Nucleotide salvage deficiencies, DNA damage and neurodegeneration

Abstract

Nucleotide balance is critically important not only in replicating cells but also in quiescent cells. This is especially true in the nervous system, where there is a high demand for adenosine triphosphate (ATP) produced from mitochondria. Mitochondria are particularly prone to oxidative stress-associated DNA damage because nucleotide imbalance can lead to mitochondrial depletion due to low replication fidelity. Failure to maintain nucleotide balance due to genetic defects can result in infantile death; however there is great variability in clinical presentation for particular diseases. This review compares genetic diseases that result from defects in specific nucleotide salvage enzymes and a signaling kinase that activates nucleotide salvage after DNA damage exposure. These diseases include Lesch-Nyhan syndrome, mitochondrial depletion syndromes, and ataxia telangiectasia. Although treatment options are available to palliate symptoms of these diseases, there is no cure. The conclusions drawn from this review include the critical role of guanine nucleotides in preventing neurodegeneration, the limitations of animals as disease models, and the need to further understand nucleotide imbalances in treatment regimens. Such knowledge will hopefully guide future studies into clinical therapies for genetic diseases.

Figure 1

Outline of purine de novo synthesis, salvage, and degradation. Red arrows indicate sysnthesis, and blue arrows indicate salvage. Black arrows indicate the conversion of nucleotides to nucleosides and bases, and the conversion of nucleosides to nucleotides. Phosphoribosyl pyrophosphate (PRPP) is inhibited by adenylate (AMP), inosylate (IMP), and guanylate (GMP), as indicated.

Figure 2

Key steps in the salvage of deoxynucleotides in the mitochondria. Arrows designate the steps in the salvage pathways; a dashed arrow indicates import into the mitochondria, and a blue arrow indicates degradation. The large oval (blue) is the mitochondria, the small oval (black) represents the p53-regulated ribonucleotide reductase (p53R2/R1). The nucleotides in the mitochondria include thymidylate (TMP), thymidine diphosphate (TDP), thymdine triphosphate (TTP), deoxyguanosine monophosphate (dGMP), deoxyguanosine diphosphate (dGDP), and deoxyguanosine triphosphate (dGTP). Thymidine kinase II (TK2) and deoxyguanosine kinase (dGK) are salvage enzymes.