Defects In Heme Synthesis Research Articles

Liver transplantation and primary liver cancer in porphyria.

The porphyrias are a heterogeneous group of metabolic disorders that result from defects in heme synthesis. The metabolic defects are present in all cells, but symptoms are mainly cutaneous or related to neuropathy. The porphyrias are highly relevant to hepatologists since patients can present with symptoms and complications that require liver transplantation (LT), and some porphyrias are associated with a high risk for primary liver cancer (PLC). Among the cutaneous porphyrias, erythropoietic protoporphyria (EPP) can lead to cholestatic liver failure where LT cures the liver disease but not the porphyria. In acute porphyria (AP), neurotoxic porphyrin precursors are produced in the liver and LT is a curative treatment option in patients with recurrent severe neuropathic attacks. Patients with AP, mainly acute intermittent porphyria, have a significantly increased risk for PLC that warrants surveillance and adequate follow-up of high-risk groups. LT is well established in both EPP with liver failure and AP with recurrent attacks, but most transplant centres have little porphyria experience and cooperation between transplant hepatologists, and porphyria experts is important in the often-difficult decisions on timing and management of comorbid conditions.

Consideration of Acute Porphyria in an Emergency Department Patient: A Case Report and Discussion of Common Pitfalls.

Porphyria refers to a group of disorders associated with defects in heme synthesis. They can be associated with severely debilitating features, including abdominal pain, psychiatric symptoms, neurological defects, and cardiovascular irregularities. Although these diseases are rare, patients with attacks often do present to the emergency department (ED) where consideration of porphyria is generally not included in the differential. Here, we examine a case of a 16-year-old male who presented to our ED for evaluation of recurring abdominal pain and auditory hallucinations in which porphyria was considered by the emergency physician. Not considering acute porphyria in patients with recurring neurovisceral symptoms in the ED can lead to missed opportunities for diagnosing such pathologies.

Differential diagnosis of inherited bone marrow failure syndromes in erythrocyte disorders

Diamond-Blackfan anemia (DBA), congenital dyserythropoietic anemia (CDA), and inherited sideroblastic anemia (ISA) are representative diseases of inherited bone marrow failure syndromes in erythrocyte diseases. DBA is primarily caused due to ribosomal dysfunctions. Furthermore, reticulocytes and erythroid progenitor cells decrease considerably within the peripheral blood and bone marrow, respectively. CDA is caused by a disturbance in red blood cell maturation and ineffective erythropoiesis due to hemolysis in the bone marrow. CDA is mainly classified into types I to III, and multinucleated erythroblasts observed in the bone marrow, typically in the internuclear bridge in type I. ISA is caused by iron metabolism dysfunction in the mitochondria due to defective heme synthesis. Sideroblasts appear ringed due to iron accumulation in the mitochondria of erythroid precursors. Gene mutation analysis is indispensable for the confirmatory diagnosis of these diseases; however, narrowing down the diagnosis, by examining the erythrocytes in the peripheral blood and the erythroblast morphology in the bone marrow, is also important.

Inherited microcytic anemias.

Inherited microcytic anemias can be broadly classified into 3 subgroups: (1) defects in globin chains (hemoglobinopathies or thalassemias), (2) defects in heme synthesis, and (3) defects in iron availability or iron acquisition by the erythroid precursors. These conditions are characterized by a decreased availability of hemoglobin (Hb) components (globins, iron, and heme) that in turn causes a reduced Hb content in red cell precursors with subsequent delayed erythroid differentiation. Iron metabolism alterations remain central to the diagnosis of microcytic anemia, and, in general, the iron status has to be evaluated in cases of microcytosis. Besides the very common microcytic anemia due to acquired iron deficiency, a range of hereditary abnormalities that result in actual or functional iron deficiency are now being recognized. Atransferrinemia, DMT1 deficiency, ferroportin disease, and iron-refractory iron deficiency anemia are hereditary disorders due to iron metabolism abnormalities, some of which are associated with iron overload. Because causes of microcytosis other than iron deficiency should be considered, it is important to evaluate several other red blood cell and iron parameters in patients with a reduced mean corpuscular volume (MCV), including mean corpuscular hemoglobin, red blood cell distribution width, reticulocyte hemoglobin content, serum iron and serum ferritin levels, total iron-binding capacity, transferrin saturation, hemoglobin electrophoresis, and sometimes reticulocyte count. From the epidemiological perspective, hemoglobinopathies/thalassemias are the most common forms of hereditary microcytic anemia, ranging from inconsequential changes in MCV to severe anemia syndromes.

S1154 Clinical Outcomes in Patients With Acute Hepatic Porphyria Treated With Givosiran Who Stopped Hemin Prophylaxis at Study Entry: A Post Hoc Analysis of Data From the Phase 3 ENVISION Study Through Month 12

INTRODUCTION: Acute Hepatic Porphyria (AHP) is a family of rare genetic diseases due to enzyme defects in heme synthesis in the liver. Accumulation of toxic heme intermediates ALA and PBG may result in neurovisceral attacks and chronic manifestations. Intravenous (IV) hemin is approved to treat acute attacks and is sometimes used off-label prophylactically. In the ENVISION study in AHP patients, givosiran, an RNAi therapeutic, reduced the composite porphyria annualized porphyria attack rate (AAR) vs. placebo (pbo). A post-hoc analysis was conducted to evaluate outcomes in AHP patients with or without prior hemin prophylaxis prior to screening. METHODS: ENVISION (NCT03338816) is an ongoing Phase 3 global, randomized, pbo-controlled study, evaluating givosiran efficacy and safety in symptomatic AHP patients in a 6-month double blind (DB) period and an open label extension (OLE) period (30 months). Patients were required to discontinue prophylactic hemin treatment at study entry, but could receive hemin for acute attacks. Outcome measures included the composite porphyria AAR (defined as attacks requiring hospitalization, urgent care, or IV hemin at home). Analyses were descriptive. RESULTS: For AHP patients on prior hemin prophylaxis (median historical AAR: 9.0), a 77% reduction in mean AAR was observed with givosiran treatment vs. pbo in the DB period (Table 1). A similar reduction (63%) in mean AAR was observed in those without prior hemin prophylaxis (median historical AAR: 7.0). In both groups, further reduction in AAR was observed in patients who continued on givosiran in the OLE period (Table 1). A similar reduction in AAR was also observed in both groups of pbo patients who received givosiran in the OLE (Table 1). The percentage of patients with 0 composite attacks increased in each group following 6-months of givosiran treatment in the OLE with 55% and 67% in the patients who had continued givosiran treatment (Table 1). Additional analyses of outcome measures between the two groups will be presented. CONCLUSION: Clinically meaningful reduction in the AAR was observed in AHP patients treated with givosiran regardless of whether they received hemin prophylaxis prior to study entry, with further AAR reduction observed in those who continued to receive givosiran during the OLE. In addition, a similar benefit was observed in pbo patients who received givosiran for 6-months during the OLE period regardless of prior hemin prophylaxis use.Table 1

Nutrients and Porphyria: An Intriguing Crosstalk.

Porphyria refers to a group of fascinating diseases from a metabolic and nutritional standpoint as it provides an example of how metabolic manipulation can be used for therapeutic purposes. It is characterized by defects in heme synthesis, particularly in the erythrocytes and liver. Specific enzymes involved in heme biosynthesis directly depend on adequate levels of vitamins and minerals in the tissues. Moreover, micronutrients that are required for producing succinyl CoA and other intermediates in the Krebs (TCA) cycle are indirectly necessary for heme metabolism. This review summarizes articles that describe the nutritional status, supplements intake, and dietary practices of patients affected by porphyria, paying special attention to the therapeutic use of nutrients that may help or hinder this group of diseases.

Pilot study of mitochondrial bioenergetics in subjects with acute porphyrias

Background and aimsThe acute porphyrias are characterized by defects in heme synthesis, particularly in the liver. In some affected patients, there occurs a critical deficiency in a regulatory heme pool within hepatocytes that leads to up-regulation of 5-aminolevulinic acid [ALA] synthase-1, which is the first and normally rate-controlling enzyme in the pathway. In earlier work, we described defects in mitochondrial functions in cultured skin fibroblasts from patients with acute intermittent porphyria [AIP]. Others described defects in livers of murine models of AIP. Here, we explored mitochondrial energetics in peripheral blood mononuclear cells [PBMCs] and platelets in persons with AIP and hereditary coproporphyria [HCP]. Our hypotheses were that there are deficits in bioenergetic capacity in acute porphyrias and that subjects with more severe acute porphyria have more pronounced reductions in mitochondrial oxygen consumption rates [OCR]. MethodsWe studied 17 subjects with acute hepatic porphyrias, 14 with classical AIP, one with severe AIP due to homozygous deficiency of hydroxymethylbilane synthase [HMBS], 2 with HCP, and 5 non-porphyric controls. We collected peripheral blood, isolated PBMCs, which we assayed either immediately or after frozen storage [80C] for up to 14 days. Using Seahorse XF-24-3, we measured OCR in the presence of glucose + pyruvate under basal condition, and after additions of oligomycin, carbonylcyanide p-trifluoromethoxyphenylhydrazone [FCCP], and antimycin+rotenone. ResultsMost subjects [13/17, 76%] were female. Subjects with moderate/severe symptoms associated with acute porphyria had significantly lower basal and maximal-OCR than those with no/mild symptoms who were the same as controls. We observed significant inverse correlation between urinary porphobilinogen [PBG] excretion and OCR. The subject with homozygous AIP had a much lower-OCR than his asymptomatic parents. Summary/conclusionsResults support the hypothesis that active acute hepatic porphyria is characterized by a deficiency in mitochondrial function that is detectable in PBMCs, suggesting that limitations in electron transport and ATP production exist in such individuals.

Heart disease in Friedreich's ataxia.

Friedreich’s ataxia (FRDA), which occurs in 1/50000 live births, is the most prevalent inherited neuromuscular disorder. Nearly all FRDA patients develop cardiomyopathy at some point in their lives. The clinical manifestations of FRDA include ataxia of the limbs and trunk, dysarthria, diabetes mellitus, and cardiac diseases. However, the broad clinical spectrum makes FRDA difficult to identify. The diagnosis of FRDA is based on the presence of suspicious clinical factors, the use of the Harding criteria and, more recently, the use of genetic testing for identifying the expansion of a triplet nucleotide sequence. FRDA is linked to a defect in the mitochondrial protein frataxin; an epigenetic alteration interferes with the folding of this protein, causing a relative deficiency of frataxin in affected patients. Frataxins are small essential proteins whose deficiency causes a range of metabolic disturbances, including oxidative stress, iron-sulfur cluster deficits, and defects in heme synthesis, sulfur amino acid metabolism, energy metabolism, stress responses, and mitochondrial function. The cardiac involvement seen in FRDA is a consequence of mitochondrial proliferation as well as the loss of contractile proteins and the subsequent development of myocardial fibrosis. The walls of the left ventricle become thickened, and different phenotypic manifestations are seen, including concentric or asymmetric hypertrophy and (less commonly) dilated cardiomyopathy. Dilated cardiomyopathy and arrhythmia are associated with mortality in patients with FRDA, whereas hypertrophic cardiomyopathy is not. Systolic function tends to be low-normal in FRDA patients, with an acute decline at the end of life. However, the literature includes only a few long-term prospective studies of cardiac progression in FRDA, and the cause of death is often attributed to heart failure and arrhythmia postmortem. Cardiomyopathy tends to be correlated with the clinical neurologic age of onset and the nucleotide triplet repeat length (i.e., markers of phenotypic disease severity) rather than the duration of disease or the severity of neurologic symptoms. As most patients are wheelchair-bound within 15 years of diagnosis, the clinical determination of cardiac involvement is often complicated by comorbidities. Researchers are currently testing targeted therapies for FRDA, and a centralized database, patient registry, and natural history study have been launched to support these clinical trials. The present review discusses the pathogenesis, clinical manifestations, and spectrum of cardiac disease in FRDA patients and then introduces gene-targeted and pathology-specific therapies as well as screening guidelines that should be used to monitor cardiac disease in this mitochondrial disorder.

ABCB6, an ABC Transporter Impacting Drug Response and Disease.

Recent findings have discovered how insufficiency of ATP-binding cassette (ABC) transporter, ABCB6, can negatively impact human health. These advances were made possible by, first, finding that ABCB6 deficiency was the genetic basis for some severe transfusion reactions and by, second, determining that functionally impaired ABCB6 variants enhanced the severity of porphyria, i.e., diseases associated with defects in heme synthesis. ABCB6 is a broad-spectrum porphyrin transporter that is capable of both exporting and importing heme and its precursors across the plasma membrane and outer mitochondrial membrane, respectively. Biochemical studies have demonstrated that while ABCB6 influences the antioxidant system by reducing the levels of reactive oxygen species, the exact mechanism is currently unknown, though effects on heme synthesis are likely. Furthermore, it is unknown what biochemical or cellular signals determine where ABCB6 localizes in the cell. This review highlights the major recent findings on ABCB6 and focuses on details of its structure, mechanism, transport, contributions to cellular stress, and current clinical implications.

Diagnosis and treatment of sideroblastic anemias: from defective heme synthesis to abnormal RNA splicing.

The sideroblastic anemias are a heterogeneous group of inherited and acquired disorders characterized by the presence of ring sideroblasts in the bone marrow. X-linked sideroblastic anemia (XLSA) is caused by germline mutations in ALAS2. Hemizygous males have a hypochromic microcytic anemia, which is generally mild to moderate and is caused by defective heme synthesis and ineffective erythropoiesis. XLSA is a typical iron-loading anemia; although most patients are responsive to pyridoxine, treatment of iron overload is also important in the management of these patients. Autosomal recessive sideroblastic anemia attributable to mutations in SLC25A38, a member of the mitochondrial carrier family, is a severe disease: patients present in infancy with microcytic anemia, which soon becomes transfusion dependent. Conservative therapy includes regular red cell transfusion and iron chelation, whereas allogenic stem cell transplantation represents the only curative treatment. Refractory anemia with ring sideroblasts (RARS) is a myelodysplastic syndrome characterized mainly by anemia attributable to ineffective erythropoiesis. The clinical course of RARS is generally indolent, but there is a tendency to worsening of anemia over time, so that most patients become transfusion dependent in the long run. More than 90% of these patients carry somatic mutations in SF3B1, a gene encoding a core component of the RNA splicing machinery. These mutations cause misrecognition of 3' splice sites in downstream genes, resulting in truncated gene products and/or decreased expression attributable to nonsense-mediated RNA decay; this explains the multifactorial pathogenesis of RARS. Variants of RARS include refractory cytopenia with multilineage dysplasia and ring sideroblasts, and RARS associated with marked thrombocytosis; these variants involve additional genetic lesions. Inhibitors of molecules of the transforming growth factor-β superfamily have been shown recently to target ineffective erythropoiesis and ameliorate anemia both in animal models of myelodysplastic syndrome and in RARS patients.

Fam210b Is Required for Optimal Cellular and Mitochondrial Iron Uptake during Erythroid Differentiation

Red cells synthesize large quantities of heme during terminal differentiation. Central to erythropoiesis is the transport and trafficking of iron within the cell. Despite the importance of iron transport during erythroid heme synthesis, the molecules involved in intracellular trafficking of iron are largely unknown. In a screen for genes that are up-regulated during erythroid terminal differentiation, we identified FAM210B, a predicted multi-pass transmembrane mitochondrial protein as an essential component of mitochondrial iron transport during erythroid differentiation.In zebrafish and mice, Fam210b mRNA is enriched in differentiating erythroid cells and liver (fetal and adult), which are tissues that require large amounts of iron for heme synthesis. Here, we report that FAM210B facilitates mitochondrial iron import during erythroid differentiation and is essential for hemoglobin synthesis. Zebrafish are anemic when fam210b is silenced using anti-sense morpholinos (Fig. A). CRISPR knockout of Fam210b caused a heme synthesis defect in differentiating Friend murine erythroleukemia (MEL) cells. PPIX levels in Fam210b deficient cells are normal, demonstrating that Fam210b does not participate in synthesis of the heme tetrapyrrole ring. Consistent with this result, supplementation of Fam210b deficient MEL cells with either aminolevulinic acid, the first committed substrate of the heme synthesis pathway or a chemical analog of protoporphyrin IX failed to chemically complement the heme synthesis defect. While Fam210b was not required for basal housekeeping heme synthesis, Fam210b deficientcells showed defective total cellular and mitochondrial iron uptake during erythroid differentiation (Fig. B). As a result, Fam210b deficient cells had defective hemoglobinization. Supplementation of Fam210b-/- MEL cells with non-transferrin iron chelates restored erythroid differentiation and hemoglobin synthesis; whereas, similar chemical complementation could not be achieved in the Tmem14c-/- cells, which have a primary defect in tetrapyrrole transport. (Fig. C). Our findings reveal that FAM210B is required for optimal mitochondrial iron import during erythroid differentiation for hemoglobin synthesis. It may therefore function as a genetic modifier for mitochondriopathies, anemias or porphyrias. [Display omitted] DisclosuresBauer:Biogen: Research Funding; Editas Medicine: Consultancy. Orkin:Editas Inc.: Consultancy.

Update on the biology of heme synthesis in erythroid cells

Heme is a prosthetic group of hemoproteins playing important roles in oxygen transport, detoxification, circadian rhythm, microRNA processing, regulation of transcription, and translation. The majority of heme (-85%) is synthesized in red blood cells mainly for hemoglobin production, whereas hepatocytes account for most of the rest, functioning primarily in the synthesis of cytochrome P450 enzymes and mitochondrial respiratory enzymes. Thus, failure of heme biosynthesis causes severe inherited or acquired disorders in humans, including porphyria and sideroblastic anemia. The heme biosynthetic pathway is composed of eight enzymes that work in either mitochondria or the cytoplasm, which have been extensively researched and frequently reviewed. On the other hand, the mechanisms governing transport and intracellular trafficking of heme intermediates, as well as their potential links to human diseases, are poorly understood. Herein, we focus on recent understanding of the heme biosynthetic pathway and on human disorders due to defective heme synthesis in erythroid cells, such as X-linked sideroblastic anemia and erythropoietic protoporphyria.

Acute intermittent porphyria.

Acute intermittent porphyria.

Biology of Heme in Mammalian Erythroid Cells and Related Disorders.

Heme is a prosthetic group comprising ferrous iron (Fe2+) and protoporphyrin IX and is an essential cofactor in various biological processes such as oxygen transport (hemoglobin) and storage (myoglobin) and electron transfer (respiratory cytochromes) in addition to its role as a structural component of hemoproteins. Heme biosynthesis is induced during erythroid differentiation and is coordinated with the expression of genes involved in globin formation and iron acquisition/transport. However, erythroid and nonerythroid cells exhibit distinct differences in the heme biosynthetic pathway regulation. Defects of heme biosynthesis in developing erythroblasts can have profound medical implications, as represented by sideroblastic anemia. This review will focus on the biology of heme in mammalian erythroid cells, including the heme biosynthetic pathway as well as the regulatory role of heme and human disorders that arise from defective heme synthesis.

TMEM14C is required for erythroid mitochondrial heme metabolism.

The transport and intracellular trafficking of heme biosynthesis intermediates are crucial for hemoglobin production, which is a critical process in developing red cells. Here, we profiled gene expression in terminally differentiating murine fetal liver-derived erythroid cells to identify regulators of heme metabolism. We determined that TMEM14C, an inner mitochondrial membrane protein that is enriched in vertebrate hematopoietic tissues, is essential for erythropoiesis and heme synthesis in vivo and in cultured erythroid cells. In mice, TMEM14C deficiency resulted in porphyrin accumulation in the fetal liver, erythroid maturation arrest, and embryonic lethality due to profound anemia. Protoporphyrin IX synthesis in TMEM14C-deficient erythroid cells was blocked, leading to an accumulation of porphyrin precursors. The heme synthesis defect in TMEM14C-deficient cells was ameliorated with a protoporphyrin IX analog, indicating that TMEM14C primarily functions in the terminal steps of the heme synthesis pathway. Together, our data demonstrate that TMEM14C facilitates the import of protoporphyrinogen IX into the mitochondrial matrix for heme synthesis and subsequent hemoglobin production. Furthermore, the identification of TMEM14C as a protoporphyrinogen IX importer provides a genetic tool for further exploring erythropoiesis and congenital anemias.

Heme and erythropoieis: more than a structural role.

Erythropoiesis is the biological process that consumes the highest amount of body iron for heme synthesis. Heme synthesis in erythroid cells is finely coordinated with that of alpha (α) and beta (β)-globin, resulting in the production of hemoglobin, a tetramer of 2α- and 2β-globin chains, and heme as the prosthetic group. Heme is not only the structural component of hemoglobin, but it plays multiple regulatory roles during the differentiation of erythroid precursors since it controls its own synthesis and regulates the expression of several erythroid-specific genes. Heme is synthesized in developing erythroid progenitors by the stage of proerythroblast, through a series of eight enzymatic reactions divided between mitochondria and cytosol. Defects of heme synthesis in the erythroid lineage result in sideroblastic anemias, characterized by microcytic anemia associated to mitochondrial iron overload, or in erythropoietic porphyrias, characterized by porphyrin deposition in erythroid cells. Here, we focus on the heme biosynthetic pathway and on human erythroid disorders due to defective heme synthesis. The regulatory role of heme during erythroid differentiation is discussed as well as the heme-mediated regulatory mechanisms that allow the orchestration of the adaptive cell response to heme deficiency.

Feigning acute intermittent porphyria.

Acute intermittent porphyria (AIP) is an autosomal dominant genetic defect in heme synthesis. Patients with this illness can have episodic life-threatening attacks characterized by abdominal pain, neurological deficits, and psychiatric symptoms. Feigning this illness has not been reported in the English language literature to date. Here, we report on a patient who presented to the hospital with an acute attack of porphyria requesting opiates. Diligent assessment of extensive prior treatment records revealed thirteen negative tests for AIP.

Tmem14c Plays An Essential Role In Mitochondrial Heme Metabolism

Red cells synthesize large amounts of heme during terminal differentiation. Central to this process is the transport and trafficking of heme synthesis intermediates within the cell. Despite the importance of transport during heme synthesis, the molecules involved in this process are largely unknown. In a screen for genes that are upregulated during erythroid terminal differentiation, we identified Tmem14c, a predicted multi-pass transmembrane protein as an essential component of the porphyrin metabolism pathway.Here, we report that Tmem14c facilitates the synthesis of mitochondrial protoporphyrin IX from coproporphyrinogen III and is thus required for heme synthesis. Tmem14c is a mitochondrial inner-membrane protein enriched in vertebrate hematopoietic tissues and is required for terminal erythropoiesis. Tmem14c gene-trap mouse embryos are severely anemic and mostly die by E13.5 (Fig. A). Fetal liver erythroid cells derived from gene-trap embryos experience maturation arrest. shRNA silencing of Tmem14c in Friend murine erythroleukemia (MEL) cells results in a significant decrease in de-novo heme synthesis. The biochemical defect is due to a decrease in mitochondrial protoporphyrin IX synthesis, while cytoplasmic porphyrin levels remain normal (Fig. B). The heme synthesis defect in Tmem14c-silenced MEL cells is complemented with a protoporphyrin IX analog. These data show the role of Tmem14c in regulating the terminal steps in mitochondrial porphyrin trafficking. [Display omitted] Our findings collectively demonstrate that Tmem14c is required for the transport of mitochondrial porphyrins in developing erythroid cells. Due to its inner-mitochondrial localization and its relative proximity to heme synthetic enzymes coproporphyrinogen oxidase and protoporphyrinogen oxidase (Rhee et al., 2013 Science), Tmem14c can function as a molecular adaptor that facilitates the interaction of proteins involved in porphyrin transport, or as a protoporphyrinogen IX transporter (Fig. C). The identification of Tmem14c as an essential regulator of porphyrin transport and heme synthesis provides a novel genetic tool for exploring erythropoiesis and disorders of heme synthesis such as porphyria and anemia. Disclosures:No relevant conflicts of interest to declare.

Erythroid Heme Biosynthesis and Its Disorders

Heme, which is composed of iron and the small organic molecule protoporphyrin, is an essential component of hemoglobin as well as a variety of physiologically important hemoproteins. During erythropoiesis, heme synthesis is induced before, and is essential for, globin synthesis. Although all cells possess the ability to synthesize heme, there are distinct differences between regulation of the pathway in developing erythroid cells and all other types of cells. Disorders that compromise the ability of the developing red cell to synthesize heme can have profound medical implications. The biosynthetic pathway for heme and key regulatory features are reviewed herein, along with specific human genetic disorders that arise from defective heme synthesis such as X-linked sideroblastic anemia and erythropoietic protoporphyria.

Abstract 161: ABCB10 Plays a Role in Heme Synthesis and Mitochondrial δ-Aminolevulinic Acid Export

Introduction: Heme is essential for cardiovascular health, as it is a component of myoglobin, cytochrome P450, electron transport chain complexes, and antioxidant enzymes. The eight steps of heme synthesis are well characterized; however, it is not known how the synthetic intermediates are transported between mitochondria and cytosol. We hypothesized that the mitochondrial ATP-binding cassette protein-B10 (ABCB10) plays a role in heme synthesis by mediating the export of δ-aminolevulinic acid (ALA) out of the mitochondria. Results: Lentiviral shRNA-based knockdown of ABCB10 in H9c2 cardiac myoblasts significantly reduced cytosolic and mitochondrial heme levels and the activities of heme-containing proteins (catalase, peroxidase and complex IV), while their mRNA levels remained unchanged. The observed heme deficiency was not due to a defect in heme synthesis, iron incorporation, or enhanced heme degradation, as ABCB10 shRNA had no effect on the expression of enzymes mediating these steps. We then tested whether ABCB10 regulates mitochondrial export of ALA. Addition of exogenous ALA completely reversed the reduction in heme levels and normalized the activities of heme containing enzymes in cells with ABCB10 knockdown. However, overexpression of ALA synthase 1 (ALAS1), an enzyme catalyzing the production of ALA in the mitochondria, resulted in increased heme levels at baseline, but failed to restore heme content in ABCB10 shRNA-treated cells. To examine the role of ABCB10 in cardiac disease, we measured ABCB10 expression in the hearts of mice subjected to myocardial infarction and in explanted human hearts with ischemic cardiomyopathy. ABCB10 protein was significantly elevated in mouse and human hearts, and adenoviral overexpression of ABCB10 in H9c2 cardiac myoblasts significantly attenuated the production of reactive oxygen species (ROS), suggesting the protective role for ABCB10 in heart disease. Conclusions: Our studies place ABCB10 between the mitochondrial synthesis of ALA by ALAS1 and the cytosolic conversion of ALA into heme, suggesting that ABCB10 plays a role ALA export out of the mitochondria. Furthermore, its overexpression exerts protective effects against oxidant induced cell death in cardiac cells.