Clonotypic Heterogeneity In Cutaneous T-Cell Lymphoma Revealed By Comprehensive Whole Exome Sequencing

Mycosis fungoides (MF), the most common type of cutaneous T-cell lymphoma, is believed to represent a clonal expansion of a transformed skin resident memory T-cell. T-cell receptor (TCR) clonality (i.e. identical sequences of rearranged TCRα, β and γ), the key premise of this hypothesis, has been difficult to document conclusively because malignant cells are not readily distinguishable from the tumor infiltrating, reactive lymphocytes, which contribute to the TCR clonotypic repertoire of MF. Here we have successfully adopted the technique of targeted whole exome and whole transcriptome sequencing (WES/WTS) to identify the repertoire of rearranged TCR genes in tumor enriched samples from patients with MF. Although most of the investigated biopsies of MF had the expected monoclonal rearrangements of TCRγ of the frequency corresponding to the frequency of tumor cells, in half of the samples we detected multiple (up to seven) TCRα and -β clonotypes by WES and WTS. Our findings are compatible with the model in which the initial malignant transformation in MF does not occur in mature, memory T-cells but rather at the level of T-lymphocyte progenitor after TCRγ rearrangement but before TCRβ or TCRα rearrangements. The WES/WTS method is potentially applicable to other types of T-cell lymphomas and enables comprehensive characterization of the TCR repertoire and mutational landscape in these malignancies.