Sirisak Chanprasert

Mitochondria: Role of citrulline and arginine supplementation in MELAS syndrome

Mitochondria are found in all nucleated human cells and generate most of the cellular energy. Mitochondrial disorders result from dysfunctional mitochondria that are unable to generate sufficient ATP to meet the energy needs of various organs. Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is a frequent maternally inherited mitochondrial disorder. There is growing evidence that nitric oxide (NO) deficiency occurs in MELAS syndrome and results in impaired blood perfusion that contributes significantly to several complications including stroke-like episodes, myopathy, and lactic acidosis. Both arginine and citrulline act as NO precursors and their administration results in increased NO production and hence can potentially have therapeutic utility in MELAS syndrome. Citrulline raises NO production to a greater extent than arginine, therefore, citrulline may have a better therapeutic effect. Controlled studies assessing the effects of arginine or citrulline supplementation on different clinical aspects of MELAS syndrome are needed.

Molecular and clinical characterization of the myopathic form of mitochondrial DNA depletion syndrome caused by mutations in the thymidine kinase (TK2) gene

Mitochondrial DNA (mtDNA) depletion syndromes (MDSs) are a clinically and molecularly heterogeneous group of mitochondrial cytopathies characterized by severe mtDNA copy number reduction in affected tissues. Clinically, MDSs are mainly categorized as myopathic, encephalomyopathic, hepatocerebral, or multi-systemic forms. To date, the myopathic form of MDS is mainly caused by mutations in the TK2 gene, which encodes thymidine kinase 2, the first and rate limiting step enzyme in the phosphorylation of pyrimidine nucleosides. We analyzed 9 unrelated families with 11 affected subjects exhibiting the myopathic form of MDS, by sequencing the TK2 gene. Twelve mutations including 4 novel mutations were detected in 9 families. Skeletal muscle specimens were available from 7 out of 11 subjects. Respiratory chain enzymatic activities in skeletal muscle were measured in 6 subjects, and enzymatic activities were reduced in 3 subjects. Quantitative analysis of mtDNA content in skeletal muscle was performed in 5 subjects, and marked mtDNA content reduction was observed in each. In addition, we outline the molecular and clinical characteristics of this syndrome in a total of 52 patients including those previously reported, and a total of 36 TK2 mutations are summarized. Clinically, hypotonia and proximal muscle weakness are the major phenotypes present in all subjects. In summary, our study expands the molecular and clinical spectrum associated with TK2 deficiency.

Impaired nitric oxide production in children with MELAS syndrome and the effect of arginine and citrulline supplementation.

Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome is one of the most frequent maternally inherited mitochondrial disorders. The pathogenesis of this syndrome is not fully understood and believed to result from several interacting mechanisms including impaired mitochondrial energy production, microvasculature angiopathy, and nitric oxide (NO) deficiency. NO deficiency in MELAS syndrome is likely to be multifactorial in origin with the decreased availability of the NO precursors, arginine and citrulline, playing a major role. In this study we used stable isotope infusion techniques to assess NO production in children with MELAS syndrome and healthy pediatric controls. We also assessed the effect of oral arginine and citrulline supplementations on NO production in children with MELAS syndrome. When compared to control subjects, children with MELAS syndrome were found to have lower NO production, arginine flux, plasma arginine, and citrulline flux. In children with MELAS syndrome, arginine supplementation resulted in increased NO production, arginine flux, and arginine concentration. Citrulline supplementation resulted in a greater increase of these parameters. Additionally, citrulline supplementation was associated with a robust increase in citrulline concentration and flux and de novo arginine synthesis rate. The greater effect of citrulline in increasing NO production is due to its greater ability to increase arginine availability particularly in the intracellular compartment in which NO synthesis takes place. This study, which is the first one to assess NO metabolism in children with mitochondrial diseases, adds more evidence to the notion that NO deficiency occurs in MELAS syndrome, suggests a better effect for citrulline because of its greater role as NO precursor, and indicates that impaired NO production occurs in children as well as adults with MELAS syndrome. Thus, the initiation of treatment with NO precursors may be beneficial earlier in life. Controlled clinical trials to assess the therapeutic effects of arginine and citrulline on clinical complications of MELAS syndrome are needed.

Glucose metabolism derangements in adults with the MELAS m.3243A>G mutation

The m.3243A>G mutation in the mitochondrial gene MT-TL1 leads to a wide clinical spectrum ranging from asymptomatic carriers to MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) at the severe end. Diabetes mellitus (DM) occurs in mitochondrial diseases, with the m.3243A>G mutation being the most common mutation associated with mitochondrial DM. The pathogenesis of mitochondrial DM remains largely unknown, with previous studies suggesting that impaired insulin secretion is the major factor. In this study we used stable isotope infusion techniques to assess glucose metabolism in vivo and under physiological conditions in 5 diabetic and 11 non-diabetic adults with the m.3243A>G mutation and 10 healthy adult controls. Our results revealed increased glucose production due to increased gluconeogenesis in both diabetic and non-diabetic subjects with the m.3243A>G mutation. In addition, diabetic subjects demonstrated insulin resistance and relative insulin deficiency, resulting in an inability to increase glucose oxidation which can explain the development of DM in these subjects. Non-diabetic subjects showed normal insulin sensitivity; and therefore, they were able to increase their glucose oxidation rate. The ability to increase glucose utilization can act as a compensatory mechanism that explains why these subjects do not have DM despite the higher rate of glucose production. These results suggest that increased gluconeogenesis is not enough to cause DM and the occurrence of combined insulin resistance and relative insulin deficiency are needed to develop DM in individuals with the m.3243A>G mutation. Therefore, multiple defects in insulin and glucose metabolism are required for DM to occur in individuals with mitochondrial diseases. The results of this study uncover previously undocumented alterations in glucose metabolism in individuals with the m.3243A>G mutation that contribute significantly to our understanding of the pathogenesis of mitochondrial DM and can have significant implications for its management.

Adult liver disorders caused by inborn errors of metabolism: Review and update

Inborn errors of metabolism (IEMs) are a group of genetic diseases that have protean clinical manifestations and can involve several organ systems. The age of onset is highly variable but IEMs afflict mostly the pediatric population. However, in the past decades, the advancement in management and new therapeutic approaches have led to the improvement in IEM patient care. As a result, many patients with IEMs are surviving into adulthood and developing their own set of complications. In addition, some IEMs will present in adulthood. It is important for internists to have the knowledge and be familiar with these conditions because it is predicted that more and more adult patients with IEMs will need continuity of care in the near future. The review will focus on Wilson disease, alpha-1 antitrypsin deficiency, citrin deficiency, and HFE-associated hemochromatosis which are typically found in the adult population. Clinical manifestations and pathophysiology, particularly those that relate to hepatic disease as well as diagnosis and management will be discussed in detail.

TK2-Related Mitochondrial DNA Depletion Syndrome, Myopathic Form

Abstract Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are a genetically heterogeneous group of disorders characterized by a significant reduction of mtDNA copy number in various tissues. The clinical manifestations are protean, yet they can be divided into myopathic, hepatocerebral, encephalomyopathic, and multisystemic forms. The molecular defects of nuclear genes responsible for biogenesis and maintenance of deoxynucleotide pools are the cause of MDS. The disorders are transmitted in an autosomal recessive fashion. Currently, there are many nuclear genes in which mutations have been associated with MDS. The majority of myopathic cases of MDS are caused by mutations in TK2. TK2 encodes thymidine kinase 2, which is an enzyme catalyzing the first and rate-limiting step of phosphorylation of pyrimidine nucleosides. Affected individuals typically present with progressive proximal muscle weakness and hypotonia at a young age. The diagnosis is supported by the elevation of creatine phosphokinase (CPK). The elevation is usually 5–10 times the upper limit of normal. Electron transport chain activity in skeletal muscle shows decreased activity in multiple complexes. The mtDNA copy number in muscle tissue is severely reduced compared to age- and tissue-matched controls. The diagnosis is confirmed by molecular genetic testing of the TK2 gene. Currently, treatment of TK2-related MDS is largely supportive. Despite supportive measures, most cases will eventually develop respiratory failure and death within a few years after diagnosis.

Hypercalcemia in Human Immunodeficiency Virus-Related Lymphoma and Valacyclovir Toxicity

To the Editor: We thank Shrayyef et al for their article entitled “Hypercalcemia in Two Patients with Sarcoidosis and Mycobacterium avium intracellulare Not Mediated by Elevated Vitamin D Metabolites,”1 which was published in the October issue of The American Journal of the Medical Sciences (Vol. 342, 4, pp 336–340). The authors wrote very excellent review on granulomatous diseases induced by hypercalcemia without elevated active vitamin D level. We found an interesting association between human immunodeficiency virus (HIV) infection, valacyclovir and hypercalcemia in the first case study. Hypercalcemiarelated granulomatous diseases and non-Hodgkin lymphomas are not uncommon in HIV-infected patients especially in patients with low CD4 counts, high HIV viral loads and a prior diagnosis of AIDS.2 In addition, Li et al3 recently reported a case of severe hypercalcemia due to valacyclovir toxicity. The dose of valacyclovir should be renally adjusted for kidney function, and physicians should be aware of the possibility of valacyclovir-induced hypercalcemia as in the first case presentation.

Coexistent Membranous Nephropathy with Doubly ANCA-Associated Crescentic Glomerulonephritis: A Case Report and Review of Literature

Introduction: Membranous nephropathy (MN) is the most common causes of the nephrotic syndrome in nondiabetic, Caucasian adults. Pauci-immune necrotizing and crescentic glomerulonephritis (PNCGN) typically present with rapidly progressive glomerulonephritis. Coexistent MN and PNCGN is a rare occurrence. We report a case of both MPO- and PR3-ANCA associated NCGN with MN that presented as rapidly progressive glomerulonephritis. Case presentation: A 46-year-old female presented with nausea and vomiting. On physical examination, the patient was a febrile and normotensive. Blood tests showed acute kidney injury and anemia. Urinalysis demonstrated numerous dysmorphic red blood cells with granular casts and nephrotic range proteinuria. Further testing showed negative ANA, positive anti-dsDNA, PR3-ANCA and MPO-ANCA. Kidney biopsy revealed the diagnosis of concurrent PNCGN with membranous nephropathy. The diagnosis of concurrent ANCA-associated NCGN with Membranous nephropathy was made. High dose intravenous methyl prednisolone was initiated. Unfortunately, the patient developed diffuse alveolar hemorrhage and underwent 6 cycles of plasmapheresis, intravenous Cyclophosphamide and pulse dose steroids with transitioned to oral prednisone and mycophenolate. On follow up, her disease seemed to be well suppressed without dialysis. Conclusion: Membranous nephropathy with PNCGN is a rare concurrent glomerulopathy, and even more rare with both MPO and PR-3 positivity. The diagnosis of MN with PNCGN should be considered in patients who present with RPGN and nephrotic range proteinuria.