Mechanistic Characterization of De Novo Generation of Variable Number Tandem Repeats in Circular Plasmids during Site-Directed Mutagenesis and Optimization for Coding Gene Application.

Site-directed mutagenesis for creating point mutations, sometimes, gives rise to plasmids carrying variable number tandem repeats (VNTRs) locally, which are arbitrarily regarded as polymerase chain reaction (PCR) related artifacts. Here, the alternative end-joining mechanism is reported rather than PCR artifacts accounts largely for that VNTRs formation and expansion. During generating a point mutation on GPLD1 gene, an unexpected formation of VNTRs employing the 31 bp mutagenesis primers is observed as the repeat unit in the pcDNA3.1-GPLD1 plasmid. The 31 bp VNTRs are formed in 24.75% of the resulting clones with copy number varied from 2 to 13. All repeat units are aligned with the same orientation as GPLD1 gene. 43.54% of the repeat junctions harbor nucleotide mutations while the rest don't. Their demonstrated short primers spanning the 3' part of the mutagenesis primers are essential for initial creation of the 2-copy tandem repeats (TRs) in circular plasmids. The dimerization of mutagenesis primers by the alternative end-joining in a correct orientation is required for further expansion of the 2-copy TRs. Lastly, a half-double priming strategy is established, verified the findings and offered a simple method for VNTRs creation on coding genes in circular plasmids without junction mutations.

High-Level Secretion of Pregnancy Zone Protein Is a Novel Biomarker of DNA Damage-Induced Senescence and Promotes Spontaneous Senescence.

Identification of unique and specific biomarkers to better detect and quantify senescent cells remains challenging. By a global proteomic profiling of senescent human skin BJ fibroblasts induced by ionizing radiation (IR), the cellular level of pregnancy zone protein (PZP), a presumable pan-protease inhibitor never been linked to cellular senescence before, was found to be decreased by more than 10-fold, while the level of PZP in the conditioned medium was increased concomitantly. This observation was confirmed in a variety of senescent cells induced by IR or DNA-damaging drugs, indicating that high-level secretion of PZP is a novel senescence-associated secretory phenotype. RT-PCR examination verified that the transcription of the PZP gene is enhanced in various cells at senescence or upregulated following DNA damage treatment in a p53-independent manner. Moreover, pretreatment with late pregnancy serum containing a high level of PZP led to inhibition of doxorubicin-induced senescence in A549 cells, and depletion of PZP in the pregnancy serum could enhance such inhibition. Finally, the addition of immuno-precipitated PZP complexes into tissue culture attenuated the growth of A549 cells and promoted the spontaneous senescence. Therefore, we revealed that high-level secretion of PZP is a novel and unique feature associated with DNA damage-induced senescence, and secreted PZP is a positive regulator of cellular senescence, particularly during the late stage of gestation.