Detection of transgene insertions carrying exogenous mouse sequences
Next, we examined transgenic samples containing the insertion of elements foreign to the mouse genome. Since these sequences are not native to the mouse genome, the associated ChIP-seq reads were not expected to align to the mouse reference genome. These unmapped reads were thus realigned to a new reference sequence collection corresponding to the exogenous inserted sequence (Section 2). If these reads align to this custom reference sequence, then the presence of the sequence has been identified in the sample of interest.
All three of the test samples for the study contained insertions foreign to the mouse genome (Table 1). The foreign insertions originated from the human genome (ACTA1 and TPM3 in GMO sample 1), a viral genome (SV40 in GMO samples 1 and 2), and a common genetic engineering transfection tool (SB10 cassette in GMO sample 3). The unmapped reads were aligned to a reference genome, the detected peaks were plotted, and qualitatively examined for similarities and differences between wild-type and GMO samples (Fig. 4).