X-linked Sideroblastic Anemia via the ALAS2 Gene

X-linked Sideroblastic Anemia (XLSA) is a disorder characterized by a defect in heme synthesis. Patients present with hallmark ringed sideroblasts in erythrocyte precursor cells from bone marrow aspirates due to deposition of iron granules in the mitochondria. Clinical symptoms include non-hemolytic anemia with variable degree of iron overload leading to fatigue, dizziness, rapid heartbeat, and hepatosplenomegaly. In severe cases, chronic iron deposition can result in advanced fibrotic liver disease, cirrhosis, hepatocellular carcinoma, atherosclerosis, arthritis, fatigue, diabetes, hypogonadism, cardiomyopathy, and diffuse skin pigmentation (Camaschella 2008). Age of onset is primarily between ages 25-35, but may present in early childhood (Liu et al. 2013; May and Bishop 1998).

XLSA is caused by mutations in the ALAS2 gene. Mutations in this gene have also been associated with a separate disorder, protoprophyria, which is characterized by acute painful photosensitivity with stinging and burning sensations upon sunlight exposure but without blistering (Bishop et al. 2013). Sideroblastic anemia is also observed in the context of other disease settings including Pearson Syndrome, mitochondrial myopathy with sideroblastic anemia (MLASA), and X-linked sideroblastic anemia ataxia (XLSA/A). Acquired forms of sideroblastic anemia include myelodysplastic syndromes, excess zinc, lead poisoning, and excessive alcohol use (Camashella 2008). Therefore, genetic testing may be helpful in distinguishing between different inherited forms of sideroblastic anemia and ruling out acquired causes of disease.

XLSA primarily affects males, accounts for nearly half of inherited cases of congenital sideroblastic anemia and is inherited in an X-linked recessive manner through mutations in the ALAS2 gene (Camaschella 2008; Campagna et al. 2014). About 20% of XLSA cases occur in females due to constitutive skewed X inactivation (Cazzola et al. 2000). XLSA/A, a form of sideroblastic anemia with ataxia, is inherited in an X-linked recessive manner through mutations in the ABCB7 gene (Allikmets et al. 1999). Sideroblastic anemia may also be inherited in an autosomal recessive manner in MLASA via PUS1 mutations or in Thiamin-responsive megaloblastic anemia via SLC19A2 mutations (Camaschella 2008).

Missense mutations occurring through the core functional regions of the protein (exons 5-11) account for the majority of causative variants to date. Nonsense, small deletions/insertions and substitutions in intron 1 affecting the GATA transcription factor binding site have also be reported in rare cases (Campagna et al. 2014; Liu et al. 2013; Ducamp et al. 2013). No gross deletions encompassing full exon(s) have been reported. De novo mutations have been reported in the ALAS2 gene, but are not common for XLSA (Cotter et al. 1999). Causative mutations in the ALAS2 gene are fully penetrant for presence of sideroblasts in bone marrow aspirates; however, the severity of anemia and iron overload varies. The ALAS2 gene encodes an erythroid specific enzyme, delta-aminolevulinic acid synthease 2, which catalyzes the first step of the heme biosynthetic pathway (May and Bishop 1998).

Mutations in the ALAS2 gene were found in 16 of 31 patients diagnosed with congenital sideroblastic anemia (Liu et al. 2013). Analytical sensitivity should be high as all mutations reported are detectable by sequencing.

This test provides full coverage of all coding exons of the ALAS2 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).