Room: Track 3
Due to the stochastic nature of ionizing radiation, mutagenic effects get averaged out in bulk-cell DNA sequencing. Single-cell whole genome sequencing (sc-WGS), however, has the potential to assess cell-by-cell the mutations induced by ionizing radiation. In this study, we show for the first time that sc-WGS can be used to characterize genomic mutations induced in individual cells by an X-ray radiotherapy beam.
Human B-lymphoblastoid cells (Coriell GM24385 cell line) were irradiated with 0.0 Gy (sham irradiation), 0.5 Gy, 1.5 Gy, or 3.0 Gy using 6 MV X-rays from a Varian TrueBeam linac. The cells were incubated for 20 hours as a trade off between fixing the DNA damage and minimizing cell division (to minimize the number of clones). 600 cells from each exposure group were isolated and sequenced with 0.03X coverage sc-WGS. Genome alignment and copy number variation analysis were performed on the sc-WGS data to a resolution of 20 kilo-base pairs (kbp) using the Cell Ranger DNA software (10xGenomics). Adjoining copy number variations (CNVs) with the same copy number were grouped together as single mutations. The number of mutations per cell and the size of each mutation were used to characterize the mutational signature of each dose of radiation.
The total number of CNVs and the average number of mutations per cell increased with dose in a logarithmic fashion. The size distribution of CNVs also varied with dose. At low doses, small CNVs were the dominant mutation size. However, the proportion of large CNVs increased to become the dominant mutation size at higher doses.
We were able to characterize and obtain the mutational signature of X-rays using sc-WGS. This technique may be useful to compare the mutational signature of other ionizing radiations, and to serve as endpoints for RBE determination.