Genetic evidence implies that primary and relapsed tumors arise from common precursor cells in primary central nervous system lymphoma

Abstract

Primary central nervous system lymphoma (PCNSL) is a rare subtype of lymphoma that arises within the brain or the eyes. PCNSL recurs within the central nervous system (CNS) in most relapsed cases, whereas extra-CNS relapse is experienced in rare cases. The present study aimed at identifying the presence of common precursor cells (CPC) for primary intra- and relapsed extra-CNS tumors, and further assessing the initiating events in bone marrow (BM). Targeted deep sequencing was carried out for five paired primary intra- and relapsed extra-CNS tumors of PCNSL. Two to five mutations were shared by each pair of intra- and extra-CNS tumors. In particular, MYD88 mutations, L265P in three and P258L in one, were shared by four pairs. Unique somatic mutations were observed in all five intra-CNS tumors and in four out of five extra-CNS tumors. Remarkably, IgH clones in the intra- and the extra-CNS tumors in two pairs were distinct from each other, whereas one pair of tumors shared identical monoclonal IgH rearrangement. In a cohort of 23 PCNSL patients, L265P MYD88 mutations were examined in tumor-free BM mononuclear cells (MNC) in which the PCNSL tumors had L265P MYD88 mutations. L265P MYD88 mutations were detected by a droplet digital PCR method in nine out of 23 bone marrow mononuclear cells. These results suggest that intra- and extra-tumors are derived from CPC with MYD88 mutations in most PCNSL, arising either before or after IgH rearrangement. The initiating MYD88 mutations may occur during B-cell differentiation in BM.

Keywords: IgH rearrangement; L265P MYD88; bone marrow; common precursor cell; primary central nervous system lymphoma.

Figure 1

Recurrently mutated genes with primary central nervous system lymphoma (PCNSL). Comparison of mutated genes between primary or relapsed extra‐central nervous system tumors of PCNSL. Numbers of mutations are indicated when more than one mutation was detected in the genes

Figure 2

Number of somatic variants in each primary central nervous system (CNS) lymphoma tumor sample. Targeted deep sequencing for 39 genes was carried out for five paired tumors. Number of mutations found in both tumors (shared mutations), those specific to primary intra‐CNS tumors, and those specific to extra‐CNS tumors is indicated

Figure 3

Overlaps in genes and somatic mutations discovered in intra‐ and extra‐central nervous system (CNS) tumors. Venn diagram shows the comparison of genes and mutations between five intra‐ and extra‐CNS tumor samples (A) Mutated genes shared between intra‐ and extra‐CNS tumors were shown in overlap of the venn diagram. Detail of shared mutated genes were written under “Common genes”. (B) Somatic mutations shared between intra‐ and extra‐CNS tumors were shown in overlap of the venn diagram. Detail of shared mutations were written under “Common mutations”.

Figure 4

IgH rearrangement status of paired tumors in patient T1. (A) Photographs of PCR gels and (B) the electropherogram of direct sequencing show the IgH rearrangement status of paired tumors in patient T1. CNS, central nervous system

Figure 5

PCR analysis of IgH rearrangement status of paired tumors in patients T5 and TP39. Photographs of PCR gels show the IGH rearrangement status of paired tumors in patients T5 and TP39. CNS, central nervous system

Figure 6

Stages of divergence from common precursor cells to tumors. Black arrows indicate the clonal pathway between samples. CLL, chronic lymphocytic leukemia; CNS, central nervous system; FL, follicular lymphoma; MCL, mantle cell lymphoma; PCNSL, primary central nervous system lymphoma