DNA Repair

By Natalia Gurule, PhD

Proliferating cell nuclear antigen (PCNA) plays a crucial role in nucleic acid metabolism as it pertains to DNA replication and repair.  Most noted for its activation of subunits of DNA polymerase, it has also been found to interact with cell-cycle progression proteins.  Modifications of PCNA as a result of cellular response put PCNA in a pivotal position with DNA replication, DNA damage, and chromatin structure and function.  In response to DNA damage, PCNA is ubiquitinated and becomes part of the RAD-6 dependent DNA repair pathway, where it acts as a substrate with a variety of p

The p53-binding protein 1 (53BP1) is a DNA damage response factor, which is recruited to nuclear structures at the site of DNA damage.  DNA double-strand breaks (DSBs) are mutations that are detrimental to cell viability and genome stability, and must be repaired either through homologous recombination (HR) or non-homologous end joining (NHEJ). 53BP1 specifically promotes both NHEJ as well as the inhibition of HR repair, yet the decision making on a molecular level between these two routes not clearly understood.

Fanconi anemia (FA) is a genetically inherited disorder that yields cytogenetic instability, hypersensitivity to DNA crosslinking compounds and defective DNA repair. A variety of genes have been identified within the FA pathway that are referred to as the Fanconi anemia complementation group.  One member of this group, FANCD2, is monoubiquitinated in response to DNA damage.  At this point, FANCD2 specifically localizes to the nucleus to represent the site of DNA repair, often times to the DNA replication fork.

The synaptonemal complex (SC) is a protein structure that forms during the synapsis of homologous chromosomes during meiosis. This structure is involved in the processes of chromosome synapsis, genetic recombination and subsequent chromosome segregation, and is essential for gametogenesis.

PARPs (poly ADP ribose polymerases) are DNA repair enzymes that promote single stranded break (SSB) repair by binding to DNA at the sites of SSBs and recruiting repair machinery. In humans, the PARP superfamily consists of 17 members, of which five play known roles in SSB repair. PARP-1, the most well-studied family member, is required for base excision repair and is thought to be responsible for 90% of PARP activity (5).

Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) is a well-known housekeeping gene with functions in glycolysis. Many biologists are familiar with the gene and use GAPDH antibodies for a loading control when performing western blots. However, this primarily cytoplasmic protein is an essential metabolic regulator and has been shown to be involved in a variety of cellular processes like DNA repair, membrane fusion, and cell death (1).

The Dnmt1 enzyme is a member of the C5-methyltransferase family responsible for repairing cytosines in double-stranded DNA (dsDNA). This enzyme uses a nucleophilic attack mechanism and is the most abundantly found mammalian DNA methyltransferase. It primarily acts upon CpG residues and prefers hemimethylated residues, but can also methylating unmethylated DNA. The cell relies upon Dnmt1 as its key methylation maintenance enzyme both for DNA replication and repair as well as for de novo methylation during somatic cell development and differentiation.

PCNA is a nuclear protein essential for DNA replication as well as DNA excision and mismatch repair pathways. It coordinates the recruitment and association of needed components during both of these processes, both of which are essential for cell cycle regulation and cell response to stress.  Through the symmetric association of three identical monomers, PCNA forms a toroidal, ring-shaped structure that encircles DNA. This serves as the scaffold upon which polymerases and other proteins dock and associate.

The genetic disorder known as Fanconi anemia (FANC) is a heterogeneous, autosomal-recessive cancer susceptibility condition characterized by a wide array of symptoms. These include congenital malformations, progressive bone marrow failure, DNA-damage hypersensitivity, and genome instability. The protein FANCD2 is a subunit of the protein complex involved in cellular resistance to DNA cross-linking and DNA synthesis arrest triggered by ionizing radiation (IR).