DiagnosisClinical DiagnosisThe term sickle cell disease (SCD) encompasses a group of disorders characterized by the presence of at least one hemoglobin S (Hb S) allele, and a second abnormal allele allowing abnormal hemoglobin polymerization leading to a symptomatic disorder. Sickle cell anemia (Hb SS) accounts for 60%-70% of sickle cell disease in the US.The other forms of sickle cell disease result from coinheritance of Hb S with other abnormal globin β chain variants, the most common forms being sickle-hemoglobin C disease (Hb SC) and two types of sickle β-thalassemia (Hb Sβ+-thalassemia and Hb Sβ°-thalassemia).
Note: The β-thalassemias are divided into β⁺-thalassemia, in which reduced levels of normal β-globin chains are produced, and β°-thalassemia, in which there is no β-globin chain synthesis.Other globin β chain variants such as D-Punjab and O-Arab also result in sickle cell disease when coinherited with Hb S.Most individuals with sickle cell disease are healthy at birth and become symptomatic later in infancy or childhood after fetal hemoglobin (Hb F) levels decrease and hemoglobin S (Hb S) levels increase. The diagnosis of sickle cell disease is suspected in infants or young children with painful swelling of the hands and feet (dactylitis or "hand-foot syndrome"), pallor, jaundice, pneumococcal sepsis or meningitis, severe anemia with splenic enlargement, or acute chest syndrome. Note: With the initiation of universal testing of newborns in the US, the diagnosis is made primarily at birth with the goal of assuring referral to specialty care prior to the onset of symptoms.TestingHematologic Testing Table 1 summarizes the relative quantity of hemoglobins observed by age six weeks and typical hematologic studies by age two years for the four most common sickle cell diseases.Table 1. Sickle Cell Disease: Diagnostic Test ResultsView in own windowAbnormal Globin β Chain Variants ^{1Hemoglobins Identified by Age Six Weeks 2 PhenotypeHematologic Studies by Age Two YearsMCV 3Hb A₂(%) 4SS (βSβS)Hb F, Hb SHemolysis and anemia by age 6-12 monthsN <3.6% S β°-thal (βSβ°)↓>3.6% 5 S β+-thal (β+βS)Hb F, Hb S, Hb AMilder hemolysis and anemia N or ↓>3.6% 5SC (βSβC)Hb F, Hb S, Hb C<3.6%Table shows typical results; exceptions occur. Some rare genotypes (e.g., SD, SOArab, SCHarlem, Lepore, E) are not included. thal = thalassemia MCV = mean corpuscular volumeN = normal ↑= increased ↓= decreased1. The β-thalassemias are divided into β+-thalassemia, in which reduced levels of normal β-globin chains are produced, and β°-thalassemia in which there is no β-globin chain synthesis.2. Hemoglobins reported in order of quantity (e.g., FSA = F>S>A)3. Normal MCV: ≥70 at 6-12 months; ≥72 at 1-2 years; ≥81 in adults4. Hb A₂ results vary somewhat depending on laboratory method.5. Hb SS with coexistent β-thalassemia causes ↓MCV and often leads to an Hb A₂ >3.6%.The diagnosis of sickle cell disease is established by demonstrating:The presence of significant quantities of Hb S by high-performance liquid chromatography, isoelectric focusing, or (less commonly) cellulose acetate or citrate agar electrophoresis; and The lack of a normal β-globin gene (see Molecular Genetic Testing). A complete blood count (CBC) and measure of iron status (e.g., zinc-protoporphyrin) help distinguish between specific diagnostic entities.High-performance liquid chromatography (HPLC)Readily separates some proteins that cannot be resolved by other means;Allows for accurate quantification of normal and variant hemoglobins at low concentrations, enabling differentiation of Hb Sβ+-thalassemia from sickle cell trait (Hb AS), as well as identification of compound heterozygous disorders such as Hb S-HPFH (hereditary persistence of fetal hemoglobin) and Hb SC;Does not identify Hb Sβ°-thalassemia (identification requires DNA testing or additional laboratory studies).Isoelectric focusing (IEF)Capable of higher resolution than other hemoglobin electrophoresisRoutine isoelectric focusing provides an efficient platform for high-throughput screening and thus is often used for newborn screening, but is less quantitative than HPLC.Capillary isoelectric focusing technology allows for separation of very small samples, quantification, and automation of sampling.Cellulose acetate and citrate agar electrophoresisUseful for quick screening of a small number of samplesProtein bands are relatively wide and many abnormal hemoglobins overlap.Quantitative densitometry of abnormal hemoglobins is inaccurate at low concentrations (i.e., Hb A2, Hb F).Peripheral blood smearSickle cells, nucleated red blood cells, and target cells may be seen. Other abnormal forms may be present depending on the specific genotype.Presence of Howell-Jolly bodies indicates hyposplenism.Neutrophil and platelet numbers are often increased.Kleihauer-Betke test. This acid-elution test detects the presence of cells with high fetal hemoglobin content and can be used to characterize coexistent hereditary persistence of fetal hemoglobin (HPFH) with sickle cell disease.The solubility test (i.e., Sickledex, Sickleprep, or Sicklequik) utilizes the relative insolubility of deoxygenated Hb S in solutions of high molarity. Hemolysates containing Hb S precipitate in the test solution, while those without Hb S remain in solution. The solubility test has no place in the diagnosis of sickle cell disease because: It does not differentiate sickle cell disease from sickle cell trait (Hb AS); False positives have been reported [Hara 1973]; High levels of Hb F may cause false negative results in neonates with sickle cell disease; andIt may miss some clinically significant forms of sickle hemoglobinopathies (e.g., Hb SC) [Fabry et al 2003].Newborn screening. Because of the high morbidity and mortality of sickle cell disease in undiagnosed toddlers, all 50 US states, the District of Columbia, Puerto Rico, and the Virgin Islands currently provide universal newborn screening for sickle cell disease. The vast majority of new cases are diagnosed at birth, allowing referral to specialty care prior to the onset of symptoms.International screening is limited: the United Kingdom and Bahrain screen regularly, while other countries such as France do limited screening. In sub-Saharan Africa where the incidence of the sickle mutation is very high, screening is minimal, costly, and regional [Tshilolo et al 2008].The majority of newborn screening programs perform isoelectric focusing of an eluate of dried blood spots. A few programs use HPLC, DNA testing, or cellulose acetate electrophoresis as the initial screening method (genes-r-us.uthscsa.edu).Hemoglobins identified by newborn screening are generally reported in order of quantity. For example, more fetal hemoglobin (Hb F) than adult hemoglobin (Hb A) is present at birth; thus, most infants show Hb FA on newborn screening.Specimens with abnormal screening results are retested using a second, complementary electrophoretic technique, HPLC, citrate agar, IEF, or DNA-based assay (genes-r-us.uthscsa.edu).Infants with hemoglobins that suggest sickle cell disease or other clinically significant hemoglobinopathies (Table 1) require confirmatory testing of a separate blood sample by age six weeks.Molecular Genetic TestingGene. The term sickle cell disease encompasses a group of symptomatic disorders associated with mutations in HBB and defined by the presence of hemoglobin S (Hb S; Glu6Val mutation). Sickle cell anemia (also known as homozygous sickle cell disease and Hb SS) accounts for 60%-70% of sickle cell disease in the US (genes-r-us.uthscsa.edu).Sickle cell disease may also result from coinheritance of the HBB Glu6Val hemoglobin S mutation with a second HBB mutation associated with another abnormal hemoglobin variant including:Hemoglobin C (Hb C; Glu6Lys mutation): sickle-hemoglobin C disease (Hb SC)β-thalassemia mutations: Sβ+-thalassemia and Sβ°-thalassemiaHemoglobin D (D-Punjab; Glu121Gln mutation)Hemoglobin O (O-Arab; Glu121Lys mutation)Clinical testingTargeted mutation analysis identifies HBB mutations Glu6Val (associated with hemoglobin S), Glu6Lys (hemoglobin C), Glu121Gln (hemoglobin D), and Glu121Lys (hemoglobin O-Arab). Testing for any of the large number of β-thalassemia mutations and other HBB mutations associated with other specific hemoglobin variants is also possible (see β-Thalassemia).
Note: The Hb S mutation (Glu6Val) destroys the recognition sites for the restriction enzymes MniI, DdeI, MstII, and others, making it easily detectable by restriction fragment length polymorphism (RFLP) analysis. Increasingly, a variety of PCR-based techniques are being used to identify the Hb S mutation.Sequence analysis. HBB sequence analysis may be used when targeted mutation analysis is uninformative or as the primary test to detect mutations associated with β-thalassemia hemoglobin variants.Table 2. Summary of Molecular Genetic Testing Used in Sickle Cell DiseaseView in own windowGene Symbol Test MethodMutations DetectedMutation Detection Frequency by Test Method 1Test AvailabilityHBBTargeted mutation analysisHemoglobin S (Glu6Val)See footnote 2Clinical Hemoglobin C (Glu6Lys)Clinical Hemoglobin D (Glu121Gln) 3Clinical Hemoglobin O (Glu121Lys)Clinical Sequence analysis 4Sequence variants 4, 5See footnote 6Clinical1. The ability of the test method used to detect a mutation that is present in the indicated gene2. Varies depending on ethnicity of the individual. In individuals known to have SCD by hematologic studies, the hemoglobin C (Glu6Lys) mutation is more common in those of western African descent; the hemoglobin D (Glu121Gln) mutation is more common in those of Mediterranean and Indian descent but also to a lesser extent in those with Thai and Turkish backgrounds [Atalay et al 2007]; the hemoglobin O-Arab (Glu121Lys) mutation is most common in those of Middle Eastern descent as well as Greek Pomaks [Zimmerman et al 1999]. These mutations have been described in a wide population distribution reflecting migration patterns of founder populations throughout the world.3. D-Punjab, also known as D-Los Angeles (globin​.cse.psu.edu) 4. Most appropriate for Hb S β-thalassemia. Includes variants detected in targeted mutation analysis in addition to other pathologic HBB variants. β-thalassemia mutations are almost exclusively point mutations in the β-globin coding sequence but can also be mutations in the promoter region affecting the level of gene expression.5. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations; typically, exonic or whole-gene deletions/duplications are not detected. 6. Detects most, but not all, sequence variants associated with β-thalassemia; some deletions will be missed.Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Information on specific allelic variants may be available in Molecular Genetics (see Table A. Genes and Databases and/or Pathologic allelic variants).Testing StrategyTo confirm/establish the diagnosis in a proband. The multiple testing strategies possible vary depending on the specific diagnosis, proband's age, family history, and availability of parents for testing. Strategies should take into account assessment of modifying factors, such as the coexistence of α-thalassemia.Newborns. When initial screening detects a clinically significant hemoglobinopathy, the result should be confirmed within six weeks with the same or, preferably, a complementary method.For compound heterozygotes (e.g., Hb SC, SD, or SO) a repeat test is adequate.Newborns with Hb F>Hb S could have homozygous sickle cell (Hb SS), Sβ°-thalassemia, or Sβ+-thalassemia with a low level of Hb A. These hemoglobinopathies can be difficult to distinguish in the newborn period when 95% of hemoglobin is Hb F. Further testing for these infants, as well as newborns diagnosed with Sβ+-thalassemia can include molecular testing and/or hematologic testing of parents. Alternatively, some diagnoses are easier to make with increased age. Timing is in part dependent on the genetic counseling needs of the parents.Infants about age one year. Regardless of the outcome of testing in the newborn period, additional testing that should be done at about age one year (once Hb F levels have fallen) includes: a CBC, reticulocyte count, some type of electrophoresis or HPLC, a measure of iron status, and inclusion body preparation with BCB (brilliant cresyl blue) stain. Together these help determine if there is a coexisting thalassemia component, and if so, if it is α-thalassemia or β-thalassemia. This is important for genetic counseling and for providing insight into disease-specific outcomes.Individuals over age one year. A one-time assessment as described for Infants about age one year should be done.Carrier testing for at-risk relatives is commonly accomplished by HPLC to screen for abnormal hemoglobins. Other methods such as IEF and molecular genetic testing may also be used and vary by laboratory.Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing alleles in the family. Genetically Related (Allelic) DisordersBeta-thalassemia is defined by an imbalance of α- and β-globin synthesis. Most commonly this is caused by HBB mutations that result in decreased or absent production of β-globin (hemoglobin A), although this can also occur with excessive α-chain production, as with the presence of triplicated α-globin genes. More than 200 different β-thalassemia mutations have been described.Alpha-thalassemia is often coinherited with the Hb S mutation (Glu6Val). Alpha-thalassemia mutations generally arise as a consequence of large deletions in one or both α-globin genes and are detected by PCR techniques. Testing for coexisting α-thalassemia is typically considered only in the presence of a low MCV in homozygous SS disease or SC disease.}

Natural HistoryThe clinical manifestations of sickle cell disease (SCD) result from intermittent episodes of vascular occlusion leading to tissue ischemia/reperfusion injury and variable degrees of hemolysis, both of which contribute to multiorgan dysfunction. The severity of disease manifestations varies from severe to minimal, even in individuals with the same HBB mutation status. Previously, median survival in the US for those with SCD was estimated to be age 42 years for men and age 48 years for women [Platt et al 1994]; however, survival of a subset of individuals with SCD to over age 60 has been described [Wierenga et al 2001, Serjeant et al 2007]. In addition, some evidence shows a shift towards older age at death over the last 20 years, marked by significant decreases in death rates during childhood [Hassell 2010]. The main causes of death are infection, acute chest syndrome, pulmonary artery hypertension, and cerebrovascular events [Steinberg et al 2003, Bakanay et al 2005]. Causes of death in children tend to differ from those in adults. Children have higher rates of death from infection and sequestration crises whereas adult mortality is related to chronic end-organ dysfunction, thrombotic disease, and treatment-related complications [Manci et al 2003]. Disease ComplicationsThe hallmarks of SCD-related disease complications are the result of chronic hemolysis and intermittent vaso-occlusive episodes. In addition, immune dysregulation and hypercoagulability contribute to disease complications. Chronic hemolysis is associated with chronic anemia as well as vascular dysfunction [Morris 2011]. Individuals with the highest rates of hemolysis, as measured by plasma levels of lactate dehydrogenase (LDH), are susceptible to developing pulmonary artery hypertension, priapism, and leg ulcers [Taylor et al 2008]. Vaso-occlusive events are associated with ischemia/reperfusion damage to tissues that lead to pain and acute or chronic injury affecting any organ system. The bones/marrow, spleen, liver, brain, lungs, kidneys, and joints are often affected [Vichinsky 2002]. Common SCD complications include the following:Dactylitis (pain and/or swelling of the hands or feet) is often the earliest manifestation of SCD. The dorsa of the extremities are most often involved; one or all four extremities can be involved. When present, dactylitis usually occurs in infants and children. Although immediate sequelae are rare, dactylitis has been implicated as a risk factor for severe disease [Miller et al 2000].Splenic sequestration is characterized by an acutely enlarging spleen with hemoglobin more than 2 g/dL below the affected individual's baseline value. Mild-to-moderate thrombocytopenia may also be present. Splenic sequestration occurs in 10%-30% of children with sickle cell disease, most commonly between age six months and three years, and may follow a febrile illness. Children with splenic sequestration may experience abdominal pain, nausea, and vomiting. Splenectomy may be required; severe splenic sequestration may progress rapidly to shock and death.Aplastic crisis is characterized by an exacerbation of the individual's baseline anemia as a result of inadequate production of red blood cells (RBCs). Individuals with SCD are dependent on increased production of new red blood cells (reticulocytes) to compensate for the shortened life span of sickle RBCs. Most aplastic crises are caused by acute infection such as by parvovirus B19, resulting in substantially decreased reticulocyte production, typically to less than 1%, which results in transient red cell aplasia and profound anemia. Aplastic crisis is more common in children than adults. Vaso-occlusive pain episodes are the most frequent cause of recurrent morbidity in sickle cell disease and account for the majority of sickle cell disease-related hospital admissions as well as school and work absences. Vaso-occlusion occurs as a result of formation of multicellular aggregates that block blood flow in small blood vessels, depriving downstream tissues of nutrients and oxygen. The end result is tissue ischemia and tissue death in the affected vascular beds. Severe pain, often requiring narcotic analgesia, results from vaso-occlusion and ischemic tissue damage. Young children more often complain of pain in their extremities, whereas older individuals more commonly experience pain in the head, chest, abdomen, and back.Acute chest syndrome (ACS) is a form of acute lung injury in individuals with a sickle hemoglobinopathy and is a major cause of mortality [Bakanay et al 2005]. ACS is a clinical diagnosis involving the presence of a new pulmonary infiltrate on chest radiography. Definitions vary but it is often defined in combination with respiratory tract symptoms, hypoxemia, and/or fever. ACS often develops in the setting of a vaso-occlusive episode or with other acute manifestations of sickle cell disease, frequently after two to three days of severe vaso-occlusive pain. ACS can progress rapidly (over several hours to days) to requiring intubation and mechanical ventilatory support. A high index of suspicion is indicated: the presenting signs and symptoms of ACS can be highly variable and affected individuals may have an initial normal physical examination [Morris et al 1999]. Multiple etiologies (e.g., fat emboli from bone marrow infarcts, infection [particularly community-acquired pneumonia, mycoplasma pneumonia, Chlamydia pneumoniae, and viral pneumonia], pulmonary infarction, and pulmonary embolus), often at the same time, can lead to acute chest syndrome [Vichinsky et al 2000, Dessap et al 2011].Neurologic complications in SCD include stroke, silent cerebral infarcts, cerebral hemorrhage, cerebral blood flow abnormalities including Moyamoya disease, and cerebral microvascular disease. It is estimated that up to 50% of individuals with SCD will manifest some degree of cerebrovascular disease by the age of 14 [Bernaudin et al 2011]. Ischemic strokes, most often seen in children and older adults [Ohene-Frempong et al 1998], are among the most catastrophic manifestations of sickle cell disease. Common presenting signs and symptoms include: hemiparesis, monoparesis, seizures, aphasia or dysphasia, cranial nerve palsies, and mental status changes. Overt strokes occur in as many as 11% of children with sickle cell disease, with the peak occurrence between ages two and nine years. Elevated flow velocity on transcranial Doppler (TCD) has been identified as a reversible risk factor for stroke in children [Adams et al 1998]. Silent cerebral infarcts occur in approximately 22%-35% of individuals with SCD [Pegelow et al 2002, Bernaudin et al 2011]. Silent cerebral infarcts are lesions identified on cerebral imaging studies without known focal neurologic symptoms. Silent infarcts have been associated with neurocognitive deficits [Schatz et al 2001] and risk for overt stroke [Miller et al 2001]. Thus, a “silent infarct” should not be thought of as a clinically insignificant condition.Complications related to hemolysis. A hyper-hemolysis syndrome is associated with leg ulcers, priapism, and pulmonary artery hypertension. Other consequences of hemolysis include: chronic anemia, jaundice, predisposition to aplastic crisis, and cholelithiasis. Those with the highest rates of hemolysis, however, are relatively protected from vaso-occlusive pain.Pulmonary hypertension. Pulmonary artery hypertension (PAH) affects approximately 6%-35% of adults with SCD [Gladwin et al 2004, Ataga et al 2006, Parent et al 2011]. In SCD, PAH has been defined by an elevated tricuspid regurgitant jet velocity (TRV) on transthoracic echocardiography (TTE). However, subsequent studies using direct measurement of PAP (pulmonary arterial pressure) by right heart catheterization indicate an overestimation of PAH by TTE [Parent et al 2011]. PAH is associated with markedly increased mortality [Gladwin et al 2004, Ataga et al 2006, De Castro et al 2008]. PAH is also associated with significant morbidity including exercise intolerance [Sachdev et al 2011]. Risk factors for PAH include markers of increased hemolysis such as LDH [Kato et al 2006] and markers of cardiac strain such as brain natriuretic peptide (BNP) [Machado et al 2006]. Some individuals are relatively asymptomatic in the early stages of PAH. The prevalence and clinical significance of PAH in children with SCD is under investigation; prevalence is thought to be high [Dahoui et al 2010, Colombatti et al 2010]. Priapism (painful, unwanted erections) commonly occurs in males with sickle cell disease, often starting in childhood and often occurring during the early morning hours. Males may have intermittent episodes of priapism lasting fewer than two to four hours (“stuttering priapism”), which are often recurrent and may precede a more severe and persistent episode. Severe episodes lasting more than two to four hours need rapid intervention because prolonged priapism may result in permanent tissue damage and impotence [Rogers 2005].Additional ComplicationsInfection. Individuals with sickle cell disease develop splenic dysfunction as early as age three months; thus, young children with sickle cell disease are at high risk for septicemia and meningitis with pneumococci and other encapsulated bacteria including Neisseria meningiditis and Haemophilus influenza. The single most common cause of death in children with sickle cell disease is Streptococcus pneumoniae sepsis, with the risk of death being highest in the first three years of life. As most children with SCD are vaccinated against these organisms and are begun on prophylactic penicillin, the incidence of these infections has decreased [Adamkiewicz et al 2003]. Individuals with sickle cell disease are also at increased risk for other infections such as osteomyelitis caused by Staphylococcus aureus or other organisms such as Salmonella species. Infectious agents implicated in acute chest syndrome include Mycoplasma pneumoniae, Chlamydia pneumoniae, and Streptococcus pneumonia, as well as viruses. Parvovirus remains an important cause of aplastic crisis. Indwelling central venous catheters confer a high risk of bacteremia in individuals with SCD [Chulamokha et al 2006, Zarrouk et al 2006].Other complications of sickle cell disease include: avascular necrosis of the femoral head, nephropathy, restrictive lung disease, cholelithiasis, retinopathy, cardiomyopathy, and delayed growth and sexual maturation. Individuals with hemoglobin SC disease are at particularly high risk for retinopathy [Powars et al 2002]. Cardiopulmonary complications represent a major mortality risk in adults [Fitzhugh et al 2010]. Individuals who receive frequent red blood cell transfusion can develop problems with iron overload with tissue iron deposition potentially damaging the liver, lungs, and heart [Kushner et al 2001] and alloimmunization that may interfere with the ability to obtain fully matched units of blood for transfusion [Vichinsky et al 1990].Heterozygotes for Hb S have hemoglobin AS (Hb AS) (also called sickle cell trait). Heterozygous individuals are not anemic and have normal red cell indices with hemoglobin S percentages typically near 40%. In regions of the world where malaria is endemic, Hb AS confers a survival advantage in childhood malaria; this is thought to be a major selective pressure for persistence of the Hb S mutation (Glu6Val).The amount of Hb S present is insufficient to produce sickling manifestations under normal circumstances and, thus, these individuals are usually asymptomatic but are at risk for several complications [Key & Derebail 2010]: Extremes of physical exertion, dehydration, and/or altitude can induce sickle cell vaso-occlusive events in some individuals with hemoglobin AS [Mitchell 2007]. It is generally recommended that individuals with known Hb AS maintain aggressive hydration during extremes of physical exertion, with no formal activity restrictions recommended. The increasing awareness of a low but significant risk for pulmonary emboli, exertional rhabdomyolysis, and sudden death with extreme exertion in individuals with HbAS has led to the mandatory offering of testing to all NCAA Division I college athletes [Bonham et al 2010]. The implications of this policy are unclear, and the role of genetic counseling in this setting is complex [Aloe et al 2011].Some people with sickle cell trait have impaired renal concentrating abilities and may have abnormal laboratory findings, such as intermittent microhematuria. Renal medullary carcinoma is an extremely rare form of kidney cancer that almost exclusively occurs in individuals with sickle cell trait [Swartz et al 2002, Hakimi et al 2007] such that a high index of suspicion for this rare diagnosis should be given for individuals with sickle cell trait who present with hematuria.HB AS may be associated with an increased risk for venous thromboembolism [Austin et al 2007].

Genotype-Phenotype CorrelationsAlthough a tremendous amount of individual variability occurs, individuals with Hb SS and Sβ°-thalassemia are generally more severely affected than individuals with Hb SC or Sβ⁺-thalassemia. Molecular and genetic factors that are responsible for this variability are being investigated [Steinberg & Adewoye 2006]. While several groups have identified variants associated with altered risk for specific complications, their role in clinical management has not been determined. Examples include genetic correlates of HbF levels [Galarneau et al 2010, Bhatnagar et al 2011], leg ulcers [Nolan et al 2006], renal nephropathy [Ashley-Koch et al 2011], stroke [Flanagan et al 2011], and disease severity [Sebastiani et al 2010].HBB Glu6Val haplotypes correlate with disease severity, with the Sen (Senegal) haplotype being the mildest, the CAR/Bantu haplotype the most severe, and the Ben (Benin) haplotype of intermediate severity [Powars 1991]. In the US, increased disease severity likely relates to an increased prevalence of the Bantu haplotype in African Americans [Solovieff et al 2011]. In practice, these haplotypes are not routinely determined and more objective predictors of disease severity are used when available. The presence of α-thalassemia may modify sickle cell disease severity. In general, α-thalassemia improves red cell survival and decreases hemolysis in the sickle cell disease syndromes. However, the clinical effect on SCD is unclear and can be variable including possible decreased complications arising from hemolysis and potentially increased complications from vaso-occlusive events [Steinberg 2005].In individuals with Hb SC:Longer red cell life span and higher hemoglobin concentration are associated with fewer vaso-occlusive pain episodes.Splenomegaly and the accompanying risk of splenic sequestration can persist well beyond early childhood.Proliferative retinopathy and avascular necrosis are more likely to develop than in those with other sickle hemoglobinopathies.

Differential DiagnosisOnce the presence of Hb S has been confirmed, the differential diagnosis is among clinically significant, less significant, and carrier states.Clinically significantHomozygous S/S (i.e., Hb SS)Compound heterozygotes, including but not limited to:Hb SC, Hb SD, Hb SO-ArabHb Sβ°-thalassemiaSome forms of Hb Sβ⁺-thalassemiaLess clinically significantSome forms of Hb Sβ⁺-thalassemiaHb SECarrier state. Sickle cell trait (Hb AS)Note to clinicians: For a patient-specific ‘simultaneous consult’ related to sickle cell disease, go to , an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).

ManagementEvaluations Following Initial DiagnosisEvaluations to establish the extent of end-organ damage associated with sickle cell disease vary with the age and clinical status of the individual:Newborns. Confirmation of diagnosis. See Molecular Genetic Testing.Infants age 9-12 months (usually when fetal hemoglobin levels have fallen to adult levels). Reticulocyte count, CBC, measurement of Hb F (%), and assessment of iron statusOlder individuals. See Surveillance.Treatment of ManifestationsLifelong comprehensive care is necessary to minimize morbidity, reduce early mortality, and maximize quality of life [NHLBI 2002].Education of parents, caregivers, and affected individuals is essential:Families must appreciate the importance of routine health maintenance visits, prophylactic medications, and early intervention for both acute and chronic complications.Warning signs of acute illness such as fever, respiratory symptoms, pallor, lethargy, splenic enlargement, and neurologic changes must be reviewed regularly.All families should have a plan in place for 24-hour access to a medical facility that can provide urgent evaluation and treatment of acute illnesses such as fever, acute chest syndrome, and splenic sequestration.A plan to deal with mild-to-moderate episodes of pain should be in place.Families should be provided baseline (steady state) laboratory values for purposes of comparison, as values often change during acute illness [NHLBI 2002].General management of specific problems [Benjamin et al 1999, Gladwin & Rodgers 2000, Walters et al 2000, NHLBI 2002, Rees et al 2003, Lottenberg & Hassell 2005, NHS 2010]:Vaso-occlusive pain episodes including dactylitisMany uncomplicated episodes of pain can be managed at home with oral hydration and oral analgesics including nonsteroidal anti-inflammatory drugs (NSAIDs) and opiates.More severe episodes of pain require hospitalization and administration of parenteral fluids and analgesics. Optimal analgesia is generally achieved with morphine, or other opiate, given around-the-clock or by patient-controlled analgesia.NSAIDs such as ketorolac, ibuprofen, naproxen, and/or acetaminophen may be used to augment the analgesic effect of opiates and to decrease inflammation that is part of the pathophysiology.Adequate but not excessive hydration with hypotonic fluids should be used to maintain euvolemia, and individuals should be monitored closely for the development of other complications such as acute chest syndrome or splenic sequestration.A thorough evaluation for infection, including blood culture, urine culture, and chest x-ray should be performed based on the clinical scenario.Pain episodes are additionally managed with a multi-model approach that may include warmth, massage, distraction, acupuncture, biofeedback, and self-hypnosis.Infection/fever. All affected individuals with temperature greater than 38.3° C or persistent temperature elevation above baseline require rapid triage and physical assessment, urgent CBC and reticulocyte count, blood culture (other cultures should be obtained as clinically indicated), and chest x-ray.
Individuals with fever should be given parenteral broad-spectrum empiric antibiotics such as ceftriaxone pending culture results:A macrolide antibiotic should be added if pneumonia / acute chest syndrome is a concern.Additional antibiotics should be added only for proven or suspected meningitis or other severe illness.Acute chest syndrome (ACS). The index of suspicion for acute chest syndrome should be high when individuals with SCD have fever, chest pain, or respiratory signs or symptoms. Given the high mortality associated with ACS, an aggressive multimodal treatment strategy should be initiated [Miller 2011].Because physical signs are variable (and can be absent), the threshold for obtaining a chest x-ray should be low [Morris et al 1999].Those suspected of having ACS should be aggressively treated with oxygen, analgesics, and antibiotics (including a macrolide). Incentive spirometry should be encouraged. Hypoxemia can progress to need for intubation and mechanical ventilatory support. Simple transfusion or exchange transfusion may be necessary. Aplastic crisis. Monitoring of hematocrit (both absolute and compared with the individual's baseline), reticulocyte count, and cardiovascular status are required. Transfusion support may be necessary. Most cases caused by parvovirus B19 will spontaneously resolve; however, if the reticulocyte count does not improve, intravenous gammaglobulin can be considered to assist in viral clearance. Any sibs or other close contacts of an individual with SCD should be monitored for red blood cell aplasia because the virus is easily transmissible. Splenic sequestration. Severe episodes of splenic sequestration may progress rapidly to cardiovascular collapse and death; thus, emergency transfusion is indicated when signs of cardiovascular instability are present. Individuals who experience multiple or severe episodes of splenic sequestration may require splenectomy.Pulmonary hypertension. No consensus regarding the optimal management for pulmonary hypertension exists; however, the following approach is reasonable:Optimization of sickle cell disease-related therapy to stop progression (e.g., chronic transfusions, hydroxyurea, and oxygen therapy if hypoxemic)Aggressive evaluation and treatment of additional etiologies contributing to pulmonary hypertension (e.g., thrombotic disease, obstructive sleep apnea)Note: Sildenafil is not currently recommended in the treatment of PAH in SCD. A recent large trial of sildenafil (a phosphodiesterase inhibitor hypothesized to be therapeutic by increasing nitric oxide levels) in SCD-associated pulmonary hypertension was recently terminated early due to increased pain episodes in the treatment arm [Machado et al 2011]. Stroke. Any history of an acute neurologic symptom or event warrants emergent evaluation including a CBC with reticulocyte count and a non-contrast CT scan. CNS hemorrhage requires immediate neurosurgical consultation. An MRI/MRA to define injury should be obtained as soon as available, but treatment should never be delayed for these results.
Treatment for children with acute ischemic stroke includes the following:Monitor neurologic status and aggressively treat increased intracranial pressure and seizures, if present.Exchange transfusion with the goal of decreasing Hb S percentage to less than 30% of the total hemoglobin followed by a chronic transfusion program can significantly decrease stroke risk [Wang et al 2000]. Without continued therapy as many as 60%-90% of individuals who have had a stroke have a second stroke within three years. Thus, in most cases, a preventive chronic transfusion protocol is initiated after a CNS event and continued indefinitely [Adams et al 2005] (see Prevention of Primary Manifestations).
No consensus regarding the management of individuals with silent infarcts exists. Priapism. Episodes of severe priapism require urgent evaluation and treatment, including hydration and analgesia, and may require aspiration and irrigation by a urologist [Mantadakis et al 2000].Prevention of Primary ManifestationsOngoing education for all individuals with SCD is essential to help minimize morbidity and mortality. Education includes a regular review of interventions including:Maintaining hydration and avoiding extremes of climateMonitoring for signs and symptoms requiring acute medical interventionEarly detection of chronic complicationsUpdates on new therapiesDisease-modulating therapies are reviewed by Vichinsky [2002] and Wang [2007]. Chronic red blood cell transfusion therapy. The initial goal of chronic red blood cell transfusion therapy is to maintain the percentage of Hb S below 30% and suppress reticulocytosis.Chronic red blood cell transfusion therapy may be warranted for the following [Wanko & Telen 2005, Josephson et al 2007, Ware 2007, Wahl & Quirolo 2009, Wun & Hassell 2009]:Primary prevention of stroke in individuals with an abnormal transcranial DopplerPrevention of stroke recurrenceTreatment of chronic pain refractory to other therapiesPulmonary hypertensionChronic renal failureRecurrent episodes of ACSSevere end-organ damageComplications of chronic red blood cell transfusion therapy include: iron overload, alloimmunization, and, rarely, infection. To limit alloimmunization and transfusion reactions, extended matching of red blood cell antigens should be performed and blood products should be leukoreduced (removal of white blood cells from the transfusion product). Red blood cells antigen matched at the full Rh locus (D, C, E) and Kell have been suggested to decrease alloimmunization rates, as well as other alleles when possible [Castro et al 2002, Lasalle-Williams et al 2011].Hydroxyurea, the most prescribed therapy for sickle cell disease, may benefit individuals with SCD via several mechanisms [Platt 2008]:Induction of Hb F synthesis resulting in decreased sickling and improved red-cell survivalLowering the white blood cell (WBC) count and platelet countMetabolism into nitric oxide, a potent vasodilatorReducing vascular inflammationAdults treated with hydroxyurea have significantly fewer acute painful episodes, fewer episodes of acute chest syndrome, decreased need for transfusion, and, most importantly, improved survival [Steinberg et al 2010, Voskaridou et al 2010, Smith et al 2011]. A similar benefit has been reported in children, and the use of hydroxyurea in childhood is now gaining more acceptance [Wang et al 2011]. It is not clear if hydroxyurea prevents the cerebrovascular complications of sickle cell disease; current clinical trials are underway to investigate this [Ware et al 2011]. Individuals treated with hydroxyurea must be monitored closely for significant myelosuppression by routine CBC. As noted above, while this is a potential toxicity, a decreased white blood cell (WBC) count is a major mechanism of action of the drug.Stem cell transplantation from a normal donor or one with sickle cell trait can be curative in individuals with sickle cell disease. The risks and morbidity associated with this procedure have limited its use to a select group of individuals with (1) significant complications, most often a history of cerebrovascular events, and (2) a matched sib stem cell donor [Walters et al 2000]. Among these individuals, more than 90% survive; and approximately 85% survive free from sickle cell disease [Bernaudin et al 2007, Panepinto et al 2007]. With the development of less toxic transplant regimens, stem cell transplant is becoming a more acceptable option for more individuals even absent severe end-organ damage [Hsieh et al 2011, Khoury & Abboud 2011]. This has led to the successful transplant of select adults, though they have required prolonged immunosuppression. Improvements in immunosuppressive regimens and management of graft-vs-host disease and other transplant-related complications are also increasing the number of individuals for whom transplantation is an option [Shenoy 2007, Bhatia & Walters 2008]. However, it is estimated that fewer than 30% of individuals with sickle cell disease have suitable matched sibling donors and fewer than 60% have suitable matched unrelated donors; thus the use of alternate donors is an active area of research [Krishnamurti et al 2003, Ruggeri et al 2011].Because the criteria, risks, and benefits of transplantation are changing rapidly, it is important for families and providers to discuss the risks and benefits with a transplantation center.Prevention of Secondary ComplicationsNewborn screening [Vichinsky et al 1988] has made presymptomatic diagnosis possible, allowing for early, aggressive education on management issues, such as management of fevers. The use of prophylactic penicillin [Powars et al 1981] and immunization have significantly decreased morbidity and mortality in children, primarily by reducing deaths from sepsis.Penicillin prophylaxis prevents 80% of life-threatening episodes of childhood Streptococcus pneumoniae sepsis [Gaston et al 1986]: By age two months, all infants with sickle cell disease should receive penicillin V potassium prophylaxis, 125 mg orally, twice a day.At age three years, the dose is increased to 250 mg orally, twice a day, and then continued until at least age five years.Erythromycin prophylaxis is an alternative for individuals allergic to penicillin.Folic acid supplementation should also be considered because of increased RBC turnover.Immunizations. Timely administration of vaccines is essential. Updated guidelines appear yearly for pediatric [Centers for Disease Control and Prevention 2012] and adult vaccination [Centers for Disease Control and Prevention 2011]. These include: Hemophilus influenzae type b (Hib) vaccineNeisseria meningitidis vaccine13-valent pneumococcal conjugate vaccine (PCV13) 23-valent pneumococcal polysaccharide vaccineAnnual influenza immunization is recommended.Iron overload. Individuals receiving transfusions are at risk for iron overload and should be monitored closely, initially by tracking the amount of blood transfused and monitoring serum ferritin concentration. Those with high exposures or documented iron overload should have an assessment of organ iron accumulation. With its increasing availability and safety and the ability to assess iron in multiple organs while avoiding sampling bias, quantitative radiographic evaluation is increasingly replacing biopsy [Vichinsky 2001, Wood 2007]. Iron chelation therapy is recommended for those with evidence of tissue iron deposition.SurveillanceSurveillance should be tailored to a specific individual's clinical history; however, most individuals benefit from routine age-dependent screening to allow for early detection and treatment of end-organ damage [NHLBI 2002, NHS 2010]. The following are general guidelines:Routine. Developmental and/or neurocognitive assessments; social work assessments with emphasis on support, resources, and impact of the disease on lifestyle; nutritional and dental evaluationsYearly. A CBC and reticulocyte count, assessment of iron status, liver function tests (LFTs), BUN, serum concentration of creatinine (Cr), and urinalysis (UA)Yearly starting at age two to three years. For all individuals with Hb SS and Hb Sβ°-thalassemia (as well as some others), transcranial Doppler (TCD) by a person certified to record velocity of arterial blood flow for comparison to national studies to determine the risk of stroke. Individuals with an abnormally high arterial blood flow velocity have a high rate of stroke, which can be prevented by chronic red blood cell transfusion therapy. Children with normal velocities require yearly reevaluation as a proportion convert to higher risk [Adams et al 2004]. Initial studies suggest that this approach is decreasing the incidence of overt stroke in individuals with SCD [Mazumdar et al 2007].Yearly starting at age seven years. Chest x-ray, pulmonary function tests (PFTs), and abdominal ultrasound examinationFor older individuals or individuals of any age with cardiac or pulmonary concerns. Typically, echocardiogram to determine the tricuspid regurgitation (TR) jet with consideration of right heart catheterization depending on symptoms, pulmonary function testing with six-minute walk test, and sleep study (to assess nighttime hypoxemia). Guidelines for initiation and frequency of screening have not been established.Additional studies should be tailored to the affected individual's clinical history.Agents/Circumstances to AvoidEducation for individuals with SCD involves learning how to control one's environment to minimize the chance of exacerbations. Environmental controls include avoiding the following:DehydrationExtremes of temperature (e.g., swimming in cold water, which can trigger a pain episode)Physical exhaustionExtremely high altitude without oxygen supplementationThe analgesic meperidine should be avoided as first-line therapy because of potential CNS toxicity.Evaluation of Relatives at RiskEarly diagnosis of at-risk family members may allow intervention before symptoms are present. See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Pregnancy ManagementPregnancy complications in SCD can be minimized with close follow up and attentive obstetric care. Pregnancy in women with SCD involves increased risks for thrombosis and infectious complications as well as increased acute painful episodes [Villers et al 2008]. However, the risk for preeclampsia, eclampsia, and pre-term labor is not increased above the general population risk [Smith et al 1996]. Similarly the risk for maternal death in SCD is not increased [Hassell 2005]. The risk for pregnancy complications increases with limited access to prenatal care [Sun et al 2001], reinforcing the importance of obstetric follow up in limiting complication rates. The use of chronic transfusion support has not been shown to be beneficial over intermittent simple transfusion as needed for clinical symptoms [Gilli et al 2007] with the possible exception of a small improvement in gestational age [Koshy et al 1988]. Over 99% of births to women with SCD occurring after 28 weeks’ gestation are live births with normal Apgar scores [Smith et al 1996]. Several studies have reported increased rates of low birth weight and intrauterine growth retardation in babies born to women with SCD [Smith et al 1996, Hassell 2005]. Attention to postnatal opiate withdrawal in the babies of mothers treated with high-dose opiates during pregnancy is warranted. Infants with SCD do not manifest disease symptoms in the antenatal, perinatal, nor immediate postnatal period until fetal hemoglobin production switches to adult hemoglobin.Therapies Under Investigation A number of ongoing clinical studies are evaluating new therapies for sickle cell disease; several studies are ongoing to identify novel therapeutic targets. While chronic transfusion has been used to treat many complications of SCD, it is unclear if transfusion support prevents the development of silent cerebral infarcts in children who have a history of overt stroke [Hulbert et al 2011]. A multicenter trial comparing observation vs a chronic transfusion regimen is underway. Many factors in sickle-cell-induced ischemic injury (e.g., blood vessel tone, leukocyte and platelet activity, and endothelial adhesion) are regulated by nitric oxide (NO) [Gladwin & Rodgers 2000, Hebbel 2000]. Nitric oxide bioavailability is decreased in sickle cell disease. While pilot studies treating individuals with NO initially showed beneficial effects, larger studies have not shown similar benefit. A recent study of inhaled NO in acute pain crisis did not show significant benefit [Gladwin et al 2011], and a trial to increase nitric oxide in SCD-associated pulmonary hypertension was terminated early because of increased pain episodes in the treatment arm [Machado et al 2011]. Several studies have examined whether NO production could be increased by administering L-arginine, a precursor of NO. While it does appear that L-arginine may increase NO levels in some situations, the clinical benefit remains unknown [Morris et al 2003, Sullivan et al 2010]. A recent study to boost NO production using supplemental arginine failed to alter respiratory function [Sullivan et al 2010]. A benefit of arginine butyrate was seen in a small study of patients with leg ulcers [McMahon et al 2010], but this has not yet been validated in a larger study. In general, these approaches may provide short-term, but not sustained, benefit. Other novel agents currently being investigated as potential therapies for sickle cell disease include: Drugs that increase HbF percentage including pomolidomide [Meiler et al 2011], HDAC inhibitors [Bradner et al 2010], and short chain fatty acids (i.e., butyrate or valproic acid). Both 5-azacytidine [Hagar & Vichinsky 2000, Atweh & Loukopoulos 2001] and 2’ deoxy 5’ azacytidine (decitabine) [Saunthararajah et al 2008] are being investigated as both are potent inducers of Hb F and reduce anemia in SCD.Drugs that block adhesive pathways such as anti-selectins [Chang et al 2010, Gutsaeva et al 2011]Anti-coagulants [Qari et al 2007]Anti-inflammatory pathway drugs [Knight-Perry et al 2009]Statins (because of their vascular protective effects) [Hoppe et al 2011]Modulators of the transport systems involved in cellular dehydration, including the Gardos channel inhibitor ICA-17043. While this drug improved RBC survival, there was minimal clinical improvement in pain episodes [Ataga et al 2011].Phytomedicines, including two mixtures of plants that have shown some initial promising results [Oniyangi & Cohall 2010]While not yet in clinical trials, a promising approach to therapy is the development of inhibitors to the protein Bcl11a. Bcl11a was identified as an HbF cell quantitative trait locus in a genome-wide association study [Menzel et al 2007]. Since then it has been shown to bind within the β-globin locus and to be critical for suppressing fetal globin gene expression in adult erythroid cells [Sankaran et al 2008, Sankaran et al 2010]. Knockdown or knockout of Bcl11a in model systems, as well as naturally occurring deletions that remove its binding site in humans, result in substantial increases in HbF. These elevated levels of HbF provide therapeutic benefits to individuals with both sickle cell disease and thalassemia [Sankaran et al 2011, Wilber et al 2011, Xu et al 2011]. Multiple approaches are being taken to inhibit Bcl11a function in vivo.Gene therapy. As sickle cell disease arises from a defined single nucleotide substitution in the β-globin gene whose expression is restricted to erythroid cells derived from bone marrow hematopoietic stem cells, sickle cell disease is an ideal candidate for gene therapy. Gene therapy provides the benefit of stem cell transplantation, but without the problems associated with the use of an allogenic source of stem cells. Ideally, gene therapy would lead to an increase in non-sickle β-like chains, while lowering the number of sickle chains, for example by replacing the Hb S mutation (Glu6Val) with a normal allele. Previously the primary focus had been on adding a normal β-like gene, potentially modified to have additional anti-sickling properties; increasingly, however, alternate strategies are being pursued. These include (1) using trans-activators to stimulate the minimally expressed delta gene or the fetal or embryonic genes; (2) inducing embryonic α-like chains that, when forming tetramers with sickle chains, are less likely to polymerize; or (3) gene correction (see Gene therapy using iPS cell lines.).Though the efficiency of gene transfer and obtaining high levels of stable expression are still a hurdle, successful expression of the human β-globin gene by retroviral vectors in a mouse model for sickle cell anemia has demonstrated the potential of these approaches [Levasseur et al 2003, Sadelain 2006]. New strategies for in vivo selection may be useful. Currently there are several clinical trials recruiting or poised to open. Gene therapy using induced pleuripotent stem (iPS) cell lines. New techniques for generating human embryonic stem (hES) cell lines from peripheral tissue (IPS cells) have opened the door to promising new approaches [Hanna et al 2007, Higgs 2008]. IPS cell lines are generated by the addition of several genes to cells from a skin biopsy taken from an affected individual. These IPS cells, which have tremendous expansion potential, can undergo Hb S mutation (Glu6Val) correction by homologous recombination, and then be assayed for toxic effects prior to being used clinically. Increasingly new methods are being developed to improve the efficiency of gene correction, often making use of nucleases engineered to cleave in the region of the mutation, enhancing the incorporation of a correction template. In theory, these cells could be differentiated into hematopoietic stem cells for transplantation providing a permanent cure, or differentiated into erythroid progenitors that could be transfused intermittently, which would produce new normal red blood cells while avoiding the risk of alloimmunization and iron overload. Although preliminary, these approaches provide a new paradigm likely to lead rapidly to the availability of treatment with Hb S mutation (Glu6Val)-corrected autologous cells. Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Molecular GeneticsInformation in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Sickle Cell Disease: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDHBB11p15​.4Hemoglobin subunit betaHbVar: A Database of Human Hemoglobin Variants and Thalassemias
HBB @ LOVDHBBData are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Sickle Cell Disease (View All in OMIM) View in own window 141900HEMOGLOBIN--BETA LOCUS; HBB 603903SICKLE CELL ANEMIAMolecular Genetic PathogenesisHemoglobin S is produced by a substitution in the second nucleotide of the sixth codon of HBB, resulting in the changing of the glutamic acid residue to a valine. In deoxygenated sickle hemoglobin, an interaction between the Glu6Val residue and the complementary regions on adjacent molecules can result in the formation of highly ordered insoluble molecular polymers that aggregate and distort the shape of the red blood cells, making them brittle and poorly deformable, increasing adherence to the endothelium. This can lead to veno-occlusion and potentially decreased tissue perfusion and ischemia. While this is thought to be the proximate defect leading to several aspects of clinical disease, there is increasing awareness that multiple pathophysiologic pathways are involved in SCD [Kato et al 2007]. Polymerized hemoglobin is also injurious to the red cell membrane, resulting in cellular dehydration, oxidative damage, and increased adherence to endothelial cells [Gladwin & Rodgers 2000, Hebbel 2000, Nagel 2001]. There is increasing awareness of a hyper-hemolysis syndrome associated with leg ulcers, priapism, and pulmonary artery hypertension. Other consequences of hemolysis include: chronic anemia, jaundice, predisposition to aplastic crisis, and cholelithiasis. Other factors contributing to the pathophysiology of sickle cell include: leukocytosis, resulting in increased production of injurious cytokines and altered blood flow; coagulation abnormalities; and abnormal vascular regulation. The net result of these abnormalities is shortened red cell life span or hemolysis and intermittent vascular occlusion and a state of chronic inflammation.Normal allelic variants. HBB, which spans 1.6 kb, contains three exons. HBB is regulated by an adjacent 5' promoter, which contains TATA, CAAT, and duplicated CACCC boxes, and an upstream regulatory element known as the locus control region (LCR). A number of transcription factors regulate the function of HBB, including the erythroid Kruppel-like factor (EKLF) which binds the proximal CACCC box and whose knockout in the mouse leads to a thalassemia-like clinical picture. Many other factors are critical, but their deletion results in milder phenotypes because of compensation by other factors. HBB is contained within the HBB gene cluster, which also includes the genes encoding the delta-globin chain, A gamma and G gamma chains, and HBBP1 (an HBB pseudogene) and epsilon.Pathologic allelic variants. Hemoglobin C results from a substitution in the second nucleotide of the sixth codon of HBB that codes for lysine instead of glutamic acid (Glu6Lys).Table 4. Selected HBB Pathologic Allelic VariantsView in own windowDNA Nucleotide Change ^{1Protein Amino Acid Change
(Standard Nomenclature 2)Reference Sequencesc.20A>TGlu6Val
(p.Glu7Val)NM_000518​.4
NP_000509​.1c.19G>AGlu6Lys
(p.Glu7Lys)c.364G>CGlu121Gln
(p.Glu122Gln)c.364G>AGlu121Lys
(p.Glu122Lys)See Quick Reference for an explanation of nomenclature. GeneReviews follows the standard naming conventions of the Human Genome Variation Society (www​.hgvs.org).1. DNA nucleotide changes follow current nomenclature guidelines where the number 1 corresponds to the first nucleotide of the initiating methionine. 2. In this column and throughout the text of the GeneReview, the protein amino acid changes (e.g., Glu6Val) follow the long-standing convention in the hemoglobin literature to begin numbering the amino acids at the second amino acid residue (Val) rather than the initiating Met. This convention was adopted many years ago because the initiating methionine is not part of the mature β-globin protein. The standard nomenclature for protein changes are given in parentheses. The Globin Gene Server (globin​.cse.psu.edu) lists variants using both numbering conventions. Normal gene product. HBB encodes the hemoglobin β chain. The normal heterotetrameric protein hemoglobin A (Hb A) is made up of two hemoglobin α chains, two hemoglobin β chains, and four heme moieties.Abnormal gene product. Sickle hemoglobin (Hb S) results from a single point mutation in which the codon determining the amino acid at position 6 of HBB has changed from a GAG codon for glutamic acid to GTG codon for valine. Hb S is a heterotetrameric protein made up of two hemoglobin α chains, two hemoglobin sickle-β chains, and four heme moieties.}