Mutations of MAP2K1 are frequent in pediatric-type follicular lymphoma and result in ERK pathway activation

Abstract

Pediatric-type follicular lymphoma (PTFL) is a B-cell lymphoma with distinctive clinicopathological features. Recently, recurrent genetic alterations of potential importance for its pathogenesis that disrupt pathways associated with the germinal center reaction (TNFRSF14, IRF8), immune escape (TNFRSF14), and anti-apoptosis (MAP2K1) have been described. In an attempt to shed more light onto the pathogenesis of PTFL, an integrative analysis of these mutations was undertaken in a large cohort of 43 cases previously characterized by targeted next-generation sequencing and copy number array. Mutations in MAP2K1 were found in 49% (20/41) of the cases, second in frequency to TNFRSF14 alterations (22/41; 54%), and all together were present in 81% of the cases. Immunohistochemical analysis of the MAP2K1 downstream target extracellular signal-regulated kinase demonstrated its phosphorylation in the evaluable cases and revealed a good correlation with the allelic frequency of the MAP2K1 mutation. The IRF8 p.K66R mutation was present in 15% (6/39) of the cases and was concomitant with TNFRSF14 mutations in 4 cases. This hot spot seems to be highly characteristic for PTFL. In conclusion, TNFRSF14 and MAP2K1 mutations are the most frequent genetic alterations found in PTFL and occur independently in most cases, suggesting that both mutations might play an important role in PTFL lymphomagenesis.

Figure 1.

Distribution of MAP2K1 mutations at protein and exon level in PTFL in comparison with other hematological neoplasias. (A) Schematic diagram of MAP2K1 mutations in PTFL,^, Langerhans cell histiocytosis (LCH),^- hairy cell leukemia^, (including HCLv and conventional HCL [HCLc] with IGHV4-34⁺), splenic diffuse red pulp small B-cell lymphoma (SDRPL), splenic marginal zone lymphoma (SMZL),^, CLL,^, according to NGS studies and/or Sanger analysis. Exons are represented by boxes on the body of MEK1 protein and the main protein domains are represented by larger colored boxes. AL, activation loop; DD, docking domain for ERK1 and ERK2; DVD, domain of versatile docking (MAP3K docking domain); NES, nuclear export sequence; NRR, negative regulatory region; PRD: proline-rich domain. (B) Immunohistochemical analysis of pERK in MAP2K1 mutated and wild-type PTFL cases. Note that the variant allelic frequency (VAF, indicated in parentheses) of MAP2K1 mutations by NGS analysis correlates with the amount of pERK-positive cells.

Figure 2.

IRF8 mutations and global mutational landscape in PTFL. (A) Sequence electropherograms from cases PTFL6, PTFL18, PTFL21, and PTFL40 showing IRF8 p.K66R (c.197A>G) mutation by Sanger sequencing. PTFL18 carried a concomitant 16q11.2-q24.3 CNN-LOH, including an IRF8 gene. (B) Schematic diagram of IRF8 mutations in FL,^- DLBCL,^, and PTFL,^, according to NGS studies. Exons are represented by boxes on the body of the protein and the main protein domains are represented by larger colored boxes. Domains of the protein are represented according to Uniprot database (www.uniprot.org). (C) Overview of the global mutational landscape in 43 PTFL cases. Each column of the heat map represents 1 PTFL case and each line 1 specific analysis. On the right side of the figure, the frequency of the particular result of the analysis is shown.