richter syndrome (RS) [3]. Diagnosis requires a histologic shift to DLBCL that must be distinguished from an “acceleration” of CLL with expansion of proliferation centers [4]. Besides the DLBCL variant, a classical Hodgkin lymphoma (cHL) variant of RS can occur, but unusual variants such as plasmablastic lymphoma and high-grade T-cell lymphoma have been described [5,6]. RS DLBCL cells invariably express CD19, but tend to lose the expression of CD5 and CD23, leading to a morphologic and immune phenotype different from the underlying CLL [7]. Using the Hans algorithm [8], 90–95% of RS DLBCL are identified as the more aggressive activated B-cell subtype (ABC DLBCL) [1,7,9]. A clonal relationship to the preceding CLL has been shown to be an important unfavorable prognostic factor [9]

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