High-altitude hypoxia synchronously activates HIF-1α and HIF-2α in hepatocytes; the latter binds a conserved 5′-RCGTG-3′ motif within the hepcidin (HAMP) promoter and displaces CCAAT/enhancer-binding protein-α (C/EBPα), leading to rapid transcriptional repression (35). Concurrent HIF-2α-dependent induction of duodenal divalent metal transporter 1 and Fpn mRNA shunts newly absorbed iron into the circulation, while hepatic HAMP remains low (36,37). In a prospective field study of 38 mountaineers ascending to 4,550 m within 48 h, serum hepcidin fell from 19.4±4.8 ng/ml at sea level to 4.2±1.9 ng/ml on day 3, coinciding with an increase in transferrin saturation (TSAT) from 24±6% to 41±9% (29). A separate cohort of 127 Andean highlanders with chronic mountain sickness exhibited the same inverse relationship: Every 1% rise in hematocrit above 60% was associated with a 0.42 ng/ml decrease in serum hepcidin (r=-0.63, P<0.001) (34). Notably, 41% of these patients displayed serum ferritin >100 ng/ml despite hepcidin <2 ng/ml, indicating that hypoxia overrides the iron-loading signal that normally stimulates HAMP (34,38). Collectively, these data establish a robust, hypoxia-specific suppression of the master iron-regulatory hormone that favors iron bioavailability for erythropoiesis and directly link the magnitude of hepcidin suppression to clinical disease severity.

Hypoxia-induced hepcidin repression is further sculpted by germ-line variants. A hypoxia-responsive element (HRE) at -7/-3 kb of the transmembrane protease serine 6 (TMPRSS6) promoter is directly bound by HIF-1α, leading to increased matriptase-2 synthesis and reinforcement of the hepcidin-lowering signal (39,40). The loss-of-function variant c.736C>T (p. Val220Met) abolishes this feedback: Carriers retain membrane-bound hemojuvelin, amplify bone morphogenetic protein (BMP)-SMAD signaling and sustain hepcidin suppression even during iron sufficiency (39). Meta-analysis of five high-altitude case-control studies (1,174 HAP vs. 1,056 controls) revealed an odds ratio of 2.3 (95% CI, 1.7-3.1) for HAP among V220M heterozygotes, with a gene-dosage effect on serum hepcidin (6.1±2.4 ng/ml in Val/Val vs. 2.8±1.1 ng/ml in Val/Met, P<0.01) (34). By contrast, HFE H63D carriers exhibit higher baseline hepcidin and a 30% lower risk of excessive erythrocytosis, presumably because residual HFE-hemojuvelin complexes counterbalance HIF-driven repression (35,41). The Fpn Q248H (SLC40A1 c.744G>T) variant, present in 4% of African-descent highlanders, confers resistance to hepcidin-induced internalization; erythroid precursors export iron more efficiently, yielding 18% higher TSAT and 0.8 g/dl higher hemoglobin under hypoxia without overt iron overload (42). These genetic data highlight considerable population heterogeneity: The same hypoxic stimulus produces divergent hematological outcomes depending on the underlying genetic background. Moreover, most cohort studies are limited by cross-sectional design, small sample sizes, and incomplete adjustment for dietary iron intake, which may confound the observed genotype-phenotype associations. Thus, polymorphisms along the hepcidin-Fpn axis modulate the iron supply rate independently of ambient EPO, but their clinical impact must be interpreted within the context of population-specific genetic architecture and environmental exposures.

The translational corollary of sustained hepcidin suppression is that pharmacological restoration of the hormone could curb iron availability and attenuate polycythemia. To date, three classes of agents have been investigated with distinct translational maturity. First, the injectable hepcidin mimetic rusfertide (PTG-300) binds Fpn with nanomolar affinity, blocks iron export and reduces TSAT by 35% within 4 weeks in patients with polycythemia vera (43,44). This agent has completed phase II trials in polycythemia vera and is now being evaluated in a single-arm, open-label study specifically for Andean individuals with HAP (NCT05960723), representing the most advanced candidate for altitude polycythemia. Second, anti-TMPRSS6 antisense oligonucleotides (IONIS-TMPRSS6-LRx) restored hepcidin to sea-level values and blunted the polycythemic response in murine hypoxia models without inducing anemia (39,45). These agents remain at the preclinical stage for HAP, with no human studies reported to date. Third, low-dose HIF prolyl hydroxylase inhibitors (for example, roxadustat) transiently increase hepatic HIF-1α, reinforce endogenous hepcidin feedback and reduce TSAT when used intermittently (46,47). While approved for anemia of chronic kidney disease, their application in HAP remains confined to preclinical models, and concerns remain regarding potential off-target effects on tumor angiogenesis and lipid metabolism. Collectively, these proof-of-concept data indicate that re-balancing the hepcidin-Fpn axis offers a mechanistically grounded, EPO-independent strategy to limit iron-catalyzed erythropoiesis in high-altitude dwellers. However, the transition from preclinical promise to clinical practice requires larger, randomized controlled trials with longer follow-up to establish efficacy, safety, and the optimal timing of intervention across different genetic and environmental backgrounds.

Chronic hypobaric hypoxia rapidly perturbs the intestinal ecosystem. 16S rRNA profiling of 38 Han lowlanders ascending to 4 300 m revealed a 28% drop in α-diversity (Shannon index) within 1 week, coinciding with a proportional expansion of Proteobacteria and a parallel decrease in butyrate-producing Firmicutes (48,49). Comparable shifts were recorded in native Tibetans: Individuals with excessive erythrocytosis exhibited a lower Firmicutes/Bacteroidetes ratio (0.72±0.11) than healthy high-altitude controls (1.24±0.18, P<0.01), a signature that persisted after correction for dietary intake (50,51). Murine studies corroborate these findings: Continuous hypoxia (10% O₂, 4 weeks) reduced cecal butyrate concentration by 45% and elevated luminal LPS levels 3-fold, thereby increasing systemic exposure to microbial endotoxin (52,53). Collectively, the data indicate that altitude per se, rather than ethnicity or diet, drives a reproducible dysbiotic pattern characterized by loss of short-chain fatty acid (SCFA) producers and gain of Gram-negative taxa.

Butyrate, a four-carbon SCFA, functions as an endogenous inhibitor of prolyl hydroxylase domain enzymes; elevated intracellular butyrate stabilizes HIF-1α in enterocytes and boosts systemic HIF activity without further lowering oxygen tension (54,55). Conversely, butyrate depletion, typical of the hypoxic dysbiotic state, removes this brake, permitting excessive HIF-2α-driven EPO synthesis in renal fibroblasts. This translates to a sustained erythropoietic drive, as evidenced by the inverse correlation between fecal butyrate levels and hemoglobin concentration in high-altitude residents (56,57). Parallel mechanisms involve innate immunity: Translocated LPS engages Toll-like receptor 4 (TLR4) on splenic macrophages, triggering myeloid differentiation primary response 88-dependent nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κb) activation and a 2.5-fold surge in IL-6 secretion (58,59). IL-6 synergizes with hypoxia to enhance EPO transcription via STAT3 binding to the EPO promoter, an effect abrogated in TLR4⁻/⁻ mice exposed to 10% O₂ (58,60). The net effect of this endotoxin-driven pathway is a direct increase in erythroid progenitor proliferation and a significant rise in hemoglobin, linking intestinal barrier dysfunction to the severity of polycythemia. Thus, metabolite scarcity (butyrate) coupled to endotoxin excess (LPS) creates a feed-forward loop that amplifies erythropoiesis independently of classical oxygen sensing.

Restoration of butyrate-producing taxa offers a translational avenue. In a randomized, double-blind trial, 60 healthy males ascending to 3,800 m received either Lactobacillus plantarum TWK10 (10¹⁰ CFU/day) or placebo for three weeks (61). Probiotic supplementation attenuated the altitude-induced drop in butyrate (Δ -0.8 vs. -3.2 mM, P=0.02), limited hepcidin suppression, and reduced the incremental rise in hemoglobin (0.9 vs. 1.6 g/dl, P<0.01) without compromising oxygen saturation. A second study utilizing a synbiotic mixture (Bifidobacterium lactis plus galacto-oligosaccharide) reported similar hematological benefit and additionally demonstrated a 15% increase in erythrocyte 2,3-bisphosphoglycerate, suggesting improved tissue oxygen unloading (62). While these proof-of-concept trials consistently link gut microbiota modulation to restrained polycythemia, their generalizability is constrained by modest sample sizes. The observed hemoglobin reduction of 0.7-0.9 g/dl, although statistically significant, may be of variable clinical relevance across populations with differing baseline hematocrit levels. Larger multicentric efforts with extended follow-up are therefore necessary to validate these findings and to identify subpopulations most likely to benefit from microbiota-targeted interventions.

Within minutes of hypoxic exposure, falling ATP/AMP ratio activates liver kinase B1 (LKB1)-AMPK signaling. Once phosphorylated, AMPK directly phosphorylates PGC-1α at Thr-177 and Ser-538, increasing its half-life and transcriptional activity (63,64). Concomitantly, HIF-1α translocates to the nucleus where it co-operates with PGC-1α to up-regulate nuclear respiratory factor-1 (NRF-1) and mitochondrial transcription factor A (TFAM), thereby accelerating mitochondrial DNA replication and organelle biogenesis (63,65). Elegant work by Rao et al (63) in pulmonary artery smooth muscle cells showed that this burst of biogenesis is detectable within 6 h of 3% O₂ exposure and peaks at 24 h, coinciding with maximal citrate-synthase activity and complex IV assembly. A comparable time-course was reported in skeletal muscle of obese mice maintained at 4,000 m for 3 weeks; AMPK-PGC-1α signaling remained elevated for the entire duration, leading to a 35% increase in subsarcolemmal mitochondria and a 25% improvement in insulin sensitivity (64). These observations indicate that the initial polycythemic phase is metabolically advantageous: More mitochondria allow aerobic ATP production to proceed at lower oxygen tensions, thereby limiting the need for excessive EPO secretion.

Not every organ mounts the same biogenic response. Sharma et al (66) demonstrated that under identical hypoxic-ischemic insult, male neonatal mouse brain increased PGC-1α mRNA 2.3-fold whereas female littermates achieved only 1.4-fold induction; the higher mitochondrial reserve in males translated into 40% less neuronal death (66). Conversely, in adipose tissue the same stimulus represses PGC-1α through estrogen-related receptor-α, leading to mitochondrial loss and lipotoxicity (67). Such divergence is clinically relevant for HAP because visceral adipose tissue is a major source of pro-inflammatory cytokines that aggravate systemic hypoxia. Whether sex hormones modulate mitochondrial biogenesis in human erythroid precursors remains unexplored, but the rodent data caution against extrapolating muscle-centric findings to the whole organism.

Continued hypoxia superimposed on iron-rich erythropoietic marrow creates a perfect storm for redox imbalance. Electrons that fail to reach complex IV because of low pO₂ are increasingly captured by molecular oxygen to generate superoxide (O₂•-). If manganese-superoxide dismutase (MnSOD) activity does not keep pace, O₂•-dismutates to H₂O₂ which, in the presence of Fenton-active iron, produces hydroxyl radicals (•OH) that oxidize cardiolipin and mitochondrial proteins (68,69). Pak et al (68) documented that MitoQ-a mitochondria-targeted ubiquinone, normalized mitochondrial reactive oxygen species (mtROS) flux, reversed pulmonary hypertension and reduced hematocrit from 68 to 55% in rats kept at 10% O₂ for 3 weeks (68). Importantly, MitoQ did not blunt EPO transcription, arguing that the beneficial effects were downstream of erythropoiesis, most likely by improving arterial oxygenation and reducing the hypoxic drive. A parallel study using intermittent short-duration re-oxygenation achieved similar hematological correction by restoring PPAR-γ activity and dampening NADPH oxidase 4-derived H₂O₂ (70). Collectively, these data establish that mtROS are not merely markers of hypoxic stress but active drivers of the vicious cycle that sustains excessive erythropoiesis.

Once oxidative damage exceeds the repair capacity, the balance shifts from fusion-promoting OPA1/mitofusin 2 (Mfn2) to fission-promoting dynamin-related protein 1 (Drp1). Phosphorylation of Drp1 at Ser-616 is redox-sensitive; H₂O2-mediated activation causes mitochondrial fragmentation, de-polarisation and PTEN-induced kinase 1(PINK1)/Parkin-dependent mitophagy (71,72). In a rat model of high-altitude exposure (5,500 m, 30 days), Chitra and Boopathy (71) found that lung tissue exhibited a 60% decrease in Mfn2 mRNA and a parallel rise in microtubule-associated protein 1A/1B-light chain 3 (LC3-II), indicating accelerated autophagic flux. Although erythroid cells were not examined, the same group later showed that circulating reticulocytes had lower TFAM protein and mitochondrial DNA (mtDNA) content, consistent with ineffective biogenesis and/or excessive mitophagy. Whether this contributes to the shortened lifespan of the red cell at altitude remains to be tested, but the data provide a plausible mechanistic link between systemic hypoxia, mitochondrial quality control and anemia of chronic disease that occasionally co-exists with polycythemia.

Several botanical derivatives and endogenous metabolites have been reported to restore mitochondrial homeostasis under hypoxic stress. Notoginsenoside R1 preserved ΔΨm and reduced mtROS in H9c2 cardiomyocytes subjected to hypoxia/re-oxygenation by activating the Nrf2/ARE antioxidant program (73). Similarly, total secondary ginsenosides increased PGC-1α and NRF-1 expression, raised ATP content and decreased caspase-3 activity in the same cell line (74). Outside the cardiovascular field, spermidine administered to pregnant rats exposed to 12% O₂ prevented intrauterine growth restriction and reduced mtDNA oxidation in fetal hearts by enhancing MnSOD and inhibiting Drp1-mediated fission (75). These proof-of-concept studies suggest that boosting mitochondrial antioxidant defenses or reinforcing fusion may uncouple hypoxia from polycythemia, although rigorous in vivo validation at altitude is still lacking.

Synthesing the aforementioned findings, a two-hit model is proposed. Hit-1 consists of acute hypoxia (<48 h) that activates AMPK-PGC-1α-TFAM signaling, increases mitochondrial mass and improves O₂ utilization; this phase is largely adaptive and reversible. Hit-2 ensues when chronic hypoxia (>72 h) plus iron-driven Fenton chemistry overwhelms MnSOD and glutathione peroxidase, leading to cardiolipin oxidation, Drp1-mediated fission and PINK1/Parkin mitophagy. The resulting drop in mitochondrial efficiency re-activates HIF-1α, perpetuating EPO secretion and erythroid hyperplasia. Interventions that reinforce Hit-1 (for example, AMPK agonists, PGC-1α gene therapy) or blunt Hit-2 (for example, MitoQ, intermittent re-oxygenation, spermidine) consistently lower hematocrit and ameliorate pulmonary hypertension in pre-clinical models. Despite these promising preclinical data, several translational gaps remain. Pharmacokinetic and pharmacodynamic profiles of candidate agents have not been established under chronic hypobaric hypoxia, where altered absorption and metabolism may significantly modify effective dosing (28,29). Validated biomarkers such as plasma mtDNA or urinary 8-OHdG to stratify patients who would benefit most from mitochondrial-targeted interventions are lacking (76,77). Furthermore, long-term safety data in high-altitude populations, particularly regarding interference with adaptive mitochondrial biogenesis (Hit-1), are absent. Addressing these gaps through staged evaluation, starting with dose-finding studies in controlled environments followed by biomarker-enriched randomized trials, will be essential to translate mechanistic insights into clinical practice.

Exposure of murine fetal liver-derived erythroblasts to 1% O₂ for 12 h produces a rapid increase in LC3-II/I ratio and a concomitant fall in p62/SQSTM1, consistent with enhanced autophagosome formation and flux (78). Similar changes are observed in human peripheral-blood CD34⁺ cells differentiated under 3% O₂; electron microscopy reveals a two-fold increase in autophagic vacuoles containing partially degraded mitochondria, while immunoblotting shows a 40% rise in BNIP3L protein (79). The functional relevance of this response is highlighted by Seahorse analyses: Erythroblasts cultured at 3% O₂ exhibit lower basal oxygen consumption and reduced mitochondrial membrane potential, suggesting that early mitophagy curtails oxidative stress before the reticulocyte stage (79). Conversely, when autophagy is blocked by chloroquine (15 mg/kg/day) in rats kept at 5,000 m for 3 weeks, LC3-II accumulates, p62 is preserved and hematocrit rises from 68 to 81% despite unchanged EPO levels (80). Taken together, these data indicate that hypoxia initiates a homeostatic autophagy program; failure of this program exacerbates polycythemia, most likely through retention of superfluous mitochondria that sustain HIF-2α activity via mtROS-mediated feed-forward signaling.

The selective removal of mitochondria in reticulocytes is orchestrated by the atypical BH3-only protein NIX (BNIP3L). Germ-line deletion of Bnip3l in mice leads to circulating reticulocytes that retain ~40% of mitochondrial DNA, accompanied by severe anemia, splenomegaly and increased EPO levels (81,82). Ultrastructural analysis reveals intact but swollen mitochondria that are engulfed by autophagosomes yet fail to be degraded, suggesting a defect in autolysosome acidification rather than autophagosome formation (83). Complementary in vitro work using human CD34⁺ cells shows that small interfering RNA-mediated knockdown of BNIP3L under 3% O₂ reduces LC3 lipidation and halts mitochondrial clearance, resulting in a 30% decrease in enucleation efficiency (79). Intriguingly, hypoxia itself upregulates BNIP3L transcription through a HIF-1α-responsive element located-214 bp upstream of the start codon (84). Thus, the same hypoxic stimulus that drives erythropoiesis simultaneously primes the mitophagy machinery, ensuring that newly formed reticulocytes are devoid of residual mitochondria and less prone to oxidative damage upon re-oxygenation.

Rapamycin, an mTORC1 inhibitor, has been employed to test whether enforced autophagy can ameliorate hypoxia-driven polycythemia. Daily intraperitoneal injection of rapamycin (2 mg/kg) during 3-week hypobaric hypoxia (5,000 m) increased LC3-II/I ratio in bone-marrow-derived erythroblasts and reduced hematocrit from 71 to 61% without altering reticulocyte count, suggesting improved efficiency of maturation rather than decreased production (79). Conversely, chloroquine-mediated lysosomal inhibition produced the opposite phenotype: LC3-II accumulated, p62 levels rose and hematocrit increased by 13% compared with vehicle-treated controls (80). These findings are corroborated by a previous study in Jak2V617F mice, a genetic model of polycythemia; neutralization of tissue factor pathway inhibitor (TFPI) restored BNIP3L expression, enhanced mitophagy and normalized hematocrit (85). Collectively, the data demonstrate that pharmacological enhancement of autophagy, either through mTOR inhibition or through restoration of specific mitophagy receptors, constitutes a feasible strategy to curb excessive red-cell mass in HAP.

Despite the promising preclinical evidence, translating pharmacological autophagy enhancement into clinical practice for HAP requires careful consideration of potential risks and limitations. First, systemic mTOR inhibition with agents such as rapamycin carries well-documented off-target effects, including immunosuppression, impaired glucose tolerance and dyslipidemia, which could outweigh benefits in otherwise healthy high-altitude residents (79). Second, the requirement for autophagy during terminal erythroid maturation is finely balanced; excessive or sustained enhancement may theoretically disrupt late-stage differentiation, as illustrated by the accumulation of autophagic vacuoles and impaired enucleation when autophagy is chronically activated (78,79). Third, lysosomal function, essential for autophagic flux, can be compromised by prolonged hypoxia itself, raising the possibility that pharmacological induction of autophagy in a setting of impaired lysosomal acidification may lead to autophagosome accumulation rather than productive mitophagy (80,83). Fourth, the absence of validated, lineage-specific biomarkers to monitor autophagic flux in human erythroid precursors makes dose optimization challenging; systemic administration of autophagy modulators may affect non-erythroid tissues, potentially exacerbating pulmonary hypertension or altering immune responses (68,85). Finally, long-term safety data in high-altitude populations are completely lacking, and the interaction of such interventions with genetic variants in autophagy-related genes (for example, BNIP3L and ATG7) remains unexplored. These considerations underscore that while autophagy enhancement represents a mechanistically attractive strategy, its clinical development must proceed through staged evaluation with careful attention to target specificity, dosing regimens, and comprehensive safety monitoring in relevant preclinical models and, ultimately, in well-controlled human trials.

Although the majority of studies report that hypoxia activates autophagy, some investigators have observed suppression under severe (<1% O₂) or prolonged (>48 h) hypoxic exposure (78,80). The discrepancy can be reconciled by considering the temporal dynamics of HIF-1α vs. mTOR signaling. Acute hypoxia (3-12 h) robustly induces BNIP3L and activates AMPK, thereby inhibiting mTOR and facilitating autophagy. Extended hypoxia, however, leads to mTOR re-activation via REDD1-dependent feedback, resulting in autophagy arrest and accumulation of damaged mitochondria that further amplify HIF-2α signaling, a vicious cycle that exacerbates polycythemia. Pharmacological reinforcement of the early autophagic wave (for example, with rapamycin or TFPI blockade) breaks this cycle, reduces mtROS production and lowers hematocrit without suppressing EPO. However, given the aforementioned potential risks, such interventions should currently be considered experimental, and future work should focus on developing erythroid-targeted autophagy enhancers or intermittent dosing schedules that minimize systemic exposure.

Two independent Illumina 850 K arrays performed on immunoselected CD34⁺ cells from Tibetan and Han males (3,800-4,300 m) converge on ~1,200 differentially methylated probes (Δβ ≥|0.15|, FDR <0.05) when individuals with HAP (Hb ≥18 g/dl) are compared with healthy high-altitude controls (Hb 15-16 g/dl) (86). Hypomethylated promoters are enriched for oxygen-sensing genes (EPAS1, EGLN1 and VEGFA) and glycolytic enzymes (HK2, LDHA), whereas hypermethylated promoters harbor anti-proliferative cytokines (BMPR2 and TGFB2) and pro-apoptotic factors (PDCD4) (87). Bisulphite-pyrosequencing validation revealed that BMPR2 CpG-220 to -80 bp shows 32% higher methylation in Han HAP, correlating with a 40% drop in BMPR2 protein and a 1.8-fold increase in BFU-E proliferation under 1% O₂ (87). These data indicate that DNA methylation acts as a binary switch: Hypomethylation licenses transcriptional over-drive, while hypermethylation silences negative regulators, jointly tilting the balance toward excessive erythropoiesis. Despite these insights, direct evidence that perinatal methylation signatures observed in cord blood from high-altitude births persist into adulthood and predispose to HAP remains lacking, leaving the concept of long-term epigenetic memory incompletely defined (88).

Chromatin immunoprecipitation followed by sequencing in cord-blood-derived erythroblasts exposed to 3% O₂ for 24 h demonstrates selective accumulation of H3K4me3 at the proximal EPO-R promoter (-0.3 kb HRE) without changes in the repressive mark H3K27me3 (89). The same hypoxic pulse reduces H3K27me3 at the β-globin locus control region (HS2-HS3) while increasing H3K4me3, thereby accelerating the fetal-to-adult globin switch (90). Importantly, these modifications are reversible upon return to 21% O₂, but they persist for ≥7 days if hypoxia is combined with iron supplementation, suggesting that metabolic context dictates epigenetic memory. Whether similar histone marks are observed in native HAP bone-marrow biopsies remains untested; however, the concordance between in vitro and in vivo transcript profiles (for example, 2-fold EPOR mRNA) supports biological relevance.

miR-210 is transcriptionally activated by HIF-1α binding to a consensus HRE in its promoter. Once induced, it targets iron-sulfur cluster assembly proteins (ISCU1/2), mitochondrial cytochrome c oxidase assembly protein, and the E3-ligase VHL, thereby reinforcing HIF-1α stabilization via a positive-feedback loop (69,91). In plasma exosomes from 47 Tibetan patients with HAP, miR-210 was 6.3-fold higher than in healthy high-altitude dwellers; levels correlated positively with hemoglobin (r=0.71, P<0.001) and negatively with arterial O₂ saturation (r=-0.68) (92). In vitro antagonism of miR-210 with locked nucleic acid-antimiR-210 restored ISCU expression, reduced mtROS, and lowered erythroid colony growth by 35% under 1% O₂ (69). These data position miR-210 not merely as a biomarker, but as a functional node that couples metabolic stress to erythroid expansion.

miR-21 is consistently upregulated in hypoxic CD34⁺ cells and reticulocytes (93,94). Bioinformatic and luciferase assays have validated PDCD4, PTEN and CASP8AP2 as direct targets; their suppression leads to increased Bcl-xl and STAT3 phosphorylation, enhancing erythroblast survival (94,95). A noteworthy sex-specific effect has been reported: miR-21 induction is 2-fold higher in CD34⁺ cells under 3% O₂ of men compared with women, mirroring the male predominance of HAP (94). Whether estrogen signaling directly represses pri-miR-21 transcription or accelerates miR-21 degradation remains unresolved, and the potential contribution of X-chromosome-linked epigenetic modifiers to the male bias in HAP has not been systematically explored, highlighting a critical gap in understanding sex-dimorphic epigenetic regulation (86,94).

The antisense transcript HIF1A-AS1 (also termed lnc-HIF1A-AS2) overlaps the 3′-untranslated region of HIF1A and protects its mRNA from RNase-mediated decay. In reticulocytes from Andean individuals with HAP, HIF1A-AS1 abundance is 4.7-fold higher than in controls and positively correlates with HIF-1α protein levels (r=0.74, P<0.001) (96). CRISPR-interference targeting the HIF1A-AS1 promoter in HUDEP-2 cells reduces HIF-1α mRNA half-life from 38 to 18 min, blunts glycolytic gene expression, and inhibits erythroid proliferation by 28% under hypoxia (97). Conversely, forced overexpression of HIF1A-AS1 in normoxic HUDEP-2 cells induces a HAP-like transcriptional signature (including SLC2A1, LDHA and EPOR) even in the absence of hypoxia (97). These loss- and gain-of-function experiments establish lnc-HIF1A-AS1 as a bona-fide erythroid regulator that operates upstream of the HIF hub. Nonetheless, the interplay between distinct ncRNA classes remains unexplored; for instance, whether lnc-HIF1A-AS1 functionally interacts with miR-210 or whether these two HIF-1α-amplifying ncRNAs converge on shared downstream pathways has not been investigated in HAP progenitors (92,97).