ABSTRACT
Cyrtomiumadenotrichum Y. Nong & R.H. Jiang (Dryopteridaceae), a new species from Guangxi, China, is described and illustrated. This new species is similar to C.nephrolepioides (Christ) Copel., C.obliquum Ching & K. H. Shing ex K. H. Shing, C.sinningense Ching & K. H. Shing ex K. H. Shing and C.calcis Liang Zhang, N.T.Lu & Li Bing Zhang in having erect rhizomes, dense, leathery lamina and rounded sori, but it can be easily distinguishable by its stipe sparsely glandular, base obvious oblique, basiscopic base truncate, acroscopic base auriculate or ovate.
ABSTRACT
The APOBEC-AID family of cytidine deaminase prefers single-stranded nucleic acids for cytidine-to-uridine deamination. Single-stranded nucleic acids are commonly involved in the DNA repair system for breaks generated by CRISPR-Cas9. Here, we show in human cells that APOBEC3 can trigger cytidine deamination of single-stranded oligodeoxynucleotides, which ultimately results in base substitution mutations in genomic DNA through homology-directed repair (HDR) of Cas9-generated double-strand breaks. In addition, the APOBEC3-catalyzed deamination in genomic single-stranded DNA formed during the repair of Cas9 nickase-generated single-strand breaks in human cells can be further processed to yield mutations mainly involving insertions or deletions (indels). Both APOBEC3-mediated deamination and DNA-repair proteins play important roles in the generation of these indels. Therefore, optimizing conditions for the repair of CRISPR-Cas9-generated DNA breaks, such as using double-stranded donors in HDR or temporarily suppressing endogenous APOBEC3s, can repress these unwanted mutations in genomic DNA.
Subject(s)
CRISPR-Cas Systems , Cytosine Deaminase/chemistry , DNA Breaks, Double-Stranded , DNA Repair , Mutation , APOBEC Deaminases , Cytidine/chemistry , Cytidine Deaminase/genetics , DNA, Single-Stranded , HEK293 Cells , HeLa Cells , Humans , INDEL Mutation , Oligonucleotides/genetics , RNA, Small Interfering/metabolism , Recombinational DNA Repair , Sequence Analysis, DNAABSTRACT
APOBECs (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like) are a family of cytidine deaminases that prefer single-stranded nucleic acids as substrates. Besides their physiological functions, APOBEC family members have been found to cause hypermutations of cancer genomes, which could be correlated with cancer development and poor prognosis. Recently, APOBEC family members have been combined with the versatile CRISPR/Cas9 system to perform targeted base editing or induce hypermutagenesis. This combination improved the CRISPR/Cas9-mediated gene editing at single-base precision, greatly enhancing its usefulness. Here, we review the physiological functions and structural characteristics of APOBEC family members and their roles as endogenous mutators that contribute to hypermutations during carcinogenesis. We also review the various iterations of the APOBEC-CRISPR/Cas9 gene-editing tools, pointing out their features and limitations as well as the possibilities for future developments.
