Determining the impact of uncharacterized inversions in the human genome by droplet digital PCR

Despite the interest in characterizing all genomic variation, the presence of large repeats at the breakpoints of many structural variants hinders their analysis. This is especially problematic in the case of inversions, since they are balanced changes without gain or loss of DNA. Here we tested novel linkage-based droplet digital PCR (ddPCR) assays on 20 inversions ranging from 3.1 to 742 kb and flanked by long inverted repeats (IRs) of up to 134 kb. Among these, we validated 13 inversions predicted by different genome-wide techniques. In addition, we have generated new experimental human population information across 95 African, European and East-Asian individuals for 16 of them, including four already known inversions for which there were no high-throughput methods to determine directly the orientation, like the well-characterized 17q21 inversion. Through comparison with previous data, independent replicates and both inversion breakpoints, we have demonstrated that the technique is highly accurate and reproducible. Most of the studied inversions are frequent and widespread across continents, showing a negative correlation with genetic length. Moreover, all except two show clear signs of being recurrent, and the new data allowed us to define more clearly the factors affecting recurrence levels and estimate the inversion rate across the genome. Finally, thanks to the generated genotypes, we have been able to check inversion functional effects in multiple tissues, validating gene expression differences reported before for two inversions and finding new candidate associations. Our work therefore provides a tool to screen these and other complex genomic variants quickly in a large number of samples for the first time, highlighting the importance of direct genotyping to assess their potential consequences and clinical implications.