Genomic landscape of testicular follicular lymphoma is characterized by frequent 1p36/TNFRSF14 alterations

TO THE EDITOR:

Testicular follicular lymphoma (T-FL) is a very rare and clinically indolent lymphoma occurring in children and young adults. It is included with classic FL in the current fifth edition of the World Health Organization classification, whereas it is considered a separate, distinct entity in the 2022 International Consensus Classification.^1,2 

Less than 30 cases of T-FL have been previously reported with a median age of occurrence at 5 years (range, 3-69), with most patients (93%) presenting in clinical stage IE (ie, lymphoma localized to testis). By immunohistochemistry, the majority of tested cases were positive for CD10 and B-cell lymphoma 6 (BCL6), and all were negative for BCL2^3-12 (supplemental Table 1).

Data on the genetic landscape of T-FL in the literature is scarce. By fluorescence in situ hybridization (FISH) studies, all tested cases were negative for t(14;18);IGH::BCL2; of 10 cases tested for BCL6 rearrangement, 1 was positive for rearrangement, and another showed extra copies of BCL6^3-5,7,11 (supplemental Table 1). Results of sequencing studies were previously reported in only 3 T-FL cases.¹² The aim of this study was to evaluate the clinical and pathologic characteristics of T-FL and to understand the genomic landscape of this rare lymphoma.

Eight cases (from 7 patients) with available formalin-fixed, paraffin-embedded (FFPE) biopsy material and clinical findings were contributed to the study by coauthors and the diagnosis was confirmed by at least 2 hematopathologists. Interphase FISH studies to assess for IRF4, BCL2, and BCL6 rearrangements and for 1p36 deletion were performed (supplemental Material). For whole exome sequencing, DNA was extracted from FFPE tissue sections using the Qiagen DNA FFPET kit (Qiagen, Valencia, CA) per the manufacturer’s recommended protocol. Library preparation and DNA sequencing were performed on the NovaSeq 6000 (supplemental Material). Additionally, Sanger sequencing, using probes for exon 2 of MAP2K1 gene, was performed (supplemental Material). The study was approved by the institutional review boards of the University of Michigan and City of Hope and was conducted according to the Declaration of Helsinki.

Clinicopathologic features of 7 patients are summarized in Table 1. The median age was 24 years (range, 4-38), and all presented with disease localized to the testis. Six of the 7 patients were treated with orchiectomy without adjuvant therapy, whereas 1 patient (case 1) received adjuvant chemoimmunotherapy and radiation to the contralateral testis. One patient (case 6) relapsed in the contralateral epididymis after 2 years and was treated with testicle-sparing surgery and radiation. After a median follow-up of 26.5 months, 6 patients were alive with no evidence of disease, and 1 patient was lost to follow-up. Treatment information was available on 27 previously published patients. Among them, only 5 underwent orchiectomy alone, whereas the remainder received various forms of adjuvant chemotherapy or chemoimmunotherapy, with 2 of them also receiving radiation. The median follow-up of previously published patients was 27.5 months, and all were in complete remission, similar to our cohort (supplemental Table 1).^3-12 Our data confirm that T-FL is an indolent disease with excellent prognosis. Although our data suggest that a watch-and-wait approach after orchiectomy might be the preferred management in these patients, this is a small cohort, and the median follow-up of patients is relatively short. Larger studies with longer clinical follow-up are needed to establish the optimal management of this rare lymphoma.

All cases in our cohort showed similar morphologic features (supplemental Figures 4 and 5) with effacement of testicular parenchyma by a nodular infiltrate, with histologic features compatible with grade 3A FL. Of the 8 cases analyzed, 5 were positive for CD10, all were positive for BCL6, and none showed BCL2 expression by immunohistochemistry (supplemental Table 2). By FISH studies, all tested cases were negative for rearrangements of IRF4, BCL2, and BCL6 (Figure 1A).

Abnormalities of the TNFRSF14 gene were the most common genetic alteration in our cohort, found in 6 of 8 samples (75%; Figure 1A; supplemental Table 3). Mutations were found in 4 cases, whereas the deletion of 1p36 (site of TNFRSF14) was detected by FISH in 3 samples. Although genetic alteration in TNFRSF14 is an important regulator of immune system development and function, it is relatively common across all types of FL, more common in t(14;18)-negative FL,¹³ and frequently seen in pediatric-type FL (PTFL; TNFRSF14 mutations in ∼50% of cases).^14-17 

PTFL and T-FL share some clinicopathologic features and mutation profiles. Clinically, they both occur in children and young adults, present with limited-stage disease, and have an indolent clinical behavior even without adjuvant treatment. TNFRSF14 mutations in our study were found in the first 3 exons, including the signal peptide and the tumor necrosis factor receptor (TNFR)-Cys 1 domain (Figure 1B), as previously reported in pediatric and adult studies.^15,16,18 Furthermore, consistent with a previous study, frameshift insertion and nonsense mutations that significantly alter protein structure were found in the signal peptide, whereas only the missense mutations resulting in single nucleotide change were found in the TNFR-Cys 1 domain.¹⁶ In the 3 cases in our study that showed 1p36 deletion by FISH, 1 case (case 3) demonstrated both a TNFRSF14 mutation and deletion, whereas we were unable to detect a TNFRSF14 mutation in the other 2 cases. These findings, reported by others in PTFL and adult FL cohorts, suggest that in mutation-negative cases, there are alternative mechanisms of TNFRSF14 inactivation.^16,18 Notably, we did not identify abnormalities in the MAP2K1 gene in our series of cases, another common mutation in PTFL (40%-50% of cases).

Mutations in IRF8, another immune system regulator and particularly of B-cell differentiation, are found in ∼30% of PTFL cases, frequently co-occurring with TNFRSF14 mutations.^13,17,19,20 In our study, we found IRF8 mutation in 2 cases, with 1 case also showing a TNFRSF14 mutation (Figure 1A). Both mutations were in the DNA-binding domain (Figure 1B), and one was in the hot spot K66R, which seems to be specific for PTFL.^17,19 

In our cohort, we observed frequent mutations (75%) in epigenetic regulators (KMT2C and/or EZH2; Figure 1A), which is also commonly seen in t(14;18)-positive classic FL but uncommon in PTFL.²¹ However, the mutational landscapes of other subtypes of t(14;18)-negative FL, such as nodal BCL2-negative FL and STAT6-mutated BCL2-negative FL, also show frequent mutations is epigenetic regulators.²² Our findings are similar to 2 previously sequenced cases of T-FL, which showed a similar mutational profile, with mutations in EZH2 and KMT2D, respectively, along with IRF8 mutation in 1 case. The third case in that study showed TNFRSF14 mutation detected by Sanger sequencing.¹² Supplemental Table 4 compares relative frequencies of recurrently mutated genes in different types of pediatric and adult FL.

In 1 patient, we performed the genetic analyses in 2 specimens: initial and relapse. This is a unique patient who had a local relapse in the contralateral epididymis; no similar case has been published. Interestingly, the mutational profiles of the initial (case 6A) and relapse (case 6B) specimens did not significantly overlap (Figure 1A; supplemental Table 3), which may suggest divergent evolution.

In summary, we report in this study the clinical and pathologic characteristics and genomic landscape in the largest cohort, to our knowledge, of T-FL to date. This very rare t(14;18)-negative lymphoma of children and young adults presents with limited clinical stage and shows a remarkably indolent clinical course. We did not observe significant differences in clinical presentation and pathologic and genetic features between prepubertal patients and young adults. Alterations in the TNFRSF14 gene are the most common genetic abnormality in this lymphoma, but mutations in epigenetic regulators are also frequently encountered. Although T-FL shows some clinicopathologic similarities to PTFL, it notably lacks MAP2K1 mutations and may be best classified as a separate entity.

Acknowledgments: This study was supported by the Toni Stephenson Lymphoma Foundation and funds from the Department of Pathology at City of Hope National Medical Center.

Contribution: W.Z., A.M.P., and J.Y.S. contributed to conception and design; W.Z., A.M.P., J.H., V.B., and J.Y.S. contributed to data analysis; W.Z., A.M.P., and J.Y.S. prepared the manuscript; and all authors contributed to collection and assembly of data and reviewed and approved the manuscript.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Joo Y. Song, Department of Pathology, City of Hope National Medical Center, 1500 E Duarte Rd, Duarte, CA 91010; email: josong@coh.org; and Anamarija M. Perry, Department of Pathology, University of Michigan, NCRC Bldg 35, 2800 Plymouth Rd, Ann Arbor, MI 48109; email: anaperry@med.umich.edu.