Deoxyguanosine Kinase Research Articles

Results of a Phase II Study of 506U78 (Nelarabine) in Refractory Indolent B-Cell or Peripheral T-Cell Lymphoma.

BACKGROUND: Compound 506U78 (Nelarabine), a water soluble pro-drug of 9-β-d-arabinofuranosyl-guanine, is demethoxylated by adenosine deaminase to ara-G in lymphoblasts. Intracellular ara-G is then phosphorylated via deoxycytidine kinase and mitochondrial deoxyguanosine kinase to its active 5′-triphosphate, ara-GTP. Ara-GTP, a potent inhibitor of DNA polymerase, also incorporates into DNA resulting in cell death. Studies in children and adult T-cell leukemia and non-Hodgkin's lymphoma (NHL) have shown activity, but with reversible neurotoxicity as the most prominent adverse event.METHODS: We report here the results of a phase II study with 506U78 in adults (median age 64, range 33–81) with relapsed or refractory indolent B-cell or peripheral T-cell NHL. Patients had received a median of 2 prior courses of chemotherapy. 506U78 was given at 1.5 g/m2/day IV on days 1, 3, and 5 every 28 days to a maximum of 6 cycles unless toxicity or progressive disease. If after 4 cycles a complete response (CR) or partial response (PR) was not achieved, then treatment was discontinued.RESULTS: There were 23 patients enrolled including 13 with T-cell and 10 with indolent B-cell NHL. Among 19 assessable for response, the overall response rate (ORR) in T-cell NHL was 4/9 (44%) with 2 CR and 2 PR, while in indolent B-cell NHL the PR was 3/9 (33%) with no CR. In responders, the median time to progression (TTP) was 8 months (range 2–22 months). Sixteen of 22 (73%) patients evaluable for toxicity had grade 3 or 4 adverse events. Neurotoxicity included one grade 3 and one grade 4 event.CONCLUSION: Compound 506U78 is active as a single agent in T-cell and B-cell NHL. Our response rate was higher than the ORR of 10.5% reported previously in the Czuczman et al. (2004) Blood 104(11) abstract in adults with T-cell NHL and there was less neurotoxicity in our study. This may be related to different dosing regimens (every 28 days vs. every 21 days). Compound 506U78 should be further evaluated in clinical trials.

Deoxyguanosine kinase mutations and combined deficiencies of the mitochondrial respiratory chain in patients with hepatic involvement

The activity of deoxyguanosine kinase (DGUOK), a mitochondrial enzyme involved in the anabolism of mitochondrial (mt) deoxyribonucleotides, governs the maintenance of the mtDNA. Deleterious mutations of the DGUOK gene are thus associated with mtDNA depletion and result in combined deficiencies of mtDNA-encoded respiratory chain enzymes. With the aim to estimate the prevalence of DGUOK mutations in a cohort of 30 patients with hepatocerebral disease and combined respiratory chain deficiencies, we studied the DGUOK gene and identified previously unreported mutations in five families. Two patients and their affected sibs, born to non-consanguineous parents, were homozygous for a missense mutation (M1T, and L250S, respectively). One patient presented a homozygous 4 pb insertion (796 insTGAT) and two other patients, and their affected sibs, were compound heterozygous (E165V/L266R and E211G/L266R, respectively). These findings allowed us to propose prenatal diagnosis in two families. In conclusion, we observed a high prevalence of DGUOK mutations (17%) in patients with hepatic involvement and combined respiratory chain deficiencies with hepatic involvement.

Hepatocerebral Mitochondrial DNA Depletion

Two novel homozygous mutations, G352A and C269T, are documented in the gene for deoxyguanosine kinase (DGK) in 3 children with hepatocerebral mitochondrial DNA depletion syndrome reported from Columbia University College of Physicians and Surgeons, New York; University of Pisa, Italy; University of Toronto, Canada; and University of Melbourne, Australia.

Clinical, biochemical and morphological features of hepatocerebral syndrome with mitochondrial DNA depletion due to deoxyguanosine kinase deficiency

The aim of this study was to delineate the specific clinical, biological and liver morphological alterations of the hepatocerebral syndrome due to alterations in the deoxyguanosine kinase gene, a rare and severe form of mitochondrial DNA depletion syndrome. We report seven cases from three unrelated families with the same mutation in the deoxyguanosine kinase gene. All the patients presented in the first weeks of life with hepatomegaly and progressive liver failure that led to death few months later. Major psychomotor delay and multidirectional nystagmus were reported shortly after onset of the disease. Severe hyperlactacidaemia was constant. Histological examination of the liver disclosed a multifocal injury of hepatocytes with irregular foamy steatosis, cholestasis, and fibrosis, associated with different degrees of hepatosiderosis and glycogen depletion. Liver respiratory chain activities were abnormal in all analysed patients and the amount of liver mitochondrial DNA was severely decreased. An identical homozygous 4bp GATT duplication was identified in the deoxyguanosine kinase gene of all the cases. These patients, together with patients reported in the literature, permit to delineate the specific features of the hepatocerebral form of mitochondrial DNA depletion syndrome and to differentiate them from other causes of neonatal liver failure.

Novel mutation in DGUOK in hepatocerebral mitochondrial DNA depletion syndrome associated with cystathioninuria

Mitochondrial depletion syndrome (MDS) refers to a heterogeneous group of mitochondrial disorders characterized by a reduction of the mtDNA copy number in affected tissues. Mutations in DGUOK encoding deoxyguanosine kinase (MIM 601465) cause the hepatocerebral form of MDS (MIM 251880). Cystathioninuria (MIM 219500) can result from mutations in CTH encoding cystathionine gamma lyase (MIM 607657) or can be a secondary finding in several diverse clinical conditions. We present three patients from two apparently unrelated old colony Mennonite families, each of whom had the hepatocerebral form of MDS together with cystathioninuria. Each affected child was homozygous for the novel DGUOK p.D255Y mutation, but had no CTH mutation, indicating that the hepatocerebral form of MDS might be associated with secondary cystathioninuria.

Docking simulation with a purine nucleoside specific homology model of deoxycytidine kinase, a target enzyme for anticancer and antiviral therapy

5′-Phosphorylation, catalyzed by human deoxycytidine kinase (dCK), is a crucial step in the metabolic activation of anticancer and antiviral nucleoside antimetabolites, such as cytarabine (AraC), gemcitabine, cladribine (CdA), and lamivudine. Recently, crystal structures of dCK (dCKc) with various pyrimidine nucleosides as substrates have been reported. However, there is no crystal structure of dCK with a bound purine nucleoside, although purines are good substrates for dCK. We have developed a model of dCK (dCKm) specific for purine nucleosides based on the crystal structure of purine nucleoside bound deoxyguanosine kinase (dGKc) as the template. dCKm is essential for computer aided molecular design (CAMD) of novel anticancer and antiviral drugs that are based on purine nucleosides since these did not bind to dCKc in our docking experiments. The active site of dCKm was larger than that of dCKc and the amino acid (aa) residues of dCKm and dCKc, in particular Y86, Q97, D133, R104, R128, and E197, were not in identical positions. Comparative docking simulations of deoxycytidine (dC), cytidine (Cyd), AraC, CdA, deoxyadenosine (dA), and deoxyguanosine (dG) with dCKm and dCKc were carried out using the FlexX™ docking program. Only dC (pyrimidine nucleoside) docked into the active site of dCKc but not the purine nucleosides dG and dA. As expected, the active site of dCKm appeared to be more adapted to bind purine nucleosides than the pyrimidine nucleosides. While water molecules were essential for docking experiments using dCKc, the absence of water molecules in dCKm did not affect the ability to correctly dock various purine nucleosides.

New DGK Gene Mutations in the Hepatocerebral Form of Mitochondrial DNA Depletion Syndrome

To document novel homozygous mutations in the gene for deoxyguanosine kinase (DGK) in 3 children with mitochondrial DNA depletion. Clinical features included liver failure, hypotonia, and nystagmus in 2 siblings, and liver cirrhosis, optic dysplasia, nystagmus, and microcephaly in the third patient. We sequenced the whole coding region of the DGK gene. We identified 2 novel homozygous mutations, G352A and C269T, that lead to truncated proteins. These data confirm that DGK mutations typically affect the liver and brain.

Deoxyribonucleoside kinases: two enzyme families catalyze the same reaction

Mammals have four deoxyribonucleoside kinases, the cytoplasmic (TK1) and mitochondrial (TK2) thymidine kinases, and the deoxycytidine (dCK) and deoxyguanosine (dGK) kinases, which salvage the precursors for nucleic acids synthesis. In addition to the native deoxyribonucleoside substrates, the kinases can phosphorylate and thereby activate a variety of anti-cancer and antiviral prodrugs. Recently, the crystal structure of human TK1 has been solved and has revealed that enzymes with fundamentally different origins and folds catalyze similar, crucial cellular reactions.

Selective Increase of dATP Pools upon Activation of Deoxycytidine Kinase in Lymphocytes: Implications in Apoptosis

Stimulation of the activity of deoxycytidine kinase (dCK), the principal deoxynucleoside salvage enzyme, has been recently considered as a protective cellular response to a wide range of agents interfering with DNA repair and apoptosis. In light of this, the potential contribution of dCK activation to apoptosis induction--presumably by supplying dATP or its analogues for the apoptosome formation--deserves consideration. Two-hour exposure of human tonsillar lymphocytes to 2-chloro-deoxyadenosine (CdA) led to a two-fold activation of dCK. This activation process was inhibited by pifithrin-alpha, a potent inhibitor of p53. When the dNTP pools were determined, both deoxypyrimidine triphosphate and dGTP pools were reduced after the treatments, while dATP levels elevated by 62%, 77% and 50% in the CdA, aphidicolin and etoposide-treated cells, respectively. We assume that dCK activation elicited by cellular damage might be a proapoptotic factor in terms of generating dATP well before the release of cytochrome c and deoxyguanosine kinase from mitochondria.

Deoxyribonucleoside Kinases in Mitochondrial DNA Depletion

AbstractFor Abstract see ChemInform Abstract in Full Text.

Activity profiles of deoxynucleoside kinases and 5′-nucleotidases in cultured adipocytes and myoblastic cells: insights into mitochondrial toxicity of nucleoside analogs

Nucleoside reverse transcriptase inhibitor (NRTI) treatment of HIV is associated with complications, including lipodystrophy (LD) and myopathy. Inhibition of mitochondrial DNA polymerase and depletion of mtDNA by NRTI triphosphates are believed to be key mechanisms in NRTI toxicity. Here, we determined the activities and mRNA levels of deoxynucleoside kinases (dNK) and 5′-nucleotidases (5′-NT) controlling the rate-limiting step in intracellular phosphorylation of NRTIs in cell models representing adipose, muscle tissue and peripheral blood cells using specific assays and Taqman RT-PCR. In vitro phosphorylation of 3′-azido-2′,3′-dideoxythymidine (AZT) and 2′,3′-didehydro-2′,3′-dideoxythymidine (d4T) in extracts was also determined. 3T3-L1 adipocytes showed similar activity of mitochondrial thymidine kinase-2 (TK2) and deoxyguanosine kinase (dGK) but 3- to 36-fold lower levels of cytosolic deoxycytidine kinase (dCK), thymidine kinase-1 (TK1) and thymidine monophosphate kinase (TMPK) and higher levels of deoxyribonucleotidase activity compared to proliferating 3T3-L1. dCK, dGK and TK2 activities correlated with their mRNA levels in proliferating, resting and differentiating 3T3-L1. Differentiated L6 myoblasts had lower activities of cytosolic dNK's and TMPK, higher dGK and similar TK2 and deoxyribonucleotidases (dNT) activities compared to proliferating myoblasts. TK2 was the limiting dNK activity while dGK was predominant in adipocytes and myocytes. Activity profiles revealed limited capacity to phosphorylate dThd and dCyd in adipocytes and myocytes compared to proliferating cells and CEM lymphocytes. Phosphorylation of AZT and d4T was low in adipocytes and myocytes, and the presence of these analogs inhibited the phosphorylation of dThd by TK2 suggesting that mitochondrial toxicity of some NRTIs in adipocytes and myocytes is due to the depletion of normal mitochondrial dNTP pools.

Selective Increase of dATP Pools upon Activation of Deoxycytidine Kinase in Lymphocytes: Implications in Apoptosis

Stimulation of the activity of deoxycytidine kinase (dCK), the principal deoxynucleoside salvage enzyme, has been recently considered as a protective cellular response to a wide range of agents interfering with DNA repair and apoptosis. In light of this, the potential contribution of dCK activation to apoptosis induction—presumably by supplying dATP or its analogues for the apoptosome formation—deserves consideration. Two‐hour exposure of human tonsillar lymphocytes to 2‐chloro‐deoxyadenosine (CdA) led to a two‐fold activation of dCK. This activation process was inhibited by pifithrin‐α, a potent inhibitor of p53. When the dNTP pools were determined, both deoxypyrimidine triphosphate and dGTP pools were reduced after the treatments, while dATP levels elevated by 62%, 77% and 50% in the CdA, aphidicolin and etoposide‐treated cells, respectively. We assume that dCK activation elicited by cellular damage might be a proapoptotic factor in terms of generating dATP well before the release of cytochrome c and deoxyguanosine kinase from mitochondria.

Deoxyribonucleoside Kinases in Mitochondrial DNA Depletion

Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are a heterogeneous group of mitochondrial disorders, manifested by a decreased mtDNA copy number and respiratory chain dysfunction. Primary MDS are inherited autosomally and may affect a single organ or multiple tissues. Mutated mitochondrial deoxyribonucleoside kinases; deoxyguanosine kinase (dGK) and thymidine kinase 2 (TK2), were associated with the hepatocerebral and myopathic forms of MDS respectively. dGK and TK2 are key enzymes in the mitochondrial nucleotide salvage pathway, providing the mitochondria with deoxyribonucleotides (dNP) essential for mtDNA synthesis. Although the mitochondrial dNP pool is physically separated from the cytosolic one, dNP's may still be imported through specific transport. Non ‐replicating tissues, where cytosolic dNP supply is down regulated, are thus particularly vulnerable to dGK and TK2 deficiency. The overlapping substrate specificity of deoxycytidine kinase (dCK) may explain the relative sparing of muscle in dGK deficiency, while low basal TK2 activity render this tissue susceptible toTK2 deficiency. The precise patho‐physiological mechanisms of mtDNA depletion due to dGK and TK2 deficiencies remain to be determined, though recent findings confirm that it is attributed to imbalanced dNTP pools.

The Expression of mRNA and Enzyme Activity of Deoxycytidine- and Deoxyguanosine Kinases in Cell from Patients with Untreated B-Cell Chronic Lymphocytic Leukemia.

Chronic lymphocytic leukemia (CLL) is the most common leukemia in Europe and North America. The purine analogues fludarabine and cladribine have earned an important place in the treatment of CLL. Yet, the alkylating agent chlorambucil still remains as cornerstone of treatment. A gradual shift towards the use of purine analogues as the central component has become apparent however it is of greatest importance to strictly compare the efficacy of fludarabine and cladribine to have a basis for future clinical studies with these agents. In this study 59 patients included in the international Phase III Trial for untreated B-cell CLL, were randomized in order to evaluate the efficacy of chlorambucil, fludarabine, and cladribine, as primary treatment of patients with symptomatic B-cell CLL. Both fludarabine and cladribine are prodrugs and must be phosphorylated intracellularly to monophosphates by the nuclear/cytosol enzyme deoxycytidine kinase (dCK) and possibly by the mitochondrial enzyme deoxyguanosine kinase (dGK). DCK plays a pivotal roll in the activation of fludarabine and cladribine and it has previously been reported that resistance to these drugs is mainly due to deficiency of the dCK.Before treatment, peripheral blood cells were isolated and the activity of dCK and dGK enzymes were analyzed in patient cells together with the dCK and dGK mRNA levels using the real-time quantitative PCR method.The dGK activity was considerably lower (6 fold) than the dCK activity. The enzyme activities in samples from CLL patients varied about 20 and 30 folds for dCK and dGK, respectively.The mRNA expression for dCK and dGK in patients showed also a large inter individual variability from 0,011 to 0,189 and 0,001 to 0,154 respectively.There was no correlation between enzyme activity and mRNA levels in the studied CLL patients. We could not found any spliced mRNA dCK variant in our materiel or any mutation, which could explain the discrepancy between dCK activity, and mRNA levels. In conclusion, the results suggest that dCK and dGK expression is regulated at the (post) translational level in leukemic cells. However, studies of the relationship between enzyme activities and/or mRNA levels with clinical response to chemotherapy are underway and will be reported.

Molecular insight into mitochondrial DNA depletion syndrome in two patients with novel mutations in the deoxyguanosine kinase and thymidine kinase 2 genes

Thymidine kinase 2 (TK2) and deoxyguanosine kinase (dGK) are the two key enzymes in mitochondrial DNA (mtDNA) precursor synthesis. Deficiencies in TK2 or dGK activity, due to genetic alteration, have been shown to cause tissue-specific depletion of mtDNA. In the case of TK2 deficiency, affected individuals suffer severe myopathy and, in the case of dGK deficiency, devastating liver or multi-systemic disease. Here, we report clinical and biochemical findings from two patients with mtDNA depletion syndrome. Patient A was a compound heterozygote carrying the previously reported T77M mutation and a novel mutation (R161K) in the TK2 gene. Patient B carried a novel mutation (L250S) in the dGK gene. The clinical symptoms of patient A included muscular weakness and exercise intolerance due to a severe mitochondrial myopathy associated with a 92% reduction in mtDNA. There was minimal involvement of other organs. Patient B suffered from rapidly progressive, early onset fatal liver failure associated with profoundly decreased mtDNA levels in liver and, to a lesser extent, in skeletal muscle. Site-directed mutagenesis was used to introduce the mutations detected in patients A and B into the TK2 and dGK cDNAs, respectively. We then characterized each of these recombinant enzymes. Catalytic activities of the three mutant enzymes were reduced to about 2–4% for TK2 and 0.5% for dGK as compared to the wild-type enzymes. Altered competition between dCyd and dThd was observed for the T77M mutant. The residual activities of the two mitochondrial enzymes correlated directly with disease development.

Conjugated Hyperbilirubinemia in Infancy (Mitochondrial DNA Depletion Syndrome, Liver)

CASE HISTORY A male infant was born at term after a normal pregnancy and was normal in early infancy. The mother in a previous marriage had 1 normal child, a girl, now 6 years old and healthy. The infant under discussion is the second born with her current spouse; the first born, a male, developed progressively worsening liver failure and died at age 9 months, and an etiologic diagnosis had not been established. This the second male infant presented at 3 months with poor feeding, irritability, lethargy, and jaundice. The infant had hepatomegaly of 4 cm. Investigations demonstrated hypoglycemia, conjugated hyperbilirubinemia, increased levels of alkaline phosphatase (574 U, normal 110–320 IU/L), aspartate aminotransferase (190 IU, normal 20–60 IU/L), alanine aminotransferase (144 IU, normal 6–50 IU/L), and gamma glutamyl transpeptidase (GGT) (624), an amino acid analytic result that was abnormal but not specific and in keeping with liver disease, normal result from organic acid analysis, increased a-fetoprotein (300,000 ng/mL), normal levels of very long-chain fatty acids, increased lactate (7.6 mmol/L), no ketosis, increased ammonia (40 lmol/L), prolonged prothrombin time, normal total carnitine, markedly decreased free carnitine (0.8, normal 30.3–40.5 nmol/L), and a normal acyl carnitine profile. All studies for an infectious etiology were negative. Muscle biopsy examined by hematoxylin and eosin, histochemistry, and electron microscopy was normal. Liver biopsy showed mitochondrial DNA depletion in the liver but not in fibroblasts. A mutation in the deoxyguanosine kinase (dGK) gene was excluded.

Anabolism of amdoxovir: phosphorylation of dioxolane guanosine and its 5′-phosphates by mammalian phosphotransferases

Amdoxovir [(−)-β-D-2,6-diaminopurine dioxolane, DAPD], the prodrug of dioxolane guanosine (DXG), is currently in Phase I/II clinical development for the treatment of HIV-1 infection. In this study, we examined the phosphorylation pathway of DXG using 15 purified enzymes from human (8), animal (6), and yeast (1) sources, including deoxyguanosine kinase (dGK), deoxycytidine kinase (dCK), high K m 5′-nucleotidase (5′-NT), guanylate (GMP) kinase, nucleoside monophosphate (NMP) kinase, adenylate (AMP) kinase, nucleoside diphosphate (NDP) kinase, 3-phosphoglycerate (3-PG) kinase, creatine kinase, and pyruvate kinase. In addition, the metabolism of 14C-labeled DXG was studied in CEM cells. DXG was not phosphorylated by human dCK, and was a poor substrate for human dGK with a high K m (7 mM). Human 5′-NT phosphorylated DXG with relatively high efficiency (4.2% of deoxyguanosine). DXG–MP was a substrate for porcine brain GMP kinase with a substrate specificity that was 1% of dGMP. DXG–DP was phosphorylated by all of the enzymes tested, including NDP kinase, 3-PG kinase, creatine kinase, and pyruvate kinase. The BB-isoform of human creatine kinase showed the highest relative substrate specificity (47% of dGDP) for DXG–DP. In CEM cells incubated with 5 μM DXG for 24 h, 0.015 pmole/10 6 cells (~7.5 nM) of DXG–TP was detected as the primary metabolite. Our study demonstrated that 5′-nucleotidase, GMP kinase, creatine kinase, and NDP kinase could be responsible for the activation of DXG in vivo.

Phosphorylation of Isocarbostyril‐ and Difluorophenyl‐Nucleoside Thymidine Mimics by the Human Deoxynucleoside Kinases

The thymidine mimics isocarbostyril nucleosides and difluorophenyl nucleosides were tested as deoxynucleoside kinase substrates using recombinant human cytosolic thymidine kinase (TK1) and deoxycytidine kinase (dCK), and mitochondrial thymidine kinase (TK2) and deoxyguanosine kinase (dGK). The isocarbostyril nucleoside compound 1‐(2‐deoxy‐β‐D‐ribofuranosyl)‐isocarbostyril (EN1) was a poor substrate with all the enzymes. The phosphorylation rates of EN1 with TK1 and TK2 were < 1% relative to Thd, where as the phosphorylation rates for EN1 were 1.4% and 1.1% with dCK and dGK relative to dCyd and dGuo, respectively. The analogue 1‐(2‐deoxy‐β‐D‐ribofuranosyl)‐7‐iodoisocarbostyril (EN2) showed poor relative‐phosphorylation efficiencies (k cat /K m ) with both TK1 and dGK, but not with TK2. The k cat /K m value for EN2 with TK2 was 12.6% relative to that for Thd. Of the difluorophenyl nucleosides, 5‐(1′‐(2′‐deoxy‐β‐D‐ribofuranosyl))‐2,4‐difluorotoluene (JW1) and 1‐(1′‐(2′‐deoxy‐β‐D‐ribofuranosyl))‐2,4‐difluoro‐5‐iodobenzene (JW2) were substrates for TK1 with phosphorylation efficiencies of about 5% relative to that for Thd. Both analogues were considerably more efficient substrates for TK2, with k cat /K m values of 45% relative to that for Thd. 2,5‐Difluoro‐4‐[1‐(2‐deoxy‐β‐L‐ribofuranosyl)]‐aniline (JW5), a L‐nucleoside mimic, was phosphorylated up to 15% as efficiently as deoxycytidine by dCK. These data provide a possible explanation for the previously reported lack of cytotoxicity of the isocarbostyril‐ and difluorophenyl nucleosides, but potential mitochondrial effects of EN2, JW1 and JW2 should be further investigated.

Mitochondrial deoxyguanosine kinase mutations and mitochondrial DNA depletion syndrome

Mitochondrial deoxyguanosine kinase (dGK) catalyzes the initial phosphorylation of purine deoxynucleosides. Mutations in the dGK gene leading to deficiency in dGK activity is one of the causes of severe mitochondrial DNA depletion diseases. We used site-directed mutagenesis to introduce the clinically observed genetic alterations in the dGK gene and characterized the recombinant enzymes. The R142K enzyme had very low activity with deoxyguanosine and no activity with deoxyadenosine. The E227K mutant enzyme had unchanged K m values for all its substrates but very low V max values. C-terminal truncated dGK proteins were inactive. These results may help to define the role of dGK in mitochondrial DNA (mtDNA) precursor synthesis.

Mitochondrial DNA depletion can be prevented by dGMP and dAMP supplementation in a resting culture of deoxyguanosine kinase-deficient fibroblasts.

Deoxyguanosine kinase is a constitutively expressed, mitochondrial enzyme of the deoxyribonucleoside salvage pathway. Deficiency of deoxyguanosine kinase causes early-onset, hepatocerebral mitochondrial DNA (mtDNA) depletion syndrome. To clarify the molecular mechanism of the disease, a skin fibroblast culture was studied from a patient carrying a homozygous nonsense mutation in the gene for deoxyguanosine kinase. In situ examination of DNA synthesis demonstrated that, although mtDNA synthesis is cell cycle independent in control fibroblasts, mtDNA synthesis occurs mainly during the S-phase in deoxyguanosine kinase-deficient cells. Consistent with this observation, it was found that the mtDNA content of exponentially growing, deoxyguanosine kinase-deficient cells is only mildly affected. When cycling is inhibited by serum-deprivation and cells are in a resting state, however, the mtDNA content drops considerably in deoxyguanosine kinase-deficient cells, yet remains stable in control fibroblasts. The decline in mtDNA content in resting, deoxyguanosine kinase-deficient cells can be prevented by dGMP and dAMP supplementation, providing conclusive evidence that substrate limitation triggers mtDNA depletion in deoxyguanosine kinase-deficient cells.