Pyridoxine-refractory congenital sideroblastic anaemia with evidence for autosomal inheritance: exclusion of linkage to ALAS2 at Xp11.21 by polymorphism analysis

Study of Glycine and Folic Acid Supplementation to Ameliorate Transfusion Dependence in Congenital SLC25A38 Mutated Sideroblastic Anemia

Congenital sideroblastic anemia (CSA) is a hematological disorder characterized by the presence of ringed sideroblasts in bone marrow erythroid precursors. Mutations in the erythroid-specific glycine mitochondrial transporter gene SLC25A38 have been found in a subset of patients with transfusion-dependent congenital CSA. Further studies in a zebrafish model identified a promising ameliorative strategy with combined supplementation with glycine and folate. We tested this combination in three individuals with SLC25A38 CSA, with a primary objective to decrease red blood cell transfusion requirements. No significant impact was observed on transfusion requirements or any hematologic parameters.

The genetics of inherited sideroblastic anemias

The sideroblastic anemias are a heterogeneous group of acquired and inherited bone marrow disorders defined by the presence of pathologic iron deposits in erythroblast mitochondria. While the pathogenesis of almost all cases of acquired sideroblastic anemia is unknown, the molecular genetic basis for several of the inherited forms have now been described. Initially, mutations in ALAS2 in X-linked sideroblastic anemia (XLSA) focused attention on the heme biosynthetic pathway as a primary cause of sideroblastic anemia. However, the subsequent description of the genes involved in XLSA with ataxia, thiamine-responsive megaloblastic anemia, and Pearson marrow-pancreas syndrome have implicated other pathways, including mitochondrial oxidative phosphorylation, thiamine metabolism, and iron-sulfur cluster biosynthesis, as primary defects in sideroblastic anemias that may only secondarily impact heme metabolism.

ABC-me: a novel mitochondrial transporter induced by GATA-1 during erythroid differentiation

Transcription factor GATA-1 is essential for normal erythropoiesis. GATA-binding sites are consistently found in promoters or enhancers of genes expressed selectively in erythroid cells. To discover novel GATA-1-regulated genes, we searched for GATA-1-activated transcripts in G1E cells, an erythroid line derived from GATA-1(-) embryonic stem cells. By subtractive analysis, we identified a new ATP-binding cassette (ABC) transporter that is strongly and rapidly induced by GATA-1. This protein, named ABC-me (for ABC-mitochondrial erythroid), localizes to the mitochondrial inner membrane and is expressed at particularly high levels in erythroid tissues of embryos and adults. ABC-me is induced during erythroid maturation in cell lines and primary hematopoietic cells, and its overexpression enhances hemoglobin synthesis in erythroleukemia cells. The ABC proteins participate in diverse physiological processes by coupling ATP hydrolysis to the transport of a variety of substrates across cell membranes. We speculate that ABC-me, a newly identified erythroid-expressed ABC superfamily member, may mediate critical mitochondrial transport functions related to heme biosynthesis.

Hematologic involvement in mitochondrial cytopathies in childhood: a retrospective study of bone marrow smears

We retrospectively analyzed the bone marrow (BM) smears of 10 children with mitochondrial cytopathies. Light microscopic examination showed large and coalescent cytoplasmic vacuolization of some BM precursors in nine cases, including two children with normal peripheral blood counts and four with sideroblastic anemia. BM ultrastructural study showed abnormal mitochondria in the erythroid lineage in all three children studied. Ultrastructural studies in two cases revealed a population of giant mitochondria with abnormal ultrastructure coexisting with a population of normal mitochondria in proerythroblasts, basophil erythroblasts, and less commonly in more mature erythroblasts. In a third child, mitochondria were normal in size with cristae either absent or exhibiting abnormal longitudinal orientation. Heteroplasmic segregation of mitochondria during cell division could account for the finding of a double population of cells on ultrastructural examination. These features suggest that cytologic and ultrastructural BM examination could be useful for the diagnosis of mitochondrial disorders. That is, when large and coalescent cytoplasmic vacuoles of BM precursor cells are present, the clinician should search for mitochondrial cytopathy in a child with unexplained cytopenia(s).

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.

Sideroblastic anemias: variations on imprecision in diagnostic criteria, proposal for an extended classification of sideroblastic anemias

Sideroblastic anemias are caused by a diversity of hereditary, congenital, or acquired disorders. Criteria used in describing sideroblastic anemias vary widely among standard medical textbooks and even so have been imprecisely applied in the literature. Recent discoveries concerning the basic pathophysiologic mechanisms involving the molecular biology of nuclear and mitochondrial DNA, erythroid ALA synthase (ALAS-2), and iron transport have made the classification of sideroblastic anemias very complex. We recommend a more precise evaluation and documentation of the components that characterize the sideroblastic abnormality and propose an extended classification of the sideroblastic anemias.

PCR induction of a TaqI restriction site at any CpG dinucleotide using two mismatched primers (CpG-PCR)

A final common pathway has been devised for analysis of de novo mutation at any CpG site. Artificial restriction sites can be introduced in known DNA sequences by using either or both sense and antisense mismatched PCR primers. Sitting of the primers directly adjacent 5' and 3' to the CpG site yields a 52-bp PCR product, resulting from the sum of the two 25-mer oligonucleotides plus the two intervening bases (C and G), and also yields consistent digestion fragments. Three out of four possible four-base palindromes (TaqI, HHaI, and MspI) were investigated for mutations R329X and E80K in the human LDL receptor gene, and for mutations R395W and R612C, and TaqI site was forced using PCR in which both primers had 3' mismatched T. Both empirically and on theoretical grounds, Taq1 is the forced restriction site of choice. The approach has been adapted to the high-throughput microplate diagonal gel electrophoresis (MADGE) system for selective analysis of mutations at CpG sites, which may account for 20% of all single base variation in the human genome.

Familial sideroblastic anemia with emergence of monosomy 5 and myelodysplastic syndrome

The case history of two sisters with pyridoxine-refractory familial sideroblastic anemia (FSA) is presented in which one developed a myelodysplastic syndrome (MDS) with monosomy for chromosome 5. Bone marrow examination of both patients at diagnosis showed erythroid hyperplasia with more than 50% ring sideroblasts. Karyotypic analysis initially showed a normal 46, XX karyotype in both of the children. Therapeutic trials with pyridoxine, prednisone, and erythropoietin were unsuccessful. The first patient required regular transfusions and developed a significant hemosiderosis. At the age of 9 years, 7.5 years after the diagnosis of FSA, refractory anemia with excess of blasts (RAEB) was diagnosed. Bone marrow cytogenetic analysis revealed a clone with monosomy for chromosome 5. Her sister's illness was detected at the age of 12 years. She has a more benign course of disease, remains largely transfusion independent and until now shows no signs of myelodysplasia. To our knowledge this is the first observation of a transition of FSA to MDS accompanied by the appearance of a chromosomal abnormality. FSA might be another type of bone marrow failure syndrome, therefore close follow-up of these patients may be necessary.

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.

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.

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.