Hemoglobin O Arab: Its Impact, Genetics, and Detection

Hemoglobin O-Arab is a rare hemoglobin variant caused by a single amino acid swap in the beta-globin chain that, on its own, causes little trouble but can produce severe disease when inherited alongside hemoglobin S or certain thalassemia mutations. Because it migrates in the same zone as hemoglobin C on standard electrophoresis, it is easily missed in routine screening, and that misidentification can delay correct diagnosis for months or years. The variant turns up most often in populations from the Middle East, the Mediterranean, and parts of Africa, though population-based screening has now documented it in East Asia as well.

The Mutation Behind Hemoglobin O-Arab

Hemoglobin O-Arab results from a point mutation at codon 121 of the beta-globin gene, swapping glutamic acid for lysine. In molecular shorthand the change is written as Glu122Lys (using the older numbering that counts the initiator methionine, or Glu121Lys in some references). A study sequencing the full beta-globin gene in Sudanese sickle cell patients confirmed the mutation as (HBB):c.364G>A (p.Glu122Lys) and found it co-inherited with the sickle mutation in about 5% of the cohort studied.¹ This single nucleotide change sits in exon 3 of the gene, and because it alters the surface charge of the hemoglobin molecule, the variant behaves differently from normal hemoglobin A on both electrophoresis and liquid chromatography, a fact that matters enormously for diagnosis.

Inheritance follows the same autosomal recessive pattern as other beta-globin variants. Carrying one copy of the O-Arab allele alongside one normal beta-globin allele (the heterozygous, or trait, state) is generally harmless. Trouble arrives when someone inherits two copies, or more commonly when O-Arab appears on one chromosome while a different pathological beta-globin variant sits on the other. That second scenario, compound heterozygosity, is where the real clinical risk lies.

Where the Variant Is Found

Despite its name, Hemoglobin O-Arab is not confined to Arab populations. The highest known allele frequency, roughly 4.4%, has been documented in the Pomak village communities of northern Greece, a small and genetically isolated population where genetic drift amplified the variant’s frequency over generations.² From that Mediterranean pocket, the variant has been traced across a wide arc: Israeli Arabs, Tunisians, Moroccans, Bulgarians, Saudi Arabians, and Sudanese populations all have documented cases.³ Population-based genetic screening in Guangxi, China, has also identified Hb O-Arab carriers, confirming that the variant has a broader global footprint than its name implies.²

The scattered distribution matters clinically. In regions where sickle cell disease is common, such as sub-Saharan Africa and parts of the Middle East, the chance of someone inheriting both a sickle allele and an O-Arab allele is higher than you might expect for a “rare” variant. A clinician in those settings who sees severe sickling disease but cannot confirm homozygous HbSS on standard testing should have O-Arab on the differential.

Hemoglobin O-Arab Trait on Its Own

People heterozygous for O-Arab and normal hemoglobin A are generally asymptomatic. A Tunisian series of 20 cases noted that the homozygous O-Arab form, where both beta-globin genes carry the mutation, was also “not very symptomatic.”⁴ However, research on red blood cell density tells a more nuanced story. In homozygous O-Arab patients, all red cells are denser than normal, similar to what is seen in homozygous hemoglobin C disease. Their reticulocytes, the youngest circulating red cells, were found in the densest fraction of whole blood, which is abnormal and suggests the cells begin dehydrating very early in their lifespan.⁵

The mechanism behind this dehydration involves abnormal ion transport across the red cell membrane, particularly a potassium-chloride cotransport system that is overactive in O-Arab red cells. The level of that overactivity was found to be close to what is seen in another severe variant, HbS-Oman, and only somewhat lower than in classic sickle cell disease cells.⁵ So while homozygous O-Arab patients are not clinically sick the way sickle cell patients are, their red cells are quietly under stress, losing water and becoming abnormally dense. This subclinical dehydration may contribute to a mildly shortened red cell lifespan even in the absence of overt symptoms.

When O-Arab Meets Hemoglobin S

The clinical picture changes dramatically when someone inherits one sickle allele and one O-Arab allele, creating the compound heterozygous state known as Hb S/O-Arab. This combination produces a severe sickling disorder that, by all clinical measures, rivals homozygous sickle cell anemia. A case series of 13 patients with Hb S/O-Arab documented a harrowing range of complications: 11 had acute chest syndrome, 10 had recurrent painful crises, 7 experienced dactylitis, and 5 developed gallstones. Kidney disease, bone infarction, leg ulcers, stroke, and eye damage also appeared. Four of the 13 patients died, including two children from overwhelming pneumococcal infection, a teenager from acute chest syndrome, and an adult from multi-organ failure.⁶

The reason for this severity lies in the way O-Arab hemoglobin interacts with sickle hemoglobin at the molecular level. In vitro studies on mixtures of Hb S and Hb O-Arab confirmed that O-Arab lowers the concentration at which sickle hemoglobin starts to polymerize, meaning sickling begins more readily than it would if the non-sickle hemoglobin were normal Hb A.⁷ The same study found that this polymerization-promoting effect was ionic-strength dependent and not quite as extreme as earlier claims had suggested. The oxygen dissociation curve was shifted to the right in S/O-Arab red cells, meaning these cells release oxygen more readily, but the shift was not as pronounced as in classic SS disease. So while S/O-Arab is severe, it is not molecularly identical to SS; it occupies its own place on the spectrum, clinically severe but with somewhat different biophysics under the hood.

This distinction matters because patients with S/O-Arab are sometimes initially diagnosed as having homozygous sickle cell disease on the basis of their clinical severity alone. When the hemoglobin pattern is later examined carefully and the O-Arab component is identified, the correct compound heterozygous diagnosis can guide family counseling and help predict risks in future children.

O-Arab Combined With Thalassemia

When O-Arab is inherited alongside a beta-thalassemia allele rather than a sickle allele, the clinical picture is generally milder but still noteworthy. A large screening study in Guangxi, China found that individuals carrying compound heterozygous Hb O-Arab and beta-thalassemia typically showed mild to moderate microcytic hypochromic anemia with reduced cell size and hemoglobin content per cell, along with elevated Hb A2 levels. Most were asymptomatic, though a few had mild jaundice and an enlarged spleen. As expected, the severity depended on which type of thalassemia allele was involved: beta-zero thalassemia, which produces no normal beta-globin at all from the affected chromosome, caused more pronounced blood count abnormalities than beta-plus thalassemia, which still produces some.²

The Tunisian case series aligned with these findings, describing compound heterozygous O-Arab/beta-thalassemia as a “mild form of thalassemia” with moderate microcytic hypochromic anemia and hemoglobin levels around 8.8 g/dL, notably lower than the trait state but not typically requiring regular transfusions.⁴ For affected individuals, this means the combination can produce chronic mild anemia that resembles beta-thalassemia intermedia rather than the life-threatening crises seen in S/O-Arab disease.

The Role of Alpha-Thalassemia in Modifying Severity

An unexpected wrinkle in O-Arab’s clinical behavior involves alpha-thalassemia, the separate genetic condition in which one or more alpha-globin genes are missing. In people heterozygous for O-Arab, the number of functioning alpha-globin genes turns out to substantially influence red cell density. Researchers found that O-Arab carriers who also had a single alpha-gene deletion showed red cells only slightly denser than normal. But O-Arab carriers who had the full complement of four alpha genes had much denser red cells, closer to what is seen in sickle cell disease, with a mix of normal-density and very dense cells.⁵

This is counterintuitive at first. Usually, losing an alpha gene is thought of as a mild deficit. But in this context, having fewer alpha genes actually protects the red cell, likely because less total hemoglobin is packed into each cell, reducing the tendency toward the dehydration that O-Arab hemoglobin promotes. The practical implication is that two people who both carry the O-Arab trait can have noticeably different blood counts depending on their alpha-globin gene status, which complicates interpretation of routine lab work.

Why Standard Screening Can Miss Hemoglobin O-Arab

One of the most significant practical issues with O-Arab is how easily it is confused with hemoglobin C during routine laboratory testing. On isoelectric focusing, a common first-line electrophoretic technique used in newborn screening and hemoglobinopathy workups, O-Arab migrates in the same zone as hemoglobin C and hemoglobin E. An authoritative review noted that these variants are “difficult to resolve” from each other by isoelectric focusing alone, requiring either a second electrophoretic method or high-performance liquid chromatography (HPLC) to tell them apart.⁸

HPLC can help, but even there, distinguishing O-Arab from Hb C requires attention to the chromatogram’s fine details. A study evaluating the Bio-Rad Variant II HPLC system found that O-Arab produces a distinctive pattern of two separate peaks appearing in two different retention-time windows (the D window and the C window), whereas Hb C produces only the C window peak. The retention time in the C window also differed significantly between the two variants, with O-Arab eluting slightly earlier at around 4.91 minutes compared to Hb C’s 5.18 minutes.⁹ For labs equipped with this specific system and trained to look for the dual-peak pattern, O-Arab can be identified without molecular testing. But many labs either use different HPLC platforms or do not routinely check for this pattern, leading to misidentification.

When HPLC and electrophoresis leave ambiguity, DNA sequencing of the beta-globin gene provides the definitive answer. The Sudanese study described earlier used full sequencing of the HBB gene to identify the O-Arab mutation in patients initially diagnosed only with sickle cell disease, underscoring that molecular methods may be the only reliable way to detect O-Arab when it co-exists with another variant.¹

Consequences of Misdiagnosis

Getting the diagnosis wrong is not just an academic problem. If a patient with Hb S/O-Arab is labeled as having Hb SC disease (a generally milder sickling condition), their clinicians may underestimate the severity of future crises and set management thresholds too loosely. Conversely, if a patient carrying the O-Arab trait is incorrectly told they have Hb C trait, genetic counseling about reproductive risk becomes inaccurate. A person carrying O-Arab trait who has a partner carrying sickle trait faces a one-in-four chance, with each pregnancy, of having a child with the severe S/O-Arab compound heterozygous disease. If that O-Arab carrier was told they carry Hb C instead, the couple might be reassured that the worst possible outcome would be Hb SC disease, a less severe condition, and might not receive the more aggressive monitoring that an S/O-Arab pregnancy warrants.

This cascade of consequences, from a misread band on a gel or a single misinterpreted HPLC peak, is why experts in hemoglobinopathy screening argue for reflexive molecular confirmation whenever an ambiguous C-zone band appears in a patient from a population where O-Arab is plausible.

Interference With Glycated Hemoglobin Measurement

An often-overlooked clinical side effect of carrying an abnormal hemoglobin variant is its potential to interfere with HbA1c testing, the standard blood sugar monitoring tool used in diabetes management. The presence of Hb O-Arab can alter the HPLC-based measurement of HbA1c, because the variant hemoglobin may co-elute with or shift the retention time of the glycated hemoglobin fraction. Studies have examined HbA1c levels in non-diabetic O-Arab carriers and flagged this as a potential source of clinically misleading results. For an O-Arab carrier who also has diabetes, this interference could mean falsely reassuring or falsely alarming HbA1c values, depending on the direction of the artifact and the specific HPLC platform used.

The practical fix is straightforward once the hemoglobin variant is known: use an alternative method for glycemic monitoring, such as fructosamine measurement or a non-HPLC-based HbA1c assay that is not affected by hemoglobin variants. But this only works if someone first identifies that the patient carries O-Arab, which circles back to the detection problem.

How O-Arab Compares to Other Problem Variants

Hemoglobin O-Arab shares certain properties with hemoglobin C, the variant it is most often confused with in the lab. Both promote red cell dehydration and increased density. However, O-Arab and Hb C achieve this through somewhat different proportions of ion channel and cotransport activity. In O-Arab homozygotes, the potassium-chloride cotransport overactivity is the dominant dehydrating force, resembling the ion transport pattern of HbS-Oman red cells.⁵ Meanwhile, the Gardos channel, another potassium exit pathway, was equally active in O-Arab, Hb S, and Hb C cells, suggesting that some dehydration pathways are shared across all three variants while others differ.

Where O-Arab diverges sharply from Hb C is in its interaction with sickle hemoglobin. Hb C combined with Hb S produces SC disease, which is clinically milder than homozygous SS in most patients. By contrast, S/O-Arab produces disease with severity comparable to SS, as the case series data discussed earlier made clear. This difference alone justifies the effort to distinguish the two variants in the lab. Two conditions that look nearly identical on a first-pass screening test can lead to profoundly different clinical outcomes.

Screening in Newborns and Prenatal Settings

In countries with universal newborn screening for hemoglobin disorders, the initial test typically involves isoelectric focusing or HPLC of a dried blood spot. Because O-Arab falls in the C window, a newborn carrying S/O-Arab may be reported as having a pattern suggesting SC disease. Some newborn screening programs include reflexive molecular testing for any sample with a C-zone band and a concurrent S band, which would catch the discrepancy. Others rely on confirmatory testing at a later clinic visit, by which time the infant may already have experienced an early complication like dactylitis.

Prenatal screening and preconception carrier testing face similar challenges. In regions where both sickle cell disease and O-Arab are present, couples identified through carrier screening as potentially at risk should be offered molecular confirmation of the exact variant each partner carries. Relying on the electrophoresis label alone introduces a risk of inaccurate counseling about the severity of disease in a future child.

Treatment Considerations for S/O-Arab Disease

Because Hb S/O-Arab clinically resembles homozygous sickle cell disease, management generally follows established sickle cell protocols. This includes prophylactic penicillin in young children to prevent the overwhelming pneumococcal infections that killed two of the 13 patients in the case series described above, hydroxyurea therapy to reduce the frequency and severity of painful crises, and vigilant screening for organ damage including kidney disease, avascular necrosis of the hip, and retinopathy.⁶ Chronic transfusion programs and, increasingly, stem cell transplant or gene therapy may be appropriate for patients with recurrent severe complications, though outcome data specifically for S/O-Arab patients is limited given the rarity of the condition.

For O-Arab combined with beta-thalassemia, treatment is generally supportive. Most patients with this combination do not need regular transfusions, though periodic monitoring of hemoglobin levels and iron stores is warranted, and folic acid supplementation is commonly recommended given the mild chronic hemolysis.

Why the Name Can Be Misleading

The “Arab” in Hemoglobin O-Arab traces back to an Arab family in Israel in whom the variant was first characterized decades ago. As population screening has expanded, it has become clear that the name reflects an accident of discovery rather than the true geographic distribution. The highest allele frequency is in a Greek mountain community, substantial numbers of carriers live in North Africa, and cases are now documented in East Asia. The naming convention for hemoglobin variants, which historically used the ethnic or geographic origin of the first reported family, has created confusion for other variants too. For clinicians and genetic counselors, the key point is that O-Arab should not be ruled out based on a patient’s ethnicity. If the lab pattern raises the possibility, molecular confirmation is the right next step regardless of the patient’s background.

1
PubMed Central. Molecular Detection of Hemoglobin O-Arab in the Sudanese Population
2
PubMed Central. The rare hemoglobin variants Hb O-Arab and Hb D-Punjab identified in population-based genetic screening throughout Guangxi, China
3
Karger. Human Haemoglobins and Haemoglobinopathies in Arabia: Hb O Arab in Saudi Arabia
4
PubMed. Hemoglobin O Arab: about 20 cases
5
PubMed. The erythrocyte effects of haemoglobin O(ARAB)
6
PubMed. Hemoglobin S/O(Arab): thirteen new cases and review of the literature
7
Springer. The interaction of hemoglobin O Arab with Hb S and beta+ thalassemia among Israeli Arabs
8
American Journal of Clinical Pathology. Molecular Diagnosis of Thalassemias and Hemoglobinopathies: An ACLPS Critical Review
9
PubMed. Hemoglobin C and hemoglobin O-Arab variants can be diagnosed using the Bio-Rad Variant II high-performance liquid chromatography system without further confirmatory tests