The Optimal Sequencing Depth of Tumor Biopsies for Identifying Clonal Cell Populations

Evolutionary route of nasopharyngeal carcinoma metastasis and its clinical significance

It is critical to understand factors associated with nasopharyngeal carcinoma (NPC) metastasis. To track the evolutionary route of metastasis, here we perform an integrative genomic analysis of 163 matched blood and primary, regional lymph node metastasis and distant metastasis tumour samples, combined with single-cell RNA-seq on 11 samples from two patients. The mutation burden, gene mutation frequency, mutation signature, and copy number frequency are similar between metastatic tumours and primary and regional lymph node tumours. There are two distinct evolutionary routes of metastasis, including metastases evolved from regional lymph nodes (lymphatic route, 61.5%, 8/13) and from primary tumours (hematogenous route, 38.5%, 5/13). The hematogenous route is characterised by higher IFN-γ response gene expression and a higher fraction of exhausted CD8⁺ T cells. Based on a radiomics model, we find that the hematogenous group has significantly better progression-free survival and PD-1 immunotherapy response, while the lymphatic group has a better response to locoregional radiotherapy.

Impact of Reducing DNA Input on Next-Generation Sequencing Library Complexity and Variant Detection

PCR amplification, a key step in next-generation sequencing (NGS) library construction, can generate an unlimited amount of product from limited input; however, it cannot create more information than was present in the original template. Thus, NGS libraries can be made from very little DNA, but reducing the input may compromise assay sensitivity in ways that are difficult to ascertain unless library complexity (ie, the number of unique DNA molecules represented in the library) and depth of coverage with unique sequence reads (those derived from input DNA molecules) versus duplicate sequence reads (those resulting from overamplification of particular molecules) are discretely measured. A series of experiments was performed to explore the impact of low DNA input on an amplicon-based NGS assay using unique molecular identifiers to track unique versus duplicate reads. At high sequencing depths, unique and total (unique plus duplicate) read coverage are not well correlated, so increasing the number of sequenced reads does not necessarily improve sensitivity. Unique coverage depth tends to improve with more input, but improvements are not consistent. Fluctuations in library complexity complicated variant detection using both standardized and clinical specimens, often resulting in technical replicates with vastly different estimates of variant allelic fraction. In conclusion, depth of coverage with unique reads must be tracked in clinical NGS to ensure that sensitivity and accuracy are maintained.