APOBEC3 Cytidine Deaminases: Viral Restriction and Cancer Therapy
The APOBEC (apolipoprotein B mRNA editing catalytic polypeptide-like) protein superfamily plays an important role in nucleic acid editing and immunity. Eleven APOBEC proteins are encoded by the human genome: APOBEC1 (A1), APOBEC2 (A2), APOBEC3 (consisting of the subfamily members A3A, A3B, A3C, A3D, A3F, A3G, and A3H), APOBEC4 (A4), and AID (activation-induced cytidine deaminase). They serve as cytidine deaminases and can catalyze deamination of cytidines to uridines in both DNA and RNA. While APOBECs perform essential protective functions by restricting viruses and mobile genomic elements, as well as regulating immunoglobulin maturity/diversity, their overexpression or misregulation can promote carcinogenesis.
Of particular interest is the A3 subfamily due to their well-known antiviral activity. APOBEC3-mediated cytidine deamination of single-strand viral DNA results in G-to-A hypermutations during subsequent complementary strand synthesis, consequently rendering the virus replication-defective. Hypermutations induced by A3 enzymes have been found to inhibit retrotransposons, retroviruses like HIV-1, and DNA viruses such as AAV, HBV, HSVs, and HPV [1-12]. APOBEC3A is unique in that, in addition to catalyzing C-to-U substitutions, it is the only A3 subfamily member that can efficiently deaminate 5-methylcytidine to thymidine, thereby facilitating the cellular clearance of methylated exogenous DNA [13-15].
While APOBEC3 enzymes are generally not expressed in healthy tissue, abnormal expression levels, particularly for A3A and A3B, are detected in approximately 50% of tumors [16-18]. These expression levels have been correlated with APOBEC3-specific mutational signatures in multiple cancers: lung, breast, bladder, cervical, and ovarian, among others [16,17,19,20]. By exploiting the genomic instability and ensuing DNA damage response (DDR) activated by APOBEC3 expression, apoptosis and cell death in A3-expressing cancer cells were induced via DDR inhibition [20,21]. Therefore, quantitative measures of APOBEC3 activity in tumors are critical for the research and development of candidate inhibitors for cancer therapy.
Regarding the current COVID-19 pandemic and the emergence of new and highly infectious variants, evidence suggests cytosine deamination as a major driving force behind the evolution of the SARS-CoV-2 coronavirus [22]. The extensive C-to-U substitutions displayed by the SARS-CoV-2 genome indeed may infer an underlying APOBEC3-driven mechanism. Interestingly, APOBEC3A mRNA has been reported among the most abundant transcripts to be identified in COVID-19 patients [23]. Detection of APOBEC3/A enzyme activity and inhibition would thus be of great benefit in virus infection control, cancer diagnostics, and the development of novel target-based anti-viral and anti-cancer therapeutics.
To this end, EpigenTek developed and offers the Epigenase™ APOBEC3 Cytidine Deaminase Activity/Inhibition Assay Kit (catalog no. P-3140) and Epigenase™ APOBEC3A Activity/Inhibition Assay Kit (catalog no. P-3142). Activity/Inhibition of total APOBEC3 enzymes or APOBEC3A enzyme can be conveniently assessed with these rapid colorimetric assays in only 4-5 hours, using cell extracts or purified enzymes as input material. An assay standard is provided in each kit for the quantification of enzyme activity. The innovative kit compositions enable background signals to be extremely low, allowing the assays to be simple, accurate, reliable, and consistent.
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