α-thalassemia arises when the α-globin genes, which encode one of the two protein chains of adult hemoglobin, are deleted or defective. Healthy people carry four copies of the α-globin gene, two on each chromosome 16. Carriers with three or two functional copies are usually asymptomatic, and the carrier alleles are extraordinarily common in malaria-endemic regions, reaching frequencies of up to roughly 70 percent because they confer protection against the parasite. But when these alleles combine, the consequences escalate. People inheriting one Southeast Asian deletion allele and one α-globin deletion develop hemoglobin H disease, in which excess β-globin chains form useless β4 tetramers, and about 10 percent of these patients become transfusion dependent. The most devastating outcome occurs in fetuses that inherit no functional α-globin genes at all, a condition known as hemoglobin Bart’s hydrops fetalis syndrome, or BHFS, which normally causes death in utero during the second or third trimester.

Current options for the most severe patients are limited. Hematopoietic stem cell transplantation is the only curative treatment, but it requires a human leukocyte antigen-matched donor, and even when a donor is available, graft-versus-host disease and transplant-related mortality remain real risks. A small but growing number of BHFS survivors, salvaged by intrauterine transfusions and intensive neonatal care, face lifelong transfusion dependence and iron chelation. This therapeutic gap is what drew the Oxford team to an unusual idea: rather than trying to replace the missing α-globin gene, why not switch back on the embryonic gene that once did its job?

The logic rests on a quirk of human development. The α-globin locus on chromosome 16 contains, in sequence, the enhancers R1 through R4, the embryonic ζ-globin gene HBZ, and the two fetal and adult α-globin genes HBA2 and HBA1. During the first, primitive wave of red blood cell formation, ζ-globin is fully expressed and assembles into functional hemoglobin tetramers, including Hb Portland 1 (ζ2γ2) and Hb Portland 2 (ζ2β2), which can substitute for fetal and adult hemoglobins. But by six to seven weeks of gestation, as definitive hematopoiesis takes over, ζ-globin is completely silenced and remains off for the rest of life. Crucially, the most common disease-causing allele, the Southeast Asian deletion, removes both α-globin genes while leaving the enhancers and the ζ-globin gene intact, meaning that in many patients the dormant embryonic gene is still sitting there, waiting to be unlocked.

To find the off switch, the researchers first built an elegant mouse model called DZIA, in which the 4.4-kilobase segment containing the ζ-globin gene was replaced with a modified α-globin gene carrying twenty silent single-nucleotide polymorphisms that allowed its transcripts to be distinguished from endogenous ones. The result was striking: the substituted gene, placed in exactly the position of the embryonic gene, was fully transcribed in definitive erythroid cells, accounting for 40 to 50 percent of total α-like globin output, roughly equivalent to the combined output of two endogenous α-globin genes. This proved that the silencing machinery is encoded locally within that 4.4-kilobase region, not in distant flanking sequences or in the gene’s position within the chromosome’s three-dimensional architecture.

Next came a CRISPR-Cas9 saturation mutagenesis screen across the ζ-globin promoter. Using a reporter mouse in which ζ-globin expression is marked by a fluorescent protein, and refining the search in an immortalized mouse proerythroblast line, the team tiled 47 guide RNAs across 700 base pairs upstream of the transcription start site. The screen converged on two discrete repressive elements: predicted binding motifs for the transcription factors BCL11A and ZBTB7A, also known as LRF. Disrupting the BCL11A motif alone derepressed ζ-globin to about 4 percent of total α-like globin, the LRF motif alone to about 18 percent, and combined disruption, introduced with a cytosine base editor, pushed expression to roughly 26 percent, marginally more than the sum of the individual effects.

The team then built a mouse model carrying these promoter edits, termed ZPE, and crossed it with a new BHFS model that precisely replicates the human Southeast Asian deletion, including its flanking sequences. Untreated homozygous deletion embryos, which produce no α-globin at all, develop normally until embryonic day 15.5, when the last primitive, ζ-globin-expressing red cells disappear from circulation. Thereafter they become pale and growth restricted, develop edema, and by day 17.5 suffer severe hemolytic anemia with large numbers of immature, nucleated erythroblasts churning out nonfunctional β4 tetramers. Embryos carrying the promoter edits on top of the deletion showed dramatic improvement at day 17.5, with near-normal enucleated red blood cells expressing about 7.5 percent ζ-globin mRNA and producing detectable Hb Portland 2. The embryonic protein was, in effect, standing in for the missing adult one.

The human work followed a parallel path. In the HUDEP-2 definitive erythroid cell line, the researchers found that the architecture of the ζ-globin promoter differs subtly from the mouse: two consensus BCL11A motifs lie 148 base pairs apart, with the proximal one overlapping the essential CCAAT box, so editing that site is ineffective. The distal BCL11A site and the LRF site, however, proved to be the productive targets. Base editing both sites raised ζ-globin to about 15 percent of total α-like globin, comparable to levels achieved by knocking out both repressor genes entirely, while leaving erythroid differentiation untouched. Mechanistically, the authors propose a competition model: LRF binding, just 18 base pairs from the CCAAT box, blocks NF-Y from opening the chromatin, and together with BCL11A it prevents the master activator GATA1 from engaging the promoter. When both repressive motifs are mutated, GATA1 newly binds and the gene fires.

The most clinically resonant experiments used primary human CD34-positive stem cells from healthy donors, patients with hemoglobin H disease and a 21-week BHFS fetus. Adenine and cytosine base editors achieved efficiencies of 25 to 90 percent at the two sites, and edited cells differentiated normally. After normalizing for editing efficiency, the ratio of α-like to β-like globin in the hemoglobin H patient samples rose from 24.8 percent, a dangerously low level, to 48.4 percent, with ζ-globin contributing about half of the α-like output. In the BHFS fetal cells, the ratio climbed from 17.1 percent to 50.5 percent. The authors argue that restoring this ratio to around 0.5 would likely be sufficient to rescue the severe α-thalassemia phenotype, and HPLC analysis of edited BHFS cells showed a corresponding reduction in the pathological β4 tetramers that cause the disease.

Caution remains appropriate. Hb Portland 2 has weaker subunit interactions and higher oxygen affinity than adult hemoglobin, and its physiological behavior in adult life must be characterized before clinical application. Editing efficiencies in primary stem cells vary, off-target effects and long-term engraftment have yet to be rigorously assessed, and the authors note that residual low-level ζ-globin expression seen in human carriers appears to involve human-specific mechanisms absent from the mouse. Still, the study establishes a clear road map: a compact, locally acting cis-regulatory module enforces embryonic gene silencing, and precise base editing can dismantle it. Beyond α-thalassemia, the work reinforces an emerging paradigm, already validated by fetal hemoglobin induction in sickle cell disease and β-thalassemia, that developmentally silenced paralogs can be unlocked to compensate for adult gene defects, offering a potentially generalizable route to autologous genome-editing therapies for severe inherited disorders that today demand lifelong transfusion or a matched donor.

Subject of Research: Reactivation of the embryonic ζ-globin gene by promoter base editing as a therapeutic strategy for severe α-thalassemia

Article Title: Reactivation of the embryonic ζ-globin gene ameliorates severe forms of α-thalassemia

Article References: Liu, S., Ejaz, A., King, A. J., Olijnik, A.-A., Sloane-Stanley, J., Scott, C., Amid, A., Biggs, D., Davies, B., Eglinton, J., Martyn, G., Crossley, M., Babbs, C., Kassouf, M. T., & Higgs, D. R. (2026). Reactivation of the embryonic ζ-globin gene ameliorates severe forms of α-thalassemia. Nature Genetics, 58(10), 2619-2629. https://doi.org/10.1038/s41588-026-02770-0

Image Credits: AI Generated

DOI: 10.1038/s41588-026-02770-0

Keywords: α-thalassemia, ζ-globin, gene editing, base editing, BCL11A, ZBTB7A, hemoglobin, erythropoiesis, hydrops fetalis, CRISPR, globin gene regulation, hemoglobinopathies

The study, led by researchers including Edgar Cristancho-Mejía and Walter Schmidt, recruited healthy, non-smoking adults who had lived at their respective altitudes for at least two years. Participants came from six locations spanning 970 to 2600 meters, including regions around Bogotá in the Central Eastern Andes and the lower Western Andes near Tuluá and Cali. Crucially, none of the participants were endurance athletes, eliminating a major confounding factor that has plagued earlier comparisons between highland populations and sea-level dwellers. The cohort included 133 men and 148 women, with ancestry distributions reflecting the regional population mix of African, European, and Native American heritage.

Measuring hemoglobin mass directly is technically demanding, and the team used the optimized carbon monoxide rebreathing method, widely regarded as the gold standard. Participants inhaled a precisely calculated dose of carbon monoxide over two minutes, and the researchers tracked how much of it bound to hemoglobin in the blood, allowing an accurate calculation of total hemoglobin mass. From this, together with venous hemoglobin concentration and hematocrit, the team derived red cell volume, plasma volume, and total blood volume. Blood samples drawn from a cubital vein provided hemoglobin concentration, hematocrit, and oxygen saturation values for each participant.

The results revealed a clear and unexpected divergence in thresholds. Hemoglobin mass rose significantly only in residents living at 2180 meters or higher, with the effect becoming firmly established at 2600 meters. Plasma volume, by contrast, was already significantly reduced at 1730 meters—roughly 450 meters lower than the erythropoietic threshold. Between the lowest and highest study locations, hemoglobin concentration increased by about 1.0 grams per deciliter in men and 1.3 grams per deciliter in women, while hematocrit rose by roughly 3.2 to 3.7 percentage points. Relative to lean body mass, hemoglobin mass was 5.8 percent higher in men and 8.9 percent higher in women at 2600 meters compared with 970 meters, while plasma volume per kilogram of lean mass fell by about 6 percent in both sexes.

The physiological logic behind these two thresholds traces back to the shape of the oxygen dissociation curve. This sigmoid curve allows arterial blood to remain nearly fully saturated with oxygen up to altitudes of roughly 2000 meters, meaning the kidneys—the primary sensors of oxygen availability—detect little reason to stimulate red cell production. Below that ceiling, the flat upper portion of the curve buffers the modest drop in ambient oxygen pressure. Once saturation begins to fall more steeply, however, the kidney responds by releasing erythropoietin, triggering the bone marrow to expand hemoglobin mass. Plasma volume regulation operates on a different logic entirely: it responds to the overall oxygen-carrying deficit rather than to a specific saturation threshold, and it does so within days rather than weeks.

When the researchers combined their data with earlier measurements taken at 420 and 3800 meters in Bolivia using the same methodology, a striking pattern emerged. Hemoglobin mass increased slowly and modestly up to about 2600 meters, then climbed sharply at higher elevations, reaching values 27 percent above near-sea-level levels at 3800 meters. Notably, within the moderate-altitude range of the current study, there was no significant relationship between hemoglobin mass and oxygen saturation—suggesting that up to roughly 2600 meters, altitude exerts only a gentle influence on erythropoiesis. Above that point, the diminishing oxygen supply becomes the dominant driver, most likely registered by oxygen-sensing mechanisms in the kidneys.

To quantify how much each component contributed to the observed rise in hemoglobin concentration, the team performed multiple regression analyses. The calculations showed that every 100-gram increase in hemoglobin mass raised concentration by 1.59 grams per deciliter, while every liter of plasma volume loss raised it by 3.07 grams per deciliter. Applying these coefficients to the altitude-related changes, the researchers found that of the 1.2 grams per deciliter elevation in hemoglobin concentration at 2600 meters, approximately 0.56 grams per deciliter was attributable to increased hemoglobin mass and 0.61 grams per deciliter to reduced plasma volume—a near-perfect split. When the team applied an alternative correction for the Fåhraeus effect, which accounts for differences between central and peripheral hematocrit, the plasma volume contribution grew even larger.

The findings carry immediate clinical weight. Hemoglobin concentration is one of the most commonly ordered diagnostic measurements in medicine, and anemia thresholds are already adjusted for altitude. Yet the hemoconcentration caused by plasma volume contraction at moderate elevations has never been factored into those thresholds. If a substantial fraction of the altitude-related rise in hemoglobin reflects a shrunken liquid blood volume rather than genuinely enhanced oxygen-carrying capacity, then current anemia criteria may misclassify residents of towns like Bogotá at 2600 meters. The study also resonates with recent work on high-altitude populations worldwide: data from Sherpa communities in the Himalayas suggest their lower hemoglobin concentrations compared with Andean highlanders stem largely from larger plasma volumes rather than smaller hemoglobin masses, underscoring that the ratio of red cells to plasma, not red cell count alone, defines the blood’s oxygen transport profile.

The authors acknowledge limitations. Sample sizes at individual altitudes were modest, and the high interindividual variability in hemoglobin mass—driven largely by differences in lean body mass—means the altitude effect, though statistically robust, is just one of several factors shaping any person’s blood profile. Ambient temperature may also play a role, since the lowland study sites were considerably warmer, and heat acclimatization is known to expand plasma volume. Plasma volume itself was calculated indirectly rather than measured directly, and the Fåhraeus correction introduces some uncertainty, though the researchers showed that an altitude-adjusted correction strengthened rather than weakened their conclusions. Nevertheless, the study provides the first systematic evidence that the blood’s response to moderate altitude unfolds in two distinct stages: plasma contracts first, at around 1730 meters, and the erythropoietic machinery follows at roughly 2180 meters. For the roughly 5 percent of humanity living above 1500 meters, that distinction may soon matter at the doctor’s office as much as on the mountain.

Subject of Research: Altitude thresholds for hemoglobin mass and plasma volume regulation and their effects on hemoglobin concentration in Andean residents

Article Title: Altitude thresholds for hemoglobin and plasma volume regulation in adult residents of different altitudes and their impact on hemoglobin concentration

Article References: Cristancho‐Mejía, E., Trompetero‐Gonzalez, A. C., Benavides‐Pinzón, W. F., & Schmidt, W. F.-J. (2026). Altitude thresholds for hemoglobin and plasma volume regulation in adult residents of different altitudes and their impact on hemoglobin concentration. Physiological Reports, 14(19), Article e71108. https://doi.org/10.14814/phy2.71108

Image Credits: AI Generated

DOI: 10.14814/phy2.71108

Keywords: hemoglobin mass, plasma volume, altitude, hypoxia, erythropoiesis, Andes, Colombia, anemia thresholds, oxygen saturation, blood volume, CO rebreathing, physiology

Darbepoetin is a long-acting analog of erythropoietin, the hormone that signals the bone marrow to manufacture red blood cells. In extremely low birth weight infants, who are born before they can mount a robust red cell response and who lose blood through frequent laboratory testing, randomized trials have shown that darbepoetin raises hematocrits and can reduce or even eliminate the need for red blood cell transfusions. That is a major clinical win, because transfusions in preterm infants have been linked in multiple studies to complications including retinopathy of prematurity and bronchopulmonary dysplasia. But erythropoiesis-stimulating agents come with a catch: they only work if the marrow has an adequate supply of iron to load into the hemoglobin of every new cell it produces. Without sufficient iron, the drug drives the production of red cells that are small, pale, and poorly equipped to carry oxygen.

The problem facing neonatologists is that assessing iron status in a premature infant is surprisingly difficult. Serum ferritin, the most commonly used storage iron marker in adults, is highly variable in neonates born before 29 weeks of gestation and does not correlate well with functional iron availability, as earlier work by the same group demonstrated. Inflammation, transfusions, and the normal physiology of early infancy all muddy the interpretation. What clinicians really want to know is not how much iron is stored somewhere in the body, but whether the erythron, the collective machinery of red cell production, is actually getting enough iron at this moment to build hemoglobin properly. That functional question is where reticulocyte-based measurements come in.

Reticulocytes are the immature red blood cells released by the bone marrow, and because they circulate for only a day or two before maturing, they offer a real-time snapshot of erythropoietic activity. One established metric, RET-He, measures the hemoglobin content of reticulocytes in picograms. A falling RET-He signals that newly produced cells are being under-filled with hemoglobin, a classic hallmark of iron-limited erythropoiesis. The Utah team tracked RET-He alongside reticulocyte counts in both darbepoetin-treated infants and placebo recipients, and the numbers they recovered are striking. Among placebo recipients, RET-He fell over the first week from 33.6 plus or minus 0.5 picograms to 27.4 plus or minus 0.2 picograms, a statistically significant decline with a p value below 0.001. Infants receiving darbepoetin fell even further, with the difference between groups reaching p below 0.0001.

At first glance, that pattern looks alarming: the drug seems to be driving infants into deeper iron deficiency. But the researchers identified something more nuanced, a reciprocal relationship between rising reticulocyte counts and falling RET-He. As darbepoetin accelerates the marrow’s output, the pool of available iron is distributed across a rapidly expanding population of new cells, so each reticulocyte carries less hemoglobin even if total iron delivery is adequate for the infant’s needs. This dilutional effect means that a low RET-He during stimulated erythropoiesis cannot be read the same way it would be in a stable patient. A single threshold value, applied without context, risks mislabeling infants as iron deficient when their iron supply is actually keeping pace with an unprecedented demand, or conversely missing true deficiency hidden behind a vigorous marrow response.

To resolve that ambiguity, the team turned to two novel erythrocyte parameters generated by modern hematology analyzers: Micro-R, which quantifies microcytosis, the proportion of red cells that are abnormally small, and HYPO-He, which quantifies hypochromia, the proportion of cells with reduced hemoglobin concentration. Unlike RET-He, which reflects the most recent cohort of cells leaving the marrow, these metrics survey the entire circulating red cell population and capture the structural fingerprints of iron-limited cell production over a longer window. Reference intervals for Micro-R and HYPO-He in newborn infants had previously been established by members of the same group using the refineR algorithm, giving the investigators a validated framework for interpreting the values they observed during darbepoetin therapy.

The central finding of the study emerged when the two measurement families were read together. Among darbepoetin recipients whose RET-He dropped below 25 picograms, a level that would traditionally raise concern for iron deficiency, the majority, 83 percent, had Micro-R and HYPO-He measurements indicating that they were in fact iron sufficient. In other words, most of the infants flagged by the conventional marker were not truly iron limited; their low reticulocyte hemoglobin content reflected the physiology of accelerated erythropoiesis rather than a genuine shortfall in iron supply. Only a minority showed the concordant microcytosis and hypochromia that would confirm real iron-restricted red cell production. This discordance is precisely why the authors advocate using traditional and novel red cell markers in combination rather than in isolation.

The clinical implications extend beyond the neonatal unit’s fascination with laboratory parameters. Iron deficiency in early life is not a benign laboratory curiosity; iron is essential for brain development, and studies have linked iron deficiency in extremely low birth weight infants to adverse neurodevelopmental outcomes at two years of age. Overtreating with iron, on the other hand, carries its own risks, including oxidative stress and potential toxicity in infants who cannot yet regulate absorption. A monitoring strategy that reliably distinguishes true iron deficiency from the benign dilution of hemoglobin across a surging reticulocyte population would allow clinicians to target supplementation precisely, giving iron to the infants who need it and avoiding unnecessary exposure in those who do not. As restrictive transfusion guidelines become more common and erythropoiesis-stimulating agents play a growing role in keeping preterm infants transfusion-free, the stakes of getting iron assessment right only increase.

The study was conducted as a retrospective, deidentified records review approved by the Intermountain Health Institutional Review Board with a waiver of informed consent, and it was performed in accordance with federal human subjects protections. The authors, including Natalie Bishara, Timothy M. Bahr, Anne Woodbury, Robert D. Christensen, and Robin K. Ohls, report no competing interests, and both the statistical code and the underlying data are available on reasonable request. Because the analysis drew on existing trial and clinical datasets rather than a prospective interventional design, the findings will need prospective validation before they change bedside practice, and the precise algorithm for integrating RET-He, Micro-R, and HYPO-He into a clinical decision rule remains to be defined.

Even so, the conceptual shift the paper describes is likely to resonate widely. For decades, iron sufficiency has been inferred from snapshots of storage or from single thresholds applied to dynamic systems. The Utah work shows that during accelerated erythropoiesis, the very condition in which accurate iron assessment matters most, the traditional markers move in ways that can mislead, and that the structural signatures written into circulating red cells by iron-limited production offer a corrective lens. If follow-up studies confirm the approach, neonatologists may soon monitor their tiniest patients not with a single number but with a coordinated panel that reads both the tempo of red cell production and the quality of the cells being produced, ensuring that every dose of darbepoetin is matched by the iron those infants need to build oxygen-carrying blood and to protect their developing brains.

Subject of Research: Iron sufficiency assessment in premature infants treated with darbepoetin using conventional and novel erythrocyte metrics

Article Title: Assessment of iron sufficiency in premature infants treated with darbepoetin using conventional and novel erythrocyte metrics

Article References: Bishara, N., Bahr, T. M., Woodbury, A., Christensen, R. D., & Ohls, R. K. (2026). Assessment of iron sufficiency in premature infants treated with darbepoetin using conventional and novel erythrocyte metrics. Journal of Perinatology. https://doi.org/10.1038/s41372-026-02889-2

Image Credits: AI Generated

DOI: 10.1038/s41372-026-02889-2

Keywords: darbepoetin, premature infants, iron deficiency, RET-He, Micro-R, HYPO-He, reticulocytes, erythropoiesis, neonatology, anemia, transfusion avoidance, extremely low birth weight