Natural history of deoxyguanosine kinase deficiency.

BACKGROUND AND OBJECTIVES: Deoxyguanosine kinase deficiency is one genetic cause of mtDNA depletion syndrome. Its major phenotypes include neonatal/infantile-onset hepatocerebral disease, isolated hepatic disease and myopathic disease. In this retrospective study, we seek to describe the natural history of deoxyguanosine kinase deficiency and identify any genotype-phenotype correlations. METHODS: Retrospective literature search and collation of data from genetically confirmed cases of deoxyguanosine kinase deficiency. RESULTS: 173 cases of DGUOK deficiency were identified. Neonatal/infantile-onset hepatocerebral disease accounted for 128 (74%) of cases. Isolated liver disease was seen in 36 (21%) and myopathic disease in 9 (5%) of cases. The most frequently involved systems were liver (98%), brain (75%), growth (46%) and gastrointestinal tract (26%). Infantile-onset disease typically presented with cholestatic jaundice and lactic acidosis. Neurological involvement included hypotonia, nystagmus and developmental delay with MRI brain abnormalities in about half of cases. Missense variants accounted for 48% of all pathogenic variants while variants resulting in truncated transcripts accounted for 39%. Prognosis was poor, especially for neonatal/ infantile-onset hepatocerebral disease for which 1 year survival was 11%. Twenty-three patients received liver transplants, of whom 12 died within 2 years of transplant. Patients with two truncating variants had a higher risk of death and were more likely to have the neonatal/infantile-onset hepatocerebral disease phenotype. No blood biomarker predictive of neurological involvement was identified. Earlier onset correlated with increased mortality. CONCLUSIONS: There is a narrow window for therapeutic intervention. For the hepatocerebral disease phenotype, median age of onset was 1 month while the median age of death was 6.5 months implying rapid disease progression.

Thymidine Kinase 2 and Mitochondrial Protein COX I in the Cerebellum of Patients with Spinocerebellar Ataxia Type 31 Caused by Penta-nucleotide Repeats (TTCCA)n.

Spinocerebellar ataxia type 31 (SCA31), an autosomal-dominant neurodegenerative disorder characterized by progressive cerebellar ataxia with Purkinje cell degeneration, is caused by a heterozygous 2.5-3.8 kilobase penta-nucleotide repeat of (TTCCA)n in intron 11 of the thymidine kinase 2 (TK2) gene. TK2 is an essential mitochondrial pyrimidine-deoxyribonucleoside kinase. Bi-allelic loss-of-function mutations of TK2 lead to mitochondrial DNA depletion syndrome (MDS) in humans through severe (~ 70%) reduction of mitochondrial electron-transport-chain activity, and tk2 knockout mice show Purkinje cell degeneration and ataxia through severe mitochondrial cytochrome-c oxidase subunit I (COX I) protein reduction. To clarify whether TK2 function is altered in SCA31, we investigated TK2 and COX I expression in human postmortem SCA31 cerebellum. We confirmed that canonical TK2 mRNA is transcribed from exons far upstream of the repeat site, and demonstrated that an extended version of TK2 mRNA ("TK2-EXT"), transcribed from exons spanning the repeat site, is expressed in human cerebellum. While canonical TK2 was conserved among vertebrates, TK2-EXT was specific to primates. Reverse transcription-PCR demonstrated that both TK2 mRNAs were preserved in SCA31 cerebella compared with control cerebella. The TK2 proteins, assessed with three different antibodies including our original polyclonal antibody against TK2-EXT, were detected as ~ 26 kilodalton proteins on western blot; their levels were similar in SCA31 and control cerebella. COX I protein level was preserved in SCA31 compared to nuclear DNA-encoded protein. We conclude that the expression and function of TK2 are preserved in SCA31, suggesting a mechanism distinct from that of MDS.

Extracorporeal Membrane Oxygenation (ECMO) for suspected neonatal genetic diagnoses with cardiorespiratory failure.

Recent data describe an increasing use of extracorporeal membrane oxygenation (ECMO) in neonates with various clinical conditions besides primary respiratory or cardiac diagnoses. Infants with underlying genetic disorders characterized by cardiopulmonary failure pose unique management challenges. When pathognomonic dysmorphic features for common genetic diagnoses are not present, the prognosis is uncertain at best when determining ECMO candidacy. Lengthy turnaround times of genetic testing often delay definitive diagnosis during the ECMO course. Clinical management pathways to guide practice and evidence to support the use of ECMO in rare genetic conditions are lacking. The decision to initiate ECMO is daunting but may be of benefit if the subsequent genetic diagnosis is non-lethal. In lethal genetic cases warranting discontinuation of care, the time spent on ECMO may still be advantageous as a bridge to diagnosis while allowing for parental bonding with the terminally ill infant. Diagnostic confirmation may also facilitate the attainment of closure for these parents. Here, we report our experience providing ECMO to three neonates presenting with cardiorespiratory failure and later diagnosed with rare genetic syndromes. We share the challenges faced, lessons learned, and outcomes of these critically ill neonates.

Compound Heterozygous Mutations Presented with Quadriparesis and Menopause. A Case Report.

Mitochondrion regulates cellular metabolism with the aid of its respiratory complexes; any defect within these complexes can result in mitochondrial malfunction and various conditions. One such mutation can occur in SLC25A10, resulting in mitochondrial DNA depletion syndrome. It should be noted that the pattern of inheritance of this syndrome is autosomal recessive. However, we present a case with compound heterozygous mutations within this gene resulting in disease. An 18-year-old female was referred to our clinic due to menopause with a medical history of hearing loss, spasticity, hypotonia and quadriparesis. The child's birth and development were uneventful until the initiation of movement reduction and hypotonia when she was 12 months old. Afterward, the hypotonia progressed to quadriparesis and spasticity throughout the years. Our patient became completely quadriplegic up to the age of 3 and became completely deaf at 10. Her puberty onset was at the age of 9, and no significant event took place until she was 17 years old when suddenly her periods, which were regular until that time, became irregular and ceased after a year; hence, a thorough evaluation began, but similar to her previous evaluations all tests were insignificant. Nonetheless, we suspected an underlying metabolic or genetic defect; thus, we ordered a whole-exome sequencing (WES) workup and found simultaneous heterozygous mutations within SLC25A10, HFE and TTN genes that could explain her condition. When all other tests fail, and we suspect an underlying genetic or metabolic cause, WES can be of great value.

The mitochondrial inner membrane protein MPV17 prevents uracil accumulation in mitochondrial DNA.

Mitochondrial inner membrane protein MPV17 is a protein of unknown function that is associated with mitochondrial DNA (mtDNA)-depletion syndrome (MDS). MPV17 loss-of-function has been reported to result in tissue-specific nucleotide pool imbalances, which can occur in states of perturbed folate-mediated one-carbon metabolism (FOCM), but MPV17 has not been directly linked to FOCM. FOCM is a metabolic network that provides one-carbon units for the de novo synthesis of purine and thymidylate nucleotides (e.g. dTMP) for both nuclear DNA (nuDNA) and mtDNA replication. In this study, we investigated the impact of reduced MPV17 expression on markers of impaired FOCM in HeLa cells. Depressed MPV17 expression reduced mitochondrial folate levels by 43% and increased uracil levels, a marker of impaired dTMP synthesis, in mtDNA by 3-fold. The capacity of mitochondrial de novo and salvage pathway dTMP biosynthesis was unchanged by the reduced MPV17 expression, but the elevated levels of uracil in mtDNA suggested that other sources of mitochondrial dTMP are compromised in MPV17-deficient cells. These results indicate that MPV17 provides a third dTMP source, potentially by serving as a transporter that transfers dTMP from the cytosol to mitochondria to sustain mtDNA synthesis. We propose that MPV17 loss-of-function and related hepatocerebral MDS are linked to impaired FOCM in mitochondria by providing insufficient access to cytosolic dTMP pools and by severely reducing mitochondrial folate pools.

Clinical and molecular characterization of three patients with Hepatocerebral form of mitochondrial DNA depletion syndrome: a case series.

BACKGROUND: Mitochondrial DNA depletion syndromes (MDS) are clinically and phenotypically heterogeneous disorders resulting from nuclear gene mutations. The affected individuals represent a notable reduction in mitochondrial DNA (mtDNA) content, which leads to malfunction of the components of the respiratory chain. MDS is classified according to the type of affected tissue; the most common type is hepatocerebral form, which is attributed to mutations in nuclear genes such as DGUOK and MPV17. These two genes encode mitochondrial proteins and play major roles in mtDNA synthesis. CASE PRESENTATION: In this investigation patients in three families affected by hepatocerebral form of MDS who were initially diagnosed with tyrosinemia underwent full clinical evaluation. Furthermore, the causative mutations were identified using next generation sequencing and were subsequently validated using sanger sequencing. The effect of the mutations on the gene expression was also studied using real-time PCR. A pathogenic heterozygous frameshift deletion mutation in DGUOK gene was identified in parents of two affected patients (c.706-707 + 2 del: p.k236 fs) presenting with jaundice, impaired fetal growth, low-birth weight, and failure to thrive who died at the age of 3 and 6 months in family I. Moreover, a novel splice site mutation in MPV17 gene (c.461 + 1G > C) was identified in a patient with jaundice, muscle weakness, and failure to thrive who died due to hepatic failure at the age of 4 months. A 5-month-old infant presenting with jaundice, dark urine, poor sucking, and feeding problems was also identified to have another novel mutation in MPV17 gene leading to stop gain mutation (c.277C > T: p.(Gln93*)). CONCLUSIONS: These patients had overlapping clinical features with tyrosinemia. MDS should be considered a differential diagnosis in patients presenting with signs and symptoms of tyrosinemia.

MPV17 mutations in juvenile- and adult-onset axonal sensorimotor polyneuropathy.

MPV17 encodes a putative channel-forming protein of the inner mitochondrial membrane and is involved in mitochondrial deoxynucleotide homeostasis. MPV17 mutations were first reported in patients with Navajo neurohepatopathy, an autosomal recessive mitochondrial DNA depletion syndrome, characterized by early-onset liver failure, failure to thrive as well as central and peripheral neurological involvement. Recently, two patients with juvenile-onset peripheral sensorimotor neuropathy associated with an MVP17 c.122G>A (p.Arg41Gln) variant have been reported. Here, we describe five additional patients from two unrelated families with sensorimotor axonal neuropathy without hepatocerebral affection caused by homozygous MPV17 variants. Patients of the first family carried the known c.122G>A variant and affected individuals of the second family had a novel c.376-9T>G near-splice variant, which was shown to result in an in-frame deletion of 11 amino acids. This report provides further evidence that MPV17 mutations should be considered in patients with pure, non-syndromic axonal neuropathy.

Identification of a single MPV17 nonsense-associated altered splice variant in 24 South African infants with mitochondrial neurohepatopathy.

MPV17-related mitochondrial neurohepatopathy is a rare genetic disorder worldwide. We report on a novel pathogenic variant in the MPV17 gene in 24 unrelated neurohepatopathic infants of non-consanguineous Black South African heritage. Exome sequencing identified homozygosity for a c.106C>T nonsense variant in exon 3 of the human MPV17 gene in 2 unrelated index patients. mRNA analysis revealed transcripts both with and without exon 3, indicating both reduced splice efficiency and premature termination as mechanisms for disease. Carrier frequency in this population was found to be 1 in 68 (95% CI; 1/122-1/38) with an estimated newborn incidence of 1 in 18 496 (95% CI; 1/59 536-1/5776). Affected infants all presented with infantile onset neurohepatopathy with none surviving beyond infancy. This description of a relatively common pathogenic variant underlying a previously uncharacterized severe neurohepatopathy in South Africa will engender increased awareness, earlier diagnosis and possibly improve outcome if preventative or specific therapeutic options can be found.

Co-occurring Down syndrome and SUCLA2-related mitochondrial depletion syndrome.

Mitochondrial DNA depletion syndrome 5 (MIM 612073) is a rare autosomal recessive disorder caused by homozygous or compound heterozygous pathogenic variants in the beta subunit of the succinate-CoA ligase gene located within the 13q14 band. We describe two siblings of Hispanic descent with SUCLA2-related mitochondrial depletion syndrome (encephalomyopathic form with methylmalonic aciduria); the older sibling is additionally affected with trisomy 21. SUCLA2 sequencing identified homozygous p.Arg284Cys pathogenic variants in both patients. This mutation has previously been identified in four individuals of Italian and Caucasian descent. The older sibling with concomitant disease has a more severe phenotype than what is typically described in patients with either SUCLA2-related mitochondrial depletion syndrome or Down syndrome alone. The younger sibling, who has a normal female chromosome complement, is significantly less affected compared to her brother. While the clinical and molecular findings have been reported in about 50 patients affected with a deficiency of succinate-CoA ligase caused by pathogenic variants in SUCLA2, this report describes the first known individual affected with both a mitochondrial depletion syndrome and trisomy 21.

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.

'The phenotypic conundrum of Trp748Ser variant in POLG gene: a report of two patients'.

We present two cases of a 23-years and 32-years old female respectively, who presented with recurrent seizures, ataxia, dysarthria, psychomotor slowing. Magnetic resonance imaging (MRI) of the brain in the first patient revealed T2/FLAIR hyperintensity in the bilateral thalamus and cerebellar white matter with diffusion restriction, with no contrast enhancement. In the second patient, magnetic resonance imaging of brain showed FLAIR hyperintensity in precuneus while CSF showed raised HSV IgG titre on first presentation leading to suspicion of infective etiology. The initial differential diagnosis included autoimmune, metabolic and demyelinating causes. However, routine laboratory investigations, cerebrospinal fluid analysis, and autoimmune panel and demyelination workup were inconclusive. Considering the possibility of a genetic-mediated metabolic disorder, genetic testing was carried out leading to the identification of the Trp748Ser variation in POLG gene associated with mitochondrial DNA depletion syndrome. These cases highlight the diagnostic challenges and complexities in identifying rare metabolic encephalopathy, emphasizing the importance of a multidisciplinary approach in such cases.

Liver transplantation for mitochondrial DNA depletion syndrome caused by MPV17 deficiency: a case report and literature review.

Objective: To study the effectiveness of liver transplantation (LT) in treating mitochondrial DNA depletion syndrome (MDS) caused by the MPV17 gene variant. Case presentation: A boy aged 2.8 years presented with edema of the lower limbs and abdomen, which persisted for over 10 days and was of unknown origin; this was accompanied by abnormal liver function, intractable hypoglycemia, and hyperlactatemia. During the second week of onset, he developed acute-on-chronic liver failure and was diagnosed with MDS due to homozygous variant c.293C>T in the MPV17 gene. Subsequently, he underwent LT from a cadaveric donor. At follow-up after 15 months, his liver function was found to be normal, without any symptoms. Additionally, a literature review was performed that included MDS patients with the MPV17 variant who underwent LT. The results demonstrated that the survival rates for MDS patients who underwent LT were 69.5%, 38.6%, 38.6%, and 38.6% at 1-year, 5-year, 10-year, and 20-year intervals, respectively. Sub-group analyses revealed the survival rate of MDS patients with isolated liver disease (83.33%, 5/6) was higher than that of hepatocerebral MDS patients (44.44%, 8/18). Fifteen variants were identified in the MPV17 gene, and patients with the c.293C>T (p.P98l) variant exhibited the highest survival rate. Conclusion: Hepatocerebral MDS patients without neurological symptoms may benefit from LT.

Case Report: A novel RRM2B variant in a Chinese infant with mitochondrial DNA depletion syndrome and collective analyses of RRM2B variants for disease etiology.

Background: There are few reports of infantile mitochondrial DNA depletion syndrome (MDDS) caused by variants in RRM2B and the correlation between genotype and phenotype has rarely been analyzed in detail. This study investigated an infantile patient with MDDS, from clinical characteristics to genetic causes. Methods: Routine physical examinations, laboratory assays, which included gas chromatography-mass spectrometry of blood and urine, and MRI scans were performed to obtain an exact diagnosis. Whole-exome sequencing was used to pinpoint the abnormal gene and bioinformatic analyses were performed on the identified variant. Results: The case presented with progressive neurologic deterioration, failure to thrive, respiratory distress and lactic acidosis. Sequencing revealed that the patient had a homozygous novel missense variant, c.155T>C (p.Ile52Thr), in exon 2 of the RRM2B gene. Multiple lines of bioinformatic evidence suggested that this was a likely detrimental variant. In addition, reported RRM2B variants were compiled from the relevant literature to analyze disease etiology. We found a distinctive distribution of genotypes across disease manifestations of different severity. Pathogenic alleles of RRM2B were significantly enriched in MDDS cases. Conclusion: The novel variant is a likely genetic cause of MDDS. It expands our understanding of the pathogenic variant spectrum and the contribution of the RRM2B gene to the disease spectrum of MDDS.

Nucleoside supplements as treatments for mitochondrial DNA depletion syndrome.

Introduction: In mitochondrial DNA (mtDNA) depletion syndrome (MDS), patients cannot maintain sufficient mtDNA for their energy needs. MDS presentations range from infantile encephalopathy with hepatopathy (Alpers syndrome) to adult chronic progressive external ophthalmoplegia. Most are caused by nucleotide imbalance or by defects in the mtDNA replisome. There is currently no curative treatment available. Nucleoside therapy is a promising experimental treatment for TK2 deficiency, where patients are supplemented with exogenous deoxypyrimidines. We aimed to explore the benefits of nucleoside supplementation in POLG and TWNK deficient fibroblasts. Methods: We used high-content fluorescence microscopy with software-based image analysis to assay mtDNA content and membrane potential quantitatively, using vital dyes PicoGreen and MitoTracker Red CMXRos respectively. We tested the effect of 15 combinations (A, T, G, C, AT, AC, AG, CT, CG, GT, ATC, ATG, AGC, TGC, ATGC) of deoxynucleoside supplements on mtDNA content of fibroblasts derived from four patients with MDS (POLG1, POLG2, DGUOK, TWNK) in both a replicating (10% dialysed FCS) and quiescent (0.1% dialysed FCS) state. We used qPCR to measure mtDNA content of supplemented and non-supplemented fibroblasts following mtDNA depletion using 20 µM ddC and after 14- and 21-day recovery in a quiescent state. Results: Nucleoside treatments at 200 µM that significantly increased mtDNA content also significantly reduced the number of cells remaining in culture after 7 days of treatment, as well as mitochondrial membrane potential. These toxic effects were abolished by reducing the concentration of nucleosides to 50 µM. In POLG1 and TWNK cells the combination of ATGC treatment increased mtDNA content the most after 7 days in non-replicating cells. ATGC nucleoside combination significantly increased the rate of mtDNA recovery in quiescent POLG1 cells following mtDNA depletion by ddC. Conclusion: High-content imaging enabled us to link mtDNA copy number with key read-outs linked to patient wellbeing. Elevated G increased mtDNA copy number but severely impaired fibroblast growth, potentially by inhibiting purine synthesis and/or causing replication stress. Combinations of nucleosides ATGC, T, or TC, benefited growth of cells harbouring POLG mutations. These combinations, one of which reflects a commercially available preparation, could be explored further for treatment of POLG patients.

A very early onset MNGIE-like syndrome with POLG1 mutation and accompanying leukoencephalopathy.

Mitochondrial neurogastrointestinal encephalopathy (MNGIE) is a well-known mitochondrial depletion syndrome. Since Van Goethem et al. described MNGIE syndrome with pathogenic POLG1 mutations in 2003, POLG1 gene became a target for MNGIE patients. Cases with POLG1 mutations strikingly differ from classic MNGIE patients due to a lack of leukoencephalopathy. Here we present a female patient with very early onset disease and leukoencephalopathy compatible with classic MNGIE disease who turned out to have homozygous POLG1 mutation compatible with MNGIE-like syndrome, mitochondrial depletion syndrome type 4b.

Methylmalonic aciduria as a biochemical marker for mitochondrial DNA depletion syndrome in patients with developmental delay and movement disorders: a case series.

Background: Mitochondrial DNA (mtDNA) depletion syndromes (MDDS) are genetically and clinically variable disorders resulting from a reduction in mtDNA content in the cells, tissues, and organ systems, leading to symptoms related to energy deficits. Deficiency of the mitochondrial succinyl-CoA ligase/synthetase enzyme secondary to pathogenic variations in the SUCLG1 and SUCLA2 genes is a subtype of MDDS that presents with neurological manifestations and a specific biochemical profile. Methods: This cross-sectional series describes five patients with MDDS secondary to pathogenic variations in the SUCLG1 and SUCLA2 genes from two tertiary care centers in Canada and India. Clinical data concerning the course, investigations, and outcome were gathered through chart reviews. Results: All subjects presented in early infancy with neurological manifestations, including movement disorder, psychomotor regression, developmental delay, hearing loss, behavioral issues, or a combination thereof. Elevated methylmalonic acid metabolites, an abnormal acylcarnitine profile, and lactic acidemia were noted in the biochemical profile of each patient (n = 5/5, 100%). Molecular genetic testing disclosed the presence of pathogenic homozygous mutations in four subjects and compound heterozygosity in one subject. Conclusion: MDDS associated with SUCLG1 and SUCLA2 genes can be detected biochemically by the presence of methylmalonic aciduria besides the elevation of lactate, C3, C4DC, and C5-OH acylcarnitine. Conducting metabolic workups including MMA and acylcarnitine profiles in patients with heterogeneity of clinical symptoms associated with the presence of this biochemical marker may potentially reduce the time to diagnosis and management.

Mitochondrial Metabolomics of Sym1-Depleted Yeast Cells Revealed Them to Be Lysine Auxotroph.

Metabolomics has expanded from cellular to subcellular level to elucidate subcellular compartmentalization. By applying isolated mitochondria to metabolome analysis, the hallmark of mitochondrial metabolites has been unraveled, showing compartment-specific distribution and regulation of metabolites. This method was employed in this work to study a mitochondrial inner membrane protein Sym1, whose human ortholog MPV17 is related to mitochondria DNA depletion syndrome. Gas chromatography-mass spectrometry-based metabolic profiling was combined with targeted liquid chromatography-mass spectrometry analysis to cover more metabolites. Furthermore, we applied a workflow employing ultra-high performance liquid chromatography-quadrupole time of flight mass spectrometry with a powerful chemometrics platform, focusing on only significantly changed metabolites. This workflow highly reduced the complexity of acquired data without losing metabolites of interest. Consequently, forty-one novel metabolites were identified in addition to the combined method, of which two metabolites, 4-guanidinobutanal and 4-guanidinobutanoate, were identified for the first time in Saccharomyces cerevisiae. With compartment-specific metabolomics, we identified sym1Δ cells as lysine auxotroph. The highly reduced carbamoyl-aspartate and orotic acid indicate a potential role of the mitochondrial inner membrane protein Sym1 in pyrimidine metabolism.

Deoxyguanosine kinase deficiency and recurrent spontaneous pneumothorax: a case report.

BACKGROUND: Deoxyguanosine kinase deficiency is mainly manifested by hepatic and neurological damage, hence it belongs to the hepatocerebral form of mitochondrial deoxyribonucleic acid depletion syndrome. The association between deoxyguanosine kinase deficiency and recurrent spontaneous pneumothorax has not currently been reported. CASE PRESENTATION: A 12-year-old Russian boy with deoxyguanosine kinase deficiency, a recipient of a liver transplant with amyotrophy secondary to his mitochondriopathy, presented with recurrent spontaneous bilateral pneumothorax refractory to drainage and surgery. CONCLUSION: To our knowledge, this is the first documented case of deoxyguanosine kinase deficiency associated with recurrent spontaneous pneumothorax, which could be considered a late complication of deoxyguanosine kinase deficiency. At this point, this is only an association and further studies and research need to be performed to help confirm the pathogenesis of this association.

Novel compound heterozygous SUCLG1 variants may contribute to mitochondria DNA depletion syndrome-9.

BACKGROUND: Succinate-CoA ligase/synthetase (SCS) deficiency is responsible for encephalomyopathy with mitochondrial DNA depletion and mild methylmalonic aciduria. Variants in SUCLG1, the nuclear gene encoding the alpha subunit of the SCS enzyme playing a pivotal role in maintaining mtDNA integrity and stability, are associated with mitochondrial DNA depletion syndrome 9 (MTDPS9). METHODS: In this study, we reported an infant with clinical features of MTDPS9 from China. Whole exome sequencing (WES) was used to identify the genetic cause. Bioinformatic analysis and mtDNA level detection were performed to assess pathogenicity. RESULTS: The proband manifested with hypotonia, lactic acidosis, mild methylmalonic aciduria, hearing loss and psychomotor retardation. WES identified new compound heterozygous SUCLG1 variants of c.601A>G (p.R201G) in exon 6 and c.871G>C (p.A291P) in exon 8. Computational analysis predicted that these missense variants might alter structure stability and mitochondrial translocation of SUCLG1. qRT-PCR showed 68% depletion of mtDNA content in proband as compared to controls. CONCLUSION: Novel compound heterozygous variants c.601A>G (p.R201G) and c.871G>C (p.A291P) in SUCLG1 may cause MTDPS9 in this family. Our finding should be helpful for molecular diagnosis, genetic counseling and clinical management of SCS deficiency disorders.

Whole-Cell and Mitochondrial dNTP Pool Quantification from Cells and Tissues.

Deoxynucleoside 5'-triphosphates (dNTPs) are the molecular building blocks for DNA synthesis, and their balanced concentration in the cell is fundamental for health. dNTP imbalance can lead to genomic instability and other metabolic disturbances, resulting in devastating mitochondrial diseases.The accurate and efficient measurement of dNTPs from different biological samples and cellular compartments is vital to understand the mechanisms behind these diseases and develop and scrutinize their possible treatments. This chapter describes an update on the most recent development of the traditional radiolabeled polymerase extension method and its adaptation for the measurement of whole-cell and mitochondrial dNTP pools from cultured cells and tissue samples. The solid-phase reaction setting enables an increase in efficiency, accuracy, and measurement scale.