Gene conversion is a process of transferring genetic material from one homologous sequence to another. Most reported gene conversions are meiotic although mitotic gene conversion is also described. When using CRISPR/Cas9 to target the human hemoglobin subunit beta (HBB) gene, hemoglobin subunit delta (HBD) gene footprints were observed in HBB gene. However, it is unclear whether these were the results of gene conversion or PCR-mediated sequence shuffling between highly homologous sequences. Here we provide evidence that the HBD footprints in HBB were indeed results of gene conversion. We demonstrated that the CRISPR/Cas9 facilitated unidirectional sequence transfer from the homologous gene without double-strand breaks (DSB) to the one with DSBs, and showed that the rates of HBD footprint in HBB were positively correlated to the HBB insertion and deletion rates. We further showed that when targeting HBD gene, HBB footprints could also be observed in HBD gene. The mitotic gene conversion was observed not only in immortalized HEK293T cells, but also in human primary cells. Our work reveals mitotic gene conversion as an often overlooked effect of CRISPR/Cas9-mediated genome editing.

中文翻译：

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CRISPR/Cas9 增加人类细胞中的有丝分裂基因转换。

基因转换是将遗传物质从一个同源序列转移到另一个序列的过程。大多数报道的基因转换是减数分裂，尽管也描述了有丝分裂基因转换。当使用 CRISPR/Cas9 靶向人类血红蛋白亚基 β (HBB) 基因时，在 HBB 基因中观察到血红蛋白亚基 δ (HBD) 基因足迹。然而，尚不清楚这些是基因转换的结果还是高度同源序列之间 PCR 介导的序列改组的结果。在这里，我们提供证据表明 HBB 中的 HBD 足迹确实是基因转换的结果。我们证明了 CRISPR/Cas9 促进了从没有双链断裂 (DSB) 的同源基因到带有 DSB 的同源基因的单向序列转移，并表明 HBB 中 HBD 足迹的比率与 HBB 插入和删除率呈正相关。我们进一步表明，当靶向 HBD 基因时，也可以在 HBD 基因中观察到 HBB 足迹。不仅在永生化 HEK293T 细胞中观察到有丝分裂基因转换，而且在人类原代细胞中也观察到。我们的工作揭示了有丝分裂基因转换是 CRISPR/Cas9 介导的基因组编辑经常被忽视的影响。