Heme-Related Blood Disorders

Heme biosynthesis in erythroid cells is intended primarily for the formation of hemoglobin. As in every cell, this synthesis requires a multi-step pathway that involves eight enzymes including the erythroid-specific δ-aminolevulinate synthase (ALAS2, the first regulated enzyme that converts glycine and succinyl CoA into ALA) and the ubiquitous ferrochelatase (FECH, the final enzyme). Heme biosynthesis also requires membrane transporters that are necessary to translocate glycine, precursors of heme, and heme itself between the mitochondria and the cytosol. Defects in normal porphyrin and/or heme synthesis and transport cause four major erythroid inherited disorders, which may or may not be associated with dyserythropoiesis (e.g., sideroblastic, microcytic anemia and/or hemolytic anemia): "X-linked" sideroblastic anemia (XLSA) and X-linked dominant protoporphyria (XLDPP) are two different and opposing disorders but related to altered gene encoding ALAS2 only. Defective activity of this enzyme due to mutations in the ALAS2 gene causes the XLSA phenotype, including microcytic, hypochromic anemia with abundant ringed sideroblasts in the bone marrow. Vice versa, gain-of-function mutations of ALAS2 are responsible of the XLDPP characterized by predominant accumulation of the hydrophobic protoporphyrin (PPIX, the last heme precursor) in the erythrocytes without anemia or sideroblasts. Furthermore, the glycine transporter (SLC25A38) and Glutaredoxin 5 genes are reported to be involved in human non-syndromic sideroblastic anemia. Congenital erythropoietic porphyria (CEP) is the rarest autosomal recessive disorder due to a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. CEP leads to excessive synthesis and accumulation of type I isomers of porphyrins in the reticulocytes, followed by intravascular hemolysis and severe anemia. The ALAS2 gene may act as a modifier gene in CEP patients (Figueras J et al, Blood. 2011;118(6):1443-51). Erythropoietic protoporphyria (EPP) results from a partial deficiency of FECH and leads similarly to XLDPP, to deleterious accumulation of PPIX in erythroid cells. Most EPP patients present intrans to a FECH gene mutation an IVS3-48C hypomorphic allele due to a splice mutation. Abnormal spliced mRNA is degraded which contributes to the lowest FECH enzyme activity and allowed EPP phenotype expression. We have identified an antisense oligonucleotide (ASO) to redirect splicing from cryptic to physiological site and showed that the ASO-based therapy mediates normal splice rescue of FECH mRNA and reduction by 60 percent of PPIX overproduction in primary cultures of EPP erythroid progenitors. Therapeutic approaches to target both ALAS2 inhibition and heme-level reduction may be useful in other erythroid disorders such as thalassemia (where reduced heme biosynthesis was shown to improve the clinical phenotype) or the Diamond-Blackfan anemia (DBA). Indeed, in some DBA patients, an unusual mRNA splicing of heme exporter FLVCR has been found, reminiscent of Flvcr1-deficient mice that develop a DBA-like phenotype with erythroid heme accumulation. Thus, FLVCR may act as a modifier gene for DBA phenotypic variability. Recent advances in understanding the pathogenesis and molecular genetic heterogeneity of heme-related disorders have led to improved diagnosis and treatment. These advances include DNA-based diagnoses for all the porphyrias and some porphyrins and heme transporters, new understanding of the pathogenesis of the erythropoietic disorders, and new and experimental treatments such as chronic erythrocyte transfusions, bone marrow or hematopoietic stem cell transplants, and experimental pharmacologic chaperone and stem cell gene therapies for erythropoietic porphyrias.