Glycine and Folate Ameliorate Models of Congenital Sideroblastic Anemia

Abstract

Sideroblastic anemias are acquired or inherited anemias that result in a decreased ability to synthesize hemoglobin in red blood cells and result in the presence of iron deposits in the mitochondria of red blood cell precursors. A common subtype of congenital sideroblastic anemia is due to autosomal recessive mutations in the SLC25A38 gene. The current treatment for SLC25A38 congenital sideroblastic anemia is chronic blood transfusion coupled with iron chelation. The function of SLC25A38 is not known. Here we report that the SLC25A38 protein, and its yeast homolog Hem25, are mitochondrial glycine transporters required for the initiation of heme synthesis. To do so, we took advantage of the fact that mitochondrial glycine has several roles beyond the synthesis of heme, including the synthesis of folate derivatives through the glycine cleavage system. The data were consistent with Hem25 not being the sole mitochondrial glycine importer, and we identify a second SLC25 family member Ymc1, as a potential secondary mitochondrial glycine importer. Based on these findings, we observed that high levels of exogenous glycine, or 5-aminolevulinic acid (5-Ala) a metabolite downstream of Hem25 in heme biosynthetic pathway, were able to restore heme levels to normal in yeast cells lacking Hem25 function. While neither glycine nor 5-Ala could ameliorate SLC25A38 congenital sideroblastic anemia in a zebrafish model, we determined that the addition of folate with glycine was able to restore hemoglobin levels. This difference is likely due to the fact that yeast can synthesize folate, whereas in zebrafish folate is an essential vitamin that must be obtained exogenously. Given the tolerability of glycine and folate in humans, this study points to a potential novel treatment for SLC25A38 congenital sideroblastic anemia.

Characterization of Human and Yeast Mitochondrial Glycine Carriers with Implications for Heme Biosynthesis and Anemia

“…Functional Characterization of Recombinant Hem25p-Because it has been hypothesized that Hem25p facilitates ALA production by importing glycine into mitochondria (13,14), the recombinant and purified Hem25p was reconstituted into liposomes, and its ability to transport glycine was tested. Proteoliposomes reconstituted with Hem25p catalyzed active […”

“…In this subgroup of patients the author identified several mutations in the SLC25A38 gene (13). Furthermore, an anemic phenotype similar to that of ALAS2 deficiency has also been observed in the knockdown of two zebrafish orthologues of SLC25A38 (slc25a38a and slc25a38b), indicating that this gene is important for red blood cell production and function (13,14). It has been demonstrated that the genetic deletion of the Saccharomyces cerevisiae SLC25A38 ortholog YDL119c (also named HEM25) produces a respiratory pheno-type, indicative of mitochondrial involvement (13).…”

“…This finding is not unrealistic, because many mitochondrial carriers display some overlap of substrate specificity, albeit with lower efficiency, and play different metabolic roles (38). Recently, it has been proposed that Ymc1p is a secondary mitochondrial glycine transporter, because the hem25⌬ymc1⌬ double mutant presented a lower level of heme content compared with the single hem25⌬ or ymc1⌬ mutant (14); however, data in the literature pointed out a role for YMC1 in carbon source utilization (39,40). Trotter et al (39) found a relationship between YMC1, its paralog YMC2, and the mitochondrial 2-oxodicarboxylate transporters, Odc1p and Odc2p.…”

“…On the basis of these findings, it was hypothesized that SLC25A38 and its yeast orthologue Hem25p facilitate ALA production by importing glycine into mitochondria or by exchanging glycine for ALA across the inner mitochondrial membrane (13). Furthermore, Fernández-Murray (14) has recently demonstrated through phenotype assessment in hem25⌬ cells that Hem25p is required to initiate heme synthesis. Human SLC25A38 and yeast HEM25 encode a protein of the inner mitochondrial membrane that belongs to the mitochondrial carrier family, a family of proteins (InterPro entry: IPR018108; PANDIT: PF00153) encoded by nuclear genes (15).…”

Estudo de proteínas que afetam a tradução mitocondrial em <i>Saccharomyces cerevisiae</i>.

“…5-ALA é um dos principais precursores da via de síntese de HEME, o que poderia explicar o fenótipo apresentado pelos pacientes portadores da síndrome (FLEMING, 2011) FERNANDEZ-MURRAY et al, 2016).…”