DGUOK-related mitochondrial DNA depletion syndrome in a child with an early diagnosis of glycogen storage disease.

A 4-month-old boy was referred to our center for hepatosplenomegaly and hypoglycemia with an early diagnosis of glycogen storage disease. This index case, the third child of consanguineous Turkish parents, was born at term following an uncomplicated pregnancy by normal vaginal delivery with a birth weight of 2900 g. Physical examination at the time of admission revealed failure to thrive, microcephaly, hypotonia, typical rotary nystagmus, hepatosplenomegaly, jaundice, and ascites. Blood testing showed conjugated hyperbilirubinemia, coagulopathy (partial thromboplastin time 19.2 seconds, international normalized ratio 1.63), and preprandial hypoketotic severe hypoglycemia (glucose level 26 mg/dL, normal values 70–110 and simultaneous blood ketone level 0.2 mmol/L). Growth hormone, insulin, and cortisol levels were normal. His aspartate aminotransferase, alanine aminotransferase, bilirubin, and γ-glutamyltransferase levels were elevated (aspartate aminotransferase 149 IU/L, alanine aminotransferase 58 IU/L, total bilirubin 8.6 mg/dL, direct bilirubin 5.8 mg/dL, γ-glutamyltransferase 193 IU/L). Tandem mass spectroscopy revealed elevations of alanine and tyrosine. Urinary organic acid profile showed nonspecific mild elevation of dicarboxylic acids. Urinary succinylacetone was not detected, and plasma amino acid analysis was remarkable for elevations of tyrosine suggestive of liver dysfunction. Transferrin isoelectric focusing was normal. His venous blood lactate level was elevated (3.9 mmol/L normal level<2 mmol/L). Further evaluation to determine the etiology of his hepatic failure revealed no evidence for bacterial and viral infections or galactosemia, but an elevated serum concentration of ferritin (463; normal values <150 ng/mL) and serum α-fetoprotein (334.5; age-matched upper limit of normal <7 ng/mL). Serum bile acid levels were also increased. Cardiologic examination showed persistent ductus arteriosus and secundum atrial septal defect. Open liver biopsy was performed at the age of 3½ months. Liver histopathology showed neocholangiole proliferation, marked cholestasis, and parenchymal injury accompanied by bridging fibrosis leading to micronodular transformation and cirrhosis. He progressed to liver failure with cholestasis, hypoalbuminemia, portal hypertension with intractable ascites, hypersplenism with thrombocytopenia, severely prolonged international normalized ratio, and an elevated partial thromboplastin time. The patient required regular fresh frozen plasma transfusions, vitamin K1, ursodeoxycholic acid, and spironolactone for ascites. We used formulas enriched with medium-chain triglyceride content and fractional meals with enteral nutrition at night for adequate nutrition to prevent hypoglycemia. He continued to deteriorate rapidly, leading to death at 4 months of age. Subsequently, a homozygous C to T mutation at nucleotide position 313 (c.313C>T) in exon 3 and introducing a stop codon mutation p.R105X was identified in the DGUOK gene. The patient is affected with mitochondrial DNA depletion syndrome (MDDS) caused by a nonfunctional DGUOK gene. Both parents are heterozygous carriers of this stop codon mutation. DISCUSSION We report the clinical and genetic findings of a 4-month-old boy of Turkish descent with a homozygous stop codon mutation p.R105X in the DGUOK gene. MDDS is an expanding group of clinically and genetically heterogeneous diseases that are now seen as a major cause of oxidative impairment in infancy or childhood. They present in a tissue-specific manner with a myopathic, hepatocerebral, or encephalomyopathic forms (1,2). Identification of the specific deficiency carries important prognostic information. Mitochondrial deoxyguanosine kinase (Online Mendelian Inheritance in Man no. 601465) is encoded by the 32-kb DGUOK gene on chromosome 2p13. It has been documented that DGUOK mutations cause nucleotide pool imbalance, which leads to inefficient mitochondrial DNA replication and, hence, to mitochondrial DNA depletion. The majority of affected individuals have a multisystem illness with hepatic disease and neurologic dysfunction evident within weeks of birth. Mutations in DGUOK cause mitochondrial DNA depletion; liver involvement seems to be the most prominent feature, leading to liver cirrhosis and early-onset liver failure. They subsequently manifest severe hypotonia, developmental regression, and typical rotary nystagmus. Nystagmus is the first sign of neuronal involvement. In contrast, the encephalopathy may be mild or absent. Conversely, the absence of eye abnormalities may portend a better prognosis and a possible beneficial effect of liver transplantation. Death in the first year of life is the usual outcome in patients with hepatocerebral involvement, although early liver transplantation has been shown to be effective in cases with isolated hepatic involvement (3–5). Episodes of low glucose concentrations (hypoketotic) were observed in this patient. The literature suggests that DGUOK gene mutations combined with impaired glucose homeostasis and pancreatic islet cell hyperplasia with numerous confluent giant islets found at autopsied infants may contribute to hypoglycemia in deoxyguanosine kinase deficiency (6). Iron overload may also damage mitochondrial DNA–depleted tissues. The presence of severe hypoglycemia, cholestatic jaundice, and wandering nystagmus suggested the diagnosis of septo-optic dysplasia, but we did not confirm hypopituitarism by using hormonal profiles and radiological techniques (7). The other noticeable laboratory abnormality in this patient was hyperferritinemia. The mitochondrion plays a prominent role in iron metabolism, and mitochondrial dysfunction can lead to pathological iron overload (8). The association of the lactate, tyrosine, ferritin, α-fetoprotein elevation, neurological involvement (hypotonia, failure to thrive and motor retardation, nystagmus), and hepatopathy was strongly suggestive of hepatocerebral MDDS. An ideal diagnostic strategy for hepatocerebral MDDS thus needs to include sequence analysis of POLG1, MPV17, and C100RF2, in addition to DGUOK gene mutations. In children with POLG1 mutations, the most prominent symptom is intractable progressive epilepsy. Liver involvement can be mild initially but may worsen rapidly, especially after exposure to valproic acid. C100RF2 (TWINKLE) is required for mitochondrial DNA replication together with POLG. Recessive defects of TWINKLE cause severe encephalohepatopathies of early-onset and mitochondrial DNA depletion in brain and liver. Epilepsy is severe and treatment resistant. Overall, TWINKLE and POLG syndromes are clinically overlapping, and both genes should be studied in early-onset encephalohepatopathies. MPV17 mutations have been described in infantile-onset hepatocerebral MDDS. The MPV17-MDS is also associated with variable degrees of demyelination both in the central and in the peripheral nervous systems (1,2). This boy had no convulsions, and brain magnetic resonance imaging was normal for age. The DGUOK gene was suspected to be the disease-causing gene based on clinical, biochemical, and radiological findings in this patient. A clinical picture with fetal growth restriction, postnatal lactacidosis, hypoglycemia, coagulopathy, and cholestasis, especially in combination with neurological symptoms or renal tubulopathy, should alert the neonatologist or hepatologist to direct investigations on mitochondrial disorder (5,8).