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Congenital sideroblastic anaemia with a novel frameshift mutation in SLC25A38
  1. Wai-shan Wong1,
  2. Hung-fan Wong1,
  3. Chi-keung Cheng2,
  4. Kai-on Chang3,
  5. Natalie Pui-ha Chan2,
  6. Margaret Heung-ling Ng2,
  7. Kit-fai Wong1
  1. 1Department of Pathology, Queen Elizabeth Hospital, Kowloon, Hong Kong
  2. 2Department of Anatomical and Cellular Pathology, Prince of Wales Hospital, The Chinese University of Hong Kong, New Territories, Hong Kong
  3. 3Department of Paediatrics and Adolescences, Queen Elizabeth Hospital, Kowloon, Hong Kong
  1. Correspondence to Dr Kit-fai Wong, Department of Pathology, Queen Elizabeth Hospital, 30 Gascoigne Road, Kowloon, Hong Kong; kfwong{at}ha.org.hk

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Introduction

Hypochromic microcytic anaemia is not an infrequent haematological problem in infants. There are a number of differential diagnoses including iron deficiency, haemoglobinopathy, chronic inflammation, lead poisoning or rare genetic diseases like congenital sideroblastic anaemia.1 ,2 The onset and severity of anaemia, transfusion requirement, family history together with simple laboratory tests like blood film examination, reticulocyte count, iron profile and haemoglobin pattern studies are able to solve the majority of the diagnostic problems. Bone marrow study is seldom indicated except when congenital sideroblastic anaemia or haematological malignancy is suspected.

Congenital sideroblastic anaemia is a rare cause of inheritable anaemia and is associated with heterogeneous genetic abnormalities and phenotypes. Perls’ staining of a marrow smear can reveal the presence of ring sideroblasts, characterised by abnormal iron deposition in the mitochondria clustering around the nuclei of the erythroblasts, and ring sideroblasts are the pathological hallmarks of the disease. Molecular study has an important role in confirmation of the diagnosis. We hereby report a case of non-syndromic congenital sideroblastic anaemia in a girl who presented with severe hypochromic microcytic anaemia requiring regular blood transfusion at infancy.

Case report

A 4-month-old Nepalese girl presented with failure to thrive, and the peripheral blood examination showed: haemoglobin 1.6 g/dL, mean cell volume (MCV) 68.0 fL, mean cell haemoglobin (MCH) 20.9 pg, red cell distribution width 31.6, reticulocytes 7.2×109/L, white cells 3.3×109/L and platelets 77×109/L. She was afebrile and no lymphoadenopathy or hepatosplenomegaly was detected. Peripheral blood film showed a dimorphic blood picture (figure 1) and occasional red cells with basophilic stippling were noted. There was no polychromasia or poikilocytosis such as blister cells, microspherocytes or schistocytes. There was no biochemical evidence of haemolysis and the bilirubin level was normal. The ferritin level was increased (1427 pmol/L, normal range 81–793). Haemoglobin pattern study showed a normal haemoglobin profile with no elevated haemoglobin F, haemoglobin variant or haemoglobin H inclusion detected.

Figure 1

Peripheral blood film of the patient, MGG×1000. A dimorphic blood picture is present.

The girl was the first baby of the family and was born prematurely at 31 weeks of gestation. She had congenital myelomeningocele which was repaired surgically shortly after birth. She also had a small patent ductus arteriosus and ventricular septal defect requiring regular echocardiogram monitoring. The physical abnormalities were probably related to her prematurity. There was no dysmorphic feature or skeletal abnormality. Her haemoglobin level and MCV at birth were 9.0 g/dL and 84.1 fL respectively, and were low for her age but it was attributed to her prematurity at that time. She had been given red cells transfusion during her stay in the paediatric intensive care unit and was discharged 2 months prior to this admission. It was also noteworthy that her parents had lived in the same village at Nepal before residing in Hong Kong and were consanguineous. There was, however, no family history of recurrent miscarriages, transfusion-dependent anaemia, genetic disease or early death. Peripheral blood examination of both parents showed normal red cells indices and morphology. Their haemoglobin patterns were also normal.

In view of the persistent and unexplained anaemia, bone marrow examination was performed. The marrow aspirate yielded a ‘dry tap’ but ring sideroblasts were noted on the Perls’ stain preparation (figure 2). The trephine biopsy showed active trilineage haematopoiesis with no overt dysplasia or abnormal infiltrate. The patient remained transfusion-dependent, requiring monthly red cell transfusion. A provisional diagnosis of congenital sideroblastic anaemia was made.3

Figure 2

Perls’ staining on the marrow aspirate (×1000). The ring sideroblasts are indicated by arrows.

Methods and results

The most common genetic defect for congential sideroblastic anaemia is a mutation in δ-aminolevulinate synthase 2 gene (ALAS2).4 ,5 However, this is X linked recessively inherited and manifests in boys usually while girls are asymptomatic carriers in heterozygous state. Furthermore, the patient's father was asymptomatic with normal red cell indices. In view of the apparent autosomal recessive pattern of inheritance, early presentation, severity of the anaemia and absence of metabolic defects, molecular study on the SLC25A38 gene was performed on the family. Genomic DNA from the patient and her parents were extracted from peripheral white blood cells using the Gentra Puregene Cell Kit (Qiagen) after informed consent. Primers according to Guernsey et al6 were employed to amplify the seven exons of SLC25A38 gene. The PCR products were resolved by 2% agarose gel electrophoresis, purified and sequenced using the BigDye Terminator V.3.1 Cycle Sequencing Kit (Life Technologies). Mutations were confirmed by repeated PCR amplification and sequencing.

The patient was homozygous for a nucleotide T insertion in exon 5 of the SLC25A38 gene (NM_017875.2:c.480_481insT), causing a frameshift mutation in the protein (p.Ile161fs). Both parents were heterozygous for the same mutation (figure 3).

Figure 3

Identification of mutation in SLC25A38 by direct sequencing of (A) the patient, (B) father and (C) mother. (A) Homozygous for c.480_481insT (arrow); (B) and (C) heterozygous for c.480_481insT.

Discussion

Congenital sideroblastic anaemia is a rare cause of inherited anaemia.4 The ring sideroblasts are formed as a result of the failure in haem biosynthesis and mitochondrial iron overload in the erythroid precursors.7 There are at least seven subgroups at the molecular level,8 ,9 all of them having in common a defective haem biosynthesis in the erythroid precursors, dysfunctional mitochondrial iron metabolism and ineffective erythropoiesis. The seven subgroups are divided into syndromic and non-syndromic forms; the former is associated with metabolic or neuromuscular defects. There are four syndromic forms, namely, (1) X linked sideroblastic anaemia and ataxia, (2) mitochondrial myopathy, lactic acidosis and sideroblastic anaemia, (3) thiamine-responsive megaloblastic anaemia with diabetes and deafness and (4) Pearson marrow-pancreas syndrome. The non-syndromic forms are associated with three genes, including ALAS2, SLC25A38 and GLRX5.

Mutations in the erythroid-specific gene encoding δ-aminolevulinate synthase 2 (ALAS2), the first enzyme of the haem synthetic pathway, are the most well known and commonly reported genetic defects in the literature.10 ,11 The enzyme is involved in the condensation of glycine with succinyl-coenzyme A to yield 5-aminolevulinic acid (ALA), carbon dioxide and coenzyme A. ALAS2-related sideroblastic anaemia is X linked inherited, manifests predominantly in boys who have mild to moderate microcytic anaemia, which may respond to pyridoxine treatment depending on the site of mutation. Mutations in mitochondrial glutaredoxin 5 (GLRX5),12 ,13 a gene that encodes a mitochondrial protein responsible for iron–sulfur cluster biosynthesis essential for mitochondrial iron utilisation, are rare autosomal recessively inherited causes of congenital sideroblastic anaemia. However, the anaemia only presents in midlife, which is contrary to our case.

Recently, SLC25A38 mutations have been found to be responsible for severe pyridoxine-refractory congenital sideroblastic anaemia with autosomal recessive pattern of inheritance in three families from the Canadian Maritime province.6 The same gene was also found to be mutated in patients of different ethnical origins.5 ,6 ,8 The SLC25A38 gene, located on chromosome 3p22.1, is a member of the SLC25 class of inner mitochondrial membrane carrier proteins which consists of 304 amino acid residuals including an N-terminal mitochondrial localisation signal and six transmembrane helices.5 ,6 It is responsible for transporting glycine into mitochondrial matrix for condensation with succinyl- coenzyme A to form 5-aminolevulinic acid (ALA) under the catalytic reaction by ALAS2, and exporting ALA to the cytoplasm for haem synthesis. Mutations in SLC25A38 gene result in defective haem synthesis and microcytic anaemia. Patients usually present in the first year of life and are characterised by severe anaemia refractory to pyridoxine treatment and transfusion dependent. They do not show specific physical abnormalities or metabolic defects. Owing to its role as amino acid carrier for mitochondria, it has been suggested that glycine supplement may ameliorate transfusion requirement in SLC25A38-associated congenital sideroblastic anaemia. Bone marrow transplantation is the only cure for the disease.5 ,14 ,15 Therefore, genetic workup of congenital sideroblastic anaemia is essential for prediction of treatment response, enabling genetic counselling and prenatal diagnosis in future pregnancy.

There are several types of mutations in SLC25A38, including missense mutation, splicing errors, frameshift mutation or nonsense mutation. Missense mutations affecting the conserved amino acids situated in transmembrane regions of the mitochondrial transporter are the commonest type of genetic defects5 and account for more than half of the cases. Interesting, two-thirds of patients are homozygous for the same mutation, possibly explained by the higher frequency in the relatively inbred populations as in the present case. Our patient is homozygous for a frameshift mutation in exon 5 which results in the production of a truncated protein of 171 amino acids due to premature termination of translation. This mutation is expected to result in loss of function of SLC25A38 as about half of the transmembrane domain is disrupted. The defective protein fails to transport glycine into mitochondrial matrix for haem synthesis, resulting in severe anaemia. SLC25A38 is the commonest gene affected in autosomal recessive congenital sideroblastic anaemia. The patient usually presents with severe anaemia and becomes transfusion-dependent at infancy or early childhood, and is refractory to oral pyridoxine treatment. Transfusion support and iron chelation therapy are indicated in this group of patients. Allogeneic stem cell transfusion may be curative but depends on availability of a suitable donor.5 ,14 It has been suggested that glycine may theoretically be helpful in alleviating the transfusion requirement5 ,14 ,15 and the patient was given glycine at 150 mg/kg/day for 6 months, but the frequency of transfusion remained the same. It is hypothesised that the extremely low haemoglobin level at diagnosis and clinical unresponsiveness to glycine treatment are related to severe disruption of SLC23A38 protein transmembrane domain as a result of the frameshift mutation and premature termination of translation.

References

Footnotes

  • Contributors WSW, KFW and HFW contributed to the planning of work and wrote the manuscript. KOC performed the clinical work up and managed the patient. CKC, HFW, NPH-C and MHL-N were responsible for the molecular study. All authors approved the version of the manuscript being submitted.

  • Competing interests None.

  • Patient consent Obtained.

  • Ethics approval Department of Pathology, Queen Elizabeth Hospital.

  • Provenance and peer review Not commissioned; externally peer reviewed.