The B-Cell Receptor of Primary Cutaneous Follicle Center Lymphoma: Implications for Pathogenesis

Introduction & Objectives: Primary Cutaneous Follicle Center Lymphoma (PCFCL) is a very indolent mature B-cell lymphoma that shares germinal center (GC) morphology with follicular lymphoma (FL) but lacks the characteristic t(14;18). Unlike FL, immunohistochemistry fails to detect expression of BCL2, CD10, and immunoglobulin in PCFCL. We investigated the B-cell receptor (BCR) to gain insight into the immunobiology of PCFCL.

Materials & Methods: Whole Genome Sequencing (WGS) and RNAseq were performed on five PCFCL biopsies. Full-length heavy and light chain BCR transcripts were amplified by unbiased ARTISAN PCR (Koning et al., BJH 2016). More than 2000 sequences per BCR transcript were sequenced as full-length single molecules on the PacBio next-generation sequencing platform.

Results: In addition to various minor translocations and deletions, WGSidentified a t(14;22) that resulted in juxtaposition of IGH and IGLL5 in one PCFCL case. No PCFCL case carried a t(14;18). Despite the absence of detectable surface Ig by immunohistochemistry, ARTISAN PCR and RNAseq-based de novo BCR assembly independently demonstrated expression of functional VDJ and VJ genes with heavily mutated V regions (VDJ: 7,1-16,0%; VJ: 4,6-11,1%) in all cases. Lack of intraclonal sequence variation indicated absence of ongoing somatic hypermutation (SHM). The t(14;22)-carrying PCFCL expressed an inconspicuous IgM. BCR of all remaining four PCFCL carried sequence motifs for N-linked glycosylation in antigen-binding regions that were apparently acquired by SHM. Three cases had undergone class switch recombination to IgG. The remaining case expressed IgM with extensive mutations. An L265P mutation in MYD88 was identified in one case, and two cases carried amplifications in chromosome 2 involving the proto-oncogene REL.

Conclusions: GC morphology, extensive SHM, and class switch recombination indicate a shared origin of GC B cells for both PCFCL and FL. Clonal BCR sequences and previously identified copy number alterations prove that PCFCL represents a neoplastic clonal expansion. However, lack of ongoing SHM indicates that the immune follicles of PCFCL are not fully functional germinal centers. Since ongoing SHM is thought to contribute to lymphomagenesis by targeting non-BCR loci, absence of both ongoing SHM and the t(14;18) may explain the relatively benign clinical course of PCFCL compared to FL. As previously described for FL, continuous BCR stimulation through glycosylation-mediated binding of lectins on resident cells of the follicular microenvironment may cause clonal expansion of PCFCL cells and could play a decisive role in maintaining the follicular microarchitecture in both FL and PCFCL. In comparison to this BCR glycosylation, acquired somatic alterations in oncogenes that recurrently involved in other types of indolent B-cell lymphomas may constitute secondary driver events. Further comparisons to define the extent of malignant transformation between PCFCL, FL, and other B-lymphomas are warranted.