DNA AND CELL BIOLOGY Volume 23, Number 12, 2004 © Mary Ann Liebert, Inc. Pp. 797–806 Deoxyribonucleotides and Disorders of Mitochondrial DNA Integrity ANN SAADA ABSTRACT Mitochondrial DNA (mtDNA) depends on numerous nuclear encoded factors and a constant supply of de- oxyribonucleoside triphosphates (dNTP), for its maintenance and replication. The function of proteins in- volved in nucleotide metabolism is perturbed in a heterogeneous group of disorders associated with depletion, multiple deletions, and mutations of the mitochondrial genome. Disturbed homeostasis of the mitochondrial dNTP pools are likely the underlying cause. Understanding of the biochemical and molecular basis of these disorders will promote the development of new therapeutic approaches. This article reviews the current knowl- edge of deoxyribonucleotide metabolism in relation to disorders affecting mtDNA integrity. 797 INTRODUCTION M ITOCHONDRIAL ENCEPHALOMYOPATHIES include a diverse and expanding group of disorders resulting from the de- creased ability of mitochondria to meet cellular demands for adenosine 5-triphosphate (ATP). Oxidative phosphorylation is carried out by the mitochondrial respiratory chain (RC), which is composed of 86 proteins assembled in five enzymatic com- plexes (I–V). Complexes I to IV transfer electrons, from re- ducing equivalents, through the inner mitochondrial membrane while generating a proton gradient, which is than utilized by complex V (ATP synthase) to produce ATP (Hatefi, 1985). The mitochondrial respiratory chain is under dual genetic control. RC dysfunction may thus arise from mutations in either the mitochondrial (mtDNA) or the nuclear genome. The human mtDNA is a 16.5-kb circular molecule containing 37 genes, of which 24 (2 ribosomal RNAs and 22 tRNAs) are needed for mtDNA translation and 13 encode proteins of the RC complexes I, III, IV and V (Clayton, 2000). The maintenance and replica- tion of mtDNA depends on numerous nuclear encoded factors and a balanced supply of the four deoxyribonucleoside triphos- phates (dNTP). A functional intergenomic communication is thus vital to maintain mtDNA integrity (Reichard, 1988; Hi- rano et al., 2001). Since the late 80s, hundreds of disease caus- ing mtDNA point mutations and several large-scale rearrange- ments were characterized (DiMauro and Schon, 2003). Recently, the focus has shifted towards the identification of mu- tations in the nuclear encoded subunits of the respiratory chain and in auxiliary proteins (Shoubridge, 2001; Rotig and Munnich, 2003). Within this scope, mutations have been dis- covered in a subset of mitochondrial disorders with defective intergenomic signaling, that is, mtDNA copy number is either reduced (mtDNA depletion) or mtDNA molecules are affected by multiple deletions. Because mtDNA depletion may also be acquired by nucleoside analogue treatment, some investigators thought that congenital defects in the mitochondrial nucleotide metabolism are a major cause for some of the disorders of in- tergenomic crosstalk. The role of deoxyribonucleotides in mitochondrial diseases is the subject of this article. An outline of the mitochondrial de- oxyribonucleotide metabolism will be followed by a review of disorders affecting mtDNA integrity and stability. MITOCHONDRIAL DEOXYRIBONUCLEOTIDE METABOLISM Mitochondrial deoxyribonucleotide synthesis (Fig. 1). Mitochondrial DNA replication is independent of nuclear DNA synthesis and occurs throughout the whole cell cycle. A constant supply of dNTPs is therefore required for mtDNA syn- thesis and maintenance (Bogenhagen and Clayton, 1976). The mitochondrial and cytosolic dNTP pools are separated by the Metabolic Disease Unit, Shaare Zedek Medical Center, Jerusalem, Israel.