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

Age-associated mitochondrial oxidative decay: improvement of carnitine acetyltransferase substrate-binding affinity and activity in brain by feeding old rats acetyl-L- carnitine and/or R-alpha -lipoic acid

We test whether the dysfunction with age of carnitine acetyltransferase (CAT), a key mitochondrial enzyme for fuel utilization, is due to decreased binding affinity for substrate and whether this substrate, fed to old rats, restores CAT activity. The kinetics of CAT were analyzed by using the brains of young and old rats and of old rats supplemented for 7 weeks with the CAT substrate acetyl-l-carnitine (ALCAR) and/or the mitochondrial antioxidant precursor R-alpha-lipoic acid (LA). Old rats, compared with young rats, showed a decrease in CAT activity and in CAT-binding affinity for both substrates, ALCAR and CoA. Feeding ALCAR or ALCAR plus LA to old rats significantly restored CAT-binding affinity for ALCAR and CoA, and CAT activity. To explore the underlying mechanism, lipid peroxidation and total iron and copper levels were assayed; all increased in old rats. Feeding old rats LA or LA plus ALCAR inhibited lipid peroxidation but did not decrease iron and copper levels. Ex vivo oxidation of young-rat brain with Fe(II) caused loss of CAT activity and binding affinity. In vitro oxidation of purified CAT with Fe(II) inactivated the enzyme but did not alter binding affinity. However, in vitro treatment of CAT with the lipid peroxidation products malondialdehyde or 4-hydroxy-nonenal caused a decrease in CAT-binding affinity and activity, thus mimicking age-related change. Preincubation of CAT with ALCAR or CoA prevented malondialdehyde-induced dysfunction. Thus, feeding old rats high levels of key mitochondrial metabolites can ameliorate oxidative damage, enzyme activity, substrate-binding affinity, and mitochondrial dysfunction.

Iron metabolism: iron deficiency and iron overload

Iron is an essential cofactor in a variety of cellular processes. Except for a few unusual bacterial species, iron is indispensable for living organisms. However, free iron is toxic because of its propensity to induce the formation of dangerous free radicals. Consequently, iron balance is tightly regulated. Disorders of iron homeostasis are among the most common afflictions of humans. This review discusses inherited iron deficiency and iron overload disorders and recent insights into their pathophysiology.

Iron homeostasis: insights from genetics and animal models

Disorders that perturb iron balance are among the most prevalent human diseases, but until recently iron transport remained poorly understood. Over the past five years, genetic studies of patients with inherited iron homeostasis disorders and the analysis of mutant mice, rats and zebrafish have helped to identify several important iron-transport proteins. With information being mined from the genomes of four species, the study of iron metabolism has benefited enormously from positional-cloning efforts. Complementing the genomic strategy, targeted mutagenesis in mice has produced new models of human iron diseases. The animal models described in this review offer valuable tools for investigating iron homeostasis in vivo.

Abnormalities of flavin monooxygenase as an etiology for sideroblastic anemia

We postulated that a deficiency of flavin monooxygenase (FMO)-a ferrireductase component of cells-could produce sideroblastic anemia. FMO is an intracellular ferrireductase which may be responsible for the obligatory reduction of ferric to ferrous iron so that reduced iron can be incorporated into heme by ferrochelatase. Abnormalities of this mechanism could result in accumulation of excess ferric iron in mitochondria of erythroid cells to produce ringed sideroblasts and impair hemoglobin synthesis. To investigate this hypothesis we obtained blood from patients with sideroblastic anemia and normal subjects. Extracts of peripheral blood lymphocytes were used to measure ferrireduction by utilization of NADPH. Lymphoid precursors are reported to accumulate iron in mitochondria similarly to erythroid precursors. Utilization of lymphoid precursors avoided the need for bone marrow aspirations. We studied three patients with sideroblastic anemia. One patient and his asymptomatic daughter had a significant decrease in ferrireductase activity. They also had markedly diminished concentrations of FMO in lymphocyte protein extracts on Western blots. This was accompanied by increased concentration of mobilferrin in the extracts. These results suggest that abnormalities of FMO and mobilferrin may cause sideroblastic anemia and erythropoietic hemochromatosis in some patients.

Interaction between succinyl CoA synthetase and the heme-biosynthetic enzyme ALAS-E is disrupted in sideroblastic anemia

The first and the rate-limiting enzyme of heme biosynthesis is delta-aminolevulinate synthase (ALAS), which is localized in mitochondria. There are 2 tissue-specific isoforms of ALAS, erythroid-specific (ALAS-E) and nonspecific ALAS (ALAS-N). To identify possible mitochondrial factors that modulate ALAS-E function, we screened a human bone marrow cDNA library, using the mitochondrial form of human ALAS-E as a bait protein in the yeast 2-hybrid system. Our screening led to the isolation of the beta subunit of human ATP-specific succinyl CoA synthetase (SCS-betaA). Using transient expression and coimmunoprecipitation, we verified that mitochodrially expressed SCS-betaA associates specifically with ALAS-E and not with ALAS-N. Furthermore, the ALAS-E mutants R411C and M426V associated with SCS-betaA, but the D190V mutant did not. Because the D190V mutant was identified in a patient with pyridoxine-refractory X-linked sideroblastic anemia, our findings suggest that appropriate association of SCS-betaA and ALAS-E promotes efficient use of succinyl CoA by ALAS-E or helps translocate ALAS-E into mitochondria.

Regulation of erythroid 5-aminolevulinate synthase expression during erythropoiesis

Erythroid tissue is the major site of heme production in the body. The synthesis of heme and globin chains is coordinated at both the transcriptional and post-transcriptional levels to ensure that virtually no free heme or globin protein accumulates. The key rate-controlling enzyme of the heme biosynthetic pathway is 5-aminolevulinate synthase (ALAS) and an erythroid-specific isoform (ALAS2) is up-regulated during erythropoiesis. Differentiation of embryonic stem cells with a disrupted ALAS2 gene has established that expression of this gene is critical for erythropoiesis and cannot be compensated by expression of the ubiquitous isoform of the enzyme (ALAS1). Interestingly, heme appears to be important for expression of globin and other late erythroid genes and for erythroid cell differentiation although the mechanism of this effect is not clear. Transcriptional control elements that regulate the human gene for ALAS2 have been identified both in the promoter and in intronic enhancer regions. Subsequent translation of the ALAS2 mRNA is dependent on an adequate iron supply. The mechanism by which transcription of the gene for ALAS2 is increased by erythropoietin late in erythropoiesis remains an interesting issue. Erythropoietin action may result in altered levels of critical erythroid transcription factors or modulate the phosphorylation/acetylation status of these factors. Defects in the coding region of the gene for ALAS2 underlie the disease state X-linked sideroblastic anemia. In this review, we focus on the regulation and function of erythroid-specific 5-aminolevulinate synthase during erythropoiesis and its role in the X-linked sideroblastic anemia.

A novel mutation of the erythroid-specific delta-aminolaevulinate synthase gene in a patient with X-linked sideroblastic anaemia

A novel missense mutation, A1754G, in exon 11 of the erythroid-specific delta-aminolaevulinate synthase gene (ALAS2) was identified in a Japanese male with sideroblastic anaemia. ALAS activity in bone marrow cells of the patient was reduced to 53.3% of the normal control. Consistent with this finding, activity of a bacterially expressed ALAS2 mutant protein harbouring this mutation was 19.5% compared with the normal control, but was increased up to 31.6% by the addition of pyridoxal 5'-phosphate (PLP) in vitro. RFLP analysis with Bsp HI restriction revealed that his mother was a carrier of the mutation. These findings suggest that A1754G mutation was inherited in this family in a manner consistent with X-linked inheritance, and is responsible for sideroblastic anaemia in the patient.

Four New Mutations in the Erythroid-Specific 5-Aminolevulinate Synthase (ALAS2) Gene Causing X-Linked Sideroblastic Anemia: Increased Pyridoxine Responsiveness After Removal of Iron Overload by Phlebotomy and Coinheritance of Hereditary Hemochromatosis

X-linked sideroblastic anemia (XLSA) in four unrelated male probands was caused by missense mutations in the erythroid-specific 5-aminolevulinate synthase gene (ALAS2). All were new mutations: T647C, C1283T, G1395A, and C1406T predicting amino acid substitutions Y199H, R411C, R448Q, and R452C. All probands were clinically pyridoxine-responsive. The mutation Y199H was shown to be the first de novo XLSA mutation and occurred in a gamete of the proband’s maternal grandfather. There was a significantly higher frequency of coinheritance of the hereditary hemochromatosis (HH)HFE mutant allele C282Y in 18 unrelated XLSA hemizygotes than found in the normal population, indicating a role for coinheritance ofHFE alleles in the expression of this disorder. One proband (Y199H) with severe and early iron loading coinherited HH as a C282Y homozygote. The clinical and hematologic histories of two XLSA probands suggest that iron overload suppresses pyridoxine responsiveness. Notably, reversal of the iron overload in the Y199H proband by phlebotomy resulted in higher hemoglobin concentrations during pyridoxine supplementation. The proband with the R452C mutation was symptom-free on occasional phlebotomy and daily pyridoxine. These studies indicate the value of combined phlebotomy and pyridoxine supplementation in the management of XLSA probands in order to prevent a downward spiral of iron toxicity and refractory anemia.

Four New Mutations in the Erythroid-Specific 5-Aminolevulinate Synthase (ALAS2) Gene Causing X-Linked Sideroblastic Anemia: Increased Pyridoxine Responsiveness After Removal of Iron Overload by Phlebotomy and Coinheritance of Hereditary Hemochromatosis

X-linked sideroblastic anemia (XLSA) in four unrelated male probands was caused by missense mutations in the erythroid-specific 5-aminolevulinate synthase gene (ALAS2). All were new mutations: T647C, C1283T, G1395A, and C1406T predicting amino acid substitutions Y199H, R411C, R448Q, and R452C. All probands were clinically pyridoxine-responsive. The mutation Y199H was shown to be the first de novo XLSA mutation and occurred in a gamete of the proband’s maternal grandfather. There was a significantly higher frequency of coinheritance of the hereditary hemochromatosis (HH)HFE mutant allele C282Y in 18 unrelated XLSA hemizygotes than found in the normal population, indicating a role for coinheritance ofHFE alleles in the expression of this disorder. One proband (Y199H) with severe and early iron loading coinherited HH as a C282Y homozygote. The clinical and hematologic histories of two XLSA probands suggest that iron overload suppresses pyridoxine responsiveness. Notably, reversal of the iron overload in the Y199H proband by phlebotomy resulted in higher hemoglobin concentrations during pyridoxine supplementation. The proband with the R452C mutation was symptom-free on occasional phlebotomy and daily pyridoxine. These studies indicate the value of combined phlebotomy and pyridoxine supplementation in the management of XLSA probands in order to prevent a downward spiral of iron toxicity and refractory anemia.

R411C mutation of the ALAS2 gene encodes a pyridoxine-responsive enzyme with low activity

A R411C missense mutation of the erythroid-specific delta-aminolaevulinate synthase (ALAS2) gene was identified in a pedigree with X-linked pyridoxine-responsive sideroblastic anaemia (XLSA). The normal and the mutant cDNAs were expressed in E. coli, and the enzyme protein was purified. ALAS activity of the mutant enzyme was 12% and 25%, when incubated in the absence and the presence of pyridoxal 5'-phosphate, respectively, compared with that of the wild-type enzyme. These findings suggest that the R411C mutation accounts for low ALAS activity and a partial pyridoxine-responsiveness of the disease in the patient.

Positional cloning of the zebrafish sauternes gene: a model for congenital sideroblastic anaemia

Many human anaemias are caused by defects in haemoglobin synthesis. The zebrafish mutant sauternes (sau) has a microcytic, hypochromic anaemia, suggesting that haemoglobin production is perturbed. During embryogenesis, sau mutants have delayed erythroid maturation and abnormal globin gene expression. Using positional cloning techniques, we show that sau encodes the erythroid-specific isoform of delta-aminolevulinate synthase (ALAS2; also known as ALAS-E), the enzyme required for the first step in haem biosynthesis. As mutations in ALAS2 cause congenital sideroblastic anaemia (CSA) in humans, sau represents the first animal model of this disease.

Deficient Heme and Globin Synthesis in Embryonic Stem Cells Lacking the Erythroid-Specific δ-Aminolevulinate Synthase Gene

The erythroid-specific isoform of δ-aminolevulinate synthase (ALAS-E) catalyzes the first step of heme biosynthesis in erythroid cells, and ALAS-E gene mutations are known to be responsible for x-linked sideroblastic anemia. To study the role of ALAS-E in erythroid development, we prepared mouse embryonic stem (ES) cells carrying a disrupted ALAS-E gene and examined the effect of the lack of ALAS-E gene expression on erythroid differentiation. We found that mRNAs for erythroid transcription factors and TER119-positive cells were increased similarly both in the wild-type and mutant cells. In contrast, heme content, the number of benzidine-positive cells, adult globin protein, and mRNA for β-major globin were significantly decreased in the mutant cells. These results were confirmed using another ES differentiation system in vitro and suggest that ALAS-E expression, hence heme supply, is critical for the late stage of erythroid cell differentiation, which involves hemoglobin synthesis.

Deficient Heme and Globin Synthesis in Embryonic Stem Cells Lacking the Erythroid-Specific δ-Aminolevulinate Synthase Gene

The erythroid-specific isoform of δ-aminolevulinate synthase (ALAS-E) catalyzes the first step of heme biosynthesis in erythroid cells, and ALAS-E gene mutations are known to be responsible for x-linked sideroblastic anemia. To study the role of ALAS-E in erythroid development, we prepared mouse embryonic stem (ES) cells carrying a disrupted ALAS-E gene and examined the effect of the lack of ALAS-E gene expression on erythroid differentiation. We found that mRNAs for erythroid transcription factors and TER119-positive cells were increased similarly both in the wild-type and mutant cells. In contrast, heme content, the number of benzidine-positive cells, adult globin protein, and mRNA for β-major globin were significantly decreased in the mutant cells. These results were confirmed using another ES differentiation system in vitro and suggest that ALAS-E expression, hence heme supply, is critical for the late stage of erythroid cell differentiation, which involves hemoglobin synthesis.

Pathogenetic Aspects of Myelodysplastic Syndromes

The myelodysplastic syndromes (MDS) are acquired clonal disorders of the bone marrow. They were clearly defined in morphological terms by the French-American-British (FAB) group in 1982, as five conditions each with their own diagnostic criteria, but with the shared characteristics of ineffective blood cell production in one or more cell line, morphological dysplasia and a variable propensity to evolve into acute myeloid leukaemia (AML). In clinical practice patients typically present in old age with macrocytic anaemia, cytopenias, monocytosis and accumulation of marrow blast cells leading in time to fatal bone marrow failure or AML. To date treatment is unable to alter the natural history of MDS except in those few individuals who are able to undergo allogeneic progenitor cell transplantation.

Heteroplasmic Point Mutations of Mitochondrial DNA Affecting Subunit I of Cytochrome c Oxidase in Two Patients With Acquired Idiopathic Sideroblastic Anemia

Mitochondrial iron overload in acquired idiopathic sideroblastic anemia (AISA) may be attributable to mutations of mitochondrial DNA (mtDNA), because these can cause respiratory chain dysfunction, thereby impairing reduction of ferric iron (Fe3+) to ferrous iron (Fe2+). The reduced form of iron is essential to the last step of mitochondrial heme biosynthesis. It is not yet understood to which part of the respiratory chain the reduction of ferric iron is linked. In two patients with AISA we identified point mutations of mtDNA affecting the same transmembrane helix within subunit I of cytochrome c oxidase (COX I; ie, complex IV of the respiratory chain). The mutations were detected by restriction fragment length polymorphism analysis and temperature gradient gel electrophoresis. One of the mutations involves a T → C transition in nucleotide position 6742, causing an amino acid change from methionine to threonine. The other mutation is a T → C transition at nt 6721, changing isoleucine to threonine. Both amino acids are highly conserved in a wide range of species. Both mutations are heteroplasmic, ie, they establish a mixture of normal and mutated mitochondrial genomes, which is typical of disorders of mtDNA. The mutations were present in bone marrow and whole blood samples, in isolated platelets, and in granulocytes, but appeared to be absent from T and B lymphocytes purified by immunomagnetic bead separation. They were not detected in buccal mucosa cells obtained by mouthwashes and in cultured skin fibroblasts examined in one of the patients. In both patients, this pattern of involvement suggests that the mtDNA mutation occurred in a self-renewing bone marrow stem cell with myeloid determination. Identification of two point mutations with very similar location suggests that cytochrome c oxidase plays an important role in the pathogenesis of AISA. COX may be the physiologic site of iron reduction and transport through the inner mitochondrial membrane.

Heteroplasmic Point Mutations of Mitochondrial DNA Affecting Subunit I of Cytochrome c Oxidase in Two Patients With Acquired Idiopathic Sideroblastic Anemia

Mitochondrial iron overload in acquired idiopathic sideroblastic anemia (AISA) may be attributable to mutations of mitochondrial DNA (mtDNA), because these can cause respiratory chain dysfunction, thereby impairing reduction of ferric iron (Fe3+) to ferrous iron (Fe2+). The reduced form of iron is essential to the last step of mitochondrial heme biosynthesis. It is not yet understood to which part of the respiratory chain the reduction of ferric iron is linked. In two patients with AISA we identified point mutations of mtDNA affecting the same transmembrane helix within subunit I of cytochrome c oxidase (COX I; ie, complex IV of the respiratory chain). The mutations were detected by restriction fragment length polymorphism analysis and temperature gradient gel electrophoresis. One of the mutations involves a T → C transition in nucleotide position 6742, causing an amino acid change from methionine to threonine. The other mutation is a T → C transition at nt 6721, changing isoleucine to threonine. Both amino acids are highly conserved in a wide range of species. Both mutations are heteroplasmic, ie, they establish a mixture of normal and mutated mitochondrial genomes, which is typical of disorders of mtDNA. The mutations were present in bone marrow and whole blood samples, in isolated platelets, and in granulocytes, but appeared to be absent from T and B lymphocytes purified by immunomagnetic bead separation. They were not detected in buccal mucosa cells obtained by mouthwashes and in cultured skin fibroblasts examined in one of the patients. In both patients, this pattern of involvement suggests that the mtDNA mutation occurred in a self-renewing bone marrow stem cell with myeloid determination. Identification of two point mutations with very similar location suggests that cytochrome c oxidase plays an important role in the pathogenesis of AISA. COX may be the physiologic site of iron reduction and transport through the inner mitochondrial membrane.

Transcriptional regulation of the human erythroid 5-aminolevulinate synthase gene. Identification of promoter elements and role of regulatory proteins

We have characterized the 5'-flanking region of the human erythroid-specific 5-amino levulinate synthase (ALAS) gene (the ALAS2 gene) and shown that the first 300 base pairs of promoter sequence gives maximal expression in erythroid cells. Transcription factor binding sites clustered within this promoter sequence include GATA motifs and CACCC boxes, critical regulatory sequences of many erythroid cell-expressed genes. GATA sites at -126/-121 (on the noncoding strand) and -102/-97 were each recognized by GATA-1 protein in vitro using erythroid cell nuclear extracts. Promoter mutagenesis and transient expression assays in erythroid cells established that both GATA-1 binding sites were functional and exogenously expressed GATA-1 increased promoter activity through these sites in transactivation experiments. A noncanonical TATA sequence at the expected TATA box location (-30/-23) bound GATA-1- or TATA-binding protein (TBP) in vitro. Conversion of this sequence to a canonical TATA box reduced expression in erythroid cells, suggesting a specific role for GATA-1 at this site. However, expression was also markedly reduced when the -30/-23 sequence was converted to a consensus GATA-1 sequence (that did not bind TBP in vitro), suggesting that a functional interaction of both factors with this sequence is important. A sequence comprising two overlapping CACCC boxes at -59/-48 (on the noncoding strand) was demonstrated by mutagenesis to be functionally important. This CACCC sequence bound Sp1, erythroid Krüppel-like factor, and basic Krüppel-like factor in vitro, while in transactivation experiments erythroid Krüppel-like factor activated ALAS2 promoter expression through this sequence. A sequence at -49/-39 with a 9/11 match to the consensus for the erythroid specific factor NF-E2 was not functional. Promoter constructs with 5'-flanking sequence from 293 base pairs to 10.3 kilobase pairs expressed efficiently in COS-1 cells as well as in erythroid cells, indicating that an enhancer sequence located elsewhere or native chromatin structure may be required for the tissue-restricted expression of the gene in vivo.

[Congenital sideroblastic anemia in an infant]

Sideroblastic anemia is rare in infants. When it occurs, it is generally hereditary and X-linked.

CASE REPORT

A microcytic anemia without iron deficiency was diagnosed in a 2-month old infant with probable acquired cytomegalic infection. Bone marrow examination disclosed ringed sideroblasts without cytomegalic inclusions. Treatment with pyridoxine was tried without effect and no other therapeutic trial was attempted. The child, now aged 5 years, remains well without blood transfusion.

CONCLUSION

Hereditary and acquired sideroblastic anemias may occur in infants and children. Whatever their cause might be, the evolution is unpredictable. The only successful treatment for the hereditary variant remains pyridoxine supplementation which brings relief in about one patient out of three. Efficiency and safety of other therapeutic regimens remain to be proven.

Pyridoxine Refractory X-Linked Sideroblastic Anemia Caused by a Point Mutation in the Erythroid 5-Aminolevulinate Synthase Gene

To elucidate how pyridoxine-refractory X-linked sideroblastic anemia (XLSA) develops, we analyzed the erythroid-specific 5-aminolevulinate synthase (ALAS-E) gene of a patient with the anemia. The activity and amount of the enzyme in bone marrow cells of the patient were found to be approximately 5% of the normal control. We identified a point mutation, which introduces an amino acid substitution from Asp 190 to Val. In transient transfection analyses using quail fibroblasts, accumulation of aberrantly processed proteins, the sizes of which were larger than that of mature ALAS-E, was found in mitochondria. The proteins were reproducibly detected in assays combining in vitro transcription/translation of ALAS-E precursor and import of the precursor into isolated mouse mitochondria. These results suggest that the mutation causing pyridoxine-refractory XLSA affects the processing of the ALAS-E precursor, thus provoking instability of the ALAS-E protein.

Pyridoxine Refractory X-Linked Sideroblastic Anemia Caused by a Point Mutation in the Erythroid 5-Aminolevulinate Synthase Gene

To elucidate how pyridoxine-refractory X-linked sideroblastic anemia (XLSA) develops, we analyzed the erythroid-specific 5-aminolevulinate synthase (ALAS-E) gene of a patient with the anemia. The activity and amount of the enzyme in bone marrow cells of the patient were found to be approximately 5% of the normal control. We identified a point mutation, which introduces an amino acid substitution from Asp 190 to Val. In transient transfection analyses using quail fibroblasts, accumulation of aberrantly processed proteins, the sizes of which were larger than that of mature ALAS-E, was found in mitochondria. The proteins were reproducibly detected in assays combining in vitro transcription/translation of ALAS-E precursor and import of the precursor into isolated mouse mitochondria. These results suggest that the mutation causing pyridoxine-refractory XLSA affects the processing of the ALAS-E precursor, thus provoking instability of the ALAS-E protein.

Expression and purification of mammalian 5-aminolevulinate synthase

We have described a procedure for production and purification of recombinant, mature-length mouse ALAS-2. The fact that E. coli utilizes the C5 path for ALA production means that there is no problem with contamination of the recombinant ALAS-2 by host cell enzyme, such as one may have with a yeast expression system. While the detailed procedure produces enzyme in good yield with relatively common protein purification techniques, future expression systems may be developed to take advantage of the rapid purification achieved by the use of a 6-histidine (His6) aminoterminal tag and metal chelate chromatography. Such approaches in this laboratory with protoporphyrinogen oxidase, coproporphyrinogen oxidase, and uroporphyrinogen decarboxylase have resulted in the production and purification of enzymes whose kinetic and physical parameters are essentially identical to those of proteins lacking the His6 tag.

Identification of an arginine452 to histidine substitution in the erythroid 5-aminolaevulinate synthetase gene in a large pedigree with X-linked hereditary sideroblastic anaemia

The coding region of the erythroid 5-aminolaevulinate synthetase gene (ALAS2) from a large pedigree with pyridoxine-responsive X-linked hereditary sideroblastic anaemia was examined for mutations. In three affected males from this pedigree, single strand conformational polymorphism (SSCP) analysis showed anomalous migration of a PCR product spanning exon 9. Sequencing of amplified genomic DNA from one of these affected males revealed a guanine to adenine transition at nucleotide 1407 of the cDNA sequence in exon 9 of the gene. This mutation results in the loss of an HhaI restriction enzyme digest site. An HhaI digest assay demonstrated the presence of this mutation in other affected males but not in unaffected males and unrelated individuals. The point mutation results in an arginine to histidine substitution at amino acid residue 452. The arginine residue is conserved in both the erythroid and housekeeping ALAS genes in all known vertebrate sequences. This arginine is located in the middle of a predicted alpha-helix.

Late-onset X-linked sideroblastic anemia. Missense mutations in the erythroid delta-aminolevulinate synthase (ALAS2) gene in two pyridoxine-responsive patients initially diagnosed with acquired refractory anemia and ringed sideroblasts

X-linked sideroblastic anemia (XLSA) is caused by mutations of the erythroid-specific delta-aminolevulinate synthase gene (ALAS2) resulting in deficient heme synthesis. The characteristic hypochromic, microcytic anemia typically becomes manifest in the first three decades of life. Hematologic response to pyridoxine is variable and rarely complete. We report two unrelated cases of highly pyridoxine-responsive XLSA in geriatric patients previously diagnosed with refractory anemia and ringed sideroblasts. A previously unaffected 77-yr-old male and an 81-yr-old female were each found to have developed severe hypochromic, microcytic anemia with ringed sideroblasts in the bone marrow, which responded dramatically to pyridoxine with normalization of hemoglobin values. Sequence analysis identified an A to C transversion in exon 7 (K299Q) of the ALAS2 gene in the male proband and his daughter. In the female proband a G to A transition was identified in exon 5 (A172T). This mutation resulted in decreased in vitro stability of bone marrow delta-aminolevulinate synthase activity. Each patient's recombinant mutant ALAS2 enzyme had marked thermolability. Addition of pyridoxal 5'-phosphate in vitro stabilized the mutant enzymes, consistent with the observed dramatic response to pyridoxine in vivo. This late-onset form of XLSA can be distinguished from refractory anemia and ringed sideroblasts by microcytosis, pyridoxine-responsiveness, and ALAS2 mutations. These findings emphasize the need to consider all elderly patients with microcytic sideroblastic anemia as candidates for XLSA, especially if pyridoxine responsiveness is demonstrated.

Acquired hypochromic and microcytic sideroblastic anaemia responsive to pyridoxine with low value of free erythrocyte protoporphyrin: a possible subgroup of idiopathic acquired sideroblastic anaemia (IASA)

Patients with idiopathic acquired sideroblastic anaemia (IASA) usually show macrocytic or normocytic anaemia and increased free erythrocyte protoporphyrin (FEP). The mean cell haemoglobin concentration is normal or slightly low. Here we report a pyridoxine-responsive IASA patient with microcytic and hypochromic anaemia and low FEL level; these features are usually seen in cases of hereditary sideroblastic anaemia. Microcytosis increased during therapy. There may be a subgroup of IASA with microcytic and hypochromic anaemia, low normal FEP and some response to pyridoxine like hereditary sideroblastic anaemia.

Linkage analysis of a large pedigree with hereditary sideroblastic anaemia

A large pedigree showing a history of pyridoxine responsive X linked sideroblastic anaemia was screened with several polymorphic DNA markers from the X chromosome. Linkage analysis between each marker and disease status was performed, giving a maximum two point lod score of 3.64 at zero recombination with the microsatellite marker PGK1P1 at Xq11.2-12. Close linkage to PGK at Xq13.3, one of the candidate regions for X linked sideroblastic anaemia, was excluded. Linkage to DNA markers distal to PGK and at Xp21 was also excluded. Multipoint linkage analysis was performed with markers located between Xq11.2-21. The maximum map specific lod score obtained was 3.56 at PGK1P1 (Xq11.2-12). Linkage remained significant over the interval 20 cM proximal to PGK1P1 and 5 cM distal to PGK1P1, with definite exclusion around the PGK locus. The most likely location of the gene involved in sideroblastic anaemia in this pedigree is therefore within the pericentromeric region of the X chromosome. This region includes the erythroid 5-aminolaevulinate synthetase gene of the haem synthesis pathway, which is a candidate gene for X linked sideroblastic anaemia located at Xp11.21.

Molecular defects of erythroid 5-aminolevulinate synthase in X-linked sideroblastic anemia

The erythroid-specific isozyme of 5-aminolevulinate synthase (ALAS2), the first and rate-limiting 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.

5-Aminolevulinate synthase and the first step of heme biosynthesis

5-Aminolevulinate synthase catalyzes the condensation of glycine and succinyl-CoA to yield 5-aminolevulinate. In animals, fungi, and some bacteria, 5-aminolevulinate synthase is the first enzyme of the heme biosynthetic pathway. Mutations on the human erythroid 5-aminolevulinate synthase, which is localized on the X-chromosome, have been associated with X-linked sideroblastic anemia. Recent biochemical and molecular biological developments provide important insights into the structure and function of this enzyme. In animals, two aminolevulinate synthase genes, one housekeeping and one erythroid-specific, have been identified. In addition, the isolation of 5-aminolevulinate synthase genomic and cDNA clones have permitted the development of expression systems, which have tremendously increased the yields of purified enzyme, facilitating structural and functional studies. A lysine residue has been identified as the residue involved in the Schiff base linkage of the pyridoxal 5'-phosphate cofactor, and the catalytic domain has been assigned to the C-terminus of the enzyme. A conserved glycine-rich motif, common to all aminolevulinate synthases, has been proposed to be at the pyridoxal 5'-phosphate-binding site. A heme-regulatory motif, present in the presequences of 5-aminolevulinate synthase precursors, has been shown to mediate the inhibition of the mitochondrial import of the precursor proteins in the presence of heme. Finally, the regulatory mechanisms, exerted by an iron-responsive element binding protein, during the translation of erythroid 5-aminolevulinate synthase mRNA, are discussed in relation to heme biosynthesis.

Myopathy, lactic acidosis, and sideroblastic anemia: a new syndrome

We describe 2 sibs (brother and sister) with myopathy, sideroblastic anemia, lactic acidosis, mental retardation, microcephaly, high palate, high philtrum, distichiasis, and micrognathia. Very low levels of cytochromes a, b, and c were detected in the patients' muscle mitochondria. Deposition of iron within the mitochondria of bone marrow erythroblasts was observed on electron microscopy. Irregular and enlarged mitochondria with paracrystalline inclusions were also seen on electron microscopy of the patients' muscle specimen. Examination of DNA from the affected sibs showed no deletions in the mitochondrial DNA nor the mutations identified in the syndromes of mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS) or myoclonus, and epilepsy associated with rugged-red fibers (MERRF). Since the parents were first cousins and 2 of 6 sibs (male and female) were affected, we suggest that the syndrome expressed by our patients represents a previously unknown autosomal recessive disorder that includes mitochondrial myopathy, lactic acidosis, and sideroblastic anemia.

Sideroblastic anaemia

The startling morphological abnormalities of sideroblastic anaemia contrasts our uncertainty about its cause. Studies are hampered by the fact that the abnormality resides in the dividing and differentiating erythroblast which is difficult to obtain pure and in large numbers, and in which many levels of metabolic control must coexist. Recent molecular biology approaches have confirmed abnormalities of erythroid delta-aminolaevulinic acid synthase as the cause of X-linked, pyridoxine-responsive sideroblastic anaemia and mitochondrial DNA deletions as the most common cause of congenital macrocytic sideroblastic anaemia. They have also identified a second X-linked sideroblastic anaemia locus linked to phosphoglycerate kinase and associated with ataxia. An association between sideroblastic anaemia and the use of an oral copper chelating agent has highlighted unexplained links between erythroid copper and iron metabolism. Management decisions in relation to pyridoxine treatment, iron reduction, family studies, genetic counselling and antenatal diagnosis have in recent years become of practical relevance to families with known cases of congenital sideroblastic anaemia and careful documentation of the clinical outcome of these cases and of other family members is invaluable. Parallel and integrated studies on the molecular biology of erythroid differentiation are revealing the range of possible controlling influences on erythroblasts and defining the circumstances for each, allowing studies on the cause of the most prevalent form of sideroblastic anaemia (the idiopathic acquired form) and those inherited forms that are not X-linked to be approached with a much clearer perspective.

Highlights in haem biosynthesis

The haem biosynthesis pathway continues to provide surprises, from the first enzyme, 5-aminolaevulinic acid synthase, the mRNA of which contains an iron-responsive element, to the last, ferrochelatase, that contains an iron sulphur cluster. 5-Aminolaevulinate dehydratase from animals are zinc-dependent enzymes while those from plants require magnesium. The first X-ray structure of a haem synthesis enzyme, porphobilinogen deaminase, has not only yielded clues about the mechanism of tetrapyrrole assembly but has also provided insight into the molecular basis of the human disease acute intermittent porphyria. Evidence is growing to suggest that a previously unsuspected alternative haem pathway may exist.

Acquired sideroblastic anaemia following progesterone therapy

We report a case of acquired sideroblastic anaemia precipitated by progesterone. On two separate occasions, over 15 years apart, the patient developed sideroblastic anaemia with iron overload shortly after the administration of progesterone. No other cause for sideroblastic anaemia was found, and treatment with folic acid, pyridoxine or androgens corrected the anaemia. In both instances removal of the progestational agent led to prompt disappearance of the anaemia as well as the ringed sideroblasts. Using a two-phase liquid culture procedure in which human peripheral blood-derived progenitor cells undergo erythroid proliferation and differentiation, we demonstrated enhanced sensitivity of the patient's erythroid progenitors to progesterone. We conclude that progesterone should be added to the list of medications known to be associated with acquired sideroblastic anaemia.