Sylvia S. Bottomley

Mutations in mitochondrial carrier family gene SLC25A38 cause nonsyndromic autosomal recessive congenital sideroblastic anemia

The sideroblastic anemias are a heterogeneous group of congenital and acquired hematological disorders whose morphological hallmark is the presence of ringed sideroblasts—bone marrow erythroid precursors containing pathologic iron deposits within mitochondria. Here, by positional cloning, we define a previously unknown form of autosomal recessive nonsyndromic congenital sideroblastic anemia, associated with mutations in the gene encoding the erythroid specific mitochondrial carrier family protein SLC25A38, and demonstrate that SLC25A38 is important for the biosynthesis of heme in eukaryotes.

Molecular Regulation of Heme Biosynthesis in Higher Vertebrates

Publisher Summary The regulation of heme biosynthesis in animals has been a topic of interest for many years. It is generally agreed that the first enzyme of the heme pathway, 5-aminolevulinate synthase, determines the rate of heme biosynthesis and the regulation of this enzyme is, therefore, a major focus of this chapter. The heme biosynthetic pathway is present in all cell types except mature erythrocytes. The final step of heme biosynthesis occurs in mitochondria and the heme is then utilized for the formation of different hemoproteins located in mitochondria, microsomes, peroxisomes, the cytosol, and probably the nucleus. Heme can control the biosynthesis of some proteins. In erythroid cells, heme controls the translation of proteins, notably α- and β-globin chains, by modulating the activity of a specific kinase. All nucleated animal cells must synthesize heme for incorporation into respiratory cytochromes, but erythroid and liver cells have the highest rates of heme synthesis. Erythroid cells synthesize about 90% of the total heme in the body for assembly into hemoglobin. Although the bulk of heme in the liver is made in situ , the liver may also obtain some heme from serum haptoglobin–hemoglobin and heme–hemopexin complexes, following intravascular hemolysis. All enzymes of the heme biosynthetic pathway, except for protoporphyrinogen oxidase, have been cloned from higher vertebrates. The genes encoding these enzymes are located on different chromosomes.

X-linked pyridoxine-responsive sideroblastic anemia due to a Thr388-to-Ser substitution in erythroid 5-aminolevulinate synthase

BACKGROUND X-linked sideroblastic anemia is usually associated with reduced 5-aminolevulinate synthase activity in erythroid cells, and some cases are responsive to treatment with pyridoxine, the precursor to the cofactor of the enzyme. The recently identified gene for an erythroid-specific 5-aminolevulinate synthase isoenzyme and its localization to the X chromosome make it likely that one or more defects in this gene underlie the anemia. METHODS Using a polymorphic dinucleotide-repeat sequence in the erythroid 5-aminolevulinate synthase gene, we confirmed the linkage of this gene to the disorder in a family with X-linked pyridoxine-responsive sideroblastic anemia. We therefore sought evidence of a nucleotide-sequence abnormality in the erythroid 5-aminolevulinate synthase gene by analyzing enzymatically amplified DNA. RESULTS DNA-sequencing studies in two affected males and one carrier female in the kindred demonstrated a cytosine-to-guanine change at nucleotide 1215 (in exon 8). This change results in the substitution of serine for threonine at amino acid residue 388, near the lysine that binds the pyridoxal phosphate cofactor. In expression studies, the activity of the mutant enzyme was reduced relative to that of the wild type, and this reduction was comparable to that in erythroid cells of the proband during relapse of the anemia; the enzyme activity expressed in the presence of pyridoxine was comparable to that in the probands marrow cells during remission. Although the affinity of the mutant enzyme for pyridoxal phosphate was not altered, the mutation appears to introduce a conformational change at the active site of the enzyme. CONCLUSIONS We identified a point mutation resulting in an amino acid change near the pyridoxal phosphate-binding site of the erythroid 5-aminolevulinate synthase isoenzyme as the underlying defect in a kindred with X-linked pyridoxine-responsive sideroblastic anemia.

CNS demyelination associated with copper deficiency and hyperzincemia.

CNS demyelination is not a previously reported feature of acquired copper deficiency. The authors report two patients with idiopathic hypocupremia and hyperzincemia, hematologic changes of copper deficiency, and extensive CNS demyelination. Hematologic recovery followed copper supplementation, both initially and after relapse off copper therapy, while serum zinc levels remained high and the neurologic abnormalities only stabilized.

Systematic Molecular Genetic Analysis of Congenital Sideroblastic Anemia: Evidence for Genetic Heterogeneity and Identification of Novel Mutations

Sideroblastic anemias are heterogeneous congenital and acquired bone marrow disorders characterized by pathologic iron deposits in mitochondria of erythroid precursors. Among the congenital sideroblastic anemias (CSAs), the most common form is X‐linked sideroblastic anemia, due to mutations in 5‐aminolevulinate synthase (ALAS2). A novel autosomal recessive CSA, caused by mutations in the erythroid specific mitochondrial transporter SLC25A38, was recently defined. Other known etiologies include mutations in genes encoding the thiamine transporter SLC19A2, the RNA‐modifying enzyme pseudouridine synthase 1 (PUS1), a mitochondrial ATP‐binding cassette transporter (ABCB7), glutaredoxin 5 (GLRX5), as well as mitochondrial DNA deletions. Despite these known diverse causes, in a substantial portion of CSA cases a presumed genetic defect remains unknown.

Mutations in TRNT1 cause congenital sideroblastic anemia with immunodeficiency, fevers, and developmental delay (SIFD)

Mutations in genes encoding proteins that are involved in mitochondrial heme synthesis, iron-sulfur cluster biogenesis, and mitochondrial protein synthesis have previously been implicated in the pathogenesis of the congenital sideroblastic anemias (CSAs). We recently described a syndromic form of CSA associated with B-cell immunodeficiency, periodic fevers, and developmental delay (SIFD). Here we demonstrate that SIFD is caused by biallelic mutations in TRNT1, the gene encoding the CCA-adding enzyme essential for maturation of both nuclear and mitochondrial transfer RNAs. Using budding yeast lacking the TRNT1 homolog, CCA1, we confirm that the patient-associated TRNT1 mutations result in partial loss of function of TRNT1 and lead to metabolic defects in both the mitochondria and cytosol, which can account for the phenotypic pleiotropy.

Molecular Defects of Erythroid 5-Aminolevulinate Synthase in X-Linked Sideroblastic Anemia

The erythroid-specific isozyme of 5-aminolevulinate synthase (ALAS2), the first and ratelimiting enzyme of heme biosynthesis, is expressed concomitantly with the differentiation and maturation of the erythroid cell in order to accommodate generation of the large amounts of heme required for hemoglobin production. During the past few years the ALAS2 gene and its transcript have been characterized and the amino acid sequence of the enzyme deduced. The human genetic disorder X-linked sideroblastic anemia, previously postulated to be caused by defects of ALAS, has now been analyzed at the molecular and tissue-specific level. A heterogeneous group of point mutations in the catalytic domain of the ALAS2 enzyme has been found to cause the disorder. Impaired activity of recombinant mutant ALAS2 enzymes has also been demonstrated. Characterization of molecular defects in individuals with X-linked sideroblastic anemia has provided improved diagnosis for at-risk family members.

Characterization and measurement of Δ-aminolaevulinate synthetase in bone marrow cell mitochondria

Abstract 1. 1. Δ-Aminolaevulinate synthesis was demonstrated in normal rabbit bone marrow and the measurement of Δ-aminolaevulinate synthetase (succinyl-CoA: glycine succinyltransferase) was accomplished in mitochondria obtained from marrow cells ruptured by sonication. The micromethod used to quantify Δ-amino laevulinate was sensitive to the range of 10 −9 mole. The optimal concentrations of glycine, α-ketoglutarate, CoA, pyridoxal phosphate and NAD + for maximum enzyme activity at pH 7.5 were identified. Succinate and succinyl-CoA were less effective as substrates than α-ketoglutarate. Neither MgCl 2 nor EDTA stimulated enzyme activity. Inhibition of the enzyme by its product, Δ-aminolaevulinate, was observed. Neither heme nor protoporphyrin inhibited the production of Δ-aminolaevulinate. 2. 2. Electronmicroscopic study of the mitochondrial preparation disclosed that the mitochondria were partially damaged. The isolated mitochondria contained succinate dehydrogenase (EC 1.3.99.1) and ferrochelatase (EC 4.99.1.1) activities. The soluble enzymes of the heme synthetic pathway were absent. 3. 3. The rate of production of Δ-aminolaevulinate observed in the present experiments approximates the rate which would be required in vivo by the normal rabbit for hemoglobin synthesis.

Analysis of mitochondrial DNA in 104 patients with myelodysplastic syndromes

OBJECTIVE To determine the frequency and spectrum of somatic mutations of mitochondrial DNA (mtDNA) in bone marrow of patients with myelodysplastic syndrome (MDS). MATERIALS AND METHODS Analysis included 104 patients with MDS (24 refractory anemia, 32 refractory anemia with ringed sideroblasts, 34 refractory anemia with excess of blasts, 7 refractory anemia with excess of blasts in transformation to acute leukemia, and 7 chronic myelo-monocytic leukemia), 3 patients with acute myeloid leukemia from MDS, and 36 patients with myeloproliferative disease (23 chronic myeloid leukemia, 9 polycythemia vera, 4 idiopathic myelofibrosis). Mutation scanning was performed using heteroduplex analysis with denaturing high-performance liquid chromatography (dHPLC). The entire mitochondrial genome was amplified in 67 overlapping polymerase chain reaction fragments carefully optimized regarding DNA melting profiles. Abnormal dHPLC findings were confirmed by DNA sequencing. RESULTS Heteroplasmic mtDNA mutations, mostly transitions, were identified in 56% of MDS and 44% of myeloproliferative disorders patients. In MDS, mutation frequency increased with age and more-advanced disease. Mutational spectra showed no hot spots and were similar in different types of MDS. Heteroplasmic mutations generally did not represent known polymorphisms, and about half of them affected conserved amino acids or nucleotides. Mutations were less frequent in protein encoding genes (50 per 10(6) base pairs) than other mitochondrial genes (transfer RNAs, ribosomal RNAs, and control region; about 80 per 10(6) base pairs). CONCLUSIONS As mitochondria often show ultrastructural abnormalities in MDS, including pathological iron accumulation, mitochondrial dysfunction may contribute to MDS pathology. We found a high frequency of acquired mtDNA mutations in MDS. However, their functional importance remains unclear, considering that genotype correlates poorly with phenotype in mitochondrial diseases. The clonally expanded mtDNA mutations in MDS support the concept of age-related damage to mtDNA in hematopoietic stem cells.

Copper Deficiency After Gastric Surgery: A Reason for Caution

Background:Acquired copper deficiency in adults leads to hematological and neurological manifestations that mimic vitamin B12 deficiency. A significant number of patients with copper deficiency syndrome have a history of gastric surgery, often remote. We sought to determine whether copper deficiency is present in a population of individuals with longstanding partial gastric resection. Methods:Serum copper, ceruloplasmin, and zinc levels were determined in 20 patients with a history of partial gastric resection and 50 controls, randomly selected from the Oklahoma City Veterans Affairs Medical Center electronic database. Results:Hypocupremia and symptoms of copper deficiency were detected in patients with partial gastric resection in contrast to controls (3/20 versus 0/50, P = 0.02). Serum copper and ceruloplasmin levels were significantly lower in individuals with partial gastric resection than in controls (P = 0.04 and P = 0.001, respectively). The mean interval between gastric surgery and testing was 20.7 years. Conclusions:Our results indicate that a significant number of individuals with longstanding history of partial gastric resection have undiagnosed hypocupremia. Screening for copper deficiency after gastric surgery may prevent the development of hematological and neurological complications in these patients.