Deoxyguanosine kinase deficiency presenting as neonatal hemochromatosis

Deoxyguanosine kinase deficiency: natural history and liver transplant outcome

Autosomal recessive pathogenetic variants in the DGUOK gene cause deficiency of deoxyguanosine kinase activity and mitochondrial deoxynucleotides pool imbalance, consequently, leading to quantitative and/or qualitative impairment of mitochondrial DNA synthesis. Typically, patients present early-onset liver failure with or without neurological involvement and a clinical course rapidly progressing to death. This is an international multicentre study aiming to provide a retrospective natural history of deoxyguanosine kinase deficient patients. A systematic literature review from January 2001 to June 2023 was conducted. Physicians of research centres or clinicians all around the world caring for previously reported patients were contacted to provide followup information or additional clinical, biochemical, histological/histochemical, and molecular genetics data for unreported cases with a confirmed molecular diagnosis of deoxyguanosine kinase deficiency. A cohort of 202 genetically confirmed patients, 36 unreported, and 166 from a systematic literature review, were analyzed. Patients had a neonatal onset (≤ 1 month) in 55.7% of cases, infantile (>1 month and ≤ 1 year) in 32.3%, pediatric (>1 year and ≤18 years) in 2.5% and adult (>18 years) in 9.5%. Kaplan-Meier analysis showed statistically different survival rates (P < 0.0001) among the four age groups with the highest mortality for neonatal onset. Based on the clinical phenotype, we defined four different clinical subtypes: hepatocerebral (58.8%), isolated hepatopathy (21.9%), hepatomyoencephalopathy (9.6%), and isolated myopathy (9.6%). Muscle involvement was predominant in adult-onset cases whereas liver dysfunction causes morbidity and mortality in early-onset patients with a median survival of less than 1 year. No genotype-phenotype correlation was identified. Liver transplant significantly modified the survival rate in 26 treated patients when compared with untreated. Only six patients had additional mild neurological signs after liver transplant. In conclusion, deoxyguanosine kinase deficiency is a disease spectrum with a prevalent liver and brain tissue specificity in neonatal and infantile-onset patients and muscle tissue specificity in adult-onset cases. Our study provides clinical, molecular genetics and biochemical data for early diagnosis, clinical trial planning and immediate intervention with liver transplant and/or nucleoside supplementation.

The genetics of portal hypertension: Recent developments and the road ahead

Portal hypertension (PH), defined as a pathological increase in the portal vein pressure, has different aetiologies and causes. Intrahepatic PH is mostly secondary to the presence of underlying liver disease leading to cirrhosis, characterized by parenchymal changes with deregulated accumulation of extracellular matrix and vascular abnormalities; liver sinusoidal endothelial cells and hepatic stellate cells are key players in PH progression, able to influence each other. However, PH may also develop independently of parenchymal damage, as occur in portosinusoidal vascular disorder (PSVD), a group of clinical and histological entities characterized by portal vasculature dysfunctions. In this particular group of disorders, the pathophysiology of PH is still poorly understood. In the last years, several genetic studies, based on genome-wide association studies or whole-exome sequencing analysis, have highlighted the importance of genetic heritability in PH pathogenesis, both in cirrhotic and non-cirrhotic cases. The common PNPLA3 p.I148M variant, one of the main determinants of the susceptibility to steatotic liver disease, has also been associated with decompensation in patients with PH. Genetic variations at loci influencing coagulation, mainly the ABO locus, may directly contribute to the pathogenesis of PH. Rare genetic variants have been associated with familiar cases of progressive PSVD. In this review, we summarize the recent knowledges on genetic variants predisposing to PH development, contributing to better understand the role of genetic factors in PH pathogenesis.

The clinical variations and diagnostic challenges of deoxyguanosine kinase deficiency: a descriptive case series

OBJECTIVES

Deoxyguanosine kinase (DGUOK) deficiency is one of the leading causes of the mitochondrial DNA-depletion syndromes (MDDS) associated with hepatocerebral involvement. Herein, we present four cases of DGUOK deficiency to emphasize the clinical variability of disease and the challenges in the diagnosis of DGUOK deficiency.

CASE PRESENTATION

Hepatomegaly, hyperlactatemia, elevated alpha fetoprotein (AFP), alanine, and transaminase levels were detected in all patients, and cholestasis, coagulopathy, and hypotonia were common findings. All patients had a low birth weight, one patient underwent liver transplantation (LT). Clinical and laboratory findings of two patients and one patient suggested neonatal hemochromatosis and type 1 tyrosinemia, respectively. All patients were diagnosed with DGUOK deficiency by performing molecular genetic analysis.

CONCLUSIONS

Mitochondrial DNA-depletion syndromes should be kept in mind in cases in which hypotonicity, lactic acidosis, and neonatal cholestasis are observed. DGUOK deficiency may present in different clinics suggesting neonatal hemochromatosis or tyrosinemia type 1.

A Combined Model of Human iPSC-Derived Liver Organoids and Hepatocytes Reveals Ferroptosis in DGUOK Mutant mtDNA Depletion Syndrome

Mitochondrial DNA depletion syndrome (MDS) is a group of severe inherited disorders caused by mutations in genes, such as deoxyribonucleoside kinase (DGUOK). A great majority of DGUOK mutant MDS patients develop iron overload progressing to severe liver failure. However, the pathological mechanisms connecting iron overload and hepatic damage remains uncovered. Here, two patients' skin fibroblasts are reprogrammed to induced pluripotent stem cells (iPSCs) and then corrected by CRISPR/Cas9. Patient-specific iPSCs and corrected iPSCs-derived high purity hepatocyte organoids (iHep-Orgs) and hepatocyte-like cells (iHep) are generated as cellular models for studying hepatic pathology. DGUOK mutant iHep and iHep-Orgs, but not control and corrected one, are more sensitive to iron overload-induced ferroptosis, which can be rescued by N-Acetylcysteine (NAC). Mechanically, this ferroptosis is a process mediated by nuclear receptor co-activator 4 (NCOA4)-dependent degradation of ferritin in lysosome and cellular labile iron release. This study reveals the underlying pathological mechanisms and the viable therapeutic strategies of this syndrome, and is the first pure iHep-Orgs model in hereditary liver diseases.

[DGUOK-related mitochondrial DNA depletion syndrome: a case report and literature review]

A boy, aged 4 months, had the major clinical manifestations of prolonged jaundice and hepatomegaly. Multiple biochemical tests revealed abnormal liver function along with elevated alpha-fetoprotein and lactate. Genetic analysis confirmed that the boy had the mutations of c.589C>T(p.Gln197Ter) and c.687G>C(p.Trp229Cys) in the DGUOK gene, both of which were novel mutations and were determined to be pathogenic and likely pathogenic respectively, by a variety of bioinformatics tools and the ACMG standard. Therefore, the boy was confirmed to have DGUOK-related mitochondrial DNA depletion syndrome. Literature review showed that onset of liver disease in infancy was the main clinical feature of this disease, and some children presented with nervous system manifestations. Abnormal laboratory results included abnormal liver function, increases in blood lactate, serum ferritin and alpha-fetoprotein, and hypoglycemia. Such children had marked heterogeneity of DGUOK gene mutations, with missense mutations as the most common type. This disease tended to have a poor prognosis, and 79.6% of the children died before the age of 3 years.

Sub-acute neonatal hemochromatosis in an infant with hypoplastic left heart syndrome on ventricular assist device awaiting transplantation

Single-ventricle pediatric patients, amongst other children waiting for OHT, are a vulnerable population, especially if candidacy is established before any palliation. NH is a rare disease with poor prognosis in the post-natal period. We present a case of sub-acute NH diagnosed in an infant with HLHS who was listed for OHT while bridged with a pulsatile paracorporeal VAD, with an emphasis on the evolution of the condition throughout the patient's clinical course and the ultimate decision for compassionate deactivation of VAD.

Clinical and biochemical footprints of inherited metabolic diseases. II. Metabolic liver diseases

Inherited metabolic diseases account for about one third of pediatric patients with hepatomegaly, acute liver failure, cirrhosis or cholestasis. Specifically for pediatric acute liver failure, they account for 10-15% of cases, with a mortality of 22-65%. The percentage of acute liver failure caused by an inherited metabolic disease in children <2-3 years of age is even higher, ranging from a third to half of all cases. Metabolic liver disease accounts for 8-13% of all pediatric liver transplantations. Despite this high burden of disease, underdiagnosis remains common. We reviewed and updated the list of known metabolic etiologies associated with various types of metabolic liver involvement, and found 142 relevant inborn errors of metabolism. This represents the second of a series of articles attempting to create and maintain a comprehensive list of clinical and metabolic differential diagnoses according to system involvement.

A case of Niemann-Pick disease type C with neonatal liver failure initially diagnosed as neonatal hemochromatosis

BACKGROUND

Niemann-Pick type C (NPC) is a lysosomal lipid storage disease with mutation of NPC1/NPC2 genes, which transport lipids in the endosome and lysosome, and various neurological symptoms. NPC patients also develop hepatosplenomegaly or liver disorder in the neonatal period, and 10% suffer severe liver failure. Neonatal hemochromatosis (NH) is a liver disorder characterized by hepatic and extrahepatic siderosis. Although the etiology of NH is unclear, recent reports suggest that the gestational alloimmune mechanism is the cause of NH. Herein, we report a Japanese NPC patient initially diagnosed as NH.

CASE REPORT

A 5-day-old boy was transferred to our hospital with severe cholestatic liver failure. Congenital infections and metabolic screening were negative, and NH was suspected. However intra and extrahepatic siderosis were not found. As his liver deteriorated rapidly, liver transplantation was performed at 19 days old. The explanted liver showed cirrhosis, and strong C5b-9 complex staining of hepatocytes, so NH was diagnosed. From the age of one and a half years, he developed regression, vertical supranuclear gaze palsy and cataplexy. Fibroblast filipin staining was strong, blood oxysterol was high, and there were compound heterozygous mutations in NPC1,p.[(F288L)];[(K1206N)]. The patient was then diagnosed as NPC and started on miglustat.

CONCLUSION

Neonatal liver failure was initially diagnosed as NH. Later, the patient developed various neurological symptoms characteristic of NPC. Neurological follow-up of children who develop NH is required.

A fatal case of mitochondrial DNA depletion syndrome with novel compound heterozygous variants in the deoxyguanosine kinase gene

The deoxyguanosine kinase (DGUOK) gene controls mitochondrial DNA (mtDNA) maintenance, and variation in the gene can alter or abolish the anabolism of mitochondrial deoxyribonucleotides. A Chinese female infant, whose symptoms included weight stagnation, jaundice, hypoglycemia, coagulation disorders, abnormal liver function, and multiple abnormal signals in the brain, died at about 10 months old. Genetic testing revealed a compound heterozygote of alleles c.128T>C (p.I43T) and c.313C>T (p.R105^*) of the DGUOK gene. c.128T>C (p.I43T) is a novel variant located in exon 1 (NM_080916) in the first beta sheet of DGUOK. Her mother was an allele c.313C>T (p.R105^*) heterozygote, which is located in DGUOK exon 2 (NM_080916) between the third and fourth alpha helixes. c.313C>T (p.R105^*) is predicted to result in a 173 amino acid residue truncation at the C terminus of DGUOK. There are as many as 112 infantile mtDNA depletion syndrome (MDS) cases in the literature related to DGUOK gene variants. These variants include missense mutations, nucleotide deletion, nucleotide insertion, and nucleotide duplication. Integrated data showed that mutations affected both conserved and non-conserved DGUOK amino acids and are associated with patient deaths.

Deoxyguanosine kinase deficiency: a report of four patients

BACKGROUND

Hepatic involvement is a common feature in childhood mitochondrial disorders. Deoxyguanosine kinase (DGUOK) deficiency is one of the mitochondrial DNA depletion syndromes associated with hepatocerebral syndrome. Hepatic disease and neurologic dysfunction occurs within weeks after birth. Low birth weight is one of the common features. This study aims to describe the clinical and laboratory features of four infants carrying four different pathogenic variants in the DGUOK gene.

CASE PRESENTATION

Common clinical findings were progressive cholestatic liver failure, hypoglycemia, hypotonia and rotatory nystagmus in our DGUOK deficiency patients. Lactic acidosis, elevated serum tyrosine and ferritin levels were the striking laboratory features. Cholestasis, iron deposits, microvesicular steatosis and fibrosis were the histopathological findings seen in liver biopsies of two patients. All patients died with multi-organ failure between the ages of 42 days and 6 months.

CONCLUSIONS

While neurologic findings may occur later in the course of the disease, elevated serum tyrosine levels may alert the physicians to a DGUOK deficiency in a baby with hepatopathy in the presence of the mentioned signs. Early diagnosis is important not only for genetic counseling but also for a possible liver transplantation.

Mitochondrial DNA maintenance defects

The maintenance of mitochondrial DNA (mtDNA) depends on a number of nuclear gene-encoded proteins including a battery of enzymes forming the replisome needed to synthesize mtDNA. These enzymes need to be in balanced quantities to function properly that is in part achieved by exchanging intramitochondrial contents through mitochondrial fusion. In addition, mtDNA synthesis requires a balanced supply of nucleotides that is achieved by nucleotide recycling inside the mitochondria and import from the cytosol. Mitochondrial DNA maintenance defects (MDMDs) are a group of diseases caused by pathogenic variants in the nuclear genes involved in mtDNA maintenance resulting in impaired mtDNA synthesis leading to quantitative (mtDNA depletion) and qualitative (multiple mtDNA deletions) defects in mtDNA. Defective mtDNA leads to organ dysfunction due to insufficient mtDNA-encoded protein synthesis, resulting in an inadequate energy production to meet the needs of affected organs. MDMDs are inherited as autosomal recessive or dominant traits, and are associated with a broad phenotypic spectrum ranging from mild adult-onset ophthalmoplegia to severe infantile fatal hepatic failure. To date, pathogenic variants in 20 nuclear genes known to be crucial for mtDNA maintenance have been linked to MDMDs, including genes encoding enzymes of mtDNA replication machinery (POLG, POLG2, TWNK, TFAM, RNASEH1, MGME1, and DNA2), genes encoding proteins that function in maintaining a balanced mitochondrial nucleotide pool (TK2, DGUOK, SUCLG1, SUCLA2, ABAT, RRM2B, TYMP, SLC25A4, AGK, and MPV17), and genes encoding proteins involved in mitochondrial fusion (OPA1, MFN2, and FBXL4).

MicroRNAs: tools of mechanistic insights and biological therapeutics discovery for the rare neurogenetic syndrome Lesch-Nyhan disease (LND)

MicroRNAs (miRNAs) are small regulatory RNAs that modulate the translation of mRNA. They have emerged over the past few years as indispensable entities in the transcriptional regulation of genes. Their discovery has added additional layers of complexity to regulatory networks that control cellular homeostasis. Also, their dysregulated pattern of expression is now well demonstrated in myriad diseases and pathogenic processes. In the current review, we highlight the role of miRNAs in Lesch-Nyhan disease (LND), a rare neurogenetic syndrome caused by mutations in the purine metabolic gene encoding the hypoxanthine-guanine phosphoribosyltransferase (HPRT) enzyme. We describe how experimental and biocomputational approaches have helped to unravel genetic and signaling pathways that provide mechanistic understanding of some of the molecular and cellular basis of this ill-defined neurogenetic disorder. Through miRNA-based target predictions, we have identified signaling pathways that may be of significance in guiding biological therapeutic discovery for this incurable neurological disorder. We also propose a model to explain how a gene such as HPRT, mostly known for its housekeeping metabolic functions, can have pleiotropic effects on disparate genes and signal transduction pathways. Our hypothetical model suggests that HPRT mRNA transcripts may be acting as competitive endogenous RNAs (ceRNAs) intertwined in multiregulatory cross talk between key neural transcripts and miRNAs. Overall, this approach of using miRNA-based genomic approaches to elucidate the molecular and cellular basis of LND and guide biological target identification might be applicable to other ill-defined rare inborn-error metabolic diseases.

Neonatal hemochromatosis: diagnostic work-up based on a series of 56 cases of fetal death and neonatal liver failure

OBJECTIVE

To define an algorithm to improve diagnosis of neonatal hemochromatosis (NH) related to gestational alloimmune liver disease (GALD), which is diagnosed by immunohistochemistry demonstrating activated complement at hepatocytes (IDACH).

STUDY DESIGN

We assessed 56 instances of fetal death or neonatal liver failure (NLF; 2006-2009), 29 (7 stillborns, 22 NLF) with NH, and 27 (5 stillborns, 22 NLF) without NH (non-NH). Immunohistochemistry was retrospectively performed in 21 cases. Cases were grouped as follows: (1) GALD as demonstrated by IDACH (n = 17); (2) indeterminate for GALD (n = 28); or (3) alternate diagnosis found (n = 11). We compared cases of immunohistochemically proven GALD with those with an alternate diagnosis.

RESULTS

Of the 12 stillborns, 7 had NH because of GALD (NH-GALD), one was undeterminate, and 4 had alternate diagnoses (GALD excluded). Of the 22 newborns with NH, 6 had NH-GALD, one had mitochondrial respiratory chain disorder (MRCD), and 15 were indeterminate for GALD. Of 22 non-NH newborns, extrahepatic siderosis (EHS) was not assessed in 13 (3 GALD, 1 alternate diagnosis [MRCD] and 9 indeterminate GALD) and excluded in 9 (5 alternate diagnoses and 4 indeterminate GALD). The only clinical features found to be associated with GALD were intrafamilial recurrence, prematurity, and EHS.

CONCLUSIONS

In unexplained fetal death or NLF, the diagnosis of subsets of NH requires tissue analysis (autopsy) to assess EHS. In patients with NH, if MRCD is ruled out, NH-GALD is likely. The rate of IDACH in the diagnosis of GALD in cases without NH requires further study.

Transient neonatal liver disease after maternal antenatal intravenous Ig infusions in gestational alloimmune liver disease associated with neonatal haemochromatosis

OBJECTIVES

Neonatal haemochromatosis is a rare gestational disease that results in severe foetal liver disease with extrahepatic iron overload, sparing the reticuloendothelial system. Recurrence can be prevented with intravenous immunoglobulin (IVIG) infusions during pregnancy, supporting an alloimmune aetiology. The aim of the study was to assess the effect of antenatal treatment with IVIG infusion on the outcome of pregnancies in women with a history of documented neonatal haemochromatosis likely owing to gestational alloimmune disease and to analyse IVIG tolerance.

METHODS

From 2004 to 2012, 8 pregnant women were treated with IVIG at 1 g/kg body weight weekly from 18 weeks' gestation until birth in a prospective multicentre study.

RESULTS

All 8 neonates born to the treated women survived. Five developed mild neonatal liver disease with hepatomegaly (n = 1), hyperechogenic liver (n = 2), abnormal liver function tests (n = 1), raised serum ferritin (n = 3) and α-fetoprotein (n = 5) levels, or mild iron overload on liver magnetic resonance imaging (n = 1). Ferritin and α-fetoprotein levels normalised before 14 days and 2 months, respectively. A per-mother-basis analysis comparing outcomes of treated (n = 8) and untreated (n = 9) gestations showed a significant improvement in the survival of neonates with gestational IVIG therapy (survival 8/8 vs 0/9, P < 0.001). Adverse effects of IVIG infusion occurred in 5 mothers leading to discontinuation of treatment in 1 case. Preterm neonates born before 37 weeks' gestation had a decreased risk of neonatal liver disease (P = 0.04).

CONCLUSIONS

Antenatal treatment with IVIG infusion in women at risk for gestational alloimmune disease recurrence improves the outcome of pregnancies despite mild signs of transient neonatal liver disease.

Iron metabolism in transplantation

Recipient's iron status is an important determinant of clinical outcome in transplantation medicine. This review addresses iron metabolism in solid organ transplantation, where the role of iron as a mediator of ischemia-reperfusion injury, as an immune-modulatory element, and as a determinant of organ and graft function is discussed. Although iron chelators reduce ischemia-reperfusion injury in cell and animal models, these benefits have not yet been implemented into clinical practice. Iron deficiency and iron overload are associated with reduced immune activation, whose molecular mechanisms are reviewed in detail. Furthermore, iron overload and hyperferritinemia are associated with poor prognosis in end-stage organ failure in patients awaiting kidney, or liver transplantation. This negative prognostic impact of iron overload appears to persist after transplantation, which highlights the need for optimizing iron management before and after solid organ transplantation. In contrast, iron deficiency and anemia are also associated with poor prognosis in patients with end-stage heart failure. Intravenous iron supplementation should be managed carefully because parenterally induced iron overload could persist after successful transplantation. In conclusion, current evidence shows that iron overload and iron deficiency are important risk factors before and after solid organ transplantation. Iron status should therefore be actively managed in patients on the waiting list and after transplantation.

Molecular diagnosis of mitochondrial respiratory chain disorders in Japan: focusing on mitochondrial DNA depletion syndrome

BACKGROUND

Although mitochondrial respiratory chain disorders (MRCD) are one of the most common congenital metabolic diseases, there is no cumulative data on enzymatic diagnosis and clinical manifestation for MRCD in Japan and Asia.

METHODS

We evaluated 675 Japanese patients having profound lactic acidemia, or patients having symptoms or signs of multiple-organ origin simultaneously without lactic acidemia on respiratory chain enzyme activity assay and blue native polyacrylamide gel electrophoresis. Quantitative polymerase chain reaction was used to diagnose mitochondrial DNA depletion syndrome (MTDPS). Mutation analysis of several genes responsible for MTDPS was also performed.

RESULTS

A total of 232 patients were diagnosed with a probable or definite MRCD. MRCD are common, afflicting one in every several thousand people in Japan. More than one in 10 of the patients diagnosed lacked lactic acidemia. A subsequent analysis of the causative genes of MTDPS identified novel mutations in six of the patients. A 335 bp deletion in deoxyguanosine kinase (DGUOK; g.11692_12026del335 (p.A48fsX90)) was noted in two unrelated families, and may therefore be a common mutation in Japanese people. The proportion of all patients with MTDPS, and particularly those with recessive DNA polymerase γ (POLG) mutations, appears to be lower in Japan than in other studies. This is most likely due to the relatively high prevalence of ancient European POLG mutations in Caucasian populations. No other significant differences were identified in a comparison of the enzymatic diagnoses, disease classifications or prognoses in Japanese and Caucasian patients with MRCD.

CONCLUSION

MTDPS and other MRCD are common, but serious, diseases that occur across all races.

Neonatal hemochromatosis

Neonatal hemochromatosis is a clinical condition in which severe liver disease in the newborn is accompanied by extrahepatic siderosis. Gestational alloimmune liver disease (GALD) has been established as the cause of fetal liver injury resulting in nearly all cases of NH. In GALD, a women is exposed to a fetal antigen that she does not recognize as "self" and subsequently begins to produce IgG antibodies that are directed against fetal hepatocytes. These antibodies bind to fetal liver antigen and activate the terminal complement cascade resulting in hepatocyte injury and death. GALD can cause congenital cirrhosis or acute liver failure with and without iron overload and siderosis. Practitioners should consider GALD in cases of fetal demise, stillbirth, and neonatal acute liver failure. Identification of infants with GALD is important as treatment is available and effective for subsequent pregnancies.

Liver pathology in infantile mitochondrial DNA depletion syndrome

Mitochondrial DNA (mtDNA) depletion syndrome is a relatively novel cause of hepatic dysfunction in the pediatric population. It is caused by mutations in either mtDNA or nuclear DNA (nDNA) that result in a quantitative reduction in mtDNA and, in turn, dysfunctional oxidative phosphorylation. In infants, it results in the hepatocerebral phenotype, characterized by hyperbilirubinemia, coagulopathy, lactic acidosis, hypoglycemia, lethargy, encephalopathy, developmental delay, and hypotonia. Three infants diagnosed with mtDNA depletion syndrome at The Children's Hospital of Philadelphia were identified, and their clinical presentation, disease course, and histologic and ultrastructural features of liver samples (pre- and postmortem) were characterized. While a different mutant gene was identified in each child, they all showed clinical evidence of metabolic dysfunction soon after birth and expired by 1 year of age. Steatosis, cholestasis, and cytoplasmic crowding by atypical mitochondria were consistent pathologic liver findings. Other findings included hepatocyte hypereosinophilia, fibrosis, and hemosiderosis. This analysis provides insight into the important clinical signs/symptoms and histopathologic and ultrastructural features of mtDNA depletion syndrome in infants and young children. Knowledge of these characteristics will facilitate early recognition and appropriate treatment of this rare disorder. Additionally, ultrastructural evaluation of liver samples by electron microscopy is an important diagnostic component of hepatic dysfunction caused by metabolic abnormalities. This type of analysis should be routinely employed in the setting of unexplained cholestasis, especially when accompanied by steatosis and hepatocyte hypereosinophilia.

Mitochondrial DNA depletion syndromes: review and updates of genetic basis, manifestations, and therapeutic options

Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are a genetically and clinically heterogeneous group of autosomal recessive disorders that are characterized by a severe reduction in mtDNA content leading to impaired energy production in affected tissues and organs. MDS are due to defects in mtDNA maintenance caused by mutations in nuclear genes that function in either mitochondrial nucleotide synthesis (TK2, SUCLA2, SUCLG1, RRM2B, DGUOK, and TYMP) or mtDNA replication (POLG and C10orf2). MDS are phenotypically heterogeneous and usually classified as myopathic, encephalomyopathic, hepatocerebral or neurogastrointestinal. Myopathic MDS, caused by mutations in TK2, usually present before the age of 2 years with hypotonia and muscle weakness. Encephalomyopathic MDS, caused by mutations in SUCLA2, SUCLG1, or RRM2B, typically present during infancy with hypotonia and pronounced neurological features. Hepatocerebral MDS, caused by mutations in DGUOK, MPV17, POLG, or C10orf2, commonly have an early-onset liver dysfunction and neurological involvement. Finally, TYMP mutations have been associated with mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease that typically presents before the age of 20 years with progressive gastrointestinal dysmotility and peripheral neuropathy. Overall, MDS are severe disorders with poor prognosis in the majority of affected individuals. No efficacious therapy is available for any of these disorders. Affected individuals should have a comprehensive evaluation to assess the degree of involvement of different systems. Treatment is directed mainly toward providing symptomatic management. Nutritional modulation and cofactor supplementation may be beneficial. Liver transplantation remains controversial. Finally, stem cell transplantation in MNGIE disease shows promising results.

Neonatal liver failure due to deoxyguanosine kinase deficiency

Deoxyguanosine kinase (dGK) deficiency, a rare severe cause of mitochondrial DNA (mtDNA) depletion, has two forms of presentation: hepatocerebral syndrome and isolated hepatic disease. The authors report three cases with neonatal liver failure due to dGK deficiency. Consanguinity was present in all patients. One patient had a brother who died with a probable diagnosis of neonatal haemochromatosis. All patients had progressive cholestatic liver failure, hypoglycaemia, hyperlactacidaemia, elevated ferritin levels and nystagmus, since first day of life. Liver tissue study revealed: cholestasis, iron deposits, microvesicular steatosis and fibrosis/cirrhosis. Only one patient was submitted to liver transplantation. The other two died, at 2 and 5 months of age. mtDNA quantification and DGUOK gene study should be considered in infants/neonates with acute liver failure and systematically performed in patients with hepatocerebral presentation. Differential diagnosis with neonatal haemochromatosis is needed. Liver transplantation might be a therapeutic option. Early diagnosis is important for genetic counselling.

Two patients with hepatic mtDNA depletion syndromes and marked elevations of S-adenosylmethionine and methionine

This paper reports studies of two patients proven by a variety of studies to have mitochondrial depletion syndromes due to mutations in either their MPV17 or DGUOK genes. Each was initially investigated metabolically because of plasma methionine concentrations as high as 15-21-fold above the upper limit of the reference range, then found also to have plasma levels of S-adenosylmethionine (AdoMet) 4.4-8.6-fold above the upper limit of the reference range. Assays of S-adenosylhomocysteine, total homocysteine, cystathionine, sarcosine, and other relevant metabolites and studies of their gene encoding glycine N-methyltransferase produced evidence suggesting they had none of the known causes of elevated methionine with or without elevated AdoMet. Patient 1 grew slowly and intermittently, but was cognitively normal. At age 7 years he was found to have hepatocellular carcinoma, underwent a liver transplant and died of progressive liver and renal failure at age almost 9 years. Patient 2 had a clinical course typical of DGUOK deficiency and died at age 8 ½ months. Although each patient had liver abnormalities, evidence is presented that such abnormalities are very unlikely to explain their elevations of AdoMet or the extent of their hypermethioninemias. A working hypothesis is presented suggesting that with mitochondrial depletion the normal usage of AdoMet by mitochondria is impaired, AdoMet accumulates in the cytoplasm of affected cells poor in glycine N-methyltransferase activity, the accumulated AdoMet causes methionine to accumulate by inhibiting activity of methionine adenosyltransferase II, and that both AdoMet and methionine consequently leak abnormally into the plasma.