Alternative titles; symbols
HGNC Approved Gene Symbol: AHSP
Cytogenetic location: 16p11.2 Genomic coordinates (GRCh38) : 16:31,527,900-31,528,803 (from NCBI)
AHSP is a molecular chaperone that stabilizes alpha-globin (see 141800) (summary by Lechauve et al., 2018).
The EDRF gene expresses a transcript that is confined to the erythroid lineage and is downregulated in transmissible spongiform encephalopathies (TSEs). To identify molecular markers of TSEs in noncentral nervous system tissues, Miele et al. (2001) compared gene expression in spleens of scrapie-infected and uninfected mice. Miele et al. (2001) used the differential display RT-PCR procedure to specifically identify genes differentially expressed as a result of TSE infection. They identified 1 cDNA representing a transcript that clearly showed a decrease in expression level in spleens from scrapie-infected C57BL mice. This sequence, called 'erythroid differentiation-related factor' (EDRF), represents a transcript of approximately 0.5 kb with a predicted open reading frame (ORF) of 102 amino acids. Northern blot analysis of RNA isolated from spleens of scrapie-infected and control mice confirmed that levels of EDRF transcript are dramatically decreased at the terminal stages of disease. The effect on EDRF transcript levels in spleen is first evident during the early stages of disease and becomes more pronounced with progression of disease. Northern blot analysis of RNA from spleens of both mice and hamsters infected with a number of different strains of TSE agents confirmed the substantial decrease in levels of EDRF transcript. In mice, EDRF is normally expressed only in spleen, bone marrow, and blood, with highest levels in bone marrow. Northern blot analysis of human tissue revealed that EDRF expression was confined to blood and bone marrow, with no expression detectable in spleen. Reduced EDRF expression was also detected in BSE-infected cattle and scrapie-infected sheep. EDRF expression is confined to the erythroid lineage, with higher levels of expression in blast-forming (BFU-E), colony-forming (CFU-E), and maturing erythroid (TER-119+) cells.
By using a screen for genes induced by the essential erythroid transcription factor GATA1 (305371), Kihm et al. (2002) identified the ERAF protein as one that stabilizes free alpha-hemoglobin and renamed it 'alpha-hemoglobin stabilizing protein,' or AHSP, on the basis of this function. AHSP is an abundant erythroid-specific protein that forms a stable complex with free alpha-hemoglobin but not with beta-hemoglobin or hemoglobin A (alpha2-beta2). Moreover, AHSP specifically protects free alpha-hemoglobin from precipitation in solution and in live cells. Kihm et al. (2002) predicted that AHSP gene dosage would modulate pathologic states of alpha-hemoglobin excess such as beta-thalassemia.
Although beta-thalassemia is considered to be a classic monogenic disease, there is considerable clinical variability between patients who inherit identical mutations in the beta-globin gene (HBB; 141900), suggesting that there may be a variety of genetic determinants influencing the clinical phenotype. It has been proposed that alleles altering the levels or function of AHSP might account for some of the clinical variability observed in patients with beta-thalassemia (Kihm et al., 2002; Luzatto and Notaro, 2002). To address this hypothesis, Viprakasit et al. (2004) studied 120 Thai patients with Hb E (141900.0071) with mild, moderate, or severe clinical phenotypes. Using gene mapping, direct genomic sequencing, and extended haplotype analysis, they found no mutation or specific association between haplotypes of AHSP and disease severity in these patients, suggesting that AHSP is not a disease modifier in Hb E. Viprakasit et al. (2004) pointed out that the AHSP gene is located on chromosome 16, and contains 3 exons.
Stumpf (2023) mapped the AHSP gene to chromosome 16p11.2 based on an alignment of the AHSP sequence (GenBank BC035842) with the genomic sequence (GRCh38).
Crystal Structure
Hemoglobin A (HbA), the oxygen delivery system in humans, comprises 2 alpha and 2 beta subunits. Free alpha-Hb is unstable, and its precipitation contributes to the pathophysiology of beta-thalassemia. In erythrocytes, AHSP binds alpha-Hb and inhibits its precipitation. Feng et al. (2004) determined the crystal structure of AHSP bound to Fe(II)-alpha-Hb. AHSP specifically recognized the G and H helices of alpha-Hb through a hydrophobic interface that largely recapitulated the alpha-1-beta-1 interface of hemoglobin. The AHSP-alpha-Hb interactions were extensive but suboptimal, explaining why beta-hemoglobin can competitively displace AHSP to form HbA. The Fe(II)-heme group in AHSP-bound alpha-Hb was coordinated by the distal but not the proximal histidine. Binding to AHSP facilitated the conversion of oxy-alpha-Hb to a deoxygenated, oxidized (Fe(III)), nonreactive form in which all 6 coordinate positions were occupied. These observations revealed the molecular mechanisms by which AHSP stabilizes free alpha-Hb.
Feng et al. (2005) reported the crystal structure of ferric alpha-Hb complexed with AHSP at 2.4-angstrom resolution. Their findings revealed a striking bis-histidyl configuration in which both the proximal and the distal histidines coordinate the heme iron atom. To attain this unusual conformation, segments of the alpha-Hb undergo drastic structural rearrangements, including the repositioning of several alpha-helices. Moreover, conversion to the ferric bis-histidine configuration strongly and specifically inhibits redox chemistry catalysis and heme loss from alpha-Hb. The observed structural changes, which impaired the chemical reactivity of heme iron, explained how AHSP stabilizes alpha-Hb and prevents its damaging effects in cells.
Using immunofluorescence analysis, Lechauve et al. (2018) showed that AHSP was coexpressed with alpha-globin in mouse and human endothelial cells (ECs) and regulated alpha-globin protein levels. Expression analysis in human coronary artery ECs and experiments with purified human proteins demonstrated that AHSP and endothelial NOS (eNOS, or NOS3; 163729) interacted with alpha-globin in a mutually exclusive manner and enhanced its accumulation in cells. However, only AHSP could stabilize oxidized Fe(3+)-alpha-globin. The authors demonstrated that eNOS rapidly reduced AHSP-bound Fe(3+)-alpha-globin via direct electron transfer from its flavin-associated reductase domain.
Kihm et al. (2002) generated mice deficient in AHSP. Ahsp -/- mice were born at expected mendelian ratios and displayed grossly normal growth and development. Although the mice exhibited normal blood hemoglobin concentrations and hematocrits, their reticulocyte counts were elevated about 3-fold, indicating a shortened erythrocyte half-life. Ahsp -/- erythrocytes exhibited an abnormal spiculated morphology that has also been observed in erythroid cells of beta-thalassemic mice. Moreover, Ahsp -/- erythrocytes contained denatured hemoglobin inclusions (Heinz bodies) that stained with crystal violet. Thus, the loss of AHSP leads to defective hemoglobin metabolism in vivo. The reticulocyte counts in Ahsp +/- mice were mildly elevated, indicating that AHSP haploinsufficiency may produce a subtle erythroid phenotype.
Kong et al. (2004) performed biochemical studies in Ahsp null mice and found that Ahsp -/- erythrocytes were short-lived and contained hemoglobin precipitates and reactive oxygen species with evidence of oxidative damage. Erythroid precursors were elevated in number but exhibited increased apoptosis. Purified recombinant AHSP inhibited the production of reactive oxygen species by alpha-hemoglobin in solution. Loss of Ahsp worsened the phenotype in beta-thalassemic mice. Kong et al. (2004) proposed an essential function for AHSP, both in normal erythropoiesis and to a greater extent in beta-thalassemia, in binding alpha-hemoglobin transiently to stabilize its conformation and render it biochemically inert prior to hemoglobin A assembly.
Lechauve et al. (2018) found that Ahsp -/- mice had reduced thoracodorsal artery contractility due to increased NO diffusion to vascular smooth muscle cells, leading to reduced systemic blood pressure.
Feng, L., Gell, D. A., Zhou, S., Gu, L., Kong, Y., Li, J., Hu, M., Yan, N., Lee, C., Rich, A. M., Armstrong, R. S., Lay, P. A., Gow, A. J., Weiss, M. J., Mackay, J. P., Shi, Y. Molecular mechanism of AHSP-mediated stabilization of alpha-hemoglobin. Cell 119: 629-640, 2004. [PubMed: 15550245] [Full Text: https://doi.org/10.1016/j.cell.2004.11.025]
Feng, L., Zhou, S., Gu, L., Gell, D. A., Mackay, J. P., Weiss, M. J., Gow, A. J., Shi, Y. Structure of oxidized alpha-haemoglobin bound to AHSP reveals a protective mechanism for haem. (Letter) Nature 435: 697-701, 2005. [PubMed: 15931225] [Full Text: https://doi.org/10.1038/nature03609]
Kihm, A. J., Kong, Y., Hong, W., Russell, J. E., Rouda, S., Adachi, K., Simon, M. C., Blobel, G. A., Weiss, M. J. An abundant erythroid protein that stabilizes free alpha-haemoglobin. Nature 417: 758-763, 2002. [PubMed: 12066189] [Full Text: https://doi.org/10.1038/nature00803]
Kong, Y., Zhou, S., Kihm, A. J., Katein, A. M., Yu, X., Gell, D. A., Mackay, J. P., Adachi, K., Foster-Brown, L., Louden, C. S., Gow, A. J., Weiss, M. J. Loss of alpha-hemoglobin-stabilizing protein impairs erythropoiesis and exacerbates beta-thalassemia. J. Clin. Invest. 114: 1457-1466, 2004. [PubMed: 15545996] [Full Text: https://doi.org/10.1172/JCI21982]
Lechauve, C., Butcher, J. T., Freiwan, A., Biwer, L. A., Keith, J. M., Good, M. E., Ackerman, H., Tillman, H. S., Kiger, L., Isakson, B. E., Weiss, M. J. Endothelial cell alpha-globin and its molecular chaperone alpha-hemoglobin-stabilizing protein regulate arteriolar contractility. J. Clin. Invest. 128: 5073-5082, 2018. [PubMed: 30295646] [Full Text: https://doi.org/10.1172/JCI99933]
Luzatto, L., Notaro, R. Haemoglobin's chaperone. Nature 417: 703-705, 2002. [PubMed: 12066171] [Full Text: https://doi.org/10.1038/417703a]
Miele, G., Manson, J., Clinton, M. A novel erythroid-specific marker of transmissible spongiform encephalopathies. Nature Med. 7: 361-364, 2001. [PubMed: 11231637] [Full Text: https://doi.org/10.1038/85515]
Stumpf, A. M. Personal Communication. Baltimore, Md. 07/19/2023.
Viprakasit, V., Tanphaichitr, V. S., Chinchang, W., Sangkla, P., Weiss, M. J., Higgs, D. R. Evaluation of alpha hemoglobin stabilizing protein (AHSP) as a genetic modifier in patients with beta thalassemia. Blood 103: 3296-3299, 2004. [PubMed: 14715623] [Full Text: https://doi.org/10.1182/blood-2003-11-3957]