Lastly, the development of human isogenic lines provides a method for testing specific variants and their effects on a phenotypic trait under conditions of reduced variability. Comparing multiple hiPSCs derived neurons from genetically unrelated humans, or even siblings, can result in large variability in measurements that are unrelated to the disease phenotype (Faulconbridge et al., 2017; Lin et al., 2015). With the discovery of efficient genome-editing techniques (Ran et al., 2013), it is now possible to genetically manipulate hiPSCs with precision and efficiency (see Figure 2) (Bassett, 2017; Bertero et al., 2016). Indeed, several studies have successfully utilized clustered regularly interspaces short palindromic repeats (CRISPR)/Cas9 nuclease genome editing (Wang et al., 2017), CRISPR activation/repression (Ho et al., 2017), and shRNA for manipulating gene expression levels (Bertero et al., 2016; Sancho-Martinez et al., 2016). With these techniques, investigators can manipulate expression of single or several genes, as well as generate control cells with identical genetic (isogenic) backgrounds (for detailed review of genome editing techniques see: Bassett, 2017).
