The involvement of DNA-damage and -repair defects in neurological dysfunction

Abstract

A genetic link between defects in DNA repair and neurological abnormalities has been well established through studies of inherited disorders such as ataxia telangiectasia and xeroderma pigmentosum. In this review, we present a comprehensive summary of the major types of DNA damage, the molecular pathways that function in their repair, and the connection between defective DNA-repair responses and specific neurological disease. Particular attention is given to describing the nature of the repair defect and its relationship to the manifestation of the associated neurological dysfunction. Finally, the review touches upon the role of oxidative stress, a leading precursor to DNA damage, in the development of certain neurodegenerative pathologies, such as Alzheimer's and Parkinson's.

Figure 1

Base Excision Repair (i) Recognition and removal of a modified base by a DNA glycosylase, leaving behind an abasic site (shown is removal of deaminated cytosine (uracil) by UNG). (ii) Cleavage at the abasic site by APE1, creating a SSB with a 5′ dRP and 3′ hydroxyl (OH) end. (iii) Gap filling at the strand break via either short-patch (left) or long-patch repair synthesis (right). In short-patch base excision repair (BER), Pol β replaces the missing nucleotide, whereas in long-patch BER, Pol β, δ, or ɛ incorporates 2–10 nucleotides via strand displacement (newly synthesized sequence in gray). PCNA and RPA assist in the process. (iv) Excision of the 5′ dRP to create ligatable ends is performed by the lyase activity of Pol β (short-patch BER) or the flap endonuclease activity of FEN1 (long-patch BER). (v) The final nick is sealed by Ligase IIIα in complex with XRCC1 (short patch BER) or by Ligase I (long patch BER) to regenerate the intact strand.

Figure 2

Nucleotide Excision Repair (i) Recognition and removal of helix-distorting adducts (e.g., thymine dimer, shown) is mediated by the XPC-HR23B complex in global genomic repair (left) or by a stalled RNAP II-CSB complex during transcription-coupled repair (right). Subsequent repair steps are similar for both GGR and TCR. (ii) XPA, RPA, and the TFIIH complex are recruited to the damage site after p8 stimulation of XPB ATPase and XPB-mediated unwinding; XPB–XPD unwind DNA to create a bubble. (iii) ERCC1-XPF and XPG are then recruited and incise 5′ and 3′, respectively, to the bubble junction, releasing an approximately 30 nucleotide stretch of DNA bearing the lesion. (iv) Repair synthesis is carried out by the PCNA-dependent Pol δ/ɛ. (v) The remaining nick is sealed by Ligase I or the XRCC1-Ligase IIIα complex (not shown).

Figure 3

Repair of DSBs (A) Homologous recombination at two-ended DSBs: (i) Detection of DSB by the MRN complex and recruitment of ATM. Other repair and cell-cycle checkpoint proteins are activated by ATM. (ii) 5′–3′ exonuclease resection of the DSB to generate a 3′ single-stranded overhang. MRE11 endonuclease may play a role in this process. (iii) Rad51-directed homology search, followed by strand invasion, displaces the complementary region of the homolog (typically a sister chromatid) and creates a D-loop (open arrowhead). Rad51 is probably assisted by RPA and other Rad family members, such as Rad52 and Rad54. (iv) Upon formation of a Holliday junction (gray arrowhead), the invading strand can extend in both directions (note long arrows in v). (v) Extension of invading strand by a DNA polymerase can lead to invasion of the homolog by the second end of the original DSB to form a double Holliday junction intermediate. (vi) Rad51C promotes the resolution of the Holliday junctions to yield either crossover (vertical arrows) or non-crossover (horizontal arrowheads) recombination products. (B) Single-strand annealing (SSA): (i) Formation of a two-ended DSB between homologous repeat sequences (black and gray bars). (ii) Exonuclease resects the ends to generate a 3′ single-strand overhang, exposing the complementary regions. (iii) Alignment and Rad52-dependent annealing of the repeat sequences leads to displacement of the 3′ tail between the repeats or creation of a gap (not shown). ERCC1/XPF is thought to digest 3′-displaced tails. (iv) Ligation of ends regenerates the intact duplex and deletes the sequence between the repeats. (C) Homologous recombination at a one-ended DSB: (i) Reversal of a stalled replication fork on encountering an obstacle such as a lesion or adduct in the template strand leads to formation of an intermediate. (ii) Endonuclease action on the intermediate can result in a collapsed replication fork with a one-ended DSB. (iii) A 5′–3′ exonuclease resects the DSB to generate a 3′ overhang capable of stand invasion. (iv) Rad51 directs strand invasion into the fully copied complementary duplex, producing a D loop structure and Holliday junction (see panel [A]) necessary to ultimately restore the replication fork. (v) Resolution of the recombination intermediate can occur either via crossover (vertical arrows) or non-crossover (horizontal arrows) events to yield recombinant products. (D) Non-homologous end-joining (NHEJ): (i) The Ku70-80 heterodimer binds each end of a two-ended DSB, aligns them, and recruits DNA-PK and its cofactor inositol-6 phosphate (IP₆) to form a bridging and signaling complex. (ii) Noncomplementary ends at the DSB may be processed by Artemis exonuclease, MRN complex, or the FEN-1 nuclease to reveal microhomology or to create ligatable ends. Gaps can be filled by Pol μ to generate ligatable nicks, and other repair enzymes such as PNKP can function to generate conventional 3′ hydroxyl or 5′ phosphate termini. (iii) The processed DSB is then sealed by the XRCC4-LigaseIV tetramer to create an intact duplex.