5‐Aminolevulinate synthase in sideroblastic anemias: mRNA and enzyme activity levels in bone marrow cells

Abstract

To examine the role of 5-aminolevulinate synthase (ALAS) in the pathogenesis of sideroblastic anemias, levels of mRNAs for erythroid and housekeeping ALAS isozymes were examined, and total ALAS activity was assessed in bone marrow cells. In two patients with X-linked sideroblastic anemia the levels of mRNA for erythroid ALAS as well as for alpha and beta globin appear to be decreased while levels of mRNA for glycophorin A in both patients were the same as in normal individuals. However, amounts of housekeeping ALAS mRNA were increased two- to threefold in these patients. Total ALAS activity was also increased two- or threefold, perhaps reflecting increased transcription of the housekeeping gene in response to diminished cellular heme in erythroid cells and/or enhanced translation of the erythroid isoform in response to iron accumulation. In a third patient with X-linked sideroblastic anemia ALAS activity was low but increased to twice the normal value after pyridoxine administration, suggesting a structural defect of the enzyme. In a fourth patient, with isolated congenital, pyridoxine-responsive sideroblastic anemia, the erythroid ALAS mRNA was normal and a low enzyme activity was strikingly enhanced by pyridoxal-phosphate albeit to subnormal levels. In idiopathic acquired sideroblastic anemia, ALAS mRNA for both isozymes was normal and enzyme activity was slightly elevated. These observations thus reflect heterogeneous aberrations of erythroid heme synthesis in the various types of sideroblastic anemia and suggest that defects involving erythroid ALAS underlie at least some of them.

Sideroblastic anemia: molecular analysis of the ALAS2 gene in a series of 29 probands and functional studies of 10 missense mutations

“…Most of these mutations were described at the genomic level without further characterization of the mutated protein either in vitro or in vivo. In some patients, reduced ALAS enzymatic activity has been reported in the bone marrow [Bottomley et al, 1992; Cotter et al, 1995; Cox et al, 1994; Harigae et al, 1999a], and in others the mutated cDNA has been expressed in Escherichia coli in order to study the activity of mutated protein [Cotter et al, 1992, 1994, 1995; Cox et al, 1994; Furuyama et al, 1997, 1998, 2006; Harigae et al, 1999a, b; Prades et al, 1995]. In addition, in 2005, the crystallographic structure of ALAS2 from Rhodobacter capsulatus was published, making it possible to map the XLSA causing mutations [Astner et al, 2005].…”

Practice guidelines for the diagnosis and management of microcytic anemias due to genetic disorders of iron metabolism or heme synthesis

“…ALAS2 (OMIM 301300) is located on chromosome X and encodes for ALA synthase 2 (ALAS2), an erythroid-specific isoform of the catalytic enzyme involved in heme synthesis in the mitochondria (Figure 1). 73 Almost all ALAS2 defects are missense mutations, most commonly in domains important for catalysis or pyridoxal phosphate (vitamin B6) cofactor binding. 74 Defects in the binding site of the transcription factor GATA1 in the first intron of ALAS2 have also recently been described.…”

Sideroblastic anaemias

“…Missense mutations of the ALAS‐2 gene produce most cases of X‐linked sideroblastic anaemia (Bottomley et al , 1992; Cotter et al , 1992b; Cox et al , 1992, 1994; Edgar et al , 1997). Years after their initial evaluation, investigators located several members of the pedigree originally described by Cooley and analysed their DNA using current techniques in molecular biology (Cotter et al , 1994).…”