Quantitative Characterization of Defective Virus Emergence by Deep Sequencing

Populations of RNA viruses can spontaneously produce variants that differ in genome size, sequence, and biological activity. Defective variants that lack essential genes can nevertheless reproduce by coinfecting cells with viable virus, a process that interferes with virus growth. How such defective interfering particles (DIPs) change in abundance and biological activity within a virus population is not known. Here, a prototype RNA virus, vesicular stomatitis virus (VSV), was cultured for three passages on BHK host cells, and passages were subjected to Illumina sequencing. Reads from the initial population, when aligned to the full-length viral sequence (11,161 nucleotides [nt]), distributed uniformly across the genome. However, during passages two plateaus in read counts appeared toward the 5' end of the negative-sense viral genome. Analysis by normalization and a simple sliding-window approach revealed plateau boundaries that suggested the emergence and enrichment of at least two truncated species having medium (∼5,900 nt) and short (∼4,000 nt) genomes. Relative measures of full-length and truncated species based on read counts were validated by quantitative reverse transcription-PCR (qRT-PCR). Limit-of-detection analysis suggests that deep sequencing can be more sensitive than complementary measures for detecting and quantifying defective particles in a population. Further, particle counts from transmission electron microscopy, coupled with infectivity assays, linked the rise in smaller genomes with an increase in truncated particles and interference activity. In summary, variation in deep sequencing coverage simultaneously shows the size, location, and relative level of truncated-genome variants, revealing a level of population heterogeneity that is masked by other measures of viral genomes and particles.