Molecular, cytogenetic, and immunophenotypic characterization of follicular lymphoma grade 3B; a separate entity or part of the spectrum of diffuse large B-cell lymphoma or follicular lymphoma?

Summary

We studied a histological homogeneous group of 29 cases with the diagnosis of follicular lymphoma (FL) grade 3B (FL3Bs). In a previous study, we subdivided this group in 3 subgroups based on (1) aberrations of the 3q27 region, (2) lack of 3q27 and t(14;18), and (3) the presence of a t(14;18). In this study, we further characterized the FL3B lymphomas that are currently part of the spectrum of FL in the WHO classification, taking into account other cytogenetical aberrations, immunohistochemistry for P53, bcl2, bcl6, and CD10, rearrangement of the proto-oncogene myc, and mutation of the tumor suppressor gene TP53. With respect to P53, bcl2, bcl6 expression, myc rearrangement, and TP53 mutation, FL3B represents a homogeneous group. CD10 expression and gain of chromosome 7, considered to be typical FL markers, were more common in the FL3B t(14;18)-positive subgroup. The lack of CD10 expression and gain of chromosome 7 in most cases in the other 2 subgroups suggest that those cases have a closer relation to diffuse large B-cell lymphomas.

Introduction

Many non-Hodgkin's lymphomas (NHLs) are characterized by clonal chromosomal abnormalities [1]. Molecular characterization of the affected breakpoint regions has led to the identification of target genes mapping at or close to the breakpoint regions [2], [3]. Several of these clonal chromosomal abnormalities have now been shown to be specific for different NHL subgroups and can be used to discriminate between different NHL subtypes. Follicular lymphomas (FL) are characterized by a translocation t(14;18) affecting the bcl-2 gene [4] and Burkitt's lymphomas by a t(8;14) involving the myc gene [5]. Translocations involving the bcl-6 gene, located on chromosome 3q27, are frequently seen in diffuse large B-cell lymphomas (DLBCLs) [6]. Several other chromosomal abnormalities have been associated with other entities. Frequently, however, these abnormalities are not as specific as initially described.

Cytogenetic studies indicate that FLs that undergo histologic transformation retain the t(14;18) translocation and generally acquire multiple, often complex secondary chromosomal abnormalities [7], [8]. Until now, no predominant genetic locus is suggested to be responsible for this histologic transformation. Follicular lymphoma grade 3B (FL3B) can be distinguished from DLBCL by the presence of at least a focal follicular pattern. In contrast to FL grade 1 or 2, the predominant cell type of FL3B is the centroblast, which is also the typical cytologic feature of DLBCL.

We previously demonstrated that 3 distinct subsets can be identified among cases with the histologic diagnosis FL3B [9]: (1) with the presence of a breakpoint 3q27/bcl-6 rearrangement without t(14;18), (2) with the presence of cytogenetic aberrations without 3q27/bcl-6 rearrangements and without a translocation t(14;18), and (3) with the presence of a translocation t(14;18) without 3q27/bcl-6 rearrangements. Based on these findings, a conservative conclusion may be that FL3B with a translocation t(14;18)/bcl-2 rearrangement are part of the same entity as the other FL grades (1, 2, or 3A). The cases with 3q27/bcl-6 rearrangements or other unrelated translocations are potentially more closely related to most DLBCLs. The cases with rearrangements affecting 3q27, an aberration also frequently observed in DLBCL, was studied in further detail to discriminate between the alternative breakpoint cluster region (ABR) and the major breakpoint region (MBR), mapped into the first noncoding exon of the bcl-6 gene. In most FL3B cases, we detected a breakpoint in the ABR [10], whereas a breakpoint in the MBR was commonly observed in a control group of DLBCL cases. Although the presence of 3q27 aberrations suggests a similar pathogenesis for this group of FL3B and DLBCL, the consistent difference observed in BCL6 breakpoint regions might suggest an alternative pathogenesis for the FL3B cases.

These observations led us to further examine FL3B cases to assess the presence of other genetic and immunophenotypic characteristics that might allow discrimination of cases related closer to either FL1, FL2, FL3A, or DLBCL. We selected characteristics that are thought to play an important role in the histologic transformation from low-grade to high-grade NHL or markers considered to be typical for FLs or DLBCL. We studied TP53 mutation analysis; presence of myc rearrangements; and bcl2, bcl6, CD10, and P53 expression. Mutation of the tumor suppressor gene TP53 frequently associates with cases resulting from the histologic transformation of FL [11], [12] and appears in low percentages in de novo DLBCL [13]. The human TP53 gene encodes a 393 amino-acid-residue long nuclear phosphoprotein, P53, which is involved in several processes such as programed cell death or apoptosis, inhibition of tumor growth, maintenance of genome integrity or stability, and cell cycle arrest [14]. The nuclear phosphoprotein myc, located on chromosome 8q24, has been implicated as a potent regulator of cellular proliferation and may also play an important role in the transformation process of FL [15], [16]. Most precursor B cells as well as a subset of follicular center cells express CD10 antigen, whereas other mature B cells, plasma cells, and B-cell lymphomas only occasionally do [17], [18]. The immunophenotype of the malignant lymphocytes in FL suggests that they are derived from germinal center cells, and mostly, they do express CD10, although some grade III FLs are negative [19].

We report the further molecular cytogenetic and immunophenotypic characterization of a group of 29 patients with the diagnosis FL3B [9] with single or combined expression or abnormalities of P53, CD10, bcl-2, bcl-6, myc rearrangement, and TP53 mutation.