Cell migration patterns and ongoing somatic mutations in the progression of follicular lymphoma

Abstract

Follicular lymphoma (FL) constitutes the neoplastic equivalent of germinal center B-cells. Like its physiological counterpart, FL grows in (atypical) follicular structures, the formation of which is as yet poorly understood. Recent data indicate that in early tumour stages, neoplastic FL cells home to and colonise reactive germinal centers. Laser microdissection (LMD) and micromanipulation techniques now allow for the molecular genetic analysis of single cell mutation patterns in FL. The purpose of the present study was the analysis of the sequence and order of somatic mutations in FL, i.e. the influence of the germinal center microenvironment on the clonal evolution in different grades of FL. By generating phylogenetic trees as calculated from tumour cell sequences, the clonal evolution from a putative progenitor cell was elucidated and finally, the tumour cell migration pattern in disease progression was assessed by analyzing biopsies at different time points in relapsed tumours. Four patients suffering from FL were included in the study. A primary FL grade 1 showed clustering of genetically related subclones in distinct follicles. A moderate interfollicular exchange of tumour cells was detected. Three cases of FL grade 2 were found to show decreased subclonal clustering in follicles and an increase in the interfollicular migration. Accumulations of replacement mutations in antigen binding domains (CDR) and silent mutations in non-antigen binding domains (FR), respectively, indicating antigen influence on hypermutation were only found in the case of FL grade 1. Our conclusion is that the microenvironment in germinal centers exercises influence on clonal evolution and tumour cell distribution patterns in FL. With increasing histologic grade during disease progression, a reduced intraclonal diversity and selection of subclones also occurs outside the setting of transformation to high-grade lymphoma. Antigen-dependent hypermutations were only seen in FL grade 1, while in progressed FL, random mutation patterns and a decrease of clonal diversity were found.