A unifying hypothesis for PNMZL and PTFL: morphological variants with a common molecular profile

Pediatric nodal marginal zone lymphoma (PNMZL) is an uncommon B-cell neoplasm affecting mainly male children and young adults. This indolent lymphoma has distinct characteristics that differ from those of conventional nodal marginal zone lymphoma (NMZL). Clinically, it exhibits overlapping features with pediatric-type follicular lymphoma (PTFL). To explore the differences between PNMZL and adult NMZL and its relationship to PTFL, a series of 45 PNMZL cases were characterized morphologically and genetically by using an integrated approach; this approach included whole-exome sequencing in a subset of cases, targeted next-generation sequencing, and copy number and DNA methylation arrays. Fourteen cases (31%) were diagnosed as PNMZL, and 31 cases (69%) showed overlapping histologic features between PNMZL and PTFL, including a minor component of residual serpiginous germinal centers reminiscent of PTFL and a dominant interfollicular B-cell component characteristic of PNMZL. All cases displayed low genomic complexity (1.2 alterations per case) with recurrent 1p36/TNFRSF14 copy number-neutral loss of heterozygosity alterations and copy number loss (11%). Similar to PTFL, the most frequently mutated genes in PNMZL were MAP2K1 (42%), TNFRSF14 (36%), and IRF8 (34%). DNA methylation analysis revealed no major differences between PTFL and PNMZL. Genetic alterations typically seen in conventional NMZL were absent in PNMZL. In summary, overlapping clinical, morphologic, and molecular findings (including low genetic complexity; recurrent alterations in MAP2K1, TNFRSF14, and IRF8; and similar methylation profiles) indicate that PNMZL and PTFL are likely part of a single disease with variation in the histologic spectrum. The term "pediatric-type follicular lymphoma with and without marginal zone differentiation" is suggested.