APEX1

APEX1 Full Name

APEX nuclease (multifunctional DNA repair enzyme) 1

APEX1 Introduction

APEX1 (also known as APE1, HAP1, or REF1) is a multifunctional protein that plays central roles in the base excision repair (BER) pathway of DNA damage repair and in the regulation of oxidative stress responses. The gene is located on chromosome 14q11.2 and encodes a protein of 318 amino acids with a molecular weight of approximately 36 kDa. APEX1 is ubiquitously expressed in all human tissues, with particularly high levels in the testis, thymus, and neurons, reflecting its essential role in maintaining genomic integrity. As the major apurinic/apyrimidinic (AP) endonuclease in human cells, APEX1 cleaves the DNA backbone immediately 5' to AP sites, which are one of the most common forms of DNA damage, generated spontaneously or by DNA glycosylases during the removal of damaged bases. In addition to its DNA repair function, APEX1 acts as a reduction-oxidation (redox) factor that regulates the DNA-binding activity of numerous transcription factors, including AP-1 (FOS/JUN), NF-κB, p53, HIF-1α, and PAX8, thereby controlling gene expression programs involved in cell survival, proliferation, inflammation, and angiogenesis. APEX1 also participates in the regulation of RNA metabolism, including RNA cleavage and the control of microRNA biogenesis. Given its dual functions in DNA repair and transcriptional regulation, APEX1 is critical for cellular resistance to genotoxic stress and has been implicated in cancer development, chemoresistance, neurodegenerative diseases, and aging.

Figure 1. Schematic structure of APEX1.

Gene Structure and Protein Architecture
The human APEX1 gene spans approximately 6 kb and contains 5 exons. The full-length protein consists of 318 amino acids and is organized into two major functional domains. The N-terminal domain (amino acids 1-127) contains the nuclear localization signal (NLS) and the redox regulatory activity. This domain is rich in lysine and arginine residues and mediates the interaction of APEX1 with numerous transcription factors. The N-terminal domain is also the site of critical post-translational modifications including acetylation, phosphorylation, ubiquitination, and sumoylation that regulate APEX1 localization, stability, and activity. The C-terminal domain (amino acids 128-318) contains the AP endonuclease active site and the DNA-binding surface. This domain adopts a globular structure with a central four-stranded β-sheet flanked by α-helices, and contains the conserved catalytic residues His309, Asp283, and Glu96 that coordinate a divalent metal ion (typically Mg²⁺) essential for endonuclease activity. The active site pocket accommodates the AP site and positions the phosphodiester backbone for cleavage. APEX1 also contains a highly conserved cysteine residue (Cys65) within the N-terminal domain that is critical for its redox function; the thiol group of Cys65 reduces oxidized transcription factors, restoring their DNA-binding activity. APEX1 exists predominantly as a monomer in solution but can dimerize under certain conditions, and it localizes to both the nucleus and mitochondria, reflecting its roles in repairing DNA damage in both compartments.

Biological Functions in DNA Repair and Redox Regulation
APEX1 is the major AP endonuclease in human cells, responsible for cleaving over 95% of AP sites in the genome. In the base excision repair (BER) pathway, DNA glycosylases remove damaged or inappropriate bases (e.g., oxidized, alkylated, or deaminated bases), generating an AP site. APEX1 then incises the DNA backbone immediately 5' to the AP site, producing a single-strand break with a 3'-hydroxyl and a 5'-deoxyribose phosphate (dRP) terminus. This nick is then processed by other BER enzymes including DNA polymerase β and DNA ligase III or I to restore the original DNA sequence. APEX1 also possesses 3'-phosphodiesterase, 3'-phosphatase, and 3'-5' exonuclease activities, allowing it to process various types of damaged DNA termini, making it a versatile repair enzyme. Beyond its repair functions, APEX1 regulates the activity of over 50 transcription factors through its redox activity. Under conditions of oxidative stress, transcription factors such as AP-1 and NF-κB become oxidized at conserved cysteine residues, which prevents their DNA binding. APEX1 reduces these cysteine residues using Cys65 as a reducing agent, restoring transcription factor activity. This redox function is independent of APEX1's DNA repair activity and is essential for the induction of cell survival genes in response to oxidative stress, inflammation, and hypoxia. APEX1 also directly binds to the promoters of genes involved in antioxidant defense, such as catalase and superoxide dismutase (SOD), and regulates their expression. In addition, APEX1 plays roles in RNA quality control, cleaving abasic RNA species, and in the regulation of microRNA processing, where it interacts with DICER and promotes the maturation of specific miRNAs.

Alternate Names for APEX1

APEX1; APEX nuclease (multifunctional DNA repair enzyme) 1; APE; APX; APE1; APEN; APEX; HAP1; REF1; DNA-(apurinic or apyrimidinic site) lyase