Deoxyguanosine kinase deficiency presenting as neonatal hemochromatosis

Abstracts

64 Mitochondrial DNA mutations provide selective advantage in hematopoietic stem cell niche Michael L. Chen a, T. Daniel Logan a, Maryann Hochberg b, Gregory E. Wilding c, Wei Tan c, Gregory Kujoth d, Tomas A. Prolla d, Martin Carroll a, James E. Thompson a,b,e,⁎ a Division of Hematology and Oncology of the Department of Medicine, University of Pennsylvania, Philadelphia, PA, 19104, United States b Departments of 2Immunology, Roswell Park Cancer Institute, Buffalo, NY, 14263, United States c Biostatistics, Roswell Park Cancer Institute, Buffalo, NY, 14263, United States d Department of Genetics and Medical Genetics, University of Wisconsin, Madison, WI, 53706, United States e Medicine, Roswell Park Cancer Institute, Buffalo, NY, 14263, United States The myelodysplastic syndromes (MDS) are a heterogeneous group of aging-associated disorders characterized by peripheral blood cytopenias with hypercellular and dysplastic bone marrow. Several lines of evidence implicate disruptions of mitochondrial physiology as an important factor in the development of human MDS, but the contribution of somatic mitochondrial DNA (mtDNA) mutations to the pathogenesis of human MDS is not well understood. We recently demonstrated that the DNA polymerase γ (Polg) “mitochondrial mutator” mouse model recapitulates the erythroid dysplasia seen in MDS [Blood (2009) vol. 114 (19) pp. 4045–53]. In this system, the proofreading activity of Polg, the sole polymerase responsible for mtDNA replication, has been ablated without impairing the rate of mtDNA replication. Homozygous mutant animals exhibit numerous features of accelerated aging, and so the development of MDS in these mice is consistent with the age-related incidence of MDS in humans (median age at diagnosis of MDS is 70). Despite the profound defects in hematopoietic function seen in these mice, their marrows remain highly cellular. These results suggest that, while mitochondrial dysfunction generates significant defects in peripheral blood cell function, the presence of mtDNA mutations in hematopoietic stem cells (HSCs) does not impair the long-term selfrenewal of these cells. We hypothesize that mtDNA defects can provide a selective advantage in the HSC niche, thereby allowing the age-related accumulation of dysfunctional mitochondria and the development of MDS in a significant minority of aging humans. In support of this hypothesis, we now demonstrate, using competitive bone marrow engraftment methods, that homozygous Polg mutant marrow has a long-term engraftment advantage over wild-type Polg competitor marrow. Experiments assessing the durability of the long-term engraftment advantage for homozygous mutant Polg HSCs, as well as the effect of various interventions on this engraftment advantage, are ongoing. Overall, our data demonstrate that mitochondrial dysfunction in the hematopoietic compartment can generate key features of human MDS, and suggest that the somatic acquisition of mtDNA mutations represent a novel class of mitochondrial disorders. The possibility that the ageassociated accumulation of mtDNA mutations is driven by positive selection for mutant mtDNA in the specialized environment of the stem cell niche has profound implications for the study of mitochondrial biology in normal human aging.

doi:10.1016/j.mito.2011.03.077

65 Deoxyguanosine kinase deficiency presenting as neonatal hemochromatosis Neil Hanchard a, Oleg A. Shchelochkov a,b, Ellen Brundage a, Eric Schmitt a, Fangyuan Li a, Lee-Jun Wong a, Fernando Scaglia a,⁎ a Department of Human and Molecular Genetics, Baylor College of Medicine, One Baylor Plaza, Houston, TX, United States b University of Iowa Hospitals and Clinics, Department of Pediatrics, Division of Genetics, Iowa City, IA, United States

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Mutations in the nuclear gene deoxyguanosine kinase (DGUOK) can result in mitochondrial DNA (mtDNA) depletion, which may present as neonatal liver failure. The association between DGUOK deficiency and neonatal hemochromatosis, however, remains underrecognized in clinical practice. Here we report a female neonate born at term to non-consanguineous parents of African American heritage, who failed her newborn screen for tyrosinemia (tyrosine of 800 μmol/L; upper cut-off 500 μmol/L). By two weeks of age she was hospitalized with progressive liver failure including elevated transaminases, hypoglycemia, direct hyperbilirubinemia, coagulopathy, thrombocytopenia, as well as an elevated serum lactate (9.4 mmol/L). Urinary succinylacetone was not observed and plasma amino acids revealed a marked elevation of tyrosine, alanine, and phenylalanine, consistent with hepatic failure. Further evaluation revealed an elevated ferritin (1627 ng/mL; upper limit of normal 391 ng/mL), which, in conjunction with an abdominal MRI suggestive of iron deposition in the liver, pancreas and adrenal glands, led to a presumptive diagnosis of neonatal hemochromatosis and the initiation of N-acetylcysteine therapy. Despite intensive treatment, her hepatic dysfunction and coagulopathy continued to worsen. She subsequently developed respiratory failure and septicemia, leading to death on day of life 44 before the possibility of liver transplantation. Given the abnormal newborn screen, severe liver dysfunction, and elevated plasma lactate, a hepatic mtDNA depletion syndrome was considered. Sequencing of the nuclear genes POLG1, MPV17 and DGUOK from blood revealed an apparent homozygous change c.572A>G (p.Y191C) in the DGUOK gene. Mitochondrial/metabolic array CGH did not detect intragenic deletions. SNP array confirmed the absence of heterozygosity at the DGUOK locus. We have previously reported this missense mutation in a neonate of Chinese ethnicity who presented with the hepatocerebral form of DGUOKrelated mtDNA depletion syndrome, and had predicted that this change would disrupt a critical tyrosine–glutamate interaction in the kinase domain of the enzyme. This report reinforces the consideration of mtDNA depletion-mediated hepatic dysfunction in the differential diagnosis of neonatal tyrosinemia, highlights the importance of considering DGUOK deficiency in neonatal hemochromatosis, and broadens the ethnic phenotype of observed DGUOK mutations to include subjects of African American heritage. doi:10.1016/j.mito.2011.03.078

66 Are pathogenic mitochondrial DNA missense mutations predictable? Renkui Bai⁎, Prachi Kothiyal, Sharon Suchy, John Compton, Sherri Bale GeneDx, Gaithersburg, MD 20877, United States Background: Mitochondrial DNA missense mutations account for more than half of the pathogenic mutations of mitochondrial DNA. When performing whole mitochondrial genome sequence analysis for patients suggestive of mitochondrial DNA diseases, it is common to find missense variants of unknown significance (VOUS). Although transmitochondrial cybrid study followed by mitochondrial functional analysis may help to determine the pathogenicity of the missense VOUS, it is costly and time-consuming, and the results from the in vitro transmitochondrial cybrid study may not represent the functional effects of the mutations in the affected tissue. Fast and accurate prediction of the pathogenicity of the VOUS is important for molecular diagnosis of mitochondrial DNA diseases. Methods: A total of 58 mitochondrial DNA missense variants, including 21 confirmed missense mutations listed in the Mitomap database (http://www.mitomap.org) and 37 known missense polymorphisms (SNPs) located at the same functional locations as the