ALAS2

I keep it my hand ,Prognosis and Therapy

SA with mutated ALAS2 may respond to pyridoxine treatment. Transfusions with therapy to manage iron overload provides relief for patients with severe anemia. Hematopoietic stem cell transplantation has been successful in some patients with congenital SA. For acquired SA, the anemia is often reversible after the offending agents have been removed (e.g., alcohol, drugs) or treatment for copper deficiency has been implemented.

Sideroblastic Anemia—Pathologic Features

Peripheral Blood

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Anemia with dimorphic red blood cell morphology

Bone Marrow

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Ring sideroblasts on Prussian blue–stained bone marrow aspirate smear

Differential Diagnosis

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Iron deficiency

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Thalassemia

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Anemia of chronic disease

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Sideroblastic anemia

A May, in Blood and Bone Marrow Pathology (Second Edition), 2011

ALA synthase 2 defect

The common form of X-linked SA (XLSA) is caused by mutations in the erythroid-specific, 5′-aminolevulinate synthase gene (ALAS2) at Xp11.21. Within the mitochondrial matrix, ALAS2 homodimer uses co-factor pyridoxal phosphate to catalyze the first step of heme synthesis.6 Decreased ALAS2 activity leads to decreased protoporphyrin production and decreased heme. Iron continues to enter the erythroblast unchecked, accumulates in the mitochondria and becomes most visible in the late erythroblasts on staining.

In male patients the severity of the anemia is variable. It is microcytic-hypochromic and may respond to pyridoxine treatment (Fig. 14.1A, B).7,8 Female carriers have the potential to produce two RBC populations, one microcytic-hypochromic and one normal, depending on which erythroblast X-chromosome is active (Fig. 14.1C). Carriers are usually unaffected but some are anemic due to skewed X chromosome inactivation against that carrying the normal allele, accounting for almost one third of probands.9,10 Anemic carriers of mild or moderately severe mutations have the same presentation as male hemizygotes (Fig. 14.2). Patients can present at any age with symptoms of anemia or iron overload.4,11

Physical examination is unremarkable although mild hepatosplenomegaly may be present. Red cell size and hemoglobin content are broadly distributed and variable proportions of microcytic-hypochromic and normocytic-normochromic cells are seen on the blood film (Fig. 14.1A,B). Anisocytosis, poikilocytosis, elliptocytosis and target cells may be present. An occasional cell with basophilic stippling or Pappenheimer bodies and an occasional late erythroblast may be seen; the numbers of these increase after splenectomy. White cell and platelet counts are normal.

In the bone marrow erythropoiesis is expanded but ineffective; iron overload from increased dietary iron absorption may develop. Transferrin saturation is usually increased; even mildly anemic patients may present with secondary iron overload in the absence of blood transfusion.12 Total erythrocyte protoporphyrin (TEP) is low/normal. Severe anemia requiring regular blood transfusions is a rare occurrence. Phlebotomy has been successful for preventing iron overload, reversing iron overload and maintaining normal iron levels in pyridoxine-responsive patients (see below). This may additionally improve the hemoglobin level.13 Iron chelation may be required for correcting iron overload. Splenectomy is not recommended because this has led to thrombotic complications.4

At least 120 cases from 70–80 families are reported (January 2011). More than 50 different mutations are involved, scattered across seven exons encoding the catalytic and C-terminal domains.4,5 Missense mutations predominate; null mutations, or those predicted to be very severe, are found only in female heterozygotes. ALAS2 variations causing anemia in one member of a family may occasionally be silent in another.14

Pyridoxal phosphate (vitamin B6) is required for ALAS2 activity and stability. Many patients respond partially to pharmacologic doses of oral pyridoxine but some do not or barely do so.13 Responsive patients remain on maintenance doses of about 25–200 mg oral pyridoxine/day for life.4 Complete correction of hematological changes with pyridoxine, although rare, has been reported.15 Pyridoxine responsiveness is most associated with mutations involving amino acids fairly close to the pyridoxal phosphate binding site that cause partial loss of enzyme function or stability.16,17 Refractoriness is associated with mutations that cause irreversible disruption of activity or cause loss of in vivo activity through faulty interaction with components required for intramitochondrial processing.18 Iron overload, or its complications, also contribute to pyridoxine refractoriness. There may be some response to folate due to secondary deficiency.

Not all ALAS2 mutations cause SA. Mutations leading to deletion or substitution of the C-terminal 19 amino acids generate truncated ALAS2 protein of increased activity and cause X-linked, dominant erythropoietic protoporphyria, not anemia.19

Female carriers of severe/null mutations that prevent erythrocyte production go undetected unless they become anemic because of either skewed but incomplete X-chromosome inactivation against the normal allele or some mechanism selecting against cells expressing the normal allele. In these cases red cells produced by residual effective erythropoiesis from expression of normal ALAS2 are macrocytic, the number of ring sideroblasts may be very low and the cause of the ineffective erythropoiesis may go unsuspected or misdiagnosed (Fig. 14.2).10,20

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Other Hereditary Red Blood Cell Disorders

Bertil Glader, in Emery and Rimoin's Principles and Practice of Medical Genetics, 2013

72.6.1 X-Linked Sideroblastic Anemia

This CSA usually occurrs in males, although skewed lyonization has resulted in affected females. This disorder is due to abnormalities of the erythrocytic isozyme for 5-aminolevulinic acid synthetase (ALAS2), the rate-limiting enzyme reaction in heme synthesis. An important cofactor for ALAS is pyridoxal phosphate. The gene for the erythrocyte-specific ALAS (ALAS2) is located on the X chromosome, and almost 50 different mutations have been identified in 80 unrelated individuals. Of interest, several of these mutations occur near the binding site for pyridoxal phosphate (160,161).

The anemia in this disorder is characterized by hypochromic microcytic RBC mixed with normal red cells, thus giving an overall picture of a dimorphic population of erythrocytes. Severe anemia can be recognized in infancy or early childhood, whereas milder cases may not become apparent until early adulthood or later. Reports of elderly women with late onset of X-linked sideroblastic anemia, presumably are a consequence of acquired skewing with age (163). Patients present with pallor, icterus, moderate splenomegaly, hepatomegaly or both. The severity of the anemia varies such that some patients require no therapy while others need regular RBC transfusions. A subset of patients manifests a hematologic response to pharmacologic doses of pyridoxine. In a few severe RBC transfusion-dependent cases, BMT has been done utilizing fully matched siblings as donors (164). Iron overload as manifested by elevated serum ferritin, elevated serum iron, and increased transferrin saturation is a major complication of this disorder. In some cases where there is little or no anemia, there still may be clinical evidence of iron overload (i.e. diabetes mellitus, liver dysfunction, etc.). The coinheritance of the gene for hereditary hemochromatosis has been shown to exacerbate the iron problems associated with sideroblastic anemia (165).

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Heme Biosynthesis and Its Disorders

Stephen J. Fuller, James S. Wiley, in Hematology (Seventh Edition), 2018

Biologic and Molecular Aspects

Approaching 40% of congenital sideroblastic anemias are molecularly unexplained.155 Erythroid cells from patients with X-linked forms of hereditary sideroblastic anemia generally exhibit low activity of ALAS230,156; however, for a minority of ALAS2 mutations this effect may be difficult to detect in vitro.155 A defect in this enzyme is firmly established in patients whose anemia responds to pyridoxine therapy, because pyridoxal phosphate is an essential cofactor for ALAS. However, even affected female patients with moderate anemia unresponsive to pyridoxine have been documented to have low levels of ALAS in bone marrow lysates. In some male patients with X-linked pyridoxine-responsive sideroblastic anemia, the low ALAS activity in bone marrow increased to levels above the normal range when the patient took pyridoxine supplements and recovered from the anemia.157 There are several possible explanations for this enhancement of ALAS activity by dietary pyridoxine supplements. The most likely is that pyridoxine (or its phosphate) may stabilize the ALAS during folding of the mutant enzyme after its synthesis.156 The gene for the ALAS2 isoenzyme has been localized to the X chromosome, and this gene is known to be the site of most mutations giving rise to X-linked pyridoxine-responsive sideroblastic anemia.158–160 Approximately 90 different mutations have been identified in individuals or families with hereditary sideroblastic anemia, and nearly all have resulted from single base alterations in DNA.161–163 A frequent mutation affects arginine at residue 452 of ALAS2, which occurs in a quarter of all pedigrees but does not affect enzyme activity measured in vitro.164 All known mutations lie between exons 5 and 11 of ALAS2, the region that codes for the catalytic domain, with most lying within exon 9, which contains the lysine at which binding of pyridoxal 5′-phosphate occurs.165 A mutation, Asp190Val, has been described in a pyridoxine-refractory patient and appears to affect the proteolytic processing of the ALAS2 during or after import into the mitochondrion.166 The variety of different mutations in the erythroid ALAS2 gene responsible for X-linked sideroblastic anemia and their pyridoxine responsiveness were reviewed in 2002 and 2010.159,167

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Iron Metabolism and Its Disorders

John W. Harvey, in Clinical Biochemistry of Domestic Animals (Sixth Edition), 2008

1 Cytoplasmic Regulation of Iron

The synthesis of a number of proteins important in iron metabolism, including apotransferrin, TfR1, apoferritin, DMT1, DcytB, ferroportin, iron responsive protein-1 (IRP1), and erythroid-specific 5-aminolevulinic acid synthase (eALAS or ALAS2), is regulated posttranscriptionally depending on intracellular iron content (Beutler, 2006b; Koury and Ponka, 2004; Latunde-Dada et al., 2006a). The mRNA for each protein contains one or more iron responsive elements (IREs), each consisting of a stem-loop structure. The IREs located at the 5′ end of mRNAs regulate translation, and IREs located at the 3′ end regulate mRNA stability. IRP1 contains an Fe-S (4Fe-4S) cluster and exhibits aconitase activity when cells are iron replete, but it lacks the Fe-S cluster and aconitase activity when cytoplasmic iron is scarce. IRP1 binds tightly to IREs when cytoplasmic iron content is low. IRP2 is closely related to IRP1, but it lacks aconitase activity. The regulation of IRP2 is mediated by proteosomal degradation when cellular iron is adequate, through binding to iron and possibly heme (Wingert et al., 2005). The binding of IRPs to IREs at the 5′ end of mRNAs (including apoferritin and eALAS) inhibits translation and protein synthesis from these mRNAs, but the binding of IRPs to IREs at the 3′ end of mRNAs (including TfR1 and intestinal DMT1) promotes mRNA stability, thereby enhancing protein synthesis from these mRNAs. When cytoplasmic iron content is high, IRPs are displaced from IREs, resulting in opposite effects on protein synthesis (Starzynski et al., 2004). Because of these controlling factors, TfR1 synthesis is higher and apoferritin synthesis is lower when cytoplasmic iron content is low, and TfR1 synthesis is lower and apoferritin synthesis is higher when cytoplasmic iron content is high (Napier et al., 2005).

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Congenital and Hereditary Disorders of the Skin

Cheryl B. Bayart, Heather A. Brandling-Bennett, in Avery's Diseases of the Newborn (Tenth Edition), 2018

Erythropoietic Protoporphyria and X-Linked Erythropoietic Protoporphyria

EPP is the most common form of porphyria in childhood and is caused by autosomal recessive mutations in the gene encoding ferrochelatase. XLP, also referred to as X-linked dominant protoporphyria, is an X-linked form that results from mutations in the ALAS2 gene, the product of which catalyzes the first committed step of heme biosynthesis (Whatley et al., 2008). XLP has a phenotype very similar to that of EPP but with higher concentrations of erythrocyte protoporphyrin and a higher incidence of liver disease (Seager et al., 2014).

Clinical manifestations of EPP include immediate painful photosensitivity to sunlight and sometimes fluorescent lighting. In infancy, this may manifest itself as episodes of crying within minutes of UV exposure, and older children may complain of stinging or a burning sensation in exposed areas. Prolonged exposure may lead to erythema, edema, and petechiae and vary rarely vesicles or bullae. Long-term UV exposure results in shallow atrophic scars and thickened, leathery skin around the mouth (pseudorhagades) and overlying knuckles.

Hepatic involvement can range from mild liver dysfunction to rare liver failure. Cholelithiasis may cause severe abdominal pain. Anemia is usually mild or absent.

Diagnosis of EPP and XLP can be made on the basis of elevated erythrocyte protoporphyrin concentration, and red blood cells will fluoresce under Wood lamp examination. Increased stool protoporphyrin concentration may also be detected. The diagnosis is confirmed by genetic testing. Management in infancy includes sun avoidance and protection, with symptomatic treatment of photosensitivity reactions. Patients should be monitored for liver disease and microcytic anemia (Lecha et al., 2009; Balwani and Desnick, 2012).

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Inherited Porphyrias

R.J. Desnick, ... Karl E. Anderson, in Emery and Rimoin's Principles and Practice of Medical Genetics, 2013

99.2.1.1 5-Aminolevulinate Synthase

The first enzyme in the pathway, 5-aminolevulinate synthase (ALA-synthase; also known as d-aminolevulinate synthase; E.C. 2.3.1.37), catalyzes the condensation of glycine (activated by pyridoxal phosphate) and succinyl coenzyme A to form ALA. Distinct human housekeeping and erythroid-specific ALA-synthase isozymes are encoded by separate genes: the ~17-kb housekeeping gene (ALAS1), located at chromosome 3p21.1, is expressed in all tissues, while the ~22-kb erythroid-specific gene (ALAS2), located at chromosome Xp11.21, is expressed only in erythroid cells to supply the large amounts of heme required for hemoglobin (see Figure 99-2). These findings provide a basis for the tissue-specific regulation of this pathway (for a review see Reference (1)). Of note, expression of the housekeeping gene ALAS1 in the liver is under negative feedback repression by the cellular heme concentration and functions to modulate the supply of heme for the hepatic cytochrome P450 enzymes and other hepatic hemoproteins (7). In acute porphyrias, the depletion of hepatic heme by various drugs, hormones, and glucose restriction, the increased synthesis of the housekeeping ALAS1 isozyme, and the generation of the large amount of the porphyrin precursors, ALA and PBG, are the biochemical hallmarks of acute neurologic attacks (2).

Mutations in the X-linked ALAS2 gene and the resultant deficient activity of the erythroid-specific isozyme cause X-linked sideroblastic anemia (1,8). Over 35 mutations in the erythroid-specific ALA-synthase gene causing X-linked sideroblastic anemia are listed in the Human Gene Mutation Database (www.hgmd.org) (6). Except for a mutation in the promoter region of the ALAS2 gene (9) and one nonsense mutation, all of the reported lesions have been missense mutations in the ALAS2 catalytic core encoded by exons 5 to 11, with the majority occurring in exons 5 and 9. Most mutations were pyridoxine responsive in vivo and when expressed in Escherichia coli. Molecular modeling of the ALAS2 isozyme, based on the crystal structure of a bacterial ALA-synthase, suggested the molecular basis for the pyridoxine responsiveness of certain mutations (10). Recently, gain of function mutations in exon 11 of ALAS2 that increase its activity have been shown to cause an X-linked form of erythropoetic protoporphyria (EPP), known as X-linked protoporphyria (XLP). To date, only two gain of function mutations in ALAS2 have been described. No deficiencies of the ALAS1 isozyme have been described; presumably, the enzymatic deficiency would be lethal.

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Heme Synthesis

G.C. Ferreira, in Encyclopedia of Biological Chemistry (Second Edition), 2013

Regulation of the Heme Biosynthetic Pathway/Heme Biosynthesis

There is not a single ubiquitous regulatory mechanism for heme biosynthesis, and so the discussion in this article focuses solely on the regulation of the pathway in mammalian cells. Considering that heme is predominantly synthesized in bone marrow (i.e., in erythroblasts and reticulocytes that still contain mitochondria) and liver cells, most of the studies on regulation have focused on these two types of cells. Predominance of heme synthesis in bone marrow and liver cells reflects the greater demands due to the syntheses of the heme-containing proteins, hemoglobin, and cytochrome P-450 enzymes, respectively. The major regulatory and rate-limiting step of the pathway corresponds to the reaction catalyzed by ALAS, the first enzyme of the pathway. Two genes, located in two different chromosomes in humans, encode the two ALAS forms of the enzyme (or isozymes): ALAS1 and ALAS2. The gene encoding ALAS1 is expressed in every cell, whereas that for ALAS2 is specifically expressed in erythroid cells. Importantly, the regulatory mechanisms for the expression of the two ALAS genes and the posttranscriptional events seem to be distinct in differentiating erythrocytes from those in nonerythroid cells. ALAS1 controls the production of heme for basic cellular functions and is feedback regulated by heme through (1) repression of transcription of the ALAS1 gene, (2) inhibition of translation of the ALAS1 message, (3) destabilization of the ALAS1 message, and (4) inhibition of import of the cytosolic precursor form of the enzyme into mitochondria. By contrast, certain drugs and hormones induce liver cells to make more ALAS, and consequently heme and cytochrome P-450, by activating transcription of the ALAS1 gene.

In differentiating erythroid cells, the syntheses of heme and globin need to be coordinated, as the production of hemoglobin requires that neither the protein (globin) component nor the nonprotein component (heme) will be in excess. And indeed, identical transcriptional elements have been identified in the genes of ALAS2 and globin. Another level of regulation of the erythroid heme biosynthetic pathway is centered at the stage of translation, which responds to the cellular iron levels. This response is mediated through specific interactions between an iron-responsive element (IRE), present in the ALAS2 messenger RNA (mRNA), and IRE-binding proteins, such that under iron limitation, the translation of ALAS2 mRNA is inhibited, while the presence of iron above the cellular threshold relieves the repression caused by IRE-binding proteins. The essential requirement for ALAS2 in heme biosynthesis and erythropoiesis was clearly demonstrated with targeted disruption of ALAS2 in the mouse, which led to no hemoglobin in cells, the accumulation of iron (albeit in the cytoplasm and not in mitochondria) and, ultimately, embryonic death.