Familial-skewed X-chromosome inactivation as a predisposing factor for late-onset X-linked sideroblastic anemia in carrier females

X-linked chronic granulomatous disease secondary to skewed X-chromosome inactivation in female patients

BACKGROUND

Chronic granulomatous disease (CGD) is a heterogeneous primary immunodeficiency. X-linked (XL) CGD caused by gene defects of CYBB is the most prevalent type of CGD.

OBJECTIVE

We aim to understand the clinical and molecule features of XL-CGD secondary to skewed X-chromosome inactivation (XCI) in female.

METHODS

We retrospectively reviewed the medical records of a female patient diagnosed with XL-CGD. Flow cytometry was used to detect the respiratory burst function. After restriction enzyme digestion of DNA, XCI was calculated by detecting fluorescent PCR products with capillary electrophoresis. The previously published female XL-CGD cases secondary to skewed XCI was summarized.

RESULTS

Clinical data were available for 15 female subjects. The median age of diagnosis was 16 years. Consistent with XL-CGD in males, infection was the most frequent manifestation in the female patients. Catalase-positive pathogens including Serratia marcescens and Staphylococcus aureus infections were the most common pathogens. Autoimmune/autoinflammation manifestations were observed in five patients. Dihydrorhodamine (DHR) assay showed that median %DHR+ values were 6.5% and the values varying with age were observed in 2 patients. All patients had a skewing XCI and there was no consistency between the daughter and carrier mother. Anti-infective treatment was effective in majority and there was no mortality reported in XL-CGD female patients to date.

CONCLUSION

XL-CGD should not be neglected in female patients manifested as CGD phenotype and it is necessary to make periodic clinical evaluation of CGD female carriers as the neutrophil oxidative function may decline with aging and increase the risk for infection.

Severe Microcytic Anemia Caused by Complex Hereditary Spherocytosis and X-Linked Sideroblastic Anemia with Mutations in SPTB and ALAS2 Genes

We report a case of severe anemia caused by complex hereditary spherocytosis (HS) and X-linked sideroblastic anemia (XLSA) with two mutations in the spectrin beta (SPTB) and 5-aminolevulinic acid synthase (ALAS2) genes. The proband was a 16-year-old male with severe jaundice and microcytic hypochromic anemia since his childhood. He had more severe anemia requiring erythrocyte transfusion, and had no response to vitamin B₆ treatment. Next-generation sequencing (NGS) revealed double heterozygous mutations, one in exon 19 (c.3936G > A:p.W1312X) of the SPTB gene and another in exon 2 (c.37A > G:p.K13E) of the ALAS2 gene, and confirmed again by Sanger sequencing. The mutation of ALAS2 (c.37A > G) is inherited from his asymptomatic heterozygous mother, causing amino acid p.K13E, and the mutation has not yet been reported. The mutation of SPTB (c.3936G > A) is a nonsense mutation, leading to a premature termination codon in exon 19, and the mutation in the SPTB gene is not found in any of his relatives, which indicates a de novo monoallelic mutation. Conclusions: The double heterozygous mutations in the SPTB and ALAS2 genes lead to the joint occurrence of HS and XLSA in this patient, and are implicated in the more severe clinical phenotypes.

Congenital sideroblastic anemia model due to ALAS2 mutation is susceptible to ferroptosis

X-linked sideroblastic anemia (XLSA), the most common form of congenital sideroblastic anemia, is caused by a germline mutation in the erythroid-specific 5-aminolevulinate synthase (ALAS2) gene. In XLSA, defective heme biosynthesis leads to ring sideroblast formation because of excess mitochondrial iron accumulation. In this study, we introduced ALAS2 missense mutations on human umbilical cord blood-derived erythroblasts; hereafter, we refer to them as XLSA clones. XLSA clones that differentiated into mature erythroblasts showed an increased frequency of ring sideroblast formation with impaired hemoglobin biosynthesis. The expression profiling revealed significant enrichment of genes involved in ferroptosis, which is a form of regulated cell death induced by iron accumulation and lipid peroxidation. Notably, treatment with erastin, a ferroptosis inducer, caused a higher proportion of cell death in XLSA clones. XLSA clones exhibited significantly higher levels of intracellular lipid peroxides and enhanced expression of BACH1, a regulator of iron metabolism and potential accelerator of ferroptosis. In XLSA clones, BACH1 repressed genes involved in iron metabolism and glutathione synthesis. Collectively, defective heme biosynthesis in XLSA clones could confer enhanced BACH1 expression, leading to increased susceptibility to ferroptosis. The results of our study provide important information for the development of novel therapeutic targets for XLSA.

X-linked sideroblastic anaemia in a female fetus: a case report and a literature review

Background

X-linked sideroblastic anaemia (XLSA) is commonly due to mutations in the ALAS2 gene and predominantly affects hemizygous males. Heterozygous female carriers of the ALAS2 gene mutation are often asymptomatic or only mildly anaemic. XLSA is usually characterized by microcytic erythrocytes (reduced mean corpuscular volume (MCV)) and hypochromia, along with increased red cell distribution width. However, in females with XLSA the characteristic laboratory findings can be dimorphic and present with macrocytic (elevated MCV) in addition to microcytic red cells.

Case presentation

We report a case of fetal anaemia, presenting in the early third trimester of pregnancy, in a female fetus. Ultrasound findings at 29 weeks were of cardiomegaly, prominent umbilical veins, a small rim of ascites, and mean cerebral artery peak systolic velocity (PSV) value above 1.5 Multiples of the Median (MoM). She underwent non-invasive prenatal testing that determined the rhesus genotype of the fetus to be rhesus B negative. No red blood cell antibodies were reported. Other investigations to determine the underlying cause of fetal anaemia included microarray comparative genomic hybridization, serology to exclude congenital infection and a peripheral blood film and fetal bilirubin to detect haemolysis. The maternal grandmother had a history of sideroblastic anaemia diagnosed at the age of 17 years. The mother had mild macrocytic anaemia with haemoglobin of 10.4 g/dl and MCV of 104 fl. The fetal anaemia was successfully treated with two in utero transfusions (IUTs), and delivery occurred via caesarean section at 37 weeks of gestation. The red cell gene sequencing in both the mother and fetus were heterozygous for an ALAS2 mutation causing in utero manifestations of XLSA. The haemoglobin on discharge to the local hospital at five days of age was 19.1 g/dl. Subsequently, the infant became anaemic, requiring regular 3–4 monthly blood transfusions and demonstrating overall normal development. Her anaemia was unresponsive to pyridoxine.

Conclusions

This is one of four cases reporting multiple female members presenting with discordant clinical features of XLSA from being entirely asymptomatic to hydropic in utero. Our report is novel in that there are no previous cases in the literature of anaemia in a female fetus heterozygous for ALAS2 mutation.

Azacitidine is a potential therapeutic drug for pyridoxine-refractory female X-linked sideroblastic anemia

A patient-derived iPSC model recapitulates defective erythroid maturation in female XLSA.

Azacitidine reactivates the silent wild-type ALAS2 allele and ameliorates inefficient erythropoiesis in iPSC-derived HPCs from female XLSA.

X-linked sideroblastic anemia (XLSA) is associated with mutations in the erythroid-specific δ-aminolevulinic acid synthase (ALAS2) gene. Treatment of XLSA is mainly supportive, except in patients who are pyridoxine responsive. Female XLSA often represents a late onset of severe anemia, mostly related to the acquired skewing of X chromosome inactivation. In this study, we successfully generated active wild-type and mutant ALAS2-induced pluripotent stem cell (iPSC) lines from the peripheral blood cells of an affected mother and 2 daughters in a family with pyridoxine-resistant XLSA related to a heterozygous ALAS2 missense mutation (R227C). The erythroid differentiation potential was severely impaired in active mutant iPSC lines compared with that in active wild-type iPSC lines. Most of the active mutant iPSC-derived erythroblasts revealed an immature morphological phenotype, and some showed dysplasia and perinuclear iron deposits. In addition, globin and HO-1 expression and heme biosynthesis in active mutant erythroblasts were severely impaired compared with that in active wild-type erythroblasts. Furthermore, genes associated with erythroblast maturation and karyopyknosis showed significantly reduced expression in active mutant erythroblasts, recapitulating the maturation defects. Notably, the erythroid differentiation ability and hemoglobin expression of active mutant iPSC-derived hematopoietic progenitor cells (HPCs) were improved by the administration of δ-aminolevulinic acid, verifying the suitability of the cells for drug testing. Administration of a DNA demethylating agent, azacitidine, reactivated the silent, wild-type ALAS2 allele in active mutant HPCs and ameliorated the erythroid differentiation defects, suggesting that azacitidine is a potential novel therapeutic drug for female XLSA. Our patient-specific iPSC platform provides novel biological and therapeutic insights for XLSA.

Understanding Sideroblastic Anemia: An Overview of Genetics, Epidemiology, Pathophysiology and Current Therapeutic Options

Sideroblastic anemia (SA) consists of a group of inherited and acquired anemias of ineffective erythropoiesis characterized by the accumulation of ring sideroblasts in the bone marrow due to disrupted heme biosynthesis. Congenital sideroblastic anemia (CSA) is rare and has three modes of inheritance: X-linked (XLSA), autosomal recessive (ARCSA), and maternal. Acquired SA is more common and can be a result of myelodysplastic syndromes (MDS) or other, generally reversible causes. The diagnostic approach to SA includes a work-up for reversible causes and genetic testing for CSA based on clinical suspicion, family history and genetic pedigree. The treatment of SA depends on the underlying etiology but remains primarily supportive with vitamin B6 supplementation for select cases of XLSA, thiamine for thiamine-responsive megaloblastic anemia subtype, red blood cell transfusions for symptomatic patients and iron chelation therapy for iron overload. The management of anemia in MDS subtypes with ring sideroblasts remains unique and includes the recently approved erythroid maturation agent, Luspatercept. Although there is currently no curative therapy for CSA, anecdotal reports of hematopoietic stem cell transplant demonstrate remissions in selective, non-syndromic cases. This review summarizes the genetics, pathophysiology, diagnosis and treatment of SA for general practitioners and clinical hematologists.

A new sex-specific underlying mechanism for female schizophrenia: accelerated skewed X chromosome inactivation

BACKGROUND

X chromosome inactivation (XCI) is the mechanism by which the X-linked gene dosage is adjusted between the sexes. Evidence shows that many sex-specific diseases have their basis in X chromosome biology. While female schizophrenia patients often have a delayed age of disease onset and clinical phenotypes that are different from those of males, it is unknown whether the sex differences in schizophrenia are associated with X-linked gene dosage and the choice of X chromosome silencing in female cells. Previous studies demonstrated that sex chromosome aneuploidies may be related to the pathogeneses of some psychiatric diseases. Here, we examined the changes in skewed XCI in patients with schizophrenia.

METHODS

A total of 109 female schizophrenia (SCZ) patients and 80 age- and sex-matched healthy controls (CNTLs) were included in this study. We evaluated clinical features including disease onset age, disease duration, clinical symptoms by the Positive and Negative Syndrome Scale (PANSS) and antipsychotic treatment dosages. The XCI skewing patterns were analyzed by the methylation profile of the HUMARA gene found in DNA isolated from SCZ patient and CNTL leukocytes in the three age groups.

RESULTS

First, we found that the frequency of skewed XCI in SCZ patients was 4 times more than that in the age- and sex-matched CNTLs (p < 0.01). Second, we found an earlier onset of severe XCI skewing in the SCZ patients than in CNTLs. Third, we demonstrated a close relationship between the severity of skewed XCI and schizophrenic symptoms (PANSS score ≥ 90) as well as the age of disease onset. Fourth, we demonstrated that the skewed XCI in SCZ patients was not transmitted from the patients' mothers.

LIMITATIONS

The XCI skewing pattern might differ depending on tissues or organs. Although this is the first study to explore skewed XCI in SCZ, in the future, samples from different tissues or cells in SCZ patients might be important for understanding the impact of skewed XCI in this disease.

CONCLUSION

Our study, for the first time, investigated skewed XCI in female SCZ patients and presented a potential mechanism for the sex differences in SCZ. Our data also suggested that XCI might be a potential target for the development of female-specific interventions for SCZ.

The X Files: "The Mystery of X Chromosome Instability in Alzheimer's Disease"

Alzheimer's disease (AD) is a neurodegenerative disease that affects millions of individuals worldwide and can occur relatively early or later in life. It is well known that genetic components, such as the amyloid precursor protein gene on chromosome 21, are fundamental in early-onset AD (EOAD). To date, however, only the apolipoprotein E4 (ApoE4) gene has been proved to be a genetic risk factor for late-onset AD (LOAD). In recent years, despite the hypothesis that many additional unidentified genes are likely to play a role in AD development, it is surprising that additional gene polymorphisms associated with LOAD have failed to come to light. In this review, we examine the role of X chromosome epigenetics and, based upon GWAS studies, the PCDHX11 gene. Furthermore, we explore other genetic risk factors of AD that involve X-chromosome epigenetics.

Molecular pathophysiology and genetic mutations in congenital sideroblastic anemia

Sideroblastic anemia is a heterogeneous congenital and acquired disorder characterized by anemia and the presence of ring sideroblasts in the bone marrow. Congenital sideroblastic anemia (CSA) is a rare disease caused by mutations in genes involved in the heme biosynthesis, iron-sulfur [Fe-S] cluster biosynthesis, and mitochondrial protein synthesis. The most prevalent form of CSA is X-linked sideroblastic anemia, caused by mutations in the erythroid-specific δ-aminolevulinate synthase (ALAS2), which is the first enzyme of the heme biosynthesis pathway in erythroid cells. To date, a remarkable number of genetically undefined CSA cases remain, but a recent application of the next-generation sequencing technology has recognized novel causative genes for CSA. However, in most instances, the detailed molecular mechanisms of how defects of each gene result in the abnormal mitochondrial iron accumulation remain unclear. This review aims to cover the current understanding of the molecular pathophysiology of CSA.

The molecular genetics of sideroblastic anemia

The sideroblastic anemias (SAs) are a group of inherited and acquired bone marrow disorders defined by pathological iron accumulation in the mitochondria of erythroid precursors. Like most hematological diseases, the molecular genetic basis of the SAs has ridden the wave of technology advancement. Within the last 30 years, with the advent of positional cloning, the human genome project, solid-state genotyping technologies, and next-generation sequencing have evolved to the point where more than two-thirds of congenital SA cases, and an even greater proportion of cases of acquired clonal disease, can be attributed to mutations in a specific gene or genes. This review focuses on an analysis of the genetics of these diseases and how understanding these defects may contribute to the design and implementation of rational therapies.

PLP1 Mutations in Patients with Multiple Sclerosis: Identification of a New Mutation and Potential Pathogenicity of the Mutations

PLP1 is located on the X-chromosome and encodes myelin proteolipid protein (PLP), the most abundant protein in central nervous system myelin. Generally, point mutations in PLP1 result in X-linked dysmyelinating disorders, such as Pelizaeus-Merzbacher disease (PMD) or spastic paraplegia type 2 (SPG2). However, several case studies have identified patients with missense point mutations in PLP1 and clinical symptoms and signs compatible with a diagnosis of multiple sclerosis (MS). To investigate if PLP1 mutations occur relatively frequently in MS, we sequenced the coding regions of PLP1 in 22 female MS patients who had developed disease after the age of 40 and in 42 healthy women, and identified a missense mutation in exon 2 of PLP1 resulting in a Leu30Val mutation in the protein in one of the MS patients. mCherry-tagged plasmids containing wild type or mutant PLP1 sequences of PLP, including two known PMD/SPG2-related mutations as positive controls, were constructed and transfected into Cos-7 cells. In comparison with cells transfected with wild type PLP1, all mutations caused significant accumulation of PLP in the endoplasmic reticulum of the cells and induction of the unfolded protein response-a mechanism that leads to apoptosis of cells expressing mutant proteins. Additionally, in silico analysis of the binding of peptides containing the Leu30Val mutation to the human leukocyte antigen (HLA) molecules carried by the patient harboring this mutation suggested that the mutation could produce several novel immunogenic epitopes in this patient. These results support the idea that mutations in myelin-related genes could contribute to the development of MS in a small proportion of patients.

Iron metabolism in erythroid cells and patients with congenital sideroblastic anemia

Sideroblastic anemias are anemic disorders characterized by the presence of ring sideroblasts in a patient's bone marrow. These disorders are typically divided into two types, congenital or acquired sideroblastic anemia. Recently, several genes were reported as responsible for congenital sideroblastic anemia; however, the relationship between the function of the gene products and ring sideroblasts is largely unclear. In this review article, we will focus on the iron metabolism in erythroid cells as well as in patients with congenital sideroblastic anemia.

Cerebral Manifestations of Mitochondrial Disorders

This review aims at summarizing and discussing previous and recent findings concerning the cerebral manifestations of mitochondrial disorders (MIDs). MIDs frequently present as mitochondrial multiorgan disorder syndrome (MIMODS) either already at onset or later in the course. After the muscle, the brain is the organ second most frequently affected in MIMODS. Cerebral manifestations of MIDs are variable and may present with or without a lesion on imaging or functional studies, but there can be imaging/functional lesions without clinical manifestations. The most well-known cerebral manifestations of MIDs include stroke-like episodes, epilepsy, headache, ataxia, movement disorders, hypopituitarism, muscle weakness, psychiatric abnormalities, nystagmus, white and gray matter lesions, atrophy, basal ganglia calcification, and hypometabolism on 2-deoxy-2-[fluorine-18]fluoro-D-glucose positron-emission tomography. For most MIDs, only symptomatic therapy is currently available. Symptomatic treatment should be supplemented by vitamins, cofactors, and antioxidants. In conclusion, cerebral manifestations of MIDs need to be recognized and appropriately managed because they strongly determine the outcome of MID patients.

Neuronal Ndrg4 Is Essential for Nodes of Ranvier Organization in Zebrafish

Axon ensheathment by specialized glial cells is an important process for fast propagation of action potentials. The rapid electrical conduction along myelinated axons is mainly due to its saltatory nature characterized by the accumulation of ion channels at the nodes of Ranvier. However, how these ion channels are transported and anchored along axons is not fully understood. We have identified N-myc downstream-regulated gene 4, ndrg4, as a novel factor that regulates sodium channel clustering in zebrafish. Analysis of chimeric larvae indicates that ndrg4 functions autonomously within neurons for sodium channel clustering at the nodes. Molecular analysis of ndrg4 mutants shows that expression of snap25 and nsf are sharply decreased, revealing a role of ndrg4 in controlling vesicle exocytosis. This uncovers a previously unknown function of ndrg4 in regulating vesicle docking and nodes of Ranvier organization, at least through its ability to finely tune the expression of the t-SNARE/NSF machinery.

Myelination is an important process that enables fast propagation of action potential along the axons. Schwann cells (SCs) are the specialized glial cells that ensure the ensheathment of the corresponding axons in the Peripheral Nervous System. In order to do so, SCs and axons need to communicate to organize the myelinating segments and the clustering of sodium channels at the nodes of Ranvier. We have investigated the early events of myelination in the zebrafish embryo. We here identify ndrg4 as a novel neuronal factor essential for sodium channel clustering at the nodes. Immuno-labeling analysis show defective vesicle patterning along the axons of ndrg4 mutants, while timelapse experiments monitoring the presence and the transport of these vesicles reveal a normal behavior. Molecular analysis unravels a novel function of ndrg4 in controlling the expression of the t-SNARE/NSF machinery required for vesicle docking and release. However, inhibiting specifically regulated synaptic vesicle release does not lead to sodium channel clustering defects. We thus propose that ndrg4 can regulate this process, at least partially, through its ability to regulate the expression of key components of the t-SNARE/NSF machinery, responsible for clustering of sodium channels along myelinated axons.

Diagnosis and treatment of sideroblastic anemias: from defective heme synthesis to abnormal RNA splicing

The sideroblastic anemias are a heterogeneous group of inherited and acquired disorders characterized by the presence of ring sideroblasts in the bone marrow. X-linked sideroblastic anemia (XLSA) is caused by germline mutations in ALAS2. Hemizygous males have a hypochromic microcytic anemia, which is generally mild to moderate and is caused by defective heme synthesis and ineffective erythropoiesis. XLSA is a typical iron-loading anemia; although most patients are responsive to pyridoxine, treatment of iron overload is also important in the management of these patients. Autosomal recessive sideroblastic anemia attributable to mutations in SLC25A38, a member of the mitochondrial carrier family, is a severe disease: patients present in infancy with microcytic anemia, which soon becomes transfusion dependent. Conservative therapy includes regular red cell transfusion and iron chelation, whereas allogenic stem cell transplantation represents the only curative treatment. Refractory anemia with ring sideroblasts (RARS) is a myelodysplastic syndrome characterized mainly by anemia attributable to ineffective erythropoiesis. The clinical course of RARS is generally indolent, but there is a tendency to worsening of anemia over time, so that most patients become transfusion dependent in the long run. More than 90% of these patients carry somatic mutations in SF3B1, a gene encoding a core component of the RNA splicing machinery. These mutations cause misrecognition of 3′ splice sites in downstream genes, resulting in truncated gene products and/or decreased expression attributable to nonsense-mediated RNA decay; this explains the multifactorial pathogenesis of RARS. Variants of RARS include refractory cytopenia with multilineage dysplasia and ring sideroblasts, and RARS associated with marked thrombocytosis; these variants involve additional genetic lesions. Inhibitors of molecules of the transforming growth factor-β superfamily have been shown recently to target ineffective erythropoiesis and ameliorate anemia both in animal models of myelodysplastic syndrome and in RARS patients.

X-linked macrocytic dyserythropoietic anemia in females with an ALAS2 mutation

Macrocytic anemia with abnormal erythropoiesis is a common feature of megaloblastic anemias, congenital dyserythropoietic anemias, and myelodysplastic syndromes. Here, we characterized a family with multiple female individuals who have macrocytic anemia. The proband was noted to have dyserythropoiesis and iron overload. After an extensive diagnostic evaluation that did not provide insight into the cause of the disease, whole-exome sequencing of multiple family members revealed the presence of a mutation in the X chromosomal gene ALAS2, which encodes 5'-aminolevulinate synthase 2, in the affected females. We determined that this mutation (Y365C) impairs binding of the essential cofactor pyridoxal 5'-phosphate to ALAS2, resulting in destabilization of the enzyme and consequent loss of function. X inactivation was not highly skewed in wbc from the affected individuals. In contrast, and consistent with the severity of the ALAS2 mutation, there was a complete skewing toward expression of the WT allele in mRNA from reticulocytes that could be recapitulated in primary erythroid cultures. Together, the results of the X inactivation and mRNA studies illustrate how this X-linked dominant mutation in ALAS2 can perturb normal erythropoiesis through cell-nonautonomous effects. Moreover, our findings highlight the value of whole-exome sequencing in diagnostically challenging cases for the identification of disease etiology and extension of the known phenotypic spectrum of disease.

Identification of novel informative loci for DNA-based X-inactivation analysis

The HUMARA assay, the most common method for evaluation of X-inactivation skewing in blood cells, has been reported to be usable in only about 80% of females, emphasizing the need for alternative methods for testing of HUMARA-uninformative individuals. We conducted an in silico search for potentially polymorphic tri-to-hexanucleotide repeats in the proximity of CpG islands located in 5' regions of X-chromosome genes to design five candidate assays (numbered I, II, III, IV, and V) combining methylation-specific restriction digest with PCR amplification in a manner similar to the HUMARA assay. The results obtained by these assays in 100 healthy females were compared to X-inactivation skewing measured by the AR-MSP method which is based on methylation-specific PCR amplification of the first exon of the AR gene. On the basis of statistical evidence, three of the novel assays (II, IV, and V), which were informative in 18%, 61%, and 55% of females in the cohort, respectively, may be used as alternatives or conjointly with the HUMARA assay to improve its reliability. The three new assays were combined with the HUMARA assay into a novel X-inactivation test leading to the increase of informative females in the cohort from 67% to 96%.

Etiologies underlying sex differences in Autism Spectrum Disorders

The male predominance of Autism Spectrum Disorders (ASD) is one of the best-known, and at the same time, one of the least understood characteristics of these disorders. In this paper we review genetic, epigenetic, hormonal, and environmental mechanisms underlying this male preponderance. Sex-specific effects of Y-linked genes (including SRY expression leading to testicular development), balanced and skewed X-inactivation, genes that escape X-inactivation, parent-of-origin allelic imprinting, and the hypothetical heterochromatin sink are reviewed. These mechanisms likely contribute to etiology, instead of being simply causative to ASD. Environments, both internal and external, also play important roles in ASD's etiology. Early exposure to androgenic hormones and early maternal immune activation comprise environmental factors affecting sex-specific susceptibility to ASD. The gene-environment interactions underlying ASD, suggested here, implicate early prenatal stress as being especially detrimental to boys with a vulnerable genotype.

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

During recent years, our understanding of the pathogenesis of inherited microcytic anemias has gained from the identification of several genes and proteins involved in systemic and cellular iron metabolism and heme syntheses. Numerous case reports illustrate that the implementation of these novel molecular discoveries in clinical practice has increased our understanding of the presentation, diagnosis, and management of these diseases. Integration of these insights into daily clinical practice will reduce delays in establishing a proper diagnosis, invasive and/or costly diagnostic tests, and unnecessary or even detrimental treatments. To assist the clinician, we developed evidence-based multidisciplinary guidelines on the management of rare microcytic anemias due to genetic disorders of iron metabolism and heme synthesis. These genetic disorders may present at all ages, and therefore these guidelines are relevant for pediatricians as well as clinicians who treat adults. This article summarizes these clinical practice guidelines and includes background on pathogenesis, conclusions, and recommendations and a diagnostic flowchart to facilitate using these guidelines in the clinical setting.

X chromosome inactivation pattern in BRCA gene mutation carriers

An association of preferential X chromosome inactivation (XCI) with BRCA gene status and breast/ovarian cancer risk has been reported. We evaluated XCI in a large group of BRCA mutation carriers compared to non-carriers and investigated associations between preferential XCI (⩾90:10) and age, mutated gene, cancer development and chemotherapy. XCI was analysed by human androgen receptor (HUMARA) assay and pyrosequencing in 437 BRCA1 or BRCA2 mutation carriers and 445 age-matched controls. The distribution of XCI patterns in the two groups was compared by logistic regression analysis. The association between preferential XCI and selected variables was investigated in both univariate and multivariate fashion. In univariate analyses preferential XCI was not significantly associated with the probability of being a BRCA mutation carrier, nor with cancer status, whereas chemotherapeutic regime and age both showed a significant association. In multivariate analysis only age maintained significance (odds ratio, 1.056; 95% confidence interval, 1.016-1.096). Our findings do not support the usefulness of XCI analysis for the identification of BRCA mutation carriers and cancer risk assessment. The increasing preferential XCI frequency with ageing and the association with chemotherapy justify extending the investigation to other categories of female cancer patients to identify possible X-linked loci implicated in cell survival.

Skewed X inactivation and survival: a 13-year follow-up study of elderly twins and singletons

In mammalian females, one of the two X chromosomes is inactivated in early embryonic life. Females are therefore mosaics for two cell populations, one with the maternal and one with the paternal X as the active X chromosome. A skewed X inactivation is a marked deviation from a 50:50 ratio. In populations of women past 55-60 years of age, an increased degree of skewing (DS) is found. Here the association between age-related skewing and mortality is analyzed in a 13-year follow-up study of 500 women from three cohorts (73-100 years of age at intake). Women with low DS had significantly higher mortality than the majority of women who had a more skewed DS (hazard ratio: 1.30; 95% CI: 1.04-1.64). The association between X inactivation and mortality was replicated in dizygotic twin pairs for which the co-twin with the lowest DS also had a statistically significant tendency to die first in the twin pairs with the highest intra-pair differences in DS (proportion: 0.71; 95% CI: 0.52-0.86). Both results suggest that lower DS is associated with higher mortality. We therefore propose that age-related skewing may be partly due to a population selection with lower mortality among those with higher DS.

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

X-linked Sideroblastic Anemia (XLSA) is the most common genetic form of sideroblastic anemia, a heterogeneous group of disorders characterized by iron deposits in the mitochondria of erythroid precursors. XLSA is due to mutations in the erythroid-specific 5-aminolevulinate synthase (ALAS2) gene. Thirteen different ALAS2 mutations were identified in 16 out of 29 probands with sideroblastic anemia. One third of the patients were females with a highly skewed X-chromosome inactivation. The identification of seven novel mutations in the ALAS2 gene, six missense mutations, and one deletion in the proximal promoter extends the allelic heterogeneity of XSLA. Most of the missense mutations were predicted to be deleterious, and 10 of them, without any published functional characterization, were expressed in Escherichia coli. ALAS2 activities were assayed in vitro. Five missense mutations resulted in decreased enzymatic activity under standard conditions, and two other mutated proteins had decreased activity when assayed in the absence of exogenous pyridoxal phosphate and increased thermosensitivity. Although most amino acid substitutions result in a clearly decreased enzymatic activity in vitro, a few mutations have a more subtle effect on the protein that is only revealed by in vitro tests under specific conditions.

Hereditary sideroblastic anemias: pathophysiology, diagnosis, and treatment

Inherited sideroblastic anemia comprises several rare anemias due to heterogeneous genetic lesions, all characterized by the presence of ringed sideroblasts in the bone marrow. This morphological aspect reflects abnormal mitochondrial iron utilization by the erythroid precursors. The most common X-linked sideroblastic anemia (XLSA), due to mutations of the first enzyme of the heme synthetic pathway, delta-aminolevulinic acid synthase 2 (ALAS2), has linked heme deficiency to mitochondrial iron accumulation. The identification of other genes, such as adenosine triphosphate (ATP) binding cassette B7 (ABCB7) and glutaredoxin 5 (GLRX5), has strengthened the role of iron sulfur cluster biogenesis in sideroblast formation and revealed a complex interplay between pathways of mitochondrial iron utilization and cytosolic iron sensing by the iron-regulatory proteins (IRPs). As recently occurred with the discovery of the SLC25A38-related sideroblastic anemia, the identification of the genes responsible for as yet uncharacterized forms will provide further insights into mitochondrial iron metabolism of erythroid cells and the pathophysiology of sideroblastic anemia.

X chromosome inactivation in clinical practice

X chromosome inactivation (XCI) is the transcriptional silencing of the majority of genes on one of the two X chromosomes in mammalian females. Females are, therefore, mosaics for two cell lines, one with the maternal X and one with the paternal X as the active chromosome. The relative proportion of the two cell lines, the X inactivation pattern, may be analyzed by simple assays in DNA from available tissues. This review focuses on medical issues related to XCI in X-linked disorders, and on the value of X inactivation analysis in clinical practice.

Analysis of X chromosome inactivation in autism spectrum disorders

Autism spectrum disorders (ASD) are complex genetic disorders more frequently observed in males. Skewed X chromosome inactivation (XCI) is observed in heterozygous females carrying gene mutations involved in several X-linked syndromes. In this study, we aimed to estimate the role of X-linked genes in ASD susceptibility by ascertaining the XCI pattern in a sample of 543 informative mothers of children with ASD and in a sample of 163 affected girls. The XCI pattern was also determined in two control groups (144 adult females and 40 young females) with a similar age distribution to the mothers sample and affected girls sample, respectively. We observed no significant excess of skewed XCI in families with ASD. Interestingly, two mothers and one girl carrying known mutations in X-linked genes (NLGN3, ATRX, MECP2) showed highly skewed XCI, suggesting that ascertainment of XCI could reveal families with X-linked mutations. Linkage analysis was carried out in the subgroup of multiplex families with skewed XCI (> or = 80:20) and a modest increased allele sharing was obtained in the Xq27-Xq28 region, with a peak Z-score of 1.75 close to rs719489. In summary, our results suggest that there is no major X-linked gene subject to XCI and expressed in blood cells conferring susceptibility to ASD. However, the possibility that rare mutations in X-linked genes could contribute to ASD cannot be excluded. We propose that the XCI profile could be a useful criteria to prioritize families for mutation screening of X-linked candidate genes.

Recent advances in the understanding of inherited sideroblastic anaemia

Sideroblastic anaemia includes a heterogeneous group of rare conditions, characterized by decreased haem synthesis and mitochondrial iron overload, which are diagnosed by the presence of ringed sideroblasts in the bone marrow aspirate. The most frequent form is X-linked sideroblastic anaemia, caused by mutations of delta-aminolevulinic acid synthase 2 (ALAS2), the enzyme that catalyses the first and regulatory step of haem synthesis in erythroid precursors and is post-transcriptionally controlled by the iron regulatory proteins. Impaired haem production causes variable degrees of anaemia and mitochondrial iron accumulation as ringed sideroblasts. The heterogeneity and complexity of sideroblastic anaemia is explained by an increasing number of recognized molecular defects. New forms have been recognized as being linked to the deficient function of mitochondrial proteins involved in iron-sulphur cluster biogenesis, such as ABCB7 and GLRX5, which are extremely rare but represent important biological models. Local mitochondrial iron overload is present in all sideroblastic anaemias, whereas systemic iron overload occurs only in the forms because of primary or secondary deficiency of ALAS2.

No evidence that skewing of X chromosome inactivation patterns is transmitted to offspring in humans

Skewing of X chromosome inactivation (XCI) can occur in normal females and increases in tissues with age. The mechanisms underlying skewing in normal females, however, remain controversial. To better understand the phenomenon of XCI in nondisease states, we evaluated XCI patterns in epithelial and hematopoietic cells of over 500 healthy female mother-neonate pairs. The incidence of skewing observed in mothers was twice that observed in neonates, and in both cohorts, the incidence of XCI was lower in epithelial cells than hematopoietic cells. These results suggest that XCI incidence varies by tissue type and that age-dependent mechanisms can influence skewing in both epithelial and hematopoietic cells. In both cohorts, a correlation was identified in the direction of skewing in epithelial and hematopoietic cells, suggesting common underlying skewing mechanisms across tissues. However, there was no correlation between the XCI patterns of mothers and their respective neonates, and skewed mothers gave birth to skewed neonates at the same frequency as nonskewed mothers. Taken together, our data suggest that in humans, the XCI pattern observed at birth does not reflect a single heritable genetic locus, but rather corresponds to a complex trait determined, at least in part, by selection biases occurring after XCI.

Candidate gene mutation analysis in idiopathic acquired sideroblastic anemia (refractory anemia with ringed sideroblasts)

BACKGROUND

For most cases of idiopathic acquired sideroblastic anemia (IASA), the molecular pathogenesis is unknown, despite the consistent morphological signature of abundant pathological ringed sideroblasts with their characteristic iron-engorged mitochondria. Moderately elevated free erythrocyte protoporphyrin (FEP) levels have been described in IASA, suggesting that the activity of ferrochelatase, the enzyme that catalyzes the final step in heme biosynthesis (incorporation of ferrous iron into protoporphyrin), might be diminished in erythroid progenitor cells from IASA patients.

METHODS

We confirmed FEP elevation in IASA, then pursued a candidate gene approach that included screening the gene encoding ferrochelatase, FECH, for promoter and coding region mutations and mRNA expression changes in bone marrow from 37 patients with IASA.

RESULTS

The analytical techniques employed detected mutations in a test cohort of previously undiagnosed patients with biochemical evidence for erythropoietic protoporphyria, a condition resulting from germline mutations in FECH, but somatic missense mutations of FECH and its promoter were not observed in IASA patients. FECH was modestly overexpressed in progenitor cells from patients with IASA, compared with MDS patients without sideroblasts and healthy controls. In addition, we analyzed ABCB7 and PUS1, genes implicated in congenital sideroblastic anemia syndromes, but again found no coding mutations in acquired cases.

CONCLUSION

We conclude that acquired mutations in the factors currently known to cause inherited sideroblastic anemias are uncommon in IASA.

X-linked sideroblastic anemia associated with a novel ALAS2 mutation and unfortunate skewed X-chromosome inactivation patterns

Historically X-linked sideroblastic anemia, with rare exceptions, was thought to be manifested only in males. Since the discovery of the erythroid-specific isoform of 5-aminolevulinate synthase (ALAS2) and the cloning of its gene (ALAS2) 15 years ago, mutation analysis has revealed that clinical expression of this X-linked disorder is prevalent in females as well. However, presence of the disease in both genders within affected kindreds appears to be very uncommon. We report a unique family with the disorder in three women who have had widely disparate clinical courses. The anemia is associated with a previously unrecognized ALAS2 mutation (Arg436Trp) and is unresponsive to pyridoxine. To clarify the varied clinical courses of the patients, X-chromosome inactivation patterns were examined in hematopoietic and non-hematopoietic cells. We observed inactivation patterns supporting the conclusions that one daughter has a mild phenotype at age 31 because of moderate constitutive skewed X-chromosome inactivation, another daughter with clinical onset at age 16 is severely affected due to extreme constitutive X-skewing, whereas the mother developed progressive anemia in the fifth decade as she acquired an age-related non-random X-inactivation in hematopoietic cells. In addition, we observed random X-inactivation in reticulocytes of all three women that contrasted with a markedly skewed inactivation pattern in bone marrow erythroid cells. This discordance is attributable to apoptosis of erythroid precursors derived from progenitor cells with an active X-chromosome bearing the ALAS2 mutation. The features of the disorder in this family are also instructive in regard to the differential diagnosis of sideroblastic anemias in women.

Disparate phenotypic expression of ALAS2 R452H (nt 1407 G → A) in two brothers, one with severe sideroblastic anemia and iron overload, hepatic cirrhosis, and hepatocellular carcinoma

We report the case of a man with severe X-linked sideroblastic anemia, severe iron overload, and hepatic cirrhosis who died of hepatocellular carcinoma. Evaluation of family members using DNA sequencing revealed that he was hemizygous for the novel ALAS2 mutation R452H (exon 9; nt 1407 G --> A). The proband's brother, an ALAS2 R452H hemizygote, had mild anemia and mild iron overload. Four female relatives were ALAS2 R452H heterozygotes, but they had mild or no anemia and no iron overload. Sequencing of TFR2, HFE, FPN1 (SLC40A1), HAMP, HJV, and the erythrocyte pyruvate kinase genes of family members was also performed. We thus detected the novel TFR2 missense mutation I449V (exon 10; nt 1345 A --> G) in the proband's wife and daughter, neither of whom had anemia or iron overload. Possible explanations for the disparate red blood cell and iron phenotypes of the proband and his family members are discussed.

Three kinships with ALAS2 P520L (c. 1559 C → T) mutation, two in association with severe iron overload, and one with sideroblastic anemia and severe iron overload

Mutations in aminolevulinate synthase 2 (ALAS2) are usually associated with sideroblastic anemia and iron overload. The objective of this study was to determine if "mild" mutations in ALAS2 might increase the severity of primary iron overload. Direct sequencing of the ALAS2 gene was performed on 24 subjects with primary hemochromatosis and one subject with sideroblastic anemia with severe iron overload. We identified a novel mutation P520L (c. 1559 C --> T) in ALAS2 in three subjects. Two had severe iron overload and no anemia: one also had HFE C282Y homozygosity, and the other was wildtype for HFE and other iron-related genes. The third subject had sideroblastic anemia with iron overload, and was hemizygous for both P520L and R560H (c. 1679 G --> A) mutations in ALAS2. The P520L mutation was found at a frequency of 0.0013 (741 alleles) in white control subjects, but was not found in 158 alleles from black control subjects. The proline in this position is highly conserved across species from humans to zebrafish. However, genotype/phenotype studies of the families demonstrate that the P520L mutation alone has no iron-associated phenotype, but it may act as a modifier of iron overload in the presence of mutations in HFE or other uncharacterized hemochromatosis genes. Thus, ALAS2 mutations might contribute to more severe iron loading in persons with primary hemochromatosis.

Arg452 substitution of the erythroid-specific 5-aminolaevulinate synthase, a hot spot mutation in X-linked sideroblastic anaemia, does not itself affect enzyme activity

Mutations of the erythroid-specific 5-aminolaevulinate synthase (ALAS2) gene are known to be responsible for X-linked sideroblastic anaemia (XLSA). An amino acid (AA) substitution for arginine at the 452 AA position of the ALAS2 protein is the most frequent mutation, which has been found in approximately one-quarter of patients with XLSA. Despite its high frequency, there has been no report on the enzymatic activity of Arg452 mutant proteins. In this study, we examined enzymatic activity in vitro of two Arg452 mutants, Arg452Cys and Arg452His, which were found in two new pedigrees of XLSA. While these mutations must be responsible for the clinical phenotype of XLSA in patients, the enzymatic activity and stability of these mutant proteins studied in vitro are indistinguishable from those of the wild type protein. These findings suggest that the Arg452 mutation of the ALAS2 gene by itself does not decrease the enzymatic activity or the stability in vitro, and that there may be an additional factor(s) in the bone marrow, which ensures the full ALAS2 activity in vivo.

Twin study of genetic and aging effects on X chromosome inactivation

To investigate the genetic influence on X chromosome inactivation and on age-related skewing of X inactivation, in particular, we analysed the X inactivation pattern (XIP) in peripheral blood cells from 118 young monozygotic (MZ) twin pairs (18-53 years), 82 elderly MZ twin pairs (55-94 years), 146 young dizygotic (DZ) twin pairs (20-54 years) and 112 elderly DZ twin pairs (64-95 years). Elderly twins had a higher frequency of skewed X inactivation (34%) than young twins (15%) (P<0.001). Our data suggest that the increase in skewing occurs after age 50-60 years. The intraclass correlation was 0.61 and 0.58 in young and elderly MZ twin pairs, and 0.08 and 0.09 in young and elderly DZ twin pairs. Biometric analysis showed that dominant genetic effects accounted for 63 and 58% of the variance of XIP in the young and elderly twin pairs, respectively. The dominant genetic effect and the shared environment for monochorionic MZ twins may explain the high intraclass correlation for the MZ twin pairs compared to the DZ twin pairs. We did not observe a significant decrease in the intraclass correlation in elderly MZ twins compared to young MZ twins, which would be expected if age-related skewing were due to stochastic factors. We conclude that the increased skewing with age implies that a genetically dependent selection of blood cells take place.

Differential allele expression of host defense genes, pulmonary surfactant protein-A and Osteopontin, in rat

Differential allele-specific expression has been observed in several genes involved in immunity. SP-A and OPN play a role in innate host defense. To determine whether SP-A and OPN are subject to differential allele-specific regulation, we investigated their gene or allele-specific expression in various tissues. The results showed: (1) Tissue-specific expression with high levels in lung (SP-A) and kidney (OPN). (2) Differences in allele-specific expression among individuals and tissues. SP-A showed an exclusively balanced biallelic expression (BB) in lung, but both BB and imbalanced biallelic (IB) expression in colon. Allele expression of OPN was more heterogeneous, e.g. in colon BB (22%), IB (64%), and monoallelic expression (MO) (14%). (3) Differential allele-specific expression was observed in all tissues studied (OPN) or in all extrapulmonary tissues (SP-A). (4) Family studies indicated that inheritable factor(s) may be involved in the regulation of allele-specific expression. (5) Analysis of co-expression of gene-specific alleles from double heterozygous rats revealed lack of coordinate allele expression among SP-A, SP-D, and OPN. We conclude that allele-specific expression occurs among genes of innate host defense. This may yet provide another level of regulatory complexity for molecules involved in the first line of defense.

Primary biliary cirrhosis: does X mark the spot?

Primary biliary cirrhosis (PBC) is an autoimmune disease of unknown etiology leading to progressive destruction of intrahepatic bile duct, with cholestasis, cirrhosis, and eventually liver failure. Epidemiological data indicate that environmental factors trigger autoimmunity in genetically susceptible individuals, although no definitive association of PBC with specific genes has been found. Further, no convincing explanation has been provided for the strong female predominance observed in the prevalence of PBC. However, we recently suggested that the enhanced monosomy X in peripheral white blood cells, and particularly in lymphocytes, of affected women might play a role in the induction of PBC. Such observations appear independent from the degree of cholestasis and specific for PBC. In this review we discuss the implications of these findings and their immunological implications.

A promoter mutation in the erythroid-specific 5-aminolevulinate synthase (ALAS2) gene causes X-linked sideroblastic anemia

X-linked sideroblastic anemia (XLSA) is caused by mutations in the erythroid-specific 5-aminolevulinate synthase gene (ALAS2). XLSA was diagnosed in a 32-year-old woman with a mild phenotype and moderately late onset. Pyridoxine therapy had no effect in the proband, but in her affected son engendered a modest increase in hemoglobin concentration and a 4-fold reduction in ferritin iron. Molecular analysis identified a C to G transversion at nucleotide -206 from the transcription start site, as defined by primer extension, in the proximal promoter region of ALAS2. No other mutations were found in the promoter region, the flanking intronic sequences, the exons, or the 3' genomic region. The same mutation was found in her affected son but not in any other of her unaffected relatives. The mutation resulted in a 94% loss of activity relative to the wild-type sequence for a luciferase reporter construct containing the proximal 293 nucleotides (nt's) of the ALAS2 promoter when transfected into human erythroid K562 cells. Confirming the mutation's deleterious effect, the ALAS2 mRNA level in the proband's erythroid precursors was reduced 87%. The mutation occurred in or near 3 different putative transcription factor binding sites of unknown erythroid importance. The dramatic decreases in reporter activity and mRNA level suggest that the region of the mutation may bind a novel and important erythroid regulatory element.

Late-onset X-linked sideroblastic anemia following hemodialysis

X-linked sideroblastic anemia (XLSA) is due to deficient activity of erythroid-specific 5-aminolevulinate synthase (ALAS2). We report here a patient who developed sideroblastic anemia at the age of 81 years while undergoing hemodialysis. The diagnosis of sideroblastic anemia was established by the presence of ringed sideroblasts in the bone marrow, and treatment with oral pyridoxine completely eliminated the ringed sideroblasts. We identified a novel point mutation in the fifth exon of this patient's ALAS2 gene, which resulted in an amino acid change at residue 159 from aspartic acid to asparagine (Asp159Asn). In vitro analyses of recombinant Asp159Asn ALAS2 revealed that this mutation accounted for the pyridoxine-responsiveness of this disease. The very late onset in this case of XLSA emphasizes that nutritional deficiencies caused either by dietary irregularities in the elderly or, as in this case, by maintenance hemodialysis therapy, may uncover occult inherited enzymatic deficiencies in the heme biosynthetic pathway.

Mitochondrial ferritin expression in erythroid cells from patients with sideroblastic anemia

The sideroblastic anemias are characterized by ring sideroblasts, that is, red cell precursors with mitochondrial iron accumulation. We therefore studied the expression of mitochondrial ferritin (MtF) in these conditions. Erythroid cells from 13 patients with refractory anemia with ring sideroblasts (RARS) and 3 patients with X-linked sideroblastic anemia (XLSA) were analyzed for the distribution of cytoplasmic H ferritin (HF) and MtF using immunocytochemical methods. We also studied 11 healthy controls, 5 patients with refractory anemia without ring sideroblasts (RA), and 7 patients with RA with excess of blasts (RAEB). About one fourth of normal immature red cells, mostly proerythroblasts and basophilic erythroblasts, showed diffuse cytoplasmic positivity for HF, but very few were positive for MtF (0%-10%). Similar patterns were found in anemic patients without ring sideroblasts. In contrast, many erythroblasts from patients with sideroblastic anemia (82%-90% in XLSA and 36%-84% in RARS) were positive for MtF, which regularly appeared as granules ringing the nucleus. Double immunocytochemical staining confirmed the different cellular distribution of HF and MtF. There was a highly significant relationship between the percentage of MtF(+) erythroblasts and that of ring sideroblasts (Spearman R = 0.90; P <.0001). Reverse transcription-polymerase chain reaction studies demonstrated the presence of MtF mRNA in circulating reticulocytes of 2 patients with XLSA but not in controls. These findings suggest that most of the iron deposited in perinuclear mitochondria of ring sideroblasts is present in the form of MtF and that this latter might be a specific marker of sideroblastic anemia.

Absent phenotypic expression of X-linked sideroblastic anemia in one of 2 brothers with a novel ALAS2 mutation

X-linked sideroblastic anemia (XLSA) is caused by mutations in the erythroid-specific 5-aminolevulinic acid synthase (ALAS2) gene. Hemizygous males have microcytic anemia and iron overload. A 38-year-old male presented with this phenotype (hemoglobin [Hb] 7.6 g/dL, mean corpuscular volume [MCV] 64 fL, serum ferritin 859 microg/L), and molecular analysis of ALAS2 showed a mutation 1731G>A predicting an Arg560His amino acid change. A 36-year-old brother was hemizygous for this mutation and expressed the mutated ALAS2 mRNA in his reticulocytes, but showed almost no phenotypic expression. All 5 heterozygous females from this family, including the 3 daughters of the nonanemic hemizygous male, showed marginally increased red-cell distribution width (RDW). Although variable penetrance for XLSA in males has been previously described, this is the first report showing that phenotypic expression can be absent in hemizygous males. This observation is relevant to genetic counseling, emphasizing the importance of gene-based diagnosis.

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.