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

Absent pulmonary valve syndrome is among the rarest of congenital cardiac anomalies. It is defined by the absence, or severe underdevelopment, of the pulmonary valve, the one-way gate that normally prevents blood from flowing backward from the pulmonary artery into the right ventricle. Without that valve, every heartbeat drives blood forward through a narrowed outflow tract and then lets it slosh backward in diastole, hammering the pulmonary arteries from both directions. The result is the condition’s signature finding: massive, sometimes aneurysmal dilation of the pulmonary arteries and their branches, which in the confined space of the fetal thorax can compress the airways and crowd out the surrounding structures. Precise incidence figures are difficult to pin down, but recent reviews suggest that absent pulmonary valve syndrome accounts for roughly 0.2 to 0.4 percent of all congenital heart disease, and that among fetuses diagnosed prenatally, more than 84 percent occur in association with tetralogy of Fallot, the classic conotruncal defect combining a ventricular septal defect, an overriding aorta, right ventricular outflow obstruction, and right ventricular hypertrophy.

The prognosis for fetuses diagnosed with this physiology before birth is sobering. In one series of 37 prenatally diagnosed fetuses, survival after initial diagnosis was only about 37 percent overall, with survival of roughly 30 percent for the tetralogy-associated variant. Against that backdrop, the new case report adds an unusual twist: the fetal heart was not only burdened by a suspected absent pulmonary valve but was also positioned abnormally, with dextrocardia, a rightward orientation of the cardiac apex within a normally positioned abdominal situs. Isolated dextrocardia of this kind is itself rare and is reported to be accompanied by complex intracardiac defects in the vast majority of cases, carrying a worse outlook than the mirror-image variety. The combination of a conotruncal malformation, absent pulmonary valve physiology, and dextrocardia in a single fetus is exceptionally uncommon, and the authors present it as a contribution to the limited literature on the prenatal natural history of this cluster of anomalies.

The mother was a 33-year-old woman in her second pregnancy, referred at 24 weeks of gestation for fetal cardiac evaluation. She carried a diagnosis of pre-gestational diabetes mellitus with demonstrably suboptimal glycemic control; her hemoglobin A1c at admission was 8.3 percent, well above the thresholds associated with elevated risk of fetal malformation. No additional maternal risk factors were identified. The referral triggered a comprehensive fetal echocardiographic workup using two-dimensional imaging, M-mode, color Doppler, and spectral Doppler, the full modern toolkit of prenatal cardiac diagnosis, and it is the meticulous serial documentation that gives this report much of its scientific value.

The initial scan at 24 weeks established the anatomy in detail. The fetal heart sat with a rightward apex, consistent with isolated dextrocardia. The main pulmonary artery was already markedly dilated, measuring 15 millimeters, and Doppler interrogation demonstrated both pulmonary stenosis and significant pulmonary regurgitation, with diastolic retrograde flow visible in the main pulmonary artery. The right ventricular free wall appeared hypertrophied, and imaging of the left ventricular outflow tract revealed a large subaortic ventricular septal defect with an overriding aorta, findings squarely compatible with a conotruncal anomaly on the tetralogy spectrum. Imaging of the pulmonary valve region showed only rudimentary ridges where normal leaflets should have been, the echocardiographic hallmark of the absent pulmonary valve variant. At that stage the fetus showed no signs of hydrops, and umbilical Doppler indices remained normal, suggesting that despite the severity of the anatomy, the circulation was still compensating.

What followed over the next four weeks was a documented unraveling of that compensation. Serial scans tracked the main pulmonary artery as it ballooned from 15 millimeters at 24 weeks to 18 millimeters at 26 weeks and 21 millimeters at 28 weeks. The cardiothoracic ratio, a measure of heart size relative to the chest, climbed from roughly 70 to 75 percent to as high as 80 percent, reflecting progressive biventricular hypertrophy and massive cardiomegaly. Doppler velocities across the stenotic, regurgitant pulmonary outflow rose in parallel. Then, at 27 weeks, the fetal echocardiogram revealed the development of hydrops fetalis, the accumulation of fluid in body cavities that signals decompensated cardiac failure. Bilateral pleural effusions appeared, followed by marked ascites. In structural fetal heart disease, the onset of hydrops is one of the most powerful adverse prognostic markers known, reflecting elevated central venous pressures and ventricular dysfunction, and reported series consistently show sharply increased fetal and neonatal loss once it supervenes.

Management options, the authors explain, were limited from the start. Given the complexity of the defects and the rapidly evolving hydrops, no intrauterine cardiac intervention was deemed feasible. The team instead pursued close surveillance with repeat echocardiography and intensive counseling of the parents about prognosis and potential perinatal outcomes. Despite maximal expectant management, the pregnancy ended at 30 weeks with intrauterine fetal demise secondary to progressive hydrops. Notably, by that point the mother’s HbA1c had improved to 6.6 percent, but the critical period of embryonic cardiac development had long since passed. Genetic assessment was incomplete: amniocentesis was not technically feasible because of the patient’s delayed presentation, although noninvasive prenatal testing had shown no evidence of trisomy 13, 18, or 21. The authors acknowledge that the absence of postmortem examination means the precise intracardiac anatomy, particularly the suspected absent pulmonary valve variant, could not be definitively confirmed, and that 22q11.2 deletion and other genetic contributors remain unassessed.

The mechanistic link to maternal diabetes is one of the most scientifically interesting dimensions of the case. Pre-gestational diabetes is a well-established, non-inherited risk factor for congenital heart disease, with particularly strong associations for conotruncal defects, exactly the spectrum observed here. The leading explanation involves hyperglycemia-driven oxidative stress during cardiac morphogenesis, the window in early pregnancy when the heart takes shape. Experimental models and human observational studies have repeatedly shown that elevated glucose impairs the survival and migration of cardiac neural crest cells, a transient embryonic cell population essential for septating the outflow tract and remodeling the aorticopulmonary structures, and disrupts endocardial cushion development. Population data reinforce the dose-response relationship: compared with normal HbA1c, values above 8 percent confer roughly a fourfold higher risk of major congenital heart disease, and even modest elevations above about 5.6 percent increase risk. Across large cohorts, pre-gestational diabetes raises the risk of all congenital heart defect subtypes, with the greatest excess for outflow-tract defects. Emerging work also suggests genetic modifiers, including reduced NOTCH1 function, that may render some fetuses more vulnerable to maternal hyperglycemia, hinting at a gene-environment interaction that could one day refine risk stratification.

For clinicians, the case is a pointed argument for targeted early fetal echocardiography, typically performed between 18 and 22 weeks and earlier when feasible, in pregnancies at elevated risk, including all those complicated by pre-gestational diabetes. Early diagnosis enables comprehensive anatomic and rhythm assessment, serial surveillance for evolving ventricular hypertrophy, outflow obstruction, and heart failure, genetic counseling and testing for conditions such as 22q11.2 deletion, and multidisciplinary delivery planning at tertiary centers. It also opens the door to realistic counseling: in published cohorts of fetal dextrocardia, most cases involve complex heart disease, and outcomes range widely depending on associated defects. But the deepest lesson of this report lies upstream of any scanner. Once hydrops appears in structural fetal heart disease, therapeutic options in utero are few, and outcomes deteriorate dramatically. The single most effective intervention to prevent cases like this one happens before conception, through tight glycemic control and maternal education. This fetus’s heart, imaged in such painful detail as it failed, stands as a vivid scientific case study and, equally, as a public health message: the fourfold elevated risk carried by an HbA1c above 8 percent is modifiable, and the time to modify it is months before a positive pregnancy test, not at 24 weeks when the heart has already formed and the damage is done.

Subject of Research: Prenatal diagnosis of suspected tetralogy of Fallot with absent pulmonary valve syndrome and dextrocardia in a pregnancy complicated by pre-gestational diabetes mellitus

Article Title: Coexistence of Suspected Tetralogy of Fallot With Absent Pulmonary Valve and Dextrocardia in Pre‐Gestational Diabetes Mellitus: A Case Report

Article References: Golbabaei, A., Kahani, V., & Mousivand, M. (2026). Coexistence of Suspected Tetralogy of Fallot With Absent Pulmonary Valve and Dextrocardia in Pre‐Gestational Diabetes Mellitus: A Case Report. Clinical Case Reports, 14(10), Article e73597. https://doi.org/10.1002/ccr3.73597

Image Credits: AI Generated

DOI: 10.1002/ccr3.73597

Keywords: tetralogy of Fallot, absent pulmonary valve syndrome, dextrocardia, pre-gestational diabetes, fetal echocardiography, congenital heart disease, hydrops fetalis, conotruncal defect, HbA1c, neural crest cells, intrauterine fetal demise, prenatal diagnosis