Mitochondrialneurogastrointestinalencephalomyopathy (MNGIE) is an autosomal recessive human disease due to mutations in the thymidine phosphorylase (TP) gene. TP enzyme catalyzes the reversible phosphorolysis of thymidine to thymine and 2-deoxy-d-ribose 1-phosphate. We present evidence that thymidine metabolism is altered in MNGIE. TP activities in buffy coats were reduced drastically in all 27 MNGIE patients compared with 19 controls. All MNGIE patients had much higher plasma levels of thymidine than normal individuals and asymptomatic TP mutation carriers. In two patients, the renal clearance of thymidine was ∼20% that of creatinine, and because hemodialysis demonstrated that thymidine is ultrafiltratable, most of the filtered thymidine is likely to be reabsorbed by the kidney. In vitro, fibroblasts from controls catabolized thymidine in medium; by contrast, MNGIE fibroblasts released thymidine. In MNGIE, severe impairment of TP enzyme activity leads to increased plasma thymidine. In patients who are suspected of having MNGIE, determination of TP activity in buffy coats and thymidine levels in plasma are diagnostic. We hypothesize that excess thymidine alters mitochondrial nucleoside and nucleotide pools leading to impaired mitochondrial DNA replication, repair, or both. Therapies to reduce thymidine levels may be beneficial to MNGIE patients.

Elevated left ventricular mass, a highly heritable trait, is an important risk factor for heart failure and death. Stuart Cook and colleagues genetically dissect a locus in the rat associated with blood-pressure-independent cardiac hypertrophy and identify endonuclease G (ENDOG) as a key regulator of hypertrophy at this locus. They further show that the Endog gene is involved in proper mitochondrial function and is modulated by ERR-α and PGC1α, master regulators of mitochondrial and cardiac function. Loss of function of ENDOG causes impaired mitochondrial respiration and increased production of reactive oxygen species, which may contribute to the observed cardiac hypertrophy. Left ventricular mass (LVM) is a highly heritable trait1 and an independent risk factor for all-cause mortality2. So far, genome-wide association studies have not identified the genetic factors that underlie LVM variation3, and the regulatory mechanisms for blood-pressure-independent cardiac hypertrophy remain poorly understood4,5. Unbiased systems genetics approaches in the rat6,7 now provide a powerful complementary tool to genome-wide association studies, and we applied integrative genomics to dissect a highly replicated, blood-pressure-independent LVM locus on rat chromosome 3p. Here we identified endonuclease G (Endog), which previously was implicated in apoptosis8 but not hypertrophy, as the gene at the locus, and we found a loss-of-function mutation in Endog that is associated with increased LVM and impaired cardiac function. Inhibition of Endog in cultured cardiomyocytes resulted in an increase in cell size and hypertrophic biomarkers in the absence of pro-hypertrophic stimulation. Genome-wide network analysis unexpectedly implicated ENDOG in fundamental mitochondrial processes that are unrelated to apoptosis. We showed direct regulation of ENDOG by ERR-α and PGC1α (which are master regulators of mitochondrial and cardiac function)9,10,11, interaction of ENDOG with the mitochondrial genome and ENDOG-mediated regulation of mitochondrial mass. At baseline, the Endog-deleted mouse heart had depleted mitochondria, mitochondrial dysfunction and elevated levels of reactive oxygen species, which were associated with enlarged and steatotic cardiomyocytes. Our study has further established the link between mitochondrial dysfunction, reactive oxygen species and heart disease and has uncovered a role for Endog in maladaptive cardiac hypertrophy.

Mitochondrial encephalomyopathies are clinically and genetically heterogeneous because mitochondria are the products of 2 genomes: mitochondrial DNA (mtDNA) and nuclear DNA (nDNA). Among the mendelian-inherited mitochondrial diseases are defects of intergenomic communication, disorders due to nDNA mutations that cause depletion and multiple deletions of mtDNA. Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive disorder of intergenomic communication and is defined clinically by 1) severe gastrointestinal dysmotility; 2) cachexia; 3) ptosis, ophthalmoparesis, or both; 4) peripheral neuropathy; and 5) leukoencephalopathy. Skeletal muscle biopsies of patients have revealed abnormalities of mtDNA and mitochondrial respiratory chain enzymes. The disease is caused by mutations in the thymidine phosphorylase (TP) gene. TP protein catalyzes phosphorolysis of thymidine to thymine and deoxyribose 1-phosphate. In MNGIE patients, TP enzyme activity is reduced drastically, and plasma thymidine and deoxyuridine are elevated dramatically. We have hypothesized that alterations of nucleoside metabolism cause an imbalanced mitochondrial nucleotide pool that leads to depletion and deletions of mtDNA. MNGIE is a recognizable clinical syndrome caused by mutations in TP. The diagnosis can be confirmed by measuring TP activity in buffy coat or plasma levels of thymidine and deoxyuridine. Reduction of circulating thymidine and deoxyuridine in MNGIE patients may be therapeutic.

Haematopoietic stem cell transplantation has been proposed as treatment for mitochondrial neurogastrointestinal encephalomyopathy, a rare fatal autosomal recessive disease due to TYMP mutations that result in thymidine phosphorylase deficiency. We conducted a retrospective analysis of all known patients suffering from mitochondrial neurogastrointestinal encephalomyopathy who underwent allogeneic haematopoietic stem cell transplantation between 2005 and 2011. Twenty-four patients, 11 males and 13 females, median age 25 years (range 10–41 years) treated with haematopoietic stem cell transplantation from related (n = 9) or unrelated donors (n = 15) in 15 institutions worldwide were analysed for outcome and its associated factors. Overall, 9 of 24 patients (37.5%) were alive at last follow-up with a median follow-up of these surviving patients of 1430 days. Deaths were attributed to transplant in nine (including two after a second transplant due to graft failure), and to mitochondrial neurogastrointestinal encephalomyopathy in six patients. Thymidine phosphorylase activity rose from undetectable to normal levels (median 697 nmol/h/mg protein, range 262–1285) in all survivors. Seven patients (29%) who were engrafted and living more than 2 years after transplantation, showed improvement of body mass index, gastrointestinal manifestations, and peripheral neuropathy. Univariate statistical analysis demonstrated that survival was associated with two defined pre-transplant characteristics: human leukocyte antigen match (10/10 versus <10/10) and disease characteristics (liver disease, history of gastrointestinal pseudo-obstruction or both). Allogeneic haematopoietic stem cell transplantation can restore thymidine phosphorylase enzyme function in patients with mitochondrial neurogastrointestinal encephalomyopathy and improve clinical manifestations of mitochondrial neurogastrointestinal encephalomyopathy in the long term. Allogeneic haematopoietic stem cell transplantation should be considered for selected patients with an optimal donor.

Proliferating cells are preferentially susceptible to infection by retroviruses. Sterile α motif and HD domain–containing protein-1 (SAMHD1) is a recently described deoxynucleotide phosphohydrolase controlling the size of the intracellular deoxynucleotide triphosphate (dNTP) pool, a limiting factor for retroviral reverse transcription in noncycling cells. Proliferating (Ki67⁺) primary CD4⁺ T cells or macrophages express a phosphorylated form of SAMHD1 that corresponds with susceptibility to infection in cell culture. We identified cyclin-dependent kinase (CDK) 6 as an upstream regulator of CDK2 controlling SAMHD1 phosphorylation in primary T cells and macrophages susceptible to infection by HIV-1. In turn, CDK2 was strongly linked to cell cycle progression and coordinated SAMHD1 phosphorylation and inactivation. CDK inhibitors specifically blocked HIV-1 infection at the reverse transcription step in a SAMHD1-dependent manner, reducing the intracellular dNTP pool. Our findings identify a direct relationship between control of the cell cycle by CDK6 and SAMHD1 activity, which is important for replication of lentiviruses, as well as other viruses whose replication may be regulated by intracellular dNTP availability.