Emergence of biased hypermutation in a heterologous additional transcription unit in recombinant lentogenic Newcastle disease virus

Recombinant Newcastle Disease virus (rNDV) strains engineered to express foreign genes from an additional transcription unit (ATU) are considered as candidate live-attenuated vector vaccines for human and veterinary use. Early during the COVID-19 pandemic we and others have generated COVID-19 vaccine candidates based on rNDV expressing a partial or complete SARS-CoV-2 spike (S) protein. In our studies, a number of the rNDV constructs did not show high S expression levels in cell culture or seroconversion in immunized hamsters. Sanger sequencing showed the presence of frequent A-to-G transitions characteristic of adenosine deaminase acting on RNA (ADAR). Subsequent whole genome rNDV sequencing revealed that this biased hypermutation was exclusively localized in the ATU expressing the spike gene.

经工程改造可通过额外转录单位（additional transcription unit, ATU）表达外源基因的重组新城疫病毒（Recombinant Newcastle Disease virus, rNDV）毒株，被视为适用于人类及兽用领域的候选减毒活载体疫苗。早在新冠（COVID-19）疫情初期，本团队与其他研究团队便开发出以表达部分或完整SARS-CoV-2刺突蛋白（spike protein, S）的rNDV为载体的新冠疫苗候选株。在本研究中，多款rNDV构建体既未在细胞培养体系中表现出较高的S蛋白表达水平，也未能使免疫后的仓鼠产生血清转换。桑格测序（Sanger sequencing）结果显示，样本中频繁出现A-to-G转换突变，这正是作用于RNA的腺苷脱氨酶（adenosine deaminase acting on RNA, ADAR）的典型突变特征。后续开展的rNDV全基因组测序进一步揭示，这种偏倚性高突变仅局限于表达刺突蛋白基因的额外转录单位（ATU）中。